Biomedical subjects

M H Stetson

Publications and source records attributed to M H Stetson.

At least 37 records · Page 2Linked to original sources

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

Ultraviolet regulation of neuroendocrine and circadian physiology in rodents.

UV wavelengths can regulate neuroendocrine and circadian responses in some rodent species. Appropriately timed UV exposures can block the short photoperiod-induced collapse of the reproductive system, cause a rapid suppression of nocturnal melatonin synthesis, regulate melatonin rhythms and phase shift wheel running rhythms. These biological effects of UV are not dependent on the Harderian gland or melanin in the eye, but appear to be related to the degree of transmission through the ocular lens. Such results are consistent with the hypothesis that elements in the retina can transduce UV stimuli for circadian and neuroendocrine regulation.

Albinism↗

PubMed1994

Dissociation of the reproductive and prolactin photoperiodic responses in male golden hamsters.

The response of prolactin (PRL) and that of the pituitary-testicular axis to inhibitory photoperiods were compared in immature and young adult hamsters. In the first experiment, 14-day-old hamsters were transferred from 14L:10D to 12L:12D or 8L:16D. At 21, 28, 35, and 49 days of age, body weights, testes weights, and plasma LH were similar in all groups. In contrast, the levels of PRL were lower in animals on 8L:16D or 12L:12D than in those on 14L:10D and 49 days of age. In a second experiment, 80-day-old hamsters were transferred from 14L:10D to either 12L:12D or 8L:16D. At 5-wk intervals, they were weighed and bled by cardiocentesis and the length of the testes was measured. Photoperiods of 12L:12D and 8L:16D induced testicular regression, which was complete by 15 wk. Thereafter, augmented levels of FSH preceded spontaneous recrudescence of the testes in animals on both photoperiods. Serum PRL levels were significantly depressed within 5 wk after transfer to either 8L:16D or 12L:12D. After 15 wk as the testes were increasing in size in both groups, circulating PRL levels increased in animals exposed to 8L:16D, whereas in animals exposed to 12L:12D, PRL remained at or below the sensitivity of the assay. In a final experiment, on the day before parturition, pregnant hamsters were transferred to either 6L:18D, 6L:30D, 6L:42D, or 6L:54D or were held on 14L:10D. There were no differences in testicular weights, plasma LH, or plasma thyroxine (T4) of the pups from these dams at 21, 36, or 48 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗

PubMed1994

Effects of photoperiod, pinealectomy, and melatonin implants on testicular development in juvenile Siberian hamsters (Phodopus sungorus).

When transferred from a long to short photoperiod, Siberian hamsters (Phodopus sungorus) undergo a number of physiological and morphological changes including suppression of gonadal activity, a change in pelage color, a decrease in body weight, and, in response to a simultaneous decrease in ambient temperature, physiological changes associated with the induction of daily torpor. All these functions can be affected by photoperiod and melatonin treatment. To investigate the interactive effects of photoperiod, pinealectomy, and melatonin on gonadal development, two experiments were performed using juvenile Siberian hamsters. In experiment 1, animals born in a long photoperiod (16L:8D) either remained in a long photoperiod or were transferred to a short photoperiod (8L:16D) from 15 days of age, when surgeries (pinealectomy and/or melatonin implantation) were performed. Testicular development was inhibited in all animals bearing melatonin implants irrespective of the presence or absence of the pineal gland. Pinealectomy blocked the inhibitory effect of short photoperiod on maturation of the reproductive system. Therefore, the pineal gland must be involved in the short photoperiod- induced inhibition of testicular maturation of juvenile Siberian hamsters. In experiment 2, a similar experimental design was employed except that the hamsters were born and raised to 15 days of age in 8L:16D. Exogenous melatonin, pinealectomy, or both retarded gonadal development in hamsters born in 8L:16D and transferred on Day 15 of age to 16L:8D. All hamsters maintained in a short photoperiod had small testes irrespective of the presence of absence of the pineal gland or of melatonin implants. Hamsters transferred to a long photoperiod after pinealectomy and/or melatonin implantation had small testes compared to those of the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Photoperiod and melatonin affect testicular growth in the marsh rice rat (Oryzomys palustris).

Reproduction in rice rats is subject to photoperiodic control and the pineal gland mediates this effect. We examined the effects of the pineal gland hormone melatonin on testicular weight when administered via implants, injections, and infusions. Testicular weight was modified by photoperiod and the size of the melatonin implant. Twenty-millimeter implants suppressed testicular weight in rice rats housed on 12- and 16-hr photoperiods, while those housed on a 14-hr photoperiod were more sensitive to melatonin; in these animals 10- and 20-mm implants inhibited testicular weight. Melatonin implants also prevented rice rats from responding to a change in photoperiod with the appropriate alteration of testicular growth. Melatonin injections inhibited testicular growth when administered before lights out on LD 14:10, but not on LD 16:8. Morning injections had no effect on either photoperiod. Finally, 12-hr duration melatonin infusions inhibited testicular growth in pinealectomized rice rats on LD 16:8, while 6-hr duration infusions were without effect. These data show that the pineal, through the secretion of melatonin, is a phototransducing organ intimately involved in testicular growth in rice rats.

