Localization of Nitric Oxide Synthase-containing Neurons in the Bat Visual Cortex and Co-localization with Calcium-binding Proteins

Microchiroptera (microbats) is a suborder of bats thought to have degenerated vision. However, many recent studies have shown that they have visual ability. In this study, we labeled neuronal nitric oxide synthase (nNOS)—the synthesizing enzyme of the gaseous non-synaptic neurotransmitter nitric oxide—and co-localized it with calbindin D28K (CB), calretinin (CR), and parvalbumin (PV) in the visual cortex of the greater horseshoe bat (Rhinolophus ferrumequinum, a species of microbats). nNOS-immunoreactive (IR) neurons were found in all layers of the visual cortex. Intensely labeled neurons were most common in layer IV, and weakly labeled neurons were most common in layer VI. Majority of the nNOS-IR neurons were round- or oval-type neurons; no pyramidal-type neurons were found. None of these neurons co-localized with CB, CR, or PV. However, the synthesis of nitric oxide in the bat visual cortex by nNOS does not depend on CB, CR, or PV.     

Colocalization of parvalbumin, calretinin and calbindin D-28k in human cortical and subcortical visual structures

Several studies have demonstrated that three calcium-binding proteins parvalbumin (PV), calbindin D-28k (CB) and calretinin (CR) mark distinct subsets of cortical interneurons. This study demonstrates, in cortical and subcortical visual structures, the coexistence of two calcium-binding proteins in some neuronal subpopulations. The human visual cortex (VC), lateral geniculate nucleus (LGN), lateral inferior pulvinar (LIP) and superior colliculus (SC) were examined by a double-labelling immunocytochemical technique. The VC showed mostly separate populations of PV, CB and CR immunoreactive (-ir) interneurons, but also small populations of double-stained PV+CR and CR+CB neurons, while PV+CB neurons were less frequent. An average of 2.5% of the immunoreactive neurons were double-stained for PV+CR and 7.1% for CR+CB in area 17, while this percentage was slightly higher in association area 18 (3.3 and 7.4%, respectively). In the LGN and LIP, double-stained neurons were scarce, but in the fibre capsule of these nuclei, as well as in the optic radiation (OR) and white matter underlying area 17, both double-stained PV+CR or CR+CB and separate populations of PV-ir, CB-ir and CR-ir neurons and fibres were observed. Unlike the thalamic regions, the SC showed some double-stained PV+CR and CR+CB neurons, scattered both in the superficial and deep layers. These findings are discussed in the light of similar observations recently reported from other regions of the human brain.     

Altered expression of parvalbumin and calbindin in interneurons within the primary visual cortex of neonatal enucleated hamsters

In the present study, we tested the hypothesis that the expression of calcium binding proteins (CaBPs), parvalbumin (PV), calretinin (CR) and calbindin (CB), is dependent upon sensory experience as emphasized in visual deprivation and deafferentation studies. The expression of CaBPs was studied in interneurons within the primary and extrastriate visual cortices (V1, V2M, V2L) and auditory cortex (AC) of adult hamsters enucleated at birth. The effects of enucleation were mainly confined to area V1 where there was a significant volume reduction (26%) and changes in the laminar distribution of PV and CB immunoreactive (IR) cells. The density of PV-IR cell bodies was significantly increased in layer IV and reduced in layer V. Moreover, the density of CB-IR neurons was inferior in layer V of V1 in enucleated hamsters (EH) compared to controls. These results suggest that some features of the laminar distribution of specific CaBPs, in primary sensory cortices, are dependent upon or modulated by sensory input.     

Projections from the visual cortex to the contralateral claustrum of the cat revealed by an anterograde axonal transport method

Contralateral projections from visual areas 17, 18, 19 and the Clare-Bishop area of the cerebral cortex to the claustrum have been investigated in the cat using intracortical injections of [3H]proline. Radioactive material was found in a dorsocaudal region of the contralateral claustrum. This region was homotopic with respect to that found for the ipsilateral projection from visual cortex. The contralateral connection is assumed to be a monosynaptic pathway. The pattern by which the corticofugal fibres terminate in the claustrum is quite similar to the one described for the opposite hemisphere [6].     

