Photoperiodic responses are modulated by voluntary exercise in Djungarian hamsters (Phodopus sungorus)

Introduction: In response to short day (SD) lengths, Phodopus sungorus reduces body mass, size and activity of the gonads and changes into a whitish winter pelage. In previous studies we reported a reversal of the seasonal reduction in body mass and an impairment of seasonal gonadal regression in Phodopus with access to a running wheel (RW) in SDs. However, the seasonal change to a whitish winter fur was not affected by RW activity, suggesting that the SD signal can be read, but is overridden by a physiological response to RW activity. To understand how RW activity influences the occurrence of SD physiology, we examined its effect on physiological parameters and genes regulated by photoperiod that are important for the seasonal adaptation of Phodopus sungorus.

Methods: Expt 1. The effect of photoperiod on the activity pattern of Phodopus with and without access to a RW was recorded by infrared motion detectors under natural photoperiod and ambient temperature. Expt 2. Hamsters were housed in artificial SD for 8 weeks with or without a RW. At the end of the experiment food intake was measured before the hamsters were killed. Liver glucose and lipids were determined by a colorimetric assay. VGF, DIO3 and SRIF mRNA levels in the dmpArc, ependymal layer of the third ventricle and ARC, respectively, were quantified by in situ hybridization. The effect of photoperiod and activity on growth hormone (GH) was determined by Northern blot analysis, carried out on RNA extracted from pituitary glands.

Results: Expt 1. In exercising hamsters the annual activity pattern shows an entrained and well-defined rhythm throughout the year. However, hamsters without a RW show a tendency towards an indistinct rhythm during times of decreasing photoperiod in autumn and winter months. Expt 2. With access to a RW, the typical SD decrease in testes and body mass is attenuated and food intake is nearly doubled. SRIF gene expression is decreased and VGF gene expression is further increased by RW activity in SD, whereas DIO3 gene expression is not affected. Liver glucose levels are increased by RW activity and GH expression in the pituitary is increased in SD.

Conclusions: The annual activity pattern and DIO3 gene expression of the RW hamsters demonstrates that the SD signal is perceived, but the hamsters show an altered physiological response. Expressions of VGF and SRIF are affected by RW activity. The latter may contribute to an increased release of GH to increase body mass, and an increase in GnRH might cause the delay of testes regression.