Interneurons containing calretinin are specialized to control other interneurons in the rat hippocampus

Gamma oscillations in the entorhinal cortex of the freely behaving rat

Gamma frequency field oscillations (40-100 Hz) are nested within theta oscillations in the dentate-hilar and CA1-CA3 regions of the hippocampus during exploratory behaviors. These oscillations reflect synchronized synaptic potentials that entrain the discharge of neuronal populations within the approximately 10-25 msec range. Using multisite recordings in freely behaving rats, we examined gamma oscillations within the superficial layers (I-III) of the entorhinal cortex. These oscillations increased in amplitude and regularity in association with entorhinal theta waves. Gamma waves showed an amplitude minimum and reversed in phase near the perisomatic region of layer II, indicating that they represent synchronized synaptic potentials impinging on layer II-III neurons. Theta and gamma oscillations in the entorhinal cortex were coupled with theta and gamma oscillations in the dentate hilar region. The majority of layer II-III neurons discharged irregularly but were phase-related to the negative peak of the local (layer II-III) gamma field oscillation. These findings demonstrate that layer II-III neurons discharge in temporally defined gamma windows (approximately 10-25 msec) coupled to the theta cycle. This transient temporal framework, which emerges in both the entorhinal cortex and the hippocampus, may allow spatially distributed subpopulations to form temporally defined ensembles. We speculate that the theta-gamma pattern in the discharge of these neurons is essential for effective neuronal communication and synaptic plasticity in the perforant pathway.

Expression of calretinin in diverse neuronal populations during development of rat hippocampus

The prenatal and postnatal expression of calretinin was studied in hippocampus of the rat using immunohistochemical procedures. Calretinin was detected as early as embryonic day 15 in the primordial hippocampus where calretinin-containing neurons and fibres were localized to the primitive plexiform layer. Upon emergence of the hippocampal plate (the prospective stratum pyramidale), large numbers of immunopositive multipolar cells were observed in the marginal zone. Fewer cells with fusiform cell bodies were observed bordering the hippocampal plate and subplate. During the perinatal period (embryonic day 20 to postnatal day 0), large numbers of immunoreactive pyramidal-like neurons were observed at the margin of the hippocampal plate with the subplate. At this same time, many calretinin-containing neurons with irregularly shaped dendrites were observed in stratum radiatum. Soon after birth (postnatal day 3), the calretinin immunoreactivity of both these later cell types rapidly declined and a new population of calretinin-immunopositive cells emerged, the Cajal-Retzius cells of stratum lacunosum-moleculare and the dentate gyrus. The Cajal-Retzius cells rapidly matured but disappeared by the second postnatal week. During the second postnatal week, calretinin interneurons of the adult hippocampal formation began to appear. Their immunoreactivity increased by postnatal day 15, when the number of calretinin-immunopositive interneurons in area CA1 and stratum radiatum of CA3 exceeded that of the adult. At this time, the soma and proximal dendrites of many calretinin interneurons were found to contact each other. The frequency of such cellular appositions decreased in adulthood. The results presented here show that calretinin immunohistochemistry can be very useful in recording the development of subpopulations of hippocampal neurons that are present during distinct embryonic and postnatal periods. Although some neuronal types may exist only briefly during hippocampal development, others appear to express calretinin transiently during restricted phases of neuronal differentiation. Surprisingly, this includes some hippocampal pyramidal cells. However, even as the adult pattern of immunostaining emerges in week 2, morphological refinement of interneurons continues to take place, which eventually leads to the population of calretinin-containing interneurons of the mature hippocampus.

