Download Thalamic POm projections to the dorsolateral striatum of rats

J Neurophysiol 108: 160 –174, 2012.
First published April 11, 2012; doi:10.1152/jn.00142.2012.
Thalamic POm projections to the dorsolateral striatum of rats: potential
pathway for mediating stimulus–response associations for sensorimotor habits
Jared B. Smith,1,2 Todd M. Mowery,1 and Kevin D. Alloway1,2
1
Department of Neural and Behavioral Sciences, Pennsylvania State University College of Medicine, Hershey, Pennsylvania;
and 2Center for Neural Engineering, Pennsylvania State University, University Park, Pennsylvania
Submitted 15 February 2012; accepted in final form 10 April 2012
basal ganglia; neuronal tracing; somatosensory; tactile stimulation;
thalamostriatal; thalamus
that the striatum is involved in
regulating the selection and execution of specific motor behaviors (McHaffie et al. 2005; Redgrave et al. 2010; Smith et al.
2011). Furthermore, although the dorsomedial striatum is important for mediating goal-directed behaviors, the dorsolateral
striatum (DLS) is necessary for executing well-learned sensorimotor habits (Aldridge and Berridge 1998; Balleine and
O’Doherty 2010; Graybiel 2008; Yin et al. 2006).
In rats, exploratory whisking is a highly repetitive behavior
that does not depend on rewarded outcomes. As such, it has the
hallmarks of a species-specific motor habit that is executed, in
part, by neural mechanisms in the DLS (Gao et al. 2001;
Welker 1964). Consistent with this view, unexpected whisker
contacts with external stimuli evoke stereotyped patterns of
whisker movements that seem to reflect the formation of a
stimulus–response association (Mitchinson et al. 2007; SachSUBSTANTIAL EVIDENCE INDICATES
Address for reprint requests and other correspondence: Kevin D. Alloway,
Center for Neural Engineering, Millennium Science Complex, Pennsylvania
State University, University Park, PA 16802 (e-mail: [email protected]).
160
dev et al. 2003). The importance of somesthetic information in
regulating whisking behavior and other well-learned motor
habits is underscored by the fact that the DLS receives dense,
overlapping projections from the primary somatosensory (SI)
barrel cortex and other somatosensory cortical areas (Alloway
et al. 2000, 2006; Brown 1998; Hoffer and Alloway 2001).
These dense corticostriatal projections have prompted recent
comparisons of cortical and DLS responses during controlled
whisker stimulation (Mowery et al. 2011; Pidoux et al. 2011;
Syed et al. 2011). Findings from our laboratory indicate that
repetitive whisker deflections evoke consistent responses in the
DLS that are qualitatively different from the responses recorded simultaneously in the SI barrel cortex (Mowery et al.
2011). Whereas SI cortical responses decline in magnitude as
the frequency of whisker stimulation increases (Ahissar et al.
2001; Chakrabarti and Alloway 2009; Khatri et al. 2004;
Melzer et al. 2006), the responses of the DLS neurons remain
relatively constant. Furthermore, analysis of the response latencies shows that DLS neurons respond either before or at the
same time as the neurons in the SI barrel cortex. These findings
strongly suggest that subcortical regions must be involved in
transmitting somesthetic information to the DLS.
Tracing studies have identified many subcortical regions that
project to the rodent striatum, including the amygdala, the
substantia nigra pars compacta (SNpc), and several intralaminar and modality-specific thalamic nuclei (Alloway et al. 2006;
Cheatwood et al. 2005; Erro et al. 2001, 2002; Kelley et al.
1982; Pan et al. 2010; Redgrave and Gurney 2006). No study,
however, has quantified the relative contributions of these
subcortical inputs to the whisker-sensitive regions in the DLS.
Furthermore, although the DLS and other parts of the striatum
receive thalamic projections from the centromedian and parafascicular nuclei (Castle et al. 2005; Deschenes et al. 1996;
Smith et al. 2009), there are conflicting data regarding whether
the DLS receives somesthetic-related projections from the
medial posterior (POm) and ventral posteromedial (VPM)
thalamic nuclei (Alloway et al. 2006; Deschenes et al. 1995;
Erro et al. 2001, 2002).
To establish whether thalamostriatal projections could transmit somesthetic information directly to DLS, we injected
retrograde and anterograde tracers into whisker-responsive
regions of the DLS and thalamus, respectively. Our results
demonstrate that POm, but not VPM, projects to whiskersensitive parts of the DLS. Furthermore, simultaneous recordings of whisker-sensitive neurons in POm and the DLS indicate
that POm responds to whisker deflections immediately before
the DLS is activated.
0022-3077/12 Copyright © 2012 the American Physiological Society
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Smith JB, Mowery TM, Alloway KD. Thalamic POm projections to the dorsolateral striatum of rats: potential pathway for
mediating stimulus–response associations for sensorimotor habits.
J Neurophysiol 108: 160 –174, 2012. First published April 11,
2012; doi:10.1152/jn.00142.2012.—The dorsolateral part of the
striatum (DLS) represents the initial stage for processing sensorimotor
information in the basal ganglia. Although the DLS receives much of
its input from the primary somatosensory (SI) cortex, peripheral
somesthetic stimulation activates the DLS at latencies that are shorter
than the response latencies recorded in the SI cortex. To identify the
subcortical regions that transmit somesthetic information directly to
the DLS, we deposited small quantities of retrograde tracers at DLS
sites that displayed consistent time-locked responses to controlled
whisker stimulation. The neurons that were retrogradely labeled by
these injections were located mainly in the sensorimotor cortex and, to
a lesser degree, in the amygdala and thalamus. Quantitative analysis
of neuronal labeling in the thalamus indicated that the strongest
thalamic input to the whisker-sensitive part of the DLS originates
from the medial posterior nucleus (POm), a somesthetic-related region
that receives inputs from the spinal trigeminal nucleus. Anterograde
tracer injections in POm confirmed that this thalamic region projects
to the DLS neuropil. In subsequent experiments, simultaneous recordings from POm and the DLS during whisker stimulation showed that
POm consistently responds before the DLS. These results suggest that
POm could transmit somesthetic information to the DLS, and this
modality-specific thalamostriatal pathway may cooperate with the
thalamostriatal projections that originate from the intralaminar nuclei.
THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
MATERIALS AND METHODS
their responses to a block of 50 or 100 stimulus-based trials were
recorded (see NEUROPHYSIOLOGY RECORDINGS). After locating an appropriate whisker-sensitive site, the tracer was iontophoretically ejected
into the DLS or thalamus by applying positive current pulses in
alternating on– off intervals of 7 s. To deposit FG, current was applied
in 1-␮A pulses for 20 min; for BDA, current was applied in 3- to
5-␮A pulses for 20 – 40 min. When both tracers were in the pipette
solution, current was applied in 3-␮A pulses for 20 –30 min.
Histochemistry. After a 7-day transport period, the injected rat was
deeply anesthetized with ketamine (60 mg/kg) and xylazine (18
mg/kg), and was transcardially perfused with physiologic saline containing 1% heparin, followed by 4% paraformaldehyde in 0.1 M
phosphate buffer (PB, pH 6.9), and 4% paraformaldehyde containing
10% sucrose. The brain was removed and, after the olfactory bulbs,
cerebellum, and caudal brain stem were removed, it was stored in 4%
paraformaldehyde with 30% sucrose. A shallow slit was made in the
ventral surface of the left hemisphere to provide a fiduciary mark for
mounting sections in the same orientation. Using a freezing microtome, coronal sections were obtained at a thickness of 60 ␮m and
stored in 0.1 M phosphate buffer saline (PBS).
For brains that received FG injections in the DLS, alternate sections
were mounted on gel-coated glass slides and dried overnight. One
series was stained with thionin to reveal the cytoarchitecture; the other
series was processed for fluorescent labeling by dehydrating the tissue
in ethanol and then defatting it with xylene before coverslipping with
Cytoseal.
For brains that received BDA injections in the thalamus, the brain
sections were divided into three series. The first series was processed for
cytochrome oxidase to reveal thalamic cytoarchitecture (Land and Simons 1985; Wong-Riley 1979). The second series was processed for
BDA as described before (Alloway et al. 1998; Kincaid and Wilson
1996). These sections were gently agitated in 0.3% H2O2 to reduce
background enzymes and then in 0.1 M PBS with 0.3% Triton X-100 (pH
7.4) before incubating for 2– 4 h in an avidin-biotin horseradish peroxidase solution (Vector Novocastra Laboratories, Burlingame, CA). After
rinsing sections with PBS, the tracer was visualized with 0.05% diaminobenzidine, 0.005% H2O2, 0.04% NiCl2, and 0.04% CoCl2 in 0.1 M Tris
buffer (pH 7.2) for 9 –12 min. The reaction was stopped by subsequent
washes in PBS and sections were mounted on gel-coated slides. The final
series was a backup to optimize BDA processing if necessary.
