Download Spatial Organization of Facial Vibrissae and Cortical Barrels in the

THE JOURNAL OF COMPARATIVE NEUROLOGY 385:515–527 (1997)
Spatial Organization of Facial Vibrissae
and Cortical Barrels in the Guinea Pig
and Golden Hamster
SEBASTIAN HAIDARLIU* AND EHUD AHISSAR
Department of Neurobiology, The Weizmann Institute of Science, Rehovot 76100, Israel
ABSTRACT
The arrangements of vibrissae in guinea pigs and golden hamsters were previously
reported to be different from those in mice and rats. Whereas the mystacial pads in mice and
rats include four straddlers and five rows of vibrissae, guinea pigs were described to possess
six rows of irregularly aligned mystacial vibrissae and no straddlers, and golden hamsters to
include seven vibrissal rows and also no straddlers. We found that all of these four species
possess similar vibrissal arrangements within the mystacial pad. To demonstrate this
similarity, we developed a new method of sinus hair visualization in flattened and cleared
preparations of the mystacial pad. Intrinsic muscles of the mystacial pad that were revealed
in thick histological preparations showed clearly the structural and functional relationships
between straddlers and vibrissal rows. To verify this finding, and to extend the knowledge of
vibrissal cortical representations in guinea pigs and golden hamsters, we have investigated
the spatial organization and the functional vibrissal representations of barrels in the
posteromedial barrel subfield (PMBSF) of these rodents. The barrel morphology was clearly
preserved in Nissl-stained sections and sections processed for cytochrome oxidase of flattened
cerebral cortices. We demonstrate that the vibrissal arrangement in the mystacial pad is
replicated in the PMBSF of guinea pigs and golden hamsters and that this arrangement is
similar to that found in mice and rats. To facilitate comparative studies, these findings
strongly recommend the use, in guinea pigs and golden hamsters, of the same classifications
and nomenclatures that are used in mice and rats to describe mystacial vibrissae and cortical
barrels. J. Comp. Neurol. 385:515–527, 1997. r 1997 Wiley-Liss, Inc.
Indexing terms: mystacial pad; somatosensory cortex; barrel field
In mammals, specializations in the periphery are often
reflected in anatomical and physiological distinctions in
the central nervous system, at the level of the brainstem,
thalamus, and cerebral cortex. Such specializations have
been identified in the cerebral cortex in a variety of
mammals for different sensory systems and are broadly
referred to as ‘‘modules.’’ The most intensively studied and
best understood system in which peripheral specializations generate functionally and structurally distinct units
in the central nervous system is the mystacial vibrissae
and their corresponding cortical barrels in rodents. Most of
the information regarding peripheral and central organization has been derived from only two species of rodents, rats
and mice, in which the anatomical relationship of whiskers
to barrels of layer IV in the somatosensory cortex have
been extensively described (Woolsey and Van der Loos,
1970; Welker, 1971; Van der Loos and Woolsey, 1973).
Relatively little is known about the peripheral organization and cortical barrels in any other species of rodents. In
fact, for some rodents, such as guinea pigs and golden
r 1997 WILEY-LISS, INC.
hamsters, the data available are scant and sometimes
even contradictory.
The presence of barrels in the somatosensory cortex of
the guinea pig was first indicated by Woolsey et al. (1975)
and then confirmed by Rice et al. (1985). However, these
studies did not define the anatomical location of the
posteromedial barrel subfield (PMBSF) or the number of
rows of barrels contained in it. Because of the absence of an
adequate description of the guinea pig vibrissae-somatosensory cortex system, an arbitrary nomenclature for the
Grant sponsor: The Minna-James Heinemann Foundation, Germany;
Grant sponsor: The Minerva Foundation, Munich, Germany; Grant sponsor: The US-Israel Binational Science Foundation, Israel; Grant number:
93-00198.
*Correspondence to: S. Haidarliu, Center for Brain Research, Department of Neurobiology, The Weizmann Institute of Science, Rehovot 76100,
Israel. E-mail: [email protected]
Received 16 December 1996; Revised 28 March 1997; Accepted April 2,
1997.
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S. HAIDARLIU AND E. AHISSAR
guinea pig vibrissal pad and its cortical barrels was
proposed by Sikich et al. (1986), who described six rows of
mystacial vibrissae, designated A–F in a dorsal to ventral
direction, and reported that these rows differ from those of
mice and rats in that they are more irregularly aligned
with respect to one another. However, this arbitrary
nomenclature had little experimental basis; intrinsic musculature was not studied to verify the lack of straddlers,
mystacial vibrissae were not differentiated from the nasal
ones, and cortical representations were not verified.
In the golden hamster, a noncorresponding relationship
was reported for the mystacial vibrissae and cortical
barrels. Mystacial vibrissae were reported to be organized
in seven rows (Wineski, 1983, 1985) but to be represented
by five rows of large barrels in the PMBSF of the somatosensory cortex (Woolsey et al., 1975; Jacquin et al., 1993).
For this study, a method was developed to improve both
barrel and whisker presentation. This was achieved by
flattening and clearing entire mystacial pads and by
combining the method of cerebral cortex and mystacial pad
flattening with paraffin embedding. We show that guinea
pigs and golden hamsters possess four straddlers and five
rows of vibrissae in their mystacial pad and a similar
distribution of related barrels in their PMBSF. This similarity was confirmed by electrophysiological mapping. In
these rodents, the row of facial sinus hairs that was
previously considered to be the most dorsal row of mystacial vibrissae is in fact a row of nasal vibrissae and has a
different musculature pattern and a different cortical
barrel representation than the mystacial vibrissae. These
findings indicate that the principles of mystacial pad
organization in the periphery, and of the cortical barrels
distribution in the PMBSF, are similar across major
lineages of rodents, even in the absence of typical whisking
behavior.
