Distribution of glutamate receptor subunit proteins GluR2(4), GluR5/6/7, and NMDAR1 in the canine and primate cerebral cortex: a comparative immunohistochemical analysis

Delving into the significance of the His289Tyr single-nucleotide polymorphism in the glutamate ionotropic receptor kainate-1 (Grik1) gene of a genetically audiogenic seizure model

Genetic abnormalities affecting glutamate receptors are central to excitatory overload-driven neuronal mechanisms that culminate in seizures, making them pivotal targets in epilepsy research. Increasingly used to advance this field, the genetically audiogenic seizure hamster from Salamanca (GASH/Sal) exhibits generalized seizures triggered by high-intensity acoustic stimulation and harbors significant genetic variants recently identified through whole-exome sequencing. Here, we addressed the influence of the missense single-nucleotide polymorphism (C9586732T, p.His289Tyr) in the glutamate receptor ionotropic kainate-1 (Grik1) gene and its implications for the GASH/Sal seizure susceptibility. Using a protein 3D structure prediction, we showed a potential effect of this sequence variation, located in the amino-terminal domain, on the stability and/or conformation of the kainate receptor subunit-1 protein (GluK1). We further employed a multi-technique approach, encompassing gene expression analysis (RT-qPCR), Western blotting, and immunohistochemistry in bright-field and confocal fluorescence microscopy, to investigate critical seizure-associated brain regions in GASH/Sal animals under seizure-free conditions compared to matched wild-type controls. We detected disruptions in the transcriptional profile of the Grik1 gene within the audiogenic seizure-associated neuronal network. Alterations in GluK1 protein levels were also observed in various brain structures, accompanied by an unexpected lower molecular weight band in the inferior and superior colliculi. This correlated with substantial disparities in GluK1-immunolabeling distribution across multiple brain regions, including the cerebellum, hippocampus, subdivisions of the inferior and superior colliculi, and the prefrontal cortex. Notably, the diffuse immunolabeling accumulated within perikarya, axonal fibers and terminals, exhibiting a prominent concentration in proximity to the cell nucleus. This suggests potential disturbances in the GluK1-trafficking mechanism, which could subsequently affect glutamate synaptic transmission. Overall, our study sheds light on the genetic underpinnings of seizures and underscores the importance of investigating the molecular mechanisms behind synaptic dysfunction in epileptic neural networks, laying a crucial foundation for future research and therapeutic strategies targeting GluK1-containing kainate receptors.

Ionotropic glutamate receptors: Which ones, when, and where in the mammalian neocortex

A multitude of 18 iGluR receptor subunits, many of which are diversified by splicing and RNA editing, localize to >20 excitatory and inhibitory neocortical neuron types defined by physiology, morphology, and transcriptome in addition to various types of glial, endothelial, and blood cells. Here we have compiled the published expression of iGluR subunits in the areas and cell types of developing and adult cortex of rat, mouse, carnivore, bovine, monkey, and human as determined with antibody- and mRNA-based techniques. iGluRs are differentially expressed in the cortical areas and in the species, and all have a unique developmental pattern. Differences are quantitative rather than a mere absence/presence of expression. iGluR are too ubiquitously expressed and of limited use as markers for areas or layers. A focus has been the iGluR profile of cortical interneuron types. For instance, GluK1 and GluN3A are enriched in, but not specific for, interneurons; moreover, the interneurons expressing these subunits belong to different types. Adressing the types is still a major hurdle because type-specific markers are lacking, and the frequently used neuropeptide/CaBP signatures are subject to regulation by age and activity and vary as well between species and areas. RNA-seq reveals almost all subunits in the two morphofunctionally characterized interneuron types of adult cortical layer I, suggesting a fairly broad expression at the RNA level. It remains to be determined whether all proteins are synthesized, to which pre- or postsynaptic subdomains in a given neuron type they localize, and whether all are involved in synaptic transmission. J. Comp. Neurol. 525:976-1033, 2017. © 2016 Wiley Periodicals, Inc.

