Evidence for G protein mediation of serotonin- and GABA_B- induced hyperpolarization of rat dorsal raphe neurons

In vitro intracellular recording techniques in the rat brain slice preparation demonstrate that both serotonin (5-HT) and baclofen (a GABA_B-receptor agonist) inhibit 5-HT neurons in the dorsal raphe nucleus by inducing a hyperpolarization of membrane potential and a decrease in apparent input resistance (R_in). Similar to previous results with 5-HT, baclofen-mediated inhibition of 5-HT neurons also shows an apparent reversal potential (E_rev) of approximately-90 mV, consistent with mediation by K channels. In slices from rats that had previously received a local injection of pertussis toxin (0.5 μg) immediately rostral to the dorsal raphe nucleus, there was a virtually complete blockade of inhibition induced by both the serotonin autoreceptor and the GABA_B-receptor. Intracellular injection of the stable GTP analog (guanosine-5-O-(3-thiotriphosphate); GTPγS) mimicked the actions of both 5-HT and baclofen. The inhibitory actions of GTPγS were not additive with those of either 5-HT or baclofen, suggesting they share some common effector system. The stable cAMP analog (8-bromo-adenosine-3′, 5′-cyclic monophosphate (8-Br-cAMP) had no effect on membrane potential or apparent input resistance and did not block the inhibitory actions mediated by 5-HT or baclofen. The local injection of pertussis toxin (0.5 γg) caused a far greater blockade of 5-HT and baclofen-mediated inhibition than the intracerebroventricular (i.c.v.) injection of pertussis toxin (1.0 γg). In parallel sets of animals with i.c.v. and local injections, we measured the pertussis toxin-mediated ADP-ribosylation of G proteins in membranes prepared from dorsal raphe nucleus. These biochemical studies showed that sensitivities to 5-HT and baclofen correlated with the concentration of remaining non-ADP-ribosylated G proteins following in vivo pertussis toxin injection. In summary, these results provide evidence for the role of a G protein(s) in the mediation of the cAMP-independent increase in potassium conductance in 5-HT neurons of dorsal raphe nucleus induced by both 5-HT_1A and GABA_B-receptors.