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 use d dual intracellular recordings with biocytin-filled microelectrodes a nd investigated synaptic interactions between pairs of interneurons in area CA1 of the adult rat hippocampus. Of four synaptically-coupled i nterneuron-to-interneuron cell pairs, three presynaptic cells were ide ntified as basket cells, preferentially innervating somata and proxima l dendrites of pyramidal cells, but one differing from the other two i n the laminar distribution of its dendritic and axonal fields. The fou rth presynaptic interneuron was located at the border between strata l acunosum moleculare and radiatum, with axon ramifying within stratum r adiatum. Action potentials evoked in all four presynaptic interneurons were found to elicit fast hyperpolarizing inhibitory postsynaptic pot entials (mean amplitude 0.35 +/- 0.10 mV at a membrane potential of -5 9 +/- 2.8 mV) in other simultaneously recorded interneurons (n=4). In addition, three of the presynaptic interneurons were also shown to pro duce similar postsynaptic responses in subsequently recorded pyramidal cells (n=4). Electron microscopic evaluation revealed one of the pres ynaptic basket cells to form 12 synaptic junctions with the perisomati c domain (seven somatic synapses and five synapses onto proximal dendr itic shafts) of the postsynaptic interneuron in addition to innervatin g the same compartments of randomly-selected local pyramidal cells (5O % somatic and 50% proximal dendritic synapses, n=12). In addition, lig ht microscopic analysis also indicated autaptic self-innervation in ba sket (12 of 12) and bistratified cells (six of six). Electron microsco pic investigation of one basket cell confirmed six autaptic junctions made by five of its boutons. Together, these data demonstrate that sev eral distinct types of interneuron have divergent output to both princ ipal cells and local interneurons of the same (basket cells) or differ ent type. The fast synaptic effects, probably mediated by GABA in both postsynaptic interneurons and principal cells are similar. These addi tional sources of GABA identified here in the input to GABAergic cells could contribute to the differential temporal patterning of distinct GABAergic synaptic networks. (C) 1997 IBRO. Published by Elsevier Scie nce Ltd.