THE JOURNAL OF EXPERIMENTAL ZOOLOGY 262:469-473 (1992)

RAPID COMMUNICATION

The Role of the Adrenal Glands in Regulating Onset of Winter Fur Growth in Mink (Mustela visoi)

JACK ROSE AND MAURITZ STERNER Department of Biological Sciences, Idaho State University, Pocatello, Idaho 83209 (J.R .), and Division of Parasitology, University of Nebraska State Museum, Lincoln, Nebraska 68508 (M.S.)

ABSTRACT The role of the adrenal glands in regulating onset of winter fur growth in mink was investigated in long-term adrenalectomized animals. Bilateral adrenalectomy of adult female stan- dard dark mink between June 23 and July 11, 1990, initiated onset of winter fur growth approxi- mately 6 weeks earlier than controls. One month following completion of the winter fur growth in adrenalectomized mink, molting and growth of a new coat was observed. The type of pelage that grew as a result of the second growth wave was less dense than the normal summer or winter fur. However, this renewed hair growth suggests that adrenal hormones not only inhibit the onset of winter fur growth but also influence the duration of inactivity following each period of hair growth. Administration of deoxycorticosterone as a mineralocorticoid supplement had no effect on initiation of fur growth. It would appear that adrenal hormones are part of the mechanism through which photoperiod regulates fur growth in the mink. The identity of the adrenal hormones and their site of action is unknown. o 1992 Wiley-Liss, Inc.

Mink exhibit a cyclic fur growth pattern that is regulated by photoperiod (Bissonnette and Wilson, '39; Hammond, '54; Duby and Travis, '72). Increas- ing daylength in the spring activates neuroendo- crine mechanisms that culminate in renewed hair growth giving rise to the summer pelage and molt- ing of the winter fur. In contrast, decreasing day- length in the fall results in growth of winter fur and molting of the summer pelage.

Part of the mechanism by which light mediates its effects on fur growth of the mink is through the pineal gland and its major secretory product, mel- atonin. Administration of this hormone to mink maintained under natural photoperiodic conditions during late June stimulates molting and growth of the winter fur 6-8 weeks earlier than normal (Allain and Rougeot, '80; Rose et al., '84, '87). It would appear that one of the effects of decreasing daylength is to stimulate increased secretion of mel- atonin. The site and mechanism of action through which this hormone regulates fur growth is incom- pletely understood. However, exogenous melatonin or artificially short photoperiod-induced growth of winter fur coincides with the ability of these treat- ments to inhibit the synthesis and/or secretion of the pituitary hormone prolactin (Allain et al., '81; Rose et al., '85). Furthermore, selectively inhibit- ing the secretion of prolactin with bromocryptine (CB-154) initiates growth of the winter fur at an

0 1992 WILEY-LISS, INC.

earlier time than is possible with reduced photo- period (6L:18D) or with 10 mg of melatonin (Mar- tinet et al., '84; Rose et al., '87). Martinet et al. ('84) demonstrated that mink maintained on a long pho- toperiod after the summer solstice exhibited a delayed reduction in plasma prolactin concentra- tions, molted, and regrew fur characteristic of the summer type. In contrast, approximately one-half of their animals that were maintained on a long photoperiod and treated with bromocryptine exhib- ited a significantly early reduction in plasma pro- lactin levels, molted, and grew the winter type pelage at an earlier time. Duncan and Goldman ('84a) demonstrated that when Djungarian ham- sters were given daily injections of ovine prolactin (oPRL) while in the winter pelage, the type of hair that grew back after plucking was pigmented and characteristic of the summer fur. Alternatively, when animals were given injections of prolactin while maintained on a short photoperiod, molting and growth of the winter fur was inhibited (Dun- can and Goldman, '84b). It is paradoxical that mink maintained on a short photoperiod and treated with prolactin still molt and grow the winter fur (Mar- tinet et al., '84; Rougeot et al., '84). The reason for this is unknown, but may be related to the use of heterologous hormone (bovine prolactin) or dosage.

Received September 17,1991; revision accepted January 30,1992.

470 J. ROSE AND M. STERNER

It has been demonstrated that mink and weasels with pituitary autographs under the kidney cap- sule grow hair of the summer type, even when exposed to environmental stimuli that induce growth of winter fur in intact animals (Rust, '65; Rust and Meyer, '68; '69). Collectively, these find- ings strongly suggest that a reduction in serum lev- els of prolactin is a prerequisite to initiation of winter fur growth and that high prolactin concen- trations are associated with summer fur growth.

