Seasonal mammals are characterised by an annual cycle of growth which is exemplified in small mammals by reversible changes in body mass including organ weights, a phenomenon first noted by Dehnel (Dehnel’s Phenomenon). To understand the mechanism by which seasonal mammals adapt growth to the seasonal changes in photoperiod we investigated the reversible growth exhibited by the Siberian hamster. Siberian hamsters in long photoperiod (16:8h light:dark, LD) or for 10 weeks in short photoperiod (8:16h light:dark, SD) before switching back to LD (SWB hamsters) were injected with SOM-230LAR (160mg/kg), a long lasting somatostatin agonist used for the treatment of acromegaly. In LD treated hamsters, SOM-230LAR reduced body weight similar to hamsters transferred from LD to SD photoperiod. The weight loss includes reduction in both lean and fat mass. The loss of lean tissue mass encompassed reductions in organ weights of kidneys, testes, brown and white adipose tissue with a concomitant reduction in circulating IGF-1. In SWB hamsters, SOM-230 inhibited the LD induction of growth with a concomitant reduction in IGF-1 and retardation of tissue mass increases. At the level of the hypothalamus, mRNA expression of photoperiod dependent genes were unchanged including tanycyte expression of type 2 and type 3 deiodinases, but reduced expression of Srif mRNA in the periventricular hypothalamus and increases in growth-hormone-releasing-hormone gene expression were evidence of an inhibition of growth hormone release and growth hormone feedback to the brain. In a separate experiment, SOM230LAR inhibited growth induced by exercise in SD hamsters, but did not affect testes recrudescence. In conclusion photoperiod regulation of the hypothalamic-pituitary-growth hormone axis is likely to be a key element in seasonally induced changes in body mass in the Siberian hamster.
Scottish Government Rural and Environment Science and Analytical Services Division;
British Society for Neuroendocrinology research travel grant to RD