Role of prolactin and the gonads in seasonal physiological changes in the collared lemming (Dicrostonyx groenlandicus)

Evidence for phenotypic plasticity in response to photic cues and the connection with genes of risk in schizophrenia

Numerous environmental factors have been identified as influential in the development of schizophrenia. Some are byproducts of modern life, yet others were present in our evolutionary past and persist to a lesser degree in the current era. The present study brings together published epidemiological data for schizophrenia and data on variables related to photic input for places of residence across geographical regions, using rainfall as an inverse, proxy measure for light levels. Data were gathered from the literature for two countries, the former Yugoslavia and Ireland, during a time in the early 20th century when mobility was relatively limited. The data for Yugoslavia showed a strong correlation between hospital census rates for schizophrenia (by place of birth) and annual rain (r = 0.96, p = 0.008). In Ireland, the hospital census rates and first admissions for schizophrenia (by place of permanent residence) showed a trend for correlation with annual rain, reaching significance for 1st admissions when the rainfall data was weighted by the underlying population distribution (r = 0.71, p = 0.047). In addition, across the years 1921-1945, birth-year variations in a spring quarter season-of-birth effect for schizophrenia in Ireland showed a trend for correlation with January-March rainfall (r = 0.80, p ≤ 0.10). The data are discussed in terms of the effect of photoperiod on the gestation and behavior of offspring in animals, and the premise is put forth that vestigial phenotypic plasticity for such photic cues still exists in humans. Moreover, genetic polymorphisms of risk identified for psychotic disorders include genes modulated by photoperiod and sunlight intensity. Such a relationship between phenotypic plasticity in response to a particular environmental regime and subsequent natural selection for fixed changes in the environmentally responsive genes, has been well studied in animals and should not be discounted when considering human disease.

Photoperiod differentially affects immune function and reproduction in collared lemmings (Dicrostonyx groenlandicus)

Many nontropical rodent species experience predictable annual variation in resource availability and environmental conditions. Individuals of many animal species engage in energetically expensive processes such as breeding during the spring and summer but bias investment toward processes that promote survival such as immune function during the winter. Generally, the suite of responses associated with the changing seasons can be induced by manipulating day length (photoperiod). Collared lemmings (Dicrostonyx groenlandicus) are arvicoline rodents that inhabit parts of northern Canada and Greenland. Despite the extreme conditions of winter in their native habitat, these lemmings routinely breed during the winter. In the laboratory, collared lemmings have divergent responses to photoperiod relative to other seasonally breeding rodents; short day lengths can stimulate, rather than inhibit, the reproductive system. Male and female collared lemmings were maintained for 11 weeks in 1 of 3 photoperiods (LD 22:2, LD 16:8, or LD 8:16) that induce markedly different phenotypes. Following photoperiod treatment, cell-mediated immune function as assessed by delayed-type hypersensitivity reactions was elevated in lemmings housed in LD 16:8 and LD 8:16 relative to LD 22:2. However, antibody production to a novel antigen was unaffected by photoperiod. Exposure to LD 8:16 induced weight gain, molt to a winter pelage, and in contrast to previous studies, regression of the male, but not the female, reproductive tract. In conclusion, these data indicate that components of immune function among collared lemmings are responsive to changes in day length.

Seasonal pelage color change: news based on a South American Rodent

Mammalian seasonalmolting and color change are known to be influencedby photoperiod changes. Calomys laucha, a South American rodent, exhibits seasonal pelage color change; however, unlike Northern hemisphere rodents, which present a gray or brown color during summer and a whitish color during winter, C. laucha pelage changes from an orange color during summer to a dark gray color during winter. Animals maintained for over a year in stationary photoperiod (LD 12:12h, 22ºC) presented orange pelage color during the summer corresponding month (January), and gray color during the winter corresponding month (July). Same age animals were evaluated during summer or winter months, and also showed different colors. Animals exposed for 12 weeks to summer or winter artificial conditions displayed color change, not according to the environmental conditions, as expected, but similar to that of animals maintained in stationary photoperiod. These results suggest that pelage color change in C. laucha is controlled by an endogenous circannual rhythm. The adaptive function of C. laucha color change is discussed.

