Distribution and patterns of connectivity of interneurons containing calbindin, calretinin, and parvalbumin in visual areas of the occipital and temporal lobes of the macaque monkey

Muscarinic acetylcholine receptors in macaque V1 are most frequently expressed by parvalbumin-immunoreactive neurons

Acetylcholine (ACh) is believed to underlie mechanisms of arousal and attention in mammals. ACh also has a demonstrated functional effect in visual cortex that is both diverse and profound. We have reported previously that cholinergic modulation in V1 of the macaque monkey is strongly targeted toward GABAergic interneurons. Here we examine the localization of m1 and m2 muscarinic receptor subtypes across subpopulations of GABAergic interneurons--identified by their expression of the calcium-binding proteins parvalbumin, calbindin, and calretinin--using dual-immunofluorescence confocal microscopy in V1 of the macaque monkey. In doing so, we find that the vast majority (87%) of parvalbumin-immunoreactive neurons express m1-type muscarinic ACh receptors. m1 receptors are also expressed by 60% of calbindin-immunoreactive neurons and 40% of calretinin-immunoreactive neurons. m2 AChRs, on the other hand, are expressed by only 31% of parvalbumin neurons, 23% of calbindin neurons, and 25% of calretinin neurons. Parvalbumin-immunoreactive cells comprise approximately 75% of the inhibitory neuronal population in V1 and included in this large subpopulation are neurons known to veto and regulate the synchrony of principal cell spiking. Through the expression of m1 ACh receptors on nearly all of these PV cells, the cholinergic system avails itself of powerful control of information flow through and processing within the network of principal cells in the cortical circuit.

A neurochemical signature of visual recovery after extrastriate cortical damage in the adult cat

In adult cats, damage to the extrastriate visual cortex on the banks of the lateral suprasylvian (LS) sulcus causes severe deficits in motion perception that can recover as a result of intensive direction discrimination training. The fact that recovery is restricted to trained visual field locations suggests that the neural circuitry of early visual cortical areas, with their tighter retinotopy, may play an important role in attaining perceptual improvements after damage to higher level visual cortex. The present study tests this hypothesis by comparing the manner in which excitatory and inhibitory components of the supragranular circuitry in an early visual cortical area (area 18) are affected by LS lesions and postlesion training. First, the proportion of LS-projecting pyramidal cells as well as calbindin- and parvalbumin-positive interneurons expressing each of the four AMPA receptor subunits was estimated in layers II and III of area 18 in intact animals. The degree to which LS lesions and visual retraining altered these expression patterns was then assessed. Both LS-projecting pyramidal cells and inhibitory interneurons exhibited long-term, differential reductions in the expression of glutamate receptor (GluR)1, -2, -2/3, and -4 following LS lesions. Intensive visual training post lesion restored normal AMPAR subunit expression in all three cell-types examined. Furthermore, for LS-projecting and calbindin-positive neurons, this restoration occurred only in portions of the ipsi-lesional area 18 representing trained visual field locations. This supports our hypothesis that stimulation of early visual cortical areas-in this case, area 18-by training is an important factor in restoring visual perception after permanent damage to LS cortex.

Gain modulation by nicotine in macaque v1

Acetylcholine is a ubiquitous cortical neuromodulator implicated in cognition. In order to understand the potential for acetylcholine to play a role in visual attention, we studied nicotinic acetylcholine receptor (nAChR) localization and function in area V1 of the macaque. We found nAChRs presynaptically at thalamic synapses onto excitatory, but not inhibitory, neurons in the primary thalamorecipient layer 4c. Furthermore, consistent with the release enhancement suggested by this localization, we discovered that nicotine increases responsiveness and lowers contrast threshold in layer 4c neurons. We also found that nAChRs are expressed by GABAergic interneurons in V1 but rarely by pyramidal neurons, and that nicotine suppresses visual responses outside layer 4c. All sensory systems incorporate gain control mechanisms, or processes which dynamically alter input/output relationships. We demonstrate that at the site of thalamic input to visual cortex, the effect of this nAChR-mediated gain is an enhancement of the detection of visual stimuli.

Specificity in inhibitory systems associated with prefrontal pathways to temporal cortex in primates

The prefrontal cortex selects relevant signals and suppresses irrelevant stimuli for a given task through mechanisms that are not understood. We addressed this issue using as a model system the pathways from the functionally distinct prefrontal areas 10 and 32 to auditory association cortex, and investigated their relationship to inhibitory neurons labeled for calbindin (CB) or parvalbumin (PV), which differ in mode of inhibition. Projection neurons in area 10 originated mostly in layers 2-3 and were intermingled with CB inhibitory neurons. In contrast, projections from area 32 originated predominantly in layers 5-6 among PV inhibitory neurons. Prefrontal axonal boutons terminating in layers 2-3 of auditory association cortex were larger than those terminating in layer 1. Most prefrontal axons synapsed on spines of excitatory neurons but a significant number targeted dendritic shafts of inhibitory neurons. Axons from area 10 targeted CB and PV inhibitory neurons, whereas axons from area 32 targeted PV inhibitory neurons. The preferential association of the 2 prefrontal pathways with distinct classes of inhibitory neurons at their origin and termination may reflect the specialization of area 10 in working memory functions and area 32 in emotional communication. These findings suggest diversity in inhibitory control by distinct prefrontal pathways.

