The objective of this research proposal is to determine the neural substrate in the rodent hindbrain through which gustatory stimuli modulate ingestive behavior. Although the regulation of ingestive behavior is exceedingly complex, involving many regions of the CNS, the basic regulatory function of taste to evoke either an ingestion or rejection response, is complete in the caudal brain stem. Chronic EMG recording experiments have characterized the behaviors of ingestion and rejection to sapid stimuli by determining the activation sequences of a subset of trigeminal, facial, ambiguus and hypoglossal innervated muscles. A rejection response to quinine monohydrochloride (bitter) is characterized by retraction of the tongue (styloglossus contraction) in conjunction with jaw opening (anterior digastric contraction) in contrast to an ingestion sequence in which tongue protrusion (genioglossus contraction) is simultaneous with jaw opening. These stereotypic motor components of ingestion and rejection are also evident in a restrained, anesthetized preparation in which central recording is possible. The neural substrate mediating these oro-motor responses is polysnaptic and anatomical experiments have delineated several regions of the brain stem reticular formation (RF) that project to all (or a subset) of the oral motor nuclei producing them. Thus, these RF regions are candidates for coordinating and sequencing the complex synergies of ingestion and rejection. Using neurophysiological techniques, neurons in these reticular regions will be studied for (1) Sensitivities to gustatory, intra-oral tactile and oral proprioceptive stimuli, (2) responsiveness during specific motor components of ingestion and rejection defined electromyographically, (3) connectivity with oro-motor nuclei using antidromic stimulation. Additional experiments using retrograde transport of horseradish peroxidase will determine if these reticular regions receive projections from gustatory nuclei in the brain stem or whether other interneurons are interposed. Ingestion and rejection represent the final behavioral decision required for replenishing energy, fluid, and electrolyte stores. Chronic disorders such as obesity, hypertension and anorexia nervosa are related to disordered ingestion. Understanding the neurological basis of this fundamental biological decision, to ingest or reject will contribute to the eventual solution of these chronic debilitating disorders.
