Ultrastructural study of cholecystokinin-immunoreactive cells and processes in area CA1 of the rat hippocampus

NECAB1 and NECAB2 are Prevalent Calcium-Binding Proteins of CB1/CCK-Positive GABAergic Interneurons

The molecular repertoire of the "Ca2+-signaling toolkit" supports the specific kinetic requirements of Ca2+-dependent processes in different neuronal types. A well-known example is the unique expression pattern of calcium-binding proteins, such as parvalbumin, calbindin, and calretinin. These cytosolic Ca2+-buffers control presynaptic and somatodendritic processes in a cell-type-specific manner and have been used as neurochemical markers of GABAergic interneuron types for decades. Surprisingly, to date no typifying calcium-binding proteins have been found in CB1 cannabinoid receptor/cholecystokinin (CB1/CCK)-positive interneurons that represent a large population of GABAergic cells in cortical circuits. Because CB1/CCK-positive interneurons display disparate presynaptic and somatodendritic Ca2+-transients compared with other interneurons, we tested the hypothesis that they express alternative calcium-binding proteins. By in silico data mining in mouse single-cell RNA-seq databases, we identified high expression of Necab1 and Necab2 genes encoding N-terminal EF-hand calcium-binding proteins 1 and 2, respectively, in CB1/CCK-positive interneurons. Fluorescent in situ hybridization and immunostaining revealed cell-type-specific distribution of NECAB1 and NECAB2 throughout the isocortex, hippocampal formation, and basolateral amygdala complex. Combination of patch-clamp electrophysiology, confocal, and STORM super-resolution microscopy uncovered subcellular nanoscale differences indicating functional division of labor between the two calcium-binding proteins. These findings highlight NECAB1 and NECAB2 as predominant calcium-binding proteins in CB1/CCK-positive interneurons.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Estradiol regulates large dense core vesicles in the hippocampus of adult female rats

Previous work has shown that the steroid hormone estradiol facilitates the release of anticonvulsant neuropeptides from inhibitory neurons in the hippocampus to suppress seizures. Because neuropeptides are packaged in large dense core vesicles, estradiol may facilitate neuropeptide release through regulation of dense core vesicles. In the current study, we used serial section electron microscopy in the hippocampal CA1 region of adult female rats to test three hypotheses about estradiol regulation of dense core vesicles: (1) Estradiol increases the number of dense core vesicles in axonal boutons, (2) Estradiol increases the size of dense core vesicles in axonal boutons, (3) Estradiol shifts the location of dense core vesicles toward the periphery of axonal boutons, potentially lowering the threshold for neuropeptide release during seizures. We found that estradiol increases the number and size of dense core vesicles in inhibitory axonal boutons, consistent with increased neuropeptide content, but does not shift the location of dense core vesicles closer to the bouton periphery. These effects were specific to large dense core vesicles (>80 nm) in inhibitory boutons. Estradiol had no effects on small dense core vesicles or dense core vesicles in excitatory boutons. Our results indicate that estradiol suppresses seizures at least in part by increasing the potentially releasable pool of neuropeptides in the hippocampus, and that estradiol facilitation of neuropeptide release involves a mechanism other than mobilization of dense core vesicles toward sites of release.

Ultrastructure of synapses in the mammalian brain

The morphology and molecular composition of synapses provide the structural basis for synaptic function. This article reviews the electron microscopy of excitatory synapses on dendritic spines, using data from rodent hippocampus, cerebral cortex, and cerebellar cortex. Excitatory synapses have a prominent postsynaptic density, in contrast with inhibitory synapses, which have less dense presynaptic or postsynaptic specializations and are usually found on the cell body or proximal dendritic shaft. Immunogold labeling shows that the presynaptic active zone provides a scaffold for key molecules involved in the release of neurotransmitter, whereas the postsynaptic density contains ligand-gated ionic channels, other receptors, and a complex network of signaling molecules. Delineating the structure and molecular organization of these axospinous synapses represents a crucial step toward understanding the mechanisms that underlie synaptic transmission and the dynamic modulation of neurotransmission associated with short- and long-term synaptic plasticity.

Terminal field and firing selectivity of cholecystokinin-expressing interneurons in the hippocampal CA3 area

Hippocampal oscillations reflect coordinated neuronal activity on many timescales. Distinct types of GABAergic interneuron participate in the coordination of pyramidal cells over different oscillatory cycle phases. In the CA3 area, which generates sharp waves and gamma oscillations, the contribution of identified GABAergic neurons remains to be defined. We have examined the firing of a family of cholecystokinin-expressing interneurons during network oscillations in urethane-anesthetized rats and compared them with firing of CA3 pyramidal cells. The position of the terminals of individual visualized interneurons was highly diverse, selective, and often spatially coaligned with either the entorhinal or the associational inputs to area CA3. The spike timing in relation to theta and gamma oscillations and sharp waves was correlated with the innervated pyramidal cell domain. Basket and dendritic-layer-innervating interneurons receive entorhinal and associational inputs and preferentially fire on the ascending theta phase, when pyramidal cell assemblies emerge. Perforant-path-associated cells, driven by recurrent collaterals of pyramidal cells fire on theta troughs, when established pyramidal cell assemblies are most active. In the CA3 area, slow and fast gamma oscillations occurred on opposite theta oscillation phases. Perforant-path-associated and some COUP-TFII-positive interneurons are strongly coupled to both fast and slow gamma oscillations, but basket and dendritic-layer-innervating cells are weakly coupled to fast gamma oscillations only. During sharp waves, different interneuron types are activated, inhibited, or remain unaffected. We suggest that specialization in pyramidal cell domain and glutamatergic input-specific operations, reflected in the position of GABAergic terminals, is the evolutionary drive underlying the diversity of cholecystokinin-expressing interneurons.

Increased cholecystokinin labeling in the hippocampus of a mouse model of epilepsy maps to spines and glutamatergic terminals

The neuropeptide cholecystokinin (CCK) is abundant in the CNS and is expressed in a subset of inhibitory interneurons, particularly in their axon terminals. The expression profile of CCK undergoes numerous changes in several models of temporal lobe epilepsy. Previous studies in the pilocarpine model of epilepsy have shown that CCK immunohistochemical labeling is substantially reduced in several regions of the hippocampal formation, consistent with decreased CCK expression as well as selective neuronal degeneration. However, in a mouse pilocarpine model of recurrent seizures, increases in CCK-labeling also occur and are especially striking in the hippocampal dendritic layers of strata oriens and radiatum. Characterizing these changes and determining the cellular basis of the increased labeling were the major goals of the current study. One possibility was that the enhanced CCK labeling could be associated with an increase in GABAergic terminals within these regions. However, in contrast to the marked increase in CCK-labeled structures, labeling of GABAergic axon terminals was decreased in the dendritic layers. Likewise, cannabinoid receptor 1-labeled axon terminals, many of which are CCK-containing GABAergic terminals, were also decreased. These findings suggested that the enhanced CCK labeling was not due to an increase in GABAergic axon terminals. The subcellular localization of CCK immunoreactivity was then examined using electron microscopy, and the identities of the structures that formed synaptic contacts were determined. In pilocarpine-treated mice, CCK was observed in dendritic spines and these were proportionally increased relative to controls, whereas the proportion of CCK-labeled terminals forming symmetric synapses was decreased. In addition, CCK-positive axon terminals forming asymmetric synapses were readily observed in these mice. Double labeling with vesicular glutamate transporter 1 and CCK revealed colocalization in numerous terminals forming asymmetric synapses, confirming the glutamatergic identity of these terminals. These data raise the possibility that expression of CCK is increased in hippocampal pyramidal cells in mice with recurrent, spontaneous seizures.

