Download Opposite Functions of Histamine H1 and H2 Receptors and H3

J Neurophysiol 96: 1581–1591, 2006.
First published May 31, 2006; doi:10.1152/jn.00148.2006.
Opposite Functions of Histamine H1 and H2 Receptors and H3 Receptor in
Substantia Nigra Pars Reticulata
Fu-Wen Zhou,1 Jian-Jun Xu,1 Yu Zhao,2 Mark S. LeDoux,2 and Fu-Ming Zhou1
1
Department of Pharmacology and 2Department of Neurology, University of Tennessee College of Medicine, Memphis, Tennessee
Submitted 12 February 2006; accepted in final form 25 May 2006
INTRODUCTION
The substantia nigra pars reticulata (SNr) is a key output
nucleus of basal ganglia motor circuitry (DeLong 1990; Hikosaka et al. 2000; Parent et al. 2000; Wilson 2004). ␥-Aminobutyric acid (GABA)– containing SNr projection neurons are
tonically active and spike spontaneously at high frequencies
(Atherton and Bevan 2005; Gulley et al. 2002; Maurice et al.
2003; Nakanishi et al. 1987; Schultz 1986; Wichmann et al.
1999; Wilson et al. 1977). Their axons innervate and inhibit the
thalamus along with oculormotor and other brain stem motor
structures (Cebrian et al. 2005; Chevalier and Deniau 1990;
Hikosaka et al. 2000). In Parkinson’s disease and movement
disorders of basal ganglia origin, SNr output is often altered in
its intensity and/or pattern (Bergman et al. 1998; Hutchison et
al. 2004; Nevet et al. 2004; Obeso et al. 2000; Walters et al.
2000). Therefore regulation of SNr neurons is likely to influence overall basal ganglia output and motor control.
Address for reprint requests and other correspondence: F.-M. Zhou, Department of Pharmacology, University of Tennessee College of Medicine, Memphis, TN 38163 (E-mail: [email protected]).
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Previous anatomical studies showed that histamine fibers
originating in the tuberomamillary histamine neurons innervate
SNr (Airaksinen and Panula 1988; Panula et al. 1989; Schwartz
and Arrang 2002). Histochemical studies indicated that histamine H1, H2, and H3 receptors are expressed in rat and guinea
pig substantia nigra including SNr (Bouthenet et al. 1988;
Pillot et al. 2002; Ryu et al. 1995; Traiffort et al. 1994; Vizuete
et al. 1997). Some of the receptors, H3 receptor in particular,
may also be expressed on afferent terminals (Pillot et al. 2002;
Threlfell et al. 2004; Vizuete et al. 1997). Postmortem studies
indicate that histamine innervation of SNr is increased in
Parkinson’s disease, although the precise relationship between
histamine and Parkinson’s disease is unknown (Anichtchik et
al. 2000). Injection of an H3 receptor agonist into SNr affected
turning behavior in rats (Garcia-Ramirez et al. 2004). In a
monkey Parkinson’s disease model, systemic administration of
an H3 receptor agonist increased parkinsonian symptoms (Gomez-Ramirez et al. 2006). These results indicate that H3
receptor may regulate SNr neuron activity critical to motion
control.
In other brain areas, activation of H1 receptor may increase
neuronal excitability by blocking a leak K⫹ conductance (Bell
et al. 2000; Gorelova and Reiner 1996; McCormick and Williamson 1991). H2 receptor activation may also increase neuronal excitability by inhibiting Ca2⫹-activated K⫹ channels
that mediate afterhyperpolarizations (AHPs) (Haas and Konnerth 1983; McCormick and Williamson 1989) and increasing
a cation conductance (McCormick and Williamson 1991). In
hippocampal interneurons, H2 receptor activation may inhibit
voltage-gated K⫹ channels and alter the output from these
interneurons (Atzori et al. 2000). H3 receptor activation may
inhibit neuronal excitability, Ca2⫹ influx, and neurotransmitter
release (Brown et al. 2001).
We hypothesize that histamine may directly increase SNr
neuron firing by activation of H1 and H2 receptors. Histamine
may also directly inhibit these neurons by H3 receptor activation. This inhibition may reduce spike frequency and render the
spike firing less regular. Herein, we tested these hypotheses
with patch-clamp techniques in well-identified SNr GABA
projection neurons.
METHODS
Patch clamp
Wild-type, 16- to 25-day-old male and female C57BL/6J mice were
used. Animal handling and use followed National Institutes of Health
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0022-3077/06 $8.00 Copyright © 2006 The American Physiological Society
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Zhou, Fu-Wen, Jian-Jun Xu, Yu Zhao, Mark S. LeDoux, and
Fu-Ming Zhou. Opposite functions of histamine H1 and H2
receptors and H3 receptor in substantia nigra pars reticulata. J
Neurophysiol 96: 1581–1591, 2006. First published May 31, 2006;
doi:10.1152/jn.00148.2006. The substantia nigra pars reticulata (SNr) is a
key basal ganglia output nucleus. Inhibitory outputs from SNr are
encoded in spike frequency and pattern of the inhibitory SNr projection
neurons. SNr output intensity and pattern are often abnormal in movement disorders of basal ganglia origin. In Parkinson’s disease, histamine
innervation and histamine H3 receptor expression in SNr may be increased. However, the functional consequences of these alterations are
not known. In this study, whole cell patch-clamp recordings were used to
elucidate the function of different histamine receptors in SNr. Histamine
increased SNr inhibitory projection neuron firing frequency and thus
inhibitory output. This effect was mediated by activation of histamine H1
and H2 receptors that induced inward currents and depolarization. In
contrast, histamine H3 receptor activation hyperpolarized and inhibited
SNr inhibitory projection neurons, thus decreasing the intensity of basal
ganglia output. By the hyperpolarization, H3 receptor activation also
increased the irregularity of the interspike intervals or changed the pattern
of SNr inhibitory neuron firing. H3 receptor–mediated effects were
normally dominated by those mediated by H1 and H2 receptors. Furthermore, endogenously released histamine provided a tonic, H1 and H2
receptor–mediated excitation that helped keep SNr inhibitory projection
neurons sufficiently depolarized and spiking regularly. These results
suggest that H1 and H2 receptors and H3 receptor exert opposite effects
on SNr inhibitory projection neurons. Functional balance of these different histamine receptors may contribute to the proper intensity and pattern
of basal ganglia output and, as a consequence, exert important effects on
motor control.
