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In many seasonally breeding rodents, reproduction and metabolism are activated by long summer days (LD) and inhibited by short winter days (SD). After several months of SD, animals become refractory to this inhibitory photoperiod and spontaneously revert to LD-like physiology. The suprachiasmatic nuclei (SCN) house the primary circadian oscillator in(More)
The tau mutation is a semi-dominant autosomal mutation which, in homozygotes, accelerates the period of the circadian activity cycle by approximately 4 h. In mammals, the circadian system contributes to seasonal photoperiodic time measurement by generating a repeated daily melatonin signal during the hours of darkness. Our earlier studies suggest an altered(More)
Seasonal Siberian hamsters lose fat reserves, decrease body weight and leptin concentrations, and suppress reproduction on short-day photoperiod (SD). Chronic leptin infusion at physiological doses caused body weight and fat loss in SD animals but was ineffective in long-day (LD) hamsters. Using ovariectomized estrogen-treated females, we tested the(More)
Syrian hamsters exhibit a marked seasonal variation in prolactin secretion. The aim of this study was to analyse the nature of the photoperiodic regulation of prolactin gene expression, and to define the role of melatonin and the pars tuberalis of the anterior pituitary in this process. Pituitary prolactin gene expression, restricted to the pars distalis,(More)
The tau mutation of Syrian hamsters induces a robust reduction in the period of circadian activity rhythms, from 24 h (wild-type; tau++) to 22 h (heterozygote; tauS+) and 20 h (homozygous mutant, tauSS). Here, we examine the effect of this mutation on circadian rhythms of LH, melatonin, and cortisol in ovariectomized hamsters. Free running circadian rhythms(More)
Leptin may play a role in appetite regulation and metabolism, but its reproductive role is less clear. In photoperiodic Siberian hamsters, seasonal changes in fatness, leptin gene expression, and metabolism occur synchronously with activation or suppression of reproduction, analogous to puberty. Here, we test the hypothesis that seasonal changes in leptin(More)
This study investigated the role of the circadian timing system (CTS) in photoperiodic time measurement by examining the response of the tau mutant hamster to programmed infusions of melatonin. The mutation is a single Mendelian gene defect which accelerates circadian period from 24 h in the wild-type (WT) to 20 h in the homozygote. If the CTS does not(More)
Oxytocin secretion is inhibited by opioids, and oxytocin is important in parturition. The effects on parturition of morphine, a relatively selective mu-opioid receptor agonist, were studied in the rat. Morphine or vehicle with or without the opiate antagonist naloxone were administered immediately after the birth of the second pup and the subsequent course(More)
The tau mutation of the Syrian hamster is one of only two mutations currently known to affect the circadian system in mammals. In order to investigate the molecular mechanisms associated with this mutation, we have compared the level of expression of the mRNA for vasoactive intestinal peptide (VIP) in the suprachiasmatic nuclei (SCN) and cerebral cortex of(More)
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