A high degree of spatial selectivity in the axonal and dendritic domains of physiologically identified local-circuit neurons in the dentate gyrus of the rat hippocampus

Spatial synaptic integration in Purkinje cell dendrites

Synaptic integration occurs within a framework of synaptic connections, and cell type-specific, intrinsic and transmitter-gated ion channels. These components are differentially distributed over the somato-dendritic membrane. Recent results from Purkinje cells and pyramidal cells exemplify some of these mechanisms of spatial synaptic integration. This paper focusses on the cerebellar Purkinje cell. In these neurons, the amplitude and distribution of single climbing fibre and parallel fibre EPSP-evoked Ca2+ influx were regulated by the transient outward, IA-like current in the distal (spiny) dendrites. The synaptically evoked Ca2+ influx was graded from a local response involving only a few terminal spiny dendrites to a propagated Ca2+ spike. The climbing fibre-evoked Ca2+ influx in the spiny dendrites was finely graded by parallel fibre-induced depolarization. Climbing fibre and parallel fibre-evoked Ca2+ influx elicited a short lasting afterhyperpolarization that affected subsequent dendritic Ca2+ influx. In addition, inhibitory synaptic input controlled dendritic Ca2+ influx. Interaction between information from different sources along the dendrites is thus controlled by intrinsic potassium conductances and IPSPs. Different electrophysiological properties are found in the cerebellar neurons. Thus, Golgi cells, stellate cells and granule cells seem to integrate on a shorter intrinsic timescale than do Purkinje cells, the output neuron of the cerebellar cortex. The specific mechanisms by which different types of presynaptic neurons specifically innervate a given dendritic compartment remain to be elucidated, but recent results provide some experimental evidence of a differential distribution of cell adhesion molecules between the axonal and the somato-dendritic membrane, suggesting one mechanism contributing to the ordered distribution of synapses during synaptogenesis.

Spontaneous and synaptic input from granule cells and the perforant path to dentate basket cells in the rat hippocampus

To characterize excitatory inputs to dentate basket cells from dentate granule cells and the perforant path, the whole-cell recording technique was used in neonatal rat hippocampal slices. Spontaneous excitatory input to basket cells was also examined and compared to that of other interneurons in the dentate gyrus. Basket cells were separable from other neurons in the dentate gyrus based on morphology and location, as determined by biocytin staining following recording, and by resting membrane potential, propensity to fire action potentials spontaneously, and miniature excitatory postsynaptic current (EPSC) characteristics. Minimal electrical stimulation of the granule cell layer evoked in basket cells short latency EPSCs that were composed of both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) components as judged by their time course, voltage dependence, and blockade by selective antagonists. Perforant path EPSCs exhibited slower kinetics than EPSCs evoked by granule cell stimulation. Like granule cell evoked EPSCs, however, perforant path EPSCs were composed of both NMDA and AMPA components. Minimal electrical stimulation of the granule cell layer and perforant path evoked monosynaptic EPSCs in only 67% and 62% of the trials, respectively, suggesting that these inputs are as unreliable as previously determined inputs from CA3 pyramidal cells (48%). Tetrodotoxin-insensitive spontaneous miniature EPSCs were frequent in basket cells and non-basket interneurons residing either at the border between the granule cell layer and the hilus or deep within the hilus. Miniature EPSCs recorded from all cells held at -70 mV were blocked completely by 3 microM 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). Though a component of the miniature EPSCs recorded from border and deep hilar interneurons at +40 mV was blocked by the NMDA receptor antagonist D-2-amino-phosphonovaleric acid (D-APV), miniature EPSCs in basket cells were insensitive to D-APV. We conclude that input from granule cells and the perforant path results in activation of basket cells via glutamatergic synapses that employ both NMDA and AMPA receptors. These inputs to basket cells likely contribute to feedback and feedforward inhibition of granule cells. The absence of an NMDA receptor component in spontaneous miniature EPSCs of dentate basket cells implies a difference in organization of excitatory synapses made onto basket cells compared with other hilar interneurons.

