Impairment of gastric adaptive relaxation reflexes is an underlying cause of gastroparesis. It may also contribute to gastroesphageal reflux disease and metaplasia (Barretts). In the hindbrain, vagal motor output to the gastric fundus is from preganglionic cholinergic neurons that are both excitatory and inhibitory. The inhibitory pathway is termed non-adrenergic non-cholinergic (NANC) and evokes relaxation of the stomach via enteric nitric oxide (NO) and vasoactive polypeptide inhibitory motor neurons. To manipulate this pathway we currently use pharmacological and molecular antisense techniques in rats. However, it is clear that the most powerful molecular tools are in mice with targeted genetic mutations. Therefore, in this exploratory proposal we will modify our physiological recordings and stereotaxic brain microinjections for use in mice. This would be a unique approach in the field of brain-gut interactions. In rats, we (and others) have successfully measured gastric relaxation by intragastric balloon pressure. However, the extent of gastric relaxation is limited by the inherent compliance of the intragastric balloon and the initial imparting pressure. This can be circumvented by use of a barostat, which measures changes in intragastric volume using an open system in large animals and human. Barostats have recently been miniaturized for rodents. The first hypothesis is that gastric relaxation is evoked by GABAb and NK1 receptors in neurons of the dorsal motor nucleus of the vagus (DMN) of mice. Mice will receive mid-collicular decerebration to circumvent potentially confounding effects of anesthesia and low baseline gastric tone. NO mediates gastric relaxation at many sites in the brain-gut 'highway'. Specifically, gastric relaxation is regulated by hindbrain NO input into the dorsal vagal complex, by preganglionic NO containing vagal neurons and by inhibitory motor neurons at the neuromuscular junction. The second hypothesis is that cholinergic tone withdrawal is retained in centrally evoked gastric relaxation in NO synthase knockout mice. We predict that the extent of gastric relaxation is impaired compared to control mice. This proposal will enable us to surmount the difficulties associated with physiological miniaturization. Thus we will be able to exploit genetically engineered mice using state-of-the-art equipment for measuring intragastric volume. This will be critical for understanding how the underlying genetic components translate into alterations in neural control of gastric motor function.