Modular organization of the monkey presubiculum

There have been previous reports of somatostatin- and acetylcholinesterase (AChE)-positive patches in the superficial layers of the presubiculum in monkeys. In this study, we show additional instances of patches in the presubiculum, as demonstrated by cytochrome oxidase (CO), by myelin and Nissl stains, and by the calcium-binding proteins calbindin (CB), calretinin (CR), and parvalbumin (PV). Markers are differentially expressed along the lateral and longitudinal axes. Comparisons of adjacent sections reacted for different markers suggest that the CB+ and CR+ patches, and CO+ and AChE+ patches generally correspond, but not the CB+ and CO+ patches. In cross section, patches are about 100–300 μm in width. Sections cut tangentially through the superficial layers indicate that the pattern of CO and AChE labeling is in fact patchlike in layer I, but that the apparent patches in layer II (CB and CR) form a reticular or lattice-like network. As patches are restricted to the presubiculum, these labeling patterns provide a convenient marker for the boundary between the presubiculum and the adjoining posteroventral retrosplenial cortex. More work is necessary to determine how this modularity may relate to the functional organization of the presubiculum. Electronic Publication     

Calcium-binding proteins in the laterodorsal thalamic nucleus during development of the guinea pig

The laterodorsal thalamic nucleus (LD) is often treated as a part of the anterior thalamic nuclei (ATN) because of its location and similar connectivity. Our previous studies have shown that distribution of three calcium-binding proteins, i.e. calbindin D_28k (CB), calretinin (CR) and parvalbumin (PV), changes within the ATN during development of the guinea pig. The aim of this study is to examine the immunoreactivity pattern of these proteins in the LD in the guinea pig ontogeny. Brains from animals ranging from 40th embryonic day to 80th postnatal day were used in the study. Two methods were applied: a single-labelling immunoenzymatic method and double-labelling immunofluorescence. No changes of the distribution pattern of the substances were observed throughout the examined developmental stages. CB and CR were the most abundantly expressed proteins in perikarya of the LD. Numerous CB- and CR-immunoreactive cell bodies were found throughout the whole extent of the nucleus. In most of these cell bodies both proteins colocalized vastly. The highest immunoreactivity of the perikarya containing CB and CR was observed in the mediodorsal part of the LD and in its rostral portion. In regard to PV, single cell bodies were observed mostly in the dorsal part of the nucleus. PV did not colocalize with the other proteins. In summary, all the studied calcium-binding proteins were already present in the LD at prenatal developmental stages and the pattern of distribution remained virtually constant until adulthood. Thus, the LD differs considerably from the ATN in an aspect of neurochemical cell differentiation.     

Parvalbumin expression in the claustrum of the adult dog. An immunohistochemical and topographical study with comparative notes on the structure of the nucleus

Although the detailed structure and function of the claustrum remain enigmatic, its extensive reciprocal connection with the cortex suggests a role in the integration of multisensory information.Claustrum samples, obtained from necropsy of four dogs, were formalin fixed for paraffin embedding. Sections were either stained for morpho-histological analysis or immunostained for parvalbumin (PV). We focused on PV because in cortical and hippocampal areas it is a marker of the fast-spiking interneurons which have an important role in the information transmission and processing. Soma area, perimeter and circularity were considered as morphological parameters to quantitatively group the PV positive somata by k-means clustering.The histological investigation revealed a superior pyramidoid puddle and a posterior puddle characterized by a “cloud” of neurons in its dorso-lateral part. Immunostaining showed positive somata and fibers throughout the rostro-caudal extent of the dog claustrum, localized principally in the dorsal region. k-Means clustering analysis enabled neuron classification according to size, identifying respectively big (radius = 11.42 ± 1.99 μm) and small (radius = 6.33 ± 1.08 μm) cells. No statistical differences in soma shape were observed. The topographical distribution of PV immunoreactivity suggests that the dog dorsal claustrum might be functionally related to the processing of visual inputs.Taken together our findings may help in the understanding the physiology of claustrum when compared with anatomical and functional data obtained in other species.     