Synaptic communication among hippocampal interneurons: properties of spontaneous IPSCs in morphologically identified cells

The properties of spontaneous IPSCs (sIPSCs) recorded with whole-cell patch-clamp techniques were investigated in various anatomically identified hippocampal CA1 interneurons and were compared with those recorded in pyramidal cells. Neurons labeled with biocytin or neurobiotin were classified on the basis of their dendritic and axonal arborizations, leading to the identification of previously unknown interneuron types projecting to the dendritic region of pyramidal cells. In most interneurons, the average sIPSCs decayed slower than did those observed in pyramidal cells. The properties of sIPSCs were homogeneous within a given morphologically identified neuron type. Many interneurons had comparable somatic size, location, and dendritic arbor but displayed extremely different axonal projections paralleled by distinct sIPSC properties. Thus, physiological comparisons are only meaningful after the complete morphological identification of the recorded cells. The decay of sIPSCs matched for amplitudes and rise times could vary over 10-fold in a given interneuron, consistent with electrotonic filtering and possibly with different GABAA receptor subunit assemblies present at distinct synapses. Our findings demonstrate an extensive connectivity among hippocampal interneurons through GABAA synapses of various properties that may underlie complex network oscillations at different frequencies.

Interneurons immunoreactive for vasoactive intestinal polypeptide (VIP) are extensively innervated by parvalbumin-containing boutons in rat primary somatosensory cortex

Sensory perception results from the synchronized action of large ensembles of cortical neurons. Receptive field properties of such neurons in sensory areas strongly depend on circuits utilizing the inhibitory amino acid transmitter gamma-aminobutyric acid (GABA). GABAergic neurons often co-localize neuropeptides and/or calcium-binding proteins in a cell-type specific manner. We have taken advantage of this fact to study the synaptic circuitry involving presynaptic parvalbumin-containing boutons (originating from horizontally extensive basket cells) and postsynaptic VIP-immunoreactive GABAergic targets which mostly have a vertically oriented axonal field. Abundant appositions between parvalbumin-immunoreactive boutons and all VIP-stained neurons were observed at the light microscopic level. The numbers of contacts ranged between three and well over 20 for single VIP cells. The higher numbers were especially frequent in the supragranular and granular layers contacting the numerous bipolar, as well as multipolar VIP cells located there; but the VIP-immunoreactive neurons in the infragranular layers were also targeted by parvalbumin-immunoreactive boutons without exception, albeit in more variable, mostly lower numbers. Correlated electron microscopic investigations revealed that virtually all of these light microscopically observed appositions resembled symmetric synaptic specializations. The vast majority were located on the soma or proximal dendrites of the VIP-positive neurons. Since pyramidal cells, in turn, represent a major target for the parvalbumin and VIP synapses, the activation of these synapses may lead to coherent oscillations providing the necessary clock function to synchronize pyramidal cell discharges, both across and within cortical columns.

The expression pattern of somatostatin and calretinin by postnatal hippocampal interneurons is regulated by activity-dependent and -independent determinants

Hippocampal interneurons form distinct populations identified on the basis of their projection pattern and neurochemical characteristics, which includes the expression of specific neuropeptides and/or calcium-binding proteins. The neurochemical maturation of hippocampal interneurons is largely a postnatal event, and factors which govern this maturation are presently unknown. Using slice cultures, we have investigated the role of neuronal activity in regulating the expression of somatostatin and calretinin during the postnatal maturation of hippocampal interneurons. Blocking inhibitory activity with bicuculline, or excitatory activity with 6,7-dinitroquinoxaline-2,3-dione, for 14 days in slice cultures from seven-day-old rat increased and decreased, respectively, the number of somatostatin-immunoreactive neurons. Withdrawal of the blocking agents resulted in a reversal of the effects on somatostatin immunoreactivity. In addition, bicuculline slightly increased the number of calretinin-positive neurons, while 6,7-dinitroquinoxaline-2,3-dione exerted no effect. However, bicuculline and 6,7-dinitroquinoxaline-2,3-dione markedly increased and decreased, respectively, the number of calretinin-labelled axons. Despite activity-linked modifications of immunoreactivity levels, no change in the organotypic location of somatostatin-labelled neurons was observed, whatever the treatment. Double labelling studies demonstrated that somatostatin and calretinin were expressed by different neurons, even when the number of labelled cells was highly increased. These results show that the levels of expression of somatostatin and calretinin in maturing hippocampal interneurons are tuned to the endogenous balance of excitatory and inhibitory activity. In contrast, the neurochemical specificity of each subtype of interneurons does not depend upon variations in neuronal activity.

