Effects of photoperiod on the secretion of growth hormone in female goats.

The aim of the present study was to clarify the effect of photoperiod on the secretion of growth hormone (GH) in goats. Adult female goats were kept at 20°C with an 8-h or 16-h photoperiod, and secretory patterns of GH for 4 h (12.00 to 16.00 hours) were compared. In addition, the goats were kept under a 16-h photoperiod and orally administered saline (controls) or melatonin, and the effects of melatonin on the secretion of GH were examined. GH was secreted in a pulsatile manner. There were no significant differences in pulse frequency between the 8- and 16-h photoperiods; however, GH pulse amplitude tended to be greater in the group with the 16-h photoperiod (P = 0.1), and mean GH concentrations were significantly greater in the 16-h photoperiod (P < 0.05). The GH-releasing response to GH-releasing hormone (GHRH) was also significantly greater for the 16-h photoperiod (P < 0.05). There were no significant differences in GH pulse frequency between the saline- and melatonin-treated groups. However, GH pulse amplitude and mean GH concentrations were significantly greater in the saline-treated group (P < 0.05). The present results show that a long photoperiod enhances the secretion of GH, and melatonin modifies GH secretion in female goats.