Changes in rat brain muscarinic receptors after inhibitory avoidance learning

It is widely accepted that cerebral acetylcholine is necessary for learning and memory, but little is known about the type of muscarinic receptors involved in these functions. To investigate this problem, [H-3]-N-methyl-scopolamine which binds to different types of muscarinic receptors, [H-3]-Pirenzepine an M(1) receptor antagonist, and [H-3]-Oxotremorine-M which binds mainly to M(2) receptors, were used as ligands to look for possible changes in muscarinic receptor density in neostriatum (NEO), hippocampus (HIP), amygdala (AMY), and temporo-parietal neocortex (CTX), after tec:ting for retention of inhibitory avoidance, trained 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 suggests a concerted pre- and postsynaptic cholinergic activation in this 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 of a cortical postsynaptic M(2) receptor.