Exogenous T3 Elicits Long Day–Like Alterations in Testis Size and the RFamides Kisspeptin and Gonadotropin-Inhibitory Hormone in Short-Day Siberian Hamsters

Two experiments were designed to assess whether the short-day-induced patterns of shallow daily torpor, body weight, and other seasonal responses (food intake and pelage pigmentation) exhibited by Siberian hamsters (Phodopus sungorus sungorus) are under the control of a "seasonal timekeeping mechanism" that is independent of reproductive status [testosterone, (T)]. We examined whether the patterning and expression of these seasonal responses were altered by decreases in serum T that accompany gonadal regression during the first 8 wk of short-day exposure (i.e., the "preparatory phase" of the torpor season) or by experimental increases in serum T after this phase. Short-day-housed, castrated hamsters bearing T implants had long-day levels of the hormone and did not exhibit torpor. Appropriate seasonal patterns and levels of torpor, body weight, pelage color stage, and food intake were exhibited after T implant removal although serum T was clamped to long-day levels during the preparatory phase. In animals that were gonad intact during the preparatory phase and were subsequently castrated and given T implants, torpor did not occur as long as the implants were in place. However, the patterns and levels of daily torpor, food intake, and body weight rapidly returned to appropriate seasonal values compared with the castrated, blank-implanted controls on T implant removal; these effects occurred whether the T implants were removed when torpor frequency was increasing, at its peak, or decreasing across the torpor season. T did not affect pelage color stage under any condition.(ABSTRACT TRUNCATED AT 250 WORDS)

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Photoperiod modulates the effects of norepinephrine on lymphocyte proliferation in Siberian hamsters

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00209.2003 ◽

2003 ◽

Vol 285 (4) ◽

pp. R873-R879 ◽

Cited By ~ 10

Author(s):

Gregory E. Demas ◽

Timothy J. Bartness ◽

Randy J. Nelson ◽

Deborah L. Drazen

Keyword(s):

Lymphocyte Proliferation ◽

Splenic Tissue ◽

Splenocyte Proliferation ◽

Short Day ◽

Siberian Hamsters ◽

Long Day ◽

Β Adrenergic Receptors ◽

Short Days

Siberian hamsters ( Phodopus sungorus) rely on photoperiod to coordinate seasonally appropriate changes in physiology, including immune function. Immunity is regulated, in part, by the sympathetic nervous system (SNS), although the precise role of the SNS in regulating photoperiodic changes in immunity remains unspecified. The goal of the present study was to examine the contributions of norepinephrine (NE), the predominant neurotransmitter of the SNS, to photoperiodic changes in lymphocyte proliferation. In experiment 1, animals were maintained in long [16:8-h light-dark cycle (16:8 LD)] or short days (8:16 LD) for 10 wk, and splenic NE content was determined. In experiment 2, in vitro splenocyte proliferation in response to mitogenic stimulation (concanavalin A) was assessed in spleen cell suspensions taken from long- or short-day hamsters in which varying concentrations of NE were added to the cultures. In experiment 3, splenocyte proliferation was examined in the presence of NE and selective α- and β-noradrenergic receptor antagonists (phenoxybenzamine and propranolol, respectively) in vitro. Short-day animals had increased splenic NE content compared with long-day animals. Long-day animals had higher proliferation compared with short-day animals independent of NE. NE (1 μM) further suppressed splenocyte proliferation in short but not long days. Last, NE-induced suppression of proliferation in short-day hamsters was blocked by propranolol but not phenoxybenzamine. The present results suggest that NE plays a role in photoperiodic changes in lymphocyte proliferation. Additionally, the data suggest that the effects of NE on proliferation are specific to activation of β-adrenergic receptors located on splenic tissue. Collectively, these results provide further support that photoperiodic changes in immunity are influenced by changes in SNS activity.