CNS sympathetic outflow neurons to white fat that express MEL receptors may mediate seasonal adiposity

Many animals show seasonal changes in adiposity that are triggered by changes in the photoperiod. For example, in short "winterlike" days, the nocturnal duration of pineal melatonin (MEL) secretion increases ultimately resulting in body fat decreases by Siberian hamsters. These decreases in body fat are mediated through increases in the sympathetic drive on white adipose tissue (WAT). The central nervous system (CNS) origins of the sympathetic outflow from brain to WAT include the suprachiasmatic nucleus (SCN), an area necessary for the reception of season-encoded MEL signals in Siberian hamsters. Therefore, we tested whether SCN neurons that are part of the sympathetic outflow to WAT also express MEL receptors (MEL(1a)). This was accomplished by labeling the sympathetic outflow from brain to WAT using a transsynaptic retrograde tract tracer, the pseudorabies virus (PRV), injected into inguinal WAT combined with labeling of brain MEL(1a) receptors using in situ hybridization. We found PRV-labeled neurons that also expressed MEL(1a)-receptor mRNA in several brain regions including the SCN. Thus the increased duration of MEL secretion in short days may increase MEL(1a)-receptor stimulation that, in turn, increases the sympathetic drive on WAT, thereby increasing lipolysis and decreasing adiposity.

Role of prolactin in regulating the onset of winter fur growth in mink (Mustela vison): A reconsideration

The objectives of this study were to determine: (1) if the onset of winter hair growth (anagen) in mink could be delayed or inhibited by elevating endogenous PRL concentrations; (2) if bilaterally adrenalectomy (ADX)-induced winter anagen occurs concomitantly with a reduction in serum PRL concentrations, and (3) if exogenous dehydroepiandrosterone (DHEA), an adrenal steroid or Delta(5)-DIOL (a peripherally produced metabolite of DHEA), would delay or inhibit the onset of winter anagen. During early July, while in the resting (telogen) stage of the hair growth cycle, mink were treated with slow release implants containing haloperidol (HAL, a dopaminergic antagonist), melatonin (MEL), deoxycorticosterone (DOC), DHEA and Delta(5)-DIOL. In addition, mink were ADX'd and supplemented with DOC and DHEA. MEL reduced PRL levels to basal levels and induced winter anagen 7 weeks earlier than controls. Surprisingly, HAL initiated winter anagen 7 weeks earlier than controls (P < 0.05), although serum PRL levels were not different between the two groups. Mink that were ADX'd or ADX + DHEA-treated exhibited winter anagen 6 weeks earlier than controls (P < 0.05), but serum PRL concentrations were not different between the three groups. The administration of DHEA or Delta(5)-DIOL to mink with intact adrenals had no effect on the time of onset of winter anagen or serum PRL levels. Our findings suggest that a reduction in circulating PRL levels is not essential for onset of winter anagen in the mink and that the apparent inhibitory effects of the adrenal glands on initiation of winter anagen is not mediated through DHEA or its metabolite Delta(5)-DIOL. J. Exp. Zool. 284:437-444, 1999.

Effect of prolactin on the attractiveness of male odors to females in meadow voles: independent and additive effects with testosterone

This study investigated the roles of testosterone and prolactin in seasonal changes in the attractiveness of odors of male meadow voles to long-photoperiod females. Prolactin was reduced in long-photoperiod males by treatment with bromocryptine (Experiment 1) and increased in short-photoperiod males by replacement with ovine prolactin (Experiment 2). Testosterone was manipulated by castration and testosterone replacement. During long photoperiods, the attractiveness of the odor produced by male meadow voles depends on the circulating titers of both prolactin and testosterone. Males with high titers of both hormones produced scents that were highly attractive to females; those with low titers of both hormones produced unattractive scents. When the concentration of one hormone was high and the other low, long-photoperiod males produced scents of intermediate attractiveness. During short photoperiods, the attractiveness of the male's scents depended strictly on his prolactin titers. Males with high prolactin titers produced attractive scents, and males with low prolactin titers produced unattractive scents, regardless of their prolactin concentrations. Thus, high titers of both testosterone and prolactin are needed to produce attractive scents in males exposed to long photoperiods, whereas high prolactin titers are sufficient in short-photoperiod males.