MATERIALS AND METHODS
Animals and experimental procedures
Adult guinea pigs, Cavia porcellus (36 female and 5
male), weighing 360–760 g, and adult golden hamsters,
Mesocricetus auratus (6 female and 6 male), weighing
110–160 g, were obtained from The Animal Breeding Unit
of The Weizmann Institute of Science (Rehovot, Israel).
Maintenance, manipulations, and surgery followed institutional animal welfare guidelines, which met the NIH
standards. Identification of cortical regions responsive to
tactile stimuli applied to vibrissae was achieved using a
multielectrode array (Haidarliu et al., 1995) for extracellular recording. Upon completion of the recording, determination of receptive fields, and marking procedures, the
animals were euthanized with Pental (1 ml/kg body weight)
and perfused transcardially with 10% formaldehyde. The
cerebral cortices and the whiskerpads were then excised.
The whiskerpads were flattened in newly developed flattening device, fixed, dehydrated in alcohol, cleared in xylene,
and used to determine gross morphology. These samples
were then embedded in paraffin for histological studies.
Cerebral cortices were flattened in the same device, embedded in paraffin, and cut either parallel or perpendicular to
the flattened surface. For delineation of three-dimensional
barrel structure in the guinea pig, coronal, parasagittal, or
oblique sections were also prepared. Cortical sections were
Nissl stained, some of them were also processed for
cytochrome oxidase. Mystacial sections were stained with
cresyl violet supplemented with thiazine red.
Surgery
A single dose of Urethane (1.5 g/kg for guinea pigs, 2 g/kg
for hamsters) was administered at the beginning of surgery. This was sufficient to maintain surgical levels of
anesthesia throughout the experiment. Guinea pigs were
also administered Rompun (8 mg/kg), a tranquilizer, every
3 hours, and hamsters were given atropine sulfate (20
mg/kg) at the beginning of surgery. Anesthetized animals
were mounted in anatomically adapted, injury-free headholders without earbars, which permit free access to all
brain regions (Haidarliu, 1996) and in which the vibrissae
remain accessible to mechanical stimulation. The headholder with the subject’s head was mounted onto a stereotaxic instrument (Narishige SR-6, Japan). A local anesthetic (2% Lidocaine, 0.5–1.0 ml) was applied s.c. to the
scalp. During the surgery and experimental manipulations, the body temperature was monitored rectally and
maintained between 37°C and 38°C by heating pad. Mystacial vibrissae were clipped to a length of 10–15 mm. A
portion of the somatosensory cortex was exposed without
removing dura. The dura was protected and pulsations of
the brain were attenuated by forming a well of dental
acrylic on the edge of the trepanned bone and filling the
well with agar-agar.
Extracellular recordings and tactile stimuli
The multielectrode array used consisted of regular tungsten-in-glass electrodes (0.2–0.8 MV at 1 kHz) and combined electrodes that each contained a tungsten electrode
(0.2–0.8 MV at 1 kHz) surrounded by six micropipettes
with orifice diameters of 1–4 µm (10–80 MV at 1 kHz;
Haidarliu et al., 1995). The combined electrodes were
included for iontophoretic drug applications; however, no
drugs were applied during the experiments reported here.
The electrodes were assembled in two different configurations within a stainless steel guide: A circular configuration included two regular and two combined electrodes,
with interelectrode distances of 300–600 µm and a linear
configuration of four regular electrodes distanced 350 1/2
32 µm from each other.
Using a microdrive system, designed and built by Y. de
Ribaupierre (The Institute of Physiology, University of
Lausanne, Lausanne, Switzerland), the microelectrode
array was brought close to the exposed dura, and each of
the four electrodes was introduced independently into the
somatosensory cortex at a site chosen according to stereotaxic coordinates. The electrodes were introduced perpendicular to the surface of the cortex and to a depth that
corresponded to layer IV. The location of each penetration
was determined in relation to the bregma by direct measurements with an ocular-micrometer and by mapping on
an enlarged photograph of the superficial blood vessels of
the particular brain being studied.
For each penetration, local field potentials (LFPs) and
up to 16 single- and/or multi-units were recorded simultaneously from four electrodes during spontaneous activity
and during response to mechanical stimulation of vibrissae or head surfaces. Each electrode channel was amplified
(310,000–40,000) and filtered (.5–10 kHz) and then fed
into a template-matching spike sorter (MSD-2, AlphaOmega Engineering, POB 2091, Nazareth, Israel). The
amplified signals were also fed into a loudspeaker through
VIBRISSAL AND BARREL ARRANGEMENT IN RODENTS
an offset circuit that removed background noise. To determine the principal vibrissae associated with the recording
sites of the different electrodes, different vibrissae and the
skin of the head were stimulated with a thin hand-held
probe while the population responses were estimated by
listening to the loudspeaker. When a responding vibrissa
was ascertained, it was inserted into a small teflon tubing
attached via a probe to a linear-displacement motor
(Schneider, 1988) that was controlled by the experimental
computer. Vibratory, square-wave stimuli (80–320 µm p-p,
1–20 Hz; see Fig. 6a) were applied to the selected vibrissa(e). Receptive field boundaries were determined according to the peri-stimulus-time-histograms (PSTHs) of the
neuronal responses. Upon completion of the extracellular
recordings and receptive field determinations, the sites of
recording were marked by passing anodal DC currents
through the electrodes, which resulted in tissue coagulation and were used for the reconstruction of the map of
penetrations in a given experiment.
