Evidence that 5-HT2A receptors in the hypothalamic paraventricular nucleus mediate neuroendocrine responses to (-)DOI

Abstract

The present study determined whether the serotonin2A (5-HT2A) receptors in the hypothalamic paraventricular nucleus mediate the neuroendocrine responses to a peripheral injection of the 5-HT2A/2C receptor agonist (-)DOI [(-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane]. The 5-HT2A receptor antagonist MDL100,907 ((+/-)-alpha(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol), the 5-HT2C receptor antagonist SB-242084 (6-chloro-5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-indoline), or vehicle were microinjected bilaterally through a chronically implanted double-barreled cannula into the hypothalamic paraventricular nucleus 15 min before a peripheral injection of (-)DOI in conscious rats. (-)DOI significantly elevated plasma levels of oxytocin, prolactin, ACTH, corticosterone, and renin. Neither the 5-HT2A receptor antagonist nor the 5-HT2C receptor antagonist, injected alone, altered the basal levels of these hormones. MDL100,907 (0.748, 7.48, and 18.7 nmol) dose dependently inhibited the (-)DOI-induced increase in all of the hormones except corticosterone. In contrast, SB-242084 (10 nmol) did not inhibit (-)DOI-increased hormone levels. To confirm the presence of 5-HT2A receptors in the hypothalamic paraventricular nucleus, 5-HT2A receptors were mapped using immunohistochemistry. Densely labeled magnocellular neurons were observed throughout the anterior and posterior magnocellular subdivisions of the hypothalamic paraventricular nucleus. Moderately to densely labeled cells were also observed in parvicellular regions. Thus, it is likely that 5-HT2A receptors are present on neuroendocrine cells in the hypothalamic paraventricular nucleus. These data provide the first direct evidence that neuroendocrine responses to a peripheral injection of (-)DOI are predominantly mediated by activation of 5-HT2A receptors in the hypothalamic paraventricular nucleus.

Fig. 1.

Histological verification of the position of the tips of the double-barreled injection cannula (30 μm; cresyl violet staining). Arrows, The tips of a double-barreled cannula. PVN, The hypothalamic paraventricular nucleus;V, the 3rd ventricle.

Fig. 2.

Dose-dependent inhibition of oxytocin (A) and prolactin (B) responses to (−)DOI by MDL100,907 microinjected into the hypothalamic paraventricular nucleus. The data represent the mean ± SEM of 7–14 rats per group. *p < 0.05, **p < 0.01, significant effect of (−)DOI compared with the saline challenge group. #p < 0.05, ##p < 0.01, significant effect of MDL100,907 compared with the vehicle–(−)DOI group (two-way ANOVA and Newman–Keuls multiple range test). Veh, Vehicle;PVN, the hypothalamic paraventricular nucleus.

Fig. 3.

Microinjection of MDL100,907 into the hypothalamic paraventricular nucleus dose-dependently inhibits ACTH (A) but not corticosterone (B) responses to (−)DOI. The data represent the mean ± SEM of 7–14 rats per group. *p < 0.05, **p < 0.01, significant effect of (−)DOI compared with the saline challenge group. ##p < 0.01, significant effect of MDL100,907 compared with the vehicle–(−)DOI group (two-way ANOVA and Newman–Keuls multiple range test). Veh, Vehicle; PVN, the hypothalamic paraventricular nucleus.

Fig. 4.

Microinjection of MDL100,907 into the hypothalamic paraventricular nucleus dose-dependently inhibited (−)DOI-induced increase in renin activity (A) and renin concentration (B). The data represent the mean ± SEM of 7–14 rats per group. **p < 0.01, significant effect of (−)DOI compared with the saline challenge group. ##p < 0.01, significant effect of MDL100,907 compared with the vehicle–(−)DOI group (two-way ANOVA and Newman–Keuls multiple range test). Veh, Vehicle;PVN, the hypothalamic paraventricular nucleus.

Fig. 5.

Microinjection of SB-242084 into the hypothalamic paraventricular nucleus does not inhibit the neuroendocrine responses to (−)DOI. The data represent the mean ± SEM of 7–14 rats per group. **p < 0.01, significant effect of (−)DOI compared with the saline challenge groups (two-way ANOVA and Newman–Keuls multiple range test). Veh, Vehicle;SB, SB-242084; PVN, the hypothalamic paraventricular nucleus.

Fig. 6.

5-HT_2A receptor immunoreactivity in the hypothalamic paraventricular nucleus (PVN).V denotes the 3rd ventricle. The numbersindicate the rostrocaudal distance from bregma (Swanson, 1992).A–D, Distribution of 5-HT_2A receptor immunoreactivity in the hypothalamic paraventricular nucleus. Scale bar, 100 μm. E, High magnification ofC. Scale bar, 10 μm. F, 5-HT_2A receptor immunoreactivity in cerebral cortex.Arrows, Pyramidal neurons; arrowheads, apical dendrites of pyramidal neurons. Scale bar, 10 μm.G, No 5-HT_2A receptor-like immunoreactivity was observed in the hypothalamic paraventricular nucleus in the absence of primary antibody. Scale bar, 100 μm.