Biomedical subjects

T J Bartness

Publications and source records attributed to T J Bartness.

At least 19 recordsLinked to original sources

PubMed1995

A role for testosterone in the maintenance of seasonally appropriate body mass but not in lipectomy-induced body fat compensation in Siberian hamsters.

The purpose of this study was to test whether serum testosterone (T) concentrations characteristic of reproductively active, long-day-housed Siberian hamsters are necessary for compensatory increases in nonexcised fat pads following removal of epididymal white adipose tissue (EWAT) and/or for the maintenance of seasonally appropriate body weights in these hamsters. Long-day-housed hamsters were castrated or left intact, sham or EWAT lipectomized, and given T or cholesterol (C) implants. All groups had ad libitum food access except for two castrated T-treated groups that were pair-fed to their C-treated counterparts to control for effects of T on food intake. C-treated castrates had decreased body weights compared with all other groups, suggesting a role of T in the maintenance of seasonally appropriate body mass. Since the T-treated hamsters pair-fed to these T-deficient animals exhibited seasonally appropriate body weights and fat pad masses, T does not appear to affect these parameters through the modulation of food intake. All fat pads of C-treated animals were smaller than those of ad libitum- or pair-fed, T-treated castrates; however, EWAT was the only fat pad that was smaller in the C-treated sham-lipectomized group than in gonad-intact sham-lipectomized hamsters. This result may indicate an enhanced sensitivity of EWAT to T. The effects of T on fat pad mass were not associated with proportionate changes in lipoprotein lipase activity, suggesting that the major effect of T on fat accumulation occurs through other mechanisms in this species. C-treated lipectomized hamsters compensated for the body fat deficit 8 weeks after lipectomy via statistically non-significant increases in retroperitoneal and inguinal WAT mass. This finding suggests that, whereas T is necessary for maintenance of seasonally-appropriate body weight, it is not necessary for fat pad compensation after EWAT lipectomy.

Adipose Tissue↗

PubMed1995

Effects of food deprivation and metabolic fuel utilization on the photoperiodic control of food intake in Siberian hamsters.

Unlike rats, Siberian hamsters exhibit seasonal changes in energy balance that are controlled by the photoperiod (daylength). In Siberian hamsters, body weight and fat, and food intake peak in long, summer-like days and reach nadirs in short, winter-like days. The purpose of the present experiments was to test whether metabolic challenges that increase food intake in laboratory rats also increase feeding in Siberian hamsters. Specifically, we asked the questions: (a) Is food intake increased following a fast?; (b) Is food intake increased following treatments that block metabolic fuel utilization, such as the glucose utilization blocker 2-deoxy-D-glucose (2 DG), or that enhance storage of metabolic fuels, such as short acting insulin (regular insulin)?; (c) Does the combination of food deprivation and decreased metabolic fuel utilization increase food intake?; and (c) Does the photoperiod affect the feeding and physiological responses to metabolic challenges? Food intake was measured in response to fasting, insulin or 2 DG treatment in adult female Siberian hamsters housed in long photoperiods. Following exposure to a short photoperiod, these hamsters were tested for their response to insulin and 2 DG, but not to fasting. Food intake did not increase following fasts of 12, 24, or 48 h. Food intake was increased in long day-housed hamsters given the lowest dose of 2 DG tested (125 mg/kg) 2, 4, and 6 h after treatment, but not in short days nor in long days with larger doses of the 2 DG. Similarly, food intake was increased by treatment with regular insulin in long days, but not short days.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Catecholaminergic innervation of white adipose tissue in Siberian hamsters.

