Inhibitory Modulation and Circuitry in the Rostral Solitary Nucleus

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PROJECT SUMMARY The sense of taste profoundly affects food selection and is thus inextricably linked to health concerns including caries exacerbated by sugar overconsumption and the sequela of obesity such as cardiovascular disease and diabetes. Understanding how taste is represented and processed in the brain thus has important clinical implications. The central processing of gustatory signals first occurs in the rostral nucleus of the solitary tract (rNST). A major difficulty in discerning how taste is represented in this nucleus is phenotypic heterogeneity. Output neurons are primarily glutamatergic and are responsible for representing the chemosensory properties of tastants and relaying this information to circuits that affect perception, behavior, and reflexes, whereas GABA/glycinergic neurons modulate the output cells. Due to technical challenges, it is thus far been impossible to correlate taste response properties with neurotransmitter phenotype. Moreover, little is known about the impact that GABAergic circuits exert on gustatory processing. Recent advances utilizing transgenic mice in combination with optogenetics can overcome these barriers by introducing the light-sensitive protein channelrhodopsin in specific cell types. This makes it possible to use neurophysiological recording techniques to identify GABAergic neurons in vivo, based on responses to light, and to simultaneously describe chemosensitive properties from the same cell. These techniques also allow temporally precise and repeatable activation or deactivation of GABAergic circuitry. The present proposal will use strains of mice that express either channelrhodopsin or archaerhodopsin in GABAergic neurons. Aim 1 will investigate the impact of optogenetically induced GABAergic modulation on non-GABAergic neurons in vivo, and contrast GABAergic modulation of rNST taste responses that arise from local neurons in the rNST, with those from the caudal NST, a local medullary source of visceral signals, or from the central nucleus of the amygdala. Aim 2 will investigate cellular mechanisms of GABAergic modulation in vitro combined with computational modeling to investigate interactions between hyperpolarization-sensitive ion channels and inhibition. A second study will test the hypothesis that inhibition differentially affects afferent signaling in neurons that contribute to an ascending pathway compared to local reflexive pathways. Aim 3 will define properties of GABAergic neurons and characterize subtypes. The first study will record in vivo from GAD65-ChR2 mice to identify GABAergic neurons by responsiveness to light and test the hypothesis that the chemosensitive responses of GABAergic rNST neurons differ from non-GABAergic neurons. A second aim will record in vitro from mice expressing tdTomato in GAD65+ neurons to identify subtypes based on location, inhibitory modulation, and the presence of the hyperpolarization-sensitive Ih current. A causal role for these currents in modulating afferent responses will be tested. These studies will provide novel insights into taste coding and ultimately contribute toward understanding how taste signaling interacts with circuits that control feeding behavior.