FUNCTIONAL ACTIVATION OF SOMATOSTATIN- AND NEUROPEPTIDE Y-CONTAINING NEURONS IN THE ENTORHINAL CORTEX OF CHRONICALLY EPILEPTIC RATS A. VEZZANI,* R. MONHEMIUS, P. TUTKA, R. MILANI and R. SAMANIN Istituto di Ricerche Farmacologiche ‘‘Mario Negri’’, Via Eritrea 62, 20157, Milano, Italy Abstract––The in vitro release of somatostatin and neuropeptide Y, their tissue concentration and immunocytochemical pattern were examined in the entorhinal cortex of chronically epileptic rats. A systemic administration of 12 mg/kg kainic acid causing generalized tonic–clonic seizures for at least 3 h after injection was used to induce, 60 days later, a chronically enhanced susceptibility to seizures in the rats. The release of both peptides under depolarizing conditions was significantly reduced by 15% on average from slices of the entorhinal cortex two days after kainic acid-induced status epilepticus. At 60 days, the spontaneous and 30 mM KCl-induced release of somatostatin was significantly enhanced by 30% on average. The release induced by 100 mM KCl was raised by 70%. The spontaneous, 30 mM and 100 mM KCl-induced release of neuropeptide Y from the same slices was increased, respectively, by 120%, 76% and 36%. The late changes were associated with an increased tissue concentration of neuropeptide Y but not of somatostatin. This was confirmed by immunocytochemical evidence showing that neuropeptide Y-, but not somatostatin-immunoreactive neurons were increased in the entorhinal cortex of kainic acid-treated rats. These results indicate that neurotransmission mediated by somatostatin and neuropeptide Y, two peptides previously shown to play a role in limbic epileptogenesis, is enhanced in the entorhinal cortex of chronically epileptic rats. Copyright  1996 IBRO. Published by Elsevier Science Ltd. Key words: somatostatin, neuropeptide Y, entorhinal cortex, epilepsy, rats, seizures. Electrophysiological, immunocytochemical and pharmacological studies indicate that somatostatin- and neuropeptide Y (NPY)-containing neurons in the hippocampus play an important role in hippo- campal excitability in chronically epileptic rats. 3,7–9,13,14,23,26–31,34,36,39,40 The modifications in peptide immunoreactivity in the rat hippocampus more than 15 days after status epilepticus induced by kainic acid are reminescent of those in patients with temporal lobe epilepsy, 8,26,34 the most common medically uncontrolled epileptic condition, and are associated with changes in the neurons’ ability to release the peptides. 24,36 The entorhinal cortex (EC) has recently attracted attention on account of its possible involvement in epilepsy because of its susceptibility to pathological changes in animal models and humans, 10,11,32,34 its predisposition to chemically or electrically induced seizures 6,15,16,35,38 and its prominent role in control- ling hippocampal excitability. 15 The EC is rich in somatostatin- and NPY-positive neurons and their binding sites. 1,4,5,17–19,34 There is an inverse relation- ship between somatostatin content in the EC and interictal paroxysmal activity in the hippocampus of human epileptics, 9 suggesting that this peptide has inhibitory action on seizures similar to that recently described in the rat hippocampus. 20,23,24 To probe the neurophysiological mechanisms involved in the control of hippocampal excitability during epileptogenesis, we investigated whether lasting functional alterations in somatostatin- and NPY-containing neurons occurred in the EC of chronically epileptic rats. We assessed the release, tissue concentration and/or the immunocytochemical distribution of these peptides in the EC of rats, two and 60 days after kainic acid-induced limbic seizures. Previous studies have shown that these animals develop a chronically enhanced susceptibility to generalized convulsions from 15 days after kainic acid. 22,24,33,36 Considering the modulatory effects of these neuropeptides on synaptic transmission, 3,7,14,28 these observations may have relevance to the control of the limbic circuit in epileptogenesis. EXPERIMENTAL PROCEDURES Kainic acid treatment Male Sprague–Dawley rats (250–280 g, Charles River, Calco, Italy) were subcutaneously injected with 12 mg/kg kainic acid or saline (controls). Experiments were conducted *To whom correspondence should be addressed. A bbreviations: DAB, 3,3-diaminobenzidine; EC, entorhinal cortex; FCS, fetal calf serum; NPY, neuropeptide Y; PBS, phosphate-buffered saline. Pergamon N euroscience Vol. 75, No. 2, pp. 551–557, 1996 Copyright  1996 IBRO. Published by Elsevier Science Ltd Printed in Great Britain 0306–4522/96 $15.00+ 0.00 PII: S0306-4522(96)00261-8 551