The prefrontal cortex and flexible behavior

The prefrontal cortex in primates guides behavior by selecting relevant stimuli for the task at hand, mediated through excitatory bidirectional pathways with structures associated with sensory processing, memory, and emotions. The prefrontal cortex also has a key role in suppressing irrelevant stimuli through a mechanism that is not well understood. Recent findings indicate that prefrontal pathways interface with laminar-specific neurochemical classes of inhibitory neurons in sensory cortices, terminate extensively in the frontal and sensory sectors of the inhibitory thalamic reticular nucleus, and target the inhibitory intercalated masses of the amygdala. Circuit-based models suggest that prefrontal pathways can select relevant signals and efficiently suppress distractors, in processes that are disrupted in schizophrenia and in other disorders affecting prefrontal cortices.

Studies of stimulus parameters for seizure disruption using neural network simulations

A large scale neural network simulation with realistic cortical architecture has been undertaken to investigate the effects of external electrical stimulation on the propagation and evolution of ongoing seizure activity. This is an effort to explore the parameter space of stimulation variables to uncover promising avenues of research for this therapeutic modality. The model consists of an approximately 800 mum x 800 mum region of simulated cortex, and includes seven neuron classes organized by cortical layer, inhibitory or excitatory properties, and electrophysiological characteristics. The cell dynamics are governed by a modified version of the Hodgkin-Huxley equations in single compartment format. Axonal connections are patterned after histological data and published models of local cortical wiring. Stimulation induced action potentials take place at the axon initial segments, according to threshold requirements on the applied electric field distribution. Stimulation induced action potentials in horizontal axonal branches are also separately simulated. The calculations are performed on a 16 node distributed 32-bit processor system. Clear differences in seizure evolution are presented for stimulated versus the undisturbed rhythmic activity. Data is provided for frequency dependent stimulation effects demonstrating a plateau effect of stimulation efficacy as the applied frequency is increased from 60 to 200 Hz. Timing of the stimulation with respect to the underlying rhythmic activity demonstrates a phase dependent sensitivity. Electrode height and position effects are also presented. Using a dipole stimulation electrode arrangement, clear orientation effects of the dipole with respect to the model connectivity is also demonstrated. A sensitivity analysis of these results as a function of the stimulation threshold is also provided.

Chemoarchitecture of the middle temporal visual area in the marmoset monkey (Callithrix jacchus): laminar distribution of calcium-binding proteins (calbindin, parvalbumin) and nonphosphorylated neurofilament

We studied the distributions of interneurons containing the calcium-binding proteins parvalbumin and calbindin D-28k, as well as that of pyramidal neurons containing nonphosphorylated neurofilament (NNF), in the middle temporal visual area (MT) of marmoset monkeys. The distributions of these classes of cells in MT are distinct from those found in adjacent areas. Similar to the primary visual area (V1), in MT, calbindin-immunopositive neurons can be objectively classified into "dark" and "light" subtypes based on optical density of stained cell bodies. Calbindin-positive dark neurons are particularly concentrated in layers 2 and 3, whereas light neurons have a more widespread distribution. In addition, a subcategory of calbindin-positive dark neuron, characterized by a "halo" of stained processes surrounding the cell body, is found within and around layer 4 of MT and V1. These cells are rare in most other visual areas. In comparison, parvalbumin-immunopositive cells in area MT have a relatively homogeneous distribution, although with a trend toward higher spatial density in lower layer 3, and are relatively uniform in terms of density of staining. Finally, MT shows a characteristic trilaminar distribution of NNF-immunopositive pyramidal cells, with stained cell bodies evident in layers 3, 5, and 6. Although the laminar distribution of cells stained for the three markers overlap to some extent, these subcategories can be readily distinguished in terms of morphology, including cell body size. Chemoarchitectural parallels observed between MT and V1 suggest comparable physiological requirements and neuronal circuitry.

Scaling of inhibitory interneurons in areas v1 and v2 of anthropoid primates as revealed by calcium-binding protein immunohistochemistry

Inhibitory GABAergic interneurons are important for shaping patterns of activity in neocortical networks. We examined the distributions of inhibitory interneuron subtypes in layer II/III of areas V1 and V2 in 18 genera of anthropoid primates including New World monkeys, Old World monkeys, and hominoids (apes and humans). Interneuron subtypes were identified by immunohistochemical staining for calbindin, calretinin, and parvalbumin and densities were quantified using the optical disector method. In both V1 and V2, calbindin-immunoreactive neuron density decreased disproportionately with decreasing total neuronal density. Thus, V1 and V2 of hominoids were occupied by a smaller percentage of calbindin-immunoreactive interneurons compared to monkeys who have greater overall neuronal densities. At the transition from V1 to V2 across all individuals, we found a tendency for increased percentages of calbindin-immunoreactive multipolar cells and calretinin-immunoreactive interneurons. In addition, parvalbumin-immunoreactive cell soma volumes increased from V1 to V2. These findings suggest that modifications of specific aspects of inhibition might be critical to establishing the receptive field properties that distinguish visual areas. Furthermore, these results show that phylogenetic variation exists in the microcircuitry of visual cortex that could have general implications for sensory processing.