Coordination of size and number of excitatory and inhibitory synapses results in a balanced structural plasticity along mature hippocampal CA1 dendrites during LTP

Enlargement of dendritic spines and synapses correlates with enhanced synaptic strength during long-term potentiation (LTP), especially in immature hippocampal neurons. Less clear is the nature of this structural synaptic plasticity on mature hippocampal neurons, and nothing is known about the structural plasticity of inhibitory synapses during LTP. Here the timing and extent of structural synaptic plasticity and changes in local protein synthesis evidenced by polyribosomes were systematically evaluated at both excitatory and inhibitory synapses on CA1 dendrites from mature rats following induction of LTP with theta-burst stimulation (TBS). Recent work suggests dendritic segments can act as functional units of plasticity. To test whether structural synaptic plasticity is similarly coordinated, we reconstructed from serial section transmission electron microscopy all of the spines and synapses along representative dendritic segments receiving control stimulation or TBS-LTP. At 5 min after TBS, polyribosomes were elevated in large spines suggesting an initial burst of local protein synthesis, and by 2 h only those spines with further enlarged synapses contained polyribosomes. Rapid induction of synaptogenesis was evidenced by an elevation in asymmetric shaft synapses and stubby spines at 5 min and more nonsynaptic filopodia at 30 min. By 2 h, the smallest synaptic spines were markedly reduced in number. This synapse loss was perfectly counterbalanced by enlargement of the remaining excitatory synapses such that the summed synaptic surface area per length of dendritic segment was constant across time and conditions. Remarkably, the inhibitory synapses showed a parallel synaptic plasticity, also demonstrating a decrease in number perfectly counterbalanced by an increase in synaptic surface area. Thus, TBS-LTP triggered spinogenesis followed by loss of small excitatory and inhibitory synapses and a subsequent enlargement of the remaining synapses by 2 h. These data suggest that dendritic segments coordinate structural plasticity across multiple synapses and maintain a homeostatic balance of excitatory and inhibitory inputs through local protein-synthesis and selective capture or redistribution of dendritic resources.

Ultrastructural analysis of hippocampal neuropil from the connectomics perspective

Complete reconstructions of vertebrate neuronal circuits on the synaptic level require new approaches. Here, serial section transmission electron microscopy was automated to densely reconstruct four volumes, totaling 670 μm(3), from the rat hippocampus as proving grounds to determine when axo-dendritic proximities predict synapses. First, in contrast with Peters' rule, the density of axons within reach of dendritic spines did not predict synaptic density along dendrites because the fraction of axons making synapses was variable. Second, an axo-dendritic touch did not predict a synapse; nevertheless, the density of synapses along a hippocampal dendrite appeared to be a universal fraction, 0.2, of the density of touches. Finally, the largest touch between an axonal bouton and spine indicated the site of actual synapses with about 80% precision but would miss about half of all synapses. Thus, it will be difficult to predict synaptic connectivity using data sets missing ultrastructural details that distinguish between axo-dendritic touches and bona fide synapses.

Synaptic cross talk between perisomatic-targeting interneuron classes expressing cholecystokinin and parvalbumin in hippocampus

Cholescystokinin (CCK)- or parvalbumin (PV)-containing interneurons are the major perisomatic-targeting interneurons in the cerebral cortex, including hippocampus, and are thought to form mutually exclusive networks. We used several techniques to test the alternative hypothesis that CCK and PV cells are coupled by chemical synapses. Triple immunofluorescence confocal microscopy revealed numerous axosomatic, axodendritic, and axoaxonic contacts stained for CCK, PV, and the presynaptic marker synaptophysin. The existence of mutual CCK and PV synapses was supported by dual EM immunolabeling. Paired whole-cell recordings detected unitary GABA(A)ergic synaptic transmission between identified CCK and PV cells, and single CCK cells could transiently inhibit action potential firing of synaptically coupled PV cells. We conclude that the major hippocampal perisomatic-targeting interneurons communicate synaptically. This communication should affect neuronal network activity, including neuronal oscillations, in which the CCK and PV cells have well established roles. The prevalence of CCK and PV networks in other brain regions suggests that internetwork interactions could be generally important.

Synaptic kainate receptors tune oriens-lacunosum moleculare interneurons to operate at theta frequency

GABAergic interneurons of the hippocampus play an important role in the generation of behaviorally relevant network oscillations. Among this heterogeneous neuronal population, somatostatin (SOM)-positive oriens-lacunosum moleculare (O-LM) interneurons are remarkable because they are tuned to operate at theta frequencies (6-10 Hz) in vitro and in vivo. Recent studies show that a high proportion of glutamatergic synapses that impinge on O-LM interneurons are mediated by kainate receptors (KA-Rs). In the present study, we thus tested the hypothesis that KA-Rs transmit afferent inputs in O-LM neurons during synaptic stimulation at theta frequency. We combined multibeam two-photon calcium imaging in hippocampal slices from SOM-enhanced green fluorescent protein (EGFP) mice, to record the activity of SOM cells as well as hundreds of neurons simultaneously, and targeted electrophysiological recordings and morphological analysis to describe the morphofunctional features of particular cells. We report that EGFP-positive O-LM neurons are the only subtype of interneuron that reliably follows synaptic stimulation of the alveus in the theta frequency range. Electrophysiological recordings revealed the crucial contribution of KA-Rs to the firing activity and to the glutamatergic response to theta stimuli in O-LM cells compared with other cell types. The reliable activation of O-LM cells in the theta frequency range did not simply result from the longer kinetics of KA-R-mediated postsynaptic events (EPSP(KA)) but presumably from a specific interaction between EPSP(KA) and their intrinsic active membrane properties. Such preferential processing of excitatory inputs via KA-Rs by distally projecting GABAergic microcircuits could provide a key role in theta band frequency oscillations.

Estrogen mobilizes a subset of estrogen receptor-alpha-immunoreactive vesicles in inhibitory presynaptic boutons in hippocampal CA1

Although the classical mechanism of estrogen action involves activation of nuclear transcription factor receptors, estrogen also has acute effects on neuronal signaling that occur too rapidly to involve gene expression. These rapid effects are likely to be mediated by extranuclear estrogen receptors associated with the plasma membrane and/or cytoplasmic organelles. Here we used a combination of serial-section electron microscopic immunocytochemistry, immunofluorescence, and Western blotting to show that estrogen receptor-alpha is associated with clusters of vesicles in perisomatic inhibitory boutons in hippocampal CA1 and that estrogen treatment mobilizes these vesicle clusters toward synapses. Estrogen receptor-alpha is present in approximately one-third of perisomatic inhibitory boutons, and specifically in those that express cholecystokinin, not parvalbumin. We also found a high degree of extranuclear estrogen receptor-alpha colocalization with neuropeptide Y. Our results suggest a novel mode of estrogen action in which a subset of vesicles within a specific population of inhibitory boutons responds directly to estrogen by moving toward synapses. The mobilization of these vesicles may influence acute effects of estrogen mediated by estrogen receptor-alpha signaling at inhibitory synapses.

Interneurons targeting similar layers receive synaptic inputs with similar kinetics

GABAergic interneurons play diverse and important roles in controlling neuronal network dynamics. They are characterized by an extreme heterogeneity morphologically, neurochemically, and physiologically, but a functionally relevant classification is still lacking. Present taxonomy is essentially based on their postsynaptic targets, but a physiological counterpart to this classification has not yet been determined. Using a quantitative analysis based on multidimensional clustering of morphological and physiological variables, we now demonstrate a strong correlation between the kinetics of glutamate and GABA miniature synaptic currents received by CA1 hippocampal interneurons and the laminar distribution of their axons: neurons that project to the same layer(s) receive synaptic inputs with similar kinetics distributions. In contrast, the kinetics distributions of GABAergic and glutamatergic synaptic events received by a given interneuron do not depend upon its somatic location or dendritic arborization. Although the mechanisms responsible for this unexpected observation are still unclear, our results suggest that interneurons may be programmed to receive synaptic currents with specific temporal dynamics depending on their targets and the local networks in which they operate.

Complementary roles of cholecystokinin- and parvalbumin-expressing GABAergic neurons in hippocampal network oscillations

In the hippocampal CA1 area, a relatively homogenous population of pyramidal cells is accompanied by a diversity of GABAergic interneurons. Previously, we found that parvalbumin-expressing basket, axo-axonic, bistratified, and oriens-lacunosum moleculare cells, innervating different domains of pyramidal cells, have distinct firing patterns during network oscillations in vivo. A second family of interneurons, expressing cholecystokinin but not parvalbumin, is known to target the same domains of pyramidal cells as do the parvalbumin cells. To test the temporal activity of these independent and parallel GABAergic inputs, we recorded the precise spike timing of identified cholecystokinin interneurons during hippocampal network oscillations in anesthetized rats and determined their molecular expression profiles and synaptic targets. The cells were cannabinoid receptor type 1 immunopositive. Contrary to the stereotyped firing of parvalbumin interneurons, cholecystokinin-expressing basket and dendrite-innervating cells discharge, on average, with 1.7 +/- 2.0 Hz during high-frequency ripple oscillations in an episode-dependent manner. During theta oscillations, cholecystokinin-expressing interneurons fire with 8.8 +/- 3.3 Hz at a characteristic time on the ascending phase of theta waves (155 +/- 81 degrees), when place cells start firing in freely moving animals. The firing patterns of some interneurons recorded in drug-free behaving rats were similar to cholecystokinin cells in anesthetized animals. Our results demonstrate that cholecystokinin- and parvalbumin-expressing interneurons make different contributions to network oscillations and play distinct roles in different brain states. We suggest that the specific spike timing of cholecystokinin interneurons and their sensitivity to endocannabinoids might contribute to differentiate subgroups of pyramidal cells forming neuronal assemblies, whereas parvalbumin interneurons contribute to synchronizing the entire network.

