Hypoprolactinemia decreases tyrosine hydroxylase activity in the tuberoinfundibular dopaminergic neurons acutely by protein dephosphorylation and chronically by changes in gene expression

Secretion and Function of Pituitary Prolactin in Evolutionary Perspective

The hypothalamo-pituitary system developed in early vertebrates. Prolactin is an ancient vertebrate hormone released from the pituitary that exerts particularly diverse functions. The purpose of the review is to take a comparative approach in the description of prolactin, its secretion from pituitary lactotrophs, and hormonal functions. Since the reproductive and osmoregulatory roles of prolactin are best established in a variety of species, these functions are the primary subjects of discussion. Different types of prolactin and prolactin receptors developed during vertebrate evolution, which will be described in this review. The signal transduction of prolactin receptors is well conserved among vertebrates enabling us to describe the whole subphylum. Then, the review focuses on the regulation of prolactin release in mammals as we have the most knowledge on this class of vertebrates. Prolactin secretion in response to different reproductive stimuli, such as estrogen-induced release, mating, pregnancy and suckling is detailed. Reproduction in birds is different from that in mammals in several aspects. Prolactin is released during incubation in avian species whose regulation and functional significance are discussed. Little information is available on prolactin in reptiles and amphibians; therefore, they are mentioned only in specific cases to explain certain evolutionary aspects. In turn, the osmoregulatory function of prolactin is well established in fish. The different types of pituitary prolactin in fish play particularly important roles in the adaptation of eutherian species to fresh water environments. To achieve this function, prolactin is released from lactotrophs in hyposmolarity, as they are directly osmosensitive in fish. In turn, the released prolactin acts on branchial epithelia, especially ionocytes of the gill to retain salt and excrete water. This review will highlight the points where comparative data give new ideas or suggest new approaches for investigation in other taxa.

Patterns of prolactin secretion

Prolactin is pleotropic in nature affecting multiple tissues throughout the body. As a consequence of the broad range of functions, regulation of anterior pituitary prolactin secretion is complex and atypical as compared to other pituitary hormones. Many studies have provided insight into the complex hypothalamic-pituitary networks controlling prolactin secretion patterns in different species using a range of techniques. Here, we review prolactin secretion in both males and females; and consider the different patterns of prolactin secretion across the reproductive cycle in representative female mammals with short versus long luteal phases and in seasonal breeders. Additionally, we highlight changes in the pattern of secretion during pregnancy and lactation, and discuss the wide range of adaptive functions that prolactin may have in these important physiological states.

Conditional Deletion of the Prolactin Receptor Reveals Functional Subpopulations of Dopamine Neurons in the Arcuate Nucleus of the Hypothalamus

Tuberoinfundibular dopamine (TIDA) neurons, known as neuroendocrine regulators of prolactin secretion from the pituitary gland, also release GABA within the hypothalamic arcuate nucleus. As these neurons express prolactin receptors (Prlr), prolactin may regulate GABA secretion from TIDA neurons, potentially mediating actions of prolactin on hypothalamic function. To investigate whether GABA is involved in feedback regulation of TIDA neurons, we examined the physiological consequences of conditional deletion of Prlr in GABAergic neurons. For comparison, we also examined mice in which Prlr were deleted from most forebrain neurons. Both neuron-specific and GABA-specific recombination of the Prlr gene occurred throughout the hypothalamus and in some extrahypothalamic regions, consistent with the known distribution of Prlr expression, indicative of knock-out of Prlr. This was confirmed by a significant loss of prolactin-induced phosphorylation of STAT5, a marker of prolactin action. Several populations of GABAergic neurons that were not previously known to be prolactin-sensitive, notably in the medial amygdala, were identified. Approximately 50% of dopamine neurons within the arcuate nucleus were labeled with a GABA-specific reporter, but Prlr deletion from these dopamine/GABA neurons had no effect on feedback regulation of prolactin secretion. In contrast, Prlr deletion from all dopamine neurons resulted in profound hyperprolactinemia. The absence of coexpression of tyrosine hydroxylase, a marker for dopamine production, in GABAergic nerve terminals in the median eminence suggested that rather than a functional redundancy within the TIDA population, the dopamine/GABA neurons in the arcuate nucleus represent a subpopulation with a functional role distinct from the regulation of prolactin secretion.