Modulation of proopiomelanocortin gene expression by ethanol in mouse anterior pituitary corticotrope tumor cell AtT20

In humans, alcoholism is associated with a decrease in basal ACTH and cortisol levels, and blunted pituitary ACTH responses to administered corticotropin-releasing hormone (CRH) during active drinking and after long-term abstinence. Preclinical studies indicate that a persistent decrease in pituitary activation after chronic exposure to ethanol is due to a direct effect of ethanol on the corticotrope of the anterior pituitary. The present studies were undertaken to determine if ethanol has effects on proopiomelanocortin (POMC) gene transcription activity in mouse anterior pituitary corticotrope tumor cell AtT20. We measured the levels of the POMC primary nuclear RNA transcript (PT), processing intermediate, and mature mRNA in the nucleus and the levels of the POMC mRNA in the cytoplasm after treatment of AtT20 cells with 5-15 mM concentrations of ethanol. After 15 mM ethanol for 60 to 120 min, the POMC PT levels were significantly decreased. This decreased POMC gene transcription activity was coupled with a significant reduction of the POMC cytoplasmic mRNA levels. After ethanol for 4h, however, both the decreases were no longer observed. After 8h, a decrease in the ACTH secretion in the medium was found. We further investigated if CRH or glutamate modulates the effects of ethanol on the POMC gene transcription activity. CRH at 10nM after 60 min increased the POMC PT levels, and 15mM ethanol attenuated the effect of CRH on the nuclear transcription activity. Glutamate receptor proteins, including NMDA receptor subtype NR1 (but not NR2A or NR2B) and GluR2, were identified by Western immunoblot analysis in AtT20 cells, with similar sizes to those in mouse hypothalamus. The inhibitory effect of 60 min ethanol at 5 to 15 mM on the POMC PT levels was attenuated by 50 μM L-glutamate. Together, our data showed that: (1) ethanol treatment in intoxicate doses significantly inhibited POMC gene transcription activity in a dose- and time-dependent manner in AtT20 cells, and (2) the POMC gene transcription activity in response to CRH or glutamate was altered by ethanol. Our results suggest that ethanol has an inhibitory effect on the POMC gene transcription activity in the anterior pituitary corticotrope, which may contribute to the persistent decrease in pituitary activation after chronic ethanol exposure.

Regulation of dendrite arborization by substrate stiffness is mediated by glutamate receptors

Brain injury or disease can initiate changes in local or global stiffness of brain tissue. While stiffness of the extracellular environment is known to affect the morphology and function of many cell types, little is known about how the dendrites of neurons respond to changes in brain stiffness. To assess how extracellular stiffness affects dendrite morphology, we took biomaterial and biomedical engineering approaches. We cultured mixed and pure hippocampal neurons on hydrogels composed of polyacrylamide (PA) of varying stiffnesses to mimic the effects of extracellular matrix stiffness on dendrite morphology. The majority of investigations of cortical and spinal cord neurons on soft hydrogels examined branching at early time points (days in vitro (DIV) 2-7), an important distinction from our study, where we include later time points that encompass the peak of branching (DIV 10-12). At DIV 12, dendrite branching was altered by stiffness for both pure and mixed neuronal cultures. Furthermore, we treated hippocampal cultures with glutamate receptor antagonists and with astrocyte-conditioned media. Blocking AMPA and NMDA receptors affected the changes in dendrite branching seen at varying rigidities. Moreover, extracellular factors secreted by astrocytes also change dendrite branching seen at varying rigidities. Thus, astrocytes and ionotropic glutamate receptors contribute to mechanosensing.

Attenuation of cocaine self-administration in squirrel monkeys following repeated administration of the mGluR5 antagonist MPEP: comparison with dizocilpine

RATIONALE

The mGluR5 antagonist MPEP has effects that suggest potential as a pharmacotherapy for cocaine addiction. MPEP can attenuate self-administration of cocaine in animals; however, studies usually involved only acute treatment with MPEP and a single dose of self-administered cocaine. Cocaine addicts use varied amounts of cocaine over long periods of time, and an effective pharmacotherapy would almost certainly require more chronic treatment.