Immunohistochemical distribution of the cannabinoid receptor 1 and fatty acid amide hydrolase in the dog claustrum

Cannabinoid receptor 1 (CB1R) and fatty acid amide hydrolase (FAAH) are part of the endocannabinoid system (ECB) which exerts a neuromodulatory activity on different brain functions and plays a key role in neurogenesis. Although many studies have reported FAAH and CB1R expression in the brain of different animal species, to the best of our knowledge they have never been described in the canine claustrum. Claustrum samples, obtained from necropsy of four neurologically normal dogs, were formalin fixed for paraffin embedding. Sections were either stained for morpho-histological analysis or immunostained for CB1R and FAAH. Analysis of adjacent sections incubated with the two antisera showed a complementary labeling pattern in the claustrum, with CB1R antibody staining fibers while anti-FAAH antibody stained cell bodies and the proximal portion of dendrites; this particular anatomical relationship suggests a retrograde endocannabinoid action via CB1R. CB1R and FAAH complementary immunostaining and their cellular localization reported here provide the first anatomical evidence for existence of the ECB in the dog claustrum.     

Visually-induced NGFI-A protein expression in the calbindin-, parvalbumin- and nitric oxide synthase-neuronal populations of the rat superior colliculus

The expression of the immediate early gene NGFI-A in the nervous system is induced by sensory stimulation and seems to be related to long-term synaptic plasticity. We have used double-labeling immunohistochemistry to identify calbindin (CB)(+), parvalbumin (PV)(+) and neuronal nitric oxide synthase (nNOS)(+) neurons that also expressed the protein encoded by this immediate early gene after light-exposure on in the superficial layers of the rat superior colliculus (sSC). The majority of the NGFI-A(+) cells were not double-labeled for the tested markers. In the stratum zonale+stratum griseum superficiale (SZ/SGS), only 17.8%, 8.0% and 12.1% of NGFI-A(+) cells were also labeled for CB, PV or nNOS, respectively. In the stratum opticum (SO), only 10.5% of the NGFI-A(+) cells were also CB(+). Furthermore, only a small subset of each population expressed the NGFI-A protein after light-exposure. In the SZ/SGS, 35.7% of the CB(+), 32.1% of the PV(+) and 26.6% of the nNOS(+) neurons also expressed the NGFI-A. In the SO, 31.7% of the CB(+) neurons also expressed the NGFI-A. The proportional distribution of the nNOS(+)/NGFI-A(+) neurons throughout the SZ/SGS layers showed a slight but significant rostro-caudal gradient. No significant difference was observed for the other markers, indicating homogeneous activation of these populations throughout the retinotopic map. Our results suggest that the visually-driven NGFI-A expression is not restricted to a specific population of the sSC and that visual processing in this structure, as assessed by the expression of this candidate-plasticity protein, involves the activation of subsets of ascending and non-ascending projection neurons.     