Preparation of mystacial pad samples
After excision of whiskerpad, the vibrissae and intervibrissal hairs (fur) were cut close to the skin. Excised
whiskerpads were placed into a simple flattening device
that consisted of a polished stainless steel base with two
fixed screws, a piece of sponge, and a cover that fit over the
base. The whiskerpad (skin-side down) was placed on the
polished surface of the base, covered with the sponge, and
pressed with the cover using two nuts. The filled flattening
device was placed in Carnoy’s fluid overnight, then transferred sequentially through ethyl alcohol solutions (70%,
90%, 95%, and 100%; 1 hour in each), taken out of the
flattening device, and placed overnight in xylene for clearing. By the next day, the whiskerpads had become transparent, except for the vibrissal sinus regions, and were
photographed. The tissue preparations were then placed
in paraffin at 56°C. After 2 hours, the paraffin-soaked
tissue was again placed into the flattening device, which
was also in the chamber maintained at 56°C, flattened,
and immediately dipped into ice-cold water. The flattened,
paraffin-soaked tissue was attached to a wooden platform,
and sections (40 or 100 µm thick) were prepared using a
microtome. These sections were stained with cresyl violet,
washed, stained supplementary with thiazine red, and
mounted. The thicker sections (100 µm) were used to
determine the intrinsic musculature.
Processing of cerebral cortex preparations
Excised brain hemispheres were cut sagittally into two
symmetrical pieces. The cerebral cortex was carefully
separated from the rest of the brain and placed into a
flattening device similar to that used to flatten whiskerpads. The outer surface of the cortex was placed on the
polished surface of the base, the subcortical white matter
was covered with a sponge, and the sample was gently
pressed, as described above for the mystacial pads. Flattened cortical tissues were embedded in paraffin and
sectioned in a fashion similar to that used for the whiskerpads. Flattened cortices were cut parallel to the pial
surface overlying the PMBSF. Some of the guinea pigs
brains were sectioned also in coronal (see Fig. 4c) and
oblique planes oriented posteromedial to anterolateral, as
recommended by Chmielowska et al. (1989) for the rat
cortex, in order to produce sections ‘‘along’’ the barrel rows.
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Serial histological sections were examined for PMBSF,
electrode tracks, and/or sites of coagulation.
In a group of six randomly selected guinea pigs, a
morphometric analysis was performed to compare two
populations of cortical barrels which represent mystacial
and nasal vibrissae. From studying the shape of the
barrels in tangential, coronal, parasagittal, and oblique
Nissl-stained sections, we concluded that the configuration of the barrels in the PMBSF approximately resembles
an ellipsoid, of which vertical axes that correspond to the
thickness of layer IV are approximately the same for all
the barrels. So, the differences calculated for the mean
area of a barrel in different groups of barrels measured in
tangential sections would reflect the differences in barrel
volumes. Barrel area was calculated according to the
formula of an ellipse: S 5 pab, where a is the large, and b is
the small radius of a barrel. Barrel radii were measured
from the calibrated camera lucida drawings of the PMBSF.
Mean areas were calculated for the four posterior barrels
representing the four largest nasal vibrissae and for the
barrels corresponding to each of the 24 usually present
mystacial vibrissae (four straddlers, two vibrissae in row
A, three in row B, and five in each of the rows C, D, and E).
Statistical significance was estimated using a two-sided
Student t-test.
In two guinea pigs and two golden hamsters, brain
tissues were also processed for CO histochemistry. The
animals were perfused transcardially with 2.5% glutaraldehyde, 0.5% paraformaldehyde, and 5% sucrose in 0.1 M
sodium phosphate (PO4) at pH 7.4. Freezing microtome
sections (60 µm) were prepared and left overnight in
perfusion solution at 4°C. The next day, sections were
incubated in oxygenated solution of 4% sucrose, 0.02%
cytochrome C (Type VI, Sigma), 0.05% diaminobenzidine
in PO4 for 2–3 hours at 37°C under constant agitation
(Land and Simons, 1985). The reaction was terminated by
PO4 change. CO-stained sections were mounted on gelatincoated slides, air-dried, and coverslipped using Entellan.
Production of photomicrographs
Photomicrographs were taken by using Wild Makroskop
M 420 with reversal Kodak Ektachrome 160T or negative
black-and-white T-MAX 100 35-mm professional film. Figures 3 and 8 were printed by using standard darkroom
methods. Figures 1, 2, 4, 5, 6a and c, 7, and 8a and c were
processed by a computer. Original photographic color
slides (for Figs. 2, 4, and 5) were scanned on a Kodak
Professional RFS 2035 Plus Film Scanner, and black-andwhite prints were scanned on a Nicon AX-1200 scanner.
Image editing was performed on a Power Macintosh
9500/132 with Adobe Photoshop 3.0 using standard tools.
Only the sharpness, contrast, and/or brightness were
adjusted. Final prints were prepared on a Kodak XLS 8600
PS printer.
RESULTS
Vibrissal arrangement and nomenclature
In cleared mystacial preparations, all large and small
sinus hairs on the face of the guinea pig and golden
hamster were easily observed (Fig. 1). Almost all the
mystacial vibrissae of these rodents were arranged in
essentially straight rows. The sizes of the vibrissae diminished from the caudal to rostral direction. The most caudal
arc was composed of four large mystacial vibrissae that
Fig. 1. Vibrissal arrangement in the mystacial pad of the guinea pig (A) and golden hamster
(B). Darkfield photographs of entire whiskerpads after clearing in xylene; all sinus hairs are
visualized. A clear arrangement of mystacial vibrissae in five rows (A–E) that are oriented from
caudal to rostral along the snout is observed. The four most caudally placed large vibrissae (a,
b, g, and d) are positioned in the spaces that would be between the rows if the rows were
continued caudally. These four vibrissae straddle vibrissal rows from A to E. C, caudal; FBP,
furry buccal pad; N, nasal vibrissae; NS, nostril; R, rostral, V, ventral. Scale bars 5 1 mm.
