Reproduction Abstracts

Recent studies have shown that microvasculature remodelling of the ovine pituitary gland is seasonally regulated through mechanisms involving the differential expression of pro-angiogenic (VEGF₁₆₅) and anti-angiogenic (VEGF_165b) isoforms of vascular endothelial growth factor (VEGF). As for other photoperiodic species, the pattern of pineal melatonin secretion mediates the effects of day length on the sheep seasonal reproductive cycle. In this study, we investigated whether timely administered melatonin can alter the pituitary microvasculature and VEGF isoform expression in seasonally anoestrous ewes exposed to natural photoperiod. Melatonin (1.5 mg/dose, i.m.) was injected twice daily at 0500 and 1530 h for 18 days during the spring; control animals were injected with vehicle (n=4/group). The effects of treatment on the pituitary microvasculature and on the expression of VEGF isoforms in the pars tuberalis (PT) and pars distalis (PD) were investigated by immunohistochemistry. Melatonin reduced the number of vascular loops connecting the PT with the infundibulum (P<0.05). In both the PT and PD, the expression of VEGF_165b was significantly increased in melatonin-treated sheep (P<0.001). In these animals, but not in controls, VEGF_165b was more abundant in the PT than in the PD (P=0.01). No difference in VEGF₁₆₅ expression was observed between groups in the PT. However, in the PD, VEGF₁₆₅ was higher in control ewes (P<0.001). Critically, the aforementioned melatonin effects were concomitant with a robust up-regulation of GnRH expression in neuronal terminals of the median eminence (P<0.001). Collectively, these findings show that the previously reported seasonal control of VEGF expression in photoperiodic species is mediated by melatonin. Specifically, the increased duration of the nocturnal melatonin rise during short days up-regulates the anti-angiogenic VEGF_165b isoform and restricts the vascular connection between the PT and the infundibulum as part of the mechanisms underlying the photoperiodic control of the annual reproductive cycle.