Dorsal raphe nucleus melanocortin signaling regulates energy homeostasis

Our recent studies have demonstrated that that activation of GABAergic MC4R neurons in the DRN increases feeding (Nectow et al., 2017; Bruschetta et al., 2020), thus suggesting that contrary to MC4R neurons located in the PVN, DRN GABAergic MC4R neurons are inhibited by its ligand, a-MSH. In support of this, we have observed that injection of a-MSH in the DRN reduces food intake (Bruschetta et al., 2020) and reduces DRN neuronal activation (Bruschetta et al., 2020) in overnight fasted mice. In agreement with these data, DREADD inhibition of DRN MC4R neurons reduces feeding in overnight fasted mice (Bruschetta et al., 2020) while DREADD activation of DRN MC4R neurons increases feeding in overnight fed mice (Bruschetta et al., 2020). Furthermore, bath application of a-MSH in DRN slice leads to hyperpolarization of MC4R neurons (Bruschetta et al., 2020) and a significant decreased of frequency of action potentials in DRN GABAerigic neurons (Nectow et al., 2017). Contrary to our published data, however, a recent study (Han et al., 2021) has reported that activation of MC4R neurons in the DRN does not affect feeding while induce decreased body weight by increasing energy expenditure and thermogenesis. While in this study the authors showed that activation of these MC4R neurons induced glutamate release, our studies found that the majority of the MC4R neurons are GABAergic in nature and their inhibition affects feeding, thus suggesting the existence of at least two DRN MC4R neuronal subpopulations, i.e. GABAergic vs glutamatergic MC4R neurons, with distinct roles in the regulation of metabolism. Notably, we have also found a significant increase in a-MSH immunoreactivity in the DRN of diet- induced obese (DIO) mice compared to standard chow-fed (SD) mice, suggesting a possible role for DRN melanocortin signaling in the development of diet-induced obesity. Thus, based on these data, we hypothesize that melanocortin signaling in the DRN plays a role in the regulation of metabolism (Nectow et al., 2017; Bruschetta et al., 2020) through the activity of 2 distinct MC4R subpopulations, GABAergic and glutamatergic MC4R neurons. In Aim 1 we will determine and compare the role of melanocortin signaling in DRN GABAergic neurons versus DRN glutamatergic neurons in the regulation of feeding and metabolism by affecting pharmacologically and genetically GABAergic and glutamatergic MC4R neurons. In Aim 2 we will profile POMC (and AgRP) projections to the DRN GABAergic and glutamatergic MC4R neuronal subpopulations and the intracellular signaling pathways in DRN (and PVN) GABAergic and glutamatergic MC4R neurons. In Aim 3 we will interrogate the role of DRN melanocortin signaling in diet-induced obesity. The proposed studies are a logical extension of our preliminary data and, their completion will unmask a novel mechanistic principle in the central regulation of metabolism.