Ghrelin and orexin interact to increase meal size through a descending hippocampus to hindbrain signaling pathway

BACKGROUND: Memory and cognitive processes influence the amount of food consumed during a meal, yet the neurobiological mechanisms mediating these effects are poorly understood. The hippocampus (HPC) has recently emerged as a brain region that integrates feeding-relevant biological signals with learning and memory processes to regulate feeding. We investigated whether the gut-derived hormone ghrelin acts in the ventral HPC (vHPC) to increase meal size through interactions with gut-derived satiation signaling. METHODS: Interactions between vHPC ghrelin signaling, gut-derived satiation signaling, feeding, and interoceptive discrimination learning were assessed via rodent behavioral neuropharmacological approaches. Downstream neural pathways were identified using transsynaptic virus-based tracing strategies. RESULTS: vHPC ghrelin signaling counteracted the food intake-reducing effects produced by various peripheral biological satiation signals, including cholecystokinin, exendin-4 (a glucagon-like peptide-1 receptor agonist), amylin, and mechanical distension of the stomach. Furthermore, vHPC ghrelin signaling produced interoceptive cues that generalized to a perceived state of energy deficit, thereby providing a potential mechanism for the attenuation of satiation processing. Neuroanatomical tracing identified a multiorder connection from vHPC neurons to lateral hypothalamic area orexin (hypocretin)-producing neurons that project to the laterodorsal tegmental nucleus in the hindbrain. Lastly, vHPC ghrelin signaling increased spontaneous meal size via downstream orexin receptor signaling in the laterodorsal tegmental nucleus. CONCLUSIONS: vHPC ghrelin signaling increases meal size by counteracting the efficacy of various gut-derived satiation signals. These effects occur via downstream orexin signaling to the hindbrain laterodorsal tegmental nucleus, thereby highlighting a novel hippocampus-hypothalamus-hindbrain pathway regulating meal size control.