Electrophysiological Classification of Somatostatin-Positive Interneurons in Mouse Sensorimotor Cortex

Classification of inhibitory interneurons is critical in determining their role in normal information processing and pathophysiological conditions such as epilepsy. Classification schemes have relied on morphological, physiological, biochemical, and molecular criteria; and clear correlations have been demonstrated between firing patterns and cellular markers such as neuropeptides and calcium-binding proteins. This molecular diversity has allowed generation of transgenic mouse strains in which GFP expression is linked to the expression of one of these markers and presumably a single subtype of neuron. In the GIN mouse (E GFP-expressing Inhibitory Neurons), a subpopulation of somatostatin-containing interneurons in the hippocampus and neocortex is labeled with enhanced green fluorescent protein (EGFP). To optimize the use of the GIN mouse, it is critical to know whether the population of somatostatin–EGFP-expressing interneurons is homogeneous. We performed unsupervised cluster analysis on 46 EGFP-expressing interneurons, based on data obtained from whole cell patch-clamp recordings. Cells were classified according to a number of electrophysiological variables related to spontaneous excitatory postsynaptic currents (sEPSCs), firing behavior, and intrinsic membrane properties. EGFP-expressing interneurons were heterogeneous and at least four subgroups could be distinguished. In addition, multiple discriminant analysis was applied to data collected during whole cell recordings to develop an algorithm for predicting the group membership of newly encountered EGFP-expressing interneurons. Our data are consistent with a heterogeneous population of neurons based on electrophysiological properties and indicate that EGFP expression in the GIN mouse is not restricted to a single class of somatostatin-positive interneuron.

Ionotropic glutamate receptor GluR1 in the visual cortex of hamster: distribution and co-localization with calcium-binding proteins and GABA

The subunit composition of the AMPA receptor is critical to its function. AMPA receptors that display very low calcium permeability include the GluR2 subunit, while AMPA receptors that contain other subunits, such as GluR1, display high calcium permeability. We have studied the distribution and morphology of neurons containing GluR1 in the hamster visual cortex with antibody immunocytochemistry. We compared this labeling to that for calbindin D28K, parvalbumin, and GABA. Anti-GluR1-immunoreactive (IR) neurons were located in all layers. The highest density of GluR1-IR neurons was found in layers II/III. The labeled neurons were non-pyramidal neurons, but were varied in morphology. The majority of the labeled neurons were round or oval cells. However, stellate, vertical fusiform, pyriform, and horizontal neurons were also labeled with the anti-GluR1 antibody. Two-color immunofluorescence revealed that many of the GluR1-IR neurons in the hamster visual cortex were double-labeled with either calbindin D28K (31.50%), or parvalbumin (22.91%), or GABA (63.89%). These results indicate that neurons in the hamster visual cortex express GluR1 differently according to different layers and selective cell types, and that many of the GluR1-IR neurons are limited to neurons that express calbindin D28K, parvalbumin, or GABA. The present study elucidates the neurochemical structure of GluR1, a useful clue in understanding the differential vulnerability of GluR1-containing neurons with regard to calcium-dependent excitotoxic mechanisms.

Diversity of laminar connections linking periarcuate and lateral intraparietal areas depends on cortical structure

Lateral prefrontal and intraparietal cortices have strong connectional and functional associations but it is unclear how their common visuomotor, perceptual and working memory functions arise. The hierarchical scheme of cortical processing assumes that prefrontal cortex issues 'feedback' projections to parietal cortex. However, the architectonic heterogeneity of these cortices raises the question of whether distinct areas have laminar-specific interconnections underlying their complex functional relationship. Using quantitative procedures, we showed that laminar-specific connections between distinct prefrontal (areas 46 and 8) and lateral intraparietal (LIPv, LIPd and 7a) areas in Macaca mulatta, studied with neural tracers, varied systematically according to rules determined by the laminar architecture of the linked areas. We found that axons from areas 46 and rostral 8 terminated heavily in layers I-III of all intraparietal areas, as did caudal area 8 to area LIPv, suggesting 'feedback' communication. However, contrary to previous assumptions, axons from caudal area 8 terminated mostly in layers IV-V of LIPd and 7a, suggesting 'feedforward' communication. These laminar patterns of connections were highly correlated with consistent differences in neuronal density between linked areas. When neuronal density in a prefrontal origin was lower than in the intraparietal destination, most terminations were found in layer I with a concomitant decrease in layer IV. The opposite occurred when the prefrontal origin had a higher neuronal density than the target. These findings indicate that the neuronal density of linked areas can reliably predict their laminar connections and may form the basis of understanding the functional complexity of prefrontal-intraparietal interactions in cognition.

Neocortical areas, layers, connections, and gene expression

Cortical patterns of gene expression provide a new approach to long standing issues of lamination, and area identity and formation. In this review, we summarize recent findings where molecular biological techniques have revealed a small number of area-specific genes in the nonhuman primate cortex. One of these (occ1) is strongly expressed in primary visual cortex and is associated with thalamocortical connections. Another gene, RBP, is more strongly expressed in association areas. It is not clear whether RBP might be linked with any particular connectional system, but several possibilities are raised. We also discuss possible roles of area-specific genes in postnatal development, and conclude with a brief sketch of future directions.

