Distribution of phosphate-activated glutaminase in the human cerebral cortex

Phosphate-activated glutaminase (PAG), which catalyses conversion of glutamine to glutamate, is a potential marker for glutamatergic, and possibly GABA, neurons in the central nervous system. A polyclonal antibody, raised in rabbits against rat brain PAG, was applied to postmortem human brain tissue to reveal the distribution of PAG in the cerebral cortex. PAG immunoreactivity was observed in pyramidal and non-pyramidal neurons but not in glial cells. In the neocortex, large to medium-sized pyramidal neurons in layers III and V were stained most intensely, while the majority of smaller pyramidal cells were labeled either lightly or moderately. Such modified pyramids as the giant Betz cells, the large pyramidal cells of Meynert, and the solitary cells of Ramón y Cajal were also stained intensely. Fusiform cells in layer VI showed moderate to intense labeling. A number of cortical non-pyramidal neurons of various sizes stained moderately to intensely. These included large basket cells which were identified by their characteristic morphology and size in primary cortical areas. Pyramidal cells in the hippocampal formation as well as basket cells of the stratum oriens stained moderately to intensely. Since pyramidal cells are believed to be glutamatergic and large basket cells GABAergic, these results suggest that PAG plays a role in generating not only transmitter glutamate, but also GABA precursor glutamate.     

Cellular and subcellular localization of protein kinase C in cat visual cortex

Polyclonal antibodies against 3 protein kinase C (PKC) subtypes (I, II and III) were applied to localize the kinase in cat visual cortex. These antibodies exclusively stained neuronal cells. Both pyramidal and non-pyramidal cells exhibiting PKC-like immunoreactivity were concentrated in layers, II, III, V and VI with relatively few cells in layer IV. Electron microscopic examination did not reveal any presynaptic localization of the kinase. PKC immunoreactivity remained normal in a zone of cortex surgically isolated from the rest of the brain by an undercut procedure. These results suggest that PKC is heterogenously distributed in adult cat visual cortex; the kinase recognized by the polyclonal antibodies is localized postsynaptically in intracortical neurons of the superficial and deep cortical layers and the expression of the kinase is not regulated by extracortical input.     

Localization of glutaminase-like and aspartate aminotransferase-like immunoreactivity in neurons of cerebral neocortex

The distribution of glutaminase (GLNase)- and aspartate aminotransferase (AATase)-immunoreactive cells was examined in the cerebral neocortex of rat and guinea pig and in the somatic sensorimotor and primary visual cortex of the Macaca fascicularis monkey. These enzymes are involved in the metabolism of glutamate and aspartate, two amino acids thought to be excitatory amino acid transmitters for cortical neurons. In each of the species examined a large percentage of layer V and VI pyramidal neurons have pronounced glutaminase-like immunoreactivity (GLNase IR). In contrast, neurons in layers I, II, and IV show little GLNase IR. Layer III in the rat and guinea pig contains only a few, densely labeled GLNase-like-immunoreactive (GLNase-Ir) pyramidal neurons, whereas in the monkey the number of GLNase-Ir cells in layer III varies between cytoarchitectonic fields. Area 3b of the primary somatic sensory cortex and area 17 (primary visual cortex) contain few GLNase-Ir cells in layer III. However, layer III contains moderate numbers of GLNase IR in cells in areas 3a, 1, 2, 5, and in the primary motor cortex. Within the motor cortex the largest pyramidal ("Betz") cells are not labeled. In marked contrast to the results with antibody to GLNase, antibody to AATase labels cells that appear nonpyramidal in form, and these cells are in all cortical layers in each of the species examined. This distribution is roughly similar throughout all areas of rodent neocortex, but in monkey visual cortex AATase-immunoreactive neurons are more numerous in layers II-III, IVc, and VI. When combined with the findings of other studies, our results suggest that GLNase IR marks pyramidal neurons that use an excitatory amino acid transmitter. Antibody to AATase appears to mark intrinsic cortical neurons. The AATase immunoreactivity of these cells could indicate that they use an excitatory amino acid transmitter. However, their form and distribution in cortex suggest that this antibody labels GABAergic neurons.     

