Gonadal Steroid Modulation of Vasotocin Concentrations in the Bullfrog Brain

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Abstract
Concentrations of the neuropeptide arginine vasotocin (AVT) are sexually dimorphic in some regions of the bullfrog brain. Since these differences in AVT content may be due to sexual differences in plasma steroid concentrations, we performed a gonadectomy-steroid replacement experiment (30-day treatment) to assess the effects of specific steroids on AVT concentrations (determined by RIA) in adult male and female bullfrogs (Rana catesbeiana). These treatments had significant effects in six brain areas. Gonad removal decreased AVT concentrations in the amygdala pars lateralis, septal nucleus and habenula of both sexes. Gonadectomy decreased AVT content in optic tectum, torus semicircularis and pretrigeminal nucleus of males only. Dihydrotestosterone treatment resulted in AVT concentrations at or above sham levels in all six areas, whether or not gonadectomy decreased content. Estradiol fully restored AVT concentrations only in the septal nucleus and habenula (of both sexes) and in the amygdala of females. Estradiol partially restored AVT levels in the amygdala of males. These results indicate that the gonads maintain AVT concentrations in several brain areas and that steroids can modulate levels of AVT in bullfrog brain. Effects of each steroid vary depending upon the region and sex of the frog. AVT cells are present in three of these regions – the amygdala pars lateralis, septal nucleus and pretrigeminal nucleus. Steroids may be directly affecting AVT synthesis in these cells and these populations may be the source of AVT fibers in the other steroid-sensitive areas.

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