Bridging the cleft at GABA synapses in the brain

A fragile balance between excitation and inhibition maintains the normal functioning of the CNS. The dominant inhibitory neurotransmitter of the mammalian brain is GABA, which acts mainly through GABAA and GABAB receptors. Small changes in GABA-mediated inhibition can alter neuronal excitability profoundly and, therefore, a wide range of compounds that clearly modify GABAA-receptor function are used clinically as anesthetics or for the treatment of various nervous system disorders. Recent findings have started to unravel the operation of central GABA synapses where inhibitory events appear to result from the synchronous opening of only tens of GABAA receptors activated by a saturating concentration of GABA. Such properties of GABA synapses impose certain constraints on the physiological and pharmacological modulation of inhibition in the brain.

Muscarinic amplification of fast excitation in hilar neurones and inhibition in granule cells in the guinea-pig hippocampus

1. Effects of the cholinergic agonist, carbachol (CCh), or the acetylcholinesterase inhibitor, eserine, on presumed inhibitory hilar neurones and on inhibition in granule cells were studied by intracellular recording in guinea-pig hippocampal slices. 2. CCh (1-5 microM) strongly enhanced the discharge activity of hilar neurones and spontaneous and evoked IPSPs in granule cells. 3. Eserine, in an atropine-sensitive manner, increased the excitability of hilar neurones through effects on membrane properties and on excitatory synaptic barrage. EPSPs readily triggered long-duration burst discharges. In granule cells, the amplitudes of evoked GABAA and GABAB receptor-mediated IPSPs were enhanced. 4. In the presence of eserine and antagonists for glutamatergic and GABAergic synaptic transmission, train stimulation evoked atropine-sensitive slow EPSPs. In contrast to those in granule cells, slow EPSPs in hilar neurones were invariably preceded by a strong burst-after-hyperpolarization. 5. We suggest that acetylcholine, released from septo-hippocampal fibres, amplifies fast synaptic excitation of inhibitory hilar neurones and inhibition of granule cells. In the dentate area, muscarinic receptor-mediated effects are faster than anticipated from the time course of the slow EPSP.

Localization of mRNAs encoding two forms of glutamic acid decarboxylase in the rat hippocampal formation

The mRNAs for two forms of glutamic acid decarboxylase (GAD65 and GAD67) were localized in the rat hippocampal formation by nonradioactive in situ hybridization methods with digoxigenin-labeled cRNA probes. Some neurons in all layers of the hippocampus and dentate gyrus were readily labeled for each GAD mRNA, and the patterns of labeling for GAD65 and GAD67 mRNAs were very similar. All major groups of previously described GAD- and GABA-containing neurons appeared to be labeled for each GAD mRNA. Such findings suggest that most GABA neurons in the hippocampal formation contain both GAD mRNAs. When the labeling of neurons in the hippocampal formation and cerebral cortex was compared in the same sections, the intensity of neuronal labeling for GAD67 mRNA was generally similar in the two regions. However, the intensity of labeling for GAD65 mRNA was generally stronger for many neurons in the hippocampal formation than for most neurons in the cerebral cortex. Neurons in the hilus of the dentate gyrus were particularly well labeled for GAD65. The nonradioactive labeling for the GAD mRNAs was confined to the cytoplasm of neuronal cell bodies, and this allowed a clear visualization of the relative number and location of labeled neurons. Several distinct patterns of GAD mRNA-containing neurons were observed among different regions of the hippocampal formation. In the hilus of the dentate gyrus, GAD mRNA-containing neurons were numerous in the regions deep to the granule cell layer as well as in more central parts of the hilus. Within CA3, the densities (quantities) of labeled neurons varied among the regions. In the inner or hilar segment of CA3, the density of labeled neurons was often lower than that in the outer part of CA3 where numerous labeled neurons were distributed throughout all layers. In CA1, GAD mRNA-labeled neurons were distributed in a relatively laminar pattern with the highest density in stratum pyramidale and moderate densities in stratum oriens and at the interface between strata radiatum and lacunosum-moleculare. Lower densities were found within the latter two layers. The prominent localization of the two GAD mRNAs in the hippocampal formation suggests that a dual system for GABA synthesis is necessary for normal GABAergic function in this brain region. Most putative GABA neurons contain relatively high levels of GAD67 mRNA as might be expected if this GAD form is responsible for the synthesis of GABA for metabolic and baseline synaptic function.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitory CA1-CA3-hilar region feedback in the hippocampus

The organization of the hippocampus is generally thought of as a series of cell groups that form a unidirectionally excited chain, regulated by localized inhibitory circuits. With the use of in vivo intracellular labeling, histochemical, and extracellular tracing methods, a longitudinally widespread, inhibitory feedback in rat brain from the CA1 area to the CA3 and hilar regions was observed. This long-range, cross-regional inhibition may allow precise synchronization of population activity by timing the occurrence of action potentials in the principal cells and may contribute to the coordinated induction of synaptic plasticity in distributed networks.

