Biomedical subjects

I Zucker

Publications and source records attributed to I Zucker.

At least 55 records · Page 3Linked to original sources

PubMed1995

Long-term reproductive effects of a single long day in the Siberian hamster (Phodopus sungorus).

Testicular regression was prevented or attenuated in Siberian hamsters exposed to a single 1- to 4-h extension of the 16-h photophase at 18 days of age and subsequently maintained in a short photoperiod (8L:16D) through Day 35. Testicular weights on Day 35 were not correlated with the duration of the active phase of wheel running or with the time of activity onset after transfer to the 8L:16D photoperiod. Wheel-running activity was not stably entrained to the light-dark cycle by 35 days of age. Progonadal effects of a single 33-h light pulse were greatest at 18 days of age, still evident at 30 days, but undetectable in older hamsters. In female hamsters, a single longer day at weaning was associated with increased fecundity several weeks later. Long photoperiods accelerated development of antral ovarian follicles, but exposure to males was necessary to induce ovulation before 60 days of age. The interval beginning shortly after weaning is one of heightened responsiveness to changes in day length (DL); exposure to increasing DL at this time may prolong the breeding season when DL decreases after the summer solstice. We suggest that the long-term effects of acute light treatments on reproduction are mediated by sustained changes in melatonin secretion induced by reprogramming of circadian oscillators.

Aging↗

PubMed1995

Seasonal variations in circadian rhythms persist in gonadectomized golden-mantled ground squirrels.

Circadian wheel-running activity was monitored in intact and gonadectomized female and male golden-mantled ground squirrels maintained in a 14L:10D photoperiod at 21 degrees C. Marked advances in phase angle of activity onset and increases in the duration of the daily active phase occurred each year in intact animals coincident with the phase of reproductive activity. Surprisingly, long-term gonadectomized squirrels of both sexes manifested similar seasonal changes in the circadian system. Seasonal circadian variations in locomotor activity are generated by a circannual clock independent from and not influenced by concurrent changes in secretion of gonadal hormones.

Animals↗

PubMed1994

Melatonin binding sites in sciurid and hystricomorph rodents: studies on ground squirrels and guinea pigs.

Little is known about the distribution of binding sites for the pineal hormone melatonin in non-myomorph rodents. We used 2-[125I]iodomelatonin (IMEL) to analyze the distribution, affinity, and specificity of binding sites in the golden-mantled ground squirrel, a sciurid rodent that reportedly lacks IMEL binding sites in the brain. Specific binding was found not only in the pars tuberalis, but also in several telencephalic and diencephalic areas including the hypothalamic suprachiasmatic region. The affinity and specificity of IMEL binding are comparable to those reported in other rodents. IMEL binding studies in a hystricomorph rodent, the guinea pig, revealed high concentrations of receptor in the nucleus accumbens and dorsolateral thalamus. Central melatonin binding sites have now been demonstrated in species of all three rodent families. The heterogeneous distribution of melatonin receptors appears similar in the species studied, and no evidence is found to link IMEL binding sites at any particular locus to photoperiodic, circannual, or non-seasonal breeding patterns.

Animals↗

PubMed1994

Gonadal growth and hormone concentrations in photoregressed Siberian hamsters: pinealectomy versus photostimulation.

In Siberian hamsters, long and short durations of nightly melatonin secretion are associated with testicular regression and development, respectively; surprisingly, the absence of a nightly melatonin signal induced by pinealectomy is less effective than a short (4-6 h) melatonin duration for promoting gonadal development. We compared the patterns of serum FSH and prolactin concentrations underlying pinealectomy-induced and photostimulated testicular growth in juvenile hamsters. Pinealectomy in photoinhibited hamsters on Day 18 produced significant gonadal development compared to that in short-day controls by Day 23. By Day 28, however, testes weights of pinealectomized hamsters were significantly lower than those of intact hamsters transferred to a long photoperiod. Pinealectomy was associated with significant elevations in serum FSH and prolactin concentrations over baseline by Day 21, but peak values were attenuated compared with those induced by exposure to long days. FSH titers of pinealectomized animals were significantly lower than those of photostimulated animals between Days 21 and 30 but were significantly greater on Days 35 and 40; prolactin values were comparable in pinealectomized and photostimulated hamsters between Days 21 and 25 but were significantly lower in the former group thereafter. Blunted elevation of FSH and prolactin secretion may underlie the slower rates of gonadal maturation observed in pinealectomized, juvenile hamsters. The neuroendocrine systems of pinealectomized hamsters appear to be in a state intermediate between those characteristic of hamsters kept in long and in short day lengths.