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Histology
Neurobiotin (0.2%) was dissolved in the pipette solution before
each experiment and allowed to passively diffuse into neurons from
the recording electrode (Zhou and Hablitz 1996). After electrophysiological recordings, brain slices were fixed in 4% paraformaldehyde
in 0.1 M phosphate buffer (PB) at 4°C overnight. Without resectioning, slices were then processed for visualization of neurobiotin-filled
neurons. Endogenous peroxidases were quenched with 10% methanol
and 3% H2O2 in phosphate-buffered saline (PBS) for 5 min at room
temperature (RT). Brain slices were rinsed well, permeabilized with
0.5% Triton X-100 (Sigma) for 2 h at RT, incubated in streptavidin
conjugated with horseradish peroxidase (Vector Laboratories, Burlingame, CA) at 4°C overnight, and visualized with nickel-intensified
diaminobenzidine (Vector) for ⱕ10 min. Between each step, slices
were thoroughly rinsed three times in PBS over 15 min. Slices were
mounted onto glass slides, coverslipped with a 1:1 mix of glycerol and
PBS (pH 7.4), and the edges sealed with nail polish.
For Nissl staining, mice were overdosed with pentobarbital (100
mg/kg, administered intraperitoneally), intracardially perfused with
0.9% NaCl saline and then 4% paraformaldehyde in 0.1 M PB. Brains
were postfixed in 4% paraformaldehyde in 0.1 M PB for 2 h at 4°C,
blocked, and incubated in a cryoprotectant solution (30% sucrose/
0.1% sodium azide/0.1 M PB, pH 7.4) for ⱖ48 h. Tissue cryosections
(20 ␮m) were dehydrated in 95% ethanol for 3 min, followed by
xylene for 10 min. After rehydration, sections were stained with cresyl
violet solution (Sigma) for 5 to 10 min, dehydrated, cleared in xylene,
and coverslipped with Permount (Sigma).
All chemicals including D-2-amino-5-phosphonopentanoic acid (DAP5), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and bicuculJ Neurophysiol • VOL
line (BIC) were purchased from Sigma–Aldrich (St. Louis, MO) or
Tocris Cookson (Ballwin, MO). D-AP5, CNQX, and BIC were present
when examining action potential firing, depolarization, and inward
current to prevent the complications from synaptic activity.
All values were expressed as means ⫾ SE. Statistical comparisons
were performed using paired t-test or Kolmogorov–Smirnov (K-S)
test (to compare the distributions of two sets of events such as
spontaneous synaptic currents). P ⬍ 0.05 is significant.
RESULTS
Identification of substantia nigra pars reticulata GABA
projection neurons
Substantia nigra pars reticulata (SNr) can be easily identified. As shown in Fig. 1 A, SNr is a fairly large structure
ventral to substantia nigra pars compacta (SNc) and dorsal to
the cerebral peduncle (Paxinos and Franklin 2001). SNr also
has a low cell density compared with the densely packed cells
in SNc. SNr is populated largely by two types of neurons: the
majority GABA projection neurons and the minority dopamine
(DA) projection neurons (Fallon and Loughlin 1995; Tepper et
al. 1995). Both cell types are relatively large and often ovalshaped neurons and cannot be distinguished based on their
appearances (Deniau et al. 1982; Grofova et al. 1982; Juraska
et al. 1977; Nelson et al. 1996). However, they have very
different electrophysiological characteristics (Diana and Tepper 2002; Ibanez-Sandoval et al. 2006; Lacey et al. 1989;
Richards et al. 1997; Shen and Johnson 1997; Yung et al.
1991).
In our present sample, the presumed SNr GABA projection
neurons (SNr GABA neurons hereafter) spiked spontaneously
at 10.8 ⫾ 0.5 Hz (n ⫽ 58; Fig. 1, A2, A3, and B). These action
potentials had a duration at the base of 0.96 ⫾ 0.03 ms (n ⫽ 58,
Fig. 1B). These GABA neurons had a very weak, hyperpolarization-activated cation current or Ih current. In contrast, the
presumed DA neurons spiked spontaneously at 1.5 ⫾ 0.2 Hz
(n ⫽ 27; Fig. 1B). DA neuron action potentials had a base
duration of 2.53 ⫾ 0.07 ms (n ⫽ 27). DA neurons displayed a
prominent Ih current. As shown in the scatter plot of Fig. 1B,
these electrophysiological properties clearly separate SNr
GABA neurons and DA neurons into two non-overlapping
groups. Therefore SNr GABA neurons and DA neurons can be
reliably identified by their electrophysiological properties. This
report focuses on SNr GABA projection neurons. DA neurons
were excluded from results presented below.
Also, SNr GABA projection neurons did not show any
significant slow afterhyperpolarization (sAHP) after a train of
high-frequency spikes evoked from their natural membrane
potentials by injecting depolarizing current pulses (Fig. 1C).
To remove any potential interference from the spontaneous
spikes, hyperpolarizing holding currents were applied to bring
the membrane potential to ⫺65 to ⫺70 mV, such that the SNr
neurons completely ceased to fire spontaneous action potentials. Under this condition, there was still no significant sAHP
after a train of spikes evoked by depolarizing current pulses
(Fig. 1D).
Histamine excites SNr GABA projection neurons by inducing
depolarization and inward current
High-frequency spike firing encodes the output from SNr
GABA projection neurons (Hikosaka et al. 2000). Therefore
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guidelines. These mice were kept at the animal facility of the University of Tennessee Health Science Center in Memphis. They had
free access to food and water. The room light was on 7:00 AM to 7:00
PM and off for the night. Under deep halothane anesthesia, mice were
decapitated and their brains were quickly dissected out. Coronal
midbrain slices (300 ␮m thickness) containing the midrostral part of
substantia nigra were prepared according to well-established procedures (Bonci and Malenka 1999; Richards et al. 1997). Coronal
sections were chosen to maximally sever afferent fibers such that SNr
neurons can be studied in relative isolation. The cutting solution
contains (in mM): 220 sucrose, 2.5 KCl, 1.25 NaH2PO4, 25 NaHCO3,
0.5 CaCl2, 7 MgCl2, and 20 D-glucose. The slices were then transferred to a holding chamber containing the normal extracellular
solution (in mM): 125 NaCl, 2.5 KCl, 1.25 NaH2PO4, 25 NaHCO3,
2.5 CaCl2, 1.3 MgCl2, and 20 D-glucose. The solution was continuously bubbled with 95% O2-5%CO2 to supply oxygen and keep pH at
7.4. Recordings were made at 30°C under visual guidance of a video
microscope (Olympus BX51W1) equipped with Nomarski optics and
⫻60 water immersion lens. Relatively large (the longest dimension of
the soma was about 25 ␮m) oval or spindle-shaped SNr neurons were
chosen for recording. These characteristics are typical of rodent SNr
GABA projection neurons (Grofova et al. 1982; Juraska et al. 1977).
