Daily injections of fluoxetine induce dose-dependent desensitization of hypothalamic 5-HT1A receptors: reductions in neuroendocrine responses to 8-OH-DPAT and in levels of Gz and Gi proteins.

The present studies examined the dose-response relationship of fluoxetine-induced desensitization of hypothalamic postsynaptic 5-HT1A receptors, as measured from the reduced neuroendocrine responses to a 5-HT1A agonist. Because hypothalamic Gz proteins mediate the ACTH and oxytocin responses to 5-HT1A receptor activation, we also determined the effect of fluoxetine on the levels of Gz proteins in the hypothalamus. Rats were injected daily for 14 days with saline or with fluoxetine doses of 0.3, 1, 3, 5, 7. 5, or 10 mg/kg/day. Fluoxetine produced a dose-dependent reduction in the oxytocin, ACTH, and corticosterone responses to the 5-HT1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT, 50 micrograms/kg, s.c.). The lowest fluoxetine dose that significantly, although incompletely, reduced the neuroendocrine responses to 8-OH-DPAT was 5 mg/kg/day. The 10 mg/kg/day dose of fluoxetine maximally inhibited all neuroendocrine responses to 8-OH-DPAT. Hypothalamic levels of Gz protein were reduced by both the 7.5 and 10 mg/kg/day doses of fluoxetine, whereas Gi1 protein levels were reduced only after the highest dose (10 mg/kg/day) of fluoxetine. Gi2, Gi3, and Go levels were not reduced by any fluoxetine dose. Cytosolic levels of Gi1 and Gz proteins were unaltered, indicating that reductions in Gz and Gi1 proteins are not caused by a redistribution of the proteins from the membrane into the cytosol. The results from the present study indicate that fluoxetine-induced desensitization of hypothalamic postsynaptic 5-HT1A receptor systems is dose-dependent and may be caused in part by reductions in the hypothalamic levels of Gz proteins.