LOSS OF HILAR SOMUTOSTATIN NEURONS FOLLOWING TETANUS TOXIN-INDUCED SEIZURES

A loss of inhibitory interneurons has been reported in the hippocampus following seizure activity in various animal models of epilepsy and i n human epileptic tissue. The question of whether particular populatio ns of inhibitory neurons are similarly affected by the chronic block o f inhibition that results after tetanus toxin injections directly into the brain has not previously been addressed. In the present study a u nilateral intrahippocampal injection of tetanus toxin into the ventral hippocampus was used to produce a chronic epileptic syndrome characte rised by brief seizures that recurred intermittently for 6-8 weeks. Th e results reveal, for the first time, the morphological changes in som atostatin interneurons following tetanus toxin-induced seizures in the rat. A bilateral short-term increase in immunoreactivity of somatosta tin neurons is present 1 week after injection. This is accompanied by an increased intensity of somatostatin-immunoreactive axon terminals i n the outer molecular layer of the dentate gyrus, which is more marked on the contralateral side. A chronic and significant loss of somatost atin-immunoreactive neurons was noted in the hilus of the dentate gyru s 2 months later. The significance of the chronic loss of the hilar so matostatin neurons in the control of excitatory activity in the dentat e gyrus and whether the acute morphological changes are due to a direc t action of the toxin on release mechanisms or as a result of seizure activity are discussed.