Anatomic analysis. Tissue processed for FG labeling was analyzed
using an Olympus BH-2 microscope with an Accustage plotting
system (St. Paul, MN) to create digital reconstructions of the retrogradely labeled neurons with respect to the outlines of the tissue
section and other anatomic landmarks. Labeled neurons were visualized with a UV filter (110000v2; Chroma Technology, Bellows Fall,
VT), and FG-labeled cells displaying one or more dendrites were
plotted. These reconstructions were overlaid onto photographic images of adjacent thionin sections using a graphics program (Canvas X;
Deneba Systems, Miami, FL). Low-magnification images of thionin
sections were acquired by an Epson V330 flatbed scanner. Photomicrographs of tissue labeling were acquired by a Retiga EX CCD digital
camera (Q-imaging, Surrey, British Columbia, Canada) mounted on the
microscope.
For retrograde tracing experiments, FG-labeled neurons were plotted in both hemispheres of each animal. Labeled neurons were
initially plotted in alternate sections of the tissue series that was
processed for fluorescent labeling. The number of plotted neurons in
each brain region was counted, and this count was normalized by
dividing it by the total number of plotted neurons in that animal. These
normalized values were averaged across animals, and statistical significance was determined using Origin software (version 8.0; Origin
Lab, Northampton, MA).
After alternate sections of the entire brain had been plotted, all
sections through the thalamus were identified and we plotted the
remaining labeled neurons in the alternate series that had not been
reconstructed the first time. Thus, in contrast to cortex and other brain
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Animals. Experiments were performed on male Sprague–Dawley rats
weighing 250 –700 (465 ⫾ 18) g. All surgeries and other procedures
complied with National Institutes of Health (NIH) guidelines and were
approved by the Institutional Animal Care and Use Committee.
Rat surgery. Rats were sedated with an intramuscular (IM) injection of ketamine (40 mg/kg) and xylazine (12 mg/kg). Atropine
methyl nitrate (0.05 mg/kg), dexamethasone sodium phosphate (5
mg/kg), and chloramphenicol (50 mg/kg) were administered IM, and
all rats were orally intubated before being placed in a stereotaxic
frame (David Kopf Instruments, Tujunga, CA). Heart rate, blood
oxygen saturation, and end-tidal CO2 were monitored continuously,
and body temperature was maintained at 37°C by a heated water pad
and a homeothermic heating blanket. The scalp was infiltrated with
lidocaine, and a midline incision was made to expose the cranial
surface. Flat machine screws were inserted over the left frontal cortex
for electrocorticography (ECoG), and over the cerebellum to provide
a ground lead for neuronal recordings and iontophoretic tracer injections. A 1-mm2 craniotomy was made over the DLS or thalamus to
enable neuronal recordings and tracer injections.
In all experiments, a data acquisition system (SciWorks, ver. 6.0;
DataWave Technologies, Broomfield, CO) provided on-line ECoG
displays to indicate the anesthetic state of the rat (Friedberg 1999).
Activity recorded from the dural surface of the frontal cortex was
amplified, filtered (0.3–300 Hz), and sampled at 256 Hz by an analog
to digital board (DT2839; Data Translation, Marlboro, MA). A colorcoded fast Fourier transform of ECoG activity was displayed and
updated once per second to visualize changes in cortical frequencies.
Power spectra dominated by 1–2 Hz were observed during deep
anesthesia when the rat was unresponsive to noxious stimuli. As
ketamine and xylazine were metabolized, the power spectra shifted to
frequencies of 5–7 Hz, which indicates a lightly anesthetized state
(Friedberg et al. 1999). In this state, whisker stimulation evoked
neuronal discharges in the DLS. Low levels of isoflurane (0.50 –1.0%)
were administered to maintain the rat in a relatively constant anesthetic plane in which ECoG activity was dominated by frequencies of
5–7 Hz for the remainder of the experiment.
Tracer injections. Anterograde and retrograde tracers were injected
using iontophoretic techniques. For retrograde tracing, a glass pipette
(20-␮m tip) filled with a 2% solution of Fluoro-Gold (FG) was
oriented 25° to the parasagittal plane and entered the DLS through a
craniotomy located 2 mm lateral and 1–2 mm caudal to bregma. For
anterograde tracing, a glass pipette (20-␮m tip) filled with a 15%
solution of biotinylated dextran amine (BDA) entered the thalamus
vertically through a craniotomy located 2.5–3 mm lateral and 3– 4 mm
caudal to bregma. In some cases, a combined solution of FG (2%) and
BDA (15%) was injected to reveal anterograde and retrograde connections from the same location. By chance, we found that adding FG
to the BDA solution also increased the retrograde transport of BDA to
a much greater degree than when BDA was injected alone, thereby
improving visualization of BDA labeling in both directions. In all
tracing experiments, a retention current (⫺5 ␮A) was applied to
prevent tracer leakage while the pipette penetrated the cortex and
external capsule.
When the target brain region was reached, the retention current was
turned off. The silver wire that was immersed in the tracer solution for
retention or ejection currents was then connected to the headstage of
an extracellular amplifier (Dagan 2200; Dagan Corp., Minneapolis,
MN) to enable recording of neuronal discharges in the target region
(i.e., DLS or thalamus). This technique allowed neuronal responses to
be recorded at precisely the same site that received the tracer deposit.
Extracellular potentials were band-pass filtered (300 Hz to 3 kHz),
amplified, and sampled at a rate of 25 kHz. The tracer-filled pipette
was slowly advanced while the contralateral whiskers were repetitively deflected by a computer-controlled stimulator (see WHISKER
STIMULATION). When whisker-sensitive neurons were encountered,
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THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
histograms (PSTHs), and statistical criteria were applied to determine
the onset and magnitude of the stimulus-induced responses. Based on
the mean rate of spontaneous activity, 99% confidence limits were
constructed and displayed on the PSTH of each neuron. Responses to
mechanical stimulation were considered statistically significant if
they exceeded the 99% confidence limits on two contiguous bins,
and the time of the first bin was defined as the response latency.
When simultaneous recordings in the DLS and thalamus were
completed, an electrolytic lesion was made prior to the animal’s
death so that the location of the deepest recording site could be
identified histologically.
Whisker stimulation. Multiple vibrissae (rows A–E, arcs 1–5) were
stimulated in tandem by a series of computer-controlled movements.
As described previously (Mowery et al. 2011), a small screen attached
to a galvanometer was positioned near the whisker pad (⬃10 mm
away) so that the whiskers protruded through the screen openings. A
waveform generator (ArbStudio; LeCroy, Chestnut Ridge, NY) controlled the movements of the galvanometer. In the initial experiments,
each trial consisted of three groups of four 50-ms triangular waves
presented at frequencies of 2, 5, and 8 Hz. In later experiments, a
sequence of four 50-ms triangular stimuli were presented in which the
interstimulus intervals decreased so that successive stimuli on each
trial were presented at intervals of 500 ms (2 Hz), 200 ms (5 Hz), and
125 ms (8 Hz). In both sets of recording experiments, the first stimulus
in each block of four stimuli was classified as a 1-Hz stimulus because
it was preceded by an interval of 1 s or longer in which no stimuli
were administered. Each stimulus moved the whiskers in the caudal
direction (1.5 mm) during the first 25 ms and then allowed them to
return to the original resting position over the next 25-ms period.
RESULTS
Results were obtained from a total of 24 rats. As shown in
Fig. 1, whisker-sensitive sites in the DLS of 7 rats received
focal deposits of FG to reveal retrogradely labeled neurons in
all brain regions that project to this part of the DLS. Subsequently, in a second group of rats (n ⫽ 10), an anterograde
tracer was placed in somesthetic-specific nuclei of the thalamus
to confirm the retrograde tracing results. In the last set of
experiments (n ⫽ 7), neuronal discharges were recorded simultaneously in POm and DLS during controlled whisker
stimulation.
Retrograde tracer injections in DLS. An example of FG
deposit at a whisker-sensitive site in the DLS is illustrated in
Fig. 2. The neuronal response at this injection site displayed
clear responses to repetitive whisker stimulation at 2, 5, or 8 Hz
and showed minimal adaptation at these frequencies (Fig. 2, A
and B). Although on-line neuronal isolation was often difficult
to achieve with tracer-filled pipettes, off-line waveform sorting
helped isolate responses that were consistent with our previous
results (Mowery et al. 2011). After recording neuronal responses to controlled whisker stimulation, the tracer was iontophoretically deposited (Fig. 2D’) from the same pipette while
it was positioned at the recording site. Occasionally, the recording-deposit site was marked by a small amount of necrosis
(Fig. 2D).
In the seven cases used to analyze retrograde labeling
patterns, tracer leakage did not appear along the pipette trajectory. The absence of tracer leakage rules out the possibility that
any labeled neurons represent projections to the tissue surrounding the electrode penetration. For all seven cases, tracer
injections were made 1.4 to 2.0 mm caudal to bregma and were
located entirely within the DLS without any diffusion into the
external capsule. Tracer deposits were located in the neuropil
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regions, we plotted all sections through the thalamus to obtain an
accurate count of labeled neurons in all of the thalamic nuclei. The
normalized proportion of labeled neurons in each thalamic nucleus
was calculated as described earlier.