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

Investigations of the regulation of specific 2-[125I]iodomelatonin binding sites in Siberian hamsters by endogenous and exogenous melatonin.

These studies used quantitative in vitro autoradiography to investigate whether endogenous or exogenous melatonin modulate specific 2-[125I]iodomelatonin binding sites in the pars tuberalis or suprachiasmatic nuclei of Siberian hamsters. Saturation analyses were conducted on tissue sections from hamsters that were pinealectomized, exposed to constant illumination (72 h), or injected with melatonin, and from hamsters that were treated as controls. High affinity (Kd approximately 20-75 pM) specific 2-[125I]iodomelatonin binding sites were detected in the suprachiasmatic nuclei and pars tuberalis of all animals. Neither pinealectomy nor constant illumination significantly affected either the affinity or the density of the specific 2-[125I]iodomelatonin binding sites in either region. Melatonin injections led to a decrease in specific 2-[125I]iodomelatonin binding to the pars tuberalis at 3-4 h after the last injection. However, washing the sections before incubation with 2-[125I]iodomelatonin eliminated this effect, suggesting that melatonin was occupying the binding sites rather than decreasing their actual number. Furthermore, when hamsters were sacrificed 18 h after the last melatonin injection, no effect of melatonin on either the affinity or density of specific 2-[125I]iodomelatonin sites was observed. These data suggest that 2-[125I]iodomelatonin binding sites in Siberian hamsters are not regulated by changes in circulating melatonin levels.

Animals↗

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↗

PubMed1993

Effect of photoperiod on gonadal maintenance and development in the marsh rice rat (Oryzomys palustris).

Reproductive responsiveness to photoperiod was examined in adult and prepubertal rice rats. In Experiment 1, adult males on 16L:8D and 14L:10D maintained testicular size, while exposure to 12L:12D and 10L:14D resulted in testicular regression, followed by spontaneous testicular recrudescence. Adult females (Experiment 2) on long photoperiods had larger uterine and ovarian weights at most times examined and maintained vaginal patency compared to females on short photoperiods. Reproductive development of male rice rats (Experiment 3) was stimulated on 16L:8D and 14L:10D with rapid testicular development occurring by 12 weeks of age, while in males reared on 12L:12D and 10L:14D reproductive development proceeded slowly and was completed at approximately 26 weeks of age. Females on 16L:8D and 14L:10D experienced rapid uterine development compared to those on 12L:12D and 10L:14D. Ovarian weights on 14L:10D were significantly greater than ovarian weights on 12L:12D. Finally, vaginal patency was attained more rapidly on 16L:8D than on 14L:10D and was markedly inhibited on 12L:12D and 10L:14D. These data show that reproduction in adult male and female rice rats is regulated, at least in part, by photoperiod and that reproductive development in juveniles is also under photoperiodic control.

Animals↗

PubMed1993

Temporary anestrus induced by injection of luteinizing hormone-releasing hormone in hamsters.

The photoperiod-induced anovulatory state of the Syrian hamster (Mesocricetus auratus) is characterized by a daily rhythm of circulating LH and FSH with a surge in blood levels occurring each afternoon. The association between anestrus and the presence of daily gonadotropin surges suggests that initiation of the characteristic daily rhythm of LH and/or FSH after transfer to short days results in blockade of ovulation. We tested whether daily injection of LHRH affects the 4-day rhythm of vaginal estrous cycles, induces daily surges of LH and FSH, blocks ovulation, and induces regression of the ovaries. LHRH (1 microgram/animal) was injected (s.c.) at 1630 h each day into females with regular 4-day estrous cycles. Injections began on the afternoon of estrus and continued for 12 days (experiment 1) or 8 days (experiments 2 and 3). In saline-injected controls, vaginal discharges occurred regularly at 4-day intervals during the injection period in all experiments. In contrast, vaginal estrus was not observed during the period of LHRH injection in any animals. Ova were found in oviducts of all saline-injected controls (8.9 +/- 0.8; n = 8) on the morning of estrus (after 8 injections), but no ova were found in oviducts of LHRH-injected females. Levels of plasma LH but not FSH increased after injection of LHRH each day, whereas the 4-day estrous cycle rhythm of gonadotropins was not disturbed in saline-injected controls. Vaginal estrous cycles and a 4-day rhythm of LH release resumed within 7 days after LHRH injections were stopped.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

A single injection of adrenergic agonists enhances pineal melatonin production in Turkish hamsters.