OBJECTIVES

The present study (1) compared the effects of repeated treatment with MPEP or the NMDA receptor antagonist dizocilpine on the reinforcing effects of a range of doses of cocaine and (2) determined the pharmacological specificity of the effects of the drugs in attenuating cocaine self-administration compared to food-reinforced behavior. An effective pharmacotherapy should selectively reduce cocaine self-administration.

MATERIALS AND METHODS

Groups of monkeys responded under a fixed-ratio schedule of i.v. cocaine self-administration or food-pellet delivery. The effects of daily treatment with MPEP and dizocilpine were determined under both the schedule of i.v. cocaine injection and food delivery.

RESULTS

Treatment with MPEP and dizocilpine significantly reduced cocaine self-administration, producing rightward and downward shifts in the ascending limb of the cocaine dose-response function. MPEP and dizocilpine selectively and significantly attenuated self-administration of a low reinforcing dose of cocaine compared to food without evidence of tolerance.

CONCLUSIONS

Both MPEP and dizocilpine functioned as partially surmountable antagonists of the reinforcing effects of cocaine. The similar effects of the two drugs raises the possibility that MPEP attenuated the reinforcing effects of cocaine, at least in part, via mGluR5-mediated inhibition of NMDA receptor activity.

Glutamate receptor subunit 3 (GluR3) immunoreactivity delineates a subpopulation of parvalbumin-containing interneurons in the rat hippocampus

Rasmussen's encephalitis is a childhood disease resulting in intractable seizures associated with hippocampal and neocortical inflammation. An autoantibody against the GluR3 subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors is implicated in the pathophysiology of Rasmussen's encephalitis. AMPA receptors mediate excitatory neurotransmission in the brain and contain combinations of four subunits (GluR1-4). Although the distributions of GluR1, GluR2, and GluR4 are known in some detail, the cellular distribution of GluR3 in the mammalian brain remains to be described. We developed and characterized a GluR3-specific monoclonal antibody and quantified the cellular distribution of GluR3 in CA1 of the rat hippocampus. GluR3 immunoreactivity was detected in all pyramidal neurons and astrocytes and in most interneurons. We quantified the intensity of GluR3 immunoreactivity in interneuron subtypes defined by their calcium-binding protein content. GluR3 immunofluorescence, but not GluR1 or GluR2 immunofluorescence, was significantly elevated in somata of parvalbumin-containing interneurons compared to pyramidal somata. Strikingly, increased GluR3 immunofluorescence was not observed in calbindin- and calretinin-containing interneurons. Furthermore, 24% of parvalbumin-containing interneurons could be distinguished from surrounding neurons based on their intense GluR3 immunoreactivity. This subpopulation had significantly elevated GluR3 immunoreactivity compared to the rest of parvalbumin-containing interneurons. Electron microscopy revealed enriched GluR3 immunoreactivity in parvalbumin-containing perikarya at cytoplasmic and postsynaptic sites. Parvalbumin-containing interneurons, potent inhibitors of cortical pyramidal neurons, are vulnerable in the brains of epileptic patients. Our findings suggest that the somata of these interneurons are enriched in GluR3, which may render them vulnerable to pathological states such as epilepsy and Rasmussen's encephalitis.

Temporal and regional expression of NMDA receptor subunit NR3A in the mammalian brain