VIBRISSAL AND BARREL ARRANGEMENT IN RODENTS
were not situated in the essentially straight rows. These
vibrissae were in the middle of the spaces formed by an
imaginary continuation of the vibrissal rows in the caudal
direction and thus could be considered as straddlers.
To verify that these vibrissae are straddlers, we studied
the intrinsic musculature of the mystacial pad in thick
(100-µm) sections of the mystacial pad (Fig. 2). In vibrissal
rows of both guinea pigs and golden hamsters, the intrinsic
muscles were only connecting the neighboring vibrissae of
the same row. Each of the most caudal four vibrissae was
connected via follicular muscles with two rows of mystacial
vibrissae. If we apply the nomenclature proposed by Van
der Loos and Woolsey (1973) for vibrissae classification in
mice and rats, the intrinsic muscles would connect straddler a with rows A and B, straddler b with rows B and C,
straddler g with rows C and D, and straddler d with rows D
and E. Intrinsic muscles that would connect the straddlers
with the most dorsal row of sinus hairs outside the
mystacial pad (nasal vibrissae) were not observed.
This pattern of muscle distribution suggests that guinea
pigs and golden hamsters possess four straddlers and five
rows of mystacial vibrissae, which are typical for mice and
rats, and that the most dorsal row of facial vibrissae of the
guinea pig and golden hamster does not share the same
structural and functional relationships that exist between
the other five rows of facial vibrissae and the straddlers,
and thus should not be considered as a part of the
mystacial vibrissae. In the guinea pig, the anterior part of
this nonmystacial vibrissal row, which is located dorsally
to the horizontal line crossing the nostrils, was previously
called nasal vibrissae (Cooper and Schiller, 1975). We
believe that these nasal vibrissae have a caudal continuation that was previously considered as row A of mystacial
vibrissae (Sikich et al., 1986). In the golden hamster, the
most dorsal row of facial vibrissae appears similar to that
of the row of nasal vibrissae in the guinea pig (Fig. 1b).
To further compare the features of the tissue containing
facial vibrissae, we studied the behavior of the mystacial
pad during tissue processing. The clearing procedure
provided additional evidence that the sinus hairs of the
most dorsal row are not part of the mystacial pad of the
guinea pig and golden hamster. This dorsal row, which
includes four to eight large sinus hairs, became clearly
visible at the beginning of clearing procedure, whereas the
mystacial pad, which is relatively rich in muscles and
connective tissues, only became transparent later (Fig. 3).
Whether the peculiar behavior during tissue processing of
the whisker follicles in this dorsal row might simply be
related to the thickness of the tissue was checked by
flattening the snout specimens during fixation and dehydration without using a sponge. The specimens thus
flattened were of even thickness. They were taken out of
the flattening device and dipped into xylene. During the
clearing procedure, the mystacial pad still remained opaque
for at least 1 or 2 hours after the tissue around the nasal
vibrissae had become transparent. Thus, the differential
clearing characteristics were a function of the type of
tissue, not of possible variations in the thickness of the
tissue.
Although the fundamentals, with regard to straight
rows and straddlers, of the spatial organization of vibrissae for guinea pig were similar to those in mice and rats,
the precise arrangement of vibrissae, especially straddlers, differed. In guinea pigs, the vertical distances between the vibrissal rows were less than the diameters of
519
the straddlers bulbs. As a result, the straddlers were
arranged in a semicircular manner, with the d-straddler
located more rostral and appearing like a continuation of
the arc formed by vibrissae A1, B1, and C1 (see Figs. 1a,
2a). Vibrissae D1 and E1, which were straddled by this
d-straddler, appeared like a continuation of the arc formed
by vibrissae A2, B2, and C2. In the golden hamster, the
straddler arrangement appeared the same as in mice and
rats, but the row of nasal vibrissae and row A of mystacial
vibrissae were overlapping: One or two of the most caudal
nasal vibrissae were located dorsal to row A, whereas the
rest of the nasal vibrissae were shifted ventrally, forming
an apparent rostral continuation of row A (see Figs. 1b, 2b).
Thus, based on these three observations [(i) a pattern of
vibrissa arrangement that is consistent with four straddlers and five rows; (ii) specific distribution of intrinsic
musculature in the mystacial pad; and (iii) peculiar behavior of the mystacial pad during tissue processing], we
conclude that the mystacial pads of the guinea pig and
golden hamster indeed contain four straddlers that straddle
five rows of mystacial vibrissae, and that the most dorsal
row of facial vibrissae is located outside the mystacial pad
and should be considered as nasal vibrissae.
We propose that the short second row of facial vibrissae
be designated as row A of mystacial vibrissae. Ventral to
this row A is a row of mystacial vibrissae we designate as
row B. In the guinea pig, this row B usually contained
three, and in the golden hamster, four vibrissae, just like
row B in rats which also contains four. In the guinea pig
and golden hamster, as in mice and rats, the next three
rows of mystacial vibrissae (C, D, and E) were the longest.
These rows usually contained four to six large vibrissae
and had a rostral continuation that consisted of smaller
sinus hairs. Ventral to rows C–E, the upper lips of the
guinea pig and golden hamster were also covered with
small sinus hairs, which were arranged in five rows.
