Differential regulation of subtypes m1-m5 of muscarinic receptors in forebrain by chronic atropine administration.

The regulation of individual muscarinic receptor subtypes in rat cerebral cortex/dorsal hippocampus was examined following 14-day administration of the nonselective antagonist atropine. Total muscarinic receptor density increased 24%, from 2196 fmol/mg to 2722 fmol/mg. The nature of this increase was examined using a panel of antisera selective for the m1 to m5 muscarinic receptors. Thus, 97% of all cortical/hippocampal receptors were accounted for by immunoprecipitation. Three subtypes were observed to increase significantly in density: m1 receptor from 824 to 982 fmol/mg (19%, P less than .05); m2 receptor from 476 to 519 fmol/mg (9%, N.S.); m3 receptor from 259 to 438 fmol/mg (69%, P less than .001); m4 receptor from 574 to 638 fmol/mg (11%, P less than .05); and the m5 receptor from 23 to 38 fmol/mg (65%, N.S.). Receptors coupled to the hydrolysis of phosphoinositides (m1, m3, m5) appeared to be preferentially up-regulated (32% over control levels, P less than .001) compared with those coupled to the inhibition of adenylyl cyclase (m2, m4; 10% over control levels, P less than .05). The absolute density of the molecularly defined m1 and m4 receptors, which reportedly possess the highest affinity for pirenzepine (M1), increased 16% (P less than .05), whereas the density of receptors having the lowest affinity for pirenzepine (m2, m3 and m5) increased 31% (P less than .001) with atropine treatment. When the increase in total receptor density was examined with [3H]pirenzepine, the 16% elevation in high-affinity (m1 and m4) sites was not detected.(ABSTRACT TRUNCATED AT 250 WORDS)