Animals↗

PubMed1994

Reduced glucose availability induces torpor in Siberian hamsters.

Siberian hamsters kept in long photoperiods (16 h light/day) at 10 degrees C do not display torpor when provided with food ad libitum but do reduce their body temperatures (Tb) from 37 degrees C to as low as 15 degrees C for several hours each day in response to food restriction. Female hamsters maintained in a long photoperiod and fed ad libitum were injected with 2-deoxy-D-glucose (2-DG), a glucose analogue that reduces cellular glucose oxidation, or mercaptoacetate (MA), a drug that disrupts fatty acid oxidation. Dosages of 2-DG in excess of 1,500 mg/kg body mass reliably induced torpor with latencies of approximately 50 min from time of injection to a decrease in Tb below 30 degrees C; 79% of females displayed torpor at a dosage of 2,500 mg/kg. MA was uniformly ineffective in inducing torpor and did not increase incidence of torpor when combined with 2-DG treatment or food restriction. Decreased availability of glucose may be the major metabolic signal for inducing torpor in Siberian hamsters; availability of fatty acids appears to play little if any role in regulating this behavior.

Animals↗

PubMed1993

Olfactory bulb removal lengthens the period of circannual rhythms and disrupts hibernation in golden-mantled ground squirrels.

Removal of the olfactory bulbs lengthened the period of circannual rhythms (CARs) of body mass and plasma testosterone (T) concentrations in male golden-mantled ground squirrels, but did not otherwise alter their expression. The period of the CAR was approximately 2 months longer in bulbectomized than in sham-operated animals. Peak values of body mass and T were unaffected by bulbectomy. All neurologically intact, but only 50% of bulbectomized, squirrels displayed normal hibernation patterns. We conclude that the olfactory bulbs are not the site of essential circannual oscillators. Circannual reproductive and hibernation cycles of ground squirrels are less subject to modulation by the olfactory bulbs than are the corresponding rhythms of several non-circannual hamster species.

Animals↗

PubMed1993

Sustained hormonal responses of Siberian hamsters (Phodopus sungorus) to a single longer day at weaning.

Siberian hamsters undergo gonadal development for several weeks after exposure to a single longer day at weaning. To characterize changes in gonadotropin secretion after a single acute light stimulus, hamsters housed in a long photoperiod (16L:8D) were given a single longer day (20L:4D) or maintained in the 16L:8D photoperiod at 19 days of age and transferred to a short photoperiod (8L:16D) on Day 20. Elevated plasma FSH concentrations were detected in male hamsters at 5, 7, and 12 but not at 17 days after the single longer day. Melatonin treatment during light exposure and on two succeeding nights blocked the stimulatory effect of light on the reproductive axis; melatonin injections limited to one night were marginally effective. Pinealectomy during the dark phase of the photocycle and the resultant truncation of the melatonin signal for one night did not stimulate a greater degree of gonadal development than pinealectomy during the light phase. We conclude that the single extra 4-h light pulse at weaning alters hypothalamic-pituitary function for approximately 2 wk. Trophic effects of the light pulse appear to be mediated by suppression of melatonin secretion for several days; one truncate melatonin signal is not sufficient to simulate the effects of a single long day on the reproductive axis.

Animals↗

PubMed1993

Prolactin and testosterone inhibit torpor in Siberian hamsters.

