GENERAL AND COMPARATIVE ENDOCRINOLOGY 65, 212-215 (1987)

Apparent Role of Melatonin and Prolactin in Initiating Winter Fur Growth in Mink’

JACKROSE,*JAMES OLDFIELD,~ANDFREDRICK STORMSHAKtq2

*Department of Zoology and tDepartment of Animal Science. Oregon State University, Corvallis, Oregon 97331

Accepted September 8, 1986

A study was conducted to determine the effects of exogenous melatonin and bromocryp- tine (CB-154), an inhibitor of prolactin synthesis and secretion, on the induction of winter fur growth in mink. Melatonin (10 and 120 mg) was administered to mink (N = S/group) via silastic implants inserted SC over the scapular area during the last week of June 1985. Treat- ment of mink (N = 5) with CB-154 alone or in combination with 10 mg melatonin (N = 5) consisted of daily SC injections of 2 mg of the drug in sterile saline from June 2.5 through July 30. Control animals (N = 5) did not receive injections of vehicle or sham implants. Admin- istration of CB-1.54 alone or in combination with 10 mg melatonin. as well as 120 mg mela- tonin alone, initiated growth of the winter fur significantly earlier than that of controls or mink treated with 10 mg melatonin (P < 0.05). These data suggest that inhibition of pro- lactin secretion by melatonin is requisite for induction of molt of summer fur and growth of winter fur of mink. Q 1987 Academic Press. 1~.

Mink exhibit seasonal fur growth cycles that are regulated by changes in photo- period (Bissonnette and Wilson, 1939; Hammond, 1954; Duby and Travis, 1972). Decreasing daylength (photoperiod) after the summer solstice stimulates molting of the summer fur and growth of the winter fur, while increasing daylength following the spring (vernal) equinox induces molting of the winter fur and growth of the summer fur. Exposing mink to an artificially re- duced daily photoperiod beginning at the end of June initiates molting of the summer fur and growth of fully prime winter fur by mid-October, 6 weeks earlier than normal (Stout and Adair, 1969; Weiss et al., 1980; Rose et al., 1984). Although our under- standing of the mechanism by which sea- sonal changes in photoperiod regulate fur growth cycles is incomplete, both the pitu- itary and the pineal glands appear to be in- volved.

1 Technical Paper No. 7900, Oregon Agricultural Experiment Station.

2 To whom reprint requests should be addressed.

It is now well established that part of the mechanism by which light regulates fur growth cycles of the mink is mediated through the pineal gland and its major se- cretory product melatonin. The administra- tion of melatonin to mink in late June, maintained under natural photoperiod, stimulates molting and growth of the winter fur, resulting in fully prime winter fur 6-8 weeks earlier than normal; these results are equal to, if not superior to, those obtained by use of an artificially short photoperiod (Allain and Rougeot, 1980; Rose et af., 1984). The site and mechanism of action of melatonin is an enigma. However, we have recently demonstrated that melatonin, or artificially short photoperiod-induced growth of winter fur, coincided with the ability of these treatments to inhibit the synthesis and/or secretion of prolactin (Rose et al., 1985). These data suggest that prolactin may play a major role in regu- lating molting and growth of winter fur in mink by inhibiting the initiation of hair growth until endogenous hormone levels decline below some critical level, which

212 0016-6480187 $1 SO Copyright 0 1987 by Academx Press, Inc. All rights of reproduction in any form reserved.

WINTER FUR GROWTH IN MINK 213

occurs naturally by mid-September. If this premise is true, then inhibition of prolactin secretion by use of other agents should mimic the effects of melatonin or an artifi- cially short photoperiod on growth of winter fur.

An objective of this study was to deter- mine if administration of bromocryptine (CB-154) would induce molting of the summer pelage and growth of the winter fur in mink maintained under natural pho- toperiodic conditions. Also, because 10 mg of melatonin had been shown to inhibit prolactin secretion (Rose et al., 1985), a higher dose of this indoleamine was tested for its effect on induction of winter fur growth.

MATERIALS AND METHODS

On June 25, 1985, 25 standard dark adult female mink were assigned randomly, in equal numbers, to one of five groups and received the following treat- ments: (1) controls (no treatment), (2) melatonin (10 mg), (3) melatonin (120 mg), (4) CB-154 (2 mgiday), (5) CB-154 (2 mg/day) plus melatonin (10 mg). Mink in groups 2, 3, and 5 were anesthetized by administering intramuscularly 0.3 ml ketamine hydrochloride (Parke Davis, Morris Plains, N.J.) and one silastic capsule (1.47 id x 10 mm long or 3.35 id x 50 mm long) con- taining 10 or 120 mg of melatonin was inserted SC over the scapular area of each animal. Mink in groups 4 and 5 were injected SC with 2 mg bromocryptine (Sandoz, East Hanover, N.J.) at approximately 1200 hr daily from June 25 through July 30. The CB-154 was pre- pared by placing it in a flask containing an equal amount of D-tartaric acid (Sigma Chemical Co., St. Louis, MO.) and adding a small volume of 95% ethanol until both compounds were dissolved. This solution was then diluted to the desired volume with sterile sa- line to achieve a concentration of 1 mg CB-154/ml. It has previously been demonstrated that the ergot alka- loid bromocryptine inhibits the synthesis and secre- tion of prolactin in mink (Martinet er al., 1981, 1982). All mink were housed individually in cages (89 x 38 x 27 cm) and exposed to natural changes in weather

and photoperiod. Mink were fed once daily a 40% protein diet consisting of a mixture of beef and chicken offal, fish scraps, and cereal, and were sup- plied with water ad /i&turn. A 2-cm2 area of fur was shaved from the right hip of each animal and fur growth was measured to the nearest millimeter at weekly intervals from July 30 through October 8, as described by Rose et a/. (1984). Data were analyzed

MELATONIN L12-2

JULY AUGUST SEPTEMBER OCTOBER

FIG. 1. Fur growth of adult female standard dark mink treated with melatonin (0, 10, and 120 mg) from June 25 through October 8, and bromocryptine (CB-154) either alone or in combination with 10 mg melatonin from June 25 through July 30. The estimate of the common standard error of the mean was 2 0.78 mm.

by repeated measures analysis of variance for an ex- periment of factorial design.