NR3A is a developmentally regulated N-methyl-D-aspartate receptor (NMDAR) subunit that was previously known as NMDAR-L or chi-1. Unlike other NMDAR subunits, NR3A inhibits the NMDAR-associated ion channel in a novel manner, and a role in synaptogenesis has been suggested for this subunit. Here, we report a comprehensive study to delineate the temporal and anatomic expression of NR3A protein in the mammalian brain by using a monoclonal anti-NR3A antibody. NR3A protein was found to peak at postnatal day (P) 8, and to decrease gradually from P12 to adulthood in the rat central nervous system. Moreover, NR3A protein was heavily expressed in all areas of the isocortex, portions of the amygdaloid nuclei, and selective cell layers and nuclei of the hippocampus, thalamus, hypothalamus, brainstem, and spinal cord. NR3A protein was also expressed in the cerebellar cortex, whereas only weak signal was detected in the previous in situ studies by using riboprobes. At an ultrastructural level, NR3A was associated specifically with asymmetrical synapses and localized to postsynaptic membranes. This information will facilitate future research on NMDARs by providing clues to possible inclusion of the NR3A subunit in NMDARs in many brain regions.

Zinc-rich synaptic boutons in human temporal cortex biopsies

The distribution of zinc-rich synaptic boutons in biopsies of the temporal cortex from epileptic patients who had undergone surgery is described. Unfixed cryostat sections were exposed to H(2)S vapour to precipitate endogenous zinc, which was subsequently shown by silver enhancement. In the temporal cortex, the stain for zinc was arranged in bands: stain was heavy in layers II and VI, moderate-to-heavy in layers I, III and V, and low in layer IV. The white matter was virtually devoid of staining. At the electron microscope level, labelling was found in synaptic boutons that made asymmetric synaptic contacts. Immunohistochemical staining for glutamate receptor subunits GluR2/3 was observed in cell bodies in layers II, III, V and VI, coincident with the layers that showed heavy staining for zinc. Immunostaining for glutamate receptor subunit GluR1 was prominent in non-pyramidal neurons in deep cortical layers. These results support findings in other mammals and indicate that the human neocortex may contain an extensive system of zinc-rich cortico-cortical connections. This system may be altered in pathological conditions.

Neuronal subclass-selective loss of pyruvate dehydrogenase immunoreactivity following canine cardiac arrest and resuscitation

Chronic impairment of aerobic energy metabolism accompanies global cerebral ischemia and reperfusion and likely contributes to delayed neuronal cell death. Reperfusion-dependent inhibition of pyruvate dehydrogenase complex (PDHC) enzyme activity has been described and proposed to be at least partially responsible for this metabolic abnormality. This study tested the hypothesis that global cerebral ischemia and reperfusion results in the loss of pyruvate dehydrogenase immunoreactivity and that such loss is associated with selective neuronal vulnerability to transient ischemia. Following 10 min canine cardiac arrest, resuscitation, and 2 or 24 h of restoration of spontaneous circulation, brains were either perfusion fixed for immunohistochemical analyses or biopsy samples were removed for Western immunoblot analyses of PDHC immunoreactivity. A significant decrease in immunoreactivity was observed in frontal cortex homogenates from both 2 and 24 h reperfused animals compared to samples from nonischemic control animals. These results were supported by confocal microscopic immunohistochemical determinations of pyruvate dehydrogenase immunoreactivity in the neuronal cell bodies located within different layers of the frontal cortex. Loss of immunoreactivity was greatest for pyramidal neurons located in layer V compared to neurons in layers IIIc/IV, which correlates with a greater vulnerability of layer V neurons to delayed death caused by transient global cerebral ischemia.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Distribution of glutamate receptor subunit NMDAR1 in the hippocampus of normal elderly and patients with Alzheimer's disease