When Siberian hamsters are transferred from long summerlike days (LDs) to short winterlike days (SDs) they decrease their body weight, primarily as body fat. These SD-induced decreases in lipid stores are not uniform. Internally located white adipose tissue (WAT) pads are depleted preferentially of lipid, whereas the more externally located subcutaneous WAT pads are relatively spared. These data suggest a possible differential sympathetic neural control over catecholamine-induced lipolysis and that lipolytic rates are greater for internal vs. external WAT pads. Moreover, if these differential rates of lipolysis are due to differential sympathetic nervous system (SNS) drives on the pads, then fat pad-specific catecholaminergic innervation may exist. Therefore, we tested whether inguinal WAT (IWAT; an external pad) and epididymal WAT (EWAT; an internal pad) were innervated differentially. In addition, we tested whether norepinephrine (NE) turnover (TO) reflected the presumed greater SNS drive on EWAT vs. IWAT after SD exposure. Injections of fluorescent tract tracers [Fluoro-Gold or indocarbocyanine perchlorate (DiI)] demonstrated projections from the SNS ganglia T13-L3 to both fat pads. Retrograde labeling revealed a relatively separate pattern of distribution of labeled neurons in the ganglia projecting to each pad. In vivo anterograde transport of DiI resulted in labeling in both IWAT and EWAT that included staining around individual adipocytes and occasionally retrogradely labeled cells. The proportionately greater decrease in EWAT compared with IWAT mass after 5 wk of SD exposure was reflected in greater EWAT NE TO than found in their LD counterparts for this pad.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗

PubMed1994

Body fat regulation after partial lipectomy in Siberian hamsters is photoperiod dependent and fat pad specific.

Siberian hamsters exhibit seasonal fluctuations in body weight (fat). Initial exposure to a short photoperiod results in body fat loss that reverses after approximately 22 wk of short-day exposure. The purpose of this study was to determine whether Siberian hamsters are able to recover body fat after surgical reduction of total lipid stores and if so, whether this ability is photoperiod dependent and fat pad specific. Either the largest pair of internal fat pads, the epididymal white adipose tissue (EWAT) or one pair of two large external depots, the inguinal (IWAT) fat pads, were removed from male hamsters housed for 22 wk in a long (LD) or short (SD) photoperiod. Retroperitoneal fat pad (RWAT) mass was increased in LD EWAT- and IWAT-lipectomized hamsters. IWAT mass also was increased in the LD EWAT-lipectomized hamsters. Neither SD-lipectomized group compensated for body fat loss in any of the measured fat pads. Increased food intake was not necessary for total body fat recovery, but undereating partially may be responsible for the lack of recovery in SDs. The results of these experiments demonstrate a photoperiod-dependent ability of male Siberian hamsters to regulate total body fat after partial lipectomy. In addition, recovery involves a fat pad-specific compensatory response to partial lipectomy, rather than a general increase in lipid deposition in all fat depots.

Adipose Tissue↗

PubMed1994

Effects of food deprivation and restriction, and metabolic blockers on food hoarding in Siberian hamsters.

Syrian hamsters do not increase their food intake following several metabolic challenges, including food deprivation and blockade of metabolic fuel utilization, in contrast to the response of other small rodents to these challenges. Perhaps hamsters respond to such challenges differently, for example by altering hoarding. In the present experiments, we have begun to question the role of food hoarding in the total energy balance of Siberian hamsters. Therefore, we developed a simulated burrow system, where food was available outside the burrow for consumption and/or hoarding during a 15-h period surrounding the 8-h dark portion of the photocycle. Food hoarding, but not food intake, increased dramatically after 32- and 56-h fasts and was greater following the longer fast. Food-restricted weight-reduced hamsters (80% of ad libitum-fed controls) were refed and given the opportunity to hoard. Initially, when body weights were low, food hoarding was maximal and then decreased gradually to control levels as body weights reached those of the ad libitum-fed controls. Food intake was not affected. Neither hoarding nor food intake was affected by treatment with long-acting protamine zinc insulin, given to enhance the storage of metabolic fuels, at any dose tested. Finally, neither food intake nor hoarding was affected by treatment with the glucose utilization blocker 2-deoxy-D-glucose, the fatty acid utilization blocker methyl palmoxirate, or a combination of the two treatments, all at doses that stimulate food intake in laboratory rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Animal Feed↗

PubMed1993

Hyperprolactinemia does not promote testicular recrudescence in photoregressed Siberian hamsters.