Bilateral adrenalectomy induces early onset of summer fur growth in mink (Mustela vison)

The purpose of this study was to determine the effects of bilateral adrenalectomy on the time at which growth of the summer pelage begins in mink. Bilateral adrenalectomy of adult female mink between 2 and 11 March 1991, supplemented with deoxycorticosterone (DOC) as a mineralocorticoid, resulted in the onset of the summer fur growth approximately 5 weeks earlier than the controls (P < 0.01). Mink with their adrenal glands intact and treated with DOC, exhibited summer fur growth 2 weeks earlier than the controls (P < 0.01). The duration of time between the first observed hair growth and attainment of maximal guard hair length, was approximately 10 days longer in mink with their adrenal glands intact and supplemented with DOC than the controls (P < 0.05). Adrenalectomized mink treated with DOC exhibited guard hair growth for approximately 13 days longer than controls (P < 0.05). Although exogenous DOC initiated hair growth, the rate of growth appeared to be reduced. These data suggest that adrenal hormones, of undetermined identity, exert a tonic inhibitory effect on the initiation of summer fur growth in mink and may be part of the mechanism through which changes in photoperiod regulate the onset of hair growth cycles.

Prolactin receptor concentrations in the skin of mink during the winter fur growth cycle

The objectives of this study were to determine: 1) if the skin of mink might be a target organ for prolactin (PRL) by establishing if PRL binding sites (receptors) exist in the cell membranes of skin, and 2) if PRL receptor concentrations change during the onset and progression of the winter fur growth cycle. Skin was collected on October 6, 1992 for characterization of PRL receptors and from July through December 1992 (N = 3 mink/month) to evaluate possible changes in PRL receptor concentrations during the fur growth cycle. PRL receptors were quantified using 125I-oPRL in a validated radioreceptor assay. Scatchard analysis of saturation data revealed a single class of high-affinity (Kd = 5.21 x 10(-11) +/- .84 M), low capacity (Bmax = 27.03 +/- 3.37 fmoles/mg) binding sites. Only oPRL (40% displacement) and to a lesser extent oGH (3% displacement) inhibited the binding of 125I-oPRL to mink skin cell membranes. No inhibition of 125I-oPRL binding to membranes occurred in the presence of a 500-fold excess of bTSH or oLH, indicating that the receptors were hormone specific. Concentrations of 125I-oPRL receptors during the onset and development of winter fur growth (July through November) exhibited no significant change. However, following completion of the winter fur growth cycle (December 1) PRL receptor concentration was significantly higher than all preceding months. The greater binding observed at that time may reflect a change in tissue sensitivity, in preparation for growth of the summer pelage.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of collared lemmings to melatonin: II. Infusions and photoperiod

Collared lemmings (Dicrostonyx groenlandicus) show much phenotypic plasticity when exposed to photoperiods of varying length. In addition to "long day" and "short day" morphologies, apparent when animals are exposed to 22L:2D and 8L:16D, respectively, animals with intermediate features appear when collared lemmings are placed on day lengths ranging from 18L:6D to 14L:10D. Body mass, body composition, pelage color and length, the presence and size of a bifid claw, and reproductive condition are all influenced by ambient day length. We tested the hypothesis that variations in the melatonin signal mediate the physiological and morphological responses observed in collared lemmings housed in various photoperiods. One group of collared lemmings was fitted with a subcutaneous cannula at weaning and infused for 8 weeks with 2, 6, or 14 hr melatonin or 14 hr vehicle daily while housed in constant light (LL). Additional groups were transferred to LL, 20L:4D, 16L:8D, or 8L:16D (all uninfused). The response of the animals to both melatonin infusion and photoperiod was graded; the degree to which the "winter" morphology developed was proportional to the length of both the melatonin infusion and the scotophase. Both the 14 hr infusion and the 8L:16D photoperiod promoted development of the characteristic "winter" traits: large body size, white pelage, bifid claw, small uterus, and low serum prolactin (PRL). Conversely, treatment with the 2 hr infusion or exposure to the long photoperiods (LL, 20L:4D) resulted in a "summer-like" morphology. Infusion of 6 hr melatonin per day or exposure to 16L:8D produced animals with an intermediate physiology. Overall, the results support the hypothesis that variations of the endogenous melatonin rhythm mediate the effect of photoperiod length on seasonal physiological and morphological changes in collared lemmings.

