POSTSYNAPTIC TARGETS OF SOMATOSTATIN-IMMUNOREACTIVE INTERNEURONS IN THE RAT HIPPOCAMPUS

Two characteristic interneuron types in the hippocampus, the so-called hilar perforant path-associated cells in the dentate gyrus and stratu m oriens/lacunosum-moleculare neurons in the CA3 and CA1 regions, were suggested to be involved in feedback circuits. In the present study, interneurons identical to these cell populations were visualized by so matostatin-immunostaining, then reconstructed, and processed for doubl e-immunostaining and electron microscopy to establish their postsynapt ic target selectivity. A combination of somatostatin-immunostaining wi th immunostaining for GABA or other interneuron markers revealed a qua si-random termination pattern. The vast majority of postsynaptic targe ts were GABA-negative dendritic shafts and spines of principal cells ( 76%), whereas other target elements contained GABA (8%). All of the ex amined neurochemically defined interneuron types (parvalbumin-, calret inin-, vasoactive intestinal polypeptide-, cholecystokinin-, substance P receptor-immunoreactive neurons) received innervation from somatost atin-positive boutons. Recent anatomical and electrophysiological data showed that the main excitatory inputs of somatostalin-positive inter neurons originate from local principal cells. The present data reveale d a massive GABAergic innervation of distal dendrites of local princip al cells by these feedback driven neurons, which are proposed to contr ol the efficacy and plasticity of entorhinal synaptic input as a funct ion of local principal cell activity and synchrony. (C) 1998 IBRO. Pub lished by Elsevier Science Ltd.