Energy, water and electrolyte balance are among the most closely regulated parameters found in complex organisms. Even a minor, uncorrected defect in the control of any one system could be life-threatening. Most of the processes involved in these systems regulate internal use and conservation. Only one process, ingestion of foods or fluids, can replenish exhausted supplies. The ultimate goal of the research outlined in this proposal is to understand the neural mechanisms that govern the decision to ingest or reject the contents of the oral cavity. Recent evidence from this and other laboratories indicates that the basis for these neuronal mechanisms in complete in the caudal brainstem -- the midbrain, pons and medulla. Sensory information from the oral cavity -- tactile, temperature, and gustatory afferent activity -- first reaches the brain in the pons and medulla. The motor neurons that produce ingestive behavior are located in the pons and medulla. Visceral afferent activity that can alter the response elicited by an oral stimulus from one of ingestion to one of rejection also reaches the brain via axons that first synapse in the caudal brainstem. Using neuroanatomical, electrophysiological, and behavioral analysis, this project will examine the function of oral sensory activity in eliciting and guiding the motor apparatus responsible for ingestion and rejection, the nature of the visceral events that control these responses, and the neural integration of oral and visceral afferent activity that supports switching from one behavior to the other. Specific experiments will examine (1) the convergence of oral tactile, thermal, and gustatory afferent activity onto single neurons in the nucleus of the solitary tract, (2) the effects on ingestive behavior of disconnecting the afferent input or efferent output of the solitary nucleus, (3) the importance of visceral afferent or efferent activity immediating hormonal initiation or satiation of ingestion, and (4) the interaction of oral and visceral afferent activity at the neuronal level in the medulla and pons. In addition to contributing toward a basic understanding of how the nervous system coordinates diverse sensory information into precise physiological and behavioral controls, this research has direct relevance to sensory mechanisms involved in the etiology of a variety of morbid conditions. Specifically, excess ingestion of carbohydrates (sugars) and salt (sodium) is highly correlated with, and may be causally related to, obesity, diabetes, and hypertension.
