Feeding signals inhibit fluid-satiation signals in the mouse lateral parabrachial nucleus to increase intake of highly palatable, caloric solutions

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.