Double-bouquet cells in the monkey and human cerebral cortex with special reference to areas 17 and 18

The detailed microanatomical study of the human cerebral cortex began in 1899 with the experiments of Santiago Ramón y Cajal, who applied the Golgi method to define the structure of the visual, motor, auditory and olfactory cortex. In the first article of this series, he described a special type of interneuron in the visual cortex capable of exerting its influence in the vertical dimension. These neurons are now more commonly referred to as double-bouquet cells (DBCs). The DBCs are readily distinguished owing to their characteristic axons that give rise to tightly interwoven bundles of long, vertically oriented axonal collaterals resembling a horsetail (DBC horsetail). Nevertheless, the most striking characteristic of these neurons is that they are so numerous and regularly distributed that the DBC horsetails form a microcolumnar structure. In addition, DBCs establish hundreds of inhibitory synapses within a very narrow column of cortical tissue. These features have generated considerable interest in DBCs over recent years, principally among those researchers interested in the analysis of cortical circuits. In the present chapter, we shall discuss the morphology, synaptic connections and neurochemical features of DBCs that have been defined through the study of these cells in different cortical areas and species. We will mainly consider the immunocytochemical studies of DBCs that have been carried out in the visual cortex (areas 17 and 18) of human and macaque monkey. We will see that there are important differences in the morphology, number and distribution of DBC horsetails between areas 17 and 18 in the primate. This suggests important differences in the microcolumnar organization between these areas, the functional significance of which awaits detailed correlative physiological and microanatomical studies.

Activity-dependent Expression of occ1 in Excitatory Neurons Is a Characteristic Feature of the Primate Visual Cortex

occ1 is a gene whose expression is particularly abundant in neurons in the macaque primary visual cortex (V1). In the present study, we report that the expression of occ1 mRNA in the macaque neocortex can be classified into two modes. The first mode is associated with excitatory neurons distributed in the major thalamocortical recipient layers that exhibit strong cytochrome oxidase activity. This is highly prominent in V1. The second mode is associated with parvalbumin-positive GABAergic interneurons and is distributed across the macaque neocortex. In V1, monocular deprivation showed that occ1 mRNA expression in excitatory neurons was markedly dependent on afferent activity, whereas that in GABAergic interneurons was not. Cross-species comparison showed specific differences in expression. In marmosets, a strong expression was observed in V1 similarly to macaques. The occ1 mRNA expression, however, was generally weak in the mouse neocortex. In rabbit and ferret cortices, the strong expression was observed only in GABAergic interneurons. We conclude that activity-dependent occ1 mRNA expression in the excitatory neurons of V1 was caused by a novel mechanism acquired by primates after their separation from other lineages.

Cortical interneurons and their origins

GABAergic interneurons regulate both the development and function of the cerebral cortex. Multiple studies have demonstrated that many cortical interneurons, possibly all of those in nonprimate mammals, originate in the subcortical telencephalon. Most of these derive from the medial ganglionic eminence, but based on varying degrees of evidence, other sources have been proposed, including the caudal and lateral ganglionic eminences, the septal region, and the cortex itself. Recent evidence also suggests that the spatial diversity of interneuron origins contributes to the remarkable diversity of their differentiated fate.

Double bouquet cell in the human cerebral cortex and a comparison with other mammals

Double bouquet cells (DBCs) are neocortical gamma-aminobutyric acid (GABA)ergic interneurons characterized by the vertical bundling of its axon, which are generally termed "bundles" or "horse-tails." Using immunocytochemistry for the calcium binding protein calbindin, we have analyzed the morphology, density, and distribution of DBC horse-tails in different cortical areas of the human cortex (Brodmann's areas 10, 4, 3b, 22, 18, and 17). Although DBC horse-tails were very numerous and regularly distributed in all cortical areas, variations were observed both in terms of morphology and density. We distinguished two major classes of DBC horse-tails: the thicker complex type (type I) that had more axon collaterals; and the simple type (type II). The density of DBC horse-tails was significantly higher in areas 17, 18, 22, and 4 than in areas 3b and 10. Moreover, the proportion of type I and type II DBC horse-tails varied in the cortical areas studied. We also examined the distribution of DBC horse-tails in frontal, parietal, and occipital areas of different mammalian species. We found DBCs to be present in carnivores but not in rodents, lagomorphs, or artiodactyls. In carnivores, relatively few DBC horse-tails can be identified and they were generally found in the occipital cortex. Therefore, there is significant variability in the morphology and distribution of DBC horse-tails in different species and cortical areas. We conclude that, although these interneurons may be an important element in the organization of cortical microcolumns in primates, this is not the case in other mammalian species.