The Distribution of Brain-derived Neurotrophic Factor and its Receptor trkB in Parvlbumin-containing Neurons of the Rat Visual Cortex

We analysed the distribution of brain-derived neurotrophic factor (BDNF) and its receptor trkB in the adult rat visual cortex, paying particular attention to a GABAergic neuronal subpopulation—the parvalburnin-positive cells. We found expression of trkB in the cell body and apical dendrite of pyramidal neurons and in the cell body of non-pyramidal neurons. Double labelling experiments revealed extensive colocalization of parvalbumin and trkB immunoreactivity in non-pyramidal neurons. Interestingly, the trkB-positive pyramidal neurons appeared surrounded by parvalbumin-labelled boutons. The use of double immunohistochemistry and in situ hybridization histochemistry showed that parvalbumin-positive neurons express trkB mRNA. BDNF rnRNA was found in several cells. Coexpression of BDNF mRNA and parvalbumin immunoreactivity was extremely rare. These data strongly suggest that BDNF synthesized by cortical neurons acts as a postsynaptically derived factor for parvalbumin-positive neurons in the adult rat visual cortex.     

The Specification of Neuronal Fate: A Common Precursor for Neurotransmitter Subtypes in the Rat Cerebral Cortex In Vitro

Neurotransmitter choice is a crucial step in neural development. In the cerebral cortex, pyramidal neurons use the excitatory neurotransmitter glutamate, whereas non-pyramidal cells use the inhibitory neurotransmitter GABA. We are interested in how these two neuronal types are generated. We labelled precursor cells from embryonic rat cerebral cortex with a retroviral vector in dissociated cell cultures, and examined the neurotransmitter phenotype of their progeny immunohistochemically after 2 weeks in vitro. We discovered, first, that precursor cells in culture generate glutamatergic and GABAergic neurons in proportions similar to those in vivo. Second, we found that neuronal precursor cells gave rise to both GABAergic and glutamatergic neurons. These results suggest that neuronal precursor cells in the cerebral cortex have the potential to generate both neuronal subtypes. Moreover, these data are consistent with a stochastic model of neurotransmitter specification.     

Synaptic responses of cortical pyramidal neurons to light stimulation in the isolated turtle visual system

The resistance of the turtle brain to hypoxic injury permits a unique in vitro preparation in which the organization and function of visual cortex can be explored. Intracellular recordings from cortical pyramidal neurons revealed biphasic responses to flashes of light, consisting of an early phase (50-100 msec) of concurrent inhibitory and excitatory activation, followed by a longer, inhibitory phase (250-600 msec) composed of summated Cl- -dependent postsynaptic potentials mediated by GABA. This response sequence results from the coactivation of pyramidal and GABAergic non-pyramidal cells, followed by feed-forward and possibly feed-back pyramidal cell inhibition, and is partly dependent on differences in the membrane properties of pyramidal and non-pyramidal neurons.     

Long-term neurochemical changes after visual cortical lesions in the adult cat

Peripheral deafferentation alters cortical function and such alterations have been shown to affect the cortical expression of the calcium-binding proteins calbindin and parvalbumin and of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). To determine whether cortical deafferentation produces similar effects, we examined the long-term consequences of cortical lesions on the neurochemistry of interconnected cortical areas. We studied the reciprocal effects of localized damage to either visual cortical areas 17 and 18, or posteromedial lateral suprasylvian (PMLS) cortex in the adult cat. These areas are strongly interconnected and play an important role in the processing of visual information. Combined lesions of areas 17 and 18 caused a marked, topographically specific decrease in the proportion of neurons expressing calbindin in supragranular layers of PMLS cortex. Similarly, lesions of PMLS cortex caused topographically restricted decreases in calbindin expression within supragranular layers of areas 17 and 18, but not in other cortical areas with which PMLS is interconnected. To categorize the calbindin-positive neurons affected by such lesions, we carried out double-labeling experiments for the inhibitory neurotransmitter GABA. This investigation showed lesions of areas 17 and 18 to affect calbindin-positive excitatory and inhibitory neurons equally, but PMLS lesions had stronger effects on inhibitory, calbindin-positive neurons. This finding may represent differential damage to feed-forward vs. feed-back projections in the two types of lesions. Finally, the expression of parvalbumin and GABA was unchanged, even in zones of decreased calbindin immunoreactivity. Our results suggest that damage to adult visual cortical areas, whether striate or extrastriate, induces neurochemical changes in the supragranular corticocortical network to which these areas belong. That changes were restricted to calbindin expression suggests cell-specific and/or biochemical pathway-specific alterations in calcium homeostasis.     

