POSTSYNAPTIC TARGETS OF SOMATOSTATIN- IMMUNOREACTIVE INTERNEURONS IN THE RAT HIPPOCAMPUS I. KATONA, L. ACSA u DY and T. F. FREUND* Institute of Experimental Medicine, Hungarian Academy of Sciences, P.O. Box 67, Budapest H-1450, Hungary Abstract––Two characteristic interneuron types in the hippocampus, the so-called hilar perforant path-associated cells in the dentate gyrus and stratum 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 somatostatin-immunostaining, then reconstructed, and processed for double-immunostaining and electron microscopy to establish their postsynaptic target selectivity. A combination of somatostatin-immunostaining with immunostaining for GABA or other interneuron markers revealed a quasi-random termination pattern. The vast majority of postsynaptic targets were GABA-negative dendritic shafts and spines of principal cells (76%), whereas other target elements contained GABA (8%). All of the examined neurochemically defined interneuron types (parvalbumin-, calretinin-, vasoactive intestinal polypeptide-, cholecystokinin-, substance P receptor- immunoreactive neurons) received innervation from somatostatin-positive boutons. Recent anatomical and electrophysiological data showed that the main excitatory inputs of somatostatin-positive interneurons originate from local principal cells. The present data revealed a massive GABAergic innervation of distal dendrites of local principal cells by these feedback driven neurons, which are proposed to control the efficacy and plasticity of entorhinal synaptic input as a function of local principal cell activity and synchrony.  1998 IBRO. Published by Elsevier Science Ltd. Key words: inhibition, non-pyramidal cells, neuropeptides, feedback inhibition, GABA, theta. Interneurons in the cerebral cortex are known to play a crucial role in controlling the activity of large ensembles of principal cells. Recent electrophysio- logical and anatomical studies shed light on the specific function of various interneuron types in the regulation of population behaviour of principal cells at different nodes in the hippocampal network (for review see Ref. 22). Intracellular labelling of interneurons with biocy- tin in vitro and in vivo 15,16,26,59,61 and immunostain- ing for selected neurochemical markers or their combinations are the most powerful tools to investi- gate the precise connectivity, and neurochemical fea- tures of an interneuron type. The former allows a direct correlation of electrophysiological properties with the axonal and dendritic arborization pattern of a given cell, whereas the latter can visualize large populations of a specific cell type and allows corre- lation of neurochemical characteristics with certain morphological features. By using these methods, many distinct interneuron types were classified in the hippocampus. It was shown that perisomatic inhibi- tory neurons (i.e. basket and axoaxonic cells) contain either the calcium-binding protein parvalbumin 35 or the neuropeptides cholecystokinin (CCK) 51 and vasoactive intestinal polypeptide (VIP). 1 Inhibitory neurons that predominantly target the dendrites of principal cells (i.e. dendritic inhibitory cells) include two major types that differ in their layer of termin- ation and a fferent input. Bistratified cells 16 innervate pyramidal cell dendrites in strata radiatum and oriens (in conjunction with Scha ffer collaterals) and a large proportion of them contains the calcium-binding protein calbindin. 59,61 The other characteristic den- dritic inhibitory neuron type has a dendritic tree restricted to those layers where local collaterals of principal cells of the given hippocampal subfield arborize, whereas it’s axon terminates on distal den- drites of principal cells in conjunction with entorhinal a fferents. These interneurons are the so-called oriens/ lacunosum-moleculare (O-LM) cells in the CA1 47,61 and CA3 regions 26 of the hippocampus and the hilar perforant path-associated (HIPP) cells in the hilus of the dentate gyrus. 15,32,59 *To whom correspondence should be addressed. A bbreviations: ABC, avidin–biotin–horseradish peroxidase complex; ACPD, 1S,3R-aminocyclopentane dicarboxylic acid; BSA, bovine serum albumin; CCK, cholecystokinin; DAB, 3,3-diaminobenzidine–4HCl; EPSP, excitatory postsynaptic potential; HIPP, hilar perforant path- associated; IPSP, inhibitory postsynaptic potential; LTP, long-term potentiation; M2, type 2 muscarinic receptor; mGluR1, type 1 metabotropic glutamate receptor; NGS, normal goat serum; NPY, neuropeptide Y; O-LM, strata oriens/lacunosum-moleculare; PB, phosphate buffer; SPR, substance P receptor; TBS, Tris-buffered saline; VIP, vasoactive intestinal polypeptide. Pergamon N euroscience Vol. 88, No. 1, pp. 37–55, 1999 Copyright  1998 IBRO. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0306–4522/99 $19.00+ 0.00 PII: S0306-4522(98)00302-9 37