RESULTS AND DISCUSSION

Mink receiving 10 mg melatonin (group 2) began to grow their winter pelage by early August, well over 1 month in advance of controls, and had fully prime fur by mid- September (Fig. 1). Mink treated with 120 mg melatonin (group 3); or those receiving daily injections of CB-154 alone (group 4) or in combination with 10 mg melatonin (group 5), all began to show signs of winter fur growth by late July, which was approxi- mately 2 weeks in advance of those treated with 10 mg melatonin and 6 weeks in ad- vance of the controls (P < 0.05). Maximum length of fur of mink in groups 3. 4, and 5 was attained approximately l-2 weeks in advance of those treated with 10 mg mela- tonin (group 2). Thus, although these agents initiated fur growth at an earlier time, they apparently did not alter the du- ration of the actively growing phase of the hair cycle. Fur growth of mink treated with CB-154 was similiar to that of mink treated with 120 mg melatonin, suggesting that this

214 ROSE, OLDFIELD, AND STORMSHAK

larger dose of melatonin was more effective than the IO-mg dose of melatonin in inhib- iting the secretion of prolactin. Also, there was no significant difference in fur growth between CB-154-treated mink (group 4) and those treated with CB-154 plus 10 mg melatonin (group 5). The fact that there was no enhancement of fur growth in mink treated with CB-154 plus 10 mg melatonin compared with that of mink treated with CB-154 alone suggests that melatonin was probably ineffective because of maximal suppression of prolactin secretion by CB-154.

These findings agree with those of Mar- tinet et al. (1984), who demonstrated that female pastel mink exposed to an artifi- cially short photoperiod after the summer solstice exhibited a rapid decrease in plasma prolactin levels followed by growth of the winter fur. Also, mink maintained on a continuous long photoperiod but treated with CB-154 displayed a rapid decrease in prolactin levels, with over one-half the an- imals molting and growing the winter fur. Unfortunately, in that study no animals were treated with CB-154 and maintained under natural photoperiodic conditions.

Duncan and Goldman (1984) have shown that daily injections of ovine prolactin in- hibit short photoperiod-induced molt and growth of winter fur of the Djungarian hamster. Moreover, inhibition of prolactin secretion by daily injections of CB-154 during long photoperiods inhibited the molt and growth of summer pelage while con- comitant administration of prolactin re- versed the effects of CB-154. Injection of CB-154 stimulated growth of winter fur in castrated hamsters exposed to artificially long photoperiods. This effect of CB-154 was also overcome by concomitant admin- istration of prolactin. Recently, Duncan et al. (1985) confirmed by measurement of serum prolactin concentrations that short photoperiod-induced growth of winter pe- lage in Djungarian hamsters was accompa- nied by reduced secretion of this peptide

hormone. In agreement with the results of the above studies, Martinet et al. (1983) have shown that exogenous prolactin ad- vanced the onset of molt and growth of summer fur of mink. By contrast, treat- ment of mink with melatonin at this time inhibited prolactin secretion and growth of summer fur (Allain et al., 1981; Martinet et al., 1981, 1983). It would appear that pro- lactin stimulates summer fur growth but in- hibits winter growth in those species exhib- iting two fur growth cycles per year.

The mechanism by which melatonin or other pineal products regulate prolactin synthesis and secretion is unknown. The dogma has been that melatonin released into the systemic circulation or cerebro- spinal fluid, in proportion to the 1ight:dark cycle, acts on the hypothalamus to alter pi- tuitary function. However, current findings by Korf et al. (1986) suggest that pinealo- cytes of the mammalian pineal gland send projections to other parts of the brain, as occurs in lower vertebrates. It is therefore possible that the pineal gland of the mink and of other photo-dependent species com- municates directly with the hypothalamo- hypophysial axis via these cellular projec- tions. If such a pathway exists it may be the melatonin released from these cellular projections rather than the systemic con- centrations of this indoleamine that regu- lates prolactin secretion and seasonal activ- ities such as reproduction and fur growth.

In summary, these data strengthen the hypothesis that photoperiodic changes acting through the pineal gland and its pro- duction of melatonin initiate winter fur growth in mink at least in part by inhibiting the synthesis and secretion of prolactin. The site and mechanism of action of pro- lactin in regulating fur growth are currently unknown.

ACKNOWLEDGMENTS

We thank John Adair, Ron Scott, and Cliff Thompson of the Experimental Fur Farm at Oregon State University for their cooperation in care of an-

WINTER FUR GROWTH IN MINK 215

imals. Special thanks are given to John Jaeger for valuable technical assistance throughout this study. The bromocryptine was generously donated by Sandoz Research Institute, East Hanover, New Jersey. This research was supported by the Mink Farmers Research Foundations, Thiensville. Wis- consin.

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