Neocortical connections in fetal cats

The major extrinsic projections to and from visual and auditory areas of cerebral cortex were examined in fetal cats between 46 and 60 days of gestation (E46-E60) using axonal transport of horseradish peroxidase either alone or in combination with tritiated proline. Projections to visual cortex from the dorsal lateral geniculate nucleus and lateral-posterior/pulvinar complex exist by E46, and those from the contralateral hemisphere, claustrum, putamen, and central lateral nucleus of the thalamus are present by E54-E56. In addition, cells in the medial geniculate nucleus project to auditory cortex by E55. At E54-E56 efferent cortical projections reach the contralateral hemisphere, claustrum, putamen, lateral-posterior/pulvinar complex and reticular nucleus of the thalamus. Cells in visual cortex also project to the dorsal and ventral lateral geniculate nuclei, pretectum, superior colliculus and pontine nuclei, and cells in auditory cortex project to the medial geniculate nucleus. Except for interhemispheric projections, all pathways demonstrated are ipsilateral, and projections linking cerebral cortex with claustrum, dorsal lateral geniculate nucleus and lateral-posterior/pulvinar complex are reciprocal. The reciprocal projections formed with the dorsal lateral geniculate nucleus, lateral-posterior/pulvinar complex and the claustrum show a greater degree of topological organization compared to the projections formed with the contralateral hemisphere and superior colliculus, which show little or no topological order. Therefore, the results of the present study show that the major extrinsic projections of the cat's visual and auditory cortical areas with subcortical structures are present by the eighth week of gestation, and that the origins and terminations of many of these projections are arranged topologically.     

Immunohistochemical localization of calbindin D28-k,parvalbumin, and calretinin in the cerebellar cortex of the circling mouse

The spontaneous mutant circling mouse has an autosomal recessive pattern of inheritance and is an animal model for deafness, which is characterized by circling, head tossing, and hyperactivity. Since the main pathology in circling mice lies in the organ of Corti, most studies on deaf mice have focused on auditory brain stem nuclei. No studies regarding behavior-related CNS changes in circling mice have been reported. The major center of sensory input for modulation of motor activity is best-studied in the cerebellum. Considering the importance of calcium homeostasis in numerous processes, calcium-binding proteins (CaBPs), such as calbindin D-28k (CB), parvalbumin (PV), and calretinin (CR), may play crucial roles in preserving cerebellar coordinated motor function. Thus, the distribution of CB, PV, and CR was determined in the cerebellum using immunohistochemical methods to compare immunoreactivity (IR) of CaBPs between wild-type (+/+), heterozygous (+/cir), and homozygous (cir/cir) mice. The IR of CB and PV was predominantly observed in the Purkinje cell layer of all three genotypes. Compared with the +/+ genotype, the relative mean density of CB and PV IR in the Purkinje cell layer and CR IR in the granular layer was significantly decreased in the cir/cir genotype. Changes in calcium homeostasis in parallel fiber/Purkinje cell synapses could diminish cerebellar control of motor coordination. A number of deficiencies among the CaBPs lead to distinct alterations in brain physiology, which may affect normal behavior.     

Parvalbumin and calbindin in the rat claustrum: An immunocytochemical study combined with retrograde tracing from frontoparietal cortex

The distribution of the calcium binding proteins parvalbumin and calbindin D-28k was examined in the claustrum of the rat by means of immunohistochemistry. The two proteins displayed a different and largely complementary pattern of distribution. Parvalbumin-immunostaining was intense in the neuropil of the dorsal claustrum and virtually absent in the neuropil of the ventral claustrum; parvalbumin-immunoreactive neuronal cell bodies were relatively numerous in the dorsal claustrum and were detected only occasionally in the ventral region. On the other hand, calbindin-immunostaining was prevalent in the ventral claustrum; very few calbindin-positive neurons were seen in the dorsal sector of the nucleus, whereas they were relatively more numerous in the ventral claustrum. The cell bodies of the majority of the claustral parvalbumin- or calbindin-immunoreactive neurons were oval or round, but immunostained polymorphous neurons were also observed. The surface of the immunopositive dendritic branches was smooth, with no evidence of spines.

Fluorescent retrograde tracing was combined with immunohistofluorescence to determine whether the parvalbumin-containing claustral cells project to the frontoparietal cortex. Neurons labelled after large fluorogold injections in frontoparietal cortical fields were highly intermingled in the dorsal claustrum with parvalbumin-immunoreactive cells but the two neuronal populations were separate. These data show that parvalbumin-immunoreactive claustral neurons do not project to the frontoparietal cortex. In addition, although these cells may project to other cortical or subcortical targets, the present findings suggest that they may represent, at least in part, local circuit claustral neurons, corresponding to the aspiny intrinsic neurons described in the rat claustrum in studies based on Golgi impregnation.     

