Dopamine receptor coupling to adenylyl cyclase in rat olfactory pathway: Acombined pharmacological-radioautographic approach

Dopamine binding sites of D-1 and D-2/D-3 subtypes had been detected in the rat peripheral olfactory system and postulated to account for dopamine-dep endent enhancement of olfactory memory and retro-inhibition of olfactory in put within the olfactory bulb, respectively. We further assessed, in the pr esent study, the mechanisms of these dopamine actions by using adenylyl cyc lase activity assay and [S-35]GTP radioautography in rat olfactory bulb and mucosa. The D-1 agonist SKF 38393 increased adenylyl cyclase activity on m embranes of the olfactory bulb, but not on those of the olfactory mucosa. S timulation of adenylyl cyclase by SKF 38393 in the olfactory bulb was dose dependent, with a half-maximal effect (EC50) at 0.16 mu M SKF 38393, reachi ng 40% over basal adenylyl cyclase activity, and was blocked by the D-1 ant agonist SCH 23390. The D-2 agonists bromocriptine and quinpirole inhibited both basal and forskolin-stimulated adenylyl cyclase activities in the olfa ctory bulb and mucosa. These adenylyl cyclase inhibitions were dose depende nt, with EC50 values of 0.1-0.3 mu M for bromocriptine and 1-3 mu M for qui npirole, equal to 25% of basal enzyme activity at concentrations of 1-10 mu M, and were blocked by the D-2 antagonist eticlopride. The D-2 antagonist was devoid of any effect on basal and forskolin-stimulated adenylyl cyclase activities in the olfactory bulb and mucosa. Odorant-induced stimulation o f adenylyl cyclase was blocked by D-2 agonist in olfactory mucosa membranes , which suggests dopaminergic regulation of odor detection in the olfactory mucosa. By using microdissected fractions of the olfactory mucosa, D-2 ago nist-induced inhibition of adenylyl cyclase was shown to occur only in lami na propria, thus co-localizing with D-2 binding sites. [S-35]GTP radioautog raphy on tissue sections revealed D-2 agonist-induced G-protein activation in olfactory nerve and glomerular layers of the olfactory bulb, and in the chorion of the olfactory mucosa. Taken together, these data demonstrate functional coupling of the dopamine receptors with adenylyl cyclase in both the olfactory bulb and mucose, and document novel aspects of dopamine's physiological involvement in olfaction and of D-2-mediated signal transduction. (C) 1999 IBRO. Published by Elsev ier Science Ltd.