Synaptic effects of identified interneurons innervating both interneurons and pyramidal cells in the rat hippocampus

GABAergic interneurons sculpt the activity of principal cells and are themselves governed by GABAergic inputs. To determine directly some of the sources and mechanisms of this GABAergic innervation, we have used dual intracellular recordings with biocytin-filled microelectrodes and investigated synaptic interactions between pairs of interneurons in area CA1 of the adult rat hippocampus. Of four synaptically-coupled interneuron-to-interneuron cell pairs, three presynaptic cells were identified as basket cells, preferentially innervating somata and proximal dendrites of pyramidal cells, but one differing from the other two in the laminar distribution of its dendritic and axonal fields. The fourth presynaptic interneuron was located at the border between strata lacunosum moleculare and radiatum, with axon ramifying within stratum radiatum. Action potentials evoked in all four presynaptic interneurons were found to elicit fast hyperpolarizing inhibitory postsynaptic potentials (mean amplitude 0.35 +/- 0.10 mV at a membrane potential of -59 +/- 2.8 mV) in other simultaneously recorded interneurons (n=4). In addition, three of the presynaptic interneurons were also shown to produce similar postsynaptic responses in subsequently recorded pyramidal cells (n=4). Electron microscopic evaluation revealed one of the presynaptic basket cells to form 12 synaptic junctions with the perisomatic domain (seven somatic synapses and five synapses onto proximal dendritic shafts) of the postsynaptic interneuron in addition to innervating the same compartments of randomly-selected local pyramidal cells (50% somatic and 50% proximal dendritic synapses, n=12). In addition, light microscopic analysis also indicated autaptic self-innervation in basket (12 of 12) and bistratified cells (six of six). Electron microscopic investigation of one basket cell confirmed six autaptic junctions made by five of its boutons. Together, these data demonstrate that several distinct types of interneuron have divergent output to both principal cells and local interneurons of the same (basket cells) or different type. The fast synaptic effects, probably mediated by GABA in both postsynaptic interneurons and principal cells are similar. These additional sources of GABA identified here in the input to GABAergic cells could contribute to the differential temporal patterning of distinct GABAergic synaptic networks.

Calretinin-containing neurons in trimethyltin-induced neurodegeneration in the rat hippocampus: an immunocytochemical study

The present study uses immunocytochemistry to investigate the behavior of the calretinin (CR)-containing neuronal subpopulation (interneurons) of the rat hippocampus in neurodegenerative processes induced by the neurotoxicant trimethyltin. Cell counts of CR-immunolabeled interneurons indicated that these cells are spared by the neurotoxicant-induced degeneration, characterized by a generalized neuronal loss, as shown by quantitative analysis after cresyl violet staining.

Synaptic connections of calretinin-immunoreactive neurons in the human neocortex

Previous immunocytochemical studies in the cerebral cortex of various species have shown that the calcium-binding protein calretinin (CR) labels specific subpopulations of nonspiny nonpyramidal cells (interneurons). The present study attempts to characterize morphologically and chemically the microcircuitry of CR-immunoreactive (CR-ir) neurons in the human temporal neocortex. Postembedding immunocytochemistry for CR and GABA and combination immunocytochemistry for CR and nonphosphorylated neurofilament protein (NPNFP) or for CR and the calcium-binding proteins parvalbumin (PV) and calbindin (CB) showed CR multiterminal endings frequently innervating the distal apical dendrite or the cell body and proximal dendrites of NPNFP-ir or CB-ir pyramidal cells, respectively. Cell bodies of interneurons immunoreactive for CB or PV were innervated only occasionally by CR multiterminal endings, whereas certain GABA neurons were surrounded by them. Furthermore, CR-ir axon terminals formed either symmetrical (the majority) or asymmetrical synapses with a variety of postsynaptic elements. These results indicate that different subpopulations of CR interneurons exist that are specialized for selective innervation of somatic or dendritic regions of certain pyramidal and nonpyramidal neurons.

