Anterior pituitary release of prolactin is inhibited by exposure to short photoperiod

Seasonality of prolactin in birds and mammals

In most animals, annual rhythms in environmental cues and internal programs regulate seasonal physiology and behavior. Prolactin, an evolutionarily ancient hormone, serves as a molecular correlate of seasonal timing in most species. Prolactin is highly pleiotropic with a wide variety of well-documented physiological effects; in a seasonal context prolactin is known to regulate annual changes in pelage and molt. While short-term homeostatic variation of prolactin secretion is under the control of the hypothalamus, long-term seasonal rhythms of prolactin are programmed by endogenous timers that reside in the pituitary gland. The molecular basis of these rhythms is generally understood to be melatonin dependent in mammals. Prolactin rhythmicity persists for several years in many species, in the absence of hypothalamic signaling. Such evidence in mammals has supported the hypothesis that seasonal rhythms in prolactin derive from an endogenous timer within the pituitary gland that is entrained by external photoperiod. In this review, we describe the conserved nature of prolactin signaling in birds and mammals and highlight its role in regulating multiple diverse physiological systems. The review will cover the current understanding of the molecular control of prolactin seasonality and propose a mechanism by which long-term rhythms may be generated in amniotes.

Prolactin receptor regulates the seasonal reproduction of striped hamsters

In this study, differential mRNA expression patterns of prolactin receptor (PRLR) in the hypothalamus and gonads, and the correlation with follicle stimulating hormone (FSH) and luteinizing hormone (LH) in striped hamster serum from spring, summer, autumn and winter were analyzed. Mature female and male striped hamsters in oestrus were used. Expression levels of PRLR in the hypothalamus, ovaries and testis from the summer and winter individuals were significantly higher compared with levels from the spring and autumn, whereas FSH and LH serum concentrations from summer and winter individuals were significantly lower compared with that from the spring and autumn. PRLR expression levels in hypothalamus, ovaries and testis were negatively correlated with FSH and LH serum concentrations, illustrating that PRLR might negatively regulate seasonal reproductive activity. PRLR expression levels in ovaries and testes were significantly higher compared with levels in the hypothalamus, suggesting that the regulative effects of PRLR in gonads might be significantly higher compared with that in the hypothalamus. Furthermore, PRLR expression levels from the spring, summer, autumn and winter seasons in the hypothalamus and gonads were significantly higher in females compared with levels in males, indicating that the regulative effect of PRLR might be sex dependent. Taken together, this study helps to understand in depth the seasonal regulative reproduction mechanism of striped hamsters to reasonably control population abundance.

Encoding and decoding photoperiod in the mammalian pars tuberalis

In mammals, the nocturnal melatonin signal is well established as a key hormonal indicator of seasonal changes in day-length, providing the brain with an internal representation of the external photoperiod. The pars tuberalis (PT) of the pituitary gland is the major site of expression of the G-coupled receptor MT1 in the brain and is considered as the main site of integration of the photoperiodic melatonin signal. Recent studies have revealed how the photoperiodic melatonin signal is encoded and conveyed by the PT to the brain and the pituitary, but much remains to be resolved. The development of new animal models and techniques such as cDNA arrays or high throughput sequencing has recently shed the light onto the regulatory networks that might be involved. This review considers the current understanding of the mechanisms driving photoperiodism in the mammalian PT with a particular focus on the seasonal prolactin secretion.

Recurrent flank alopecia in a Tibetan Terrier

Recurrent flank alopecia is described in a 2-year-old, male, neutered Tibetan Terrier with concurrent atopic dermatitis. The diagnosis of recurrent flank alopecia was made after 3 consecutive years of localised, winter-onset alopecia. The diagnosis was based on history, compatible clinical signs and supportive histopathology. The diagnosis was complicated by the presence of concurrent pruritic dermatitis. To our knowledge this is the first report of recurrent flank alopecia in this breed.

