Peptide receptor function in the nervous system <p> My primary research interests are concerned with expression and function of peptides and peptide receptors in sensory and autonomic neurons. This research program has been using as a major model system the enteric nervous system, the neural network that innervates the digestive tract, which compromises sensory and autonomic neurons. The main trust of my research program is to understand how the enteric nervous system regulates digestive functions including secretion and motility. The main lines of research are currently concerned with 1) the expression and trafficking of peptide receptors, and 2) the expression and functional role of neuronal growth factors in the enteric nervous system. We have demonstrated that receptor subtypes for several peptides are expressed in distinct populations of enteric neurons including motor neurons, interneurons and perhaps sensory neurons, and in non-neuronal cells as well, including interstitial cells of Cajal, which have been regarded as muscle contraction pacemakers. These findings are providing important insights into the sites and mechanisms of action of peptides, which act as transmitters/modulators in the digestive system. Recent studies have focused on the expression and trafficking of the mu opioid receptor, the preferred receptor for potent analgesics with high potential for drug abuse. This receptor is expressed in motor neurons and perhaps interneurons, and undergoes rapid, specific and agonist-selective endocytosis upon stimulation in vivo. This agonist selectivity suggests the existence of different mechanisms regulating cellular responsiveness to ligands and provides insights for understanding the mechanisms underlying opioid effects in the nervous system including tolerance and addiction. Finally, our studies on neurotrophic factors, have demonstrated that neurotrophin 3 and its main receptor, trkC, represent a major neurotrophin system in the adul