Cholecystokinin increases GABA release by inhibiting a resting K+ conductance in hippocampal interneurons

Cholecystokinin (CCK) is found co-localized with the inhibitory neurotransmitter GABA in interneurons of the hippocampus. Also, CCK receptors are found in abundance in this brain region. The possibility that CCK alters interneuron activity was examined using whole-cell current- and voltage-clamp recordings from visualized interneurons in the stratum radiatum of area CA1 in rat hippocampal slices. The effect of CCK on GABA-mediated IPSCs was also determined in pyramidal neurons. The sulfated octapeptide CCK-8S increased action potential frequency or generated inward currents in the majority of interneurons. These effects of CCK persisted in the presence of tetrodotoxin and cadmium, suggesting that they were direct. Current-voltage plots revealed that CCK-8S inhibited a conductance that was linear across command potentials and reversed near the equilibrium potential for K+ ions. The K+ channel blocker tetraethylammonium (10 mM) generated inward currents similar to those initiated by CCK, and it occluded the effect of the peptide. BaCl2 (1 mM) and 4-aminopyridine (2 mM) did not alter the effect of CCK. The CCKB receptor antagonist PD-135,158 completely blocked the inward currents generated by CCK-8S. CCK also resulted in an increase in spontaneous action potential-dependent IPSC frequency, but no changes in action potential-independent miniature IPSCs or evoked IPSCs in pyramidal neurons. These results provide evidence that CCK can depolarize hippocampal interneurons through the inhibition of a resting K+ conductance, leading to increased tonic inhibition of pyramidal neurons. This action of CCK may contribute to its anticonvulsant properties, as observed in limbic seizure models.

Calretinin neurons in human medial prefrontal cortex (areas 24a,b,c, 32', and 25)

The calcium-binding protein calretinin (CR) is present in a subpopulation of local-circuit neurons in the mammalian cerebral cortex containing gamma-aminobutyric acid. This light microscopic investigation provides a detailed qualitative and quantitative morphological analysis of CR-immunoreactive (CR+) neurons in the medial prefrontal cortex (mPFC; areas 24a,b,c, 32', and 25) of the normal adult human. The morphology of CR+ neurons and their areal and laminar distributions were consistent across human mPFC. The principal organisational features of CR+ labelling were the marked laminar distribution of immunoreactive somata and the predominantly vertical orientation of labelled axon-like and dendritic processes. Several types of CR- neurons were present in layer 1, including horizontally aligned Cajal-Retzius cells. In layers 2-6, CR+ neurons displayed a variety of morphologies: bipolar cells (49% of CR+ population), vertically bitufted cells (35%), and horizontally bitufted cells (3.5%). These neuron types were mainly located in layer 2/upper layer 3, and their dendritic processes were commonly aspiny and sometimes highly beaded. Aspiny (8%) and sparsely spiny multipolar (5%) CR+ neurons were also found. The mean somatic profile diameter of CR+ cells was 11.6 +/- 0.3 microm (mean +/- S.D). CA+ puncta formed pericellular baskets around unlabelled circular somatic profiles in layers 2/3 and around unlabelled pyramidal-shaped somata in layers 5/6. The somatic sizes of these unlabelled cell populations were significantly different. Immunolabelled puncta were also found in close contact with CR+ somata. Cortical depth distribution histograms and laminar thickness measurements defined the proportions of the overall CR- cell population in each layer: 7% in layer 1, 78% in layers 2/3, 14% in layers 5/6, and 1% in the white matter. In the tangential plane, CR+ neurons were distributed uniformly at all levels of the cortex. By using stereological counting procedures on immunoreacted Nissl-stained sections, CR+ neurons were estimated to constitute a mean 8.0% (7.2-8.7%) of the total neuron population in each cortical area. These data are compared with similar information obtained for the mPFC in monkey and rat (Gabbott and Bacon [1996b] J. Comp. Neurol. 364:657-608; Gabbott et al., [1997] J. Comp. Neurol. 377:465-499). This study provides important morphological insights into a neurochemically distinct subclass of local-circuit inhibitory neurons in the human mPFC.