Serum prolactin (PRL) concentrations, as well as body, epididymal white adipose tissue (EWAT), and testes weights, decrease in Siberian hamsters (Phodopus sungorus sungorus) following short-photoperiod exposure. Previously, we have shown that lesions of the suprachiasmatic nucleus of the hypothalamus (SCNx) block regression of the testes and decreases in body weight and EWAT caused by short day-like, timed daily subcutaneous melatonin infusions in pinealectomized Siberian hamsters and elevate dramatically serum PRL concentrations. We also have shown that SCNx, as well as lesions of the paraventricular nucleus of the hypothalamus (PVNx) and an area immediately ventral to the PVN (subPVN), promote accelerated testicular recrudescence, increases in EWAT and body weights, and increases in serum PRL concentrations, in short-day (SD)-housed, photoregressed Siberian hamsters. The stimulation of the testes seen in these previous studies could have been due to the lesion-induced increases in serum PRL concentrations. Therefore, the purpose of the present experiment was to test whether experimentally induced hyperprolactinemia could stimulate testicular recrudescence. This was accomplished by giving photoregressed, SD-housed Siberian hamsters chronic subcutaneous infusions of ovine PRL (oPRL) to mimic either long-day- or lesion-induced serum concentrations of hamster prolactin (hPRL). No increase in testes, body, or EWAT weights were observed following 5 weeks of oPRL infusions in either group compared with their vehicle-infused counterparts. These data suggest that hyperprolactinemia was not solely responsible for the stimulation of testicular recrudescence in SCNx or PVNx photoregressed, or SCNx pinealectomized hamsters receiving timed melatonin infusions seen previously.

Animals↗

PubMed1993

The timed infusion paradigm for melatonin delivery: what has it taught us about the melatonin signal, its reception, and the photoperiodic control of seasonal responses?

This review summarizes the evidence showing that the duration of the nocturnal secretory profile of pineal melatonin (MEL) is critical for eliciting seasonally appropriate reproductive physiological and behavioral responses in mammals. We review experiments using the timed infusion paradigm (TIP) to deliver MEL either systemically or centrally to pinealectomized hamsters and sheep. In this paradigm, MEL is infused, usually once daily, for a specific number of hours and at a predetermined time of day. This experimental strategy tests most directly those features of the MEL signal that are necessary to trigger photoperiodic responses. The data suggest that the duration of the MEL stimulation is the critical feature of the MEL signal for both inhibitory and stimulatory effects of the hormone on the photoperiodic control of reproductive development in juvenile Siberian hamsters, and for the photoperiodic control of reproductive and metabolic responses in adult Siberian and Syrian hamsters and sheep. The use of the TIP reveals the importance of the frequency of the signal presentation of MEL and suggests the importance of a period of low-to-absent circulating concentrations of the hormone. The TIP also reveals that the characteristics of the MEL signal that regulate male sexual behavior are similar to those that are critical for reproductive and metabolic responses in Syrian hamsters. We summarize the locations of possible functional MEL target sites identified by combining the TIP with traditional brain lesion techniques. Evidence from such studies suggests that the integrity of the suprachiasmatic nucleus (SCN) region in Siberian hamsters and the anterior hypothalamus in Syrian hamsters is necessary for the response to short-day MEL signals. The TIP has been used to deliver MEL to putative target sites for the hormone in the brain of juvenile and adult Siberian hamsters. The results of these preliminary experiments suggest that the regions of specific MEL binding in this species, especially the SCN, are effective sites where MEL may stimulate short-day-type responses. In contrast, results from intracranial application of MEL in sheep suggest the medial basal hypothalamus as a critical site of action. Finally, we also discuss potential applications of the TIP for identification of brain MEL target sites, understanding of other photoperiodic phenomena and responses, and resolution of the cellular/molecular basis underlying the reception and interpretation of MEL signals.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Effects of thyroxine on the photoperiodic control of energy balance and reproductive status in Siberian hamsters.