The target tissue and mechanism by which pro- lactin influences fur growth is unknown. It is pos- sible that the hormone regulates the production of other as yet unknown substances that are the prox- imal regulators of hair growth. Steroid hormones, for example, of adrenal and ovarian origin have, in general, an inhibitory effect on hair growth in most species investigated thus far (Johnson, '58a,b,c; Mohn, '58; Ebling and Hale, '83). It would, there- fore, not be unreasonable to suspect that the inhib- itory actions of prolactin are mediated through the adrenal glands. Receptors for prolactin have been found in adrenal tissue of a variety of mammals (Marshall et al., '79, '76, '75; Posner et al., '74; McDonough and Ewig, '82). Although controversy exists, some investigators have demonstrated a relationship between serum prolactin levels and production of the adrenal androgen dehydroepi- androsterone sulfate (DHAS), suggesting that pro- lactin might stimulate the synthesis of the steroid (Bassi et al., '77; Vermeulen and Ando, '78; Ver- meulen et al., '77; Lob0 et al., '80; Higuchi et al., '84; Schiebinger et al., '86). It is unknown if this hormone has a physiological affect on hair growth.

Rust et al. ('65) bilaterally adrenalectomized mink in stages between April 6 and 20 and observed earlier growth of the summer pelage when com- pared to intact mink. Unfortunately, life expectancy of the adrenalectomized mink was only 17 to 34 days so observations of the winter fur growth could not be made. Therefore, the objective of this study was to determine if the adrenal glands influence the time at which winter fur growth begins in the mink.

MATERIALS AND METHODS Beginning the first week of June 1990,18 adult

standard dark female mink were moved to our indoor animal facility on the Idaho State Univer- sity campus. The animals were subjected to a daily light-dark cycle that approximated natural photo- periodic changes. This was accomplished through the use of an Electronic Astronomic Time Switch, Model ET816CR (Intermatic Corp., Spring Grove, IL). To duplicate as closely as possible the natural

wavelengths of sunlight, animals were illuminated with General Electric Full Spectrum Chroma-50, Model F40C50 lamps. Mink were fed daily a mix- ture of chicken and fish by-products and received water ad libitum.

On June 23,1990, the animals were assigned ran- domly to an experiment of 3 x 13 factorial design. Mink in group 1 received no treatment and repre- sented controls. Mink in groups 2 and 3 were anes- thetized with ketamine hydrochloride (30 mg/kg; Aveco Co., Fort Dodge, IA) and received a silastic implant containing deoxycorticosterone (DOC; Sigma Chemical Co., St. Louis, MO), inserted subcutaneously over the scapular region. Deoxycor- ticosterone is an adrenal steroid with mineralocor- ticoid actions and administration of the steroid to adrenalectomized mink was essential in order to maintain proper water and electrolyte balance, without which the animals would have died. Implants were constructed according to the procedures of Moore ('81), measured 50.00 mm long x 3.35 mm ID x 4.64 mm OD, and contained a column of hor- mone that weighed between 200 and 300 mg. Forty- eight hours following the administration of the implant, the animals in group 2 were again anes- thetized with ketamine hydrochloride (50 mg/kg) and acepromazine (10 mgkg; Fort Dodge Lab., Inc., Fort Dodge, IA), and bilaterally adrenalectomized through two lateral incisions. Mink in group 3 were anesthetized with ketamine hydrochloride only (30 mg/kg). At the time of surgery, animals in groups 2 and 3 each received a second DOC implant inserted into the peritoneal cavity. At the comple- tion of the treatments each animal had a 5-cm square area of fur shaved from the right hip. Begin- ning July 23 guard hair measurements were taken to the nearest millimeter at approximately weekly intervals until November 8, 1990. Data collected from animals that survived until the completion of the study were evaluated by one-way analysis of variance for an experiment of factorial design (Zar, '84).

RESULTS AND DISCUSSION Mink, with intact adrenals, exposed to a natu-

ral photoperiodic light schedule and treated with or without DOC began to exhibit winter hair growth in early September, with completion of the winter fur growth cycle occurring in early November (Fig. 1). There was no significant difference between either of the two groups with respect to the time of initiation of winter fur growth or completion of the fur growth cycle. These data confirm previous find- ings suggesting that exogenous deoxycorticoster-

WINTER FUR GROWTH IN MINK 47 1

July Aug. Sept. Oct. Nov.

Fig. 1. Fur growth of adult female standard dark mink subjected to bilateral adrenalectomy and treated with deoxy- corticosterone (ADRENEXIDOC, N = 5), treated with deoxy- corticosterone (DOC, N = 4) only, and Controls (N = 6) from July 23 to November 8,1990. The common estimate of the stan- dard error ofthe mean was k 1.10 mm.

oids have no influence on hair growth (Mohn, '58; Montagna and Parakkal, '74). Mohn ('58) injected rats daily and weekly with 1 mg of DOC acetate in oil for 1 month and observed no change in hair growth induced by plucking or spontaneous re- placement.

Mink that were bilaterally adrenalectomized and treated with DOC began to exhibit hair growth within 7-10 days after surgery and were judged to have fully prime winter fur by early September, approximately 6 weeks earlier than controls or unoperated mink treated with DOC (P < 0.0001, Fig. 1). These findings are consistent with those for other species such as the rat in which adrenal- ectomy advanced the spontaneous eruption of new hairs in females and accelerated the passage of the growth wave over the body in both sexes (John- son, '58~).