Similar hairs were designated as labial sinus hairs in mice
(Dörfl, 1982) and as rows F, G, H, I, and J in rats (Brecht et
al., 1997). From the lateral part of the upper lip, the small
sinus hairs appeared to pass onto the inner surface of the
upper lip (see Fig. 1), thus forming a furry buccal pad
(FBP) similar to the one described in the rat (Welker,
1976). In both guinea pigs and golden hamsters, the FBP
was represented in the somatosensory cortex anterior to
the PMBSF.
When comparing the described patterns of vibrissa
arrangement in the guinea pig and golden hamster with
those of mice and rats, a striking similarity emerges. This
similarity further supports our proposal and provides
support for the same principles of classification and nomenclature to be used in these four species of rodents.
Structural representation
of the vibrissae in the cortex
The patterns of barrel arrangement, including the
PMBSF, in sections of cerebral cortices of the guinea pig
and golden hamster were examined. In Nissl- and COstained sections of the cortex cut parallel to the surface of
the flattened cortex, the PMBSF was composed of visible
spheroidal barrels arranged in five rows that were directed
postero-medial to rostro-lateral (Figs. 4a,b, 5). In the
guinea pig, usually, two large barrels were present in row
A. Row B, which was medio-rostral to row A, usually
consisted of three or four barrels in an almost straight line.
Parallel to row B, in the medio-anterior direction, were
Fig. 2. Cresyl violet supplemented with thiazine red staining of thick (100-µm) sections of
flattened mystacial pads of the guinea pig (a) and golden hamster (b). The rostral segment of
the follicles of each large mystacial vibrissa is surrounded, in a sling-like fashion, by intrinsic
muscles (arrows). The ends of the slings are oriented caudally. a, b, g, and d are straddlers; A–E,
rows of vibrissae; N, nasal vibrissae; R, rostral; V, ventral. Scale bars 5 1 mm.
VIBRISSAL AND BARREL ARRANGEMENT IN RODENTS
521
Fig. 3. Mystacial pads of the guinea pig (a) and golden hamster (b)
during incubation in xylene. Centrally located opaque discs correspond to the whiskerpads, while the sinus hairs of the upper lip (UL)
and nasal vibrissae (N1–N6) are cleared. a, b, g, and d indicate the
locations of the four straddlers, which are not yet cleared. D, dorsal;
NS, nostril; R, rostral. Scale bars 5 1 mm.
rows C, D, and E, which each contained four to five barrels.
These five rows of barrels were at an angle of about 40° to
the longitudinal axis of the brain. The row of barrels
representing the nasal vibrissae was located laterally to
row A and oriented at a bigger angle to the brain axis.
Barrels representing the nasal vibrissae were significantly
smaller than barrels representing mystacial vibrissae. A
morphometric analysis has shown that the mean area of a
barrel corresponding to mystacial vibrissae was 64 · 103 6
7 · 103 µm2, whereas that of a barrel corresponding to
nasal vibrissae was 28 · 103 6 6 · 103 µm2 (P , .001).
In sections of the guinea pig cortex cut perpendicularly
to the surface of the cortex, and especially in coronal
sections (Fig. 4c,d), barrels were also easily visualized, and
possessed a spheroidal, rather than the dubbed shape
previously reported for rodents and some lagomorphs
(Woolsey and Van der Loos, 1970; Weller, 1972; Woolsey et
al., 1975).
In Nissl-stained sections of the flattened cortex of the
golden hamster cut parallel to pial surface, the PMBSF
was composed of five rows of hollow barrels, arranged in a
fashion demonstrated previously by Woolsey et al. (1975)
with thionin-stained sections. Because the hollow barrels
of the golden hamster were less prominent than those of
the guinea pig and occupied only the deeper part of layer
IV in the barrel field, the entire PMBSF could not be
clearly seen in a single Nissl-stained tangential section;
thus several cortical sections were used to prepare camera
lucida drawings (see Fig. 8d below). These peculiarities of
the golden hamster barrels were previously mentioned by
Rice et al. (1985) and Rice (1995). In CO-stained tangential
sections, we could obtain all the barrels of the PMBSF in a
single section (Fig. 5). Previous reports seem to demonstrate an incomplete picture in 2-DG- and CO-labeled
tangential sections through layer IV of the hamster barrel
cortex (Jacquin et al., 1993).
Electrophysiological verification
of the cortical representation
of the mystacial vibrissae
Cortical barrels were related to specific vibrissae by
mapping the receptive fields of single neurons and of
groups of neurons. Multiple penetrations at intervals of
about 300 µm, starting from the site where the response to
straddler stimulation was observed, were performed.
Stimulation of a single vibrissa usually evoked response in
only one, and in some cases in two, of the electrodes
introduced into the PMBSF. This enhanced neuronal activity was evident in the unfiltered electrode signal feeding
the loudspeaker, in the LFP and multi-unit responses, and
was usually evident in the response of at least one of the
single units recorded. Responses of multi-units, simultaneously recorded from four electrodes that were arranged
in a linear array, are demonstrated in Figure 6. The units
were recorded from layer IV, and the recording sites were
marked by electrolytic lesions (Fig. 6b). The PSTHs (Fig.
6a) show a clear functional segregation between the cortical barrels. The neurons recorded from the g-barrel (el. 1)
responded to stimulations of vibrissae g and C1 and not to
stimulations of vibrissae B1 or A2. C1-barrel neurons (el.