Siberian hamsters (Phodopus sungorus sungorus) exhibit a variety of seasonal responses when exposed to short days (SD), including decreases in body weight and fat, gonadal regression, and changes in several nonsteroid serum hormone concentrations. One such SD-induced hormonal change is a modest decrease in serum thyroxine (T4) concentrations. In an attempt to determine any underlying hormonal influences for the SD-induced decreases in body weight and fat, we investigated the possible role of T4 and triiodothyronine (T3). This potential paradoxical effect of these hormones on body weight and fat, as compared with most other rodent species, is not without precedent in Siberian hamsters. Specifically, changes in the gonadal steroids have opposite effects on body weight and fat in Siberian hamsters compared with laboratory rats and mice, and Syrian hamsters. SD serum thyroid hormone concentrations were elevated to long-day (LD) levels via subcutaneous T4 injections. Vehicle- and noninjected controls were included, as well as three similar LD-housed groups. Although we found a trend towards decreased T4 serum concentrations in noninjected control hamsters following 9 weeks of SD exposure, this effect did not reach statistical significance. SD-housed, T4-injected hamsters had similar decreases in body, fat pad, and paired testes weights compared to the SD-housed, vehicle- and noninjected controls despite having LD-like serum T4 and T3 concentrations. Thus, no paradoxical effect of the thyroid hormones on body weight (fat) was found, nor do these hormones appear to play a role in the effects of SDs on reproductive status in this hamster species.

Adipose Tissue↗

PubMed1992

Reversal of high-fat diet-induced obesity in female rats.

The purpose of the present study was to test whether the degree of obesity or the duration of the obese state affects the reversibility of diet-induced obesity. This was accomplished by initially feeding adult female Wistar rats either a low-fat diet (Chow) or one of two high-fat diets (HFDs; 30 and 60% of total calories as dietary fat; 30% HFD and 60% HFD, respectively). Fifty-four days, reversal 1 (R1), or ninety-seven days, reversal 2 (R2), later the HFDs were substituted with the low-fat control diet in subgroups of rats. Animals from all groups were sampled at three intervals: the start of R1 (R1 start), and the completion of R1 (R1 end) and R2 (R2 end). At the end of each interval the 60% HFD-fed group had increased body weight, carcass lipid content, and retroperitoneal and parametrial white adipose tissue (RWAT and PWAT) pad weight, fat cell diameter, and fat cell volume, but not fat cell number (FCN), compared with the other groups. The 60% HFD-fed rats also exhibited a marked and persistent hyperphagia that continued even as most of the indexes of obesity approached their maximal values (R1 end). The 60% HFD group had a transient increase in RWAT and PWAT lipoprotein lipase activity that followed the development of most obesity indicators. A clear intermediate level of obesity did not develop in the 30% HFD-fed group. Instead, these animals had nonsignificant increases in these measures of adiposity, making it impossible to test whether the severity of the obesity affected its reversibility in age-matched groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗

PubMed1992

Rapid gonadal recrudescence and body and lipid mass increases with hypothalamic lesions in photoregressed Siberian hamsters.

The effects of lesions (x) of the suprachiasmatic nucleus (SCN) or paraventricular nucleus (PVN), or pinealectomy (PINX) on gonadal recrudescence, body and fat pad weights, and food intake were examined in photoregressed male Siberian hamsters (Phodopus sungorus sungorus). Blood was sampled weekly for serum follicle-stimulating hormone (FSH) and prolactin (PRL) measurement. Lesions were classified as complete if greater than 80% of the nuclei were destroyed and designated as 'hits', whereas incomplete lesions were designated as 'misses'. Five weeks postlesion, hamsters with PVNx or SCNx hits and SCNx misses (lesions generally located caudal and dorsal to the SCN) had increased testes, epididymal white adipose tissue and body weights, increased food intake, and progressively increasing serum PRL, but not FSH concentrations compared with PINX, PVNx misses and intact short day controls. SCNx hamsters with complete lesions had sparse or arrhythmic locomotor activity patterns in subsequent tests under constant conditions. Although no single area was identified histologically as the locus for this effect, the hyperprolactinemia and rapid gonadal recrudescence was consistent with varied degrees of damage to the periventricular area. These results suggest a novel central control of PRL secretion by an area caudal and dorsal to the SCN, and extending to and including the PVN. This area may be involved with maintaining short day responses.