This selection was biased against smaller neurons that are potential
interneurons.
Conventional whole cell patch-clamp techniques were used (Zhou
and Hablitz 1999). Patch electrodes had resistances of 2–3 M⍀ when
filled with an internal solution containing (in mM): 130 KCl, 0.5
EGTA, 10 HEPES, 2 Mg-ATP, 0.2 Na-GTP, and 4 Na-phosphocreatine. pH was adjusted to 7.3 with NaOH. Axopatch 200B and
Multiclamp 700B amplifiers, pClamp 9.2 software, and Digidata
1322A interface (Axon Instruments) were used to acquire and analyze
data. Signals were digitized at 5–20 kHz and analyzed off-line. The
Mini Analysis Program (Synaptosoft, Fort Lee, NJ) was also used to
analyze spontaneous events. Recordings with access resistance increase of ⬎15% were rejected. Whole cell conductance was measured
by 100-ms voltage pulses, from ⫺70 to ⫺80 mV.
HISTAMINE REGULATION OF BASAL GANGLIA OUTPUT
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our first goal was to investigate whether histamine affected
action potential firing in well-identified and synaptically isolated SNr neurons. In the presence of 20 ␮M D-AP5, 10 ␮M
CNQX, and 10 ␮M BIC to remove complications of synaptic
activity, bath application of histamine (10 ␮M) reliably increased the firing rate of SNr neurons by 37.2 ⫾ 3.5%, from
10.4 ⫾ 0.8 to 14.1 ⫾ 1.0 Hz (n ⫽ 11, P ⬍ 0.001, Fig. 2, A and
C). This effect was fully reversed after prolonged wash. Histamine did not affect spike amplitude (67.8 ⫾ 3.4 mV under
control vs. 67.4 ⫾ 3.6 mV during histamine, n ⫽ 11) and base
duration (0.97 ⫾ 0.07 vs. 0.98 ⫾ 0.08 ms). The fast AHP (fAHP,
20.1 ⫾ 2.1 vs. 19.9 ⫾ 2.4 mV) and medium AHP (mAHP, 10.6 ⫾
1.5 vs. 10.6 ⫾ 1.7 mV) were also not affected (Fig. 2B). These
results indicate that histamine was not affecting voltage-gated
Na⫹ and K⫹ channels or Ca2⫹-activated K⫹ channels that are
responsible for spike generation and repolarization.
The increased spike firing was accompanied by a small
depolarization and a small increase in whole cell conductance.
However, these two modest changes were difficult to monitor
when the neuron was spiking at high frequency. Thus we
blocked action potentials with 0.5 ␮M tetrodotoxin (TTX), a
specific voltage-gated Na⫹ channel blocker. In the presence of
TTX, membrane potential was stable in SNr neurons (Fig. 2D).
Under this condition, bath application of 10 ␮M histamine
caused a slowly developing depolarization of 3.8 ⫾ 0.2 mV,
from baseline membrane potential of ⫺50.6 ⫾ 1.1 to ⫺46.8 ⫾
1.1 mV (n ⫽ 7, P ⬍ 0.001, Fig. 2D). This depolarization is
likely the primary mechanism underlying histamine’s enhancement of SNr neuron firing.
The depolarization was associated with an increase in whole
cell conductance, indicating that histamine was opening ion
channels in SNr neurons. To test this idea, SNr neurons were
voltage clamped at ⫺70 mV. At this holding potential, bath
application of histamine (10 ␮M) induced an inward current
J Neurophysiol • VOL
(39.6 ⫾ 4.0 pA, n ⫽ 10), increasing the holding current from
⫺164.9 ⫾ 20.9 to ⫺204.5 ⫾ 23.3 pA (Fig. 2E). The majority
of the relatively large baseline holding current arose from a
tonic inward current (Atherton and Bevan 2005). Whole cell
conductance, monitored with 10-mV voltage pulses, was also
significantly increased from 5.32 ⫾ 0.46 nS under control to
7.21 ⫾ 0.75 nS (n ⫽ 19, P ⬍ 0.01) during histamine application, suggesting an opening of ion channels. Voltage ramp
experiments revealed that histamine increased the whole cell
current. The current was linear between ⫺90 and ⫺20 mV
with no signs of voltage-dependent activation or inactivation
and reversed its polarity at ⫺42.8 ⫾ 2.1 mV (n ⫽ 8, Fig. 3A).
These results indicate that histamine was enhancing a tonic,
voltage-independent current.
Consistent with the above results that histamine increased
SNr neuron firing and reports that SNr neurons innervate each
other by their relatively sparse intranigral axonal collaterals, in
addition to receiving other GABA inputs (Celada et al. 1999;
Deniau et al. 1982; Mailly et al. 2003), histamine enhanced
spontaneous inhibitory postsynaptic currents (sIPSCs) in SNr
neurons. BIC (10 ␮M)-sensitive sIPSCs were recorded at a
holding potential of ⫺70 mV in the presence of D-AP5 (20
␮M) and CNQX (10 ␮M) to block glutamate-mediated synaptic current. Under these conditions, bath application of 10 ␮M
histamine increased the sIPSC frequency by 38.4 ⫾ 5.6% Hz
(P ⬍ 0.001, n ⫽ 10) and sIPSC amplitude by 31.3 ⫾ 7.7%
(P ⬍ 0.001) (Fig. 4, A–D). On the other hand, histamine
induced a minimal, 5% decrease in the frequency of miniature
IPSCs (mIPSCs) recorded in 0.5 ␮M TTX with no change in
amplitude. These results indicate that the predominant histamine effect was at the cell body area of SNr neurons and the
histamine effect on GABA terminals in SNr was minor.
