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.
© 2012 The Authors. Animal Science Journal © 2012 Japanese Society of Animal Science.