Prolactin rhythms-oscillators' response to photoperiodic cues is age and circadian time dependent

In vivo prolactin release patterns exhibit a compound rhythm with circadian (24 h), semicircadian (12 h) and ultradian (6-8 h) periods. Changes in these rhythmic patterns were observed at different photoperiodic conditions, and in elderly. Since in vitro prolactin release was related to the photoperiodic history of the animal, we studied the effect of varying photoperioda upon the in vitro rhythmic output of prolactin release from young and old male rat pituitaries, isolated at different circadian times from animals housed at LD 12:12, 18:6 for 10 days or 6 weeks. The results indicate that, both, mean levels and rhythmic prolactin release in vitro are determined by the age of the animal, the circadian time of pituitary isolation, the photoperiodic conditions in which the animal was housed, and the duration of housing in the long day conditions. The change of the rhythmic output pattern is gradual, reflecting a process by which the oscillators respond to the external cues to fit prolactin release pattern to the environmental conditions. Each of the oscillators (e.g. circadian, semicircadian, ultradian) shows different sensitivity to the changing photoperiodic signal and is regulated at the level of phase and amplitude but not the period. In old rats the response of the oscillators to the change in photoperioda is attenuated and not sufficient to induce a change in the output of prolactin release suggesting a loss in adaptation ability.

Just the two of us: melatonin and adenosine in rodent pituitary function

The function of the pituitary gland is tightly controlled by neuronal and hormonal afferents of the brain. In this review, the role of the neurohormone melatonin and the neuromodulator adenosine for rodent pituitary function will be elucidated. Adenosine is known as an important paracrine modulator for pituitary endocrine and folliculostellate cells, with availability regulated by local metabolic cellular activity. In general, adenosine inhibits the cyclic adenosine monophosphate (AMP) pathway in pituitary cells by binding to A1-, and A3-adenosinergic receptors, and activates it via A2-adenosinergic receptors. The neurohormone melatonin integrates time-of-day and time-of-year into pituitary function via binding to MT1-melatonin receptors. Melatonin impacts at the hypothalamic level neurons that synthesize releasing and release-inhibiting hormones, and at the pituitary level only cells of the hypophyseal pars tuberalis (PT). Thereby, the daily changes in the duration of the nocturnal melatonin surge are decoded and subsequently relayed to the pars distalis to adapt gonadotropin and prolactin release, respectively, to season. An exciting integration of time within the regulation of pituitary function was deciphered by analysing transmembrane signalling events in cells of the hypophyseal PT: a consecutive daily impact of initially the neurohormone melatonin and later the neuromodulator adenosine on rodent PT cells leads to a circadian rhythm in the transcription of cyclic-AMP-sensitive genes.

Noradrenergic regulation of prolactin secretion at the level of the paraventricular nucleus of the hypothalamus: functional significance of the alpha-1b and beta-adrenergic receptor subtypes

Previous research has demonstrated that in the Siberian hamster, both photoperiod and estrous cyclicity alter the profile of noradrenergic activity with the paraventricular nucleus of the hypothalamus (PVN), and that noradrenergic activity is correlated with changes in circulating levels of prolactin. Work from our laboratory has demonstrated an inhibitory role for norepinephrine (NE) acting at the alpha-2 receptor subtype within the PVN on serum prolactin levels; however, the functional significance of other adrenergic receptor subtypes on this system is unknown. The purpose of this study was to investigate the functional significance of the alpha-1b and beta-adrenergic receptor subtypes at the level of the PVN on circulating levels of prolactin. These experiments were performed in male Siberian hamsters using reverse microdialysis coupled with serial blood sampling. In Experiment 1, infusion of l-phenylephrine hydrochloride (alpha-1b agonist) initiated a dose-dependent increase in circulating prolactin, whereas infusion of chloroethylclonidine (alpha-1b antagonist) induced a significant dose-dependent decline in prolactin. In Experiment 2, intraparaventricular administration of propranolol (beta antagonist) initiated a significant increase in prolactin levels in a dose-dependent manner, whereas isoproterenol (beta agonist) induced a dose-dependent decline in prolactin. The results of this study indicate that both the alpha-1b and beta-adrenergic receptor subtypes have a significant role in regulating circulating levels of prolactin at the level of the PVN in the Siberian hamster.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

The pars tuberalis: the missing link in the photoperiodic regulation of prolactin secretion?

The endocrine function of the pars tuberalis of the pituitary gland has been an enigma for many years. Recent work suggests that one of its primary functions in seasonal mammals is to mediate photoperiodically regulated changes in prolactin secretion via an unidentified factor called tuberalin.