Effects of photoperiod on the secretion of growth hormone and prolactin during nighttime in female goats

The aim of the present study was to clarify the effect of photoperiod on nighttime secretion of growth hormone (GH) in goats. Adult female goats were kept at 20 degrees C with an 8 h or 16 h dark photoperiod, and secretory patterns of GH for 8h in the dark period were examined with the profile of prolactin (PRL) secretion. GH was secreted in a pulsatile manner in the dark period. There were no significant differences in pulse frequency between the 8- and 16-h dark photoperiods; however, pulse amplitude tended to be greater in the group with the 16-h dark photoperiod (P=0.1), and mean GH concentrations were significantly greater in the same photoperiod (P<0.05). PRL secretion increased quickly after lights off under both photoperiods. The PRL-releasing responses were weaker in the 8-h than 16-h dark photoperiod. The secretory response to photoperiod was more obvious for PRL than GH. The present results show that a long dark photoperiod enhances the nighttime secretion of GH in female goats, although the response is not as obvious as that for PRL.