Reproduction Abstracts

In photoperiodic species, the pattern of pineal melatonin secretion translates the effects of day length on the annual reproductive cycle. Continuous delivery of exogenous melatonin to sheep during the non-breeding season is known to activate the gonadotrophic axis, and to suppress prolactin secretion through a mechanism involving the pars tuberalis (PT) of the pituitary gland. Recent studies have revealed that vascular endothelial growth factor (VEGF), a major regulator of angiogenesis and vascular permeability, is present in PT cells expressing MT1 melatonin receptors. In the current study, we investigated whether timely administered melatonin to ewes in the non-breeding season can induce differential expression of pro-angiogenic (VEGF₁₆₅) and anti-angiogenic (VEGF_165b) isoforms of VEGF in the pituitary and affect the prolactin and gonadotrophic axes. Melatonin was injected twice daily for 18 days during the spring; the injections (1.5 mg/dose, i.m.) were given in the afternoon (1530 h) and early morning (0500 h) to extend the nocturnal endogenous rise. Control animals received vehicle (n=4/group). Blood samples were collected every 3 h for 24 h on day 4 to determine prolactin concentrations, and every 10 min for 8 h on day 15 to determine concentrations of LH and FSH. The effects of treatment on pituitary VEGF isoform expression were investigated by immunohistochemistry. Melatonin significantly suppressed the nocturnal prolactin rise (P<0.05). The frequency of LH pulses was 1.5±0.29 and 0.75±0.48 pulses/8 h for melatonin and control ewes, respectively, whereas mean FSH concentrations for the same groups were 0.7±0.16 and 0.58±0.06 ng/ml; these differences did not reach statistical significance (P>0.05). However, VEGF_165b expression was significantly increased in the pituitary of melatonin-treated animals (P<0.05). The results show that melatonin targets the mechanisms underlying pituitary micro-vascular remodelling and the prolactin axis before fully activating the gonadotrophic axis when timely administered to anoestrus ewes.