Neurotrophins and activity-dependent plasticity of cortical interneurons

Neocortical and hippocampal GABA-containing interneurons are susceptible to activity-dependent modulation, such as regulation of soma size, numbers of synaptic contacts, and levels of GABA or neuropeptide expression. In vitro, the effects of neuronal activity on morphology and gene expression of GABA-containing neurons are mimicked, in part, by members of the neurotrophin gene family, such as brain-derived neurotrophic factor (BDNF). In the neocortex and hippocampus, BDNF is synthesized and secreted in an activity-dependent manner by pyramidal neurons, the target cells of GABA-containing neurons, suggesting that BDNF is an activity-dependent, target-derived trophic factor for these interneurons. In support of this, neuronal activity fails to upregulate the expression of neuropeptide Y in hippocampal cultures from BDNF-deficient mice. We, therefore, hypothesize that neurotrophins might mediate some of the actions of neuronal activity on GABA-containing neurons.

Distribution of nonprincipal neurons in the rat hippocampus, with special reference to their dorsoventral difference

In the present study we examined the distribution of chemically identified subpopulations of nonprincipal neurons in the rat hippocampus, focusing on the dorsoventral differences in their distributions. The subpopulations analyzed were those immunoreactive for parvalbumin, calretinin, nitric oxide synthase, somatostatin, calbindin D28K, vasoactive intestinal polypeptide and cholecystokinin. Using a confocal laser scanning light microscope, we could confirm that the penetration of each immunostaining, except that of calbindin D28K, was complete throughout 50 microns thick sections under our immunostaining conditions. We counted numbers of immunoreactive somata according to the 'dissector' principle, measured areas of hippocampal subdivisions and the thickness of sections, and estimated the approximate numerical densities of these subpopulations, especially for those neurons immunoreactive for nitric oxide synthase, calretinin, somatostatin and parvalbumin. Generally speaking, neurons immunoreactive for parvalbumin showed no significant dorsoventral differences in the numerical densities in any of the subdivisions of the hippocampus, whereas the numerical densities of somata immunoreactive for calretinin, nitric oxide synthase and somatostatin were significantly larger in ventral levels than at dorsal levels of the hippocampus. The numerical density of somatostatin neurons was significantly larger in ventral levels than in dorsal levels of the denate gyrus, and, although not prominent, of the CA1 region. That of nitric oxide synthase positive neurons was significantly larger in ventral levels than in dorsal levels of the CA3 region as well as of the DG but not of the CA1 region. The numerical density of calretinin positive neurons was larger in ventral levels than in dorsal levels of all hippocampal subdivisions. The present study also revealed that dorsal and ventral levels of the hippocampus differ from each other in the composition of their nonprincipal neurons.

Local-circuit neurones in the medial prefrontal cortex (areas 25, 32 and 24b) in the rat: morphology and quantitative distribution

