Differential effects of short photoperiod on 2-[125I]iodomelatonin binding in the testis and brain of quail

The affinities and densities of 2-[125I]iodomelatonin binding sites in the brains and gonads of male Japanese quail following short photoperiod treatment were studied. At 6 weeks old, control quail were placed under a 14 hour light/10 hour dark photo-stimulatory cycle and experimental quail were housed under a 7 hour light/17 hour dark photo-inhibitory lighting regime. Eighteen weeks after photic manipulation, the birds were killed at mid-light. The photo-inhibited quail had very small testes. Brains and testes of control and experimental quail were collected for receptor binding studies. The maximum number of binding sites (Bmax) of 2-[125I]iodomelatonin determined by saturation studies and the number of 2-[125I]iodomelatonin binding sites determined by a one-point binding assay in the testes of short-day quail were significantly lower (p < 0.05) than those of the testes in reproductively active birds kept under long photoperiod. There was no significant difference (p > 0.05) between the testicular Kd (equilibrium dissociation constant) values of these two groups. As for the Kd and Bmax of 2-[125I]iodomelatonin binding sites in the whole brain, there were no significant differences (p > 0.05) between the two groups. The higher level of testicular 2-[125I]iodomelatonin binding sites in photostimulated birds may be related to an up-regulation of melatonin receptors by the suppressed pineal melatonin secretion under long photoperiod. The lower testicular 2-[125I]iodomelatonin binding sites under short photoperiod may be the result of down-regulation of melatonin receptors by the stimulated melatonin pattern in the photo-inhibited birds.(ABSTRACT TRUNCATED AT 250 WORDS)