Evidence that GZ proteins couple to hypothalamic 5-HT1A receptors in vivo

Abstract

Using in situ hybridization and immunoblot analysis, the present studies identified Gz mRNA and Gz-protein in the hypothalamic paraventricular nucleus. The role of Gz-proteins in hypothalamic 5-HT1Areceptor signaling was examined in vivo. Activation of 5-HT1A receptors increases the secretion of oxytocin and ACTH, but not prolactin. Intracerebroventricular infusion (3–4 d) of Gz antisense oligodeoxynucleotides, with different sequences and different phosphorothioate modification patterns, reduced the levels of Gz-protein in the hypothalamic paraventricular nucleus, whereas missense oligodeoxynucleotides had no effect. Neither antisense nor missense oligodeoxynucleotide treatment altered basal plasma levels of ACTH, oxytocin, or prolactin, when compared with untreated controls. An antisense-induced decrease in hypothalamic Gz-protein levels was paralleled by a significant decrease in the oxytocin and ACTH responses to the 5-HT1A agonist 8-hydroxy-dipropylamino-tetralin (8-OH-DPAT). In contrast, the prolactin response to 8-OH-DPAT (which cannot be blocked by 5-HT1A antagonists) was not inhibited by Gz antisense oligodeoxynucleotides. Gz-proteins are the only members of the Gi/Go-protein family that are not inactivated by pertussis toxin. In a control experiment, pertussis toxin treatment (1μg/5 μl, 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 Gi/Go-proteins do not mediate the 5-HT1A receptor-mediated increase in ACTH and oxytocin secretion. Combined, these studies provide the firstin vivo evidence for a key role of Gz-proteins in coupling hypothalamic 5-HT1Areceptors to effector mechanisms.