2) responded only to vibrissa C1, neurons recorded from
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Fig. 4. Nissl (a,c) and cytochrome oxidase (b,d) staining of the
sections (60 µm) of cerebral cortices of guinea pigs. Tangential sections
through the posteromedial barrel subfield (PMBSF) of flattened
cortices of right hemispheres (a,b), and coronal sections through the
PMBSF of left hemispheres (c,d). Roman numerals demarcate cortical
S. HAIDARLIU AND E. AHISSAR
laminae. Labels in a and b are centered inside the barrels. a, b, g, and d
represent straddlers; A–E, rows of mystacial vibrissae; N, nasal
vibrissae; SO, supraorbital vibrissa. White arrowheads point to barrels; black arrowheads indicate the loci of lesions. M, medial; R,
rostral; W, white matter. Scale bars 5 1 mm.
VIBRISSAL AND BARREL ARRANGEMENT IN RODENTS
523
Fig. 5. Cytochrome oxidase staining of a tangential section (60 µm) through layer IV of the hamster
left barrel cortex. Conventions and abbreviations as in Figure 4.
the border between barrels B1 and B2 (el. 3) responded to
both vibrissae B1 and B2 (the latter is not shown), and
A2-barrel neurons (el. 4) responded to both vibrissae A2
and B2 (the latter is not shown). Note that most of the
responses depended upon the direction of vibrissa movement (retraction vs. protraction, lower trace). Both spatial
segregation and directional dependencies were typical for
all PMBSF recordings. Another example of the observed
spatial segregation of cortical responses is depicted in
Figure 7. In this figure, the average firing rates of two
single-units, recorded from layer IV during the penetrations marked in Figure 4b, are plotted as a function of
time. During the simultaneous stimulation of straddler b
and vibrissa B1, the firing rates of both neurons were
significantly increased. However, when vibrissa B1 was
stimulated alone, only the neuron recorded near barrel B1
(el. 4) was activated.
Figure 8 summarizes the spatial arrangements of the
sinus hairs on the snouts of the guinea pig (Fig. 8a) and
golden hamster (Fig. 8c) and the spatial arrangements of
the barrels in the PMBSF of these two species (Fig. 8b and
d, respectively). This figure demonstrates the proposed
adaptation, for these two species, of the nomenclature
used for the vibrissa arrangement of the mouse and rat
(Van der Loos and Woolsey, 1973). In the rostral part of the
barrel field (data not shown), both the vibrissal follicles
and the hairs that surround them were represented, which
is similar to the situation in mice (Nussbaumer and Van
der Loos, 1985).
DISCUSSION
In order to understand the general principles of how
structures in the periphery generate topographic maps at
higher levels of the nervous system, and how these representations interconnect to form processing networks, a
detailed description of such systems in a variety of mammals is essential. Such descriptions will elucidate similarities in peripheral and cortical organization, as well as
differences that exist as a result of different location of
specialized receptor structures on the sensory surface, and
the function or use of such structures. This study was
initiated to (i) delineate the general patterns of vibrissa
and barrel arrangement in guinea pigs and golden hamsters, (ii) determine the cortical representation of each
whisker of the whiskerpad of these two species, and (iii)
compare the general organizational principles of the whiskerpad-PMBSF system of the guinea pig and golden
hamster with those of the mouse and rat. A highly reproducible map of both the vibrissal distribution in the mystacial
pad and of the barrel arrangement in the PMBSF of the
guinea pig and golden hamster was obtained, and their
functional correlation was determined. These findings
support application to the guinea pig and golden hamster
524
S. HAIDARLIU AND E. AHISSAR
Fig. 7. Effect of vibrissa stimulation on the firing rates of two
individual neurons in the PMBSF of the guinea pig. Neuronal
activities were recorded simultaneously by electrode 3 located in
barrel b and electrode 4 which was touching barrel B1 (see Fig. 4b).
Thick traces indicate the average firing rates of the single-units,
recorded from layer IV, as a function of time (bin 5 10 seconds). Thin
traces indicate a moving estimation of the mean 1/2 2*std, computed
for every data point from its consequent 30 bins, not including the
stimulation periods. Last 30 bins of the records were duplicated for
this computation. Thick horizontal bars indicate the time of stimulation of the vibrissae. Stimuli were continuous square-wave vibratory
movements at 5 Hz and 160 µm p-p (see Fig. 6a, bottom trace).
Fig. 6. Effect of rhythmic vibratory stimulation (5 Hz) of individual
whiskers from the left side of the face on firing probabilities of
multi-units recorded simultaneously from different barrels of the right
PMBSF of a guinea pig. a: peri-stimulus-time-histograms (PSTHs;
1/2 100 ms, bin 1 ms) were computed from continuous stimulus cycles
for the multi-units recorded from each of the electrodes (el. 1–4) and
for each stimulated vibrissa. A single stimulus cycle is depicted at the
bottom trace. Number of stimulus cycles: g, 288; C1, 289; B1, 272; A2,
289. All PSTHs sharing the same rows have the same scale. b: A
camera lucida drawing of the PMBSF of the same animal is shown.
Electrolytic lesions were produced by delivering a DC current between
the tips of the recording electrodes and the ground electrode. After
subsequent staining for cytochrome oxidase, the PMBSF was reconstructed from 5 consecutive 60-µm-thick sections. Arrowheads indicate the loci of lesions. Labels are located inside the barrels. M, medial;
R, rostral. Abbreviations and conventions as in Figure 4. Scale bar 5
1 mm.
of the nomenclature for vibrissal and barrel arrangement
previously elaborated in mice and rats (Van der Loos and
Woolsey, 1973).