These results indicate that histamine can directly excite SNr
neurons. In other brain areas, H1 and H2 receptors are often
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FIG. 1. Identification of substantia nigra pars reticulata
(SNr) ␥-aminobutyric acid (GABA) projection neurons. A1:
Nissl-stained coronal mouse brain section showing that SNr, as
marked by the red circle, is a distinct, relatively large, and
uniquely located brain structure. Arrowhead points to the dense
cell band of SNc. d, dorsal; m, medial. A2: example of frequently seen SNr neurons that were chosen for recording. This
neuron fired fast spikes and therefore was a presumed GABA
projection neuron. Roughly 85% of the SNr neurons chosen this
way were fast-spiking neurons or presumed GABA projection
neurons and the rest were typical of dopamine (DA) neurons.
A3: photograph showing the neuron in A2 stained with neurobiotin. Scale bar in A2 applies to A3. Gross morphology is
typical of SNr GABA projection neurons, although it is not
distinctively different from SNr DA neurons. B: scatterplot
showing that SNr DA and GABA neurons can be unequivocally
distinguished based on their spontaneous action potential duration and frequency. GABA neurons formed a cluster with high
frequency and short duration, whereas DA neurons formed
another cluster with low frequency and long duration. Note that
these 2 clusters do not overlap. Inset: example action potentials
from a DA neuron and a GABA neuron, respectively. Scale bar:
2 ms and 20 mV. C: at their natural membrane potential, SNr
GABA projection neurons lack the slow afterhyperpolarization
(sAHP). D: injection of ⫺100-pA hyperpolarizing current
brought the membrane potential to ⫺70 mV and the SNr neuron
ceased to fire spontaneous action potentials. In the absence of
the interference of spontaneous spikes, there was still no significant sAHP after a train of spikes evoked by a pulse of
depolarizing current.
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ZHOU, XU, ZHAO, LEDOUX, AND ZHOU
excitatory and H3 receptor is always inhibitory (for review see
Haas and Panula 2003). Multiple histamine receptors are expressed in SNr (Pillot et al. 2002; Traiffort et al. 1994; Vizuete
et al. 1997). Thus we hypothesize that H1 and H2 receptor
activation may underlie histamine’s direct excitation of SNr
neurons. We also reason that H3 receptor activation may inhibit
SNr neurons, although this inhibition is likely to be masked
and dominated by the H1 and H2 receptor–induced excitation.
These hypotheses are tested in the following sections.
Histamine H1 receptor activation excites SNr GABA
projection neurons
No selective H1 receptor agonists are commercially available. Therefore we studied the potential effects of H1 receptor
activation by first blocking H2 receptor with 5 ␮M ranitidine or
tiotidine and H3 receptor with 100 nM clobenpropit (van der
Goot and Timmerman 2000). As will be discussed in later
sections, these H2 and H3 receptor antagonists at the concentrations used here completely blocked H2 and H3 receptor
agonist–induced effects, indicating that these H2 and H3 receptor antagonists were able to fully inhibit H2 and H3 receptors.
J Neurophysiol • VOL
FIG. 4. Histamine increases spontaneous inhibitory postsynaptic currents
(sIPSCs) in SNr GABA projection neurons. A and B: specimen recordings of
sIPSCs in an SNr GABA projection neuron voltage clamped at ⫺70 mV under
control conditions (A) and during bath application of 10 ␮M histamine (B). C
and D: group data showing that histamine increased the amplitude (C) and
frequency (D) of sIPSCs in SNr GABA neurons. Also shown here are the
effects of H1, H2, and H3 receptor activation. H1 receptor activation was
achieved by histamine after blocking H2 and H3 receptors because there is no
commercially available specific H1 agonist. Amthamine and imetit are agonists
for H2 and H3 receptors, respectively. H1 and H2 receptor activation increased
sIPSC amplitude and frequency, whereas H3 receptor activation decreased
them. *P ⬍ 0.05, **P ⬍ 0.01, ***P ⬍ 0.001 vs. control.
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FIG. 2. Histamine excites SNr GABA projection neurons. A: sample recordings of spontaneous action potentials in an SNr GABA projection neuron
under control and during 10 ␮M bath-applied histamine. In this and all
following figures, spikes were truncated. B: 2 individual spikes under control
and during bath application of 10 ␮M histamine displayed alone or superimposed at slow and fast timescales, showing that histamine did not affect spike
duration and amplitude, the fast AHP (fAHP), or the medium AHP (mAHP).
C: group data showing that the SNr GABA neuron firing frequency was clearly
increased during histamine application. Frequency was binned every 12 s and
normalized to better illustrate the change induced by histamine. D: example of
10 ␮M histamine-induced depolarization in an SNr GABA neuron under
current-clamp recording condition. Synaptic transmission and action potentials
were blocked with 20 ␮M D-2-amino-5-phosphonopentanoic acid (D-AP5), 10
␮M 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 10 ␮M bicuculline (BIC),
and 0.5 ␮M tetrodotoxin (TTX). E: an example of 10-␮M histamine-induced
inward current in an SNr GABA neuron voltage clamped at ⫺70 mV. Holding
current was relatively large in SNr GABA neurons and much of it was
attributed to a physiologically important tonic inward current. Note that the
histamine-induced firing increase, depolarization, and inward current have
similar time courses.
FIG. 3. Voltage ramp experiments reveal the current–voltage (I–V) relationships of the currents induced by activation of different histamine receptors.
Net current was obtained by digital subtraction of the whole cell current under
a histamine ligand by the whole cell current under control condition. A: 10 ␮M
histamine increased the whole cell current and the net current was linear with
no signs of voltage-dependent activation or inactivation. B and C: H1 receptor
activation and H2 receptor agonist amthamine also increased whole cell current
that was linear and voltage independent. D: H3 receptor agonist imetit reduced
the whole cell current and the net current was linear. Note the direction of H3
receptor–induced current is opposite to H1 and H2 receptor–mediated currents.
Because imetit reduced the whole cell current, H3 receptor was inhibiting an
existing current that is the mirror image of apparent H3 receptor–induced
current as indicated by the gray trace. Also, all these currents reversed their
polarity near ⫺40 mV.
HISTAMINE REGULATION OF BASAL GANGLIA OUTPUT
FIG. 6. Histamine H2 receptor activation excites SNr GABA projection
neurons. A: sample recordings of spontaneous action potentials recorded in an
SNr GABA neuron under control and during 10 ␮M amthamine, a specific H2
receptor agonist. B: group data showing that the SNr GABA neuron firing
frequency was clearly increased during application of amthamine (10 ␮M).
