BRAIN AND PINEAL GLAND MEDIATION OF PHOTOPERIOD-INDUCED HORMONE CONCENTRATIONS IN CATTL

Concentrations of prolactin (PRL) in serum were 2 to 3-fold greater in prepubertal bulls exposed to 24 (24 Light:0 Dark) or 16 (16L:8D) h of daily light as compared with 8L:16D. However, PRL attained maximum concentrations approximately 3 wk later in calves exposed to 24L:0D.;In other bulls, PRL was 2.5-fold greater after 6-wk exposure to 16L:8D compared with 8L:16D, while testosterone was concurrently increased 2-fold by 16L:8D. However, after 6 additional wk of repetitive application of 16L:8D bulls became refractory in terms of PRL while testosterone concentrations plateaued. Photoperiod did not affect serum luteinizing hormone concentrations. Shifting from 8L:16D to 16L:8D increased serum concentrations of PRL equally in castrated and gonadally intact males.;Dopamine concentrations in anterior pituitary glands were greater in calves exposed to 8L:16D compared with 16L:8D, while plasma catecholamine concentrations were not affected by photoperiod.;In other experiments, serum melatonin concentrations varied episodically and were 60 to 405% greater during dark than light periods, while 24L:0D abolished the nocturnal increase. Exposure to 8-h of darkness coincidently elevated serum melatonin, whereas melatonin is elevated for only the first 12 to 14-h of a 16-h dark period. Serum melatonin is probably not responsible for photoperiod-induced changes in serum PRL since PRL concentrations are equivalent in bulls infused daily with melatonin for 8 or 16-h.;Pinealectomy (PX) of bull calves does not abolish photoperiod-induced changes in serum PRL concentrations; however, testosterone concentrations in PX bulls are lower at 20 wk of age than in sham-PX bulls.;It is concluded that photoperiod-induced changes in PRL concentrations does not involve melatonin, the pineal gland or the testes, but may be mediated by dopamine in prepubertal bull calves. Serum PRL concentrations in calves become refractory to prolonged 16L:8D photo-stimulation. Finally, long-days hasten puberty onset in bulls and the pineal gland may be involved.