Defined types of cortical interneurone structure space and spike timing in the hippocampus

The cerebral cortex encodes, stores and combines information about the internal and external environment in rhythmic activity of multiple frequency ranges. Neurones of the cortex can be defined, recognized and compared on the comprehensive application of the following measures: (i) brain area- and cell domain-specific distribution of input and output synapses, (ii) expression of molecules involved in cell signalling, (iii) membrane and synaptic properties reflecting the expression of membrane proteins, (iv) temporal structure of firing in vivo, resulting from (i)-(iii). Spatial and temporal measures of neurones in the network reflect an indivisible unity of evolutionary design, i.e. neurones do not have separate structure or function. The blueprint of this design is most easily accessible in the CA1 area of the hippocampus, where a relatively uniform population of pyramidal cells and their inputs follow an instantly recognizable laminated pattern and act within stereotyped network activity patterns. Reviewing the cell types and their spatio-temporal interactions, we suggest that CA1 pyramidal cells are supported by at least 16 distinct types of GABAergic neurone. During a given behaviour-contingent network oscillation, interneurones of a given type exhibit similar firing patterns. During different network oscillations representing two distinct brain states, interneurones of the same class show different firing patterns modulating their postsynaptic target-domain in a brain-state-dependent manner. These results suggest roles for specific interneurone types in structuring the activity of pyramidal cells via their respective target domains, and accurately timing and synchronizing pyramidal cell discharge, rather than providing generalized inhibition. Finally, interneurones belonging to different classes may fire preferentially at distinct time points during a given oscillation. As different interneurones innervate distinct domains of the pyramidal cells, the different compartments will receive GABAergic input differentiated in time. Such a dynamic, spatio-temporal, GABAergic control, which evolves distinct patterns during different brain states, is ideally suited to regulating the input integration of individual pyramidal cells contributing to the formation of cell assemblies and representations in the hippocampus and, probably, throughout the cerebral cortex.

Immunocytochemically defined interneuron populations in the hippocampus of mouse strains used in transgenic technology

Transgenic mice are overtaking the role of model animals in neuroscience. They are used in developmental, anatomical, and physiological as well as experimental neurology. However, most results on the organization of the nervous system derive from the rat. The rat hippocampus and its neuronal elements have been thoroughly investigated, revealing remarkable functional and morphological diversity and specificity among hippocampal interneurons. Our aim was to examine the properties of distinct hippocampal interneuron populations, i.e., those immunoreactive for calcium-binding proteins (parvalbumin, calbindin, and calretinin), neuropeptides (cholecystokinin, neuropeptide Y, somatostatin, vasoactive intestinal polypeptide), and certain receptors (metabotropic glutamate receptor 1alpha, cannabinoid receptor type 1) in four strains of mice widely used in transgenic technology, and to compare their properties to those in the rat. Our data indicate that the distribution as well as the dendritic and axonal arborization of mouse interneurons immunoreactive for the different markers was identical in the examined mouse strains, and in most respects are similar to the features found in the rat. The postsynaptic targets of neurons terminating in the perisomatic (parvalbumin), proximal (calbindin), and distal (somatostatin) dendritic region, as well as on other interneurons (calretinin), also matched those found in the rat. However, a few significant differences could also be observed between the two species in addition to the already described immunoreactivity of mossy cells for calretinin: the absence of spiny calretinin-immunoreactive interneurons in the CA3 region, sparse contacts between calretinin-immunoreactive interneurons, and the axon staining for somatostatin and neuropil labeling for cholecystokinin. We can conclude that the morphofunctional classification of interneurons established in the rat is largely valid for mouse strains used in transgenic procedures.

Convergence of excitatory and inhibitory inputs onto CCK-containing basket cells in the CA1 area of the rat hippocampus

The number and distribution of excitatory and inhibitory inputs affect the integrative properties of neurons. These parameters have been studied recently for several hippocampal neuron populations. Besides parvalbumin- (PV) containing cells that include basket and axo-axonic cells, cholecystokinin (CCK)-containing interneurons also form a basket cell population with several properties distinct from PV cells. Here, at the light microscopic level, we reconstructed the entire dendritic tree of CCK-immunoreactive (IR) basket cells to describe their geometry, the total length and laminar distribution of their dendrites. This was followed by an electron microscopic analysis of serial ultrathin sections immunostained against gamma-aminobutyric acid, to estimate the density of excitatory and inhibitory synapses on their somata, axon initial segments and different subclasses of dendrites. The dendritic tree of CCK-IR basket cells has an average length of 6300 microm and penetrates all layers. At the electron microscopic level, CCK basket cells receive dendritic inputs with a density of 80-230 per 100 microm. The ratio of inhibitory inputs is relatively high (35%) and increases towards the soma (83%). The total numbers of excitatory and inhibitory synapses converging onto CCK-IR cells are approximately 8200. Comparison of the two, neurochemically distinct basket cells reveals that CCK-containing basket cells receive much less synaptic input than PV cells; however, the relative weight of inhibition is higher on CCK cells. Additional differences in their anatomical and physiological properties predict that CCK basket cells are under a more diverse, elaborate control than PV basket cells, and thus the function of the two populations must be different.

Postnatal development and migration of cholecystokinin-immunoreactive interneurons in rat hippocampus

The development of cholecystokinin-immunoreactive (CCK-IR) interneurons in the rat hippocampus was studied using immunocytochemical methods at the light and electron microscopic levels from early (P0-P8) to later postnatal (P12-P20) periods. The laminar distribution of CCK-IR cell bodies changed considerably during the studied period, which is suggested to be due to migration. CCK-IR cells appear to move from the molecular layer of the dentate gyrus to their final destination at the stratum granulosum/hilus border, and tend to concentrate in the distal third of stratum radiatum in CA1-3. The density of CCK-IR cells is rapidly decreasing during the first 4 postnatal days without any apparent reduction in their total number, therefore it is due to the pronounced growth of hippocampal volume in this period. Axons of CCK-IR interneurons formed symmetrical synapses already at P0, and by far the predominant targets were dendrites of presumed principal cells in all subfields of the hippocampus. These axon arbors began to concentrate around pyramidal cell bodies only at P8, at earlier ages CCK-IR axons crossed stratum pyramidale at right angles, and gave rise to varicose collaterals only outside this layer. The dendrites and somata of CCK-IR cells received synapses already at P0, but those were mostly symmetrical, apart from a few immature asymmetrical synapses. At P4, mature asymmetrical synapses with considerable amounts of synaptic vesicles were already commonly encountered. Thus, the innervation of CCK-IR interneurons apparently develops later than their output synapses, suggesting that they may be able to release transmitter before receiving any considerable excitatory drive. We conclude that CCK-IR cells represent one, if not the major, interneuron type that assists in the maturation of glutamatergic synapses (activation of N-methyl-D-aspartate receptors) via GABAergic depolarization of principal cell dendrites, and may contribute to the generation of giant depolarizing potentials. CCK-IR cells will change their function to perisomatic hyperpolarizing inhibition, as glutamatergic transmission in the network becomes operational.

Functional localization of cannabinoid receptors and endogenous cannabinoid production in distinct neuron populations of the hippocampus

The possible localization of cannabinoid (CB) receptors to glutamatergic and GABAergic synaptic terminals impinging upon GABAergic interneurons in the CA1 region of the rat hippocampus was examined using the electrophysiological measurement of neurotransmitter release in brain slices. Whereas activation of cannabinoid receptors via the application of the cannabinoid agonist WIN55,212-2 significantly and dose-dependently reduced evoked IPSCs recorded from interneurons possessing somata located in the stratum radiatum (S.R.) and stratum oriens (S.O.) lamellae, evoked glutamatergic EPSCs were unaffected in both neuronal populations. However, in agreement with previous reports, WIN55,212-2 significantly reduced EPSCs recorded from CA1 pyramidal neurons. Additional experiments confirmed that the effects of WIN55,212-2 on IPSCs were presynaptic and that they could be blocked by the CB1 receptor antagonist SR141716A. The involvement of endogenous cannabinoids in the presynaptic inhibition of GABA release was also examined in the interneurons and pyramidal cells using a depolarization-induced suppression of inhibition (DSI) paradigm. DSI was observed in CA1 pyramidal neurons under control conditions, and its incidence was greatly increased by the cholinergic agonist carbachol. However, DSI was not observed in the S.R. or S.O. interneuron populations, in either the presence or absence of carbachol. Whereas DSI was not present in these interneurons, the inhibitory inputs to these cells were modulated by the synthetic cannabinoid WIN55,212-2. These data support the hypothesis that cannabinoid receptors are located on inhibitory, but not excitatory, axon terminals impinging upon hippocampal interneurons, and that CA1 pyramidal neurons, and not interneurons, are capable of generating endogenous cannabinoids during prolonged states of depolarization.