This increase in firing was blocked when a specific H2 receptor antagonist,
ranitidine (5 ␮M), was applied. Frequency was binned every 12 s and
normalized to better illustrate the histamine-induced change. C and D: after
blocking action potentials with 0.5 ␮M TTX, 10 ␮M amthamine induced a
depolarization under current clamp (C) or an inward current under voltage
clamp (at ⫺70 mV) (D).
indicating that H1 receptor activation was inducing the depolarization and inward currents. Voltage ramp experiments
showed that H1 receptor activation increased the whole cell
current and this current was linear between ⫺90 and ⫺20 mV
without any voltage dependency and reversed its polarity at
⫺41.3 ⫾ 1.7 mV (n ⫽ 8, Fig. 3B). These results indicate that
H1 receptor activation was enhancing a tonic current in SNr.
Consistent with the excitatory effect of H1 receptor activation, histamine (10 ␮M), in the presence of ranitidine (5 ␮M)
and clobenpropit (100 nM) to block both H2 and H3 receptors,
increased sIPSCs frequency by 15.8 ⫾ 4.6% (n ⫽ 5, P ⬍ 0.05)
and amplitude by 16.6 ⫾ 5.1% (P ⬍ 0.01) (Fig. 4, C and D).
However, when action potentials were blocked with 0.5 ␮M
TTX, mIPSCs were not significantly altered by H1 receptor
activation, indicating a lack of functional H1 receptors on
GABA axon terminals innervating SNr neurons.
Histamine H2 receptor activation enhances SNr GABA
projection neurons
FIG. 5. Histamine H1 receptor activation excites SNr GABA projection
neurons. A: sample recordings of spontaneous action potentials recorded in an
SNr GABA projection neuron under control and during 10 ␮M histamine after
blocking H2 and H3 receptors. This arrangement was necessary because there
is no commercially available specific H1 agonist. B: group data showing that
SNr GABA projection neuron firing frequency was clearly increased during
histamine application after blocking H2 receptor with 5 ␮M ranitidine and H3
receptor with 100 nM clobenpropit. These 2 blockers were present during the
entire recording. Increase in firing was blocked when a specific H1 receptor
antagonist trans-triprolidine (2 ␮M) was applied. Frequency was binned every
12 s and normalized to better illustrate the histamine-induced change. C and D:
after blocking H2 and H3 receptors with ranitidine and clobenpropit and action
potentials with 0.5 ␮M TTX, 10 ␮M histamine, by activation of H1 receptor,
induced a depolarization under current clamp (C) or an inward current under
voltage clamp (at ⫺70 mV) (D).
J Neurophysiol • VOL
To examine the potential involvement of H2 receptor in
histamine-induced excitation of SNr neurons, we used the
highly selective H2 receptor agonist amthamine (van der Goot
and Timmerman 2000). After establishing a stable baseline
recording, bath application of 10 ␮M amthamine had a clearly
significant excitatory effect on SNr neurons (Fig. 6, A–D). The
spontaneous action potential firing rate was increased by
33.0 ⫾ 8.8%, from 11.5 ⫾ 1.9 Hz in control to 15.3 ⫾ 2.5 Hz
during the treatment of amthamine (n ⫽ 7, P ⬍ 0.01, Fig. 6, A
and B). This effect was blocked by a selective H2 receptor
antagonist ranitidine at 5 ␮M (Fig. 6B). Clearly, H2 receptor
activation has excitatory effects on SNr neurons.
Next, we investigated the mechanisms by which H2 receptor
activation enhanced SNr neuron firing. Because amthamine did
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Consequently, after incubation with these H2 and H3 receptor
antagonists, only H1 receptor can still respond to histamine.
Under these conditions, bath application of 10 ␮M histamine
increased the firing rate of SNr neurons by 19.6 ⫾ 2.6%, from
11.1 ⫾ 1.5 to 13.4 ⫾ 2.0 Hz (n ⫽ 10, P ⬍ 0.01, Fig. 5, A and
B). This enhancement was blocked by 2 ␮M trans-triprolidine,
a specific H1 antagonist (van der Goot and Timmerman 2000),
further confirming that H1 receptor activation was responsible
for this histamine-induced excitation of SNr neurons. Also,
under these conditions, histamine did not significantly affect
action potential shape or the fAHP or mAHP, suggesting that
the main effect of H1 receptor activation was not that of
affecting voltage-gated Na⫹ and K⫹ channels and Ca2⫹-activated K⫹ channels in SNr neurons.
To further study how H1 receptor activation increased SNr
neuron firing, action potentials were blocked with 0.5 ␮M TTX
such that a stable membrane potential was established and
small changes in membrane potential can be reliably detected.
After blocking H2 and H3 receptors with 5 ␮M ranitidine and
100 nM clobenpropit, 10 ␮M histamine caused a slowly
developing depolarization of 2.0 ⫾ 0.3 mV, from a baseline
membrane potential ⫺50.2 ⫾ 0.9 to ⫺48.2 ⫾ 1.1 mV (n ⫽ 5,
P ⬍ 0.001, Fig. 5C). Similarly, when voltage clamped at ⫺70
mV, bath application of 10 ␮M histamine in the presence of
ranitidine and clobenpropit induced an inward current of
23.7 ⫾ 4.1 pA (n ⫽ 7, Fig. 5D). Whole cell conductance was
increased from 5.59 ⫾ 0.43 nS under control to 7.17 ⫾ 0.85 nS
during H1 receptor activation (n ⫽ 8, P ⬍ 0.05). All these
effects were blocked by 2 ␮M H1 antagonist trans-triprolidine,
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ZHOU, XU, ZHAO, LEDOUX, AND ZHOU
Histamine H3 receptor activation inhibits SNr GABA
projection neurons
H3 receptor is known to be an inhibitory autoreceptor on
histamine neurons (Brown et al. 2001). Modest levels of H3
receptor are expressed in SNr (Pillot et al. 2002; Ryu et al.
1995; Vizuete et al. 1997). We hypothesize that H3 receptor
activation may induce a mild, direct inhibition of SNr neurons.
To test this hypothesis, we did the following experiments using
strategies similar to those for H2 receptor.
First, we examined the effects of an H3 receptor agonist,
imetit (van der Goot and Timmerman 2000). If H3 receptor
activation produces inhibitory effects, then imetit should inhibit SNr neurons. Indeed, bath application of 100 nM imetit
significantly decreased SNr neuron firing rate by 15.6 ⫾ 3.7%
(n ⫽ 11, P ⬍ 0.05, Fig. 7, A and B). This inhibitory effect was
recovered after prolonged wash (Fig. 7B). Furthermore, imetitinduced inhibition of SNr GABA neuron firing was completely
blocked by a selective H3 receptor antagonist clobenpropit
(100 nM). Histamine (10 ␮M) induced similar effects in the
presence of 2 ␮M H1 blocker trans-triprolidine and 5 ␮M H2
receptor blocker ranitidine (n ⫽ 5). These findings suggested
that H3 receptor activation mildly inhibited SNr neurons. H3
receptor activation by imetit also did not significantly affect the
action potential shape of fAHP or mAHP in SNr GABA
neurons.
