Evidence that G(z)-proteins couple to hypothalamic 5-HT1A receptors in vivo

Using in situ hybridization and immunoblot analysis, the present studies id entified G(z) mRNA and G(z)-protein in the hypothalamic paraventricular nuc leus. The role of G(z)-proteins in hypothalamic 5-HT1A receptor signaling w as examined in vivo. Activation of 5-HT1A receptors increases the secretion of oxytocin and ACTH, but not prolactin. Intracerebroventricular infusion (3-4 d) of G(z) antisense oligodeoxynucleotides, with different sequences a nd different phosphorothioate modification patterns, reduced the levels of G(z)-protein in the hypothalamic paraventricular nucleus, whereas missense oligodeoxynucleotides had no effect. Neither antisense nor missense oligode oxynucleotide treatment altered basal plasma levels of ACTH, oxytocin, or p rolactin, when compared with untreated controls. An antisense-induced decre ase in hypothalamic G(z)-protein levels was paralleled by a significant dec rease in the oxytocin and ACTH responses to the 5-HT1A agonist 8-hydroxydip ropylamino- tetralin (8-OH-DPAT). In contrast, the prolactin response to 8- OH-DPAT (which cannot be blocked by 5-HT1A antagonists) was not inhibited b y G(z) antisense oligodeoxynucleotides. G(z)-proteins are the only members of the G(i)/G(o)-protein family that are not inactivated by pertussis toxin . In a control experiment, pertussis toxin treatment (1 mu g/5 mu l, i.c.v. ; 48 hr before the 8-OH-DPAT challenge) did not inhibit the ACTH response, potentiated the oxytocin response, and eliminated the prolactin response to 8-OH-DPAT. Thus, pertussis toxin-sensitive G(i)/G(o)-proteins do not media te the 5-HT1A receptor-mediated increase in ACTH and oxytocin secretion. Co mbined, these studies provide the first in vivo evidence for a key role of G(z)-proteins in coupling hypothalamic 5-HT1A receptors to effector mechani sms.