GABA(B) receptor-mediated stimulation of adenylyl cyclase activity in membranes of rat olfactory bulb

1 Previous studies have shown that GABA(B) receptors facilitate cyclic AMP formation in brain slices likely through an indirect mechanism involving in tracellular second messengers. In the present study, we have investigated w hether a positive coupling of GABA(B) receptors to adenylyl cyclase could b e detected in a cell-free preparation of rat olfactory bulb, a brain region where other G(i)/G(o)-coupled neurotransmitter receptors have been found t o stimulate the cyclase activity. 2 The GABA(B) receptor agonist (-)-baclofen significantly increased basal a denylyl cyclase activity in membranes of the granule cell and external plex iform layers, but not in the olfactory nerve-glomerular layer. The adenylyl cyclase stimulation was therefore examined in granule cell layer membranes . 3 The (-)-baclofen stimulation (pD(2)=4.53) was mimicked by 3-aminopropylph osphinic acid (pD(2)=4.60) and GABA (pD(2)=3.56), but not by (+)-baclofen, 3-aminopropylphosphonic acid, muscimol and isoguvacine. The stimulatory eff ect was counteracted by the GABA(B) receptor antagonists CGP 35348 (pA(2) = 4.31), CGP 55845 A (pA(2)= 7.0) and 2-hydroxysaclofen (pK(i) =4.22). Phaclo fen (1 mM) was inactive. 4 The (-)-baclofen stimulation was not affected by quinacrine, indomethacin , nordihydroguaiaretic acid and staurosporine, but was completely prevented by pertussis toxin and significantly reduced by the alpha subunit of trans ducin, a beta gamma scavenger. The beta gamma subunits of transducin stimul ated the cyclase activity and this effect was not additive with that produc ed by (-)-baclofen. 5 In the external plexiform and granule cell layers, but not in the olfacto ry nerve-glomerular layer, (-)-baclofen enhanced the adenylyl cyclase stimu lation elicited by the neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) 38. 6 Conversely, the adenylyl cyclase activity stimulated by either forskolin or Ca2+/calmodulin(Ca2+/CaM) was inhibited by (-)-baclofen in all the olfac tory bulb layers examined. 7 These data demonstrate that in specific layers of rat olfactory bulb acti vation of GABA(B) receptors enhances basal and neurotransmitter-stimulated adenylyl cyclase activities by a mechanism involving beta gamma subunits of G(i)/G(o). This positive coupling is associated with a widespread inhibito ry effect on forskolin- and Ca2+/CaM-stimulated cyclic AMP formation.