Biomedical subjects

B A Gower

Publications and source records attributed to B A Gower.

48 records · Page 3Linked to original sources

PubMed1997

Alteration of testicular response to long photoperiod by transient exposure to short photoperiod in collared lemmings (Dicrostonyx groenlandicus).

The reproductive response of collared lemmings (Dicrostonyx groenlandicus) to photoperiod is unique for rodents. Whereas most reproductively photoresponsive rodents show maximal gonadal growth when exposed to long photoperiod (long day), collared lemmings show delayed maturation when born and maintained under this condition. However, transfer of lemmings from short photoperiod (short day) to long day results in maximal gonadal growth, indicating that the response to long day depends upon photoperiod history. We hypothesized that the slowing of maturation observed in animals born and maintained on long day reflects an inability to respond fully to long day, resulting from the absence of previous exposure to short day. To determine whether young lemmings born in long day are capable of being stimulated by long day, we exposed them at weaning (19 days of age) to 1, 6 or 10 weeks of short day, and then challenged them with a second exposure to long day. Relative to animals transferred permanently to short day at weaning, lemmings exposed to 6 weeks of short day showed accelerated gonadal growth after both 5 and 10 weeks of subsequent exposure to long day, and those exposed to 10 weeks of short day had larger testes after 6 weeks of long day. Thus, during transient exposure to short day, the animals acquired sensitivity to the stimulatory effects of long day. The responses of body mass, bifid claw width and pelage colour to the photoperiod manipulations did not parallel that of the gonads, indicating independent regulation of somatic and reproductive parameters. The unique way in which the reproductive system of collared lemmings responds to photoperiod may reflect evolution in an environment where the production of offspring during periods of unchanging long day (for example, the Arctic summer) is not selectively advantageous.

Animals↗

PubMed1996

Influence of photoperiod, time, and sex on hormone concentrations in collared lemmings (Dicrostonyx groenlandicus).

Collared lemmings (Dicrostonyx groenlandicus) of both sexes show seasonal changes in body mass and body composition. Previous studies using single-point sampling indicated that, in young males, these photoperiod-mediated changes are associated with changes in circulating growth hormone (GH), corticosterone (B), and thyroid hormones. The present study was conducted to (1) examine daily fluctuations in serum levels of GH, B, and thyroxine (T4) in animals exposed to long (22L:2D, "LD"), intermediate (16L:8D, "ID"), and short (8L:16D, "SD") photoperiods, (2) confirm that conclusions based on single-point sampling are valid when photoperiod-related differences in hormone concentration are examined over 24 hr, (3) examine the effect of photoperiod on hormone concentrations in adults of both sexes, and (4) characterize the daily pineal melatonin rhythm in this species. Adult male and female collared lemmings housed in SD had higher levels of GH, and lower levels of B and T4, even when the diurnal variations in serum concentrations of these hormones were taken into account. A significant effect of time was observed on serum B (ID animals only) and serum T4. ID lemmings had B levels that were similar to those of SD animals, but concentrations of GH that more closely resembled those of LD animals. Females had lower GH and T4 than males. Pineal melatonin concentration closely tracked the dark phase of the day in each of the three photoperiods. Photoperiod-mediated changes in melatonin synthesis may mediate observed day length-related differences in serum concentrations of metabolic hormones, which in turn may contribute to the seasonal changes in body composition observed in collared lemmings.

Animals↗

PubMed1994

Effect of photoperiod, testosterone, and estradiol on body mass, bifid claw size, and pelage color in collared lemmings (Dicrostonyx groenlandicus).

