Episodic prolactin and growth hormone secretion not related to the actual suckling activity in lactating rats

Mood, Food, and Fertility: Adaptations of the Maternal Brain

Successfully rearing young places multiple demands on the mammalian female. These are met by a wide array of alterations in maternal physiology and behavior that are coordinated with the needs of the developing young, and include adaptations in neuroendocrine systems not directly involved in maternal behavior or lactation. In this article, attenuations in the behavioral and neuroendocrine responses to stressors, the alterations in metabolic pathways facilitating both increased food intake and conservation of energy, and the changes in fertility that occur postpartum are described. The mechanisms underlying these processes as well as the factors that contribute to them and the relative contributions of these stimuli at different times postpartum are also reviewed. The induction and maintenance of the adaptations observed in the postpartum maternal brain are dependent on mother-young interaction and, in most cases, on suckling stimulation and its consequences for the hormonal profile of the mother. The peptide hormone prolactin acting on receptors within the brain makes a major contribution to changes in metabolic pathways, suppression of fertility and the attenuation of the neuroendocrine response to stress during lactation. Oxytocin is also released, both into the circulation and in some hypothalamic nuclei, in response to suckling stimulation and this hormone has been implicated in the decrease in anxiety behavior seen in the early postpartum period. The relative importance of these hormones changes across lactation and it is becoming increasingly clear that many of the adaptations to motherhood reviewed here reflect the outcome of multiple influences. © 2016 American Physiological Society. Compr Physiol 6:1493-1518, 2016.

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

Suckling-induced serum prolactin levels are modified by interference with milk ejection in lactating rats

The aim of this study was to determine whether suckling-induced prolactin (PRL) levels were modified when milk ejection was impaired. Milk ejection impairment was achieved in two experimental models: a) depriving the dam of sleep during suckling and b) increasing the nonsuckling intervals in lactating dams. Sleep deprivation blocked milk ejection and enhanced suckling-induced PRL levels in dams that had been previously separated from their pups. When milk ejection is blocked in litter-deprived dams, mammary glands are not evacuated and they remain engorged. Suckling stimuli were not the cause of the difference in suckling-induced serum PRL levels in control and sleep-deprived dams. The engorgement of the mammary glands may play a role, as a maximum suckling-induced PRL increase was not observed in nonseparated SD dams with nonengorged mammary glands. Moreover, suckling-induced PRL levels were decreased when engorged mammary glands of SD dams were evacuated through an oxytocin injection. A parallel increase between suckling-induced PRL levels and mammary gland weight was observed in the experiments in which milk ejection was impaired through an increase in the intervals of nonsuckling, providing additional support for a relationship between mammary gland engorgement and the regulation of suckling-induced PRL levels.

Characterization of growth hormone nycthemeral plasma profiles in catheterized rainbow trout (Oncorhynchus mykiss)

This study was conducted in order to characterize the nycthemeral plasma profiles of growth hormone (GH) in 41 prepubertal (2+) rainbow trout (Oncorhynchus mykiss). The possible influences of day-night alternation and of the food factor (quantity ingested, timing of the feed) on these plasma profiles were also analysed. Blood samples were taken every hour during a 24-hour period through a catheter inserted in the dorsal aorta. An assay of cortisol levels showed that the fish were in satisfactory physiological condition. The plasma concentrations of GH fluctuated significantly during the course of a nycthemere. The basal level was very low (0.32 +/- 0.01 ng/ml), and was interrupted by peaks that occurred at a rate of 2.1 +/- 0.1 peaks/24 hr, an amplitude of 2.0 +/- 0.3 ng/ml, and a duration of 3.5 +/- 0.2 hr. As a result, mean GH levels over a 24-hr period were low (0.7 +/- 0.1 ng/ml). Peaks occurred irregularly and asynchronously in individual fish and displayed no rhythmicity. Our study shows that there is a sexual difference in GH profiles in rainbow trout. No significant difference was observed between daily and nightly levels of GH (0.6 +/- 0.07 vs. 0.8 +/- 0.1 ng/ml). No influence of the diet on the plasma profiles of GH was observed. Average levels of GH over a 24 hr period are not significantly correlated with the growth rate of the fish. This study shows that circulating levels of GH in rainbow trout are markedly lower than in other vertebrates.