Next, we blocked spikes with 0.5 ␮M TTX to stabilize the
membrane potential such that the imetit-induced small hyperJ Neurophysiol • VOL
FIG. 7. H3 receptor activation changes SNr GABA projection neuron firing
pattern. A: example recordings of spontaneous action potentials under control
conditions (left) and during bath application of 100 nM imetit, a specific H3
receptor agonist (right). Note that the firing of spontaneous action potentials
was more irregular under imetit than under control condition. B: group data of
imetit inhibition of SNr GABA projection neuron firing. C and D: regularity of
spontaneous action potential firing was quantified by calculating the interspike
interval (ISI). Under control condition, the ISI distribution was narrow (C).
Mean ISI was 0.1032 s with CV of 0.1782. Under imetit, ISI distribution
became wider (D). Mean ISI was 0.1603 s with CV of 0.2605. E and F: after
blocking action potentials with 0.5 ␮M TTX, 100 nM imetit induced a
hyperpolarization under current clamp (E) or an outward current under voltage
clamp (at ⫺70 mV) (F).
polarization can be characterized. After blocking action potentials, membrane potential was stable. Bath application of 100
nM imetit caused a slowly developing hyperpolarization of
⫺2.7 ⫾ 0.5 mV, from baseline membrane potential ⫺49.7 ⫾
1.5 to ⫺52.4 ⫾ 1.2 mV (n ⫽ 7, P ⬍ 0.01, Fig. 7E). Similarly,
when SNr neurons were voltage clamped at ⫺70 mV, bath
application of 100 nM imetit caused an outward current of
26.6 ⫾ 3.0 pA (n ⫽ 12, Fig. 7F). The hyperpolarization and
outward current were associated with a decrease in whole
conductance from 5.68 ⫾ 0.48 nS under control to 4.71 ⫾ 0.38
nS (n ⫽ 10, P ⬍ 0.01), indicating a closing of ion channels.
Voltage ramp experiments revealed that imetit reduced the
whole cell current and the imetit-induced current was linear
between ⫺90 and ⫺20 mV without any voltage dependency
and reversed its polarity at ⫺41.7 ⫾ 1.4 mV (n ⫽ 6, Fig. 3D).
These results indicate that H3 receptor activation was inhibiting
a tonic current.
Histamine H3 receptor activation increases the irregularity
of SNr GABA projection neuron spiking
During imetit treatment, the decrease in firing frequency or
increase in interspike interval (ISI) was also accompanied by
an increase in irregularity in ISI. To quantify this irregularity,
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not significantly affect SNr neuron action potential shape or
AHPs, we reasoned that H2 receptor activation was not affecting voltage-gated Na⫹ and K⫹ channels or Ca2⫹-activated K⫹
channels in SNr neurons. However, we noticed a small depolarization accompanied the amthamine-induced increase in
firing frequency. To characterize this small depolarization and
further study how H2 receptor activation increased SNr neuron
firing, action potentials were blocked with 0.5 ␮M TTX such
that a stable membrane potential was established and small
changes in membrane potential can be reliably detected. Under
these conditions, bath application of 10 ␮M amthamine induced a slowly developing depolarization of 3.9 ⫾ 0.6 mV,
from baseline membrane potential ⫺49.4 ⫾ 1.4 to ⫺45.5 ⫾
1.4 mV (n ⫽ 6, P ⬍ 0.01, Fig. 6C). When the neurons were
voltage clamped at ⫺70 mV, 10 ␮M amthamine induced an
inward current of 38.8 ⫾ 4.1 pA (n ⫽ 7, P ⬍ 0.01, Fig. 6D).
Whole cell conductance was increased from 5.39 ⫾ 0.49 nS
under control conditions to 7.58 ⫾ 0.81 nS during 10 ␮M
amthamine (n ⫽ 12, P ⬍ 0.01). Voltage ramp experiments
revealed that amthamine increased the whole cell current and
the amthamine induced a linear current between ⫺90 and ⫺20
mV with a reversal potential at ⫺42.2 ⫾ 2.4 mV (n ⫽ 6, Fig.
3C). These results indicate that H2 receptor activation was
enhancing a tonic current.
Consistent with the excitatory effect of H2 receptor activation, bath application of H2 specific agonist amthamine (10
␮M) increased sIPSC frequency and amplitude by 28.1 ⫾ 8.7
and 24.2 ⫾ 5.1%, respectively (P ⬍ 0.01, Fig. 4, C and D).
However, mIPSCs recorded in the presence of 0.5 ␮M TTX
were not significantly increased by amthamine treatment, indicating a lack of functional H2 receptors on GABA axon
terminals innervating SNr neurons.
HISTAMINE REGULATION OF BASAL GANGLIA OUTPUT
1587
this idea, mIPSCs were recorded in SNr neurons in the presence of 0.5 ␮M TTX to block action potentials. Bath application of 100 nM imetit decreased the frequency of mIPSCs from
7.0 ⫾ 1.7 Hz under control to 6.1 ⫾ 1.5 Hz during imetit
application (P ⬍ 0.001; n ⫽ 5, Fig. 9B). This effect was almost
fully recovered after washing out imetit (Fig. 9B). mIPSC
amplitude was not significantly affected (49.1 ⫾ 10.4 pA in
control and 48.3 ⫾ 11.1 pA in imetit treatment) (P ⬎ 0.05, Fig.
9C). Imetit inhibition of mIPSCs was prevented by a selective
H3 receptor antagonist clobenpropit (100 nM, n ⫽ 3). These
results indicate that H3 receptor may inhibit GABA vesicle
release from axon terminals synapsing onto SNr neurons.
values of the coefficient of variation (CV) of ISI under control
and during imetit treatment were compared. By definition, CV
was computed by dividing the SD of ISI by the mean ISI
(Bennett and Wilson 1999; Motulsky 1995). Under normal
conditions, SNr neurons in coronal brain slices fired action
potentials in a regular pattern such that ISI distribution is
narrow [Fig. 7, A (left) and C]. Bath application of 100 nM
imetit increased the CV of ISI from 0.176 ⫾ 0.023 to 0.394 ⫾
0.046 [n ⫽ 12, P ⬍ 0.001, Fig. 7, B (right) and D]. Furthermore, the ISI distribution also became much wider under imetit
than under control conditions (compare Fig. 7, C and D). These
results indicate that H3 receptor activation increased irregularity of SNr neuron spiking.