Dendritic development of dentate granule cells in the absence of their specific extrinsic afferents

Dendrites and spines are postsynaptic structures that develop in association with presynaptic fibers. Recent studies have shown that granule cells of the fascia dentata survive in slice cultures and differentiate in a manner known from in situ studies. However, all extrinsic afferent fibers are absent under culture conditions. In the present study, we study whether dendrites and spines of granule cells in slice cultures differentiate normally, although they are not contacted by their normal layer-specific afferents. Slices of hippocampus were prepared from rat pups at the day of birth. After 5, 10, 15, and 20 days of incubation, granule cells in these cultures were Golgi impregnated. For comparison, perfusion-fixed hippocampal sections of 5-, 10-, 15-, and 20-day-old rats were impregnated the same way. Our results show that the total density of spines on granule cell dendrites in culture increased as in perfusion-fixed animals. However, after 20 days of incubation, the absolute number of dendritic spines on cultured neurons was reduced because of a reduction of peripheral dendrites. This reduction was accompanied by an increase in the number of stem dendrites originating from the perikaryon. The density of spines on these proximal dendrites was larger in cultured granule cells than in controls. Our results suggest that the lack of major extrinsic (entorhinal) afferents that normally terminate on peripheral granule cell dendrites causes retraction of these dendrites. At the same time, there is growth of proximal dendritic portions. Proximal dendrites are targets of associational fibers, which are known to sprout under these culture conditions.

Diverse sources of hippocampal unitary inhibitory postsynaptic potentials and the number of synaptic release sites

Dual intracellular recordings from microscopically identified neurons in the hippocampus reveal that the synaptic terminals of three morphologically distinct types of interneuron act through GABAA receptors. Each type of interneuron forms up to 12 synaptic contacts with a postsynaptic principal neuron, but each interneuron innervates a different domain of the surface of the postsynaptic neuron. Different kinetics of the postsynaptic effects, together with the strategic placement of synapses, indicate that these GABAergic interneurons serve distinct functions in the cortical network.

Organization of the embryonic and early postnatal murine hippocampus. I. Immunocytochemical characterization of neuronal populations in the subplate and marginal zone

Immunocytochemical techniques were used to characterize the neuronal populations in the hippocampal subplate and marginal zone from embryonic day 13 (E13) to postnatal day 5 (P5). Sections were processed for the visualization of microtubule-associated protein 2 (MAP2) and other antigens such as neurotransmitters, neuropeptides, calcium-binding proteins and a synaptic antigen (Mab SMI81). At E13-E14, only the ventricular zone and the primitive plexiform layer were recognized. Some cells in the later stratum displayed MAP2-, gamma-aminobutyric acid (GABA)- and calretinin immunoreactivities. From E15 onwards, the hippocampal and dentate plates became visible. Neurons in the plexiform layers were immunoreactive at E15-E16, whereas the hippocampal and dentate plates showed immunostaining two or three days later. Between E15 and E19 the following populations were distinguished in the plexiform layers: the subventricular zone displayed small neurons that reacted with MAP2 and GABA antibodies; the subplate (prospective stratum oriens) was poorly populated by MAP2- and GABA-positive cells; the inner marginal zone (future stratum radiatum) was heavily populated by multipolar GABAergic cells; the outer marginal zone (stratum lacunosum-moleculare) displayed horizontal neurons that showed glutamate- and calretinin immunoreactivities, their morphology being reminiscent of neocortical Cajal-Retzius cells. Thus, each plexiform layer was populated by a characteristic neuronal population whose distribution did not overlap. Similar segregated neuronal populations were also found in the developing dentate gyrus. At perinatal stages, small numbers of neurons in the plexiform layers began to express calbindin D-28K and neuropeptides. During early postnatal stages, neurons in the subplate and inner marginal zones were transformed into resident cells of the stratum oriens and radiatum, respectively. In contrast, calretinin-positive neurons in the stratum lacunosum-moleculare disappeared at postnatal stages. At E15-E19, SMI81-immunoreactive fibers were observed in the developing white matter, subplate and outer marginal zone, which suggests that these layers are sites of early synaptogenesis. At P0-P5, SMI81 immunoreactivity became homogeneously distributed within the hippocampal layers. The present results show that neurons in the hippocampal subplate and marginal zones have a more precocious morphological and neurochemical differentiation than the neurons residing in the principal cell layers. It is suggested that these early maturing neurons may have a role in the targeting of hippocampal afferents, as subplate cells do in the developing neocortex.