The purpose of this study was to determine whether the pineal gland of Turkish hamsters (Mesocricetus brandti) responds to adrenergic agonists with an increase in melatonin production, and, if it does, whether the sensitivity of the pineal gland to agonists would differ throughout the dark phase. Adult Turkish hamsters weighting 110-210 g received a subcutaneous injection of isoproterenol (ISO, 1 mg/kg B.W.) or norepinephrine (NE, 1 mg/kg B.W.) at different times of night. Animals exposed to LD 16:8 responded to ISO or NE with increased pineal melatonin content only when injected at dawn, when endogenous melatonin is at basal or near-basal levels. When the 8 hr scotophase was entirely replaced with light, the responsiveness to ISO injections at dawn disappeared. In animals exposed to light from 30 min prior to injection to the time of sacrifice, ISO injections increased pineal melatonin content (P < 0.005, three-way ANOVA), which varied, depending on the specific time of injection (effect of time of night, P < 0.05, three-way ANOVA). These results demonstrate that (1) adrenergic agonists enhance the production of pineal melatonin in Turkish hamsters, (2) this stimulatory effect takes place late, but not early in the 8 hr scotophase, and (3) the adrenergic induction of pineal melatonin production in Turkish hamsters requires priming by darkness during the appropriate circadian phase.

Animals↗

PubMed1992

Daily rhythms of follicle-stimulating hormone in adult anestrous and prepubertal female Turkish hamsters (Mesocricetus brandti).

We tested the hypothesis that daily FSH surges are correlated with the anestrous condition in adult Turkish hamsters. Blood samples were collected from anestrous hamsters on short (12L:12D) and long photoperiods (16L:8D) by cardiac puncture. Both photoperiod-induced anestrous (PIA) and spontaneously anestrous adult Turkish hamsters had a daily rhythm of FSH secretion with maximum hormone concentrations occurring late in the afternoon. We also hypothesized that daily FSH and progesterone surges are correlated with the reproductive response to short photoperiod in prepubertal female Turkish hamsters. Prepubertal hamsters reared on 16L:8D were sampled at 13, 15, 17, 19, and 21 days of age. Only at 19 days was a rhythm of FSH present. Prepubertal female hamsters reared on 12L:12D were sampled at 30, 35, 55, 75, 95, and 115 days of age or until estrous cycles were established. Those hamsters that delayed puberty in response to this short photoperiod had a daily rhythm of FSH and progesterone secretion. Conversely, no daily rhythms were observed in female hamsters that failed to respond to the inhibitory photoperiod with a delay in puberty. These results indicate that the daily rhythm of FSH secretion in the female Turkish hamster is correlated with reproductively inhibited physiological states.

Age Factors↗

PubMed1992

Maternal transfer of photoperiodic information in Siberian hamsters. V. Effects of melatonin implants are dependent on photoperiod.

Photoperiodic information is transferred from female Siberian hamsters to their fetuses during gestation. Although maternal melatonin is known to be essential for the transfer of prenatal photoperiodic information, its specific role is not well defined. The duration of the daily melatonin signal, expressed as an elevation of serum melatonin levels in the maternal circulation, has been hypothesized to convey day length information to the fetus. If this hypothesis is valid, it predicts that identical maternal melatonin signals should affect the fetuses identically, regardless of the prenatal photoperiod. To test this hypothesis, adult females received melatonin in beeswax or beeswax alone. They were paired with males and housed in photoperiods of 12L:12D or 16L:8D. On the day of parturition, mother and young were transferred to constant light (LL). Young males were killed on Day 28 of life, and weights of testes were determined. Prenatal treatment with beeswax alone did not affect the nature of the signal transferred from mother to fetus; young gestated in 12L:12D and reared in LL developed small testes, while those gestated in 16L:8D had large testes. On the other hand, the effect of the prenatal melatonin treatment on postnatal testicular development in LL was inversely dependent on the prenatal photoperiod: testicular growth was stimulated in young gestated in 12L:12D, but inhibited in young gestated in 16L:8D. To verify that the melatonin pellets produced equivalent serum melatonin levels in adult females in 12L:12D and 16L:8D, unmated adult females were killed 6-10 wk after receiving melatonin pellets. Serum levels were elevated in both groups throughout the day and night.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Effects of short photoperiod on ATPase activities in the testis of the immature Siberian hamster.

The effects of changes in photoperiod length upon body weight; spleen, thymus, and testis weights; testis protein content; testis cation pump enzyme activities; and plasma testosterone were studied in the developing Siberian hamster, Phodopus sungorus. Male hamsters were exposed to a cycle of 16L:8D (long-day), until Day 18 when half were switched to a 10L:14D (short-day) cycle, until killed 0, 2, 4, 7, 10, 12, or 15 days later. Body weight and relative testis weight (expressed as percentage of body weight) increased steadily during the first week of exposure. After 10 days, the long-day hamsters consistently weighed more (p less than 0.05). Relative testis weights in the short-day group began to decrease (p less than 0.005) within 10 days and continued to decline. Testis homogenate K(+)-pNPPase- (as a measure of Na+,K(+)-ATPase) and Mg(2+)-pNPPase-specific activities closely paralleled testis weight, with the short-day animals (p less than 0.05) differing after 10 days. Plasma testosterone levels remained below adult levels through exposure Day 15, but were relatively lower (p less than 0.05) in the short-day group after 10 days. Spleen weights were similar for the long- and short-day groups. The short-day group had larger thymus weights after 12 days (p less than 0.05), but thymus enzyme activities did not differ between the two groups. We conclude that cation pump activities in the Siberian hamster testis are significantly affected by changes in photoperiod length.

Adenosine Triphosphatases↗