Oxytocin has a role in gonadotrophin regulation in rats

Abstract

We previously demonstrated that oxytocin stimulates LH release from rat pituitary cells in vitro and advances follicular development and ovulation in mice in vivo. This study reports an investigation of rat LH levels following in-vivo administration of oxytocin. Injection of oxytocin (10 mIU/g, i.p.) to rats at 07.00, 08.00 and 09.00 h of pro-oestrus or at 09.00, 10.00 and 11.00 h of pro-oestrus advanced the onset of the LH surge (P less than 0.005) and attainment of peak concentrations of LH (P less than 0.02) in peripheral blood. On the other hand, the descending phase of the LH surge and the surge amplitude were not altered by oxytocin. Treatment at 05.00, 06.00 and 07.00 h of pro-oestrus or at 11.00, 12.00 and 13.00 h of pro-oestrus had no effect on the LH profile. A higher oxytocin dose (20 mIU/g) inhibited LH release when treatment was begun at 05.00, 07.00 or 09.00 h of pro-oestrus. A lower dose (5 mIU/g) was ineffective in altering LH concentrations. In addition, injections of oxytocin (10 mIU/g) at oestrus, metoestrus or dioestrus had no effect on the release of LH. Thus the efficacy of oxytocin in altering concentrations of LH was dose dependent and also critically affected by the day of the oestrous cycle and the time of pro-oestrus. Removal of endogenous oxytocin activity by the use of an oxytocin receptor antagonist abolished the pro-oestrous LH surge, indicating that oxytocin is a vital physiological component of the LH-releasing mechanism in rats. The study provides unequivocal evidence that oxytocin induces LH release in vivo, but the manifestation of oxytocin activity is dependent upon conditions of exposure.

Sex, parturition and motherhood without oxytocin?

“…However, there are considerable interspecies differences in the regulation of luteolysis, and there is no evidence that gonadal oxytocin plays a role in rodents similar to that documented for ruminants. Previous studies in the mouse on the role of oxytocin in gonadal function indicate a capacity to stimulate ovulation and weak expression in granulosa cells; at the anterior pituitary, oxytocin can advance the luteinizing hormone preovulatory surge in the rat (Robinson & Evans 1990). Evidence has also been put forward that oxytocin can stimulate blastocyst development in vitro (Furuya et al 1995), act on seminiferous tubule motility (Nicholson et al 1986) and possibly stimulate, via vasopressin receptors, testosterone production (Tahri-Joutei & Pointis 1989), although the oxytocin gene is not naturally expressed in the mouse testis (Ang et al 1991).…”

Changes of Receptor mRNAs for Oxytocin and Estrogen during the Estrous Cycle in Rat Uterus

“…In rats the regulation of the luteolysis is different from the ruminants because the involvement of ovarian oxytocin for the luteolysis has not been reported, although oxytocin injection increased the PGF 2α levels in the uterus [3]. On the other hand, some evidences that oxytocin affect the regulation of preovulatory secretions of luteinizing hormone (LH) [14,32] and prolactin (PRL) from the pituitary in rats [34] suggest that oxytocin plays important roles in reproductive function on proestrus.Considering the stimulatory effect of estrogen on OTR induction, it is likely that uterine OTR mRNA levels predominantly increase at proestrus, but the changes in OTR mRNA levels during proestrus is low compared with that during labor [20]. These results lead to the possibility that oxytocin may exert some effect on proestrus in the uterus, however its regulation may be different from that during labor.…”

Pup removal suppresses estrogen-induced surges of LH secretion and activation of GnRH neurons in lactating rats

“…Oxytocin stimulates basal pituitary secretion of LH (Evans et al 1989) and augments the response of LH secretion to GnRH (Evans et al 1995). Furthermore, peripheral administration of oxytocin antagonists prevents the estrogen-induced LH surge normally observed on the afternoon of proestrus in cyclic rats (Johnston & Negro-Vilar 1988, Robinson & Evans 1990). Based on these findings, oxytocin that originates from an unidentified source and acts on pituitary gonadotrophs has been implicated in the regulation of the estrogen-induced LH surge in cyclic female rats.…”