Mu opioid receptors are in discrete hippocampal interneuron subpopulations

In the rat hippocampal formation, application of mu opioid receptor (MOR) agonists disinhibits principal cells, promoting excitation-dependent processes such as epileptogenesis and long-term potentiation. However, the precise location of MORs in particular inhibitory circuits, has not been determined, and the roles of MORs in endogenous functioning are unclear. To address these issues, the distribution of MOR-like immunoreactivity (-li) was examined in several populations of inhibitory hippocampal neurons in the CA1 region using light and electron microscopy. We found that MOR-li was present in many parvalbumin-containing basket cells, but absent from cholecystokinin-labeled basket cells. MOR-li was also commonly in interneurons containing somatostatin-li or neuropeptide Y-li that resembled the "oriens-lacunosum-moleculare" (O-LM) interneurons innervating pyramidal cell distal dendrites. Finally, MOR-li was in some vasoactive intestinal peptide- or calretinin-containing profiles resembling interneurons that primarily innervate other interneurons. These findings indicate that MOR-containing neurons form a neurochemically and functionally heterogeneous subset of hippocampal GABAergic neurons. MORs are most frequently on interneurons that are specialized to inhibit pyramidal cells, and are on a limited number of interneurons that target other interneurons. Moreover, the distribution of MORs to different neuronal types in several laminae, some relatively far from endogenous opioids, suggests normal functional roles that are different from the actions seen with exogenous agonists such as morphine.

Cholecystokinin-immunopositive basket and Schaffer collateral-associated interneurones target different domains of pyramidal cells in the CA1 area of the rat hippocampus

Two types of GABAergic interneurone are known to express cholecystokinin-related peptides in the isocortex: basket cells, which preferentially innervate the somata and proximal dendrites of pyramidal cells; and double bouquet cells, which innervate distal dendrites and dendritic spines. In the hippocampus, cholecystokinin immunoreactivity has only been reported in basket cells. However, at least eight distinct GABAergic interneurone types terminate in the dendritic domain of CA1 pyramidal cells, some of them with as yet undetermined neurochemical characteristics. In order to establish whether more than one population of cholecystokinin-expressing interneurone exist in the hippocampus, we have performed whole-cell current clamp recordings from interneurones located in the stratum radiatum of the hippocampal CA1 region of developing rats. Recorded neurones were filled with biocytin to reveal their axonal targets, and were tested for the presence of pro-cholecystokinin immunoreactivity. The results show that two populations of cholecystokinin-immunoreactive interneurones exist in the CA1 area (n=15 positive cells). Cholecystokinin-positive basket cells (53%) preferentially innervate stratum pyramidale and adjacent strata oriens and radiatum. A second population of cholecystokinin-positive cells, previously described as Schaffer collateral-associated interneurones [Vida et al. (1998) J. Physiol. 506, 755-773], have axons that ramify almost exclusively in strata radiatum and oriens, overlapping with the Schaffer collateral/commissural pathway originating from CA3 pyramidal cells. Two of seven of the Schaffer collateral-associated cells were also immunopositive for calbindin. Soma position and orientation in stratum radiatum, the number and orientation of dendrites, and the passive and active membrane properties of the two cell populations are only slightly different. In addition, in stratum radiatum and its border with lacunosum of perfusion-fixed hippocampi, 31.6+/-3.8% (adult) or 26.8+/-2.9% (postnatal day 17-20) of cholecystokinin-positive cells were also immunoreactive for calbindin. Therefore, at least two populations of pro-cholecystokinin-immunopositive interneurones, basket and Schaffer collateral-associated cells, exist in the CA1 area of the hippocampus, and are probably homologous to cholecystokinin-immunopositive basket and double bouquet cells in the isocortex. It is not known if the GABAergic terminals of double bouquet cells are co-aligned with specific glutamatergic inputs. However, in the hippocampal CA1 area, it is clear that the terminals of Schaffer collateral-associated cells are co-stratified with the glutamatergic input from the CA3 area, with as yet unknown functional consequences. The division of the postsynaptic neuronal surface by two classes of GABAergic cell expressing cholecystokinin in both the hippocampus and isocortex provides further evidence for the uniform synaptic organisation of the cerebral cortex.

Physiological and morphological diversity of immunocytochemically defined parvalbumin- and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus

To investigate the electrophysiological properties, synaptic connections, and anatomy of individual parvalbumin-immunoreactive (PV-IR) and cholecystokinin-immunoreactive (CCK-IR) interneurones in CA1, dual intracellular recordings using biocytin-filled microelectrodes in slices of adult rat hippocampus were combined with fluorescence labelling of PV- and CCK-containing cells. Of 36 PV-IR cells, 29 were basket cells, with most of their axonal arbours in the stratum pyramidale (SP). Six were bistratified cells with axons ramifying throughout stratum oriens (SO) and stratum radiatum (SR). One was a putative axo-axonic cell with an axonal arbour confined to half of the SP and a narrow adjacent region of the SO. Of 27 CCK-IR neurones, 13 were basket cells, with most of their axonal arbours in the SP, and included basket cells with somata in the SP (6), SO (3), and SR (2) and at the border between the stratum lacunosum-moleculare (SLM) and the SR (2). In addition, several dendrite-targeting cell classes expressed CCK-IR: 4 of 9 bistratified cells with axons ramifying in the SO and SR; all five Schaffer-associated cells whose axons ramified extensively in the SR; both cells classified as quadrilaminar because their axons ramified in the SO, SP, SR, and SLM; one SO-SO cell whose dendritic and axonal arbours were contained within the SO; and one perforant path-associated cell with axonal and dendritic arbours within the distal SR and SLM. The majority (31 of 36) of PV-IR neurones recorded were fast-spiking, and most fast-spiking cells tested (25 of 29 basket, 1 axo-axonic, and 5 of 6 bistratified cells) were PV-IR. However, 1 of 6 regular-spiking basket, 1 of 4 regular-spiking bistratified, and 3 of 5 burst-firing basket cells were also PV-IR. In contrast, the majority (17 of 27) of the CCK-IR neurones recorded were regular-spiking, 3 were burst-firing, and 7 were fast-spiking. These data confirm that the majority of PV-IR and CCK-IR axon terminals innervate proximal portions of CA1 pyramidal cells. Most PV-IR cells are fast-spiking, whereas most CCK-IR cells are regular-spiking. In both neurochemical classes basket cells predominate, but both groups included subpopulations of dendrite-targeting cells. Despite these similarities, the two populations exhibited very different somatic distributions, and each contained cellular morphologies not represented in the other.

Kinetic differences between synaptic and extrasynaptic GABA(A) receptors in CA1 pyramidal cells

GABA(A)-mediated IPSCs typically decay more rapidly than receptors in excised patches in response to brief pulses of applied GABA. We have investigated the source of this discrepancy in CA1 pyramidal neurons. IPSCs in these cells decayed rapidly, with a weighted time constant tau(Decay) of approximately 18 msec (24 degrees C), whereas excised and nucleated patch responses to brief pulses of GABA (2 msec, 1 mM) decayed more than three times as slowly (tau(Decay), approximately 63 msec). This discrepancy was not caused by differences between synaptic and exogenous transmitter transients because (1) there was no dependence of tau(Decay) on pulse duration for pulses of 0.6-4 msec, (2) responses to GABA at concentrations as low as 10 microM were still slower to decay (tau(Decay), approximately 41 msec) than IPSCs, and (3) responses of excised patches to synaptically released GABA had decay times similar to brief pulse responses. These data indicate that the receptors mediating synaptic versus brief pulse responses have different intrinsic properties. However, synaptic receptors were not altered by the patch excision process, because fast, spontaneous IPSCs could still be recorded in nucleated patches. Elevated calcium selectively modulated patch responses to GABA pulses, with no effect on IPSCs recorded in nucleated patches, demonstrating the presence of two receptor populations that are differentially regulated by intracellular second messengers. We conclude that two receptor populations with distinct kinetics coexist in CA1 pyramidal cells: slow extrasynaptic receptors that dominate the responses of excised patches to exogenous GABA applications and fast synaptic receptors that generate rapid IPSCs.