Female Siberian hamsters maintained in a winter photoperiod (8 h light/day) ceased to undergo daily torpor during infusion of prolactin (PRL) from osmotic minipumps; winter torpor was reinstated within 3 days of discontinuation of treatment. By contrast, PRL infusion was ineffective in suppressing daily torpor elicited by restricting food intake in female hamsters housed in a summer photoperiod (16 h light/day). Summer daily torpor was, however, completely inhibited in long-day gonadectomized male hamsters treated with testosterone (T). We suggest that the hyperprolactinemia, which in previous studies is characteristic of hamsters that sustain ablation of the suprachiasmatic nucleus, is incompatible with winter torpor. Summer torpor may be controlled by a mechanism less responsive to variations in plasma PRL concentration. Both winter and summer torpor are inhibited by exogenous T; it remains uncertain, however, whether sustained decreases in endogenous T secretion are as essential for the expression of summer as they are for winter torpor.

Animals↗

PubMed1993

Torpor shortens the period of Siberian hamster circadian rhythms.

We investigated the influence of ambient and body temperature (Ta and Tb) on circadian rhythms of gonadectomized male Siberian hamsters. Animals that entered torpor (Tb < 30 degrees C) had significantly shorter circadian periods (tau s) than did nontorpid hamsters at a Ta of 13 degrees C (24.17 +/- 0.05 vs. 24.33 +/- 0.04 h). The tau s of homeothermic hamsters were not affected by Ta change. Short-term decreases in Tb, rather than changes in Ta, appear to affect tau. Access to activity wheels inhibited expression of torpor in short daylengths and was associated with significant increases in body mass. Running wheel activity can mask or block specific short-day responses.

Animals↗

PubMed1992

Accelerated reproductive development in juvenile male ground squirrels fed a high-fat diet.

Male golden-mantled ground squirrels held at 23 degrees C were fed high-fat (HF) or standard (chow) diets. In December, ambient temperature was reduced to 6 degrees C, food was removed, and frequency and duration of torpor bouts were monitored continuously by radiotelemetry. Reproductive condition and body composition were assessed upon terminal arousal in the spring. Juvenile males fed the HF diet weighed more than chow-fed controls before and throughout the hibernation season and had significantly greater lipid masses at terminal arousal. Testes masses and plasma testosterone concentrations were substantially higher in HF than in chow-fed juveniles. The accelerated reproductive development of fatter squirrels was not contingent upon increases in the total number of days spent in torpor, number of torpor bouts, or the average duration of each arousal from torpor. Access to the HF diet had no effect on body mass, adiposity, or reproductive status of adult male ground squirrels in spring. Threshold levels of white adipose tissue and associated differences in availability of metabolic fuels may be permissive for testicular growth during the hibernation season. Juveniles exceed this threshold only when fed the HF diet.

Aging↗

PubMed1992

Estradiol phase-shifts circannual rhythms of golden-mantled ground squirrels.

Ovariectomized golden-mantled ground squirrels (Spermophilus lateralis) were implanted with estradiol benzoate (EB)-filled or empty capsules. Body weight was monitored for at least 1 yr before, during, and after hormone treatment. EB treatment of 6-7 mo duration, restricted to the weight-gain phase, decelerated weight gain and delayed attainment of peak weight by 34 days; the period of the circannual rhythm, measured between successive trough weights, was 36 days longer in squirrels treated with estradiol than in untreated animals. EB treatment of 2- to 4-mo duration, restricted to the weight-loss phase, accelerated weight loss, advanced attainment of trough weights by 57 days, and decreased the period of the circannual rhythm by 66 days. Estradiol-induced phase delays and advances were preserved in the year after discontinuation of hormone treatment. These results favor the existence of circannual phase-response curves to hormones and provide a system for pharmacological manipulation of circannual rhythms.

Animals↗

PubMed1992

Daily torpor in the absence of the suprachiasmatic nucleus in Siberian hamsters.