Several weeks after completion of the winter fur growth (October 5) in the adrenalectomized mink, we began to notice that hair was becoming loose and could be easily pulled out. Suspecting that this might indicate molting as a consequence of renewed hair growth, we sheared an area of the opposite hip and observed fur growth through November 21. All animals exhibited rapid hair growth in the newly sheared area. However, the type of fur that grew was less dense, especially with regard to the under- fur and was not representative of a mature sum- mer or winter coat. Under normal conditions in our laboratory the hair follicle is inactive following completion of the winter fur growth cycle from mid- November through early May, a duration of ap-

proximately 5.5 months. In adrenalectomized mink this inactive period was reduced to approximately 1 month. This observation is consistent with that for the rat, where following adrenalectomy, not only is the spontaneous replacement of hair initiated immediately, but the quiescent period which pre- cedes each hair growth cycle is significantly short- ened (Baker, '51; Dieke, '48).

Most evidence would suggest that cortisone is the adrenal hormone responsible for inhibition of hair growth. Administration of cortisone and similar cor- ticosteroids to rats and mice inhibits hair growth (Mohn, '58; Davis, '63; Ebling and Hale, '83). Cor- tisone also inhibits wool growth in sheep and reduces the diameter of the resulting fibers (Lind- ner and Ferguson, '56; Ferguson et al., '65). Mink that were administered daily subcutaneous injec- tions of 1 or 1.5 mg of hydrocortisone while in the winter pelage failed to molt and grow the summer pelage (even after plucking) until injections were discontinued (Rust et al., '65). The inhibitory actions of these hormones may be mediated directly on the hair follicle since topical application of such hormones inhibits hair growth only in the area of application (Baker et al., '48; Whitaker and Baker, '48, '5 1).

Whiteley ('58) demonstrated that the adminis- tration of cortisone to rabbits had a more pro- nounced inhibitory effect on the underfur fibers than on guard hairs. It was postulated that cortisone inhibited regrowth by interfering with revascular- ization of the follicles and because the vasculature of the underfur follicles is much less extensive than that of the guard hairs (only the guard hair folli- cles have a vascularized papilla), they were most affected. This observation is of interest because the major difference between summer and winter fur of mink is that the winter type fur contains a greater number of underhair fibers. Growth of win- ter type fur in mink may result from the gradual reduction in adrenal hormone secretion or action allowing for greater vascularization of the skin, increased nutrient flow, and a greater number of hairs developing, especially the underfur type hairs. Unfortunately, our knowledge of seasonal produc- tion of specific adrenal hormones in mink is incom- plete. Weiss et al. ('80) documented three peaks in total blood glucocorticoid concentrations of female mink throughout the year (January, May, and Sep- tember). In addition, after subjecting mink to an artificially short daylength in late June, the time at which the glucocorticoid peak occurred in Sep- tember was not altered, but resulted in higher hormone concentrations. It would appear that glu-

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cocorticoid production is not correlated with sea- sonal changes in photoperiod in this animal.

Although the physiological role of the adrenal glands in regulating hair growth is uncertain,

suggest that adre- nal hormones inhibit reactivation of the hair folli- CleS and therefore Onset of winter fur growth in mink. It is possible that the adrenal hormones

J. ROSE AND M. STERNER

Bissonnette, T.H., and E. Wilson (1939) Shortening daylength periods between May 15 and September 12 and the pelt cycle Of the mink. Science, 89:418-419.

Davis, B.K. (1963) Quantitative morphological studies upon the influence of the endocrine system on the growth of hair by white mice. Acta Endocrinol. (Copenh.), Suppl. 85, &:1-102.

Dieke, S.H. (1948) The effect of removing various endocrine glands on the hair cycles of black rats. Endocrinology, 42:315-319.

Of Our research

directly inhibit hair growth or are permissive and regulate the production of other factors that act directly on the hair follicle. Nevertheless, it would appear that adrenal hormones are part of the phys- iological mechanism by which hair growth is reg- ulated in the mink. We speculate that the inhibitory actions of prolactin on hair growth in this animal are mediated through adrenal hormones or their metabolites. Studies are currently in progress to determine if prolactin binding sites exist in mink adrenal glands and to identify the inhibitory adre- nal hormones and their site of action.

ACKNOWLEDGMENTS This research was supported by the Mink Farm-

ers Research Foundation of America, Thiensville, Wisconsin. The investigators are most grateful to Mr. Lee Moyle of Moyle Fur Farms, Heyburn, Idaho, for donating the animals and feed used in this study and for advice on mink husbandry in an indoor facil- ity. We appreciate the assistance of Ms. Lani Pal- auni and Mr. Robert Glapinski in preliminary experiments. We would like to thank Mr. Jim Peck, Superintendent of the Animal Facility at Idaho State University, for his cooperation and Mr. Bob Horn and Mr. Rick Mortensen for the daily feed- ing and care of the mink. Special thanks are extended to Mr. Tracy Bowlin for statistical advice.

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