Anatomical evidence for glutamate and/or aspartate as neurotransmitters in the geniculo-, claustro-, and cortico-cortical pathways to the cat striate cortex

Data obtained by using various experimental approaches suggest that in the mammalian brain, most neurons within the visual system projecting to the striate cortex employ excitatory amino acids as transmitters. In order to investigate further the neurotransmitter phenotype of the ipsilateral afferents to area 17 of the cat, we have injected D-[³H]-aspartate, a retrograde tracer which selectively reveals putative glutamatergic and/or aspartatergic pathways, into this area. Retrogradely labelled neurons were observed in the dorsal lateral geniculate nucleus, visual claustrum, cortical areas 18, 19, 21a, and in both posteromedial and posterolateral parts of the suprasylvian areas but not in other known thalamic afferents such as the lateral posterior-pulvinar complex and the intralaminar nuclei. The distribution and localization of the labelled cells in all these regions were similar to that observed by using the non-selective tracer horseradish peroxidase conjugated to wheat germ agglutinin, though the number of cells was higher with the latter. Our findings provide additional evidence for the presence of excitatory amino acids as neurotransmitters in the major afferents to the cat striate cortex. © 1996 Wiley-Liss, Inc.     

Excitatory transmitter amino acid-containing neurons in the rat visual cortex: a light and electron microscopic immunocytochemical study

The distribution and morphology of neurons labelled with antisera to glutamate or aspartate were examined, at the light and electron microscope levels, in the rat visual cortex. Using widely accepted light microscopic features as well as well-established nuclear, cytoplasmic, and synaptic criteria, we noted that glutamate-immunoreactive neurons were pyramidal cells distributed in layers II-VI, with an increased concentration in layers II and III. Aspartate immunoreactivity was localized chiefly to pyramidal neurons in layers II-VI. However, approximately 10% of immunolabeled cells were nonpyramidal neurons scattered throughout the cortex. Cell-body measurements revealed that, for both groups of neurons, layer V contained the largest labelled neurons, whereas layers IV and VI contained the smallest. Furthermore, in every layer, aspartate-stained neurons were larger than glutamate-positive cells. Finally, glutamate- and aspartate-labelled axon terminals formed asymmetrical synapses, which are presumably excitatory in nature, primarily with dendritic spines. These findings, together with recent detailed studies of the projections of glutamate- and aspartate-labelled cortical neurons, may provide essential background information for studies aimed to elucidate the function(s) of excitatory amino acids in the cortex and their role in pathological conditions.     

Appearance of putative amino acid neurotransmitters during differentiation of neurons in embryonic turtle cerebral cortex.

Pyramidal and nonpyramidal neurons can be recognized early in the development of the cerebral cortex in both reptiles and mammals, and the neurotransmitters likely utilized by these cells, glutamate and gamma-aminobutyric acid, or GABA, have been suggested to play critical developmental roles. Information concerning the timing and topography of neurotransmitter synthesis by specific classes of cortical neurons is important for understanding developmental roles of neurotransmitters and for identifying potential zones of neurotransmitter action in the developing brain. We therefore analyzed the appearance of GABA and glutamate in the cerebral cortex of embryonic turtles using polyclonal antisera raised against GABA and glutamate. Neuronal subtypes become immunoreactive for the putative amino acid neurotransmitters GABA and glutamate early in the embryonic development of turtle cerebral cortex, with nonpyramidal cells immunoreactive for GABA and pyramidal cells immunoreactive for glutamate. The results of controls strongly suggest that the immunocytochemical staining in tissue sections by the GABA and glutamate antisera corresponds to fixed endogenous GABA and glutamate. Horizontally oriented cells in the early marginal zone (stages 15-16) that are GABA-immunoreactive (GABA-IR) resemble nonpyramidal cells in morphology and distribution. GABA-IR neurons exhibit increasingly diverse morphologies and become distributed in all cortical layers as the cortex matures. Glutamate-immunoreactive (Glu-IR) cells dominate the cellular layer throughout development and are also common in the subcellular layer at early stages, a distribution like that of pyramidal neurons and distinct from that of GABA-IR nonpyramidal cells. The early organization of embryonic turtle cortex in reptiles resembles that of embryonic mammalian cortex, and the immunocytochemical results underline several shared as well as distinguishing features. Early GABA-IR nonpyramidal cells flank the developing cortical plate, composed primarily of pyramidal cells, shown here to be Glu-IR. The earliest GABA-IR cells in turtles likely correspond to Cajal-Retzius cells, a ubiquitous and precocious cell type in vertebrate cortex. Glutamate-IR projection neurons in vertebrates may also be related. The distinctly different topographies of GABA and glutamate containing cells in reptiles and mammals indicate that even if the basic amino acid transmitter-containing cell types are conserved in higher vertebrates, the local interactions mediated by these transmitters may differ. The potential role of GABA and glutamate in nonsynaptic interactions early in cortical development is reinforced by the precocious expression of these neurotransmitters in turtles, well before they are required for synaptic transmission.(ABSTRACT TRUNCATED AT 400 WORDS)     

