ABSTRACT
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. All peptides were infused via the third ventricle (i.c.v.). Food intake was measured, and the proportion of time spent interacting with food, active or resting was scored. TRH (5 microg) significantly reduced food intake without producing associated changes in activity in hamsters maintained in both LP (p < 0.001) and SP (p < 0.05). A lower dose of TRH (0.5 microg) only decreased feeding significantly (p < 0.01) in hamsters exposed to SP, indicating that there may be an underlying difference in sensitivity to TRH depending on metabolic state. RX77368 (1 microg) produced substantial hypophagia (p < 0.001) and decreased the proportion of time spent interacting with food, but, unlike TRH, may produce this via an increase in locomotor activity. TRH-Gly (5 microg) produced a small decrease in food intake (p < 0.05), lasting for 6 h. We conclude that TRH and TRH analogues possess anorexigenic capacities in this species, with a likely site of action in the hypothalamus. Increased sensitivity to the hypophagic effects of central TRH may contribute to the long-term catabolic state induced by short photoperiods.