Evidence that Melatonin Acts in the Pituitary Gland through a Dopamine‐independent Mechanism to Mediate Effects of Daylength on the Secretion of Prolactin in the Ram

Abstract

A previous study provided evidence that melatonin acts in the pituitary gland to mediate the effects of daylength on the secretion of prolactin in sheep. This was based on the observation that hypothalamo-pituitary disconnected (HPD) Soay rams showed normal patterns in the changes in the peripheral blood concentrations of prolactin in response to alterations in photoperiod (10-fold higher concentrations under long than short days), and in response to exogenous melatonin (rapid decline following the administration of a constant-release implant of melatonin). The purpose of this study was to establish whether dopamine (DA) might be involved in mediating the effects of melatonin on the secretion of prolactin. Groups of HPD (n = 7) and control Soay rams (n = 8) were treated with vehicle (control, 2.0 ml 0.1 M tartaric acid/saline sc), bromocriptine (DA agonist, 0.06 mg/kg sc) or sulpiride (DA antagonist, 0.6 mg/kg sc), and the acute prolactin responses were measured over the next 4 h. Treatments were carried out under short days (8L: 16D, low prolactin), long days (16L: 8D), high prolactin), and under long days in the presence of a constant-release implant of melatonin (low prolactin). The prolactin response to TRH (1.25 micrograms/kg iv) was also measured. Bromocriptine caused a decrease in the plasma concentrations of prolactin in both HPD and control rams under short and long days. Sulpiride had no effect in the HPD rams on any occasion, but caused a very marked increase in the plasma concentrations of prolactin in the control rams under short days, long days, and under long days + melatonin. TRH caused an acute increase in the plasma concentrations of prolactin in the HPD rams under both long and short days although the responses were notably reduced compared with the controls especially under long days + melatonin. Overall, the inhibitory response to the DA agonist in HPD rams indicates the presence of DA D2 receptors linked to functional lactotrophs in the isolated pituitary gland. However, the total lack of a response to the DA antagonist indicates the absence of endogenous DA mechanisms regulating the secretion of prolactin in the HPD rams. The conclusion is that melatonin acts directly on the pituitary gland to mediate effects of photoperiod through a DA-independent mechanism.

Encoding and Decoding Photoperiod in the Mammalian Pars Tuberalis

“…Strong evidence shows that the melatonin duration signal does not target the PD [59] or hypothalamic factors such as dopamine [60,61] to regulate the seasonal PRL rhythm. Indeed the PT is the major tissue target in this process (reviewed in [56,62]) (fig.…”

Does melatonin treatment during lactation influence milk production in Lacaune and Assaf ewes?

“…Sheep milk production is a direct reflection of reproductive periodicity. Prolactin, which is probably the most important hormone controlling milk production, shows a seasonal variation throughout the year, associated with daylength changes (Thimonier et al, 1978), and it has been demonstrated that the seasonal rhythm of prolactin secretion is under pineal control by the secretion of melatonin (Reiter, 1991), which exerts a suppressive effect on prolactin (Lincoln and Clarke, 1995). Altered photoperiod influences milk production in dairy cattle, but evidence for this relationship in sheep is scarce.…”

Melatonin Modulation of the Daily Prolactin Secretion in Intact and Ovariectomized Ewes

“…Chronic administration of exogenous melatonin during long days shapes the secretion of the above-mentioned hormones in a similar manner [4, 5, 6]. Such response to the melatonin signal requires however long-term actions of the amine on the hypothalamus [6, 7]or on the pituitary gland [8, 9]. The mechanism of this action is still under investigation.…”