Regulation in pregnant mink (Mustela vison) of plasma progesterone and prolactin concentrations and regulation of onset of the spring moult by daylight ratio and melatonin injections

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Abstract
In pregnant or pseudopregnant mink maintained under natural daylight conditions and mated early in March, plasma prolactin and progesterone levels began to increase after the vernal equinox. The onset of spring moult was observed a few days later. When the females were transferred to a 15 h light (L): 9 h dark (D) photoperiod just after mating, prolactin and progesterone increase was advanced by several days; when the mink were transferred to an 11 h L: 13 h D schedule, these increases, as well as the onset of spring moult, were delayed or inhibited. A daily afternoon injection of melatonin mimicked the effects of short photoperiod. A positive relation was observed between plasma prolactin on the one hand and progesterone secretion or onset of moulting on the other. These results suggest that short photoperiod or melatonin effects might act by inhibiting prolactin secretion.