Differential regulation of parvalbumin and calretinin interneurons in the prefrontal cortex during adolescence

Determining the normal developmental trajectory of individual GABAergic components in the prefrontal cortex (PFC) during the adolescent transition period is critical because local GABAergic interneurons are thought to play an important role in the functional maturation of cognitive control that occurs in this developmental window. Based on the expression of calcium-binding proteins, three distinctive subtypes of interneurons have been identified in the PFC: parvalbumin (PV)-, calretinin (CR)-, and calbindin (CB)-positive cells. Using biochemical and histochemical measures, we found that the protein level of PV is lowest in juveniles [postnatal days (PD) 25–35] and increases during adolescence (PD 45–55) to levels similar to those observed in adulthood (PD 65–75). In contrast, the protein expression of CR is reduced in adults compared to juvenile and adolescent animals, whereas CB levels remain mostly unchanged across the developmental window studied here. Semi-quantitative immunostaining analyses revealed that the periadolescent upregulation of PV and the loss of the CR signal appear to be attributable to changes in PV- and CR-positive innervation, which are dissociable from the trajectory of PV- and CR-positive cell number. At the synaptic level, our electrophysiological data revealed that a developmental facilitation of spontaneous glutamatergic synaptic inputs onto PV-positive/fast-spiking interneurons parallels the increase in prefrontal PV signal during the periadolescent transition. In contrast, no age-dependent changes in glutamatergic transmission were observed in PV-negative/non fast-spiking interneurons. Together, these findings emphasize that GABAergic inhibitory interneurons in the PFC undergo a dynamic, cell type-specific remodeling during adolescence and provide a developmental framework for understanding alterations in GABAergic circuits that occur in psychiatric disorders.     

The Claustrum of the Pig: An Immunohistochemical and a Quantitative Golgi Study

Despite increasing interest in the claustrum (Cl) over the last decades, its function is still a puzzling problem. Among the experimental species of potential use in Cl research, the pig is considered an interesting model, because of the similarities of its brain with the corresponding cortical and subcortical human structures. The swine Cl presents a peculiar morphology, characterized by a wide posterior enlargement, ideal for physiological investigations. There is a wealth of data on general anatomy, cytoarchitecture, and chemo architecture of the Cl, but much less is known about the dendritic morphometry of its neurons. Dendritic length and branching pattern are key features to understand the organization of the microcircuitry, and thus the delineation of the structure–function relationships of the Cl. To this effect, we undertook (a) a quantitative study of the dendrites of the spiny neurons of the swine Cl, employing the Golgi staining; and (b) an immunohistochemical analysis to describe the distribution of the parvalbumin (PV)-immunoreactive interneurons throughout the same nucleus. Taken together, the results that we report here show that the dendritic architecture and the distribution of the PV expressing interneurons change when the Cl of this species changes its shape along the rostro-caudal axis, thus suggesting a potentially specific function for the large posterior puddle. Anat Rec, 302:1638–1646, 2019. © 2019 American Association for Anatomy     