Role of the hypophyseal neurointermediate lobe in the dynamic release of prolactin

The dynamic release of prolactin (PRL) occurs in a region of the anterior pituitary lobe (AP) that receives its blood supply primarily from the hypophyseal neurointermediate lobe (NIL) via the short portal vessels. This relationship, coupled with a growing body of supportive evidence, suggests that the hypophysiotropic signal mediating the massive discharge o f AP PRL derives from the NIL rather than the hypothalamic median eminence, as previously believed. Moreover, a major component of this signal appears to be a-MSH.

Nutritional status, suckling behavior, and prolactin release during lactation

This experiment was conducted to determine whether suckling-induced prolactin (PRL) values in chronically food-restricted lactating rats resulted from the inability of their litters to suckle vigorously. Rats were randomly assigned to be fed ad lib (AL) or to be chronically restricted (CR) to 50% of AL before and during pregnancy and lactation. Rats nursed their own litter (own) or a litter of the opposite dietary treatment group (other) for 90-min periods at early or peak lactation. Blood was collected regularly through a catheter implanted several days earlier. Suckling contributed to differences in PRL between AL and CR dams in early lactation as demonstrated by an interaction between dietary treatment and type of litter. However, tests of AL own vs. AL other and CR own vs. CR other were not significant. This is probably because the inattentive behavior of CR dams attenuated the PRL response in this group. At peak lactation, PRL values were related to recovery time from catheter implantation; therefore, determinants of PRL release change over the course of lactation.

Restoration of lactation in bromocriptine-treated rats by prolactin replacement: comparison of constant versus pulsatile infusion and intrahepatic versus intrajugular routes of delivery

The effectiveness of pulsed vs constant infusion of ovine(o) prolactin (PRL), given by different schedules, at restoring lactation in PRL-suppressed rats was compared, and the possibility that the liver participates in the restorative effects of the infused hormone was investigated. Lactating dams were given subcutaneous injections of bromocriptine (BC) between days 7 and 12 postpartum to suppress endogenous PRL secretion. Osmotic minipumps were used to infuse the oPRL into either the jugular vein or the hepatic portal vein. The latter route would expose the liver to higher concentrations of PRL than would intrajugular infusion. Constant infusion of oPRL in different doses was, overall, more effective at restoring lactation (i.e. litter weight gain) than was giving pulses, regardless of the site of delivery. Infusion of the PRL at 100 micrograms/rat/day in pulses of 1h duration was ineffective at frequencies of either 4 or 8/day, whereas pulses of 2h duration were effective at both of these frequencies. Infusing that dose of oPRL was equally effective whether it was given in 4 or 8 pulses/day of 2 h duration. Intrahepatic infusion of oPRL was not more effective than intrajugular delivery regardless of the schedule of administration. These results indicate that pulse duration is a more important determinant of the effectiveness of the galactopoietic action of PRL in the lactating rat than is pulse frequency. No evidence was obtained that the liver participates in the galactopoietic effects of PRL.

Metabolism of pregnant-lactating rats is adapted to pregnancy rather than to lactation

In pregnant-lactating rats implantation was induced on day 4 of lactation so that, as an exception, lactation coincided with the period of high fetal growth. The already present suckling litters of these animals lagged behind in growth, but the "second" litters were at birth normal in size and weight. Such pregnant-lactating rats were tested in vivo with intravenous glucose loads and compared with cyclic and lactating rats. Glucose tolerance was unaffected by the reproductive state. Pregnant-lactating rats showed, just as during their first pregnancy, low basal glucose levels. Their basal insulin levels and insulin responses, however, were decreased in comparison with the first pregnancy and resembled those of lactating rats. This may be due to an increased insulin turnover, because in vitro insulin responsiveness and insulin content of both "pregnant-lactating" and "pregnant" islets were increased in comparison with "cyclic" and "lactating" islets. It was concluded that the metabolism of pregnant-lactating rats is adapted to the pregnant rather than to the lactational state.

Ovarian oxytocin and progesterone are secreted independently of one another

Experiments were performed with cultured bovine granulosa cells to examine the relationship between the secretions of oxytocin and progesterone and to determine whether progesterone could be responsible for the progressive refractoriness of these cells to stimulation by ascorbic acid. Aminoglutethimide suppressed progesterone secretion by 95% but it neither reduced oxytocin secretion nor restored the cellular response to delayed ascorbate treatment. Addition of a high concentration of progesterone to the culture medium also failed to affect oxytocin secretion, its stimulation by ascorbate, or the endogenous secretion of the steroid. It is concluded that oxytocin and progesterone can be independently secreted and that progesterone regulates neither its own secretion nor that of oxytocin.