Evidence for G protein mediation of serotonin- and GABAB-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 GABAB-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 (Rin). Similar to previous results with 5-HT, baclofen-mediated inhibition of 5-HT neurons also shows an apparent reversal potential (Erev) 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 microgram) immediately rostral to the dorsal raphe nucleus, there was a virtually complete blockade of inhibition induced by both the serotonin autoreceptor and the GABAB-receptor. Intracellular injection of the stable GTP analog (guanosine-5'-O-(3-thiotriphosphate); GTP gamma S) mimicked the actions of both 5-HT and baclofen. The inhibitory actions of GTP gamma S 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 microgram) caused a far greater blockade of 5-HT and baclofen-mediated inhibition than the intracerebroventricular (i.c.v.) injection of pertussis toxin (1.0 micrograms). 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-HT1A- and GABAB-receptors.