CHANGES IN RAT-BRAIN MUSCARINIC RECEPTORS AFTER INHIBITORY AVOIDANCE-LEARNING

It is widely accepted that cerebral acetylcholine is necessary for lea rning and memory, but little is known about the type of muscarinic rec eptors involved in these functions. To investigate this problem, [H-3] -N-methyl-scopolamine which binds to different types of muscarinic rec eptors, [H-3]-Pirenzepine an M(1) receptor antagonist, and [H-3]-Oxotr emorine-M which binds mainly to M(2) receptors, were used as ligands t o look for possible changes in muscarinic receptor density in neostria tum (NEO), hippocampus (HIP), amygdala (AMY), and temporo-parietal neo cortex (CTX), after tec:ting for retention of inhibitory avoidance, tr ained with high or low footshock intensities. After low reinforcement there was an M(1) postsynaptic receptor up-regulation in NEO, HIP, and CTX, and an M(2) presynaptic receptor down-regulation in HIP, which s uggests a concerted pre- and postsynaptic cholinergic activation in th is area. An up-regulation of both M(1) and M(2) receptors was detected in CTX of low and footshocked animals, which indicates the presence o f a cortical postsynaptic M(2) receptor.