Electrophysiological and morphological differentiation of chandelier and basket cells in the rat hippocampal formation: a study combining intracellular recording and intracellular staining with biocytin

Using standard intracellular recording techniques 38 nonpyramidal cells or interneurons have been sampled in hippocampal slices of the rat. Among 38 physiologically identified interneurons, all 27 cells labeled with biocytin were morphologically demonstrated to be nonpyramidal and nongranule cells. The vast majority of these cells showed typical fast spiking discharges, i.e., a shorter duration action potential followed by a brief but prominent after hyperpolarisation potential without frequency adaptation in response to prolonged depolarizing current injection. However, 4 cells clearly exhibited frequency adaptation. Based on their axonal arborizations, the former group included basket interneurons innervating the principle cell body layers and axodendritic interneurons projecting to the molecular layer of the dentate gyrus; whereas the latter 4 cells belonged to chandelier interneurons selectively terminating the axon initial segments of principle cells. These results support the notion that interneurons in the hippocampal formation are heterogeneous with respect to their morphology and electrophysiological characteristics, suggesting that the electrophysiological behavior of hippocampal interneurons may be associated with their functional activities.

Divergence of hippocampal mossy fibers

By connecting the fascia dentata with the hippocampus proper, the axons of the granule cells, the mossy fibers, represent an important element of the main excitatory, trisynaptic pathway of the hippocampal formation. In this review the various synaptic connections of the mossy fibers are discussed. It turns out that the mossy fibers do not only establish synapses with the pyramidal neurons of regio inferior as traditionally assumed, but a variety of local circuit neurons as well as projection cells are also contacted by the mossy fibers. Thus there is an underestimated divergence of the impulse flow within the "trisynaptic" pathway at the level of the mossy fibers. Similarly, the pattern of afferent input to the granule cells, especially that of GABAergic neurons, is more complex than previously assumed. In this respect the concept of a unidirectional "trisynaptic" pathway certainly is an oversimplification. In particular, the hilus of the fascia dentata, that the mossy fibers traverse on their way to regio inferior, is often neglected in this concept. The hilar region comprises a large variety of morphologically and functionally distinct neuronal types that, to a large extent, are targets of hilar mossy fiber collaterals. By focusing on the mossy fiber system, an attempt is made in this review to summarize new data on hippocampal circuitries that have been accumulated since the original description of the trisynaptic pathway. This concept, which originally comprised the synapses of the perforant path fibers on dentate granule cells, the mossy fiber synapses on CA3 pyramidal neurons, and the synapses of the Schaffer collaterals on CA1 pyramidal cells, has been of great heuristic value but needs to be modified in view of recent morphological and physiological data.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Complete axon arborization of a single CA3 pyramidal cell in the rat hippocampus, and its relationship with postsynaptic parvalbumin-containing interneurons

The complete axon arborization of a single CA3 pyramidal cell has been reconstructed from 32 (60 microns thick) sections from the rat hippocampus following in vivo intracellular injection of neurobiotin. The same sections were double-immunostained for parvalbumin--a calcium-binding protein selectively present in two types of GABAergic interneurons, the basket and chandelier cells--in order to map boutons of the pyramidal cell in contact with dendrites and somata of these specific subsets of interneurons visualized in a Golgi-like manner. The axon of the pyramidal cell formed 15,295 boutons, 63.8% of which were in stratum oriens, 15.4% in stratum pyramidale and 20.8% in stratum radiatum. Only 2.1% of the axon terminals contacted parvalbumin-positive neurons. Most of these were single contacts (84.7%), but double or triple contacts (15.3%) were also found. The majority of the boutons terminated on dendrites (84.1%) of parvalbumin-positive cells, less frequently on cell bodies (15.9%). In order to estimate the proportion of contacts representing synapses, 16 light microscopically identified contacts between boutons of the filled pyramidal cell axon and the parvalbumin-positive targets were examined by correlated electron microscopy. Thirteen of them were found to be asymmetrical synapses, and in the remaining three cases synapses between the labelled profiles could not be confirmed. We conclude that the physiologically effective excitatory connections between single pyramidal cells and postsynaptic inhibitory neurons are mediated by a small number of contacts, mostly by a single synapse. This results in a high degree of convergence and divergence in hippocampal networks.