This paper is a light microscopical study describing the detailed morphology and quantitative distribution of local circuit neurones in areas 25, 32, and 24b of the medial prefrontal cortex (mPFC) in the rat. Cortical interneurones were identified immunocytochemically by their expression of calretinin (CR), parvalbumin (PV), and calbindin D-28k (CB) immunoreactivity. Neurones immunoreactive for gamma-aminobutyric acid (GABA) were also investigated, as were interneurones containing reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase activity. Several distinct classes of CR+, PV+, and CB+ neurones were identified; the most frequent were: bipolar/bitufted CR+ cells in upper layer 3; multipolar PV+ neurones in layers 3 and 5; and bitufted/multipolar CB+ neurones in lower layer 3. CB+ neurones resembling Martinotti and neurogliaform cells were also present in layers 5/6. The morphologies and depth distributions of each cell type were consistent across the three areas of mPFC studied. Seven classes of diaphorase-reactive mPFC neurone are described; these cells were composed about 0.8% of the total neurone population and had a peak distribution located in mid- to lower layer 5 in each area. In areas 32 and 25, three defined bands of diffuse NADPH diaphorase staining were located in layer 2 and in upper and deep layer 5. Diaphorase reactivity was very infrequently colocalised with either CR, PV, or CB immunoreactivities. The numerical densities of neurones (N(V), number of cells per mm3) in each layer were calculated stereologically. The mean total neuronal N(V) estimate for areas 25, 32, and 24b was 51,603 +/- 3,324 (mean +/- S.D.; n = 8). Significant interareal differences were detected. From cortical thickness data and neuronal N(V) estimates, the absolute number of neurones under 1 mm2 of cortical surface (N(C)) have been derived. The mean N(C) value for areas 25, 32, and 24b was 57,328 +/- 7,505 neurones. In immunolabelled Nissl-stained sections, CR+ neurones constituted an overall 4.0%, PV+ cells 5.6%, and CB+ 3.4% of the total neurone populations in mPFC. GABA+ cells represented a mean of 16.2% (14.8-17.2%) of neurones in areas 25, 32 and 24b. The absolute numbers of CR+, PV+, CB+, and GABA+ neurones within individual layers in a column of cortex under 1 mm2 of cortical surface (N(L)) have also been derived, with significant interareal differences in N(L) values being detected. The data provide the structural basis for a qualitative and quantitative definition of local cortical circuits in the rat mPFC.

Epilepsy in the developing brain: lessons from the laboratory and clinic

Children with epilepsy present unique challenges to the clinician. In addition to having differences in clinical and EEG phenomena, children differ from adults in regard to etiological factors, response to antiepileptic drugs (AEDs), and outcome. It is now recognized that the immature brain also differs from the mature brain in the basic mechanisms of epileptogenesis and propagation of seizures. The immature brain is more prone to seizures due to an imbalance between excitation and inhibition. gamma-Aminobutyric acid (GABA), the major CNS inhibitory neurotransmitter in the mature brain, can lead to depolarization in the hippocampal CA3 region in very young rats. There are also age-related differences in response to GABA agonists and antagonists in the substantia nigra, a structure important in the propagation of seizures. These age-related differences in response to GABAergic agents provide further evidence that the pathophysiology of seizures in the immature brain differs from that in the mature brain. Although prolonged seizures can cause brain damage at any age, the extent of brain damage after prolonged seizures is highly age dependent. Far less histological damage and fewer disturbances in cognition result from prolonged seizures in the immature brain than from seizures of similar duration and intensity in mature animals. However, detrimental effects of AEDs may be greater in the immature brain, than in the mature brain. These lessons from the animal laboratory raise questions about the appropriateness of current therapeutic approaches to childhood seizure disorders.

Gamma oscillation by synaptic inhibition in a hippocampal interneuronal network model

Fast neuronal oscillations (gamma, 20-80 Hz) have been observed in the neocortex and hippocampus during behavioral arousal. Using computer simulations, we investigated the hypothesis that such rhythmic activity can emerge in a random network of interconnected GABAergic fast-spiking interneurons. Specific conditions for the population synchronization, on properties of single cells and the circuit, were identified. These include the following: (1) that the amplitude of spike afterhyperpolarization be above the GABAA synaptic reversal potential; (2) that the ratio between the synaptic decay time constant and the oscillation period be sufficiently large; (3) that the effects of heterogeneities be modest because of a steep frequency-current relationship of fast-spiking neurons. Furthermore, using a population coherence measure, based on coincident firings of neural pairs, it is demonstrated that large-scale network synchronization requires a critical (minimal) average number of synaptic contacts per cell, which is not sensitive to the network size. By changing the GABAA synaptic maximal conductance, synaptic decay time constant, or the mean external excitatory drive to the network, the neuronal firing frequencies were gradually and monotonically varied. By contrast, the network synchronization was found to be high only within a frequency band coinciding with the gamma (20-80 Hz) range. We conclude that the GABAA synaptic transmission provides a suitable mechanism for synchronized gamma oscillations in a sparsely connected network of fast-spiking interneurons. In turn, the interneuronal network can presumably maintain subthreshold oscillations in principal cell populations and serve to synchronize discharges of spatially distributed neurons.