Immunocytochemical techniques were employed to study the distribution and cytological features of NMDAR1-immunoreactive elements in the human hippocampal formation. Subjects with Alzheimer's disease (AD), presenting with a wide range of neuropathology and classified into six Braak stage (I-VI), and nondemented age-matched controls were examined. In control cases, the most intense NMDAR1 immunoreactivity was observed within the soma and dendrites of granule cells in the dentate gyrus and pyramidal neurons in Ammon's horn. Whereas small variations in the pattern of immunoreactivity were noted in control cases, AD subjects were characterized with intersubject variability which in most instances correlated with neuropathologic severity. For example, AD cases, particularly those with mild/modest pathology (Braak I-III), were indistinguishable from controls in the overall pattern of immunolabeling. In contrast, in those more severe AD cases (Braak IV-VI) the intensity of immunolabeling within the CA fields was greater than observed in controls and those with mild AD pathology. In addition, in pathologically severe cases numerous NMDAR1-positive pyramidal neurons were characterized by unique morphologic features including long and often tortuous apical dendrites. These latter findings were most prevalent in the CA1 region and subiculum. In contrast to the marked increase in immunolabeling in the CA fields, in the dentate gyrus we observed a reduction in NMDAR1 labeling particularly within the outer molecular layer (i.e., termination zone of the perforant pathway). This latter region was also the site of a number of NMDAR1-labeled plaques. Notably, the overall pattern of NMDAR1 immunoreactivity is distinct from that observed with antibodies against AMPA receptor subunits and suggests a differential role of various inotropic glutamate receptors in hippocampal plasticity in AD.

Selective coexpression of NMDAR2A/B and NMDAR1 subunit proteins in dysplastic neurons of human epileptic cortex

NR1 and NR2 are the two gene families for the NMDA receptor. In vitro studies show that while NR2 alone is nonfunctional, NR1 alone produces weak currents to glutamate or NMDA. We previously showed by immunocytochemistry (ICC) that in normal appearing, nonepileptic human cortical neurons, only NR1 and not NR2 proteins were expressed, in contrast to the presence of both NR1 and NR2 in normal rat cortical neurons. We also showed, in dysplastic epileptic cortex, that both NR1 and NR2 were highly expressed using ICC on adjacent 30-microm sections. However, the relative coexpressions of NR1 and NR2 proteins in single neurons in single sections of human epileptic cortex were unknown. In this study, we used double-labeled immunofluorescence and confocal microscopy to examine the distribution and coexpression of subunit proteins for NR1 and NR2A/B in both nondysplastic (control comparison) and dysplastic regions of human brain resected for the treatment of intractable epilepsy (11 patients). In nondysplastic regions, cortical neurons did not have immunoreactivity (ir) for NR2A/B, whereas NR1-ir was abundant. By contrast, dysplastic neurons in the regions with epileptic cortical dysplasia showed intense NR2A/B-ir in the somata and their dendritic processes. These same NR2A/B-ir dysplastic neurons were colabeled by NR1. These results demonstrate directly that dysplastic neurons express both NR2A/B and NR1 proteins, whereas nondysplastic cortical neurons express only NR1 proteins. Selective coexpression of NR2A/B and NR1 in dysplastic neurons suggests that NR2A/B may form heteromeric NR1-NR2 coassemblies and hyperexcitability in dysplastic neurons that could contribute to focal seizure onset.

Altered hippocampal kainate-receptor mRNA levels in temporal lobe epilepsy patients

This study determined whether hippocampal kainate (KA) receptor mRNA levels were increased or decreased in temporal lobe epilepsy patients compared with nonseizure autopsies. Hippocampal sclerosis (HS; n = 17), nonsclerosis (non-HS; n = 11), and autopsy hippocampi (n = 9) were studied for KA1-2 and GluR5-7 mRNA levels using semiquantitative in situ hybridization techniques, along with neuron densities. Compared with autopsy hippocampi, HS and non-HS cases showed decreased GluR5 and GluR6 hybridization densities per CA2 and/or CA3 pyramid. Furthermore, HS patients demonstrated increased KA2 and GluR5 hybridization densities per granule cell compared with autopsy hippocampi. These findings indicate that chronic temporal lobe seizures were associated with differential changes in hippocampal KA1-2 and GluR5-7 hybridization densities that vary by subfield and pathology group. In temporal lobe epilepsy patients, these results support the hypothesis that pyramidal cell GluR5 and GluR6 mRNA levels are decreased as a consequence of seizures, and in HS patients granule cell KA2 and GluR5 mRNA levels are increased in association with aberrant fascia dentata mossy fiber sprouting and/or hippocampal neuronal loss.