Adipose Tissue↗

PubMed1991

Effects of high fat diets on hibernation and adipose tissue in Turkish hamsters.

The effects of dietary fat saturation and fat content on hibernation and several properties of white and brown adipose tissue (WAT and BAT, respectively) were investigated in Turkish hamsters (Mesocricetus brandti). Male hamsters were housed in a long photoperiod (LD 16:8) at 23 degrees C and fed one of three diets: (1) chow (6.5% fat per weight), (2) chow + 13.5% vegetable oil (OIL, 20% fat per weight [largely unsaturated fat]) and (3) chow + 13.5% vegetable shortening [(SHORTENING, 20% fat per weight (largely saturated fat)]. Five weeks later body weights had stabilized and the animals were transferred to a short photoperiod (LD 8:16) at 3 degrees C. At the peak of the hibernation season (17 weeks) the animals were sacrificed within 24 h of arousal. Chow-fed hamsters had the greatest percentage of animals hibernating and days found torpid compared with the two fat-fed groups, with no differences found between the latter two groups for these measures. There were no differences between hibernating (HIB) and non-hibernating (NON-HIB) hamsters across or within the diet groups for any of the BAT measures [uncoupling protein content, mitochondrial mass, lipoprotein lipase (LPL) activity, and in vivo lipogenesis], nor were there significant effects of the diet on these measures. CHOW- and OIL-fed HIB hamsters showed decreases in body weight. All HIB groups had decreases in each carcass component, several fat pad weights, testes weight, and food intake. No consistent differences in WAT LPL activity or in vivo lipogenesis were found between HIB and NON-HIB hamsters.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗

PubMed1991

Effects of diabetes and insulin on photoperiodic responses in Siberian hamsters.

Short day pineal melatonin secretion patterns cause gonadal regression in Siberian hamsters, an effect due to alterations in gonadotropin secretion that ultimately affects the gonads. An analogous endocrine response sequence may underlie short day (melatonin)-induced decrease in body weight (fat) in this species. A potential mediating hormone for this effect of melatonin may be insulin. The purpose of the present experiments was to determine whether alterations in insulin status might affect the photoperiodic control of body weight (fat) and reproductive condition. Insulin was controlled by inducing experimental diabetes mellitus via injections of streptozotocin across several days, followed by treatment with one of two doses of long-lasting insulin. Both insulin doses normalized urinary glucose, but not body weight in long days. Seven weeks of short day exposure caused gonadal regression in all groups regardless of insulin status. The magnitude of the short day-induced decrease in body weight and carcass lipid was directly related to long day baseline body weights; however, the body weight nadir reached was similar among the groups. A 'regulation' of seasonally appropriate body weight (fat) is implied by these findings.

Adipose Tissue↗

PubMed1991

SCN lesions block responses to systemic melatonin infusions in Siberian hamsters.

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.

Animals↗

PubMed1991

Suprachiasmatic and paraventricular control of photoperiodism in Siberian hamsters.

The effects of lesions of the suprachiasmatic (SCN) and paraventricular nuclei (PVN) of the hypothalamus on photoperiodic responses were examined in adult Siberian hamsters. SCN lesions reduced nocturnal water intake in long days, whereas PVN lesions increased body weight and food intake in both short and long days. SCN or PVN lesions blocked short-day-induced decreases in body, fat pad, and testes weights and in food intake. Serum prolactin (PRL), but not follicle-stimulating hormone, levels were increased. The distribution of immunostained neurons and fibers for gonadotropin-releasing hormone (GnRH), beta-endorphin, arginine vasopressin (AVP), and vasoactive intestinal polypeptide (VIP) resembled that of other rodent species. Short-day exposure reduced AVP staining in lateral septum, medial amygdala, and bed nucleus of the stria terminalis but not in the PVN of the thalamus or the SCN. Short-day-exposed hamsters had fewer beta-endorphin-positive arcuate nucleus cells and tended to have fewer GnRH-positive preoptic cells than long-day controls. VIP staining was unaffected by photoperiod. Most day length effects on immunostaining were eliminated by either lesion. These results establish the importance of the SCN and PVN in the photoperiodic control of several seasonal responses in Siberian hamsters.