Siberian hamsters express torpor spontaneously after several weeks of exposure to short days. In long days, torpor is expressed only when food intake is restricted. Hamsters maintained in a long photoperiod (16 h light/day) at 15 degrees C expressed daily torpor during food restriction both before and after bilateral ablation of the suprachiasmatic nucleus (SCN). Hamsters housed in short days (8 h light/day, ambient temperature 15 degrees C) and fed ad libitum displayed torpor before, but not after, ablation of the SCN (SCNX). Torpor was reinstated in all short-day SCNX hamsters during postoperative food restriction and persisted in several animals even after ad libitum feeding was reinstated. Torpor was entrained to the light-dark cycle in both long- and short-day hamsters preoperatively but appeared to occur in a temporally random fashion in SCNX animals. SCNX hamsters, unlike control animals, displayed multiple torpor bouts per 24 h. The SCN is not essential for the expression of torpor but plays a crucial role in its temporal organization.

Animals↗

PubMed1992

Influence of gonadal hormones on odours emitted by male meadow voles (Microtus pennsylvanicus).

Free-living male meadow voles (Microtus pennsylvanicus) emit odours that are attractive to females at the beginning, but not at the end, of the breeding season. The effect of gonadal hormones on female-attractant cues was examined in males born and reared in long (14 h light day-1) and short (10 h light day-1) photoperiods that simulate daylengths in the breeding and nonbreeding seasons, respectively. Gonadectomy affected the attractant properties of odours emitted by long photoperiod, but not short photoperiod, males. Long photoperiod females preferred odours of intact rather than those of gonadectomized long photoperiod males, and odours of gonadectomized long photoperiod males rather than those of intact short photoperiod males. Females did not show a preference between the odours of intact and castrated short photoperiod males. Gonadal hormone replacement in males affected female responses to the odours emitted by long photoperiod, but not short photoperiod, gonadectomized males. Long photoperiod females did not display a preference between odours of intact long photoperiod males and gonadectomized long photoperiod males treated with testosterone or oestradiol. We conclude that in spring and summer gonadal hormones increase attractiveness of male odours; this effect may require aromatization of testosterone to oestradiol. Substrates that control attractiveness of odour cues in male voles appear to be unresponsive to androgens during the nonbreeding season.

Animals↗

PubMed1991

Ovarian hormones influence odor cues emitted by female meadow voles, Microtus pennsylvanicus.

During the spring-summer breeding season female meadow voles emit odors that are preferred by males, whereas in the autumn-winter season of reproductive quiescence females emit odors that are not preferred by males, but are attractive to females. The effects of daylength and ovarian hormones on salience of female odors were determined by assaying male responses to odors. Females housed in long and short photoperiods transmitted odors that elicited responses similar to those of spring and autumn female voles, respectively. The odor cues emitted by ovariectomized (OVX) females, irrespective of photoperiodic history, were similar to those generated by females during the nonbreeding season. In the absence of ovarian hormones, long daylengths were not sufficient to induce females to broadcast the spring odors preferred by males. Spring-type odor cues were, however, emitted by OVX voles housed in either photoperiod and treated with estradiol. Ovarian hormones appear necessary and sufficient to generate breeding season odor cues and sufficient to induce production of such cues during the nonbreeding season. We conclude that daylength affects odor cues emitted by females by altering ovarian hormone activity.

Animal Communication↗

PubMed1991

Suprachiasmatic nucleus and photic entrainment of circannual rhythms in ground squirrels.

