Differential adaptive changes in serotonin 2A 2A ] receptor signaling during treatment may be one mechanism involved in the latency of therapeutic improvement with antidepressants, such as fluoxetine. We examined the effects of fluoxetine (2, 3, 7, 21, or 42 days) on hypothalamic 5-HT 2A receptor signaling. The hormone responses to an injection of the 5-HT 2A receptor agonist (Ϯ)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane HCl (DOI) were used as an index of hypothalamic 5-HT 2A receptor function. Treatment with fluoxetine for 21 or 42 days produced diminished adrenocorticotropic hormone (ACTH) and oxytocin (but not corticosterone) responses to DOI injections (2.5 mg/kg i.p.; 15 min postinjection). Regulators of G protein signaling 4 and G ␣q protein levels in the hypothalamic paraventricular nucleus were not altered during fluoxetine treatment. Because previous studies indicate that treatment with fluoxetine for 21 days resulted in increased hormone responses to DOI when measured at 30 min after injection, we examined the effect of fluoxetine (21 days) on DOI-induced increase hormone levels at 15, 30, and 60 min after DOI injection. Fluoxetine decreased the oxytocin response at 15 but not at 30 min post-DOI injection, and potentiated the ACTH and corticosterone responses at 30 min post-DOI injection. For comparison, we examined the effect of fluoxetine on 5-HT 2A receptor-mediated increase in phospholipase C (PLC) activity in the frontal cortex. 5-HT-stimulated, but not guanosine 5Ј-O-(3-thio)triphosphate-stimulated PLC activity was increased after 21 days of fluoxetine-treatment. Overall, these results indicate that chronic fluoxetine treatment can potentiate 5-HT 2A receptor signaling in frontal cortex but differentially alters 5-HT 2A receptor signaling in oxytocin-containing neurons and corticotropin-releasing factor-containing neurons in the paraventricular nucleus.

This study examined the time course and possible mechanisms of agonist-induced desensitization of 5-hydroxytryptamine serotonin 2A receptors in the rat frontal cortex and hypothalamic paraventricular nucleus after 1, 4, and 7 days of treatment with (Ϫ)-1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane HCl [(Ϫ)-DOI] (1 mg/kg i.p.), a selective 5-HT 2A/2C receptor agonist. In the frontal cortex, 5-HT-mediated phospholipase C (PLC) enzyme activity decreased by 24 to 30% after 4 to 7 days of (Ϫ)-DOI treatment without any significant changes in the guanosine 5Ј-3-O-(thio)triphosphate-mediated PLC enzyme activity. Additionally, treatment with (Ϫ)-DOI did not significantly change the levels of G ␣11 , regulator of G protein signaling (RGS)4, or RGS7 proteins in the frontal cortex, whereas G ␣q protein levels in the frontal cortex decreased (47%) only after 7 daily (Ϫ)-DOI injections. The functional status of 5-HT 2A receptors in the hypothalamic paraventricular nucleus was examined using 5-HT 2A receptor-mediated increases in plasma hormone levels. Plasma adrenocorticotrophic hormone (ACTH) and oxytocin measurements showed that 5-HT 2A receptor desensitization began after only 1 day of (Ϫ)-DOI treatment, and the desensitization continued to increase after 4 and 7 days of treatment (ACTH response decreased 64.2-67.7%; oxytocin response decreased 82.3-90.1%). There were no significant alterations in levels of G ␣q or G ␣11 proteins in the hypothalamic paraventricular nucleus. In conclusion, these results suggest that chronically administered (Ϫ)-DOI induces desensitization of 5-HT 2A receptors in vivo, via a reduction in the ability of 5-HT 2A receptors to activate G proteins without consistently altering levels of G ␣ proteins or RGS proteins.

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The 5-hydroxytryptamine 2A and 2C (5-HT 2A and 5-HT 2C ) receptors are so closely related that selective agonists have not been developed until recently with the advent of (S)-2-(chloro-5-fluoro-indol-l-yl)-1-methylethylamine fumarate (Ro 60-0175), a putatively selective 5-HT 2C receptor agonist. In the present study, Ro 60-0175 was used to analyze the importance of 5-HT 2C receptors in hormone secretion. Injection of Ro 60-0175 (5 mg/kg s.c.) produced a maximum increase in plasma levels of adrenocorticotrophic hormone, oxytocin, and prolactin at 15 min postinjection and a maximum increase in plasma corticosterone levels at 60 min postinjection. Ro 60-0175-mediated increases in plasma hormone levels were dose-dependent (corticosterone ED 50 ϭ 2.43 mg/kg; oxytocin ED 50 ϭ 4.19 mg/kg; and prolactin ED 50 ϭ 4.03 mg/kg). To assess the role of 5-Hydroxytryptamine 2 (5-HT 2A and/or 5-HT 2C ) receptors play important roles in depression, obsessive compulsive disorder, eating disorders, and schizophrenia (Blier and de Montigny, 1999;Aghajanian and Marek, 2000). Both 5-HT 2A and 5-HT 2C receptors are G protein-linked receptors that couple to phospholipase C as a second messenger and thereby increase diacylglycerol, inositol trisphosphate, and intracellular Ca 2ϩ levels (for review, see Barnes and Sharp, 1999). The lack of selective agonists for each 5-HT 2 receptor subtype has hindered a precise differentiation between 5-HT 2A and 5-HT 2C receptor-mediated effects.The involvement of the 5-HT 2C and/or 5-HT 2A receptors in the regulation of hormone secretion has been examined with a variety of 5-HT 2 receptor agonists, the most common being (Ϯ)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane HCl (DOI). DOI increases the plasma levels of adrenocorticotrophic hormone (ACTH), corticosterone, oxytocin, prolactin, and renin (Rittenhouse et al., , 1994Van de Kar et al., 2001). DOI was shown to stimulate the secretion of ACTH, corticosterone, oxytocin, prolactin, and renin by specifically activating the 5-HT 2A receptor as the increase in plasma levels of all these hormones was blocked by very low doses of the 5-HT 2A receptor antagonist MDL 100,907 (Van de Kar et al., 2001). Moreover, evidence suggests that 5-HT 2A receptors mediate hormone responses to other 5-HT 2C/2A agonists, such as mchlorophenylpiperazine (m-CPP), MK-212, and quipazine. The increase in plasma corticosterone levels after injection of MK-212, m-CPP, and quipazine are blocked by MDL 100,907. On the other hand, SB 242084 and SB 200646A, 5-HT 2C receptor antagonists, do not inhibit the m-CPP-, MK-212-, and quipazine-mediated increases in plasma corticosterone levels (Hemrick-Luecke and Fuller, 1996;Hemrick-Luecke and Evans, 2002). Both studies provide evidence for the importance of the 5-HT 2A receptor subtype in the regulation of neuroendocrine responses. However, there is no direct evidence to evaluate the role of the 5-HT 2C receptor subtype in