To explore how H3 receptor activation altered the SNr
neuron firing pattern, hyperpolarizing currents were directly
injected into these neurons. Membrane hyperpolarization decreased the firing frequency (Fig. 8, A and B). More important,
the direct hyperpolarizing current injection also made the spike
firing significantly more irregular, as indicated by the broadening of ISI distribution and the increased CV of ISI (Fig. 8, C
and D). Thus direct hyperpolarizing current injection appeared
to mimic the effects of H3 receptor activation, suggesting that
H3 receptor was altering the firing pattern primarily by hyperpolarizing SNr neurons such that these neurons reach spike
threshold less reliably and consequently spike less regularly.
Endogenous histamine release induces a tonic excitation in
SNr GABA projection neurons
Like other neurotransmitters, histamine may be released
spontaneously from histamine terminals and induce a low
level, tonic activation of histamine receptors and exert a tonic
influence on SNr neurons. Consequently, blocking histamine
receptors may have detectable effects in SNr neurons. We did
the following experiments to test this idea.
After establishing stable baseline recording of spontaneous
action potential firing, H1, H2, and H3 antagonists (2 ␮M
trans-triprolidine, 5 ␮M ranitidine, and 100 nM clobenpropit)
were individually tested; however, none of the antagonists
induced statistically significant change in SNr neuron firing.
This is not surprising because even the large doses of exogenous histamine agonists induced only mild effects as described
earlier. Because both H1 and H2 receptors are excitatory, we
reasoned that combined application of H1 and H2 receptor
antagonists might induce detectable effects. Indeed, a combined application of 2 ␮M trans-triprolidine (H1 receptor
antagonist) and 5 ␮M ranitidine (H2 receptor antagonist) induced a small hyperpolarization of 1.3 ⫾ 0.3 mV (n ⫽ 6) and
significantly decreased SNr neuron firing frequency by 9.2 ⫾
Histamine H3 receptor activation diminishes inhibitory
synaptic inputs to SNr GABA projection neurons by
presynaptic mechanisms
As expected from H3 receptor’s hyperpolarizing effect on
SNr neurons, bath application of H3 receptor agonist imetit
(100 nM) slightly but significantly decreased sIPSCs. The
sIPSC frequency was reduced by 17.9 ⫾ 2.5% (n ⫽ 5, P ⬍
0.05) and the amplitude by 9.8 ⫾ 3.6% (P ⬍ 0.05) (Fig. 4, C
and D). These results indicate that a fraction of action potential– dependent sIPSCs disappeared during imetit activation of
H3 receptor.
We also hypothesized that H3 receptor may act as an
inhibitory presynaptic receptor on GABA terminals. To test
J Neurophysiol • VOL
FIG. 9. H3 receptor activation inhibits miniature IPSCs (mIPSCs) in SNr
GABA projection neurons. A: example of mIPSCs under control conditions
and during 100 nM imetit application recorded at ⫺70 mV. TTX (0.5 ␮M) was
present during the entire recording. Although the outward current is clear, as
indicated by the dotted line, the effect on mIPSCs is small and difficult to
discern visually. B and C: quantification of the effects of H3 receptor activation
on mIPSCs. H3 receptor agonist imetit increased the interval of mIPSCs [P ⬍
0.001, Kolmogorov–Smirnov (K-S) test, control vs. histamine] but did not alter
mIPSC amplitude (P ⬎ 0.05, K-S test), indicating a decrease in the frequency
of spontaneous vesicular GABA release. Recovery after washing out histamine
was obtained.
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FIG. 8. Direct hyperpolarizing current injection increases SNr GABA neuron firing irregularity. A: spontaneous spikes recorded in an SNr GABA neuron
under control condition. B: when 45-pA hyperpolarizing current was applied
and the neuron was hyperpolarized by about 3 mV, spontaneous spikes became
slower in frequency. C: histogram of ISIs under control condition. ISI distribution was narrow. Mean ISI was 0.1062 s with CV of 0.1331. D: histogram
of ISI after the neuron was injected with 45-pA hyperpolarizing current. ISI
distribution became wider. Mean ISI was now 0.2050 s with CV of 0.2901.
Similar changes were seen in the other 3 SNr GABA neurons.
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ZHOU, XU, ZHAO, LEDOUX, AND ZHOU
3.2% (n ⫽ 6, P ⬍ 0.05, Fig. 10A). At the same time, the CV
for ISI was increased by 11.0% (P ⬍ 0.05). These results
suggest that spontaneously released endogenous histamine has
a modest tonic excitatory effect on SNr neurons by activating
H1 and H2 receptors that tend to keep these neurons spike more
regularly (Fig. 10B).
DISCUSSION
The main findings of this study are that H1 and H2 receptor
activation depolarizes SNr GABA projection neurons and
helps these neurons fire action potentials reliably and regularly,
whereas H3 receptor activation hyperpolarizes these neurons
and renders their spiking less reliable and regular. Consequently, histamine may alter the intensity and pattern of basal
ganglia output in opposite directions with the net effect of
histamine being dependent on the functional balance of the
different histamine receptors.
H1 and H2 receptor activation increases SNr GABA
projection neuron output
We found that activation of H1 receptors induced an inward
current and increased spike firing in SNr neurons (Figs. 3 and
5). These effects were accompanied by increased whole cell
conductance, indicating an opening of unknown type(s) of ion
channels. In the cortex, hippocampus, septum, striatum, and
thalamus, activation of H1 receptors increases neuronal excitability by blocking a leak K⫹ conductance or decreasing whole
cell conductance (Bell et al. 2000; Gorelova and Reiner 1996;
McCormick and Williamson 1991).
Activation of H2 receptors also induced an inward current
and enhanced spike firing in SNr neurons (Figs. 3 and 6). These
effects were also accompanied by increased whole cell conductance, indicating opening of ion channels. In thalamocortiJ Neurophysiol • VOL
H3 receptor activation increases SNr projection neuron
spiking irregularity
Our data clearly demonstrate that activation of H3 receptor
induced a small hyperpolarization or a small outward current
and consequently an inhibition of SNr neuron firing (Fig. 7).
