Effects of photoperiod on salsolinol-induced prolactin secretion in goats

The aim of the present study was to clarify the relation between salsolinol (SAL)-induced prolactin (PRL) release and photoperiod in goats. A single intravenous (i.v.) injection of SAL was given to adult female goats under short (8 h light, 16 h dark) or long (16 h light, 8 h dark) photoperiod conditions at two different ambient temperatures (20°C or 5°C), and the PRL-releasing response to SAL was compared to that of thyrotropin-releasing hormone (TRH) or a dopamine (DA) receptor antagonist, sulpiride. SAL, as well as TRH or sulpiride, stimulated the release of PRL promptly after each injection in both 8- and 16-h daily photoperiods at 20°C (P<0.05). The area under the response curve (AUC) of PRL for the 60-min period after injections of saline (controls), SAL, TRH and sulpiride in the 16-h daily photoperiod group was greater than each corresponding value in the 8-h daily photoperiod group (P<0.05). There were no significant differences in the AUC of PRL among the values produced after the injection of SAL, TRH and sulpiride in 16-h daily photoperiod group; however, the values produced after the injection of TRH were smallest among the three in the 8-h daily photoperiod group (P<0.05). The PRL-releasing responses to SAL, TRH and sulpiride under a short and long photoperiod condition at 5°C resembled those at 20°C. These results show that a long photoperiod highly enhances the PRL-releasing response to SAL as well as TRH or sulpiride in either medium or low ambient temperature in goats.