Hippocampal nicotinic cholinergic mechanisms mediate spontaneous alternation and fear during ontogenesis but not later in the rat.

Spontaneous alternation was examined in young rats following microinjections of antinicotinic agents into one of the 4 hippocampal sites: anterodorsal, or posteroventral dentate gyrus, hippocampal gyrus, or entorhinal cortex. In control and saline-injected animals, the alternation rate was shown to grow suddenly from 40 to 80% between days 15 and 17 (the adult level being 85-90%), to regress partly (to 55%) between days 20 and 30, and return to a near-adult level (75%) by day 40. Meanwhile fear responses to environment (defecation and vocalization) emerged between days 20 and 25, increased to a maximum until day 30, and returned to the typically low adult level by day 40. Injections of mecamylamine (5, 20 micrograms) or hexamethonium (5, 20 micrograms) into any of the 4 sites significantly reduced the rate of alternation from as early as day 10 on, but were no longer effective from day 30 on; on the other hand, they did not alter the level of defecation, but had a tendency to lower the level of vocalization on day 30 only. These results indicate that hippocampal nicotinic cholinergic mechanisms play a role in spontaneous alternation and appear to be involved in the control of one fear reaction (vocalization) until day 30.