Galanin exerts excitatory and inhibitory effects on gastrointestinal motility either directly by acting on the muscle or indirectly by acting on neuronal pathways. Galanin actions are mediated by three subtypes of galanin receptors (GAL-R), GAL- R1, GAL-R2 and GAL-R3, which are G protein-coupled receptors with distinct pharmacological profiles that utilize different intracellular signaling pathways. These observations strongly suggest that different galanin receptors mediate the different actions of galanin on gastrointestinal functions, including motility. The neurally-mediated inhibitory actions of galanin on gastrointestinal motility and the galanin receptor subtypes involved in this inhibitory modulation by galanin are the focus of this proposal. The hypothesis to be tested is that GAL-R1 and GAL-R3 mediate the indirect inhibitory effect of galanin on gastrointestinal motility primarily through the inhibition of transmitter release from excitatory pathways, and that GAL-R2 stimulates the inhibitory pathway by inducing inhibitory transmitter release. These studies will utilize the rat and guinea pig as model systems and will combine morphological and functional approaches including in vitro and in vivo studies.
Specific aim 1 will a) elucidate the cellular localization of GAL-Rs in the gastrointestinal tract of the rat and guinea pig, and b) identify the neuronal pathways expressing these receptors.
Specific aim 2 will characterize the role of GAL-Rs on galanin inhibition of neurogenic transmission using the muscle-myenteric plexus preparations of the guinea pig ileum in vitro. These studies will a) evaluate whether GAL-R1 and GAL-R3 mediate the inhibitory effect of galanin on neurotransmission by determining the diminution of electrically-stimulated muscle twitch and the reduction of acetylcholine release, and b) investigate whether GAL-R2 activation induces a neurally-mediated inhibitory response.
Specific aim 3 will investigate the role of galanin and GAL-Rs on the ascending excitatory reflex and peristalsis in the guinea pig ileum in vitro by determining a) the effects of galanin on physiological parameters Of peristalsis, b) the sites of action of galanin on peristaltic reflex, c) which GAL-Rs are involved and d) whether galanin is released during peristalsis.
Specific aim 4 will investigate the role of GAL-R activation in the inhibition of gastrointestinal motility and transit in vivo in the rat. These studies will provide insights into the role and site of action of galanin and its receptors in modulating gastrointestinal motility and the basis for better understanding pathways involved in gastrointestinal dysmotility.
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