For anterograde tracing experiments, sections through the DLS
were examined for the presence of BDA-labeled beaded varicosities
that represent en passant synapses (Kincaid and Wilson 1996; Meng
et al. 2004; Voight et al. 1993). Labeling of thalamic projections to the
DLS and cortex were photographed, and digital reconstructions of the
BDA-labeled varicosities were plotted relative to anatomical landmarks as described previously (Alloway et al. 2009).
Neurophysiology recordings. Neuronal discharges were recorded
from whisker-sensitive neurons in all rats. In the tracing experiments,
tracer-filled pipettes were used to record neuronal discharges from
whisker-sensitive regions in the thalamus or DLS before the tracer
was injected. In the remaining experiments, whisker-sensitive neurons
in the DLS and POm were recorded simultaneously with two highimpedance (1–2 megohms) tungsten electrodes to compare the onset
of well-isolated neuronal responses in these regions during controlled
whisker stimulation.
Prior to simultaneous recordings from the DLS and POm, an
acrylic headstage was constructed over the occipital ridge while the
rat’s head was held by stereotaxic ear bars. After exposing the
cranium, holes were drilled for electrode penetrations into the DLS
and POm. After applying dental acrylic (Hygenic, Akron, OH) over
small screws in the occipital ridge, two small bolts were placed head
down in the acrylic, approximately 10 mm apart. After the acrylic
cured for 10 min, a gooseneck manipulator (Flexbar Machine Corp.,
Islandia, NY) was fastened to each bolt with a nut. Subsequently, the
stereotaxic ear bars were withdrawn to remove nociceptive inputs
originating from the external auditory meatus. Consequently, low
concentrations of isoflurane (0.5–1.0%) were sufficient to maintain
each rat in a stable, lightly anesthetized plane that facilitated detection
of whisker-evoked neuronal responses in the DLS and POm. Although
isoflurane produces a dose-dependent suppression of thalamocortical
transmission, the concentration of isoflurane used for simultaneous
recordings from the POm and DLS was minimized and was well
below levels that produce significant suppression of thalamic responses (Detsch et al. 1999, 2002; Masamoto et al. 2009).
Craniotomies for simultaneous recordings in the DLS and POm
were made at the same coordinates used to inject tracers into these
regions (see TRACER INJECTIONS). A single electrode penetration was
made in each brain region, and isolated neuronal discharges were
recorded during whisker pad stimulation at successive depths in both
the DLS (3– 6 mm below pia) and POm (4.5– 6 mm below pia).
Mechanical stimulation of the contralateral whiskers was conducted
as each electrode advanced to its target brain region. This enabled
detection of physiologic cues that indicate the location of the electrode
in its trajectory toward the whisker-sensitive neurons in each region.
In the case of thalamic recordings, for example, quiescent neuronal
activity was encountered as the electrode entered the ventricular space
below the hippocampus, and whisker-sensitive neurons were subsequently encountered at successive depths that correspond to POm and
VPM (Paxinos and Watson 2005).
In all rats, including those receiving tracer injections, stimulusevoked extracellular waveforms in the DLS and thalamus were visualized on a digital oscilloscope (Tektronix DPO4034; Tektronix,
Beaverton, OR) while listening to the discharges on acoustic speakers.
Trial-based neuronal responses were stored on hard disk and were
replayed to enable sorting of waveforms by amplitude, width, and
other criteria. Biphasic extracellular discharges were plainly evident
in the waveforms recorded by the tracer-filled pipettes in the anatomic
experiments. Although the waveforms were not always as well isolated as the waveforms recorded by tungsten electrodes in subsequent
experiments, most DLS discharges matched the firing patterns associated with medium spiny neurons. Neuronal discharges were timestamped to a resolution of 0.1 ms, displayed as peristimulus time
THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
163
and appeared to avoid the fiber fascicles that contain corticothalamic and thalamocortical projections.
Retrograde labeling throughout the CNS. Neuronal cell
bodies labeled by FG injections in the DLS were plotted in
both hemispheres for all brain regions. Differences in the
spatial extent of the tracer injections in the DLS led to differences in the total number of labeled neurons. Among the seven
rats that received FG deposits in the DLS, we plotted an
average of 10,012 ⫾ 3,619 (mean ⫾ SE) neurons throughout
the brain. Although the number of FG-labeled neurons varied
with the size of the tracer injections, virtually the same brain
regions contained labeled neurons in each case. Brain regions
that consistently contained neuronal labeling included sensorimotor cortex, globus pallidus (GP), amygdala, substantia
nigra pars compacta (SNpc), the dorsal raphe, and several
thalamic nuclei. In cases that received the largest DLS injec-
tions, some neuronal labeling appeared in limited parts of
cortex that were interpreted to be the insular, auditory, and
visual cortical regions.
The densest retrograde labeling in the cortex appeared in
areas that are associated with sensorimotor functions (Paxinos
and Watson 2005). In these regions, cortical labeling was most
prominent in layer Va (Fig. 3, A, B, B’, and C). This labeling
was distributed bilaterally, but most of the labeled neurons
were in the ipsilateral hemisphere, which is consistent with our
previous results (Alloway et al. 2006).
In contrast to that previous study, in which tangential cortical sections were processed for cytochrome oxidase, the
boundaries of many cortical areas were not clearly demarcated
in the coronal sections obtained in the present study. Nonetheless, cortical labeling patterns in the two studies appeared
similar. As in our previous report (Alloway et al. 2006),
Fig. 2. Injection of FG in a whisker-responsive part of the DLS. A: segment of a fast Fourier transform of electrocorticographic (ECoG) activity displayed on-line
in this experiment. Dominant frequencies of 5–7 Hz indicate a lightly anesthetized preparation that permits whisker-evoked responses in the DLS. B: peristimulus
time histogram (PSTH) illustrating the DLS response to 100 trials of whisker deflections at 2, 5, and 8 Hz. Waveform scales: 200 ␮V, 1 ms. PSTH bins: 2 ms.
C: thionin-stained coronal section 2.0 mm caudal to bregma; rectangle indicates region in D showing necrosis at the FG injection site. D’: adjacent section
showing the focal size of the FG injection in the DLS. Scale bars: 1 mm in C; 250 ␮m in D.
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Fig. 1. Location of Fluoro-Gold (FG) injections in the dorsolateral striatum (DLS). The FG injections in the DLS of seven rats are represented by color-coded
outlines on a composite map of the striatum surrounded by the external capsule (ec), internal capsule (ic), and globus pallidus (GP). Numbers indicate distance
caudal from bregma.
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THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
approximately 40% of the cortical neuronal labeling appeared
in regions identified as motor cortex (Paxinos and Watson
2005), and slightly more than half of the cortical labeling was
in somatosensory regions directly above the DLS. This densely
labeled region probably represented the combination of the SI
and SII cortical areas. The remaining cortical labeling was
more scattered and appeared laterally in regions that represent
the parietal ventral and perirhinal regions.
Compared with labeling in the presumed SI and SII regions,
which was predominantly on the ipsilateral side, neuronal
labeling in MI was more evenly distributed across both hemispheres. This finding is consistent with the presumed roles of
MI cortex and the striatum in coordinating the bilateral, synchronous movements of the whiskers during behavioral exploration (Alloway et al. 2009). As shown in Fig. 3, FG-labeled
neurons appeared in the amygdala of both hemispheres but
were predominantly on the ipsilateral side. On each side,
neuronal labeling in the amygdala was restricted to the magnocellular and intermediate portions of the basal nucleus; the
central, lateral, medial, and cortical nuclei did not contain any
labeled neurons.
The ipsilateral SNpc contained many large, brightly labeled
neurons with oblong soma and intermingled dendritic processes that were predominantly oriented in the mediolateral
dimension (Fig. 3, F, F’, G, and H). In all rats, labeled
neurons in the SNpc appeared in two separate clusters that
resided in the dorsal and lateral subnuclei. Based on their
projections to the DLS, labeled neurons in the SNpc were
presumed to be dopaminergic, but this could not be confirmed because sections were not processed for the presence
of tyrosine hydroxylase.
Although we occasionally observed FG-labeled neurons in
the SNpc located contralateral to the DLS injection sites, the
number of midline-crossing nigrostriatal projections accounted
for ⬍1% of all labeled neurons in the SNpc. This result is
consistent with reports indicating that few dopaminergic neurons in the SNpc project to the striatum in the contralateral
hemisphere (Consolazione et al. 1985; Pritzel et al. 1983).
Additional retrograde labeling was observed in the GP and
raphe nuclei. In agreement with other studies (Pan et al. 2010),
only a small number of labeled neurons appeared in the midline
dorsal raphe. Although more labeled neurons appeared in the
GP than in the dorsal raphe, the combined sum of labeled
neurons in both regions accounted for only 1% of the total
number of labeled neurons plotted in each rat.
Retrograde labeling in the thalamus. Retrogradely labeled
neurons were observed in several thalamic nuclei of each rat.
