Hypothalamic thyroid hormone catabolism acts as a gatekeeper for the seasonal control of body weight and reproduction.

Seasonal adaptations in physiology exhibited by many animals involve an interface between biological timing and specific neuroendocrine systems, but the molecular basis of this interface is unknown. In this study of Siberian hamsters, we show that the availability of thyroid hormone within the hypothalamus is a key determinant of seasonal transitions. The expression of the gene encoding type III deiodinase (Dio3) and Dio3 activity in vivo (catabolism of T(4) and T(3)) is dynamically and temporally regulated by photoperiod, consistent with the loss of hypothalamic T(3) concentrations under short photoperiods.

Photoperiodic regulation of cellular retinol binding protein, CRBP1 [corrected] and nestin in tanycytes of the third ventricle ependymal layer of the Siberian hamster.

Tanycytes in the ependymal layer of the third ventricle act both as a barrier and a communication gateway between the cerebrospinal fluid, brain and portal blood supply to the pituitary gland. However, the range, importance and mechanisms involved in the function of tanycytes remain to be explored. In this study, we have utilized a photoperiodic animal to examine the expression of three unrelated gene sequences in relation to photoperiod-induced changes in seasonal physiology and behaviour.

Role of melanocortin in the long-term regulation of energy balance: lessons from a seasonal model.

Siberian hamsters express photoperiod-regulated seasonal cycles of body weight and food intake, providing an opportunity to study the role of melanocortin systems in regulating long-term adaptive changes in energy metabolism. These hamsters accumulate intraperitoneal fat reserves when kept in long summer photoperiods, but show a profound long-term decrease in food intake and body weight when exposed to a short winter photoperiod. Icv administration of a MC3/4-R agonist (MTII) potently suppresses food intake in hamsters in both the obese and lean state, indicating the potential for melanocortin systems to regulate energy metabolism in the hypothalamus of the Siberian hamster.

Photoperiodic regulation of histamine H3 receptor and VGF messenger ribonucleic acid in the arcuate nucleus of the Siberian hamster.

To survive winter the Siberian hamster has evolved profound physiological and behavioral adaptations, including a moult to winter pelage, regression of the reproductive axis, onset of daily torpor and increased capacity for thermogenesis. However, one of the most striking adaptations is the catabolism of intraabdominal and sc fat reserves contributing to the loss of up to 40% of body weight. These physiological and behavioral adaptations are photoperiodically driven, yet neither the site(s) in the brain nor the molecular mechanism(s) involved in the regulation of these profound adaptations is known.

Feeding and behavioural effects of central administration of the melanocortin 3/4-R antagonist SHU9119 in obese and lean Siberian hamsters.

Siberian hamsters accumulate fat reserves in long photoperiods, but show a long-term decrease in food intake and body weight when exposed to a short winter photoperiod. The aim of this study was to determine the role of central melanocortin 3/4 receptors (MC3/4-R) in generating this chronic catabolic state by investigating the effects of SHU9119, a MC3/4-R antagonist, on food intake and associated behaviours. In adult male hamsters, intra-cerebroventricular infusions of SHU9119 significantly increased food intake in a dose-dependent manner.

Photoperiodic regulation of hypothalamic retinoid signaling: association of retinoid X receptor gamma with body weight.

This study reports novel events related to photoperiodic programming of the neuroendocrine hypothalamus. To investigate photoperiod-responsive genes, Siberian hamsters were maintained in long or short photoperiods that generate physiological states of obesity or leanness. Microarray expression analysis first identified CRBP1 as a photoperiod-responsive gene, and then further studies using in situ hybridization and immunocytochemistry revealed that expression levels of several related retinoid-signaling genes were modulated in response to photoperiod changes.

Central administration of thyrotropin releasing hormone (TRH) and related peptides inhibits feeding behavior in the Siberian hamster.

Centrally acting thyrotropin releasing hormone (TRH), independent of endocrine action, has been shown to regulate several metabolic and behavioral parameters in rats, including food intake and locomotor activity. The present study investigated and compared the effects of central TRH on feeding behavior in Siberian hamsters exposed to long (LP) or short (SP) photoperiods, which induce natural physiological states of obesity and leanness respectively. The effects of two TRH analogues, RX77368 (a metabolically stable TRH analogue) and TRH-Gly (an endogenous precursor to TRH with putative preferential action at the central TRH receptor, TRH-R2), were also investigated.

Decrease of food intake by MC4-R agonist MTII in Siberian hamsters in long and short photoperiods.

We investigated the role of the hypothalamic melanocortin system in the regulation of food intake in the Siberian hamster, which shows a profound seasonal decrease in food intake and body weight in short photoperiod (SP). In male hamsters maintained in long photoperiod (LP), intracerebroventricular injection of melanotan II (MTII) just before lights off significantly decreased food intake relative to vehicle treatment over the 6-h observation period. Similar effects were observed in age-matched hamsters after exposure to a short daylength for 9 wk, when body weight had significantly decreased.

Entrainment of locomotor activity rhythm in pinealectomized adult Syrian hamsters by daily melatonin infusion.

Melatonin entrains circadian rhythms in several species of rodents, but a role for melatonin as a Zeitgeber in the adult Syrian hamster is debated. The aim of this study was to define the conditions of daily programmed melatonin infusion in which an entrainment of the locomotor activity rhythm is obtained in adult male Syrian hamsters. The animals were pinealectomized, cannulated with a subcutaneous infusion system and submitted to dim red light conditions.