Intracerebroventricular administration of the prolactin (PRL) receptor antagonist, S179D PRL, disrupts parturition in rats

Prolactin promotes oxytocin and vasopressin release by activating neuronal nitric oxide synthase in the supraoptic and paraventricular nuclei

Prolactin (PRL) stimulates the secretion of oxytocin (OXT) and arginine AVP as part of the maternal adaptations facilitating parturition and lactation. Both neurohormones are under the regulation of nitric oxide. Here, we investigate whether the activation of neuronal nitric oxide synthase (nNOS) in the hypothalamo-neurohypophyseal system mediates the effect of PRL on OXT and AVP release and whether these effects operate in males. Plasma levels of OXT and AVP were measured in male rats after the intracerebroventricular injection of PRL or after inducing hyperprolactinemia by placing two anterior pituitary glands under the kidney capsule. NOS activity was evaluated in the paraventricular (PVN) and supraoptic (SON) hypothalamic nuclei by NADPH-diaphorase histochemistry and in hypothalamic extracts by the phosphorylation/inactivation of nNOS at Ser847. Elevated central and systemic PRL correlated with increased NOS activity in the PVN and SON and with higher OXT and AVP circulating levels. Notably, treatment with 7-nitroindazole, a selective inhibitor of nNOS, prevented PRL-induced stimulation of the release of both neurohormones. Also, phosphorylation of nNOS was reduced in hyperprolactinemic rats, and treatment with bromocriptine, an inhibitor of anterior pituitary PRL secretion, suppressed this effect. These findings suggest that PRL enhances nNOS activity in the PVN and SON, thereby contributing to the regulation of OXT and AVP release. This mechanism likely contributes to the regulation of processes beyond those of female reproduction.

Mother counts: how effects of environmental contaminants on maternal care could affect the offspring and future generations

Various compounds of anthropogenic origin represent environmental contaminants (EC) that penetrate the food chain and are frequently detected in human milk and maternal blood at the time of delivery. These ECs can affect the development of the fetus and can be transferred to the newborn during lactation. Many studies have used animal models to study the impact of ECs on the development of the nervous system and have reported effects of early exposure on neural and neuroendocrine systems and on behavior, when the exposed animals are tested as adults. Some of these effects persist across generations and may involve epigenetic mechanisms. The majority of these studies in developmental toxicology treat the pregnant or lactating animal with ECs in order to deliver the contaminants to the developing offspring. Almost universally, the mother is viewed as a passive conduit for the ECs, and maternal behavior is rarely assessed. Here we review the literature on the effects of ECs on maternal care and find mounting evidence that important components of the care given to the offspring are affected by maternal exposure to different ECs. Some of these changes in maternal behavior appear to be secondary to changes in the behavior and/or stimulus properties of the exposed offspring, but others are likely to be direct effects of the ECs on the maternal nervous and endocrine systems. Considering the extent to which the quality of maternal care affects the development of the offspring, it becomes imperative to determine the contributions that changes in maternal behavior make to the deficits traditionally ascribed solely to direct effects of ECs on the developing organism. Given the complexity and importance of mother-infant interactions, future research on developmental toxicology must consider the effects of ECs not only on the offspring, but also on the mother and on the interactions and social bond between mother and infant.

Endocrinologic control of normal canine ovarian function

In dogs, the termination of the 3-10-month obligate anoestrus involves selection of a cohort of LH-sensitive follicles, presumably from a wave of dominant small antral follicles that would otherwise undergo atresia. The number and size of such follicles appears to increase, especially during the last 50 days of anoestrus when the already elevated concentrations of FSH become further elevated. The final selection and eventual terminal development of these follicles is caused by an increased frequency of high-amplitude LH pulses at the end of anoestrus. Concomitant increases in FSH are typically small or negligible. High concentrations of FSH in anoestrus are likely to be important in maintaining, if not stimulating, overlapping waves of dominant follicles throughout anoestrus, their expression of aromatase activity and basal oestradiol secretion sufficient to suppress LH by negative feedback. An attractive hypothesis is that late anoestrus increases in LH-stimulate synthesis of precursor androgen for already available FSH-dependent aromatase. After 7 or more days of elevated LH, and perhaps 2-5 days of semi-autonomous growth, with maximal oestradiol production reached, follicle capacity to further increase oestradiol becomes limited and excess progesterone becomes increasingly secreted. The pre-ovulatory LH surge and oestrus onset are then triggered - often synchronously and in concert with the terminal maturation of the follicles - by central effects of the large decrease in the oestrogen to progestin ratio. Follicular endocrine and paracrine events during and following the LH surge are likely similar to those reported for other species. The prolonged luteal phase lengths of 55-75 days in non-pregnant bitches bracket the 64 +/- 1 day in pregnancy and represent a genetically programmed luteal cell lifespan approximating gestation length as occurs in the luteal phase of hysterectomized animals of most polyoestrous artiodactyls and rodents. The 30-40-day slow regression after day 20 to 30 involves periodic cell death, diminution in cell size, low levels of apoptosis and minimal or modest involvement of endogenous prostaglandin F (PGF) production. The canine corpus luteum (CL) is dependent on both LH and prolactin as stimulating luteotrophins by day 15, and as required luteotrophins by days 20-25, if not earlier. Thereafter, both luteotrophins likely have cellular mechanisms of action similar to those reported for other species. Progesterone secretion during pregnancy is greatly enhanced by characteristic, and probably relaxin-stimulated, increases in prolactin concentration starting at or after day 25, and persisting to term. Near term, foetoplacental maturation results in the placental release of large, luteolytic amounts of PGF for 1-2 days pre-partum. Pre-partum luteolysis, like that induced by exogenous prostaglandin, likely involves a cascade enhanced by the removal of progesterone inhibition of PGF release and some degree of intra-luteal PGF synthesis. That a likely twofold or greater increase in progesterone production by the CL of pregnancy does not result in significantly higher serum progesterone than in non-pregnant metoestrus relates to several biological changes, including a large increase in plasma volume of distribution, increased metabolism of progesterone by increased uterine, placental and mammary masses and increased liver clearance and excretion of progesterone and progesterone metabolite. Anoestrus length and ovarian cycle intervals, variable within and among bitches, are likely affected by neuroendocrine components of an endogenous circannual cycle, albeit only photo-entrained in the Basenji breed. This may be modified by the prior luteal phase, exposure to oestrus female pheromones and as yet unknown mechanisms that likely operate via inhibitory opioidergic and/or stimulatory dopaminergic hypothalamic pathways affecting late anoestrus increases in LH.