Consistent with the known topography of thalamostriatal projections (Berendse and Groenewegen 1990), labeled neurons
were located in the parafasicular (Pf), centromedian (CM), and
ethmoid nuclei. In addition to these intralaminar regions, many
labeled neurons also appeared in modality-specific regions,
including the ventrolateral (VL), ventral posterolateral (VPL),
ventral posteromedial (VPM), lateral posterior (LP), and POm
nuclei.
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Fig. 3. Retrogradely labeled neurons produced by the FG injection shown in Fig. 2. A: section of SI cortex illustrating the region magnified in subsequent panels.
B, B’: adjacent sections showing the primary somatosensory (SI) cortex and the location of FG-labeled corticostriatal neurons in layer Va. Box indicates the
region in C. D, E’: bilateral sections showing retrogradely labeled neurons in the basolateral amygdala. F, G’: cytoarchitecture of substantia nigra and FG-labeled
neurons in the dorsal and lateral subnuclei of the pars compacta. Arrowheads indicate common blood vessels. Scale bars: 1 mm in A; 250 ␮m in B, D, and F; 100
␮m in C and G.
THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
retrograde transport of BDA, thereby enabling BDA transport in
both the anterograde and retrograde directions.
Thalamostriatal projections from POm. As indicated in
Table 1, a total of four rats received tracer deposits entirely
within the POm nucleus. Figure 6 illustrates a case that received the largest injection of BDA in the POm. Consistent
with other reports (Diamond et al. 1992b), neurons recorded at
this and other POm injection sites displayed responses to
multiple whiskers. As indicated by Fig. 6A, POm neurons
occasionally contained more than one response for each stimulus when whiskers were deflected at 2 Hz. In view of evidence
that POm receives both descending projections from the SI
cortex and ascending projections from the trigeminal nuclei
(Ahissar 1998; Alloway 2008), these dual responses could
reflect corticothalamic feedback that is asynchronous with
respect to the ascending sensory input. Regardless of the exact
mechanism that mediates this temporal pattern, our histology
confirmed that BDA was injected into the POm and that it
diffused throughout much of the nucleus (Fig. 6, C and C’).
Consistent with a previous report (Deschenes et al. 1998),
inspection of the thalamus revealed large numbers of BDA-
Fig. 4. Retrogradely labeled neurons in the posterior medial (POm) and parafasicular (Pf) thalamic nuclei produced by the FG injection shown in Fig. 2. A: coronal view
of the thalamus located 4.25 mm caudal to bregma; rectangle indicates the view in B and the locations of labeled neurons reconstructed in B’. C, D: photomicrographs
of nucleus Pf showing a dense cluster of FG-labeled neurons. E, F: photomicrographs showing a dense cluster of FG-labeled neurons in POm but comparatively few
labeled neurons in ventral posteromedial (VPM) thalamic nuclei. Scale bars: 1 mm in A; 500 ␮m in B; 250 ␮m in C; 100 ␮m in D.
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As shown by the photomicrographs and plotted reconstructions in Figs. 4 and 5, neuronal labeling was most prominent in
the POm, Pf, and CM nuclei. Coronal sections that contained
the Pf and POm nuclei usually contained labeled neurons in
both of these regions. In the Pf nucleus, scattered clusters of
neurons with small, brightly lit cell bodies appeared just lateral
to the fasciculus retroflexus (Fig. 4, C’ and D). More laterally,
larger numbers of FG-labeled neurons were present throughout
the nucleus POm, with the densest clusters located near the
VPM border (Fig. 4, E’ and F).
Anterograde tracer injections in the thalamus. Both POm
and VPM process somesthetic information received from the
trigeminal nuclei (Chiaia et al. 1991; Peschanski 1984;
Timofeeva et al. 2004; Veinante et al. 2000a). Consequently,
either of these thalamic nuclei could transmit whisker-related
information directly to the DLS. To rule out the possibility that
the retrogradely labeled neurons in these thalamic nuclei might
represent tracer uptake by fibers of passage, we performed some
anterograde tracing experiments. The anterograde tracer BDA or
a mixture of BDA and FG was injected into POm, VPM, or VPL
of 10 rats (see Table 1). Mixing FG with BDA enhanced the
165
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THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
labeled fibers originating from POm that traversed the VPM en
route to the DLS and cortex. In addition, we also observed
retrogradely labeled neurons in the interpolaris and other spinal
trigeminal nuclei, which is consistent with tracer injections in
the POm (Chiaia et al. 1991; Peschanski 1984).
The mixture of FG and BDA revealed both anterograde and
retrograde labeling in SI barrel cortex (see Fig. 6, D, E, and
E’). Compared with adjacent sections processed for cytochrome oxidase, the BDA-labeled terminals were densest in
layer Va, and this pattern replicates previous anterograde
tracing data (Wimmer et al. 2010). Like other reports that
analyzed corticothalamic projections to the POm (Killackey
and Sherman 2003; Veinante et al. 2000b), retrogradely labeled
neurons were predominantly in layer Vb and VIb of the SI
cortex, with additional neuronal labeling in layer VIa of the
septal-aligned columns. Terminal labeling from the POm injections was also observed in motor cortical areas that contain
the whisker representations (Alloway et al. 2009).
Injections of BDA into the POm revealed dense bundles of
axons and their terminal arbors throughout the posterior DLS
(Fig. 7). High-power photomicrographs revealed labeled terminals in the DLS neuropil that displayed small (⬍1 ␮m)
beaded varicosities representing en passant synaptic contacts
(Fig. 7C). Reconstructions of these axonal enlargements indicate that thalamostriatal projections from POm terminate most
Table 1. Summary of thalamic tracer injections
Case
Tracer1
Nucleus2
TS01
TS03
TS04
TS05
TS08
TS09
TS12
TS13
TS15
TS16
BDA
BDA
BDA
BDA
BDA
BDA
BDA
BDA
FG/BDA
FG/BDA
VPL
LP/POm
VPM
VPM
POm
POm
POm
VPM/POm
VPM
POm
1
BDA, biotinylated dextran amine; FG, Fluoro-Gold.2POm, posteromedial;
VPM, ventroposteromedial; VPL, ventroposterolateral; LP, lateral posterior.
densely along the edge of the DLS that adjoins the external
capsule. Furthermore, in every case that received a BDA
deposit in the POm (n ⫽ 5), small puffs of BDA-labeled
terminal arbors were apparent in the DLS neuropil as described
previously (Deschenes et al. 1995). These data validate the
retrograde tracing results showing that POm projects to the
whisker-sensitive parts of the DLS.
Absence of thalamostriatal projections from VPM. As indicated in Table 1, a total of three rats received tracer deposits
that were located entirely within the VPM. Figure 8 illustrates
a tracer deposit at a VPM site that responded primarily to
deflections of a single whisker, and similar receptive field
properties were recorded in other VPM cases. As indicated by
the PSTH obtained at the injection site (see Fig. 8A), the
neuronal response to repetitive whisker stimulation was robust
and displayed little adaptation to progressive increases in
stimulus frequency. In all cases, tracer deposits in the VPM
produced dense thalamocortical terminal labeling in the layer
IV barrels and labeled many corticothalamic neurons whose
soma were located in layer VIa (see Fig. 9). These results
corroborate previous reports that characterized VPM connections with the SI barrel cortex (Killackey and Sherman 2003;
Veinante et al. 2000b; Wimmer et al. 2010).
Microscopic inspection of the DLS revealed bundles of
labeled fibers en route to the overlying cortex, but did not
reveal any axonal arbors or beaded varicosities in the DLS
neuropil. Similar results were obtained in all cases in which
tracers were injected into the VPM or VPL nuclei. These
findings support previous data indicating that VPM neurons
send their axonal projections through the striatum en route to
the SI cortex, but do not innervate the DLS neuropil (Deschenes et al. 1996). Thus, retrogradely labeled VPM neurons
produced by tracer injections in the DLS are probably due to
tracer uptake by fibers of passage.
Quantitative analysis of thalamic labeling. Based on our
retrograde and anterograde tracing results, we performed several statistical analyses to determine the relative contributions
of brain regions that project to the whisker-sensitive part of the
DLS. Figure 10A, for example, depicts the normalized distri-
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Fig. 5. Plotted reconstructions of FG-labeled neurons in a series of coronal sections through the thalamus following the tracer injection depicted in Fig 2. Numbers
in top right indicate the distance caudal from bregma. Scale bar: 1 mm. APT, anterior pretectal; eth, ethmoid; LD, lateral dorsal; LGN, lateral geniculate; LP,
lateral posterior; ml, medial lemniscus; VM, ventral medial (VM); ZI, zona incerta.
THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
167
bution of retrogradely labeled neurons after placing FG in the
DLS. As the bar graphs in that figure indicate, the cerebral
cortex contained approximately two thirds of the labeled neurons, and the vast majority of these were located in SI, MI, and
other sensorimotor cortical regions. By comparison, the
amygdala and thalamus contained the second (13.6%) and third
(8.8%) largest proportion of labeled neurons, respectively.