Whole cell conductance was decreased, indicating a closing of
an unknown type ion channel. These results suggest that H3
receptor is on the somatodendritic area of SNr neurons. In
addition, H3 receptor activation also slightly reduced the frequency of mIPSCs in SNr neurons, indicating a low level of H3
receptor expression on GABA axon terminals synapsing onto
SNr neurons. This is consistent with histochemical studies
(Pillot et al. 2002; Vizuete et al. 1997) and also with previous
findings that H3 receptor serves as an inhibitory presynaptic
receptor (Arrang et al. 1983; Brown and Haas 1999; Jang et al.
2001; Threlfell et al. 2004; for review see Haas and Panula
2003).
Importantly, the small hyperpolarization induced by H3
receptor activation was able to alter the pattern of SNr neuron
firing, making the firing of these neurons more irregular (Fig.
7, A–D). Apparently, under normal conditions SNr neurons are
depolarized sufficiently and can spike reliably and regularly.
When hyperpolarized by H3 receptor activation or direct negative current injection (Fig. 8), these neurons are no longer
sufficiently depolarized, such that they reach action potential
threshold less reliably and consequently spike more irregularly.
The molecular identities of the ion channel(s) affected by H3
receptor and also those affected by H1 and H2 receptors are not
known. Our results show that histamine did not affect the usual
suspects such as the leak K⫹ conductance and AHPs in SNr
GABA neurons. Instead, our observations suggest that histamine may modulate a tonically active, Na⫹-dependent inward
current that was critical to the spontaneous firing of SNr
GABA neurons (Atherton and Bevan 2005). Specifically, H1
and H2 receptors appeared to upregulate this tonic inward
current, whereas H3 receptor seemed to downregulate it. How-
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FIG. 10. Tonic histamine receptor activation enhances SNr GABA projection neuron firing. A: simultaneous bath application of H1 and H2 receptor
antagonists trans-triprolidine (2 ␮M) and ranitidine (5 ␮M) induced a small
but significant decrease in the frequency of spontaneous action potential firing
in SNr GABA neurons. Insets: examples of spikes under control and during
trans-triprolidine and ranitidine. Spikes were truncated for display. Scale bar:
100 ms and 20 mV. B: schematic diagram showing that SNr projections may
be tonically influenced by spontaneously released histamine.
cal neurons, H2 receptor activation enhanced the hyperpolarization-activated Ih current (McCormick and Williamson
1991). However, Ih is very small in SNr neurons. Furthermore,
this Ih usually starts to activate when the membrane potential is
more negative than ⫺60 mV. In contrast, the histamineinduced current was linear between ⫺90 and ⫺20 mV with no
signs of voltage-dependent activation or inactivation (Fig. 3),
indicating that Ih is not likely histamine’s target conductance in
SNr neurons. Also, neither H1 nor H2 receptor activation
affected the Ca2⫹-activated K⫹ channel–mediated fAHP and
mAHP in SNr neurons. H2 receptor activation inhibits sAHP in
hippocampal and cortical neurons (Haas and Konnerth 1983;
McCormick and Williamson 1989; Yanovsky and Haas 1998),
but SNr GABA neurons lack sAHP (Fig. 1, C and D). Clearly,
H1 receptor and H2 receptor in SNr neurons are likely coupled
to different effectors or ion channels compared with those in
other brain areas.
An extracellular recording study reported that exogenous
histamine slightly increased the firing rate of SNr neurons by
H1 receptor activation and that H2 receptor was not involved,
although H3 receptor’s effects were not studied (Korotkova et
al. 2002). Apparently, these authors missed some important
histamine effects in SNr.
HISTAMINE REGULATION OF BASAL GANGLIA OUTPUT
ever, a definitive answer requires cloning of the channel
conducting the tonic Na⫹-dependent inward current (for an
example of histamine regulation of molecularly identified K⫹
channel see Atzori et al. 2000).
ganglia output and consequently contribute to multiple aspects
of movement disorders of basal ganglia origin.
ACKNOWLEDGMENTS
We thank L. Lu and R. Williams for help with histology and S. Tavalin and
S. Matta for comments.
Tonic activation of histamine receptors by
endogenous histamine
GRANTS
Functional implications
Our present study indicates that the direct effects of histamine on SNr GABA projection neurons are a mild, H1 and H2
receptor-mediated excitation and a weak, H3 receptor-mediated
inhibition. Functional balance of these different histamine
receptors may alter the intensity and pattern of SNr GABA
neuron activity. Consequently, basal ganglia output and movement control may also be affected. Although the situation is
likely to be more complex in vivo because histamine may also
affect afferents to SNr neurons (Brown et al. 2001; Hass and
Panula 2003; Threlfell et al. 2004), the direct histamine effects
on SNr neurons described herein are likely to be important.
Indeed, selective activation of H3 receptors by injection of an
H3 receptor agonist into SNr has been shown to influence
motor behavior in rats (Garcia-Ramirez et al. 2004). In addition, a recent study found that systemic administration of an H3
receptor agonist worsened parkinsonian symptoms in a primate
model of Parkinson’s disease (Gomez-Ramirez et al. 2006). In
aggregate, these findings indicate that H3 receptors may regulate the activity of SNr GABA projection neurons and basal
ganglia output. In Parkinson’s disease, histamine levels in the
substantia nigra (both SNc and SNr) were substantially increased (Anichtchik et al. 2000; Rinne et al. 2002). Nigral H3
receptor expression was also increased in patients with Parkinson’s disease (Anichtchik et al. 2001) and a rodent model of
the disease (Ryu et al. 1994). Because H3 activation causes
hyperpolarization, decreases firing rates, and increases the
irregularity of spike firing, abnormally high levels of histamine
innervation and H3 receptor expression in SNr in parkinsonian
brain may adversely alter the intensity and pattern of the basal
This work was supported by grants from the National Alliance for Research
on Schizophrenia and Depression and National Institutes of Health Grants
MH-067119 to F.-M. Zhou and NS-04858 to M. S. LeDoux.
DISCLOSURE
The authors declare no conflict of interest.
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We found that blockade of H1 and H2 receptors induced a
small hyperpolarization, decreased firing frequency, and increased the firing irregularity in SNr neurons (Fig. 10). These
results indicate that spontaneously released endogenous histamine may induce a low-level, tonic activation of histamine
receptors and influence SNr neuron activity. This is not surprising because other neurotransmitters such as acetylcholine,
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