Distribution of calcium binding proteins in visual and auditory cortices of hamsters

The morphology and distribution of neurons immunoreactive (ir) to parvalbumin (PV), calretinin (CR) and calbindin (CB) were studied in the primary visual (V1) and auditory (A1) cortices of hamsters. Cortical cell populations were labelled immunohistochemically using a glucose oxidase-diaminobenzidine-nickel combined revelation method. Quantitative analysis revealed significant differences between V1 and A1 in the density and distribution of their neuronal population. CBir cells exhibited several typologies in both cortical regions. Most cells were multipolar even though many of them had bitufted or bipolar morphologies. These cells were distributed in layers II/III and in layer V of both A1 and V1, but were more numerous in layer V of V1. CRir cells were of the fusiform type with long bipolar dendritic arbours. These were similarly distributed in both cortices with a peak in superficial layers II/III. PVir cells were also found in both cortices and had round or oval-shaped somata with multipolar processes. They were mostly located in layer V for V1 and in layers III/IV for A1. Visual and auditory primary cortices can thus be differentiated on the basis of their immunoreactivity to specific calcium binding proteins.

Relationship of prefrontal connections to inhibitory systems in superior temporal areas in the rhesus monkey

The prefrontal cortex selects relevant signals and suppresses irrelevant signals in behavior, as exemplified by its functional interaction with superior temporal cortices. We addressed the structural basis of this process by investigating quantitatively the relationship of prefrontal pathways to inhibitory interneurons in superior temporal cortices. Pathways were labeled with neural tracers, and two neurochemical classes of inhibitory interneurons were labeled with parvalbumin (PV) and calbindin (CB), which differ in mode of inhibitory control. Both markers varied significantly and systematically across superior temporal areas. Calbindin neurons were more prevalent than PV neurons, with the highest densities found in posterior high-order auditory association cortices. Axons from anterior lateral, medial prefrontal and orbitofrontal areas terminated in the anterior half of the superior temporal gyrus, targeting mostly the superficial layers (I to upper III), where CB neurons predominated. Reciprocal projection neurons were intermingled with PV neurons, and emanated mostly from the deep part of layer III and to a lesser extent from layers V-VI, in proportions matching the laminar density of inhibitory interneurons. In marked contrast, prefrontal connections in temporal polar cortex were found mostly in the deep layers, showing mismatch with the predominant upper laminar distribution of interneurons. Differences in the relationship of connections to inhibitory neurons probably affect the dynamics in distinct superior temporal cortices. These findings may help explain the reduced efficacy of inhibitory control in superior temporal areas after prefrontal cortical damage.

Region- and age-specific deficits in γ-aminobutyric acidergic neuron development in the telencephalon of theuPAR-/- mouse

We have previously shown that in adult mice with a null mutation in the urokinase-type plasminogen activator receptor (uPAR) gene, maintained on a C57BL/6J/129Sv background, there is a selective loss of GABAergic interneurons in anterior cingulate and parietal cortex, with the parvalbumin-expressing subpopulation preferentially affected. Here, we performed a more detailed anatomical analysis of uPAR(-/-) mutation on the congenic C57BL/6J background. With glutamic acid decarboxylase-67 and gamma-aminobutyric acid (GABA) immunostaining, there is a similar region-selective loss of cortical interneurons in the congenic uPAR(-/-) mice from the earliest age examined (P21). In contrast, the loss of parvalbumin-immunoreactive cells is observed only in adult cortex, and the extent of this loss is less than in the mixed background. Moreover, earlier in development, although there are normal numbers of parvalbumin cells in the uPAR(-/-) cortex, fewer cells coexpress GABA, suggesting that the parvalbumin subpopulation migrates appropriately to the cortex, but does not differentiate normally. Among the other forebrain regions examined, only the adult hippocampus shows a loss of GABAergic interneurons, although the somatostatin, rather than the parvalbumin, subpopulation contributes to this loss. The data suggest that uPAR function is necessary for the normal development of a subpopulation of GABAergic neurons in the telencephalon. It is likely that the late-onset parvalbumin phenotype is due to the effects of an altered local environment on selectively vulnerable neurons and that the extent of this loss is strain dependent. Thus, an interplay between complex genetic factors and the environment may influence the phenotypic impact of the uPAR mutation both pre- and postnatally.

Localization of calcium-binding proteins in physiologically and morphologically characterized interneurons of monkey dorsolateral prefrontal cortex

In the primate neocortex, little is known about the possible associations between functional subclasses of GABA neurons, their morphological properties and calcium-binding protein (CaBP) content. We used whole-cell current clamp recordings, combined with intracellular labeling and fluorescence immunohistochemistry, to determine these relationships for interneurons in layers 2-3 of monkey prefrontal cortex (PFC). Eighty-one interneurons were included in the analysis. Thirty-eight of these cells showed immunoreactivity for one of the three CaBPs tested. Co-localization of more than one CaBP was not observed in any of the interneurons examined. Interneurons with different CaBPs formed distinct populations with specific physiological membrane properties and morphological features. Parvalbumin (PV)-positive cells had the physiological properties characteristic of fast-spiking interneurons (FS) and the morphology of basket or chandelier neurons. Most calretinin (CR)-containing cells had the physiological properties ascribed to non-fast-spiking cells (non-FS) and a vertically oriented axonal morphology, similar to that of double bouquet cells. Calbindin (CB)-positive interneurons also had non-FS properties and included cells with double bouquet morphology or with a characteristic dense web of axonal collaterals in layer 1. Classification of the interneurons based on cluster analysis of multiple electrophysiological properties suggested the existence of at least two distinct groups of interneurons. The first group contained mainly PV-positive FS cells and the second group consisted predominantly of CR- and CB-positive non-FS interneurons. These findings may help to illuminate the functional roles of different groups of interneurons in primate PFC circuitry.