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Regional differences in fat pad responses to short days in Siberian hamsters

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.1989.257.6.r1533 ◽

1989 ◽

Vol 257 (6) ◽

pp. R1533-R1540 ◽

Cited By ~ 14

Author(s):

T. J. Bartness ◽

J. M. Hamilton ◽

G. N. Wade ◽

B. D. Goldman

Keyword(s):

Body Weight ◽

Body Fat ◽

Subcutaneous Fat ◽

Short Photoperiod ◽

Fat Pad ◽

Initial Exposure ◽

Short Day ◽

Siberian Hamsters ◽

Long Day ◽

Induced Changes

Siberian hamsters exhibit decreased body weight and fat after initial exposure to short photoperiods and increased body weight and fat after extended short photoperiod exposure. The purpose of the present experiments was to determine if uniform changes in white adipose tissue (WAT) pad weights and lipid metabolism correspond to these short photoperiod-induced changes in body fat. Carcass lipid content and testes and fat pad weights [retroperitoneal WAT (RWAT), epididymal WAT (EWAT), and inguinal and dorsal subcutaneous WAT, respectively] were decreased in male hamsters relative to their long day counterparts after 6 and 12 wk of short-day exposure. Moreover, EWAT and RWAT weight, EWAT specific lipoprotein lipase activity, and specific and total lipogenesis were disproportionately decreased relative to the subcutaneous fat pads. The changes in fat pad weight and metabolism were generally reversed coincident with the return to a long-day-like reproductive status after prolonged short-day exposure (24 and 30 wk). In a less detailed experiment, female Siberian hamsters had decreased body, fat pad, and uterine weights after 6 wk of short-day exposure; however, no fat pad-specific changes in weight were observed. The results of these experiments demonstrate that short-day-exposed male Siberian hamsters may be a useful model for examining mechanisms underlying fat pad-specific responses. In addition, gender appears to influence the pattern of short-day-induced lipid depletion in this species.

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Metabolic stress suppresses humoral immune function in long-day, but not short-day, Siberian hamsters (Phodopus sungorus)

Journal of Comparative Physiology B ◽

10.1007/s00360-006-0133-4 ◽

2006 ◽

Vol 177 (3) ◽

pp. 339-347 ◽

Cited By ~ 28

Author(s):

Devin A. Zysling ◽

Gregory E. Demas

Keyword(s):

Immune Function ◽

Metabolic Stress ◽

Phodopus Sungorus ◽

Humoral Immune ◽

Short Day ◽

Siberian Hamsters ◽

Long Day

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Exogenous T3 mimics long day lengths in Siberian hamsters

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00713.2006 ◽

2007 ◽

Vol 292 (6) ◽

pp. R2368-R2372 ◽

Cited By ~ 50

Author(s):

David A. Freeman ◽

Brett J. W. Teubner ◽

Carlesia D. Smith ◽

Brian J. Prendergast

Keyword(s):

Gonadal Development ◽

Day Length ◽

Intermediate Step ◽

Iodothyronine Deiodinase ◽

Short Day ◽

Siberian Hamsters ◽

Long Day ◽

Long Duration ◽

Reproductive Response

Siberian hamsters ( Phodopus sungorus) exhibit seasonal cycles of reproduction driven by changes in day length. Day length is encoded endogenously by the duration of nocturnal melatonin (Mel) secretion from the pineal gland. Short-duration Mel signals stimulate reproduction and long-duration signals inhibit reproduction. The mechanism by which Mel signals are decoded at the level of neural target tissues remains uncharacterized. In Siberian hamsters, exposure to short day lengths or injections of Mel in long days results in a decrease in hypothalamic expression of type 2 iodothyronine deiodinase (Dio2) mRNA. Dio2 catalyzes the conversion of the thyroid hormone thyroxine to triiodothyronine (T3). Thus exposure to short and long day lengths should decrease and increase hypothalamic T3 concentrations, respectively. We tested the hypothesis that exogenous T3 administered to short-day hamsters would mimic exposure to long day lengths with respect to gonadal stimulation. Hamsters gestated and raised in short day lengths that exhibited photoinhibition of the testes were given daily subutaneous injections of T3 or saline vehicle for 4 wk beginning at week 12 of life. The results indicate that exogenous T3 induced gonadal growth in short-day hamsters and delayed spontaneous gonadal development by an interval equal to the number of weeks during which T3 was administered. T3 injections delayed gonadal regression if given coincident with the transfer of hamsters from long to short day lengths. These results suggest that T3 mimics long day exposure in Siberian hamsters and may serve as an intermediate step between the Mel rhythm and the reproductive response.

