HORMONES THAT INFLUENCE SEXUAL BEHAVIOR.EVIDENCE FROM ANIMAL RESEARCH
The body produces a great variety of hormones and metabolites, the functions of which are often poorly understood. Delineation of the effects of individual hormones is difficult; one of the major problems is that hormones do not operate in isolation but are part of a complex endocrine system in which many components influence each other’s production, release, target tissue effects, and metabolism. Nevertheless, systematic experimental and clinical observation make it possible to identify certain physiologic or behavioral events that are influenced greatly, although usually not exclusively, by a given individual hormone. With regard to sexual behavior, it seems obvious that all hormones of the hypothalamic-pituitary-gonadal axis need to be screened for behavioral effects. Sex steroids produced by the adrenals also must be considered. Although many other hormones have been shown to interact with sex steroids, their respective influences on sexual behavior are largely unknown and will not be discussed here.
Testosterone of gonadal origin appears to be the major hormone in both the pre- and perinatal organization as well as the pubertal activation and adult maintenance of male sexual behavior in subhuman mammals. A major, continuing controversy concerns the mechanism of action that is important to the choice of androgen metabolites to be measured in psychoendocrine studies. In the rat, 5-alpha-reduced metabolites, especially dihydrotestosterone, appear to be effective mainly in peripheral target tissues and in the hypo-thalamic-pituitary regulation of gonadotropin release (negative feedback) but not in brain systems which regulate sexual behavior. The latter is believed by some researchers to depend on the aromatization of androgens to estradiol on the target cell level. However, findings in other lower mammals and in rhesus monkeys negate such a simple dichotomy, and recent data suggest that even in the rat not aromatization but 19-hydroxylation of androgens may be the decisive metabolic step for behavioral effects. In contrast to testosterone, adrenal androgens seem to have a negligible role in male sexual behavior: in several lower mammalian species, adrenal hormones do not account for the persistence of sexual behavior after castration. LH-RH, the hypothalamic polypeptide hormone that stimulates pituitary gonadotropin (especially LH) secretion and thereby regulates gonadal steroid production, has been shown to facilitate sexual behavior in lower mammals, but the effects are clearly weaker than those of the gonadal steroids.
For the major components of female sexual behavior, i.e. attractiveness, proceptivity, and receptivity, me estrogens, especially estradiol, are clearly the most important hormones in lower mammals. In some species, progesterone is needed in addition to optimize female sexual behavior, but under certain conditions progesterone has inhibitory effects. Testosterone increases components of male behavior (e.g., mounting) in females of lower mammalian species. In female primates, estrogens increase all aspects of sexual behavior, while progesterone lowers them. Testosterone has also been shown to have facilitory effects. There are a few reports showing a positive effect of adrenal androgen therapy on sexual behavior in adrenalectomized monkeys: Everitt and Herbert found that dehydroepian-drosterone was ineffective, while androstenedione had marked effects. It is unclear if the latter’s effects are mainly due to the adrenal androgen itself or to its conversion product, testosterone. As in males, females of some mammalian species exhibit facilitory effects of LH-RH on sexual behavior.
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