含Parvalbum in和Calbind in D-28k样免疫阳性神经元在大鼠三叉神经本体觉中枢通路的分布

本研究应用免疫组织化学技术对Ｐａｒｖａｌｂｕｍ ｉｎ（ＰＶ）和Ｃａｌｂｉｎｄｉｎ Ｄ ２８ｋ（ＣＢ）样免疫阳性神经元在大鼠三叉神经本体觉中枢通路上的分布状况作了普查。结果表明： 这两种钙结合蛋白在该通路上具有下列的分布特点：（１）三叉神经中脑核神经元几乎全部为ＰＶ 样免疫阳性,胞体和突起染色强或中等强度,胞体大（１８～３０ μｍ ）,多为假单级神经元,偶见中等大小的多极神经元。（２）在三叉神经脊束核吻侧亚核尾侧段水平的三叉神经脊束核吻侧亚核背内侧部及邻接的网状结构中可看到由ＰＶ 样免疫阳性神经元构成的两团深染区,外侧者大,内侧者小,胞体呈圆形、三角形或不规则形（９～１６ μｍ ）;在此处ＣＢ样免疫阳性细胞亦呈类似分布。（３）“带状区”内ＰＶ 样免疫阳性细胞多为具有短突的多极神经元;在三叉上核尾外侧部,ＰＶ 样神经元分布稀疏,但在感觉主核背内侧部则呈高密度分布;ＡＶＭ 、ＡＤＯ 等两个位于“带状区”腹侧部的新发现的小核团亦可观察到阳性细胞和突起。此“带状区”内ＣＢ样免疫阳性细胞数目较少,分布也稀疏,但胞体较大（１１～１８ μｍ ）,呈三角形、圆形或不规则形,轴突上可见少量的串珠状膨体。在ＡＤＯ 中ＣＢ样免疫阳性细胞的分布较Ｐ?     

The combined retrograde transport and unbiased stereological study of the claustrocortical connections in the rabbit

The quantitative analysis of the claustrocortical connections in the rabbit, labeled with the fluorescent retrograde tracer Fluoro-Gold (FG), was conducted by means of unbiased stereology. The FG was injected into selected regions of the motor, somatosensory, auditory and visual cortices and then a comparison of the various claustrocortical projections was carried out. This was achieved by comparing (1) the numerical densities of projecting neurones for each claustral projection zone and (2) the distribution of the labeled neurones throughout the rostro-caudal extent of the claustrum. No significant differences between the numerical densities of labeled neurones in the various projection zones are reported. The motor and primary somatosensory projections dominated in the anterior and central parts of the claustrum, whereas the secondary somatosensory, auditory and visual projections--in the posterior part. The difference in the distributions was significant (p < 0.001). Summarizing, the cortical projections in the claustrum, although varying topographically, do not reveal a quantitative differentiation. This may speak in favour of the integrative and modulating function of this structure in relationship to the neocortex.     

Strain differences in the effect of rTMS on cortical expression of calcium-binding proteins in rats

Using a rat model to study the cellular effects of repetitive transcranial magnetic stimulation (rTMS) with regard to changes in cortical excitability, we previously described opposite effects of continuous and intermittent theta-burst stimulation (cTBS, iTBS) on the expression of the calcium-binding proteins (CaBP) parvalbumin (PV), calbindin (CB) and calretinin (CR) in Dark Agouti rats (DA). While iTBS significantly reduced the number of cortical PV+ cells but did not affect the CB+ cells, cTBS resulted in a decrease in CB+ cells with no effects on PV+ cells. We concluded that activity of these classes of cortical interneurons is differently modulated by iTBS and cTBS. When testing two further rat strains, Sprague–Dawley (SD) and Long Evans (LE), we obtained deviating results. In SD, iTBS reduced PV and CB expression, while cTBS only reduced PV expression. In contrast, reanalysed DA showed reduced CB expression after cTBS and reduced PV expression after iTBS, while LE shows an intermediate reaction. CR expression was unaffected in any case. Interestingly, we found significantly different basal expression patterns of the CaBPs for the strains, with DA and LE showing much higher numbers of PV+, CB+ and CR+ cells than SD, and with particularly higher number of CB+ and CR+ cells in DA compared to the other two strains. These findings demonstrate that inhibitory systems may be either differently developed in rats belonging to diverse strains or show different basal levels of activity and CaBP expression and may therefore be differently sensitive to the rTMS protocols.     

