Evidence for effects of nesfatin-1, alpha-melanocyte stimulating hormone, and glucose on the excitability of individual neurons in the nucleus of the solitary tract
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The nucleus of the solitary tract (NTS) is a medullary autonomic center with essential roles in the regulation of energy homeostasis. Understanding how the NTS contributes to the coordination of ingestive behaviors and autonomic processes is therefore critical to provide insight into the control of feeding in physiological and pathological states. Thus, here we have investigated the integration of metabolically relevant factors in individual NTS neurons. We uncovered direct depolarizing (42% of responses, mean 7.83±0.84 mV, n=39) and hyperpolarizing (21% of responses, mean -8.23±1.00 mV, n=20) effects of the anorexigenic peptide nesfatin-1 on NTS neurons. Our reverse transcription PCR studies demonstrated exclusively depolarizing effects of nesfatin-1 on neurons expressing NUCB2, the gene which encodes nesfatin-1, and the orexigenic neuropeptide Y. Furthermore, in vivo microinjections of nesfatin-1 into the NTS elicited tachycardic and pressor effects. We also delineated an NTS circuit engaged by the anorexigenic peptide α-melanocyte stimulating hormone (α-MSH). We established the depolarizing effects (39% of responses, mean 6.14±0.54 mV, n=16) of α-MSH are due to postsynaptic actions on the membrane potential of NTS cells, while hyperpolarizations (22% of responses, mean -6.79±1.02 mV, n=9) are indirect and caused by enhancement of GABAergic neurotransmission. Indeed, 50% of NTS neurons showed an increase in inhibitory postsynaptic current (IPSC) frequency in response to α-MSH. Another subset of NTS neurons was non-responsive to α-MSH due to simultaneous direct depolarizing and indirect hyperpolarizing effects of the peptide. Finally, we established 57% of NTS neurons exhibit physiologically relevant glucose sensing properties (n=81, 35% glucose excited, 22% glucose inhibited). Moreover, the effects of nesfatin-1 on the excitability of NTS neurons were consistent across glycemic conditions and categories of glucose sensing cells. α-MSH, however showed preferential effects on glucose responsive NTS neurons, and depolarized all NTS cells in hypoglycemic conditions, in contrast to the heterogeneous effects elicited in normo- and hyperglycemic states. Taken together, our studies highlight the integration of metabolic signals in individual NTS neurons. We have also revealed critical effects of glucose concentrations on brain slice studies, findings which have important implications for electrophysiological investigations in the NTS and in other neuronal centers.