Seasonal prolactin secretion and its role in seasonal reproduction: a review

Abstract

The majority of seasonally breeding mammals show a seasonal pattern of prolactin secretion with peak concentrations in spring or summer and a nadir in autumn or winter. Photoperiod influences prolactin secretion via its effects on the secretion of the pineal hormone melatonin. Preliminary evidence suggests that the effects of melatonin on both prolactin and gonadotrophin secretion are via a common target area, possibly within the anterior hypothalamus, and that differences in response to photoperiod may be due to differences in the processing and/or interpretation of the melatonin signal. In contrast to seasonal gonadotrophin secretion, the seasonal changes in prolactin are not due to changes in the sensitivity of a feedback loop and so must be due to direct effects on the hypothalamic pathways that control prolactin secretion. Little else can be said with confidence about the neuroendocrine mechanisms that lead to the seasonal changes in prolactin secretion. Dopamine and noradrenaline turnover in the arcuate nucleus and median eminence decrease under short daylength. If catecholamine turnover in these structures is positively correlated with catecholamine concentrations in the long or short hypophysial portal vessels, it is unlikely that the decrease in prolactin concentration in winter is due to the effects of increased concentrations of dopamine or noradrenaline in the portal vessels. There is, however, evidence for increased pituitary sensitivity to dopamine under short daylength, so increased dopamine concentrations may not be required for suppression of prolactin secretion at this time. In addition to the diminished secretion of prolactin under short daylength, rate of prolactin synthesis and pituitary content of prolactin also decline although the mechanisms that regulate these changes are poorly understood.Although all seasonal breeders show a seasonal change in prolactin secretion, there are continuously breeding species in which prolactin secretion is also under photoperiodic control. It is likely therefore that a seasonal pattern of prolactin secretion is only evidence of neuroendocrine sensitivity to changing photoperiod. Depending upon the species, this sensitivity to the seasonal changes in daylength may or may not be accompanied by seasonal changes in a biological endpoint such as seasonal reproduction or indeed other adaptations. Whether the seasonal change in prolactin secretion is an endocrine mediator of such adaptations remains in contention. Certainly in some species this signal does have a role in reproduction. For example, in species with an obligate seasonal embryonic diapause, the seasonal increase in prolactin can act as a luteotrophin (mink and western spotted skunk) or luteostatin (Bennett's and tammar wallabies). In species where seasonal anoestrus or oligo-or azoospermia are due to a direct effect of photoperiod on gonadotrophin secretion, the seasonal changes in prolactin levels may also have been used by some species to influence seasonal reproduction. For example, the increase in ...

Prolactin regulation of testosterone secretion and testes growth in DLS rams at the onset of seasonal testicular recrudescence

“…Diminished testosterone production by the testes in this stage is mainly due to atrophy of Leydig cells (Hochereau-de Reviers et al 1985, 1992 and decreases in the activity of three regulatory proteins that are highly dependent upon LH, namely STAR protein and the cytochrome P450 cholesterol side-chain cleavage (CYP11A1, P450scc) and 17a-hydroxylase C17-C20 lyase (CYP17A1, P450c17) enzymes (Price et al 2000). While LH is the primary hormone regulating the structural and functional integrity of Leydig cells (Mendis-Handagama 1997, Lejeune et al 1998), present results indicate that, as in male rodents (Bole-Feysot et al 1998, Lejeune et al 1998, PRL assists in maintaining the competency of Leydig cells in the ram.…”

“…It has numerous actions in mammalian biological systems (Bole-Feysot et al 1998, Freeman et al 2000, including the reproductive processes of species that breed seasonally such as golden hamsters and black bears (spring), and sheep and deer (autumn) (Curlewis 1992, Bartke 2004. Sheep exhibit a robust annual cycle in PRL secretion that is closely related to day length.…”

Seasonal changes in ovulatory activity, plasma prolactin, and melatonin concentrations, in Mouflon (Ovis gmelini musimon) and Manchega (Ovis aries) ewes

“…Several factors could explain the discrepancy with this earlier study. Among others, female steroids are known to influence the release of prolactin [10], thus, gender may be a source of variation for prolactin. Also, melatonin can suppress prolactin secretion [27,41] and Manchega ewes had higher melatonin concentrations during the summer solstice.…”

Implication of D2-like dopaminergic receptors in the median eminence during the establishment of long-day inhibition of LH secretion in the ewe

“…For this, the effects on LH secretion, used as an index of LHRH secretion, of D1-like and D2-like agonists (after blockade of dopamine synthesis) and antagonists infused locally in the ME were studied in OVX+E ewes exposed to 30-50 long days, when ME dopamine has been shown to be critical for the inhibition of LH secretion (Bertrand et al 1998). Because prolactin (PRL) secretion is strongly regulated by photoperiod (Thimonier et al 1978) and dopamine is involved in this regulation (Curlewis 1992), the effects of the different treatments on PRL secretion were also assessed.…”