Callosal projections between areas 17 in the adult tree shrew (Tupaia belangeri)

Summary In the primary visual cortex (area 17) of the tree shrew (Tupaia belangeri) neurons projecting to the contralateral area 17 via the corpus callosum were identified by horseradish peroxidase histochemistry (HRP, WGA-HRP). The distribution of homotopic and heterotopic connections was studied. We found that a narrow stripe of area 17 close to the dorsal area 17/18 border — which corresponds to the visual field along the vertical meridian — is connected via homotopic callosal projections. The adjacent dorsal part of area 17, which largely corresponds to the binocular visual field, is connected via homotopic as well as heterotopic projections. Heterotopic projections originate in the cortical stripe along the area 17/18 border and their contralateral targets are displaced medially. Callosal neurons are located mostly in supragranular but also occur in infragranular layers. The supragranular neurons in general are pyramidal cells. In addition to these findings, we confirmed earlier reports on ipsilateral projections of the primary visual area to the dLGN, the claustrum, area 18 and other visual areas.The authors wish to dedicate this paper to Prof. W. Lierse in honour of his 60th birthday     

Major defects in neocortical GABAergic inhibitory circuits in mice lacking the fragile X mental retardation protein

This study focused on the cytoarchitectonic and morphological differences in GABA-releasing interneurons between adult Fmr1 knock-out (FMR1KO) and wild-type (WT) mice in the somatosensory cortex. Our results showed a robust reorganization of neocortical, but not hippocampal inhibitory circuits in the FMR1KO mouse. The reorganization is characterized by a significant reduction (20%, p<0.001) in the densities of parvalbumin (PV)-positive, but not calbindin (CB) and calretinin (CR)-positive interneurons. A significant enlargement of soma size and an altered lamina distribution of PV but not CR and CB cells was also observed. Additionally, there was a modest but significant increase in TrkB-immunoreactivity in PV-positive cells in the FMR1KO mouse. These results provide the first report showing significant alterations of GABA-releasing interneurons in the mouse model of fragile X syndrome. Uncovering the changes in specific GABAergic inhibitory circuits could help understand mechanisms underlying the behavior deficits of fragile X syndrome and autism.     

Immunohistochemical characterization of parvalbumin-containing interneurons in the monkey basolateral amygdala

Interneurons expressing the calcium-binding protein parvalbumin (PV) are a critical component of the inhibitory circuitry of the basolateral nuclear complex (BLC) of the mammalian amygdala. These neurons form interneuronal networks interconnected by chemical and electrical synapses, and provide a strong perisomatic inhibition of local pyramidal projection neurons. Immunohistochemical studies in rodents have shown that most parvalbumin-positive (PV+) cells are GABAergic interneurons that co-express the calcium-binding protein calbindin (CB), but exhibit no overlap with interneuronal subpopulations containing the calcium-binding protein calretinin (CR) or neuropeptides. Despite the importance of identifying interneuronal subpopulations for clarifying the major players in the inhibitory circuitry of the BLC, very little is known about these subpopulations in primates. Therefore, in the present investigation dual-labeling immunofluorescence histochemical techniques were used to characterize PV+ interneurons in the basal and lateral nuclei of the monkey amygdala. These studies revealed that 90–94% of PV+ neurons were GABA+, depending on the nucleus, and that these neurons constituted 29–38% of the total GABAergic population. CB+ and CR+ interneurons constituted 31–46% and 23–27%, respectively, of GABAergic neurons. Approximately one quarter of PV+ neurons contained CB, and these cells constituted one third of the CB+ interneuronal population. There was no colocalization of PV with the neuropeptides somatostatin or cholecystokinin, and virtually no colocalization with CR. These data indicate that the neurochemical characteristics of the PV+ interneuronal subpopulation in the monkey BLC are fairly similar to those seen in the rat, but there is far less colocalization of PV and CB in the monkey. These findings suggest that PV+ neurons are a discrete interneuronal subpopulation in the monkey BLC and undoubtedly play a unique functional role in the inhibitory circuitry of this brain region.