SIGNIFICANCE STATEMENT

Using a novel conditional deletion of the prolactin receptor, we have identified functional subpopulations in hypothalamic dopamine neurons. Although commonly considered a uniform population of neuroendocrine neurons involved in the control of prolactin secretion, we have shown that approximately half of these neurons express GABA as well as dopamine, but these neurons are not necessary for the feedback regulation of prolactin secretion. The absence of tyrosine hydroxylase in GABAergic nerve terminals in the median eminence suggests that only the non-GABAergic dopamine neurons are involved in the control of pituitary prolactin secretion, and the GABAergic subpopulation may function as interneurons within the arcuate nucleus to regulate other aspects of hypothalamic function.

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Prolactin regulates tuberoinfundibular dopamine neuron discharge pattern: novel feedback control mechanisms in the lactotrophic axis

Balance in the body's hormonal axes depends on feedback onto neuroendocrine hypothalamic neurons. This phenomenon involves transcriptional and biosynthetic effects, yet less is known about the potential rapid modulation of electrical properties. Here, we investigated this issue in the lactotrophic axis, in which the pituitary hormone prolactin is tonically inhibited by tuberoinfundibular dopamine (TIDA) neurons located in the hypothalamic arcuate nucleus. Whole-cell recordings were performed on slices of the rat hypothalamus. In the presence of prolactin, spontaneously oscillating TIDA cells depolarized, switched from phasic to tonic discharge, and exhibited broadened action potentials. The underlying prolactin-induced current is composed of separate low- and high-voltage components that include the activation of a transient receptor potential-like current and the inhibition of a Ca(2+)-dependent BK-type K(+) current, respectively, as revealed by ion substitution experiments and pharmacological manipulation. The two components of the prolactin-induced current appear to be mediated through distinct signaling pathways as the high-voltage component is abolished by the phosphoinositide 3-kinase blocker wortmannin, whereas the low-voltage component is not. This first description of the central electrophysiological actions of prolactin suggests a novel feedback mechanism. By simultaneously enhancing the discharge and spike duration of TIDA cells, increased serum prolactin can promote dopamine release to limit its own secretion with implications for the control of lactation, sexual libido, fertility, and body weight.

Expression of mRNA for prolactin receptor (long form) in dopamine and pro-opiomelanocortin neurones in the arcuate nucleus of non-pregnant and lactating rats

Under most conditions, prolactin secretion from the pituitary gland is subject to negative-feedback regulation. Prolactin stimulates dopamine release from tuberoinfundibular (TIDA) neurones in the arcuate nucleus of the hypothalamus, which in turn suppresses the production of prolactin. However, during late pregnancy and continuing into lactation, this feedback mechanism becomes less responsive to prolactin and, as a result, a hyperprolactinaemic state develops. We investigated whether long-form prolactin receptor (PRL-R(L)) mRNA is present on TIDA neurones in nonpregnant and lactating rats. In addition, we examined whether PRL-R(L) mRNA is colocalized on hypothalamic pro-opiomelanocortin (POMC) neurones. Dual-label in situ hybridizations using an (35)S-labelled cRNA probe specific for long-form PRL-R, together with a digoxigenin-labelled RNA probe that encoded either tyrosine hydroxylase (TH) or POMC mRNA, were performed on brain sections. In both nonpregnant and lactating rats, the majority of TH mRNA-positive cells (> 90%) were found to express long-form PRL-R mRNA. In sections from nonpregnant rats, few non-TH positive cells expressed PRL-R(L) mRNA. By contrast, during lactation, the proportion of PRL-R(L) mRNA-positive cells that were not TH mRNA-positive increased to approximately 70%. Only a small number of neurones in this subpopulation of PRL-R(L) mRNA-positive neurones were found to be positive for POMC mRNA. These data show that the loss of responsiveness to prolactin occurring during lactation is not due to down regulation of long-form PRL-R gene expression on TIDA neurones. Moreover, the persistent expression of PRL-R(L) in arcuate neuroendocrine circuits suggests that PRL-R-mediated signalling continues to be important in these neurones during lactation.