Relation to previous findings
Histochemical delineation of the forth layer in the
somatosensory cortex of the guinea pig was first reported
by Friede (1960). The general organization of the somatic
afferent areas, including the representation of the mystacial pads, in the cortex of the guinea pig was illustrated by
Zeigler (1964), and Zeigler’s data were confirmed by Campos and Welker (1976) and Rapisarda et al. (1990).
Unlike the vigorous whisking of rats and mice, guinea
pigs move their vibrissae in brief irregular bursts (Rice,
1995). This might be the reason why only few studies
examined the guinea pig’s vibrissa/somatosensory system.
Upon discovery of barrels in the somatosensory cortex of
the mouse (Woolsey and Van der Loos, 1970) and their
precise description in the rat (Welker, 1971; Welker and
Woolsey, 1974), the presence of such barrels in other
experimental species of rodents was examined. Hollow
barrels were observed in the guinea pig (Woolsey et al.,
1975; Rice et al., 1985), but the exact map and the
nomenclature of the PMBSF was not obtained. Sikich et al.
(1986) stated that the appropriate electrophysiological and
lesion studies, which were needed to definitively identify
individual architectonic units as the representations of
specific vibrissae, had not been performed in guinea pigs.
These authors concluded, however, that the arrangement
of vibrissae on the face and the patterns and locations of
the barrels in the central nervous system of the guinea pig
differed from those of mice and rats in that (i) there were
six rather than five rows of mystacial vibrissae and of
vibrissal representations and (ii) the rows were more
irregularly aligned.
In the present study, we observed that the guinea pig
has four straddlers and that the most dorsal row of the
previously suggested six rows of mystacial vibrissae differs
from the other five rows by (i) not being straddled (Figs. 1a,
8a), (ii) not being connected via follicular muscles to a
straddler or to other vibrissa in the mystacial pad (Fig. 2a),
(iii) responding differently to the clearing procedure (Fig.
3a), which indicates a different histological structure, (iv)
being located dorsal to the horizontal line crossing the
nostril, and (v) being represented by cortical barrels that
are significantly smaller than those representing the other
five rows, and that are often oriented in a different
direction (Fig. 4a,b). Furthermore, the apparent irregularity in the orientation of both the vibrissae and barrels
VIBRISSAL AND BARREL ARRANGEMENT IN RODENTS
525
Fig. 8. Mapping of the sinus hairs onto the posteromedial barrel
subfield. Photomicrographs of unstained cleared mystacial pads of the
guinea pig (a) and golden hamster (c). The rows of vibrissae are
marked A–E in the dorsal to ventral direction and numbered caudal to
rostral. The straddlers, which are designated a, b, g, and d, straddle
rows A–B, B–C, C–D, and D–E, respectively, of vibrissae. In the
camera lucida drawings of the corresponding barrels in the PMBSF of
the guinea pig (b) and golden hamster (d), barrels are represented as
irregular elliptical figures of appropriate shapes and dimensions. The
barrels that correspond to the straddlers form one arc, which starts at
the caudo-lateral edge and is directed medio-rostrally along the
posterior border of the PMBSF. M, medial; N, nasal vibrissae; NS,
nostril; P, posterior; R, rostral; V, ventral. Scale bars 5 1 mm.
disappears if the presence of four straddlers is considered.
With this consideration, four straddlers and five almost
straight rows of vibrissae are clearly seen in the whiskerpad of the guinea pig. In light of the presence of these four
straddlers, the number of vibrissae cited by Sikich et al.
(1986) for each row should be revised.
Unlike the guinea pig, the golden hamster is a typical
whisking rodent. Nevertheless, its vibrissae/somatosen-
sory system was not studied in detail. By measuring the
distances between adjacent vibrissae of the golden hamster under a dissecting microscope and plotting a grid map
for them on a diagram of the head (Wineski, 1983),
Wineski (1985) reported that in the golden hamster, vibrissae are organized in a gridlike pattern of seven rows and
seven columns along the anteroposterior and dorsoventral
planes of the snout. This appeared to contradict a previous
526
indication that the hamster’s vibrissae are represented in
the somatosensory cortex by only five rows of barrels
(Woolsey et al., 1975). Nevertheless, Wineski (1983, 1985)
developed new classification principles for vibrissae mapping, in which all the hamster vibrissae are arranged in
seven rows (from A to G) in anteroposterior direction of the
mystacial pad and seven columns (1 to 7) from the dorsal to
ventral direction, with no allowances made for straddlers.
In the golden hamster, as in the guinea pig, we also clearly
demonstrated four straddlers and five rows of mystacial
vibrissae. The most dorsal row of facial vibrissae (sometimes considered as two rows, e.g., Wineski, 1985) has a
rostral continuation and is a row of nasal vibrissae. We
demonstrate the same distribution of barrels in the PMBSF.
These results are in agreement with Woolsey et al., (1975)
and with previous CO histochemistry in the hamster
which also demonstrated only five rows of barrels in the
PMBSF (Jacquin et al., 1993).
Why were nasal vibrissae confused with mystacial ones
in previous studies? We believe that the main reason
involves the inconsistent pattern of distribution of nasal
vibrissae among different rodents. There seem to be at
least two different patterns: an elliptic arrangement of
four or more vibrissae dorsal to the nostril that is characteristic for mice and rats, and an almost straight horizontal row of four to eight vibrissae that is characteristic for
guinea pigs and golden hamsters. In these latter rodents,
the nasal vibrissae row starts dorsal to vibrissa A1 (in the
guinea pig) or close to vibrissa A2 (in the golden hamster),
and finishes dorsal to the nostril in both species.
