GABAB 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 intracellular second messengers. In the present study, we have investigated whether a positive coupling of GABA(B) receptors to adenylyl cyclase could be 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 to stimulate the cyclase activity. 2 The GABA(B) receptor agonist (-)-baclofen significantly increased basal adenylyl cyclase activity in membranes of the granule cell and external plexiform 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-aminopropylphosphinic acid (pD(2)=4.60) and GABA (pD(2)=3.56), but not by (+)-baclofen, 3-aminopropylphosphonic acid, muscimol and isoguvacine. The stimulatory effect 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). Phaclofen (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 transducin, a beta gamma scavenger. The beta gamma subunits of transducin stimulated the cyclase activity and this effect was not additive with that produced by (-)-baclofen. 5 In the external plexiform and granule cell layers, but not in the olfactory nerve-glomerular layer, (-)-baclofen enhanced the adenylyl cyclase stimulation 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 olfactory bulb layers examined. 7 These data demonstrate that in specific layers of rat olfactory bulb activation 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 inhibitory effect on forskolin- and Ca2+/CaM-stimulated cyclic AMP formation.