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Effects of melatonin on long-day responses in short-day housed adult Siberian hamsters

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.1988.255.5.r823 ◽

1988 ◽

Vol 255 (5) ◽

pp. R823-R830 ◽

Cited By ~ 3

Author(s):

T. J. Bartness ◽

B. D. Goldman

Keyword(s):

Short Photoperiod ◽

Serum Prolactin ◽

Short Day ◽

Siberian Hamsters ◽

Long Day ◽

Secretion Profile ◽

Total Body Fat ◽

Photoperiodic Responses ◽

Epididymal White Adipose Tissue ◽

Total Body

Testis growth is stimulated when short photoperiod-regressed Siberian hamsters are exposed to a lengthening photoperiod, an effect presumably mediated by the pineal gland through a decrease in the peak nocturnal duration of secretion of its hormone melatonin (MEL)(D. S. Carter and B. D. Goldman, Endocrinology 113: 1268-1273, 1983). We examined this stimulatory or "progonadal" effect of MEL in short photoperiod-regressed, adult male Siberian hamsters that were pinealectomized (PINX) and given timed daily subcutaneous 1) injections of MEL (1 or 10 micrograms/day) or saline or 2) infusions of MEL that were "long day-like" (4 h, 10 or 100 ng/day), "short day-like" (10 h, 10 ng/day), or control saline infusions (4 h/day). Photoregressed sham PINX hamsters were transferred to long days at this time. After 5 wk of treatment, 1-microgram MEL-injected hamsters and both groups of 4-h MEL-infused hamsters had stimulatory responses that mimicked those of the long-day-exposed, sham PINX group [i.e., increased testes, body, and epididymal white adipose tissue (EPIWAT) weights, total body fat, EPIWAT lipoprotein lipase activity, and serum prolactin and follicle-stimulating hormone levels]. These effects were not observed in 10-micrograms MEL- or saline-injected and 10-h MEL- or saline-infused hamsters. Thus the peak nocturnal duration of serum MEL is the critical parameter of the MEL secretion profile for stimulating a variety of photoperiodic responses when photoregressed hamsters are exposed to lengthening daylengths.

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SCN lesions block responses to systemic melatonin infusions in Siberian hamsters

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.1991.260.1.r102 ◽

1991 ◽

Vol 260 (1) ◽

pp. R102-R112 ◽

Cited By ~ 1

Author(s):

T. J. Bartness ◽

B. D. Goldman ◽

E. L. Bittman

Keyword(s):

Serum Prolactin ◽

Suprachiasmatic Nuclei ◽

Fat Pad ◽

Short Day ◽

Siberian Hamsters ◽

Long Day ◽

Testicular Regression ◽

Body Weights ◽

Short Days

The role of the suprachiasmatic nuclei (SCN) in the response to short-day melatonin (MEL) signals was examined in long-day-housed pinealectomized (PINX) Siberian hamsters. Five- or ten-hour MEL infusions that mimicked the peak nocturnal durations of serum MEL levels in long or short days, respectively, or control saline infusions were given for 5 wk. Half the hamsters in each infusion group also received bilateral electrolytic SCN lesions. The 10-h MEL infusions reduced testes weight, body and fat pad weights, and serum prolactin (PRL) and follicle-stimulating hormone (FSH) concentrations in unoperated controls. These short-day-type effects were blocked by SCN lesions, which often produced hyperprolactinemia. Circadian rhythms of locomotor activity were disrupted or sparse in hamsters with lesions in or near the SCN. In a second experiment, 5 wk of long-day-like, short-duration (5-h) MEL infusions were administered to hamsters that had been PINX after 8 wk of short-day exposure. Control hamsters given 5-h MEL infusions, but not 10-h MEL or saline infusions, exhibited testicular growth and increased serum PRL levels. Hamsters with SCN lesions showed similar responses, regardless of the duration or type of infusion. Although the blockade of 10-h MEL infusion-induced testicular regression by SCN lesions in experiment 1 may have been due to stimulation of the testes by PRL, it is unlikely that the hyperprolactinemia accounted for the ability of SCN lesions to block effects of 10-h MEL infusions on fat pad and body weights. Therefore, the SCN and/or neighboring structures may participate in the response to short-day MEL signals in Siberian hamsters.

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