Why were straddlers overlooked previously in guinea
pigs and hamsters? The reasons are probably different for
the two species. In the guinea pig, the straddlers are not
arranged in a straight line, because of their large dimensions. As a result, their straddling position is not evident
from the position on the skin but rather from the arrangement of the intrinsic musculature of the mystacial pad (see
Fig. 2). In the hamster, the spaces between the vibrissal
rows are so small that straddlers can be easily brought in
line with the corresponding rows of vibrissae. The barrels
corresponding to straddlers are very large and barely
delineated, especially in Nissl-stained sections, and can be
easily overlooked.
Comparison of the vibrissae/somatosensory
systems of the guinea pig, golden hamster,
mouse, and rat
The presence of straddlers and the arrangement of
vibrissae in five almost straight rows in the mystacial pads
of guinea pigs, hamsters, mice, and rats support a similar
vibrissal distribution. Mapping of the receptive fields of
the vibrissae-associated barrels in these species also corroborates this similarity. Furthermore, three vibrissal
rows (C, D, and E) in the guinea pig (Fig. 1a), golden
hamster (Fig. 1b), and rat (Welker, 1971) have a direct
rostral continuation that is composed of small nonvibrissal
sinus hairs. The distribution of intrinsic muscles in the
mystacial pad of guinea pigs, golden hamsters, and mice is
also similar.
In guinea pigs, as in mice (Van der Loos and Dörfl, 1978),
an extra vibrissa can occasionally be found in the mystacial pad. In only two of the 41 guinea pigs studied was an
additional vibrissa observed. This small extra vibrissa,
which was rostro-ventrally to row A and on the apparent
border of the mystacial pad (Fig. 1a, not labeled), was
S. HAIDARLIU AND E. AHISSAR
represented in the PMBSF by an additional barrel in row
A. In the golden hamster, a third vibrissa in row A was seen
in over half the preparations.
The vibrissal barrel field of the mouse can be divided
into compartments (Nussbaumer and Van der Loos, 1985):
the PMBSF, which receives only vibrissal input, and an
anterior part, which is populated with hollow barrels, and
receives inputs from vibrissal follicles, intervibrissal hairs,
and skin. In the present study, similar compartments were
observed in the guinea pig and golden hamster.
Are those structural similarities accompanied by functional similarities? At least for the rat and the guinea pig,
it seems to be the case. In both species, many neurons in
the somatosensory cortex exhibit spontaneous oscillations
around 10 Hz, which can be entrained by periodic vibrissal
stimulations between, usually, 3 and 12 Hz (Ahissar et al.,
1996). The functional details characterizing both the spontaneous and the entrained neuronal behaviors appeared to
be similar in rats and guinea pigs. These similarities
suggest that the difference in whisking behavior between
these two species is probably mainly due to motor, rather
than sensory, differences. Furthermore, these structural
and functional similarities should constrain models assigning a critical role for development in the formation of
cortical barrels or other sensory modular organizations.
Difficulties encountered during
determination of the arrangement
of vibrissae and cortical barrels
During determination of the vibrissal arrangement in
the whiskerpad of rodents, difficulties arise if the positions
of the vibrissae change during processing due to stretching, excessive flattening etc. We found that proper flattening and paraffin-embedding of the mystacial pad preserved
the actual positions of the vibrissae for visualization.
During transformation of the convex surface of the mystacial pad into a planar one by our new technique for
flattening and embedding, the artifacts that appeared
were minimal. The dimensions of the sinus hairs and the
direction of the vibrissal axes must be taken into account
when determining the section thickness and cutting plan.
The clarity of vibrissal representation in the PMBSF is
limited by vertical displacements of some barrels (see the
arrowheads in Fig. 4c,d), in addition to the usual limitations of histological procedures. In tangential sections,
these vertically displaced barrels were barely seen, because their hollows did not lie in the plane of the single
section.
The differential appearance of the Nissl-stained barrels
in row E affected the imaging of barrels. In some cases, the
hollows of the barrels E1 and E2 were not evident, whereas
the hollows of E3 and E4 were always clearly evident.
However, staining for CO is providing a better imaging of
the PMBSF including all the barrels of row E. The
variability in the distribution of the small barrels that
represent the nonmystacial row of sinus hairs located
dorsally to the whiskerpad is high, with frequent extra
barrels and variations in the location of the area occupied
by their representation.
CONCLUSIONS
The organizational principles of, and the correspondences between, the vibrissae in the mystacial pad and the
barrels in the PMBSF of the guinea pig and golden
VIBRISSAL AND BARREL ARRANGEMENT IN RODENTS
hamster are similar to those of the mouse and rat. Two
templates of vibrissa arrangement for the nasal vibrissae,
one characteristic for the guinea pig and golden hamster
and one characteristic for mice and rats, may account for
seemingly contradictory classifications previously proposed. The differences in the neuronal mechanisms that
are associated with the different motor patterns of vibrissal touch possessed by guinea pigs compared to hamsters,
mice, and rats are not reflected in the anatomical structures, proposing that the differences in whisking patterns
originate from functional and not structural differences.
ACKNOWLEDGMENTS
We thank Drs. M. Segal, R. Malach, and B. Schick for
helpful comments on this manuscript. We are also grateful
to M. Harel for her skilled CO histochemistry, and to K.O.
Johnson for kindly providing the vibrotactile stimulator.
This work was supported by the Minna-James Heinemann
Foundation, Germany; the Minerva Foundation, Munich,
Germany; and by grant 93-00198 from The US-Israel
Binational Science Foundation, Israel. S.H. was supported
by the Gileadi program, Ministry of Absorption, Israel, and
E.A. was supported by an Alon Fellowship, Israel.
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