Nicotinic receptor activation excites distinct subtypes of interneurons in the rat hippocampus

We examined the function of nicotinic acetylcholine receptors (nAChRs) in interneurons of area CA1 of the rat hippocampus. CA1 interneurons could be classified into three categories based on nicotinic responses. The first class was depolarized by alpha7 nAChRs, found in all layers of CA1 and as a group, had axonal projections to all neuropil layers of CA1. The second class had both fast alpha7 and slow non-alpha7 nAChR depolarizing responses, was localized primarily to the stratum oriens, and had axonal projections to the stratum lacunosum-moleculare. The third group had no nicotinic response. This group was found in or near the stratum pyramidale and had axonal projections almost exclusively within and around this layer. Low concentrations (500 nM) of nicotine desensitized fast and slow nAChR responses. These findings demonstrate that there are distinct subsets of interneurons with regard to nicotinic receptor expression and with predictable morphological properties that suggest potential cellular actions for nicotinic receptor activation in normal CNS function and during nicotine abuse.

GABAergic axon terminals at perisomatic and dendritic inhibitory sites show different immunoreactivities against two GAD isoforms, GAD67 and GAD65, in the mouse hippocampus: a digitized quantitative analysis

Glutamic acid decarboxylase (GAD), the gamma-aminobutyric acid (GABA)-synthetic enzyme, consists of two isoforms, GAD67 and GAD65. Although distributions of the two GAD isoforms at the somatic level are known to be heterogeneous among different subpopulations of GABAergic neurons, those at the synaptic level have not been investigated. In order to analyze quantitatively the two GAD-isoform immunoreactivities in axon terminals, we combined confocal laser scanning microscopy with digitized image analysis to measure the gray levels of immunofluorescent signals for the two GAD isoforms in a large number of individual boutons in each hippocampal and dentate layer of the mouse. Synaptic boutons exhibited lamina-specific immunoreactivities against the GAD isoforms. Boutons in the principal cell layers (stratum pyramidale of the hippocampus proper and the granule cell layer of the dentate gyrus) showed more intense immunoreactivity against GAD67 than those in the dendritic layers (strata lacunosum-moleculare, radiatum, and oriens of the hippocampus proper and the molecular layer of the dentate gyrus). By contrast, boutons in the dendritic layers showed more intense immunoreactivity against GAD65 than those in the principal cell layers. Such differential distributions could be correlated to the GAD-isoform immunoreactivities in the axon terminals originating from parvalbumin-containing neurons, a particular subpopulation of hippocampal GABAergic neurons mainly innervating the perisomatic domain of principal neurons. In addition to previously reported physiological and pharmacological differences between the GABAergic synapses on perisomatic domain and those on distal dendrites, the present results suggest a functional differentiation of GABAergic synapses between these two inhibitory sites.

Neurocalcin-immunoreactive cells in the rat hippocampus are GABAergic interneurons

Neurocalcin (NC) is a recently described calcium-binding protein isolated and characterized from bovine brain. NC belongs to the neural calcium-sensor proteins defined by the photoreceptor cell-specific protein recoverin that have been proposed to be involved in the regulation of calcium-dependent phosphorylation in signal transduction pathways. We analyzed the distribution and morphology of the NC-immunoreactive (IR) neurons in the rat dorsal hippocampus and the coexistence of NC with GABA and different neurochemical markers which label perisomatic inhibitory cells [parvalbumin (PV) and cholecystokinin (CCK)], mid-proximal dendritic inhibitory cells [calbindin D28k (CB)], distal dendritic inhibitory cells [somatostatin (SOM) and neuropeptide Y (NPY)], and interneurons specialized to innervate other interneurons [calretinin (CR) and vasoactive intestinal polypeptide (VIP)]. NC-IR cells were present in all layers of the dentate gyrus and hippocampal fields. In the dentate gyrus, NC-IR cells were concentrated in the granule cell layer, especially in the hilar border, whereas in the CA fields they were most frequently found in the stratum radiatum. NC-IR cells were morphologically heterogeneous and exhibited distinctive features of non-principal cells. In the dentate gyrus, pyramidal-like, multipolar and fusiform (horizontal and vertical) cells were found. In the CA3 region most NC-IR cells were multipolar, but vertical and horizontal fusiform cells also appeared. In the CA1 region, where NC-IR cells showed most frequently vertically arranged dendrites, multipolar, bitufted and fusiform (vertical and horizontal) cells could be distinguished. All the NC-IR cells were found to be GABA-IR in all hippocampal layers and regions, and they represented about 19% of the GABA-positive cells. NC/CB, NC/CR and NC/VIP double-labeled cells were found in all hippocampal regions, and represented 29%, 24% and 18% of the NC-IR cells, respectively. NC and CCK did not coexist in the dentate gyrus; however, 9% of the NC-IR cells in the CA fields also contained CCK. No coexistence of NC with PV, SOM or NPY was found in any hippocampal region. We conclude that NC is exclusively expressed by interneurons in the rat hippocampus. NC-IR cells are a morphologically and neurochemically heterogeneous subset of GABAergic non-principal cells, which, on the basis of the known termination pattern of the colocalizing markers, are also functionally heterogeneous and are mainly involved in feed-forward dendritic inhibition in the commissural-associational and Schaffer collateral termination zones (CB containing cells), in innervation of other interneurons (CR- and VIP-containing cells), and in perisomatic inhibition (CCK-containing cells). NC is never present in perisomatic inhibitory PV-containing cells, or in feed-back distal dendritic inhibitory SOM/NPY-containing cells.

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.

Nucleus reuniens thalami modulates activity in hippocampal field CA1 through excitatory and inhibitory mechanisms

The nucleus reuniens thalami (RE) originates dense projections to CA1, forming asymmetrical synapses on spines (50%) and dendrites (50%). The hypothesis that RE input modulates transmission in CA1 through excitation of both pyramidal cells and interneurons was tested using electrophysiological methods in the anesthetized rat. The RE-CA1 afferents were selectively stimulated at their origin; evoked field potentials and unit activity were recorded in CA1. RE-evoked depth profiles showed a prominent negative deflection in the stratum lacunosum-moleculare and a positive one in the stratum radiatum. The lacunosum-moleculare sink-radiatum source configuration is compatible with RE-elicited depolarization of apical dendrites of pyramidal cells. Despite a consistent and robust paired pulse facilitation of RE-evoked field potentials, population spikes in the stratum pyramidale were not detected at any tested condition. This indicates the inability of RE-CA1 input to discharge pyramidal cells. However, stimulation of RE-elicited spiking of extracellularly recorded units in strata oriens/alveus and distal radiatum, indicative of the activation of local interneurons. Thus, RE seems to modulate transmission in CA1 through a (subthreshold) depolarization of pyramidal cells and a suprathreshold excitation of putative inhibitory oriens/alveus and radiatum interneurons. RE-evoked monosynaptic or disynaptic field potentials were associated with stimulation of rostral or caudal RE, respectively. Anatomically, a projection from caudal to rostral RE was demonstrated that can account for the disynaptic RE-CA1 input. Because caudal RE receives input from the hippocampus via the subiculum, we propose the existence of a closed RE-hippocampal circuit that allows RE to modulate the activity in CA1, depending on hippocampal output.

The 5-HT3 receptor is present in different subpopulations of GABAergic neurons in the rat telencephalon

The type 3 serotonin receptor (5-HT3R) is a ligand-gated ion channel whose presence in the CNS has been established by radioligand binding, in situ hybridization, and immunohistochemical analysis. To analyze further the role of the 5-HT3R in the CNS, we used in situ hybridization and immunocytochemistry to determine that 5-HT3R-expressing neurons are mainly GABA-containing cells in the rat telencephalon. We determined that 5-HT3R/GABA-containing neurons do not exhibit somatostatin immunoreactivity but often contain cholecystokinin (CCK) immunoreactivity. 5-HT3R-expressing cells with CCK immunoreactivity were observed in the neocortex, olfactory cortex, hippocampus, and amygdala. The 5-HT3R/CCK interneurons represent between 35 and 66% of the total population of CCK-containing cells in the neocortex. Further characterization of the 5-HT3R/GABAergic neurons was based on their calcium-binding protein immunoreactivity and showed that these neurons lack parvalbumin (PV) and represent a subpopulation of calbindin (CB)-containing interneurons that were preferentially present in the CA1-CA3 subfield of the hippocampus. Although some 5-HT3R/GABAergic neurons with calretinin (CR) were found in the neocortex, olfactory cortex, hippocampus, and amygdala, these neurons were more often present in the agranular insular and piriform cortices. We conclude that the neuronal expression of the 5-HT3R is selective within the GABA neuron population in the rat telencephalon. These 5-HT3R-expressing interneurons might contain CCK, CB, and CR. We suggest that serotonin through the 5-HT3R may regulate GABA and CCK neurotransmission in the telencephalon.

