The Ovine Pars Tuberalis does not Appear to be Targeted by Melatonin to Modulate Luteinizing Hormone Secretion, but may be Important for Prolactin Release

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Abstract
The pineal hormone, melatonin, transduces photoperiodic information to the neuroendocrine axis of seasonally breeding mammals to regulate reproduction. It is not known where or how melatonin achieves this effect, but the recent identification of the pars tuberalis (PT) as the area with the highest density of melatonin binding sites suggests that this pituitary subdivision may be an important target for the actions of this indoleamine on luteinizing hormone (LH) and prolactin release. The present study was designed to test this hypothesis. Ovariectomized oestradiol‐implanted ewes were exposed to inhibitory long days for 85 days and then received melatonin micro‐implants (Day 0) in the mediobasal hypothalamus (MBH; n = 7) or PT (Melatonin‐PT; n = 5). The effect of these micro‐implants was compared to ewes receiving empty micro‐implants in the PT (Sham‐PT; n = 5). For LH, bi‐weekly jugular blood samples were collected and for prolactin, samples were collected every 20 min for 5 h, with the first hour discarded, on Days ‐4, 26 and 69. Melatonin implanted in the MBH stimulated LH secretion in 3 ewes by Day 46±0 after implantation, and one ewe by Day 67 after implantation. In contrast, no Melatonin‐PT or Sham‐PT ewes exhibited an increase in LH secretion by the end of the study (Day 70). A subsequent experiment, in which the Sham‐PT ewes were implanted with melatonin both subcutaneously and in the PT showed that the micro‐implants did not impair the ability of the ovine reproductive neuroendocrine axis to respond to melatonin. For prolactin, both the Melatonin‐PT ewes and MBH ewes (which had displayed an increase in LH secretion) exhibited a significant decrease in prolactin secretion by Day 26 and this decrease persisted until the end of the study. This result suggests that melatonin may have two sites of action on prolactin secretion; the MBH and PT, or, if melatonin diffused from the MBH to the PT, or vice versa, that only one of these sites is important. In contrast, these data suggest that the PT is not an important target for the action of melatonin on LH secretion.