Role of Arcuate AgRP Neurons in Ghrelin Action on Food Reward
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Media coverage of the current obesity epidemic often ascribes stress as a major cause of excessive food intake in humans, with a particular increase in calorically dense 'comfort foods.' In the current study, the hormonal and neural link between stress and feeding behavior was further investigated using behavioral tests to monitor the rewarding effects of high fat diet in a novel transgenic mouse model. In particular, the octanoylated peptide hormone, ghrelin, plays a key role in appetite regulation as well as a behavioral role in the stress response. Synthesized and secreted by a small population of gastric epithelial cells, ghrelin is the only circulating hormone known to stimulate appetite. Importantly, ghrelin levels are significantly and persistently elevated in response to chronic stress. Also, ghrelin signaling enables the development of stress-induced conditioned place preference (CPP) for high fat diet, a reward-based eating behavior, and ghrelin minimizes stress-induced depressive-like behavior. The ghrelin receptor, known as the growth hormone secretagogue receptor (GHSR), is highly expressed in hypothalamic brain regions involved in homeostatic feeding, such as the arcuate nucleus (Arc). Preliminary studies in the lab indicate that acute ghrelin-induced feeding, which is not observed in GHSR-deficient (GHSR-null) mice, is partially restored in mice with GHSR expression only in Arc AgRP neurons. We now hypothesize that the GHSRs present on Arc AgRP neurons are sufficient to mediate the stress-induced development of food reward behavior in mice. To test this hypothesis, the lab has generated a novel, genetically engineered mouse line in which GHSR expression is limited only to Arc AgRP neurons. Such is possible due to a tamoxifen-inducible Cre recombinase, which in turn removes a loxP-flanked transcriptional blocking cassette from an altered GHSR gene, thus allowing tamoxifen-dependent, AgRP neuron-selective reactivation of GHSR expression. For these studies, littermates of 4 genotypes were used: wild-type mice, wild-type mice expressing Cre recombinase, GHSR-null mice, and mice with GHSRs expressed only in AgRP neurons, as described above. All mice were treated with tamoxifen for five days to activate Cre recombinase, allowed to recover for three weeks and monitored for ad-lib food intake for the last nine days of recovery. Mice were then subjected to a ten-day chronic social defeat stress (CSDS) protocol and afterward tested for depressive-like behavior using the social interaction (SI) test. Following the SI test, mice underwent a CPP for HFD protocol, which includes a pretest, twelve days of conditioning, and a test day. Data for an initial cohort of animals has been obtained and analyzed, and studies with additional cohorts are ongoing. The new mouse model with GHSRs expressed only in the Arc will assist in determining whether AgRP neurons in the Arc are sufficient to mediate ghrelin's effects on stress-induced food reward-behavior and stress-associated depressive-like behavior. Further work may include a similar tamoxifen-inducible, Cre-recombinase system expressing GHSRs only in the VTA or only in the hippocampus to narrow down where ghrelin has its food reward behavior effects.