Consistent with these regional variations, a two-way ANOVA
indicated significant differences in neuronal labeling across
brain regions (F ⫽ 19.50, P ⬍ 0.0001) and indicated that
labeling was significantly higher in the ipsilateral than in the
contralateral hemisphere (F ⫽ 31.75, P ⬍ 0.0001). In fact,
paired t-tests confirmed that neuronal labeling was higher on
the ipsilateral side for both cortex (t ⫽ 22.31, P ⬍ 0.0001) and
the amygdala (t ⫽ 4.50, P ⬍ 0.001).
Variations in thalamic labeling were apparent because of
differences in the size and locations of the DLS tracer injections (see Fig. 1), but the normalized distributions were highly
similar in each rat. As indicated by Fig. 10B, approximately a
third of all labeled neurons in the thalamus were in the nucleus
POm, and this proportion was nearly twice that observed in the
Pf (17.3%) or Cm (16.5%) nuclei. These values, however,
reflect the inclusion of VPM and VPL in the overall distribution of thalamic labeling.
In view of our anterograde tracing results, we reanalyzed the
distribution of thalamic labeling after removing all counts of
labeled neurons that appeared in VPM and VPL. As shown in Fig.
10B, making this correction revealed that 40% of the thalamostriatal projections originated from POm, and that the PF and CM
nuclei each contributed slightly ⬎20% of the thalamic projections
to the DLS injection sites. A one-way ANOVA revealed significant differences in the regional distribution of labeled neurons in
the thalamus (F ⫽ 25.5, P ⬍ 0.0001). A paired t-test confirmed
that the proportion of labeled neurons was significantly higher in
POm than in the CM (t ⫽ 3.69, P ⬍ 0.05) or Pf (t ⫽ 3.25, P ⬍
0.05) nuclei.
Simultaneous recordings in POm and DLS. To determine
whether POm could be responsible for the rapid transmission
of somesthetic information to the DLS, neuronal responses to
computer-controlled whisker deflections were simultaneously
recorded in POm and the DLS. Results from one of these
experiments are illustrated in Fig. 11. As the PSTHs in Fig. 11
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Fig. 6. Injection of biotinylated dextran amine (BDA) in a whisker-responsive part of POm. A: PSTH response to 50 trials of whisker deflections at 2, 5, and
8 Hz. Figurine indicates the neuron’s receptive field. Waveform scales: 200 ␮V, 1 ms. PSTH bins: 2 ms. B and C’: cytochrome oxidase (CO) processing reveals
the POm–VPM boundary. C’: location of BDA tracer deposit in POm. Labeling in VPM is due to tracer transport through axons originating from POm as they
course through VPM en route to the SI cortex and the DLS. D and E’: BDA labeling in the SI cortex shows a distinct pattern of terminals in layer Va and neurons
in layer VI, predominantly in the septal columns. Scale bars: 1 mm in B; 500 ␮m in C and D; 250 ␮m in E.
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THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
indicate, whisker-sensitive neurons in POm and the DLS respond to repetitive stimulation with minimal amounts of sensory adaptation (see Fig. 11, B and D). Importantly, comparison of the latencies of these neuronal responses indicate that
activation of the DLS neuron was preceded by a response from
the POm neuron (Fig. 11E).
Simultaneous neuronal recordings from the DLS and POm
of seven rats generated whisker-evoked responses from 18
Fig. 8. Injection of BDA in the VPM nucleus. A: PSTH response to 100 trials of whisker stimulation at 2, 5, and 8 Hz. Figurine indicates the neuron’s receptive
field. Waveform scales: 200 ␮V, 1 ms. PSTH bins: 2 ms. B: coprocessed section of thalamus showing VPM. Inset: magnified view in C. C’: focal BDA injection
in VPM; arrows indicate common blood vessels. Scale bars: 1 mm in B; 500 ␮m in C.
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Fig. 7. Terminal labeling in the DLS produced by the BDA injection in POm shown in Fig. 6. A: labeled terminals in the reticular nucleus of the thalamus. Inset: location
of B. B: labeled terminals and their varicosities in the DLS; inset indicates C. C: magnified view of a dense plexus of BDA-labeled terminals and varicosities in the lateral
edge of the DLS. D: reconstructions of labeled varicosities in DLS of this case. Scale bars: 500 ␮m (A); 100 ␮m (B); 50 ␮m (C); 1 mm (D).
THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
169
POm and 20 DLS neurons. In this sample, the latency of
stimulus-evoked responses in POm ranged from 9 to 20 ms,
whereas the corresponding values in the DLS ranged from 10
to 22 ms. As indicated by Fig. 11D, mean response latencies
were at least 1.7 ms shorter in the POm than in the DLS for
each of the stimulus frequencies tested. These differences in
latency were significant when whiskers were deflected at 1 Hz,
2 Hz, or 5 Hz (t ⬎ 1.71 P ⬍ 0.05, one-tailed t-test), but failed
to reach statistical significance at 8 Hz.
DISCUSSION
In contrast to studies that relied only on stereotaxic coordinates to locate tracer injection sites in the striatum, we injected
retrograde tracers only at those DLS locations in which neurons responded to whisker stimulation. This procedure revealed that two thirds of all neurons projecting to the whiskersensitive DLS are located in the cerebral cortex, and that the
vast majority of these are in the sensorimotor regions of the
ipsilateral hemisphere. Neurons in the amygdala accounted for
approximately 15% of all labeled neurons, making it the
second largest fraction of inputs to the whisker-sensitive DLS.
These results indicate that the cortex and amygdala must
influence striatal processing, but these structures are several
synapses away from the periphery and are unlikely to represent
the fastest route for transmitting sensory information to the
DLS.
The thalamus contained the next highest proportion of labeled neurons, and several pieces of evidence suggest that the
POm nucleus is a major source of the short-latency responses
that we recorded in the DLS during whisker stimulation. First,
our retrograde tracing results revealed that POm contained
more labeled neurons than any other thalamic nucleus. Subsequent experiments with anterograde tracers confirmed that
whisker-sensitive sites in POm project to the DLS. Finally, our
physiology experiments indicate that POm responds to whisker
deflections at latencies that precede the responses of whiskersensitive neurons in the DLS. Collectively, these results
strongly suggest that stimulus-induced somesthetic inputs are
conveyed directly to the DLS by projections from the POm
nucleus.
Although our data indicate that the whisker-sensitive DLS
region receives more inputs from POm than from any other
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Fig. 9. Cortical and striatal labeling produced by the BDA injection shown in Fig. 8. A and B’: coprocessed section showing layer IV barrel field and BDA labeling
in SI cortex. C: magnified view of colabeled barrels from the rectangle in B. C’: adjacent section shows BDA terminal labeling in layer IV barrels (D) and
retrogradely labeled neurons in layer VIa (E). F–H: inspection of the DLS reveals BDA-labeled fiber bundles but no labeled varicosities in the DLS. Scale bars:
1 mm (A); 500 ␮m (B,F); 250 ␮m (C,G); 100 ␮m (D,H).
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THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
thalamic nucleus, other thalamic nuclei are more dominant in
other parts of the striatum. Thalamostriatal projections are
topographically organized (Groenewegen and Witter 2004;
Smith et al. 2004), and our BDA injections in POm indicate
that this thalamic nucleus projects mainly to the DLS. By
contrast, the midline and intralaminar nuclei innervate more
widespread parts of the striatum (Berendse and Groenewegen
1990; Groenewegen and Witter 2004; Smith et al. 2004), many
of which receive few if any projections from POm.
Sensory responsiveness in the POm. Substantial controversy
surrounds the functional role of the POm in processing somesthetic information. Much of this controversy stems from
reports indicating that POm responses are weaker and have
latencies that are longer and more variable than the responses
recorded from VPM neurons (Ahissar et al. 2000; Diamond et
al. 1992b; Masri et al. 2008; Sosnik et al. 2001). In addition,
POm neurons become less responsive if the SI cortex is
inactivated (Diamond et al. 1992a).
Several facts can account for the apparent discrepancy between the strong POm responses that we observed and the
weak POm responses that others have reported. First, we
recorded POm and DLS responses from rats in a very lightly
anesthetized state, as indicated by ECoG activity that was
dominated by frequencies of 5–7 Hz (Friedberg et al. 1999).
The anesthetic state has a significant impact on DLS responsiveness, and stimulus-evoked neuronal discharges are rarely
recorded in the DLS of more deeply anesthetized rats (Pidoux
et al. 2011; West 1998). Indeed, we found that increasing the
concentration of isoflurane ⬎1% suppressed neuronal responses in both POm and the DLS to a similar extent (data not
shown); this further supports the view that DLS responses to
somesthetic stimuli depend, in part, on inputs from POm.
In contrast to studies that relied on air-jet stimulation to
evoke whisker-related responses in POm (Masri et al. 2008;
Sosnik et al. 2001), we used direct mechanical contact to
simultaneously deflect multiple whiskers. Direct contact with
the whiskers removes variations in the onset of whisker movement and, by moving multiple whiskers at the same time,
simultaneously activates convergent inputs to the POm. This is
important because POm neurons have large receptive fields
that extend across multiple whiskers (Diamond et al. 1992b).