Immunohistochemical study of glutaminase-containing neurons in the cerebral cortex and thalamus of the rat

In an attempt to identify glutamatergic neurons, the cerebral cortex and thalamus of the rat were examined immunohistochemically by using a monoclonal antibody against phosphate-activated glutaminase (PAG), a major synthetic enzyme of transmitter glutamate in the central nervous system. In both the neocortex and mesocortex, pyramidal cells in layers V and VI showed intense PAG-like immunoreactivity (PAG-LI), whereas neuronal cell bodies in layers I-IV showed weak PAG-LI. At the deep border of layer VI, neurons with horizontally elongated cell bodies showed PAG-LI. In the pyriform and entorhinal cortices, neurons with intense to moderate PAG-LI were seen in layer II as well as in the deeper layers. In the hippocampal formation, pyramidal cells in CA1, CA2, and CA3 and polymorphic cells in CA4 showed PAG-LI; PAG-LI was most intense in pyramidal cells of CA3. Fine granules with weak PAG-LI were also seen on and/or within the cell bodies of granule cells in the dentate gyrus. In the thalamus, neurons with PAG-LI were distributed in all nuclei, although regional differences were observed in the distribution pattern of neurons with PAG-LI and in the intensity of PAG-LI in individual neurons. The largest neurons in each thalamic nucleus showed intense PAG-LI; these were considered to be projection neurons. In addition to perikaryal labeling, many fine, PAG-like immunoreactive granules were distributed in the neuropil of both the cerebral cortex and thalamic nuclei. Some of these fine granules with PAG-LI in the neuropil were assumed to represent fiber terminals with PAG-LI, because the distribution pattern of the deposits in the primary somatosensory and primary visual cortices resembled that of thalamocortical fiber terminals. Glutamate is rather ubiquitous in the mammalian central nervous system, and it is still debatable whether the monoclonal antibody to PAG from brain mitochondria can distinguish transmitter-related glutaminase from the other metabolism-related ones. In the present study, however, large neurons in the thalamic nuclei, as well as pyramidal neurons in the cerebral cortex, showed PAG-LI most intensely, supporting the assumption that projection neurons of the cerebral cortex and thalamus are primarily glutamatergic.     

Morphology and distribution of neurons and glial cells expressing beta-adrenergic receptors in developing kitten visual cortex.

The morphology and distribution of cells expressing beta-adrenergic receptors has been studied in developing kitten visual cortex using a monoclonal antibody which recognizes both beta-1 and beta-2 adrenergic receptors. We found specific populations of neurons and glial cells which express beta-adrenergic receptor immunoreactivity in the kitten visual cortex. In adult animals, the receptors are most concentrated in the superficial and deep cortical layers (layers I, II, III and VI). About 50% of the stained neural cells in adult cat visual cortex are glial cells. Most of the immunoreactive neurons in layers III and V are pyramidal cells while those in layers II and IV are more likely to be nonpyramidal cells. In neonatal kittens, staining is weaker than that in adult cats and it appears to be concentrated in neurons of the deep cortical layers and in the subcortical plate and white matter. Only a few immunoreactive glial cells were found at this age. Receptor numbers increase after birth and by 24 days of age, the laminar distribution of beta-adrenergic receptors approaches that of adult animals. Immunoreactive glial cells in the white matter show a progressive increase in number throughout postnatal development.     

