IDENTIFICATION AND CHARACTERIZATION OF MUSCARINIC RECEPTORS POTENTIATING THE STIMULATION OF ADENYLYL-CYCLASE ACTIVITY BY CORTICOTROPIN-RELEASING HORMONE IN MEMBRANES OF RAT FRONTAL-CORTEX

In membranes of the rat frontal cortex, acetylcholine (ACh) and other cholinergic agonists were found to potentiate the stimulation of adeny lyl cyclase activity elicited by corticotropin-releasing hormone (CRH) . Oxotremorine-M, carbachol and methacholine were as effective as ACh, whereas oxotremorine and arecoline were much less effective. The faci litating effect of Ach was potently blocked by the M-1 antagonists R-t rihexyphenidyl, telenzepine and pirenzepine and by the M-3 antagonists hexahydro-sila-difenidol and p-fluorohexahydro-sila-difenidol, wherea s the M-2 and M-4 antagonists himbacine, methoctramine, AF-DX 116 and AQ-RA 741 were less potent. The mamba venom toxin MT-1, which binds wi th high affinity to M-1 receptors, was also a potent blocker. The phar macological profile of the muscarinic potentiation of CRH receptor act ivity was markedly different from that displayed by the muscarinic inh ibition of forskolin-stimulated adenylyl cyclase, which could be detec ted in the same membrane preparations. Moreover, the intracerebral inj ection of pertussis toxin impaired the muscarinic inhibition of cyclic AMP formation and reduced the Ach stimulation of [S-35]GTP gamma S bi nding to membrane G proteins but failed to affect the facilitating eff ect on CRH receptor activity. The latter response was also insensitive to the phospholipase C inhibitor U-73122, the protein kinase inhibito r staurosporine and to the inhibitors of arachidonic acid metabolism i ndomethacin and nordihydroguaiaretic acid. These data demonstrate that in the rat frontal cortex, muscarinic receptors of the M-1 subtype po tentiate CRH transmission by interacting with pertussis toxin-insensit ive G proteins.