Although corticothalamic feedback contributes to POm respon-
siveness (Ahissar 1998; Diamond et al. 1992a), strong sensory
activation of convergent projections from the spinal trigeminal
nuclei should evoke rapid responses in POm. Neurons in the
POm have bushy radiating dendrites and extensive axonal
arbors that terminate in well-defined bands in the DLS (Deschenes et al. 1995, 1998). These structural features should
promote synchronous activation of clusters of POm neurons,
thereby providing effective excitatory drive to neuronal targets
in the DLS.
Thalamostriatal projections. Most discussions of thalamostriatal projections have focused on projections from the intralaminar nuclei, especially the centromedian and parafascicular
(CM/Pf) nuclear complex (McHaffie et al. 2005; Smith et al.
2009). Consistent with this focus, the CM/Pf complex contains
a large fraction of the thalamic neurons that project to the
whisker-sensitive part of the DLS. The function of the thalamostriatal projections from the CM/Pf complex is poorly understood, however, because very few studies have characterized
neuronal response properties in the intralaminar nuclei. In
monkeys, some PF neurons display short-latency responses to
multiple modalities, including visual, auditory, and somatosensory inputs (Minamimoto and Kimura 2002). In rats, Pf neurons and other intralaminar nuclei respond to whisker deflections, but response latencies to peripheral stimulation have not
been measured (Krauthamer et al. 1992). Importantly, superior
colliculus lesions reduce the detectability of somesthetic responses in PF, whereas direct electrical stimulation of the
superior colliculus effectively activates Pf and other intralaminar regions (Grunwerg and Kauthamer 1992). These and other
findings have prompted the hypothesis that the superior colliculus transmits highly salient, multimodal sensory inputs to
midline and intralaminar thalamic nuclei that are involved in
regulating the striatal-based “decisions” that underlie behavioral selection (McHaffie et al. 2005; Smith et al. 2009).
Relevant to this discussion, the striatum also receives inputs
from higher-order thalamic nuclei that process sensory-specific
information. The pulvinar and lateral posterior nuclei, for
example, receive visual information directly from the retina
(Boire et al. 2004; Cowey et al. 1994; Itaya and van Hoesen
1983) and from the upper layers of the superior colliculus
(Abrahamson and Chalupa 1988; Harting et al. 2001a). In turn,
these extrageniculate thalamic nuclei project to the striatum,
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Fig. 10. Bar histograms illustrating mean proportion of FG-labeled neurons in different brain regions after injecting FG into the DLS of seven rats. A: distribution
of cortical and subcortical labeling in the ipsilateral and contralateral hemispheres after reconstructing alternate sections throughout entire brain. B: distribution
of labeled neurons in different thalamic nuclei after reconstructing every coronal section through the thalamus. Data are expressed for all plotted neurons (Raw)
and after subtracting labeled neurons appearing in VPM and VPL (Corrected). Brackets indicate SE.
THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
171
presumably to convey vision-related information (Day-Brown
et al. 2010; Harting et al. 2001b; Lin et al. 1984; Takada et al.
1985).
These findings suggest that transmission of multimodal sensory
information through the multisynaptic tecto-thalamo-striatal circuits could be augmented by parallel sets of thalamostriatal
pathways that originate in modality-specific higher-order thalamic
nuclei. Thus, projections from POm to the DLS could represent a
higher-order thalamostriatal pathway that augments the tectothalamo-striatal circuit connections. Whereas thalamostriatal projections from intralaminar nuclei are thought to be important for
conveying unexpected sensory signals needed to redirect attention, we propose that sensory-specific thalamostriatal projections
from higher-order nuclei (e.g., pulvinar or POm) cooperate with
the thalamostriatal projections from the intralaminar nuclei. Such
cooperation could increase both the salience of a peripheral
stimulus and, in turn, the likelihood of activating postsynaptic
targets in the DLS. In addition, these thalamostriatal projections
could rapidly provide the modality-specific information needed to
select and initiate a specific sensory-guided response to an external stimulus.
Corticostriatal projections. Our quantitative tracing data,
both here and in a previous report (Alloway et al. 2006),
indicate that sensorimotor cortex sends the most inputs to the
DLS. These findings, however, could easily be misinterpreted
as indicating that the sensorimotor cortex is the most influential
source of somesthetic inputs to the DLS. This is not necessarily
correct because only a small fraction of the sensorimotor
cortical population is likely to be synchronized at any given
time during different sensorimotor behaviors. By contrast,
dense clusters of POm neurons with widespread overlapping
dendrites should become synchronized in response to stimuli
that simultaneously deflect multiple whiskers, and this could
effectively activate whisker-sensitive neurons in the DLS (Deschenes et al. 1995).
The precise role of corticostriatal projections from SI and
other sensorimotor regions during peripheral somatosensory
stimulation remains unclear. The exact nature and importance
of corticostriatal contributions to DLS responses evoked by
peripheral stimulation require additional research. For example, determining the contribution of thalamostriatal inputs on
DLS responsiveness could be achieved by recording stimulus-
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Fig. 11. Comparison of response latencies recorded simultaneously in the whisker-sensitive parts of POm and the DLS. A: microlesion at a whisker-sensitive recording
site in POm (circle) and the PSTH showing the corresponding neuronal response recorded prior to making the lesion. B: microlesion (circle) made at a site in DSL where
a whisker-sensitive medium spiny neuron was recorded simultaneously (PSTH) with the POm response shown in A. C: PSTHs showing the mean response to the 2-Hz
stimulus presentations. Arrows indicate the latency of the responses as defined by the first bin to exceed the 99% confidence limits (dashed line). D: response latencies
at each stimulus frequency for the pair of POm and DLS responses displayed in A and B. E: mean response latencies for the sample of neurons recorded in POm and
the DLS. Error bars represent SE. Asterisks indicate significant differences in latencies for POm and DLS neurons (*P ⬍ 0.05).
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THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
ACKNOWLEDGMENTS
We thank J. Harrold for helping with the rat preparation for the neurophysiology experiments.
GRANTS
This work was supported by National Institute of Neurological Disorders
and Stroke Grant NS-37532 awarded to K. D. Alloway.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
AUTHOR CONTRIBUTIONS
J.B.S., T.M.M., and K.D.A. conception and design of research; J.B.S. and
T.M.M. performed experiments; J.B.S., T.M.M., and K.D.A. analyzed data;
J.B.S. and K.D.A. interpreted results of experiments; J.B.S., T.M.M., and
K.D.A. prepared figures; J.B.S. drafted the manuscript; J.B.S., T.M.M., and
K.D.A. edited and revised the manuscript; J.B.S., T.M.M., and K.D.A. approved the final version of the manuscript.
REFERENCES
Abramson BP, Chalupa LM. Multiple pathways from the superior colliculus
to the extrageniculate visual thalamus of the cat. J Comp Neurol 271:
397– 418, 1988.
Ahissar E. Temporal-code to rate-code conversion by neuronal phase-locked
loops. Neural Comput 10: 597– 650, 1998.
Ahissar E, Sosnik R, Bagdasarian K, Haidarliu S. Temporal frequency of
whisker movement. II. Laminar organization of cortical representations. J
Neurophysiol 86: 354 –367, 2001.
Ahissar E, Sosnik R, Haidarliu S. Transformation from temporal to rate
coding in a somatosensory thalamocortical pathway. Nature 406: 302–306,
2000.
Aldridge JW, Berridge KC. Coding of serial order by neostriatal neurons: a
“natural action” approach to movement sequence. J Neurosci 18: 2777–
2787, 1998.
Alloway KD. Information processing streams in rodent barrel cortex: the
differential functions of barrel and septal circuits. Cereb Cortex 18: 979 –
989, 2008.
Alloway KD, Lou L, Nwabueze-Ogbo F, Chakrabarti S. Topography of
cortical projections to the dorsolateral neostriatum in rats: multiple overlapping sensorimotor pathways. J Comp Neurol 499: 33– 49, 2006.
Alloway KD, Mutic JJ, Hoffer ZS, Hoover JE. Overlapping corticostriatal
projections from the rodent vibrissal representations in primary and secondary somatosensory cortex. J Comp Neurol 428: 51– 67, 2000.
Alloway KD, Mutic JJ, Hoover JE. Divergent corticostriatal projections from
a single cortical column in the somatosensory cortex of rats. Brain Res 785:
341–346, 1998.
Alloway KD, Smith JB, Beauchemin KJ, Olson ML. Bilateral projections
from rat MI cortex to the neostriatum, thalamus, and claustrum: forebrain
circuits for modulating whisking behavior. J Comp Neurol 515: 548 –564,
2009.
Balleine BW, Killcross AS, Dickinson A. The effects of lesions of the
basolateral amygdala on instrumental conditioning. J Neurosci 23: 666 –
675, 2003.
Balleine BW, O’Doherty JP. Human and rodent homologies in action control:
corticostriatal determinants of goal-directed and habitual action. Neuropsychopharmacology 35: 48 – 69, 2010.