Division of labor among distinct subtypes of inhibitory neurons in a cortical microcircuit of working memory

A conspicuous feature of cortical organization is the wide diversity of inhibitory interneurons; their differential computational functions remain unclear. Here we propose a local cortical circuit in which three major subtypes of interneurons play distinct roles. In a model designed for spatial working memory, stimulus tuning of persistent activity arises from the concerted action of widespread inhibition mediated by perisoma-targeting (parvalbumin-containing) interneurons and localized disinhibition of pyramidal cells via interneuron-targeting (calretinin-containing) interneurons. Moreover, resistance against distracting stimuli (a fundamental property of working memory) is dynamically controlled by dendrite-targeting (calbindin-containing) interneurons. The experimental observation of inverted tuning curves of monkey prefrontal neurons recorded during working memory supports a key model prediction. This work suggests a framework for understanding the division of labor and cooperation among different inhibitory cell types in a recurrent cortical circuit.

Distinct GABAergic targets of feedforward and feedback connections between lower and higher areas of rat visual cortex

Processing of visual information is performed in different cortical areas that are interconnected by feedforward (FF) and feedback (FB) pathways. Although FF and FB inputs are excitatory, their influences on pyramidal neurons also depend on the outputs of GABAergic neurons, which receive FF and FB inputs. Rat visual cortex contains at least three different families of GABAergic neurons that express parvalbumin (PV), calretinin (CR), and somatostatin (SOM) (Gonchar and Burkhalter, 1997). To examine whether pathway-specific inhibition (Shao and Burkhalter, 1996) is attributable to distinct connections with GABAergic neurons, we traced FF and FB inputs to PV, CR, and SOM neurons in layers 1-2/3 of area 17 and the secondary lateromedial area in rat visual cortex. We found that in layer 2/3 maximally 2% of FF and FB inputs go to CR and SOM neurons. This contrasts with 12-13% of FF and FB inputs onto layer 2/3 PV neurons. Unlike inputs to layer 2/3, connections to layer 1, which contains CR but lacks SOM and PV somata, are pathway-specific: 21% of FB inputs go to CR neurons, whereas FF inputs to layer 1 and its CR neurons are absent. These findings suggest that FF and FB influences on layer 2/3 pyramidal neurons mainly involve disynaptic connections via PV neurons that control the spike outputs to axons and proximal dendrites. Unlike FF input, FB input in addition makes a disynaptic link via CR neurons, which may influence the excitability of distal pyramidal cell dendrites in layer 1.

Parvalbumin-, calbindin-, and calretinin-immunoreactive neurons in the prefrontal cortex of the owl monkey (Aotus trivirgatus): a standardized quantitative comparison with sensory and motor areas

Recent studies have revealed regional variation in the density and distribution of inhibitory neurons in different cortical areas, which are thought to reflect area-specific specializations in cortical circuitry. However, there are as yet few standardized quantitative data regarding how the inhibitory circuitry in prefrontal cortex (PFC), which is thought to be involved in executive functions such as cognition, emotion and decision making, compares to that in other cortical areas. Here we used immunohistochemical techniques to determine the density and distribution of parvalbumin (PV)-, calbindin (CB)-, and calretinin (CR)-immunoreactive (ir) neurons and axon terminals in the dorsolateral and orbital PFC of the owl monkey (Aotus trivirgatus), and compared them directly with data obtained using the same techniques in 11 different visual, somatosensory and motor areas. We found marked differences in the density of PV-ir, CB-ir, and CR-ir interneurons in several cortical areas. One hundred and twenty eight of all 234 possible between-area pair-wise comparisons were significantly different. The density of specific subpopulations of these cells also varied among cortical areas, as did the density of axon terminals. Comparison of PFC with other cortical areas revealed that 40 of all 66 possible statistical comparisons of the density of PV-ir, CB-ir, and CR-ir cells were significantly different. We also found evidence for heterogeneity in the pattern of labeling of PV-ir, CB-ir, and CR-ir cells and axon terminals between the dorsolateral and orbital subdivisions of PFC. These data are likely to reflect basic differences in interneuron circuitry, which are likely to influence inhibitory function in the cortex.

Distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptor subunits (GluR2/3) along the ventral visual pathway in the monkey

By using immunohistochemical methods, we examined the distribution of cells expressing subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-selective glutamate receptors (GluR2/3) in the cortical areas of the occipitotemporal pathway in monkeys. GluR2/3-immunoreactive (-ir) cells were primarily pyramidal cells; this category, however, also included large stellate cells in layer IVB of the striate cortex (V1) and fusiform cells in layer VI of all the areas examined. GluR2/3 immunoreactivity differed among the areas in laminar distribution and intensity. In V1, GluR2/3-ir cells were identified mainly in layers II, III, IVB, and VI. The prestriate areas V2 and V4 and the inferior temporal areas TEO and TE contained GluR2/3-ir cells in layers II, III, and VI. In the TE, GluR2/3-ir cells were also abundant in layer V. In area 36 of the perirhinal cortex, neurons in layers II, III, V, and VI were labeled in a similar manner to the TE labeling, but with greater staining intensity and numbers, especially in layer V. Thus, GluR2/3 immunoreactivity increased rostrally along the pathway. Within V1 and V2, cells strongly stained for GluR2/3 formed clusters that colocalized with cytochrome oxidase (CO)-rich regions. These distinct laminar and regional distribution patterns of GluR2/3 expression may contribute to the specific physiological properties of neurons within various visual areas and compartments.