Decreased expression of fos-related antigens (FRAs) in the hypothalamic dopaminergic neurons after immunoneutralization of endogenous prolactin

In our previous studies we found that administration of exogenous prolactin increased dopamine turnover in the terminal areas of the hypothalamic dopaminergic neurons controlling prolactin secretion from pituitary lactotrophs. In this study we investigated the effect of immunoneutralization of endogenous prolactin on the expression of FRAs in the tuberoinfundibular dopaminergic (TIDA), tuberohypophysial dopaminergic (THDA), and periventricular hypothalamic dopaminergic (PHDA) subpopulations of the hypothalamic dopaminergic neurons. Female rats were ovariectomized on d 0 of the experiment. At 1000 h of d 10, all animals were injected with 20 microg of 17-beta-estradiol sc to induce a proestrous-like surge of prolactin at 1700 h the next day. At 1000 h on d 11, half of the animals were injected with 200 microL of rabbit anti-rat prolactin antiserum ip, while the controls received normal rabbit serum. Groups of animals were sacrificed for immunocytochemistry in 2 h intervals between 1300 and 2100 h. Double-label immunocytochemistry for FRAs and tyrosine hydroxylase (TH) was performed and the results are presented as percentage of TH-immunoreactive neurons expressing FRAs. In the control animals, expression of FRAs decreased at 1500 h, gradually increased by 1900 h, but was lower than the basal levels by 2100 h. Expression of FRAs was significantly lower at 1900 h in the PHDA, THDA and TIDA neurons of prolactin antiserum treated rats than in the controls. These results indicate that elimination of endogenous prolactin from the circulation lowers the activity and/or prevents the reactivation of neuroendocrine dopaminergic neurons at the beginning of the dark phase.

Initial expression of the common alpha-chain in hypophyseal pars tuberalis-specific cells in spontaneous recrudescent hamsters

When exposed to short-day conditions, hamsters and other long-day breeders undergo gonadal regression. With chronic exposure to short days, however, the animals become photorefractory and gonadal recrudescence occurs. The underlying mechanism for this insensitivity is still unknown. There is growing evidence, however, that specific cells of the pituitary pars tuberalis (PT) mediate these photoperiod/nonphotoperiod-dependent changes as a direct or indirect "Zeitgeber" for the endocrine system. We investigated messenger RNA (mRNA)/protein formation for several hypophyseal hormones (beta-TSH, beta-LH, PRL, common alpha-chain) in the pars distalis (PD) and PT of female Djungarian hamsters in long photoperiod (LP) and after 18, 28, and 38 weeks of short photoperiod (SP). As indicated by gonadal and body weight, the hamsters displayed gonadal regression after 18 and 28 weeks of SP; after 38 weeks of SP, all animals showed recrudescence. At 18 and 28 weeks of SP, only PRL mRNA and protein levels were significantly reduced in the PD and returned to LP values after 38 weeks of SP. The expression of hypothalamic tyrosine hydroxylase in the arcuate nucleus that was determined by immunocytochemistry and by in situ hybridization was also down-regulated in SP18 and SP28 with increasing levels at SP38. Urinary 6-sulfatoxymelatonin levels were elevated in SP with highest levels in the SP18 group. In the PT, beta-TSH mRNA and protein were not detectable in all SP groups compared with the moderate signal intensity in LP. The common alpha-chain mRNA and protein, however, which were also reduced in the animals of the SP18 group, were already elevated after 28 weeks of SP and nearly reached LP-levels after 38 weeks of SP. These results show that, in contrast to LH and TSH, PRL expression in the PD is a sensitive indicator for photoperiod dependent changes of the endocrine system and seems to be tyrosine hydroxylase independent. The increase of common alpha-chain expression in PT-specific cells depending upon duration of SP that precedes the hormonal changes in the PD leads us to speculate that PT-specific cells initiate spontaneous recrudescence via a PT-PD pathway.