Synaptic target selectivity and input of GABAergic basket and bistratified interneurons in the CA1 area of the rat hippocampus

To assess the position of interneurons in the hippocampal network, fast spiking cells were recorded intracellularly in vitro and filled with biocytin. Sixteen non-principal cells were selected on the basis of 1) cell bodies located in the pyramidal layer and in the middle of the slice, 2) extensive labeling of their axons, and 3) a branching pattern of the axon indicating that they were not axo-axonic cells. Examination of their efferent synapses (n = 400) demonstrated that the cells made synapses on cell bodies, dendritic shafts, spines, and axon initial segments (AIS). Statistical analysis of the distribution of different postsynaptic elements, together with published data (n = 288) for 12 similar cells, showed that the interneurons were heterogeneous with regard to the frequency of synapses given to different parts of pyramidal cells. When the cells were grouped according to whether they had less or more than 40% somatic synaptic targets, each population appeared homogeneous. The population (n = 19) innervating a high proportion of somata (53 +/- 10%, SD) corresponds to basket cells. They also form synapses with proximal dendrites (44 +/- 12%) and rarely with AISs and spines. One well-filled basket cell had 8,859 boutons within the slice, covering an area of 0.331 mm2 of pyramidal layer tangentially and containing 7,150 pyramidal cells, 933 (13%) of which were calculated to be innervated, assuming that each pyramidal cell received nine to ten synapses. It was extrapolated that the intact axon probably had about 10,800 boutons innervating 1,140 pyramids. The proportion of innervated pyramidal cells decreased from 28% in the middle to 4% at the edge of the axonal field. The other group of neurons, the bistratified cells (n = 9), showed a preference for dendritic shafts (79 +/- 8%) and spines (17 +/- 8%) as synaptic targets, rarely terminating on somata (4 +/- 8%). Their axonal field was significantly larger (1,250 +/- 180 microns) in the medio-lateral direction than that of basket cells (760 +/- 130 microns). The axon terminals of bistratified cells were smaller than those of basket cells. Furthermore, in constrast to bistratified cells, basket cells had a significant proportion of dendrites in stratum lacunosum-moleculare suggesting a direct entorhinal input. The results define two distinct types of GABAergic neuron innervating pyramidal cells in a spatially segregated manner and predict different functions for the two inputs. The perisomatic termination of basket cells is suited for the synchronization of a subset of pyramidal cells that they select from the population within their axonal field, whereas the termination of bistratified cells in conjunction with Schaffer collateral/commissural terminals may govern the timing of CA3 input and/or voltage-dependent conductances in the dendrites.

Distribution of mRNA for CCK-B receptor in the brain of Mastomys natalensis: abundant expression in telencephalic neurons

The distribution of cholecystokinin B (CCK-B) receptors in the Mastomys brain was studied using Northern blot analysis and in situ hybridization technique. By Northern blot analysis using 32P-labeled cDNA probe, the cortex had the highest hybridization signal of CCK-B receptor mRNA in the brain. The olfactory bulb and hippocampus showed a moderate level of signals. In situ hybridization using 35S-labeled cRNA probes revealed a wide and region-specific distribution of CCK-B receptor mRNA in the telencephalon. Throughout the cerebral cortex, labeled cells were found in all layers, with higher intensities in layers II, V and VI. Pyramidal cells of the layer II of the piriform cortex showed the highest level of signals in the brain. In the hippocampus, most of the pyramidal cells of the Ammon's horn were labeled, although labeled cells were not detected in other layers. Distinct signals were also detected in the various amygdaloid nuclei, caudate-putamen, reticular thalamic nucleus, hypothalamic ventromedial nucleus and inferior colliculus. This distribution pattern may further support the prominent existence of CCK-B receptors in the brain particularly in the telencephalon.

Convergence and segregation of septal and median raphe inputs onto different subsets of hippocampal inhibitory interneurons

The convergence and segregation of medial septal and median raphe afferents in the innervation of different subpopulations of GABAergic interneurons was investigated in the rat hippocampal formation. Following local injections of 5,7-dihydroxytryptamine into the median raphe nucleus destroying all serotonergic neurons, iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHAL) into the medial septum resulted in anterograde labelling of axons in the hippocampus. The labelled varicose fibres made multiple contacts with calbindin D28K-, parvalbumin-, and cholecystokinin-immunoreactive interneurons. These results disproved the possibility that PHAL-labelled afferents innervating hippocampal interneurons following septal PHAL injections would have been raphe axons passing through the injection site. In the second set of experiments a double anterograde tracing technique (PHAL from the septum and biotinylated-PHAL from the median raphe) and a triple or double immunostaining procedure was used to determine the types of interneurons (calbindin D28K-, parvalbumin-, or cholecystokinin-immunoreactive) innervated by one, or the other, or both pathways. The results showed that parvalbumin-containing neurons were innervated by septal afferents but avoided by raphe axons, whereas calbindin D28K-containing cells, and to a smaller extent cholecystokinin-containing cells served as targets for both pathways. In some cases the same individual calbindin D28K- or cholecystokinin-containing neurons received multiple contacts from afferents of both septal and raphe origin. Thus, our results indicate that different subcortical nuclei modulate largely different inhibitory circuits in the hippocampal formation. However, considering the occasional convergence of the two subcortical nuclei not only onto the same type, but even onto the same individual calbindin D28K-containing interneurons, we propose that a particular inhibitory function, most probably feed-forward inhibition in the distal dendritic region, is under the control of both pathways.

Subpopulations of GABAergic neurons containing parvalbumin, calbindin D28k, and cholecystokinin in the rat hippocampus

The possible coexistence of calbindin D28k with parvalbumin and of calbindin D28k with cholecystokinin was studied in nonpyramidal cells of the rat dorsal hippocampal formation. Neighbouring Vibratome sections were immunostained either for calbindin D28k and parvalbumin or for calbindin D28k and cholecystokinin. The cells, halved during sectioning, were identified in both sections immunostained for different antigens. The coexistence of calbindin D28k and parvalbumin in the same neuron was rare throughout the hippocampal formation with the exception of stratum oriens of the CA1 region, where 9.6% of the parvalbumin-immunoreactive cells also contained calbindin D28k. In stratum radiatum of the CA3 region, calbindin D28k and cholecystokinin coexisted in 12.5% and 21.2% of the calbindin D28k and cholecystokinin-immunoreactive cells, respectively. In other regions of the hippocampal formation, the two markers coexisted in less than 5% of the cells of either type. The present results demonstrate that calbindin D28k-, parvalbumin- and cholecystokinin-containing nonpyramidal cells represent largely nonoverlapping cell populations and may thus be involved in different inhibitory circuits.

Distribution of cells containing mRNA encoding cholecystokinin in the rat central nervous system

The distribution of cells containing mRNA encoding cholecystokinin was studied in the rat central nervous system by in situ hybridization histochemistry. Cholecystokinin mRNA containing neurons were considerably more numerous than the cholecystokinin-like immunoreactive neurons detected by immunocytochemistry even after colchicine pretreatment and appeared to be heavily, moderately, or lightly labeled. Such neurons were present in the olfactory bulb, olfactory nuclei, layers II-III and V-VI of the cerebral cortex, amygdaloid nuclei, subiculum, hippocampus, claustrum, endopiriform nucleus, several hypothalamic nuclei, most of the thalamic nuclei, ventral tegmental area, substantia nigra, interfascicularis nucleus, linearis rostralis, central gray, Edinger-Westphal nucleus, superior and inferior colliculi, parabrachial nucleus, reticular formation, raphe nuclei, and spinal trigeminal nucleus. This distribution partly confirmed and partly extended the previous immunohistochemical descriptions. Several brain areas such as the thalamus and the colliculi contain cholecystokinin mRNA but are devoid of perikarya exhibiting cholecystokinin-like immunoreactivity. The cerebral cortex and the hippocampus present a far higher density of cholecystokinin mRNA containing cells, including pyramidal neurons, than of perikarya containing cholecystokinin-like immunoreactivity. These results suggest that cholecystokinin or cholecystokinin-related peptides could have a functional role in numerous cerebral pathways including long projections such as cortical or thalamic projections.

Cholecystokinin blocks some effects of kainic acid in CA3 region of hippocampal slices

We investigated the relationship between the effects of cholecystokinin (CCK) and kainic acid (KA) in the CA3 region of hippocampal slices from rats. As has been reported previously, KA in nanomolar concentrations caused spontaneous epileptiform discharges (bursts) and an excitatory shift of the input/output (I/O) curve. CCK octapeptide (100-200 nM) applied alone had no effect on spontaneous activity or I/O curves. Pretreatment of slices with sulfated CCK blocked the effect of KA on synaptic transmission, but had no effect on KA-induced bursting. Pretreatment with nonsulfated CCK had no effect.