Distribution of calbindin D-28k in the entorhinal, perirhinal, and parahippocampal cortices of the macaque monkey

We examined the distribution of calbindin D-28k-immunoreactive (CB-IR) neurons, fibers, and neuropil in the entorhinal (area 28), perirhinal (areas 35 and 36), and parahippocampal (areas TH and TF) cortices in the macaque monkey. Two main findings are reported. First, except for CB-IR neurogliaform cells that are only observed in the parahippocampal cortex, the morphology of CB-stained pyramidal and nonpyramidal cells were similar across the three cortical areas examined. Second, we find that the topography of CB staining differed between the three areas. The entorhinal cortex exhibits the most striking gradient of CB staining such that the most anterior and medial portions are most strongly labeled, whereas posterior and lateral areas exhibit only weak labeling. The labeling throughout the perirhinal and parahippocampal cortices is more homogeneous. Area 35 contains only lightly stained neuropil and few CB-IR cells. Area 36 and areas TH and TF of the parahippocampal cortex contain a moderate to high density of CB-IR cells and fibers throughout their full rostrocaudal extents, although each area exhibits unique laminar patterns of staining. In all areas examined, the highest density of CB-positive cells and fibers is observed in superficial layers with lower densities of CB-positive cells and fibers present in deep layers. These findings, taken together with our current understanding of the connections of these areas may have implications for understanding the circuit properties of the entorhinal, perirhinal, and parahippocampal cortices areas in both normal and disease states.

Cortical interneurons: from Cajal to 2001

After the collective work of many investigators, beginning with the early studies of Cajal, the following main [figure: see text] conclusions may be drawn regarding the morphology, biochemical characteristics and synaptic connections of interneurons: 1. Interneurons show a great variety of morphological, biochemical and physiological types. They constitute approximately 15-30% of the total population of neurons. 2. Because of the heterogeneity of interneurons and the lack of consensus as to which characteristics are essential for an individual neuron to be considered a member of a given cell type, there is no definitive classification of interneurons. Nevertheless, certain interneurons can be readily recognized by their unique morphological characteristics, or they can be more generally divided into subgroups on the basis of their biochemical characteristics, patterns of axonal arborization, or synaptic connections with pyramidal cells. 3. All interneurons have a more or less dense axonal arborization distributed near the cell body, mainly within the area occupied by their dendritic field. However, some interneurons may display, in addition, prominent long, horizontal or vertical axonal collaterals. [figure: see text] 4. Most interneurons form symmetrical synapses with both pyramidal cells and other interneurons, with the exception of chandelier cells, which only form synapses with the axon initial segment of pyramidal cells. Furthermore, interneurons are not only connected by chemical synapses (unidirectional connections), but they may also form electrical synapses through gap junctions (bidirectional) in a specific manner. 5. With the exception of chandelier cells, other types of interneurons include among their synaptic targets more than one type of postsynaptic element. But the degree of preference for these postsynaptic elements varies markedly between different types of interneurons. 6. The number of synapses made by a single axon originating from a given interneuron on another neuron is on the order of ten or less. Since, in general, cortical neurons receive many more interneuronal (symmetrical) synapses (on the order of a few hundred or thousand), a considerable convergence of various types of interneurons to pyramidal cells and interneurons appears to occur. 7. Most interneurons are GABAergic and also express a number of different neurotransmitters (or their synthesizing enzymes), neuropeptides and calcium-binding proteins. Thus, interneurons are, biochemically, widely heterogeneous. 8. Some of the morphologically identifiable neurons can be characterized by their particular biochemical characteristics, and some biochemically definable subgroups of interneurons display a particular morphology. However, different morphological types of GABAergic neurons may share one or several neurotransmitters, neuroactive substances and/or other molecular markers. Therefore, a great variety of subgroups of morphologically and biochemically identifiable neurons exist. 9. Some interneurons appear to be common to all species and, therefore, may be considered as basic elements of cortical circuits, whereas others may represent evolutionary specializations which are characteristic of particular mammalian subgroups and, thus, cannot be taken as essential, or general, features of cortical organization. 10. Given the complexity of cortical circuits and the areal and species differences, it is impossible to draw a "sufficiently" complete basic diagram of cortical microcircuitry that is valid for all cortical areas and species.

AMPA GluR2 subunit is differentially distributed on GABAergic neurons and pyramidal cells in the macaque monkey visual cortex

The cellular and synaptic distribution of the AMPA receptor subunit GluR2 was analyzed in the monkey primary visual cortex (area V1), by immunocytochemistry and postembedding immunogold methods. GluR2 immunoreactivity was widely distributed in all of the layers of area V1. A quantitative double labeling analysis in layers II and III revealed that the vast majority of GABAergic interneurons in this area also contained GluR2. Postembedding immunogold analysis revealed that GluR2 immunoreactivity was present at asymmetric synapses on both GABAergic interneurons and pyramidal cells. A quantitative study indicated that the number of GluR2 immunogold particles at asymmetric synapses on pyramidal cells was significantly higher than that on GABAergic interneurons. These results from the primate neocortex are in agreement with and extend our previous studies on the rat hippocampus and amygdala. In view of the dominant role of the GluR2 subunit in regulating calcium flux through AMPA receptors, the differential synaptic distribution of GluR2 on different neuronal types might provide a mechanism for cell-specific response properties to glutamate as well as clues to selective neuronal vulnerability and cell death mediated by calcium-dependent excitotoxic mechanisms.

