Philip J. Ryan

Chemogenetic activation of oxytocin receptor‐expressing neurons in the parabrachial nucleus (Oxtr PBN neurons) acts as a satiation signal for water. In this research, we investigated the effect of activating Oxtr PBN neurons on satiation for different types of fluids. Chemogenetic activation of Oxtr PBN neurons in male and female transgenic Oxtr Cre mice robustly suppressed the rapid, initial (15‐min) intake of several solutions after dehydration: water, sucrose, ethanol and saccharin, but only slightly decreased intake of Ensure®, a highly caloric solution (1 kcal/mL; containing 3.72 g protein, 3.27 g fat, 13.42 g carbohydrates, and 1.01 g dietary fibre per 100 mL). Oxtr PBN neuron activation also suppressed cumulative, longer‐term (2‐h) intake of lower caloric, less palatable solutions, but not highly caloric, palatable solutions. These results suggest that Oxtr PBN neurons predominantly control initial fluid‐satiation responses after rehydration, but not longer‐term intake of highly caloric, palatable solutions. The suppression of fluid intake was not because of anxiogenesis, but because Oxtr PBN neuron activation decreased anxiety‐like behaviour. To investigate the role of different PBN subdivisions on the intake of different solutions, we examined FOS as a proxy marker of PBN neuron activation. Different PBN subdivisions were activated by different solutions: the dorsolateral PBN similarly by all fluids; the external lateral PBN by caloric but not non‐caloric solutions; and the central lateral PBN primarily by highly palatable solutions, suggesting PBN subdivisions regulate different aspects of fluid intake. To explore the possible mechanisms underlying the minimal suppression of Ensure® after Oxtr PBN neuron activation, we demonstrated in in vitro slice recordings that the feeding‐associated agouti‐related peptide (AgRP) inhibited Oxtr PBN neuron firing in a concentration‐related manner, suggesting possible inhibition by feeding‐related neurocircuitry of fluid satiation neurocircuitry. Overall, this research suggests that although palatable beverages like sucrose‐ and ethanol‐containing beverages activate fluid satiation signals encoded by Oxtr PBN neurons, these neurons can be inhibited by hunger‐related signals (agouti‐related peptide, AgRP), which may explain why these fluids are often consumed in excess of what is required for fluid satiation.