Collared lemmings undergo several photoperiod-mediated seasonal physiological changes. When exposed to short photoperiod, lemmings increase in size, develop a bifid claw, and molt to a white pelage. Previous data indicate that body mass, claw size, and pelage color are influenced by hormones of testicular origin, suggesting that, on a seasonal basis, changes in production of, or sensitivity to, testicular hormones may play a role in the development of the phenotype characteristic of the ambient photoperiod. The present study was designed to determine if the active testicular hormone(s) is testosterone (T) and/or estradiol (E2) and if seasonally changing physiological traits in female lemmings are also influenced by gonadal status. Fifty-day-old lemmings, reared in 22L:2D (long day), 16L:8D (intermediate day), or 8L:16D (short day), were either gonadectomized or sham operated and given either empty Silastic implants or implants containing T (4 or 10 mm; castrated males), E2 (4 mm undiluted or diluted 1:4 with cholesterol; ovariectomized females), the aromatase inhibitor ATD (androsta-1,4,6-triene-3,17-dione; 2 x 20 mm; intact animals of both sexes), or ATD plus a 10-mm T implant (castrated males). After a 6-week treatment period, changes in body mass, bifid claw width, pelage color stage, and serum prolactin (PRL) were assessed. The effects of gonadectomy and steroid treatment depended upon photoperiod. Whereas gonadectomy increased mass gained by both sexes under intermediate and short day, under long day only females showed the positive mass response to gonadectomy. Treatment with T and E2 reversed the effect of gonadectomy on body mass under intermediate day and decreased the amount of mass gained under short day. Treatment with ATD (males) and E2 (females) indicated that E2 was the hormone responsible for the tonic, inhibitory effect of the gonads on body mass in both sexes. Claw size was most sensitive to steroid manipulation in animals housed in long day, in which all treatments had a negative influence. Gonadectomy under short day resulted in the development of a whiter pelage in both sexes. The effect of gonadectomy on pelage in female lemmings, and its reversal by E2 treatment, may have been partially due to alteration of serum PRL.

Animals↗

PubMed1994

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.

Animals↗

PubMed1994

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.

Animals↗

PubMed1994

Pre- and postnatal effects of photoperiod on collared lemmings (Dicrostonyx groenlandicus).

We investigated the hypothesis that photoperiod information received during the prenatal and/or early postnatal periods influences subsequent development in collared lemmings. Pregnant lemmings were exposed to either a short [8:16-h light-dark cycle (8:16)] or long (22:2) photoperiod throughout gestation. On the day of birth, pups were cross-fostered to dams housed in either short or long photoperiod. After an 11-day experimental lactational period (LACT), all animals were transferred to an intermediate photoperiod (16:8), the response to which depends on prior photoperiod exposure. Pups remained on this photoperiod until death at postnatal day 90. Information received during gestation (GEST) influenced offspring growth, pelage color, guard hair length, presence of the bifid claw at weaning, testes mass at 10 wk postweaning, and serum prolactin (PRL). Parameters measured at weaning reflected directly the photoperiod experienced during GEST (e.g., short-day GEST pups had white pelage). Conversely, parameters measured at 10 wk postweaning reflected the change in photoperiod experienced between GEST and 16:8 (e.g., short-day GEST young had gray pelage, indicative of an increase in photoperiod). Information received during LACT influenced growth and the presence of the bifid claw at weaning.

Aging↗

PubMed1993

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.

Animals↗

PubMed1993

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.

Animals↗

PubMed1993

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

On a seasonal basis, collared lemmings undergo a number of physiological and morphological changes. Short photoperiod exposure results in a molt to a white pelage, an increase in body weight, a reduction in relative body fat content, an increase in relative water content, and the development of a bifid claw. Treatment with the dopamine agonist, CB-154, resulted in a reduction in serum prolactin and the development of the white pelage in lemmings housed under 16L:8D, while treatment with the dopamine antagonist, sulpiride, prevented the winter molt in animals transferred to 8L:16D. Castration under 16L:8D resulted in an increase in body weight and an enlargement of the bifid claw. Castrated animals also molted more readily when treated with CB-154 and developed a relatively greater carcass water content. Treatment with CB-154 increased relative carcass fat content. These findings suggest that, in the collared lemming, seasonal changes in pelage parameters are regulated by prolactin, with gonadal hormones playing a modulating role. Body weight, water content, and bifid claw size appear to be influenced by gonadal hormones.

Animals↗

PubMed1992

Role of photoperiod in reproductive maturation and peripubertal hormone concentrations in male collared lemmings (Dicrostonyx groenlandicus).