Sprouting of sympathetic fibers in the hippocampus in the absence of major target cell candidates

The distribution and morphology of functionally identified neurons were examined in the visual cortex of Long Evans pigmented rats. The results, based on qualitative and quantitative analysis of single cell spike activity, have shown that neurons in the rat visual cortex have well-defined receptive field properties and are similar to those reported for animals with more highly developed visual systems. Unlike the cat and monkey, the distribution of receptive field types appeared even throughout the visual cortex. Exception was provided by layer IV which, similar to the more 'visual' animals, contained the largest percentage of simple cells. Horseradish peroxidase injected into single, physiologically identified neurons allowed for detailed morphological characterization of functional cell types. Of the cells successfully filled with horseradish peroxidase, complex cells were pyramidal in morphology and located in layers II through VI. Simple cells were both pyramidal and non-pyramidal in appearance and were located in layers II + III and IV. Finally, hypercomplex cells were pyramidal in appearance and their perikarya were situated in layers II + III and V.     

Patterns of neuronal degeneration in the motor cortex of amyotrophic lateral sclerosis patients

Summary We examined patterns of neuronal degeneration in the motor cortex of amyotrophic lateral selerosis (ALS) patients using traditional cell stains and several histochemical markers including neurofilament, parvalbumin, NADPH-diaphorase, ubiquitin, Alz-50 and tau. Three grades of ALS (mild, moderate, severe) were defined based on the extent of Betz cell depletion. Non-phosphorylated neurofilament immunoreactive cortical pyramidal neurons and non-pyramidal parvalbumin local circuit neurons were significantly depleted in all grades of ALS. In contrast, NADPH-diaphorase neurons and Alz-50-positive neurons were quantitatively preserved despite reduced NADPH-diaphorase cellular staining and dendritic pruning. The density of ubiquitin-positive structures in the middle and deep layers of the motor cortex was increased in all cases. Axonal tau immunoreactivity was not altered. These histochemical results suggest that cortical degeneration in ALS is distinctive from other neurodegenerative diseases affecting cerebral cortex. Unlike Huntington's disease, both pyramidal and local cortical neurons are affected in ALS; unlike Alzheimer's disease, alteration of the neuronal cytoskeleton is not prominent. The unique pattern of neuronal degeneration found in ALS motor cortex is consistent with non-N-methyl-Dxxx-aspartate glutamate receptor-mediated cytotoxicity.Supported in part by a Muscular Dystrophy Association Research Development grant     

Immunoreactivity for Taurine Characterizes Subsets of Glia,GABAergic and non-GABAergic Neurons in the Neo- and Archicortex of the Rat,Cat and Rhesus Monkey: Comparison with Immunoreactivity for Homocysteic Acid

The cerebral cortex is an area rich in taurine (2-aminoethanesulphonic acid), but only limited information exists regarding its cellular distribution. We therefore examined taurine-like immunoreactivity in the cerebral cortex of the rat, cat and macaque monkey using antiserum directed against glutaraldehyde-conjugated taurine. Immunostaining was assessed at the light and electron microscopic level, and patterns obtained in light microscopic studies were compared to those produced with antiserum to gamma-aminobutyric acid (GABA) and homocysteic acid (HCA). In all three species, strong taurine-like immunoreactive perivascular endothelial cells, pericytes and oligodendrocytes were found. These cells were located throughout the neuropil, which itself showed a low level of immunoreactivity. In rats and cats, a small number of weakly taurine-enriched neurons were observed, particularly in superficial layers. In all cortical areas of the macaque, however, glial staining was matched by strong, selective staining of subpopulations of cortical neurons which were distributed in a bilaminar pattern involving layers II/III and VI. In addition, in primary visual cortex, area 17, immunopositive neurons were also present in sublayer IVCbeta, while in the hippocampus strongly taurine-positive neurons were most conspicuous in the granule cell layer of the dentate gyrus. In all regions, strongly taurine-positive neurons constituted only a subpopulation of the neurons occupying a given layer. Examination of adjacent sections for GABA immunoreactivity showed that the most strongly taurine-positive neurons in layers II/III were immunoreactive for GABA. The cells located in layers IVCbeta and VI, and the granule cells of the dentate gyrus, however, were GABA-negative. The morphological features of these latter groups suggested that the antiserum to taurine identifies subsets of spiny stellate, small pyramidal and dentate granule cells. None of these neurons showed immunoreactivity with antiserum to HCA in the primate; HCA-positive glia were found along the pial and white matter boundaries of the cortex, and showed no overlap with strongly taurine-positive glial elements. Although a transmitter role for taurine may be unlikely, particularly in view of its enrichment in subpopulations of both inhibitory and excitatory cells, the capacity of taurine to influence membrane-associated functions in excitable tissues, and its selective distribution demonstrated here, provides the potential for a contribution to communication between cortical cells.     