Response of collared lemmings to melatonin: I. Implants and photoperiod

We examined the effect of constant-release melatonin capsules on the physiology and morphology of female collared lemmings exposed to either chronic long (22L:2D) or short (8L:16D) photoperiod, or to a change in photoperiod. When animals were maintained on unchanging long or short photoperiod, subcutaneous melatonin implants were without effect. However, when animals were reared on either 22L:2D or 8L:16D and transferred to the alternate photoperiod at weaning, melatonin (implanted at weaning) prevented most photoperiod-related responses. At sacrifice (after 8 weeks of treatment), melatonin-implanted animals exposed to a change in photoperiod did not differ from animals remaining in the original photoperiod with respect to pelage color, bifid claw size, uterine mass, or serum prolactin (PRL). In contrast, regardless of treatment, animals exposed to a photoperiod transfer developed a body mass that partially or fully reflected that characteristic of the secondary photoperiod; i.e., both control- and melatonin-implanted animals transferred from long to short photoperiod developed a large body mass. These results indicate that masking the endogenous melatonin rhythm via constant-release melatonin implants renders collared lemmings unable to respond to a change in photoperiod with respect to most physiological parameters. However, the striking seasonal change in body mass experienced by collared lemmings appears to be at least partially independent of a melatonin signal.

Development of collared lemmings, Dicrostonyx groenlandicus, is influenced by pre- and postweaning photoperiods

We examined the role of pre- and postweaning photoperiod on postweaning development of collared lemmings. Lemmings were gestated and reared to weaning (19 days of age) in one of three photoperiods: 22L:2D (22 hr of light:2 hr of dark), 16L:8D, and 8L:16D. At weaning, lemmings were either maintained in their natal photoperiod or transferred to one of the other two photoperiods. At the termination of the experiment (10 weeks postweaning) data were collected on somatic characters (body weight, bifid claw width, pelage stage, and guard hair length), serum prolactin (PRL), and reproductive parameters (testes, seminal vesicle, and uterine weights). Somatic characters were predominantly influenced by postweaning photoperiod, when that photoperiod was either long (22L:2D) or short (8L:16D). When lemmings were exposed to an intermediate postweaning photoperiod (16L:8D), development of somatic characters was significantly influenced by the preweaning photoperiod; animals reared in 8L:16D regarded 16L:8D as a long day, whereas those reared in 22L:2D regarded 16L:8D as a short day. Serum PRL responded to photoperiod changes, often reflecting either the increase or decrease in day length, rather than simply the absolute number of light hours per day. Whereas reproductive indices in both sexes were stimulated by transfer from short to long photoperiod, chronic exposure to long photoperiod inhibited male development. No other photoperiod manipulations significantly influenced reproductive parameters. These observations suggest that, in the collared lemming, the neural and/or humoral factors regulating somatic and reproductive characters differ in their response to photoperiod. These results also suggest that the postweaning responses to photoperiod are programmed by earlier (gestational and/or neonatal) photoperiod exposure of the mother and/or the neonates.

Threshold photoperiods for the induction of short day traits in collared lemmings (Dicrostonyx groenlandicus)

When exposed to short photoperiod collared lemmings undergo a number of physiological and morphological changes including an increase in body weight, a change in body composition, development and enlargement of the bifid "digging" claw, and a molt to a white winter pelage. We investigated the threshold photoperiods for the induction of these traits in male and female lemmings born and raised under a 22L:2D photoperiod and transferred at weaning (19 days of age) to various other photoperiods. Male lemmings showed the characteristic increase in body weight when exposed to 16 hr of light or less per day whereas females required a photoperiod of 14 hr of light or less per day to elicit an increase in body weight. The threshold photoperiods for the increase in bifid claw size were 16 and 18 hr of light per day in male and female lemmings, respectively. The molt to the white winter pelage began under a longer photoperiod in females (16L:8D) than in males (14L:10D). Testes and seminal vesicles were significantly inhibited by photoperiods of 22L:2D and 20L:4D. Likewise, uterine weights were inhibited by maintenance on a photoperiod of 22L:2D. Taken together, these data show that the threshold photoperiods for the induction of the short day traits in collared lemmings are both trait- and sex-dependent. These observations also suggest that under some conditions, exposure to long photoperiods can inhibit sexual maturation.