Evidence That G_z-Proteins Couple to Hypothalamic 5-HT_1AReceptorsIn Vivo

Abstract

Using in situ hybridization and immunoblot analysis, the present studies identified G z mRNA and G z -protein in the hypothalamic paraventricular nucleus. The role of G z -proteins in hypothalamic 5-HT 1A receptor signaling was examined in vivo. Activation of 5-HT 1A receptors increases the secretion of oxytocin and ACTH, but not prolactin. Intracerebroventricular infusion (3-4 d) of G z antisense oligodeoxynucleotides, with different sequences and 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 oligodeoxynucleotide treatment altered basal plasma levels of ACTH, oxytocin, or prolactin, when compared with untreated controls. An antisense-induced decrease in hypothalamic G z -protein levels was paralleled by a significant decrease in the oxytocin and ACTH responses to the 5-HT 1A agonist 8-hydroxydipropylamino-tetralin (8-OH-DPAT). In contrast, the prolactin response to 8-OH-DPAT (which cannot be blocked by 5-HT 1A antagonists) was not inhibited by 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 g/5 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 toxinsensitive G i /G o -proteins do not mediate the 5-HT 1A receptormediated increase in ACTH and oxytocin secretion. Combined, these studies provide the first in vivo evidence for a key role of G z -proteins in coupling hypothalamic 5-HT 1A receptors to effector mechanisms.

Differential Regulation of 5-HT1A Receptor-G Protein Interactions in Brain Following Chronic Antidepressant Administration

“…Chronic administration of the SSRI fluoxetine produces a gradual reduction in the levels of G i1 , G i3 and G z protein in hypothalamus that matches the time course of desensitization of hypothalamic 5-HT 1A receptors (Li et al 1996;Raap et al 1999). Thus, fluoxetineinduced desensitization of hypothalamic postsynaptic 5-HT 1A receptor systems may be caused in part by reductions in Gz, which mediates hypothalamic 5-HT 1A receptor-stimulated ACTH and oxytocin secretion (Serres et al 2000). Thus, antidepressant efficacy and the regulation of 5-HT 1A receptor function may be based on compensatory changes distal to the receptor, such as regulation of G protein expression or as discussed above reduced capacity of the receptor to activate G protein due to regulatory processes (e.g.…”

Sustained treatment with a 5-HT2A receptor agonist causes functional desensitization and reductions in agonist-labeled 5-HT2A receptors despite increases in receptor protein levels in rats

“…The PVN was dissected from a 700 μm coronal section obtained using a cryostat (−10°C) as previously described (Serres, et al, 2000). Tissues were homogenized in 10 mM Tris buffer containing 0.1 M NaCl, 0.1 M EDTA, and a protease inhibitor cocktail (1:1000) (Sigma Chemical Co., St. Louis, MO).…”

Agonist-Induced Serotonin 2A Receptor Desensitization in the Rat Frontal Cortex and Hypothalamus

“…Punches of the hypothalamic paraventricular nucleus from the treatment groups that did not receive a challenge injection were used for the measurements of G ␣q , and G ␣11 protein. The paraventricular nucleus was dissected from a 700-m coronal section obtained using a cryostat (Ϫ10°C) as described previously (Serres et al, 2000). Both tissues were homogenized in 10 mM Tris buffer containing 0.1 M NaCl, 0.1 M EDTA, and a protease inhibitor cocktail (1:1000) (Sigma-Aldrich, St. Louis, MO).…”