Lighting Conditions Affect Testosterone Feedback Sensitivity in Castrated Rats

Abstract

It has been shown in the Syrian hamster that a short photoperiod sensitizes the hypothalamo-hypophyseal axis of castrated animals to the negative feedback effect of testosterone. There is some evidence that even the reproductive system of the rat, which is generally considered not to be very sensitive to light, can respond to changes in illumination. Therefore, we found it of interest to examine whether alterations in lighting conditions produce changes of sensitivity in the negative feedback effect of testosterone in the rat. We kept intact, castrated, and castrated testosterone-treated animals either in periodic (L:D 12:12) or constant light for 7 days starting 4 weeks after castration. In all 3 testosterone-injected groups, serum luteinizing hormone (LH) was lower in constant than in periodic light. Exogenous testosterone did not decrease the castration-induced elevations of pituitary LH and folliclestimulating hormone (FSH). On the contrary, testosterone increased the pituitary contents of LH and FSH, especially in constant light. We conclude that, in constant light, the hypothalamo-hypophyseal axis of the castrated rat becomes more sensitive to the negative feedback action of testosterone.

Hypothalamic LHRH concentration decreases in intact but increases in castrated rats in constant light

“…In the hypothalamo-pituitary axis of the male rat, androgens have several sites of actions with varying thresholds (Kalra 1985b, Levine & Duffy 1988. We have previously found that exogenous testosterone in castrated rats inhibits the release of gonadotropins from the pituitary gland with a smaller dose in constant than in periodic light (Porkka-Heiskanen et al 1989). One site' of testosterone action is the hypothalamic LHRH synthesis (Kalra et al 1984, Kalra, 1985, and in the present experiment the sensitivity of this androgen effect to constant light was examined.…”

“…In the course of castration the hypothalamus becomes less responsive to testosterone (Kalra 1985 a). Under constant light castrated rats are more responsive to testosterone than under periodic light (Porkka-Heiskanen et al 1989). The previous finding that the post-castration rise in serum LH and FSH is higher in constant than in periodic light (Guisado et ul.…”

“…We have recently shown that testosterone prevents gonadotropin release from the pituitary gland of castrated male rats more effectively in constant than in periodic light (Porkka-Heiskanen et al 1989). Since there are no earlier studies about the effects of constant light on hypothalamic LHRH, and since this hormone may mediate the effects of light on reproductive function, the present study was undertaken to test the effect of constant light on the hypothalamic LHRH concentration in normal male animals and in animals exposed to melatonin or testosterone administration or to gonadectomy.…”

Effect of Neonatal Androgenization on the Testosterone Feedback Sensitivity in Adult Rats in Two Lighting Conditions

“…Orchidectomized rats were more sensitive to testos terone in constant than in periodical light [12], but neo natally androgenized rats were not. On the contrary, in addition to the pituitary resistance to testosterone, se rum FSH became unresponsive to testosterone injec tions in constant light.…”

“…We found earlier that the hypothalamo-pituitary axis of castrated male rats reacts more readily to testos terone in constant light than in periodical light [12], In the present experiment, we studied the effect of neona tal androgenization and constant light on adult male rats.…”