The efficacy of photoperiod as a zeitgeber for entrainment of circannual body weight and estrous rhythms was tested in female golden-mantled ground squirrels maintained for 3 or more years in either a simulated natural photoperiod (SNP) or a fixed LD 14:10 photoperiod (FP). The role of the retinohypothalamic tract--suprachiasmatic nucleus (RHT-SCN) projection in photic entrainment was assessed in animals that sustained destruction of the SCN (SCNX). Circannual rhythms were lengthened by the SNP as compared to the FP. Mean periods (tau's) for neurologically intact animals in the third year of testing were 49.6 +/- 0.3 weeks and 43.1 +/- 1.2 weeks (p less than 0.001) for the SNP and FP groups, respectively; furthermore, 56% and 7% of animals in these groups had tau's not significantly different from 365 days (p less than 0.005), and within-group variability was lower for SNP than for FP squirrels (p less than 0.01). SCNX squirrels differed from animals with the SCN intact (SCNC), as evidenced by higher within-group variability (p less than 0.001); only 29% of SCNX squirrels had tau's not different from 365 days (p less than 0.03 compared to the SCNC group). The coupling between estrous and body weight rhythms that was evident in SCN-intact SNP and FP squirrels was disrupted in SCNX animals. The RHT-SCN pathway is implicated in entrainment and in maintenance of normal phase relations among the several circannual rhythms. In a second experiment, female squirrels were maintained for 2.5 years in an accelerated SNP that compressed two normal annual photocycles into each calendar year. Of 12 squirrels, 3 had tau's that did not differ significantly from 6 months; 6 had tau's equivalent to 12 months; and 3 had tau's significantly different from both 6 months and 12 months. The data suggest that photoperiod is a major zeitgeber for entrainment of golden-mantled ground squirrels circannual rhythms.

Animals↗

PubMed1991

Seasonal control of odour preferences of meadow voles (Microtus pennsylvanicus) by photoperiod and ovarian hormones.

During the spring-summer breeding season, female meadow voles prefer odours of males over those of females, but in the autumn-winter season of reproductive quiescence this preference is reversed. Females housed in long (14 h light/day) and short (10 h light/day) photoperiods, respectively, had odour preferences comparable to those of spring and autumn voles, respectively. The preference of long-photoperiod voles for male over female odours was reversed by ovariectomy and restored by treatment with oestradiol. By contrast, neither ovariectomy nor oestradiol affected odour preferences of short-photoperiod voles. Long days appear to influence olfactory preferences by altering ovarian hormone secretion. The failure of oestradiol to affect odour preferences in short photoperiods suggests that the neural substrates mediating this behavioural response are refractory to oestrogens during the nonbreeding season.

Animals↗

PubMed1990

Long day lengths enhance myelination of midbrain and hindbrain regions of developing meadow voles.

Rates of brain growth differed in meadow voles maintained in long (LP) or short (SP) photoperiods postnatally. At 35 days of age, brain weight was greater by 6.6% in LP males and by 4.7% in LP females as compared to their SP counterparts. Whole brain galactolipid content, an index of brain myelin, was greater by 15.6% in LP as compared to SP males. At 70 days of age, brains of LP males were 4% heavier than those of SP males. Differences attributable to photoperiod were most pronounced in midbrain and hindbrain (8% and 14%, respectively). DNA and galactolipid contents were greater by 11% and 15%, respectively, in hindbrain of LP males. Photoperiod did not affect any of these measures in diencephalon, striatum, or cerebellum. Short day lengths reduce myelination in meadow voles, presumably by decreasing proliferation rates of oligodendroglia. This is one facet of a general delay in somatic development associated with being born at the end of the normal breeding season when day lengths are decreasing or below a critical threshold.

Animals↗

PubMed1990

Long day lengths promote brain growth in meadow voles.

Male meadow voles kept in a long photoperiod (LP) from birth to 70 days of age have heavier brains than those kept in a short photoperiod (SP). Brain weights of male voles kept in the LP first exceeded those of SP animals at 20 days of age; differences were greatest at 35 days (5.8%) and persisted through 140 days of age (2%), although the magnitude of the difference declined progressively. Accelerated compensatory increases in brain weight were observed in voles transferred from the SP to the LP at 70 days of age. Total brain DNA content, an index of cell number, was not significantly affected by initial or final photoperiod, although it increased 7.8% within 70 days after voles were transferred from the SP to the LP. Brain weights (but not DNA content) of males exceeded those of females, but this sex difference was present only in the LP. We suggest that short day lengths retard brain development by reducing rates of myelination and possibly reducing cell size as well; this is part of a general retardation of somatic growth associated with a delayed onset of puberty that can be reversed by a stimulatory LP but, ordinarily, occurs spontaneously as voles become refractory to short day lengths.

Aging↗