Deficits in passive avoidance learning in young rats following mecamylamine injections in the hippocampo-entorhinal area.

Young rats 11, 13, 16, and 20 days old were injected bilaterally with the nicotinic antagonist mecamylamine hydrochloride (5, 50, and 100 micrograms on each side) into the posteroventral hippocampo-entorhinal (VHE) area and trained on a "cool-draft-stimulus" passive avoidance task. The data showed impaired acquisition and reduced resistance to extinction. The deficits observed were age- and dose-dependent, rats being highly sensitive to the drug when 11 and 13 days old and decreasingly responsive up to day 20. The results may indicate that nicotinic cholinergic sites in the VHE area mediate passive avoidance learning in the young rat as soon as acquisition emerges. Muscarinic cholinergic mechanisms only develop later in this region, becoming progressively more important for passive avoidance behavior.