Graduate Thesis Or Dissertation
 

Endocrinology of fur growth and reproduction in mink

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  • Experiments were conducted to determine: 1) the effects of exogenous melatonin and reduced photoperiod on molting and growth of the winter pelage (fur) and 2) whether changes in serum prolactin levels of treated and control mink during this same period were correlated with growth of winter fur. Adult and kit female and male mink were utilized and treatments were initiated in late June. Regardless of sex or age, mink treated with 10 mg melatonin or a 6 hour light:18 hour dark photoperiod molted the summer pelage and attained prime winter pelage 6 weeks earlier than controls (P <.001). Both reduced photoperiod and exogenous melatonin caused serum prolactin concentrations to decline from a mean high of 5.9 ng/ml in mid-July to basal levels of less than 1.5 ng/ml by late July-early August, 6 to 8 weeks earlier than controls. These findings indicate that the photoperiodic effects on growth of the winter pelage of mink may be mediated through the pineal gland and its secretion of melatonin. These data also suggest that part of the mechanism of action of melatonin may be through the regulation of prolactin synthesis and secretion. Experiments were also designed to determine: 1) if prolactin receptors were present in the ovaries and uterus of the mink and 2) the relationship of changes in serum prolactin concentrations to levels of ovarian prolactin receptors during embryonic diapause. Concentrations of uterine and ovarian prolactin receptors of mink were quantified during 1) anestrus (uterus only), 2) when blastocysts were entering a state of diapause, and 3) during the period of blastocyst reactivation, just prior to implantation. Mink were also bled at periodic intervals during gestation and the serum levels of prolactin determined by use of radioimmunoassay. Scatchard analysis revealed a single class of binding sites for prolactin with K[subscript d] values of 8.25 x 10 [superscript -11] ± .68 M (uterus) and of 6.14 x 10 [superscript -11] ± .93 M (ovaries). The total concentration of uterine prolactin receptors during anestrus was 28 fmol/mg protein. The total concentration of ovarian receptors during anestrus was 90 fmol/mg protein, which increased significantly during embryonic diapause to 484 fmol/mg protein, and then declined to 16 fmol/mg protein during blastocyst reactivation. Serum prolactin concentrations began to increase soon after the vernal equinox (March 21) and were significantly elevated by March 27. The results of these studies clearly establish that the ovaries and possibly the uterus of the mink are target organs for prolactin.
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