Evidence for the Role of α-MSH in the Induction of Pseudopregnancy in the Rat

Abstract

In a previous study we have demonstrated that cervical stimulation (CS) induces α-MSH release. The present experiments were undertaken to (1) examine the pattern of serum α-MSH during CS-induced pseudopregnancy (PSP) and (2) assess the possibility that α-MSH contributes to the induction and maintenance of PSP. Throughout PSP serum α-MSH fluctuated in a cyclic manner demonstrating two daily surges which occurred between 12.00 and 13.00 h (diurnal surge) and between 24.00 and 04.00 h (nocturnal surge). Chronic exposure of animals to α-MSH administered via minipumps (24 µg/day, starting on the morning of estrus), induced PSP as determined by deciduoma formation and persistence of a characteristic diestrous vaginal cytology. Furthermore, insertion of an α-MSH-containing minipump at diestrus 1 (Dl) resulted in progesterone and prolactin (PRL) levels on the afternoon of diestrus 2 (D2) similar to those levels found on day 2 of PSP. Uterine weight was significantly decreased in α-MSH-treated rats and pseudopregnant rats as compared with cyclic D2 controls. α-MSH was found to release PRL indirectly, through stimulation of adrenal progesterone. This effect, however, necessitates the presence of the ovaries as a source of estradiol (EB) since it is demonstrable in intact and acutely ovariectomized rats, but not in chronically ovariectomized animals. EB treatment of chronically ovariectomized rats is capable of restoring the sequence. These results indicate that, as demonstrated for PRL, cervical stimulation initiates rhythmic daily surges of α-MSH secretion which are maintained through PSP. In addition, they suggest that α-MSH may contribute to inducing and maintaining PSP by facilitating the release of adrenal progesterone and by inducing PRL release via the increase in progesterone secretion from the adrenal.

A central action of α-melanocyte-stimulating hormone on serum levels of LH and prolactin in rats

“…At first it was thought that the reduction in serum -MSH may be responsible for the inhibition of LH and prolactin, since Celis (1975) showed that serum a-MSH levels were raised on the day of pro-oestrus and that the rise was involved in stimulating the release of LH and prolactin by enhancing the secretion of pro¬ gesterone (Volosin & Celis, 1984;Celis, 1985). However, when progesterone was administered in the present experiments it did not modify the results shown in Fig.…”

Interaction of Oestradiol, α-Melanotrophin and Noradrenaline within the Ventromedial Nucleus in the Control of Female Sexual Behaviour

“…Several series of studies have suggested that it is involved in reproductive processes, perhaps acting to enhance the action of gonadal steroids [1][2][3][4]. We have shown that aMSH can stimulate female sex ual behaviour in ovariectomised-adrenalectomised rats primed with low doses of oestradiol [4][5][6][7], The effect is a central one and noted at relatively low concentrations (20-100 ng/rat) after administration into the third ventri cle [4], This may be of physiological importance as aMSH concentrations are raised selectively in the ventromedial nucleus (VMN) in sexually receptive rats [7].…”

The effect of the blockade of α-melanocyte-stimulating hormone on LH release in the rat

“…Likewise, when it is injected on the day of prooestrus, the number of ova is increased as a conse¬ quence of the increase in serum LH levels (Celis, 1985). On the contrary, the chronic administration of -MSH on oestrus or on dioestrus-2 induces blockade of the LH ovulatory peak at pro-oestrus, inducing pseudopregnancy (Volosin & Celis, 1984). The purpose of this paper is to report the effect of the blockade of -MSH on pro-oestrus on LH release and ovulation on the next day of oestrus.…”

“…These results show that the blockade of \g=a\-MSHinhibits LH release and the next ovulation in rats treated with PLG or with concentrated antiserum against \g=a\-MSH, through a blockade of progesterone of adrenal origin. Journal of Endocrinology (1993) 137,[197][198][199][200][201][202] The role of a-melanocyte-stimulating hormone (a-MSH) in the control of different aspects of reproduc¬ tion has been studied extensively (Cehovic, 1965;Tomatis & Taleisnik, 1968;Runnenbaum et al 1973;Celis, 1975Celis, , 1985Aide & Celis, 1980;Khorram et al l9Ua,b;Volosin & Celis, 1984, 1988Ferreira & Celis, 1986;Durando et al I9c\9a,b;Scimonelli & Celis, 1990).…”