The axon terminals of vasoactive intestinal polypeptide (VIP)-containing bipolar cells in rat visual cortex

In vasoactive intestinal polypeptide (VIP)-immunoreacted preparations, bipolar neurons are the cells most commonly labelled. The VIP-positive axon terminals form symmetrical synapses, and their most common postsynaptic targets are small and medium sized dendrites. These are of both smooth and spiny types. Additionally, there is a concentration of VIP-positive axon terminals around the cell bodies of pyramidal neurons, and it is suggested that an important function of VIP-labelled bipolar cells is to inhibit vertically oriented groups of pyramidal cells. In order to further examine the features of axon terminals that label with VIP antibodies, conventionally prepared material was examined by electron microscopy. Those terminals which label with VIP antibody are characterized by irregular profiles of varying sizes and shapes, and by containing closely packed pleomorphic vesicles. Such terminals form symmetrical synapses. The junctions are not well marked by associated cytoplasmic densities, but there is an inherent density within the synaptic cleft. It is suggested that these features characterize all axon terminals in which GABA coexists with peptides in cerebral cortex.

Innervation of different peptide-containing neurons in the hippocampus by GABAergic septal afferents

The termination pattern of septohippocampal axons visualized by anterograde transport of Phaseolus vulgaris leucoagglutinin was studied in the hippocampal formation in the rat, with special reference to the innervation of neurons immunoreactive for the neuroactive peptides cholecystokinin, somatostatin or vasoactive intestinal polypeptide. The type I, GABAergic, septohippocampal afferents were shown to terminate on neurons immunoreactive for each of the three peptides. The cholecystokinin-like immunoreactive neurons in all regions, and the somatostatin-immunoreactive cells in stratum oriens of CA1 region were the most preferred targets. Cholecystokinin-immunoreactive cells, especially those in the granule cell layer of the dentate gyrus, were often seen to be contacted by type II (presumed cholinergic) axons as well. The somatostatin-immunoreactive cells in the hilus were also innervated by type I septohippocampal axons, although less frequently than those in stratum oriens of the CA1 subfield. Each type of peptidergic neuron received multiple symmetrical synaptic input from the Phaseolus vulgaris leucoagglutinin-labelled septal afferents, as confirmed by correlated electron microscopy. The majority of these neuropeptide-containing cells are known to be GABAergic, and to have distinct input and output relationships. Thus, the present results demonstrate that the GABAergic septohippocampal pathway can control a wide range of putative inhibitory circuits, and thereby influence the pattern of electrical activity in the hippocampal formation.

Neuronal localization of cholecystokinin mRNA in the rat brain by using in situ hybridization histochemistry

The distribution of cholecystokinin (CCK) mRNA in the rat brain was determined by means of in situ hybridization histochemistry. Our results demonstrate a widespread distribution of neurons containing CCK mRNA throughout the rat brain. Hybridization-positive neurons were distributed throughout the neocortex, olfactory bulb, claustrum, amygdala, the dentate gyrus and hippocampus proper, and several subnuclei of the thalamus and the hypothalamus. The most abundant and most heavily labeled neurons were found in the endopiriform/piriform cortex, tenia tecta, and the ventral tegmental area. The distribution of neurons positive for CCK mRNA paralleled that of CCK-like immunoreactive neurons. These results detail the distribution of CCK mRNA and clearly identify the existence of CCK-synthesizing neurons in regions such as the paraventricular and supraoptic nuclei of the hypothalamus, where the presence of CCK cell bodies was previously uncertain.

Early postnatal development of cholecystokinin-immunoreactive structures in the visual cortex of the cat

The early postnatal development of cholecystokinin-immunoreactive (CCK-ir) neurons was analyzed in visual areas 17 and 18 of cats aged from postnatal day 0 to adulthood. Neurons were classified mainly by axonal criteria. According to their chronology of appearance neurons are grouped into three neuronal populations. The first population consists of five cell types which appear perinatally in areas 17 and 18. Four of them have axons terminating in layer VI. Neurons with columnar dendritic fields of layers IV and V display a conspicuous dendritic arborization with the long dendrites always arranged parallel to each other. This way they form a vertically oriented dendritic column. The neurons differentiate at around P 2 and are present until the end of the second postnatal week. They disappear possibly by degeneration and cell death. Multipolar neurons of layer VI have long dendrites and axonal domains of up to 800 micron in diameter. Three percent of these neurons send out two axons instead of only one. Neurons differentiate at P 0 and the cell type persists into adulthood. Bitufted to multipolar neurons of layer V constitute a frequent type; 10% of these cells issue two axons. They differentiate at P 2 and the type survives into adulthood. Bitufted to multipolar neurons of layers II/III appear at P 2 and send their axons into layer VI. So, early postnatally an axonal connection from superficial cortical layers to layer VI is established. The cell type persists into adulthood. The fifth cell type of the first population is constituted by the neurons of layer I with intralaminar axons which differentiate at P 2. Although they derive from the early marginal zone, the cell type survives into adulthood. The second population consists of two cell types which appear around the end of the second and during the third postnatal week in areas 17 and 18. Multipolar neurons of layer II have horizontally or obliquely arranged basket axons which, during the second postnatal month, form patches of high fiber and terminal density along the layer I/II border. Neurons with descending main axons issuing horizontal and oblique collaterals of layers II-IV form broad axonal fields. The third population in area 17 is constituted by three cell types: Bitufted neurons with axons descending in form of loose bundles of layers II/III differentiate during the fifth postnatal week. Small basket cells of layers II/III with locally restricted axonal plexuses and somewhat larger basket cells of layer IV appear during the sixth and seventh week.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional distribution of cholecystokinin receptors in macaque medial temporal lobe determined by in vitro receptor autoradiography

Cholecystokinin (CCK) binding sites were localized in the hippocampus, amygdala, and medial temporal cortices of macaque monkeys by using techniques of in vitro receptor autoradiography. Binding sites were labeled with 3H-CCK-8 and 125I-CCK-33, and nonspecific binding was assessed in the presence of 1 microM CCK-8. Comparison of autoradiograms with Nissl-stained sections allowed precise correlation of autoradiographic grain distribution with cytoarchitecture. CCK binding in the amygdala varied among nuclear subdivisions. It was dense in the lateral, basomedial, endopiriform, and cortical nuclei, in the parvicellular portion of the accessory basal nucleus, the periamygdaloid cortex, the cortical transition area, and in the amygdalohippocampal area. Labeling was sparse in the central, medial, and basolateral nuclei as well as in the magnocellular accessory basal nucleus. In the hippocampal formation, a single dense band of CCK binding was observed over the granule cell layer and adjacent few millimeters of the molecular layer of the dentate gyrus, while in the polymorph and remaining portions of this layer binding was of very low density. Prominent label over the pyramidal layer in the presubiculum clearly distinguished this region from the adjacent subiculum in which binding just exceeded background levels. Moderate to light label was observed in the hilus and stratum pyramidale of CA3, CA2, and CA1, while other hippocampal layers showed minimal specific binding. Variation in CCK binding in the medial temporal cortex showed close correspondence to cytoarchitectonic subdivisions. In entorhinal cortex, for example, binding was concentrated in layers III-VI while label in area 35 was prominent in all laminae except layer IV. Area TH of von Bonin and Bailey ('47) was distinguished from other regions by evenly distributed binding across all layers, while in area TF a bilaminar pattern of label in layers II and IV was observed. The highly specific patterns of CCK binding in amygdala and transitional cortices of the medial temporal lobe can be related to terminal fields of neo- and allocortical afferents to these regions, while label in the hippocampal formation coincides with the terminals of intrinsic neurons which ramify among the somata of cells that are targets of neocortical afferents. Thus, in all structures of the medial temporal lobe the disposition of peptidergic binding sites suggests that CCKergic systems may be important in the modulation of cortical afferents.