Berendse HW, Groenewegen HJ. Organization of the thalamostriatal projections in the rat, with special emphasis on the ventral striatum. J Comp
Neurol 299: 187–228, 1990.
Boire D, Matteau I, Casanova C, Ptito M. Retinal projections to the lateral
posterior-pulvinar complex in intact and early visual cortex lesioned cats.
Exp Brain Res 159: 185–196, 2004.
Brown LL, Smith DM, Goldbloom LM. Organizing principles of cortical
integration in the rat neostriatum: corticostriate map of the body surface is
an ordered lattice of curved laminae and radial points. J Comp Neurol 392:
468 – 488, 1998.
Castle M, Aymerich MS, Senchez-Escobar C, Gonzalo N, Obeso JA,
Lanciego JL. Thalamic innervation of the direct and indirect basal ganglia
J Neurophysiol • doi:10.1152/jn.00142.2012 • www.jn.org
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on June 18, 2017
evoked DLS responses before and after selective inactivation
of SI and surrounding somatosensory cortical areas.
Amygdalostriatal projections. Consistent with our previous
work (Alloway et al. 2006), the present study indicates that the
whisker-sensitive DLS receives bilateral inputs from the
amygdala. This agrees with previous data showing that the
amygdala projects bilaterally to the posterior DLS (Kelly et al.
1982). Although labeling was substantially greater in the ipsilateral hemisphere, each amygdala contained neuronal labeling in
the magnocellular and, to a lesser extent, in the intermediate
portion of the basal nucleus.
Most studies examining amygdalostriatal projections in the
rat have emphasized the strong amygdaloid projections to the
ventral striatum while also noting the relative lack of projections to the rostral portion of the DLS (Kelly et al. 1982;
Russchen and Price 1984). Yet these studies also indicate that
amygdalostriatal topography is complex. In fact, careful inspection of the labeling patterns in these reports indicates that
the basolateral nuclei in the amygdala send dense projections to
posterior parts of the DLS that correspond to the whiskersensitive regions that we injected in the present study.
Several lesion-behavioral studies indicate that distinct parts
of the amygdaloid complex are differentially involved in goaldirected and habitual motor behaviors (Balleine et al. 2003;
Corbit and Balleine 2005; Lingawi and Balleine 2012). Although the basolateral amygdaloid has been implicated in
goal-directed behaviors that depend on the dorsomedial striatum, the anterior portion of the central amygdaloid nucleus
appears to be involved with the acquisition of behavioral habits
that depend on DLS processing. In view of these findings and
the presumption that whisking behavior is a well-learned sensorimotor habit, it is surprising that the whisker-sensitive part
of the DLS receives projections from the basal (or basolateral)
amygdala but not from the central amygdaloid nucleus. Evidence indicating the association between the central amygdala
and the DLS has prompted the view that the central nucleus
exerts an indirect influence on the DLS by virtue of its
connections with the nigrostriatal projection system (Lingawi
and Balleine 2012). In fact, the central nucleus represents a
major source of amygdaloid projections to other brain regions,
and intraamygdala connections enable all its nuclear components, including the basolateral nuclei, to influence the central
nucleus (Pitkanen et al. 1997; Sah et al. 2003). In this context,
it is noteworthy that whisking behavior is tightly coordinated
with sniffing (Kepecs et al. 2006; Welker 1964), and this
prompts speculation that the whisker-sensitive part of the DLS
might receive direct inputs from the amygdala to enable coordination of these sensorimotor and limbic-related behaviors.
Interpretative limitations. Together with our anatomic findings, our preliminary physiologic results suggest that the POm
could transmit somesthetic information directly to the DLS.
Our results do not, however, address the potential impact of
other thalamostriatal pathways in transmitting somatosensory
information to the DLS. Determining whether POm cooperates
with the CM/Pf complex in activating the DLS requires systematic characterization of peripherally evoked responses in
these thalamic nuclei and in the DLS simultaneously. Such an
analysis, along with manipulations that suppress the influence
of corticostriatal projections, will indicate more completely the
functional roles and relative contributions of the different
thalamostriatal pathways.
THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
Kelley AE, Domesick VB, Nauta WJ. The amygdalostriatal projection in the
rat: an anatomical study by anterograde and retrograde tracing methods.
Neuroscience 7: 615– 630, 1982.
Kepecs A, Uchida N, Mainen ZF. The sniff as a unit of olfactory processing.
Chem Senses 31: 167–179, 2006.
Khatri V, Hartings JA, Simons DJ. Adaptation in thalamic barreloid and
cortical barrel neurons to periodic whisker deflections varying in frequency
and velocity. J Neurophysiol 92: 3244 –3254, 2004.
Killackey HP, Sherman SM. Corticothalamic projections from the rat primary somatosensory cortex. J Neurosci 23: 7381–7384, 2003.
Kincaid AE, Wilson CJ. Corticostriatal innervation of the patch and matrix in
the rat neostriatum. J Comp Neurol 374: 578 –592, 1996.
Krauthamer GM, Krol JG, Grunwerg BS. Effect of superior colliculus
lesions on sensory unit responses in the intralaminar thalamus of the rat.
Brain Res 576: 277–286, 1992.
Land PW, Simons DJ. Cytochrome oxidase staining in the rat SmI barrel
cortex. J Comp Neurol 238: 225–235, 1985.
Lin C-S, May PJ, Hall WC. Nonintralaminar thalamostriatal projections in
the gray squirrel (Sciurus carolinensis) and tree shrew (Tupaia glis). J Comp
Neurol 230: 33– 46, 1984.
Lingawi NW, Balleine BW. Amygdala central nucleus interacts with dorsolateral striatum to regulate the acquisition of habits. J Neurosci 32: 1073–
1081, 2012.
Masamoto K, Fukuda M, Vazquez A, Kim S. Dose-dependent effect of
isoflurane on neurovascular coupling in rat cortex. Eur J Neurosci 30:
242–250, 2009.
Masri R, Bezdudnaya T, Trageser JC, Keller A. Encoding of stimulus
frequency and sensor motion in the posterior medial thalamic nucleus. J
Neurophysiol 100: 681– 689, 2008.
McHaffie JG, Stanford TR, Stein BE, Coizet W, Redgrave P. Subcortical
loops through the basal ganglia. Trends Neurosci 28: 401– 407, 2005.
Melzer P, Sachdev RN, Jenkinson N, Ebner FF. Stimulus processing in
awake rat barrel cortex. J Neurosci 26: 12198 –12205, 2006.
Meng Z, Li Q, Martin JH. The transition from development to motor control
function in the corticospinal system. J Neurosci 24: 605– 614, 2004.
Minamimoto T, Kimura M. Participation of the thalamic CM-Pf complex in
attentional orienting. J Neurophysiol 87: 3090 –3101, 2002.
Mitchinson B, Martin CJ, Grant RA, Prescott TJ. Feedback control in
active sensing: rat exploratory whisking is modulated by environmental
contact. Proc Biol Sci 274: 1035–1041, 2007.
Mowery TM, Harrold JB, Alloway KD. Repeated whisker stimulation
evokes invariant neuronal responses in the dorsolateral striatum of anesthetized rats: a potential correlate of sensorimotor habits. J Neurophysiol 105:
2225–2238, 2011.
Pan WX, Mao T, Dudman JT. Inputs to the dorsal striatum of the mouse
reflect the parallel circuit architecture of the forebrain. Front Neuroanat 4:
1–14, 2010.
Paxinos G, Watson C. The Rat Brain in Stereotaxic Coordinates (6th ed.).
Amsterdam: Elsevier Academic, 2005.
Peschanski M. Trigeminal afferents to the diencephalon in the rat. Neuroscience 12: 465– 487, 1984.
Pidoux M, Mahon S, Deniau JM, Charpier S. Integration and propagation of
somatosensory responses in the corticostriatal pathway: an intracellular
study in vivo. J Physiol 589: 263–281, 2011.
Pitkanen A, Savander V, LeDoux JE. Organization of intra-amygdaloid
circuitries in the rat: an emerging framework for understanding functions of
the amygdala. Trends Neurosci 20: 517–523, 1997.
Pritzel M, Sarter M, Morgan S, Huston JP. Interhemispheric nigrostriatal
projections in the rat: bifurcating nigral projections and loci of crossing in
the diencephalon. Brain Res Bull 10: 385–390, 1983.
Redgrave P, Gurney K. The short-latency dopamine signal: a role in discovering novel actions? Nat Rev Neurosci 7: 967–975, 2006.
Redgrave P, Rodriguez M, Smith Y, Rodriguez-Oraz MC, Lehericy S,
Bergman H, Agid Y, DeLong MR, Obeso JA. Goal-directed and habitual
control in the basal ganglia: implications for Parkinson’s disease. Nat Rev
Neurosci 11: 760 –772, 2010.
Russchen FT, Price JL. Amygdalostriatal projections in the rat. Topographical organization and fiber morphology shown using the lectin PHA-L as an
anterograde tracer. Neurosci Lett 47: 15–22, 1984.
