Dopamine receptor coupling to adenylyl cyclase in rat olfactory pathway: A combined 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-dependent enhancement of olfactory memory and retro-inhibition of olfactory input within the olfactory bulb, respectively. We further assessed, in the present study, the mechanisms of these dopamine actions by using adenylyl cyclase activity assay and [S-35]GTP radioautography in rat olfactory bulb and mucosa. The D-1 agonist SKF 38393 increased adenylyl cyclase activity on membranes of the olfactory bulb, but not on those of the olfactory mucosa. Stimulation 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, reaching 40% over basal adenylyl cyclase activity, and was blocked by the D-1 antagonist SCH 23390. The D-2 agonists bromocriptine and quinpirole inhibited both basal and forskolin-stimulated adenylyl cyclase activities in the olfactory bulb and mucosa. These adenylyl cyclase inhibitions were dose dependent, with EC50 values of 0.1-0.3 mu M for bromocriptine and 1-3 mu M for quinpirole, 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 of 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 agonist-induced inhibition of adenylyl cyclase was shown to occur only in lamina propria, thus co-localizing with D-2 binding sites. [S-35]GTP radioautography 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 Elsevier Science Ltd.