Reproduction Abstracts

In photoperiodic species, remodelling of pituitary microvasculature appears to play a key role in the regulation of the annual reproductive cycle. Recent studies have revealed seasonal changes in the vascular loops of the ovine pituitary stalk. Microvasculature remodelling is under the control of pro-angiogenic (VEGF₁₆₅a) and anti-angiogenic (VEGF₁₆₅b) isoforms of vascular endothelial growth factor (VEGF). Here, we examined the differential expression of these VEGF isoforms in the pars tuberalis (PT) and pars distalis (PD) of the pituitary gland of gonadal intact (stallions) and orchidectomised (geldings) thoroughbred horses, during the breeding season (BS) and non-BS (NBS). VEGF isoform expression was investigated by double immunohistochemistry. Statistically significant effects of season, gonadal status, and the interaction between them were observed in both regions of the pituitary. In stallions, an increase in VEGF₁₆₅a was detected in the PD during the NBS (P<0.01). Conversely, the expression of VEGF₁₆₅b in this region was increased during the BS (P<0.01). Similar changes were observed in the PT. Critically, robust seasonal effects on VEGF isoform expression were detected in the PD of geldings, with significant changes in VEGF₁₆₅a opposite to those recorded in intact animals, i.e. increased expression in the BS (P<0.01). In these animals, alterations in VEGF isoform in the PT were also apparent, with increased expression of VEGF₁₆₅a in the BS (P<0.01). The results show that a mechanism for vascular remodelling in the equine pituitary is highly sensitive to seasonal regulation, and that this mechanism is modulated by gonadal feedback.