Direction selectivity of excitatory and inhibitory neurons in ferret visual cortex

Direction selectivity is a characteristic feature of neurons in the visual cortex of higher mammals. Excitatory and inhibitory cortical neurons receive different patterns of synaptic connections resulting in different receptive field properties. We have analyzed the direction tuning of excitatory and inhibitory neurons of ferret visual cortex using single unit recordings. Direction tuning was constant among neurons in a vertical column. The majority (> 80%) of excitatory (regular spiking) neurons were direction tuned or direction biased. Fast spiking (inhibitory) neurons were orientation, but only weakly or not direction tuned. This indicates that excitatory and inhibitory neurons have different functions in visual processing and their different integration in thalamocortical and intracortical circuits results in a diversification of receptive field properties.

Pyramidal cell axons show a local specialization for GABA and 5-HT inputs in monkey and human cerebral cortex

Various mechanisms are thought to control excitation of pyramidal cells of the cerebral cortex. With immunocytochemical methods, we found that the proximal portions of numerous pyramidal cell axons (Pyr-axons) in the human and monkey neocortex are immunoreactive for the serotonin (5-HT) receptor 5-HT-(1A). With double-labeling experiments and confocal laser microscopy, we found that most (93.4%) of the 5-HT(1A)-immunoreactive Pyr-axons present in layers II and III were innervated by parvalbumin-immunoreactive chandelier cell axon terminals. In addition, Pyr-axons were compartmentalized: 5-HT-(1A) receptors were found proximal to inputs from chandelier cells. Although we found close appositions between GABAergic chandelier cell axon terminals and Pyr-axons, suggesting synaptic connections, we did not observe 5-HT-immunoreactive fibers in close proximity to the Pyr-axons. These results suggested that Pyr-axons are under the influence of 5-HT in a paracrine manner (via 5-HT-(1A) receptors) and, more distally, are under the influence of gamma-aminobutyric acid (GABA) in a synaptic manner (through the axons of chandelier cells). The local axonal specialization might represent a powerful inhibitory mechanism by which the responses of large populations of pyramidal cells can be globally controlled by subcortical serotonin afferents, in addition to local inputs from GABAergic interneurons.

Laminar sources of synaptic input to cortical inhibitory interneurons and pyramidal neurons

The functional role of an individual neuron within a cortical circuit is largely determined by that neuron's synaptic input. We examined the laminar sources of local input to subtypes of cortical neurons in layer 2/3 of rat visual cortex using laser scanning photostimulation. We identified three distinct laminar patterns of excitatory input that correspond to physiological and morphological subtypes of neurons. Fast-spiking inhibitory basket cells and excitatory pyramidal neurons received strong excitatory input from middle cortical layers. In contrast, adapting inhibitory interneurons received their strongest excitatory input either from deep layers or laterally from within layer 2/3. Thus, differential laminar sources of excitatory inputs contribute to the functional diversity of cortical inhibitory interneurons.

Cellular heterogeneity in cerebral cortex: a study of the morphology of pyramidal neurones in visual areas of the marmoset monkey

The morphological characteristics of the basal dendritic fields of layer III pyramidal neurones were determined in visual areas in the occipital, parietal, and temporal lobes of adult marmoset monkeys by means of intracellular iontophoretic injection of Lucifer yellow. Neurones in the primary visual area (V1) had the least extensive and least complex (as determined by Sholl analysis) dendritic trees, followed by those in the second visual area (V2). There was a progressive increase in size and complexity of dendritic trees with rostral progression from V1 and V2, through the "ventral stream," including the dorsolateral area (DL) and the caudal and rostral subdivisions of inferotemporal cortex (ITc and ITr, respectively). Neurones in areas of the dorsal stream, including the dorsomedial (DM), dorsoanterior (DA), middle temporal (MT), and posterior parietal (PP) areas, were similar in size and complexity but were larger and more complex than those in V1 and V2. Neurones in V1 had the lowest spine density, whereas neurones in V2, DM, DA, and PP had similar spine densities. Neurones in MT and inferotemporal cortex had relatively high spine densities, with those in ITr having the highest spine density of all neurones studied. Calculations based on the size, number of branches, and spine densities revealed that layer III pyramidal neurones in ITr have 7.4 times more spines on their basal dendritic fields than those in V1. The differences in the extent of, and the number of spines in, the basal dendritic fields of layer III pyramidal neurones in the different visual areas suggest differences in the ability of neurones to integrate excitatory and inhibitory inputs. The differences in neuronal morphology between visual areas, and the consistency in these differences across New World and Old World monkey species, suggest that they reflect fundamental organisational principles in primate visual cortical structure.