Immunocytochemical study of GABAA receptors in the cat visual cortex

The laminar distribution and morphological structures associated with GABA_A receptor immunoreactivity in the cat visual cortex were studied by using two different polyclonal antibodies directed either against the purified GABA_A receptor protein (antibody “967”) or against a specific domain of the β₁-subunit of the GABA_A receptor (antibody “Q”). Immunoblots of cat visual cortex tissue with these antibodies revealted that antibody “Q” recognizes only one subunit, namely the β₁-subunit of the GABA_A receptor, and that antibody “967” recognizes three subunits. Both antibodies produced very similar staining patterns, indicating that the β₁-subunit may be an essential component of the GABA_A receptor in the cat visual cortex. The typical staining pattern showed a clear membrane structure around neuronal somata. Using cell body shape criteria, immunopositive neurons included both pyramidal cells in cortical layers II, III, and V, and nonpyramidal cells in all cortical layers. Immunopositive neurons were uniformly distributed in layers II to VI, whereas the density of immunopositive cells in layer I was lower. Some immunopositive neurons were also found in the white matter underlying the visual cotex. In gray matter, immunopositive structures also included dendrites, especially the proximal dendrites, and axon initial segments of pyramidal neurons. The immunopositive processes usually ran vertically toward the pial surface. Some astrocytes were also immunostained. They were localized in layer I and in the white matter. The overall pattern of immunostaining was similar in areas 17, 18, and 19. © 1993 Wiley-Liss, Inc.     

Corticocortical connections of cat primary auditory cortex (AI): laminar organization and identification of supragranular neurons projecting to area AII

The laminar distribution and structure of the supragranular cells projecting from primary auditory cortex (AI) to the second auditory cortex (AII) in the cat were studied with horseradish peroxidase. Injections in AII retrogradely labeled somata in ipsilateral cortical layers I-VI of AI. A bimodal laminar disposition was found, with approximately 40% of the labeled cells in layer III, 25% in layer V, and 10-15% each in layers II, IV, and VI; only a few cells were found in layer I. The labeled cells were scattered in small aggregates between which unlabeled neurons were interspersed. There was some, though not a strict, topographical distribution of the labeled cells according to the locus of the injection in AII. Injections in the caudal part of AII labeled cells in more rostral AI, while rostral AII injections labeled cells in more caudal AI. Ventral AII injections labeled more ventrally located AI cells, while more dorsal AII injections labeled more dorsally situated AI cells. AII injections also labeled cells in other auditory cortex subdivisions, including the posterior ectosylvian, ventroposterior, temporal, and dorsal auditory zone/suprasylvian fringe cortical areas, and in some non-auditory cortical areas. In layers II and III, both pyramidal and non-pyramidal cells were labeled. More pyramidal cells were labeled in layer III than layer II (80% vs. 62%), and the proportion of non-pyramidal cells in layer II was more than twice that in layer IV (27% vs. 12%). The types of labeled cells were distinguished from one another on the basis of size, somatic and dendritic shape, and laminar distribution. The profiles of labeled cells in these experiments were compared to, and correlated with, those in Golgi-impregnated material. In layer II, the classes of corticocortical projecting cells consisted of small and medium-sized pyramidal, bipolar, and multipolar cells. Those in layer III included small, medium-sized, and large pyramidal neurons, and bipolar and multipolar cells. The average somatic area of the labeled cells did not differ significantly from that of the unlabeled cells, and both were about equal in somatic size to neurons accumulating tritiated gamma-aminobutyric acid in layers II and III. These findings suggest that there is convergent, ipsilateral input onto AII from every layer in AI, and from other cortical auditory and non-auditory areas. A morphologically heterogeneous population of cells in AI contributes to these projections. Diversity in the cytological origins of corticocortical projections implies functional differences between layers II and III since the latter also projects commissural, while layer II in the cat, does not.     

