Regional distribution of the muscarinic cholinoceptor and acetylcholinesterase in guinea pig brain

The muscarinic receptors in membranes prepared from guinea pig brain were studied using a radiolabeled antagonist, [³H]quinuclidinyl benzilate (QNB). The apparent dissociation constant of the QNB-receptor complex (K_d) was similar in all regions, but the concentration of receptors was highest in the striatum, cerebral cortex, and hippocampus and lowest in the cerebellum. Similar distributions have been reported for other species, although the concentration of receptors in guinea pig brain is higher than in other species. Acetylcholine inhibited QNB binding with a Hill coefficient of 0.4–0.6. The concentration of acetylcholine required to inhibit binding by 50% (I₅₀) was lowest in the brain stem and more than 10 times higher in the hippocampus. Similar results have been reported for mouse brain. The activity of acetylcholinesterase was highest in the striatum, where the concentration of muscarinic receptors is highest, but did not vary greatly in other brain regions.