Analysis of Variance↗

PubMed1990

Effects of the opiate antagonists diprenorphine and naloxone and of selected opiate agonists on feeding behavior in guinea pigs.

Opiate-sensitive feeding behavior has now been demonstrated in a number of species. We sought information on which opioid receptors might be involved in the observed feeding behaviors. Guinea pigs are known to have higher concentrations of the opioid kappa receptor than any other laboratory animal, so we compared the feeding suppressive potency of the general opiate antagonist, diprenorphine to that of the relatively more mu-specific antagonist, naloxone in that species. We found that diprenorphine was over twenty times more effective than naloxone in suppressing feeding in guinea pigs, suggesting the importance of receptors other than mu in feeding initiation in the guinea pig. Confirmatory evidence for the role of kappa receptors was sought, but not found, in comparisons of the effectiveness of different types of opiate agonists in promoting feeding in these animals. These agonists suppressed, rather than stimulated feeding. We conclude that no feeding stimulatory effects of opiates can be demonstrated in guinea pigs. This observation may indicate that opioids play little role in the natural regulation of feeding in this species or that opioids result in prolonged sedation during which the animals fail to eat. The greater feeding suppressive potency of diprenorphine, a general opiate antagonist, versus naloxone, a mu-preferential antagonist, indicates that to whatever extent opiates are involved in guinea pig feeding, the opiate effect is probably not a mu receptor effect.

Animals↗

PubMed1990

Species-specific changes in the metabolic control of food intake: integrating the animal with its environment.

Research on the metabolic control of feeding has been almost exclusively restricted to the study of laboratory rats. This poses no problem if all animals respond to metabolic challenges similarly; however, they do not. Unlike rats, Syrian, Turkish, and to a lesser degree, Siberian hamsters do not increase their food intake following food restriction. It has been argued that hamsters do not increase their food intake following a fast in laboratory tests because in their natural habitat they presumably would have a hoard of food in their burrows to buffer them from any shortfalls in the food supply above ground. An alternative hypothesis is proposed based on the self-imposed fast that occurs during a bout of hibernation in hamsters and the subsequent absence of compensation of food intake upon arousal. This fixed rate of feeding during the hibernation season may facilitate utilization of their food cache. Therefore, it may be that the failure to increase feeding following an experimenter-induced fast in the laboratory is due to the expression of this fixed pattern of feeding that normally accompanies hibernation. The relationship between food restriction and reproductive status is also discussed in terms of species- and gender-specific responses. Hamsters also differ from rats in that they do not increase their food intake following severe alterations in carbohydrate metabolism (via acute injections of 2-deoxy-D-glucose (2DG) or insulin), lipid metabolism (via administration of the fatty acid oxidation blocker methyl palmoxirate (MP), or from alterations in both (via combined administration of MP and 2DG). Perhaps food intake in hamsters is controlled by the waxing and waning of inhibitory rather than stimulatory factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

Mammalian pineal melatonin: a clock for all seasons.

The central role of the pineal gland and its hormone melatonin (MEL) in mammalian photoperiodic responses is discussed in terms of: 1) evidence for the involvement of MEL in photoperiodism, 2) which feature of the MEL secretion profile might be most important for regulating photoperiodic responses, 3) evidence for the modulation of responses to changes in daylength based on previous photoperiod exposure (i.e., photoperiodic history) and 4) how the MEL signal might be processed at its target sites to elicit physiological responses.

Animals↗

PubMed1989

Control of torpor and body weight patterns by a seasonal timer in 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)

Animals↗