Multiple loci of intra- and extra-hypothalamic leptin receptor expression and action in the mouse

The increasing incidence of obesity in developed nations represents an ever-growing challenge to health care by augmenting the collateral occurrences of diabetes, and other chronic conditions. The discovery of the satiety hormone, leptin, over a decade ago has facilitated the acquisition of new knowledge regarding the regulation of energy homeostasis. A great deal remains to be discovered regarding the specific actions of leptin at different regions within the brain, however. Our overarching goals were to develop a reliable method to detect long form leptin receptor (LepRb), to generate a comprehensive inventory of sites of LepRb-expressing cells in the brain and to characterize leptin action at two such sites. Here, we first define central sites of LepRb expression in vivo using "reporter" mice, in which LepRb- neurons express the fluorescent marker, enhanced green fluorescent protein (EGFP). We identified LepRb expression in neuronal populations both within and outside the hypothalamus and further characterized two such LepRb populations, those within the midbrain ventral tegmental area and linear raphe (VTA/RLi), and LepRb cells within the ventral premammillary nucleus of the hypothalamus (PMv). We used a novel tract tracing system combined with standard techniques to determine regions innervated exclusively by LepRb VTA, LepRb RLi or LepRb PMv neurons, thus revealing LepRb VTA neuronal projections to regions of the amygdala, LepRb RLi projections to amygdala and nucleus accumbens (NAc), and LepRb PMv projections to the preoptic area (POA). We have further shown the retrograde accumulation in LepRb PMv neurons of a trans-synaptic tracer from GnRH neurons, revealing the direct innervation of gonadotropin releasing hormone (GnRH) neurons by many LepRb PMv neurons. Additionally, we demonstrate activation of this population by leptin and opposite-sex odors. Thus, LepRb VTA/RLi neurons are well positioned to couple energy balance cues with the mesolimbic dopamine reward circuit, while LepRb PMv neurons unite metabolic and sexual odorant cues and directly innervate GnRH neurons to regulate the reproductive axis. Finally, in preliminary studies representing future analyses of the functional roles of LepRb PMv cells we delete Lepr in nitric oxide synthase-1 containing cells and observe a delay in estrus onset as well as delayed onset obesity. Understanding the regulation and physiological function of these and other sites of central leptin action will be a crucial advancement in the quest to understand mechanisms of leptin action on physiology.