Sachdev RN, Berg RW, Champney G, Kleinfeld D, Ebner FF. Unilateral
vibrissa contact: changes in amplitude but not timing of rhythmic whisking.
Somatosens Mot Res 20: 163–169, 2003.
Sah P, Faber ESL, Lopez de Armentia M, Power J. The amygdaloid
complex: anatomy and physiology. Physiol Rev 83: 803– 834, 2003.
J Neurophysiol • doi:10.1152/jn.00142.2012 • www.jn.org
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on June 18, 2017
pathways in the rat: ipsi- and contralateral projections. J Comp Neurol 483:
143–153, 2005.
Chakrabarti SC, Alloway KD. Differential response patterns in the SI barrel
and septal compartments during mechanical whisker stimulation. J Neurophysiol 102: 1632–1646, 2009.
Cheatwood JL, Corwin JV, Reep RL. Overlap and interdigitation of cortical
and thalamic afferents to the dorsocentral striatum in the rat. Brain Res
1036: 90 –100, 2005.
Chiaia NL, Rhoades RW, Bennett-Clarke CA, Fish SE, Killackey HP.
Thalamic processing of vibrissal information in the rat. I. Afferent input to
the medial ventral posterior and posterior nuclei. J Comp Neurol 314:
201–216, 1991.
Consolazione A, Bentivoglio M, Goldstein M, Toffano G. Evidence for
crossed catecholaminergic nigrostriatal projections by combining wheat
germ agglutinin-horseradish peroxidase retrograde transport and tyrosine
hydroylase immunocytochemistry. Brain Res 338: 140 –143, 1985.
Corbit LH, Balleine BW. Double dissociation of basolateral and central
amygdala lesions on the general and outcome-specific forms of Pavlovianinstrumental transfer. J Neurosci 25: 962–970, 2005.
Cowey A, Stoerig P, Bannister M. Retinal ganglion cells labeled from the
pulvinar nucleus in macaque monkeys. Neuroscience 61: 691–705, 1994.
Day-Brown JD, Wei H, Chomsung RD, Petry HM, Bickford ME. Pulvinar
projections to the striatum and amygdala in the tree shrew. Front Neuroanat
4: 1–11, 2010.
Deschenes M, Bourassa J, Doan VD, Parent A. A single-cell study of the
axonal projections arising from the posterior intralaminar thalamic nuclei in
the rat. Eur J Neurosci 8: 329 –343, 1996.
Deschenes M, Bourassa J, Parent A. Two different types of thalamic fibers
innervate the rat striatum. Brain Res 701: 288 –292, 1995.
Deschenes M, Veinante P, Zhang ZH. The organization of corticothalamic
projections: reciprocity versus parity. Brain Res Rev 28: 286 –308, 1998.
Detsch O, Kochs E, Siemers M, Bromm B, Vahle-Hinz C. Differential
effects of isoflurane on excitatory and inhibitory synaptic inputs to thalamic
neurons in vivo. Br J Anaesth 89: 294 –300, 2002.
Detsch O, Vahle-Hinz C, Kochs E, Siemers M, Bromm B. Isoflurane
induces dose-dependent changes of thalamic somatosensory information
transfer. Brain Res 829: 77– 89, 1999.
Diamond ME, Armstrong-James M, Budway MJ, Ebner FF. Somatic
sensory responses in the rostral sector of the posterior group (POm) and in
the ventral posterior medial nucleus (VPM) of the rat thalamus: dependence
on the barrel field cortex. J Comp Neurol 319: 66 – 84, 1992a.
Diamond ME, Armstrong-James M, Ebner FF. Somatic sensory responses
in the rostral sector of the posterior group (POm) and in the ventral posterior
medial nucleus (VPM) of the rat thalamus. J Comp Neurol 318: 462– 476,
1992b.
Erro E, Lanciego JL, Arribas J, Gimenez-Amaya JM. Striatal input from
the ventrobasal complex of the rat thalamus. Histochem Cell Biol 115:
447– 454, 2001.
Erro E, Lanciego JL, Gimenez-Amaya JM. Re-examination of the thalamostriatal projections in the rat with retrograde tracers. Neurosci Res 42:
45–55, 2002.
Friedberg MH, Lee SM, Ebner FF. Modulation of receptive field properties
of thalamic somatosensory neurons by the depth of anesthesia. J Neurophysiol 81: 2243–2252, 1999.
Gao P, Bermejo R, Zeigler HP. Whisker deafferentation and rodent whisking
patterns: behavioral evidence for a central pattern generator. J Neurosci 21:
5374 –5380, 2001.
Graybiel AM. Habits, rituals, and the evaluative brain. Ann Rev Neurosci 31:
359 –387, 2008.
Groenewegen HJ, Witter MP. Thalamus. In: The Rat Nervous System (3rd
ed.), edited by Paxinos G. New York: Elsevier, 2004, p. 407– 453.
Grunwerg BS, Krauthamer GM. Sensory responses of intralaminar thalamic
neurons activated by the superior colliculus. Exp Brain Res 88: 541–550,
1992.
Harting JK, Updyke BV, van Lieshout DP. The visual-oculomotor striatum
of the cat: functional relationship to the superior colliculus. Exp Brain Res
136: 138 –142, 2001a.
Harting JK, Updyke BV, van Lieshout DP. Striatal projections from the cat
visual thalamus. Eur J Neurosci 14: 893– 896, 2001b.
Hoffer ZS, Alloway KD. Organization of corticostriatal projections from the
vibrissal representations in the primary motor and somatosensory cortical
areas in rodents. J Comp Neurol 439: 87–103, 2001.
Itaya SK, van Hoesen GW. Retinal projections to the inferior and medial
pulvinar nuclei in the Old-World monkey. Brain Res 269: 223–230, 1983.
173
174
THALAMIC PROJECTIONS TO THE DORSOLATERAL STRIATUM
Veinante P, Jacquin MF, Deschenes M. Thalamic projections from the
whisker-sensitive regions of the spinal trigeminal complex in the rat. J Comp
Neurol 420: 233–243, 2000a.
Veinante P, Lavallee P, Deschenes M. Corticothalamic projections from
layer 5 of the vibrissal barrel cortex in the rat. J Comp Neurol 424: 197–204,
2000b.
Voigt T, De Lima AD, Beckmann M. Synaptophysin immunohistochemistry
reveals inside-out pattern of early synaptogenesis in ferret cerebral cortex. J
Comp Neurol 330: 48 – 64, 1993.
Welker WI. Analysis of sniffing in the albino rat. Behaviour 12: 223–244,
1964.
West MO. Anesthetics eliminate somatosensory-evoked discharges of neurons
in the somatotopically organized sensorimotor striatum of the rat. J Neurosci
18: 9055–9068, 1998.
Wimmer VC, Bruno RM, de Kock CP, Kuner T, Sakmann B. Dimensions
of a projection column and architecture of VPM and POm axons in rat
vibrissal cortex. Cereb Cortex 20: 2265–2276, 2010.
Wong-Riley M. Changes in the visual system of monocularly sutured or
enucleated cats demonstrable with cytochrome oxidase histochemistry.
Brain Res 171: 11–28, 1979.
Yin HH, Knowlton BJ, Balleine BW. Inactivation of dorsolateral striatum
enhances sensitivity to changes in the action-outcome contingency in instrumental conditioning. Behav Brain Res 166: 189 –196, 2006.
J Neurophysiol • doi:10.1152/jn.00142.2012 • www.jn.org
Downloaded from http://jn.physiology.org/ by 10.220.32.247 on June 18, 2017
Smith Y, Raju D, Nanda B, Paré JF, Galvan A, Wichmann T. The
thalamostriatal systems: anatomical and functional organization in normal
and parkinsonian states. Brain Res Bull 78: 60 – 68, 2009.
Smith Y, Raju DV, Paré JF, Sidibe M. The thalamostriatal system: a highly
specific network of the basal ganglia circuitry. Trends Neurosci 27: 520 –
527, 2004.
Smith Y, Surmeier DJ, Redgrave P, Kimura M. Thalamic contributions to
basal ganglia–related behavioral switching and reinforcement. J Neurosci
31: 16102–16106, 2011.
Sosnik R, Haidarliu S, Ahissar E. Temporal frequency of whisker movement. I. Representations in brain stem and thalamus. J Neurophysiol 86:
339 –353, 2001.
Syed EC, Sharott A, Moll CK, Engel AK, Kral A. Effect of sensory
stimulation in rat barrel cortex, dorsolateral striatum and on corticostriatal
functional connectivity. Eur J Neurosci 33: 461– 470, 2011.
Takada M, Itoh K, Yasui Y, Sugimoto T, Mizuno N. Topographical
projections from the posterior thalamic regions to the striatum in the cat,
with reference to possible tecto-thalamo-striatal connections. Exp Brain Res
60: 385–396, 1985.
Timofeeva E, Lavallee P, Arsenault D, Deschenes M. Synthesis of multiwhisker-receptive fields in subcortical stations of the vibrissa system. J
Neurophysiol 91: 1510 –1515, 2004.