Identification and characterization of neuroendocrine pathways involved in the regulation of seasonal body weight cycles

The present dissertation describes the identification and characteristics of neuroendocrine pathways involved in the regulation of seasonal body weight cycles. In different investigations, we provide new and supportive information on the central hypothalamic pathways mediating the effect of photoperiod and leptin in a seasonal animal, the Djungarian hamster. The first study (chapter II) focused on the distribution and characterisation of selected neuropeptides (CART, MCH, OXB) involved in the regulation of energy balance. There appears to be a neuroanatomical basis for CART-MCH-OXB neuronal system to indirectly influence the generation of circadian rhythmicity. The second study (chapter III) focused on determining a possible interaction between neurons containing neuropeptides (OXB) and the main pathway, the geniculohypothalamic tract arising from NPY-containing neurons in the IGL. In the IGL, there was apposition of OXB-ir terminals on NPY-ir neurons, indicating the existence of a feedback input from neuropeptides to the circadian timing system. The effect of seasonal acclimation on the distribution and expression of CART was investigated in chapter IV. Results of this study revealed increased numbers of CART within the rostral- and ventrolateral part of the ARC (peri-ARC). These regions are known to mediate the inhibitory effect of leptin on food intake in other species like rats and mice. We therefore focused our study in chapter V to identify hypothalamic structures mediating the effect of leptin in Djungarian hamsters. Results of this study revealed that leptin induces cellular activity (induction of fos) within several hypothalamic structures including the rostral- and ventrolateral part of the ARC (peri-ARC). Combined, these findings indicate that the ARC provides a central anatomic substrate for integration of both body fat and photoperiodic information in the Djungarian hamster. The final part of the thesis comprises a general discussion of the obtained results and future directions.