Regional distribution of glutamate receptor mRNA in the monkey hippocampus and temporal cortex: influence of estradiol

The distribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) glutamate receptor subunits was examined in the hippocampus and temporal cortex of adult ovariectomized female rhesus macaques, some of which received estradiol replacement. In situ hybridization revealed a generalized overlap of NR1, GluR1, and GluR2 subunit mRNAs, but no effect due to estradiol treatment. However, regional differences in expression were noted for each subunit.

Distribution of glutamate receptor subunits in the primate temporal cortex and hippocampus

The distribution of subunits for the N-methyl-D-aspartate (NR1, NR2A/B), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (GluR1, GluR2/3, GluR4) and low affinity kainate (GluR5/6/7) ionotropic glutamate receptors was examined by immunocytochemistry in the temporal cortex and hippocampus of the rhesus macaque (Macaca mulatta). Neurons expressing NR1, NR2A/B, GluR2/3, and GluR4 subunits were widely distributed in all of the cortical layers but the overall density of the GluR4-immunopositive neurons was very low. Neurons expressing the GluR1 subunit were found predominantly in cortical layers V and VI while those expressing the GluR5/6/7 subunits were concentrated in layer V and were readily distinguishable by the thick elongate shape of their primary apical dendrites. Subcellular differences in the immunostaining pattern were also noted between the different glutamate receptor subunits. NR1 and NR2A/B immunoreactivity was most pronounced in somatic and primary dendritic compartments and to a lesser extent in cortical and hippocampal molecular layers. GluR1 immunoreactivity was more intense than GluR2/3 in the hippocampal molecular layers whereas GluR4 was undetectable. GluR5/6/7 immunoreactivity was very intense in the dentate molecular layer, and the CA1 pyramidal cells had a subcellular distribution of GluR5/6/7 that was similar to the cortical neurons. Overall, the distribution patterns of the different glutamate receptor subunits was identical in animals that had been ovariectomized and in ovariectomized animals that had subsequently undergone estradiol or estradiol/progesterone hormone replacement. Taken together, these findings demonstrate a differential spatial arrangement of glutamate receptor subunits in the primate temporal cortex and hippocampus, which may have functional significance for the integration of excitatory inputs to these areas. Furthermore, they show that in adult macaques, sex steroids do not play a major role in determining the distribution patterns of these receptor subunits.

Distribution of neuronal populations containing neurofilament protein and calcium-binding proteins in the canine neocortex: regional analysis and cell typology

Neurophysiological experiments in carnivores have revealed the existence of a large number of cortical regions and an organization of sensory systems quite similar to that found in primates. However, the cyto- and chemoarchitecture of the cerebral cortex is relatively poorly known in carnivores. We analyzed the distribution and typology of classes of neurons containing neurofilament protein or the calcium-binding proteins parvalbumin, calbindin, and calretinin in six neocortical regions of the dog. In all these areas, neurofilament protein was present in a subpopulation of medium-to-large size pyramidal neurons predominantly distributed in layers III and V. Parvalbumin was present in a large population of morphologically diverse interneurons. Small ovoid and multipolar neurons were observed throughout the cortical layers, but predominated in layers II and IV. Layers III and V-VI were characterized by the presence of larger and intensely immunoreactive neurons with bitufted or multipolar morphology, and layers V-VI also contained large multipolar neurons. Calbindin was observed in small round and multipolar interneurons in layer II, and typical double bouquet cells in layer III. Layers IV-VI contained isolated double bouquet cells and large multipolar neurons. A few calbindin-immunoreactive pyramidal neurons were also observed in layer V. Calretinin was localized in bipolar and double bouquet cells in layers II and upper III. The lower part of layer III and layers IV-VI contained rare calretinin-immunoreactive neurons. In some areas, layer III displayed a few large isolated multipolar neurons and pyramidal neurons containing calretinin. In addition, the results show that there is a substantial degree of variability in the distribution of these proteins among cortical regions, and that although they are found in morphologically comparable neuronal types in dog, monkeys, and humans, many differences exist in their regional distribution patterns between carnivores and primates.