Noradrenergic inhibition of definitive POMC neurons through direct and indirect mechanisms

Norepinephrine is a key neuromodulator of hypothalamic circuits that regulate energy balance. Previous studies suggested that norepinephrine inhibits proopiomelanocortin (POMC) neurons of the arcuate nucleus via α2a-adrenoceptors (ADRA2A), but the underlying mechanisms and physiological relevance of this pathway were not assessed. We therefore investigated how ADRA2 activation regulates POMC neuron activity and whether Adra2a expressed in POMC neurons contributes to energy and glucose homeostasis in vivo. We used whole-cell patch clamp electrophysiology in male and female mice to evaluate the impact of norepinephrine and the ADRA2 agonist UK 14,304 on definitive POMC neurons in the arcuate nucleus. We also generated and validated a novel Adra2a-flox mouse line, which was crossed with Pomc-CreERT2 mice to produce inducible POMC-specific Adra2a knockout mice (POMC^KOA2A). These mice were used for both electrophysiological analyses and in vivo assessment of energy and glucose homeostasis. Multiplex RNAscope confirmed that Adra2a was highly expressed in ~50% of POMC neurons, with significantly higher expression in females. Norepinephrine and the selective ADRA2 agonist UK 14,304 robustly inhibited POMC neurons in hypothalamic slices, producing hyperpolarization, reduced firing, and decreased input resistance independent of sex. These effects were mediated in part by activation of multiple potassium conductances, as blockade of K_ir and K_ATP channels attenuated the response. They were also partly indirect, as blockade of synaptic transmission reduced the proportion of neurons that were inhibited. Deletion of Adra2a in definitive POMC neurons had little effect on body weight, food intake, or adiposity, but modestly impaired glucose tolerance in males. Electrophysiological studies revealed that loss of Adra2a in POMC neurons prevented UK 14,304-induced inhibition in approximately half of POMC neurons, supporting the involvement of both direct and indirect effects of UK 14,304 on POMC neuron activity. In conclusion, ADRA2A robustly inhibits hypothalamic POMC neurons through both direct and indirect mechanisms. However, Adra2a expression in POMC neurons is largely dispensable for the regulation of energy balance. These findings suggest that noradrenergic inhibition of POMC neuron activity involves additional cellular targets or network-level pathways beyond POMC neurons themselves.