Onset of sexual maturation was determined in weanling male collared lemmings exposed to one of three experimental regimens of different photoperiods before and after weaning. Animals gestated in photoperiods of either 16 h light:8 h dark or 8 h light:16 h dark. Those from 16 h light:8 h dark were transferred at 19 days of age to either 20 h light:4 h dark or 8 h light:16 h dark; those gestated under 8 h light: 16 h dark remained in that photoperiod throughout the experiment. After exposure for 15, 20, 25 or 30 days to the postweaning photoperiod, animals were killed and the following parameters assessed: body weight, testes weight, seminal vesicle weight, the presence or absence of epididymal spermatozoa and serum concentrations of prolactin, testosterone and corticosterone. All parameters except serum testosterone were significantly influenced by photoperiod. Animals housed under 8 h light:16 h dark had significantly greater body weights than those housed under 20 h light:4 h dark, a response that differs from that reported for other arvicoline rodents. The group gestated on 16 h light:8 h dark and transferred on day 19 to 8 h light:16 h dark had lower testes and seminal vesicle weights than the other two groups, and mature spermatozoa in the epididymides appeared 5 days later than in the 20 h light:4 h dark group. Serum prolactin was largely undetectable in animals from both 8 h light:16 h dark groups, but all males housed in 20 h light:4 h dark had 2.0-15.0 ng prolactin ml-1. Concentration of serum corticosterone was higher in animals weaned into long photoperiod, and decreased with age. These data indicate that weanling male D. groenlandicus are reproductively photoresponsive, but use a decrease in photoperiod, not static short-photoperiod exposure, to alter the rate of development. Prolactin was largely undetectable in animals exposed to short photoperiod, indicating that high concentrations of this hormone are not important for maturation. Low prolactin concentrations in animals in short photoperiods may mediate the annual moult to white pelage. The short-photoperiod-mediated decrease in corticosterone may play a role in seasonal changes in body weight and composition.

Animals↗

PubMed1990

Reproductive responses of male Microtus montanus to photoperiod, melatonin, and 6-MBOA.

Juvenile male Microtus montanus were examined for the effects of photoperiod, melatonin, and the naturally occurring reproductive stimulant 6-MBOA on growth and sexual maturation. 6-MBOA, present in sprouting grass, is an important environmental cue used for the initiation of reproduction in natural populations of this species. Long photoperiod (16:8) was stimulatory to body, testes, and seminal vesicle growth, while short photoperiod (8:16) inhibited these parameters. The pineal hormone melatonin, administered via daily afternoon injections (5 micrograms), was also inhibitory to all of the above parameters as well as to serum testosterone. 6-MBOA, administered via injection (0.0001-100 micrograms) or dietary means (0.1 or 1.0 microgram/gm unsifted chow), appeared unable to augment the rate of maturation in long-photoperiod-stimulated animals. When short-photoperiod animals were treated with high doses of the compound (100 micrograms injected or 1.0 micrograms/gm sifted chow), body and gonadal growth was inhibited to a greater extent than when animals were exposed to short photoperiod alone, and serum LH was reduced. Lower doses of the compound had no effect. Melatonin-treated animals experienced less maturational inhibition when simultaneously given a low dose of 6-MBOA-coated chow (1.0 micrograms/gm unsifted chow). A higher dose of 6-MBOA (1.0 micrograms/gm sifted chow) was ineffective in preventing the response to melatonin. These results indicate that 1) male M. montanus utilize photoperiod, rather than 6-MBOA, as a primary environmental cue, 2) high doses of 6-MBOA can be inhibitory under short photoperiod, 3) juvenile male voles are highly sensitive to the inhibitory effects of exogenously administered melatonin, and 4) 6-MBOA can partially prevent the inhibitory effects of melatonin on growth and sexual maturation.

Analysis of Variance↗

PubMed1986

Beta-adrenergic binding is increased by melatonin and alpha-adrenergic compounds.

Binding of the beta-adrenergic ligands [3H]dihydroalprenolol and [125I]cyanopindolol to pineal particulate fractions was increased 1- to 3.5-fold by addition of low concentrations of melatonin, alpha-adrenergic agonists, or alpha-adrenergic antagonists. Minimum concentrations of melatonin or alpha-adrenergic compounds which increased beta-adrenergic binding were between 1 pM and 0.1 nM. The increased binding of [3H]dihydroalprenolol caused by melatonin (0.1 muM) was attributed to a major increase in Bmax, which persisted in protein fractions after removal of melatonin. Melatonin enhancement of [3H]dihydroalprenolol binding was apparent after 5 to 7 min (30(0], was was optimal between 20 and 40 min, and decreased at longer times. Alpha-Adrenergic receptors are unchanged during beta-receptor enhancement.

Adrenergic alpha-Agonists↗