Precision and variability in postsynaptic target selection of inhibitory cells in the hippocampal CA3 region

Non-pyramidal cells were filled intracellularly with biocytin in the CA3 region of the guinea-pig hippocampus in vitro, within or close to stratum pyramidale. On the basis of camera lucida reconstructions and electron microscopy, six different cell types with distinct laminar distribution of axon terminals could be distinguished. The axon of three axo-axonic cells, three typical basket cells, and atypical basket cells of two types arborized in the perisomatic and proximal dendritic region of CA3 pyramidal cells. Two cells with axons innervating the distal dendritic segments of pyramidal cells were also found; one terminated in stratum radiatum and the other in stratum lacunosum-moleculare. Electron microscopy demonstrated that symmetrical synapses were formed by the labelled boutons on axon initial segments, somata, and proximal or distal dendrites of mostly pyramidal neurons. Axo-axonic cells showed absolute target selectivity for axon initial segments, whereas for the other cells the distribution of contacted elements was determined by the laminar distribution of axon terminals. In two cases, where additional cells were labelled with biocytin, multiple (up to nine) light microscopically identified contacts (presumed synaptic contacts) were established by the interneurons on several pyramidal cells and on an axo-axonic cell. Our results show that a restricted set of inhibitory cells, with somata within or close to CA3 stratum pyramidale, possess variable patterns of axonal arborization. Various types of postsynaptic elements are contacted, but precision in selecting certain targets and ignoring others is maintained within a particular cell type and layer. In contrast to the diversity of axonal arbors the structure of the dendritic trees shows no consistent differences, suggesting that the cells may be activated by a similar set of afferents. It seems probable that the innervation of precise regions of postsynaptic pyramidal cells by different types of interneurons--often in conjunction with particular excitatory afferents (Han et al., Eur. J. Neurosci., 5, 395-410, 1993)--underlies functional differences in inhibitory synaptic actions.

Heterogeneity in presynaptic regulation of GABA release from hippocampal inhibitory neurons

Release of GABA from the terminals of hippocampal inhibitory neurons is inhibited by activation of GABAB autoreceptors and mu opioid receptors. However, it is not known whether these presynaptic processes affect all inhibitory synapses equally. We examined the effects of the GABAB receptor agonist baclofen and the mu opioid receptor agonist DAGO on postsynaptic currents evoked by minimal stimulation of inhibitory fibers (meIPSCs) in area CA3. Baclofen reversibly depressed approximately half of the meIPSCs evoked in the stratum pyramidale. The remaining meIPSCs were unaffected despite a coincident depression of spontaneous IPSCs. In contrast, all meIPSCs were depressed by DAGO. In addition, minimal stimulation in the stratum radiatum evoked meIPSCs that were always depressed by baclofen. These results indicate that regulation of GABA release by GABAB autoreceptors occurs at a subset of inhibitory synapses and that GABAB-resistant inhibitory synapses are located on pyramidal neuron somata. Hippocampal inhibitory neurons may be heterogeneous with respect to presynaptic receptor-mediated regulation of GABA release.

The behavior of mossy cells of the rat dentate gyrus during theta oscillations in vivo