Fine structure of identified neurons in the primate hippocampus: a combined Golgi/EM study in the baboon

The hippocampi of two 1-year-old female baboons (Papio anubis) were used for a combined Golgi/electron microscope (EM) study of characteristic cell types in the hippocampus proper and fascia dentata. Results were compared with previous Golgi/EM studies of hippocampal neurons in small laboratory animals. Cell bodies of pyramidal neurons in CA1 were more loosely distributed than known from studies on the rat or guinea pig. Numerous basal and horizontal dendrites originating from the perikaryon filled in the space between neighboring cell bodies. Apical stem dendrites were varying in length, depending on the position of the parent cell body in outer or inner portions of the pyramidal layer. Dendrites were densely covered with spines which in the EM showed very complex synaptic contacts. In contrast to our observations in rats and guinea pigs, CA3 pyramidal cells in the monkey hippocampus exhibited numerous large spines or excrescences not only on apical dendrites but also on basal dendrites running through stratum oriens. These excrescences appeared to be more complex than in small rodents. They often branched, protruding deeply into presynaptic mossy fiber boutons, and formed multiple asymmetric synaptic contacts. Granule cells of the monkey fascia dentata, in contrast to those of the rodent, occasionally had basal dendrites extending into the hilar region. In the EM, granule cells either with or without basal dendrites exhibited fine structural characteristics that were very similar to those described in Golgi/EM studies of granule cells in the rat fascia dentata. Of the various types of nonpyramidal neurons the horizontal cells in stratum oriens with dendrites parallel to the alveus were analyzed. As seen in rats, these cells exhibited large amounts of rough endoplasmic reticulum, indentations of the nuclear membrane, and nuclear inclusions. Numerous terminals formed synaptic contacts on dendritic shafts. In contrast to rodents, numerous spines arose from dendrites and cell bodies of these neurons. In the EM, often single spines were found to establish synaptic contacts with several presynaptic boutons. In summary, our correlated light and EM study of four characteristic cell types, which are present in both nonprimates and primates, demonstrates a much more complex dendritic pattern and synaptic organization of these neurons in primates than in commonly studied small laboratory animals.

Immunocytochemical study of GABAergic neurons containing the calcium-binding protein parvalbumin in the rat hippocampus

The structural features of PV-immunoreactive (PV-I) neurons, a particular subpopulation of GABAergic neurons, in the hippocampus were studied by immunocytochemistry. The PV-I cell bodies were concentrated within the stratum pyramidale (SP) and stratum oriens (SO) in the hippocampus. PV-I puncta were frequent in SP, while they were rarely seen in other layers. The dendritic arborization of PV-I neurons resembled that of some of the nonpyramidal cells observed after Golgi-impregnation. The most commonly observed PV-I neurons had their perikarya located in SP with dendrites extending into SO and the stratum radiatum (SR). Most of the dendrites in SR had typical beaded or varicose segments. The dendrites extending into SO had few beaded parts. There were many bipolar and multipolar neurons with smooth dendrites in SO, but only a small number of such multipolar neurons in SR. An electron microscopic analysis revealed that PV-I products were located to perikarya, dendrites, myelinated axons and synaptic boutons. The perikarya of PV-I neurons exhibited several ultrastructural features of nonpyramidal cells, e.g., abundant cisternae of endoplasmic reticulum, mitochondria and other perikaryal organelles, an infolded nuclear envelope and intranuclear inclusions. They received many asymmetric synapses with round presynaptic vesicles. There were numerous PV-I boutons, presumably axonal endings, covering the pyramidal cell bodies. The PV-I boutons also contacted the axon initial segments and proximal dendrites of the pyramidal cells. In addition PV-I terminals were found on somata and dendrites of both PV-I or PV-negative nonpyramidal cells. The results suggest that PV-containing neurons include basket and axo-axonic cells.

CCK-immunoreactive terminals form different types of synapses in the rat and monkey hippocampus

Electronmicroscopic immunocytochemical analysis of the types and patterns of synaptic contacts formed by cholecystokinin (CCK)-containing terminals in the CA1 and CA3 region of the rat and monkey hippocampus reveals numerous symmetric synaptic contacts on cell bodies and dendritic shafts of pyramidal cells in both species. In the monkey, however, CCK-immunoreactive terminals also form asymmetric synaptic contacts with dendritic spines, such contacts are absent or very rare in the rat hippocampus. The present finding in primate hippocampus provides evidence that the same neuropeptides can be found in both symmetric and asymmetric contacts and may be added to other evidence challenging the traditional concept that symmetric synapses mediate exclusively inhibitory and asymmetric exclusively excitatory transmission. Furthermore, although our comparative analysis confirms considerable similarities in the distribution of CCK-containing elements in primate and rodent hippocampus it also revealed a potentially important difference in synaptoarchitecture that should be taken into account in extrapolations from one species to the other.

Increased neuronal responsiveness to cholecystokinin and dopamine induced by lesioning mesolimbic dopaminergic neurons: an electrophysiological study in the rat

In the rat, cholecystokinin (CCK) and dopamine (DA) coexist in a subpopulation of neurons of the ventral tegmental area (VTA) projecting to the nucleus accumbens. However, in the dorsal hippocampus, dopaminergic projections from the VTA do not contain CCK, the latter neurotransmitter being mainly localized in intrinsic hippocampal neurons. The present experiments were undertaken in order to compare the interactions of CCK and DA and the effects of lesioning VTA dopaminergic neurons in a region where these neurotransmitters coexist and in one where they do not. The effects of microiontophoretic applications of CCK, kainate (KA), glutamate (GLU) and DA were determined in control rats and in rats pretreated with a local injection of 6-hydroxydopamine (6-OHDA) in the VTA. In the nucleus accumbens and in the hippocampus of intact rats, DA exerted a similar depressant effect whether applied during CCK-, KA- or GLU-induced activations. The 6-OHDA lesion enhanced responsiveness of accumbens neurons to KA, GLU and CCK (the responsiveness to this latter peptide being increased by more than 15-fold) and the depressant effect of DA when applied during neuronal activation by KA or GLU but not when the same neurons were activated with CCK. In the dorsal hippocampus, the 6-OHDA lesion enhanced neuronal responsiveness to KA and DA in the CA1, but not in the CA3 region, whereas the responsiveness to CCK remained unchanged in both regions. These results suggest a physiological role for the coexistence of CCK and DA in the nucleus accumbens. The induction of a supersensitivity to DA in the CA1, but not in the CA3, region of the dorsal hippocampus following a VTA lesion is consistent with the regional distribution of the dopaminergic innervation in this structure.

Distribution of immunoreactive cholecystokinin in the human hippocampus

The distribution of cholecystokinin immunoreactive (CCK-IR) nerve cell bodies and processes is reported in the human hippocampus by using the peroxidase-antiperoxidase technique of Sternberger. The CCK-immunoreactivity occurs in three major classes of interneurons: small (10-20 microns) horizontal multipolar neurons of the alveus and stratum oriens; small vertically oriented bipolar or multi-polar neurons in the stratum oriens and stratum pyramidale of Ammon's horn, layers II and III of the subicular system and the entorhinal area; large (20-35 microns) bipolar neurons in the hilus. Each region of the hippocampus is distinct in its CCK-IR nerve fibers content. Those fibers are particularly abundant around pyramidal cells of the CA2 and CA3 subfields of the Ammon's horn and around granular cells suggesting synaptic interaction between the CCK nerve terminals and glutamate neurons of these two regions. No CCK-IR fiber is detected in the fimbria and only a few number of CCK-IR beaded fibers are seen in the angular bundle. These anatomical data suggest that CCK interacts in the functional circuitry of the human hippocampus.

GABAergic neurons containing the Ca2+-binding protein parvalbumin in the rat hippocampus and dentate gyrus

The distribution of Ca2+-binding protein, parvalbumin (PV), containing neurons and their colocalization with glutamic acid decarboxylase (GAD) were studied in the rat hippocampus and dentate gyrus using immunohistochemistry. PV immunoreactive (PV-I) perikarya were concentrated in the granule cell layer and hilus in the dentate gyrus and in the stratum pyramidale and stratum oriens in the CA3 and CA1 regions of the hippocampus. They were rare in the molecular layer of the dentate gyrus, in the stratum radiatum and in the stratum lacunosum-moleculare of the hippocampus. PV-I axon terminals were restricted to the granule cell layer, the stratum pyramidale and the immediately adjoining zones of these layers. Almost all PV-I neurons were also GAD immunoreactive (GAD-I), whereas only about 20% of GAD-I neurons also contained PV. The percentages of GAD-I neurons which were also immunoreactive for PV were dependent on the layer in which they were found; i.e. 40-50% in the stratum pyramidale, 20-30% in the dentate granule cell layer and in the stratum oriens of the CA3 and CA1 regions, 15-20% in the hilus and in the stratum lucidum of CA3 region and only 1-4% in the dentate molecular layer and in the stratum radiatum and the stratum lacunosum-moleculare of the CA3 and CA1 regions. PV-I neurons are a particular subpopulation of GABAergic neurons in the hippocampal formation. Based on their morphology and laminar distribution, they probably include basket cells and axo-axonic cells.