Neurochemical gradients along monkey sensory cortical pathways: calbindin-immunoreactive pyramidal neurons in layers II and III

We examined the distribution of neurons containing immunoreactivity for three calcium-binding proteins, calbindin, parvalbumin and calretinin, as well as nonphosphorylated neurofilament protein, in cortical areas along the ventral and dorsal cortical visual pathways, and in ventrally-directed somatosensory and auditory cortical pathways. Calbindin-immunoreactive pyramidal neurons showed the most prominent regional differences. They were largely restricted to layers II and III and their number monotonically increased from the primary sensory areas to the anteroventral areas along the ventral visual pathway and along the ventrally-directed somatosensory and auditory pathways. The number of calbindin-immunoreactive pyramidal neurons in layers II and III also increased along the dorsal visual pathway, but the number in the last recognized stage of the dorsal visual pathway (area 7a) was significantly smaller than that at the corresponding stage in the ventral visual pathway (TE). The number of calbindin-immunoreactive pyramidal neurons was highest in layers II and III of areas 35/36, TG, and TF/TH, which represent terminal cortical regions of the pathways. These results show neurochemical differences between cortical areas located at early and late stages along serial corticocortical pathways, as well as confirming differences between pyramidal neurons in the supragranular and infragranular layers.     

Glutamate and aspartate immunoreactive neurons of the rat basolateral amygdala: Colocalization of excitatory amino acids and projections to the limbic circuit

The basolateral amygdala has projections to several structures that take part in the limbic cortico-striato-pallido-thalamic circuit, including the prefrontal cortex, ventral striatum, and mediodorsal thalamic nucleus. The present investigation used a technique that combines retrograde tract tracing with immunohistochemistry for glutamate and aspartate to determine if amygdaloid neurons projecting to different targets in the limbic circuit can be distinguished on the basis of their content of excitatory amino acids. Cell counts revealed that at least 85–95% of the neurons in the basolateral nucleus projecting to the prefrontal cortex or ventral striatum were pyramidal cells that exhibited glutamate or aspartate immunoreactivity. Colocalization studies indicated that 94–100% of aspartate-immunoreactive neurons in the basolateral nucleus were also glutamate positive and that 92–94% of glutamate-immunoreactive neurons were also aspartate positive. A small number of glutamate-positive pyramidal neurons in the anterior subdivision of the cortical nucleus were found to project to the mediodorsal thalamic nucleus. However, the great majority of amygdaloid neurons with projections to the mediodorsal nucleus did not exhibit glutamate or aspartate immunoreactivity. The absence of glutamate and aspartate immunoreactivity in these cells suggests that these neurons do not use excitatory amino acids as neurotransmitters. The finding of high levels of glutamate and aspartate in basolateral amygdaloid neurons projecting to the prefrontal cortex and ventral striatum is consistent with previous reports indicating that these neurons may use excitatory amino acids as neurotransmitters, but is not a definitive criterion for this determination. © 1996 Wiley-Liss, Inc.     

Distribution and morphology of functionally identified neurons in the visual cortex of the rat

The distribution and morphology of functionally identified neurons were examined in the visual cortex of Long Evans pigmented rats. The results, based on qualitative and quantitative analysis of single cell spike activity, have shown that neurons in the rat visual cortex have well-defined receptive field properties and are similar to those reported for animals with more highly developed visual systems. Unlike the cat and monkey, the distribution of receptive field types appeared even throughout the visual cortex. Exception was provided by layer IV which, similar to the more ‘visual’ animals, contained the largest percentage of simple cells.Horseradish peroxidase injected into single, physiologically identified neurons allowed for detailed morphological characterization of functional cell types. Of the cells successfully filled with horseradish peroxidase, complex cells were pyramidal in morphology and located in layers II through VI. Simple cells were both pyramidal and non-pyramidal in appearance and were located in layers II + III and IV. Finally, hypercomplex cells were pyramidal in appearance and their perikarya were situated in layers II + III and V.