STIMULATION OF GUANOSINE 5'-O-(3-[S-35] THIOTRIPHOSPHATE) BINDING BY CHOLINERGIC MUSCARINIC RECEPTORS IN MEMBRANES OF RAT OLFACTORY-BULB

In membranes of rat olfactory bulb, a brain region in which muscarinic agonists increase cyclic AMP formation, the muscarinic stimulation of guanosine 5'-O-(3-[S-35]thiotriphosphate) ([S-35]GTP gamma S) binding was used as a tool to investigate the receptor interaction with the g uanine nucleotide-binding regulatory proteins (G proteins). The stimul ation of the radioligand binding by carbachol (CCh) was optimal (three fold increase) in the presence of micromolar concentrations of GDP and 100 mM NaCl. Exposure to N-ethylmaleimide and pertussis toxin markedl y inhibited the CCh effect, whereas it increased the relative stimulat ion of [S-35]GTP gamma S binding elicited by pituitary adenylate cycla se-activating polypeptide (PACAP). On the other hand, membrane treatme nt with cholera toxin curtailed the PACAP stimulation of [S-35]GTP gam ma S binding but did not affect the response to CCh. Like CCh, a numbe r of cholinergic agonists stimulated [S-35]GTP gamma S binding in a co ncentration-dependent and saturable manner. The antagonist profile of the muscarinic stimulation of [S-35]GTP gamma S binding was highly cor related with that displayed by the muscarinic stimulation of adenylyl cyclase. These data indicate that the olfactory bulb muscarinic recept ors couple to G(i)/G(o), but not to G(s), and support the possibility that activation of G(i)/G(o) mediates the stimulatory effect on adenyl yl cyclase activity.