Hormonal regulation of the annual pelage color cycle in the Djungarian hamster, Phodopus sungorus. II. Role of prolactin

This study investigated whether photoperiod‐induced changes in circulating prolactin levels, which have been observed in the Djungarian hamster (Yellon and Goldman, '83; Duncan and Goldman, '83a), might be involved in seasonal pelage color changes in this species. Injection of ovine prolactin (100 μg/day) inhibited the short photoperiod‐induced winter molt. This finding indicated that the suppression of endogenous prolactin levels normally occuring in short photoperiod‐housed hamsters (Duncan and Goldman, '83a) may induce the winter molt. Suppression of prolactin secretion with bromoergocryptine (200 μg/day) strongly inhibited the spring molt, while concomitant treatment with ovine prolactin (100 μg/day) overcame this effect of bromoergocryptine. Injection of bromoergocryptine (200 μg/day) stimulated the winter molt in castrated hamsters housed in long photoperiod; concomitant injection of prolactin (100 μg/day) reversed this effect as well. These findings strongly suggested that an increase in endogenous prolactin levels may be necessary for the development and maintenance of the summer pelage.