Dense GABAergic input on somata of parvalbumin-immunoreactive GABAergic neurons in the hippocampus of the mouse

GABAergic neurons in the hippocampus proper are greatly diverse in their morphological and physiological features. In the present study we examined whether or not they are also diverse regarding the density of GABAergic input on their somata. GABAergic neurons were immunocytochemically identified with antibodies against glutamic acid decarboxylase (GAD), and the densities of GAD-immunoreactive (GAD-IR) boutons that abutted on GAD-IR somata were estimated by conventional light microscopic, combined light and electron microscopic, and confocal laser scanning microscopic analyses. GAD-IR somata were apparently diverse regarding the density of GABAergic input on them, and those surrounded by higher densities of GAD-IR boutons were distributed mainly in the strata pyramidale and oriens of the CA3 and CA1 regions and could be correlated to a parvalbumin (PV)-IR subpopulation of GABAergic neurons. Quantitative analysis clearly revealed the statistically significant difference between PV-positive and PV-negative GAD-IR neurons in the densities of their somatic GAD-IR boutons. Particularly, most of PV-IR neurons in the CA3 stratum pyramidale as well as some in other layers are characterized by an exceedingly high density of perisomatic GAD-IR boutons. Furthermore, the majority of GAD-IR boutons on PV-IR somata in the stratum pyramidale were also PV-IR. Bilateral transection of the fimbria-fornix, which was supposed to remove GABAergic afferents from the septum, had only partial effects on the densities of PV-IR boutons on PV-IR somata, indicating these PV-IR boutons mainly originated from intrinsic PV-IR neurons. These observations indicate the dense mutual connection between PV-IR GABAergic neurons through perisomatic synaptic contacts, particularly in the stratum pyramidale.

Pyramidal cell dendrites are the primary targets of calbindin D28k-immunoreactive interneurons in the hippocampus

The axonal arborization and postsynaptic targets of calbindin D28k (CB)-immunoreactive nonprincipal neurons have been studied in the rat dorsal hippocampus. Two types of neurons were distinguished on the basis of soma location, the characteristics of the dendritic free, and the axon arborisation pattern. Type I cells were located in stratum radiatum of the CA1 and CA3 regions and occasionally in strata pyramidale and oriens. These cells had multipolar or bitufted dendritic trees primarily located in stratum radiatum. Their axons could be followed for a considerable distance, arborised within stratum radiatum, and were covered with regularly spaced small boutons. As demonstrated with postembedding immunogold staining, their axon terminals were gamma-aminobutyric acid (GABA) immunoreactive, and formed symmetrical synapses predominantly on proximal and distal dendrites of pyramidal cells (28% and 58%, respectively), and occasionally on spines (9%) or on GABA-positive dendrites (5%). Type II cells were found exclusively in stratum oriens of the CA1 and CA3 regions and possessed large, fusiform cell bodies and long, horizontally oriented dendrites. Their axon initial segments turned towards the alveus and disappeared in a myelin sheet, which was often possible to follow into the white matter. We conclude that type I CB-immunoreactive cells are likely to represent a major source of inhibitory synapses in the dendritic region of pyramidal cells, which are responsible for the control of dendritic electrogenesis. The distribution of local collaterals of type II cells-if they have any-remains unknown, but their main axon is likely to project to the medial septum.