SYNCHRONIZATION of neuronal activity is fundamental in the operation of cortical networks. With respect to an ongoing synchronized oscillation, the precise timing of action potentials is an attractive candidate mechanism for information coding. Networks of inhibitory interneurons have been proposed to have a role in entraining cortical, synchronized 40-Hz activity. Here we demonstrate that individual GABAergic interneurons can effectively phase spontaneous firing and subthreshold oscillations in hippocampal pyramidal cells at 0 frequencies (4-7 Hz). The efficiency of this entrainment is due to interaction of GABAA-receptor-mediated hyperpolarizing synaptic events with intrinsic oscillatory mechanisms tuned to this frequency range in pyramidal cells. Moreover, this GABAergic mechanism is sufficient to synchronize the firing of pyramidal cells. Thus, owing to the divergence of each GABAergic interneuron, more than a thousand pyramidal cells may share a common temporal reference established by an individual interneuron.

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.

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)

1. Hippocampal non-principal neurons at the stratum radiatum-stratum lacunosummoleculare border (R-LM interneurons) of the CA1 area may constitute several cell classes and have been implicated in the generation of GABAergic unitary IPSPs. Using biocytinfilled electrodes we recorded R-LM interneurons intracellularly in vitro and determined their postsynaptic effects in concomitantly recorded pyramidal cells. 2. Light microscopic analysis revealed four populations of R-LM interneurons with distinct axons: (1) basket cells (n = 4) with axons predominantly ramifying in the pyramidal cell layer; (2) Schaffer collateralÏcommissural pathway-associated interneurons (n = 10) stratifying in stratum radiatum and, to a lesser extent, stratum oriens; (3) perforant pathway-associated interneurons (n = 6) innervating the perforant path termination zone in stratum lacunosum-moleculare of the CA1 area as well as equivalent portions of the dentate gyrus and subiculum; and (4) neurogliaform interneurons (n = 2) characterized by their dense, compact axonal and dendritic arbour. 3. Random electron microscopic sampling of synaptic targets revealed a preponderance of pyramidal neurons as postsynaptic elements. Basket cells had a synaptic target preference for somata and proximal dendrites, whereas the remainder of R-LM interneurons innervated dendritic shafts and spines. The axon of dendrite-targeting cells formed up to six putative contacts with individual postsynaptic pyramidal cells. 4. Anatomically recovered R-LM interneurons (n = 22) had a mean resting membrane potential of −56·7 ± 3·6 mV, a membrane time constant of 12·9 ± 7·7 ms and an input resistance of 86·4 ± 29·2 MÙ. Depolarizing current pulses generally elicited overshooting action potentials (70·8 ± 6·9 mV) which had a mean duration, when measured at halfamplitude, of 0·7 ± 0·1 ms. In response to prolonged ( > 200 ms) depolarizing current pulses all R-LM interneurons displayed (a varying degree of) spike frequency adaptation. 5. Basket cells, Schaffer-associated and neurogliaform interneurons elicited small-amplitude (< 2 mV), short-latency IPSPs in postsynaptic pyramids (n = 5, 13 and 1, respectively). Those interactions in which an effect was elicited with the repetitive activation of the presynaptic neuron (n = 13) showed a substantial degree of postsynaptic response summation. Unitary IPSPs had fast kinetics and, whenever tested (n = 5; 1 basket cell and 4 Schaffer-associated interneurons), were abolished by the GABAA receptor antagonist bicuculline. 6. Thus, R-LM interneurons comprise several distinct populations which evoke fast GABAA receptor-mediated IPSPs. The domain-specific innervation of postsynaptic pyramidal cells suggests functionally diverse effects on the integration of afferent information in functionally non-equivalent compartments of pyramidal cells.

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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 μm) in the medio‐lateral direction than that of basket cells (760 ± 130 μm). The axon terminals of bistratified cells were smaller than those of basket cells. Furthermore, in contrast 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. © 1996 Wiley‐Liss, Inc.

Hippocampal pyramidal cells receive GABA-mediated synaptic input from several distinct interneurons. In order to define the effect of perisomatic synapses, intracellular recordings were made with biocytin-containing microelectrodes from synaptically connected inhibitory and pyramidal cell pairs in subfields CA1 and CA3 of the rat hippocampus. Subsequent physiological analysis were restricted to the category of cells, here referred to as basket cells (n = 14), which had an efferent synaptic target profile (n = 282 synaptic contacts) of predominantly somatic (48.2%) and proximal dendritic synapses (45.0%). Electron microscopic analysis revealed that in two instances identified postsynaptic pyramidal cells received a total of 10 and 12 labelled basket cell synapses respectively. At an average membrane potential of -57.8 +/- 4.6 mV, unitary inhibitory postsynaptic potentials (IPSPs; n = 24) had a mean amplitude of 450 +/- 238 microV, a 10-90% rise time of 4.6 +/- 3.2 ms and, measured at half-amplitude, a mean duration of 31.6 +/- 18.2 ms. In most instances (n = 19) the IPSP decay could be fitted with a single exponential with a mean time constant of 32.4 +/- 18.0 ms. Unitary basket cell-evoked IPSPs (n = 5) was extrapolated to be at -74.9 +/- 6.0 mV. Averages of unitary IPSPs had a mean calculated conductance of 0.95 +/- 0.29 nS, ranging from 0.52 to 1.16 nS. Unitary basket cell IPSPs (n = 3) increased in amplitude by 26.6 +/- 19.9% following bath application of the GABAB receptor antagonist CGP 55845A [correction of CGP 35845A] (1-4 microM), whereas subsequent addition of the GABAA receptor antagonist bicuculline (10-13 microM) reduced the IPSP amplitude to 13.5 +/- 3.1% of the control response. Rapid presynaptic trains of basket cell action potentials resulted in the summation of up to four postsynaptic responses (n = 5). However, any increase in the rate of tonic firing (2- to 10-fold) led to a > 50% reduction of the postsynaptic response amplitude. At depolarized membrane potentials, averaged IPSPs could be followed by a distinct depolarizing overshoot or postinhibitory facilitation (n = 4). At firing threshold, pyramidal cells fired postinhibitory rebound-like action potentials, the latter in close temporal overlap with the depolarizing overshoot. In conclusion, hippocampal basket cells have been identified as one source of fast, GABAA receptor-evoked perisomatic inhibition. Unitary events are mediated by multiple synaptic release sites, thus providing an effective mechanism to avoid total transmission failures.