Intracellular current clamp recordings were obtained from mossy cells (n = 6, identified by intracellular injection of biocytin) of the dorsal dentate gyrus from rats under ketamine-xylazine anesthesia. During electroencephalographic theta rhythm (4-6 Hz), recorded with a macroelectrode placed in the contralateral dorsal hippocampus near the fissure, mossy cells displayed intracellular membrane potential oscillations at low frequencies (4-6 Hz) which appeared to be phase locked to the electroencephalographic theta rhythm. The frequency of the intracellular theta rhythm was independent of the membrane potential. However, the phase difference between the intracellular and the electroencephalographic theta rhythms as well as the amplitude of the intracellular theta oscillations were voltage-dependent. These findings are consistent with the hypothesis that rhythmic GABAA receptor-mediated inhibitory postsynaptic potentials contribute to the genesis of the intracellular theta rhythm. Indeed, mossy cells displayed an early, fast inhibitory postsynaptic potential in response to electrical stimulation of the entorhinal cortex, which most likely represents a GABAA receptor-mediated event, indicating that mossy cells possess functional GABAA receptors. At the resting membrane potential, mossy cells did not fire at each cycle of the electroencephalographic theta rhythm but fired only rarely (< 1 Hz). However, when they did fire they did so preferentially in phase with the peak positivity of the electroencephalographic theta rhythm. Reconstruction of two mossy cells with axonal projections to the inner molecular layer showed that the spatial extent of the influence such weakly discharging mossy cells may have on other dentate gyrus neurons during theta oscillations can be several millimeters in the septotemporal direction. In conclusion, these findings show that mossy cells of the rat hilus during ketamine-xylazine anesthesia participate in theta oscillations of the hippocampal formation, during which their low-frequency firing may contribute to the phase-locking of a large number of spatially distributed postsynaptic neurons with postsynaptic sites in the inner molecular layer of the dentate gyrus.

The metabotropic glutamate receptor (mGluR1 alpha) is concentrated at perisynaptic membrane of neuronal subpopulations as detected by immunogold reaction

An antiserum to mGluR1 alpha labeled a 160 kd protein in immunoblots of membranes derived from rat brain or cells transfected with mGluR1 alpha. Immunoreactivity for mGluR1 alpha was present in discrete subpopulations of neurons. The GABAergic neurons of the cerebellar cortex were strongly immunoreactive; only some Golgi cells were immunonegative. Somatostatin/GABA-immunopositive cells in the neocortex and hippocampus were enriched in mGluR1 alpha. The hippocampal cells had spiny dendrites that were precisely codistributed with the local axon collaterals of pyramidal and granule cells. Electron microscopic immunometal detection of mGluR1 alpha showed a preferential localization at the periphery of the extensive postsynaptic densities of type 1 synapses in both the cerebellum and the hippocampus. The receptor was also present at sites in the dendritic and somatic membrane where synapses were not located.

Subdivisions in the multiple GABAergic innervation of granule cells in the dentate gyrus of the rat hippocampus

The sources of GABAergic innervation to granule cells were studied to establish how the basic cortical circuit is implemented in the dentate gyrus. Five types of neuron having extensive local axons were recorded electrophysiologically in vitro and filled intracellularly with biocytin (Han et al., 1993). They were processed for electron microscopy in order to reveal their synaptic organization and postsynaptic targets, and to test whether their terminals contained GABA. (1) The hilar cell, with axon terminals in the commissural and association pathway termination field (HICAP cell), formed Gray's type 2 (symmetrical) synapses with large proximal dendritic shafts (n = 18), two-thirds of which could be shown to emit spines, and with small dendritic branches (n = 6). Other boutons of the HICAP neuron were found to make either Gray's type 1 (asymmetrical) synapses (n = 4) or type 2 synapses (n = 6) with dendritic spines. Using a highly sensitive silver-intensified immunogold method for the postembedding visualization of GABA immunoreactivity, both the terminals and the dendrites of the HICAP cell were found to be immunopositive, whereas its postsynaptic targets were GABA-immunonegative. The dendritic shafts of the HICAP cell received synapses from both GABA-negative and GABA-positive boutons; the dendritic spines which densely covered the main apical dendrite in the medial one-third of the molecular layer received synapses from GABA-negative boutons. (2) The hilar cell, with axon terminals distributed in conjunction with the perforant path termination field (HIPP cell), established type 2 synapses with distal dendritic shafts (n = 17), most of which could be shown to emit spines, small-calibre dendritic profiles (n = 2) and dendritic spines (n = 6), all showing characteristics of granule cell dendrites. The sparsely spiny dendrites of the HIPP cell were covered with many synaptic boutons on both their shafts and their spines. (3) The cell with soma in the molecular layer had an axon associated with the perforant path termination field (MOPP cell). This GABA-immunoreactive cell made type 2 synapses exclusively on dendritic shafts (n = 20), 60% of which could be shown to emit spines. The smooth dendrites of the MOPP cell were also restricted to the outer two-thirds of the molecular layer, where they received both GABA-negative and GABA-positive synaptic inputs.(ABSTRACT TRUNCATED AT 400 WORDS)