Effects of photoperiod on lactotrophs and on dopaminergic and 5-hydroxytryptaminergic neurones in bull calves

This study was conducted to determine whether photoperiod-induced changes in serum concentrations of prolactin in cattle were associated with changes in activity of dopamine or 5-hydroxytryptamine (5-HT) neurones in the infundibulum/pituitary stalk and the secretion rate and number of lactotrophs in the anterior pituitary gland. Sixteen prepubertal bull calves (approximately 8 weeks of age) were divided into two groups. One group of eight was maintained on a photoperiod of 8 h light:16 h darkness (8L:16D) and the other group was exposed to 16L:8D for 4 weeks. At this time calves were injected with a decarboxylase inhibitor (m-hydroxybenzylhydrazine dihydrochloride, NSD 1015) which blocks the conversion of dihydroxyphenyl-alanine (DOPA) to dopamine and of 5-hydroxy-tryptophan (5-HTP) to 5-HT. Calves were killed with pentobarbital 15 min later. Accumulations of DOPA and 5-HTP in selected brain regions were used as indices of activity of dopamine and 5-HT neurones respectively. Secretory rate and number of prolactin-secreting lactotrophs were determined by reverse haemolytic plaque assay. Relative to calves exposed to 8L:16D, exposure to 16L:8D increased serum concentrations of prolactin by eightfold, anterior pituitary gland weight by 23%, release of prolactin from pituitary explants by 57% and the area of the plaque for prolactin-secreting lactotrophs by 70%. There was no difference in the rates of accumulation of DOPA and 5-HTP in the infundibulum/pituitary stalk of animals exposed to 4 weeks of 16L:8D or 8L:16D. It was concluded that increased serum concentrations of prolactin in bulls exposed to a photoperiod of 16L:8D for 4 weeks were associated with increased secretion of prolactin from lactotrophs which was not the result of a coincident reduction in activity of dopamine neurones or an activation of 5-HT neurones that terminate in the infundibulum/pituitary stalk.