The increased prevalence of obesity and its associated pathologies, including cardiovascular disease and diabetes mellitus, account for a large percentage of healthcare costs in the United States. Neurocircuitry within the brainstem provides critical controls of food intake and energy homeostasis. In the caudal brainstem the nucleus of the solitary tract (NTS) integrates vagal afferent information arriving from across visceral organ systems to initiate homeostatic reflex pathways, including those essential for the controls of food intake. Centrally, vagal afferents converge to form the solitary tract (ST) and contact second order NTS neurons via strong excitatory synapses. At ST-NTS synapses action-potential invasion releases multiple glutamate vesicles that are precisely synchronized with terminal depolarization. This robust 'synchronous' form of glutamate release is thought to be the predominate mode of fast neurotransmission at the ST-NTS synapse. Recently, however, we identified a novel form of activity-dependent 'asynchronous' glutamate release from a subgroup of vagal afferents. In contrast with synchronous release, this additional form of neurotransmission was only loosely coordinated with depolarization and continued for many seconds, effectively doubling the synaptic strength. As a result of the additional charge transfer the postsynaptic excitatory period was significantly extended, dramatically transforming the nature of information transfer. ST afferents are divided into myelinated (A-fiber) and unmyelinated (C-fiber) phenotypes with physiologically distinct functions. One important difference between subtypes is that C-fiber afferents express the calcium permeable non-selective ion channel 'transient receptor potential vanilloid type 1' (TRPV1). In our preliminary experiments we found all afferents with activity-dependent asynchronous release were also activated by the TRPV1 agonist capsaicin. Further, antagonism of TRPV1 activity selectively reduced the asynchronous release profile with no effect on synchronous. An attenuated asynchronous release process persists in TRPV1 KO mice and is reduced by ruthenium red. Together these findings suggest membrane depolarization endogenously activates TRPV1, and other thermosensitive-TRP channels, expressed in the central terminals of vagal afferents resulting in asynchronous glutamate release.
The aims of the current application are 1. to delineate the mechanisms of TRPV1 activation resulting in asynchronous glutamate release, 2. determine the extent to which other thermo- TRPs participate in asynchronous neurotransmission, and 3. utilize selective antagonists and genetic KO mouse models to determine the contribution of asynchronous glutamate release in the control of food intake. The findings from this project will determine the role of asynchronous glutamate release from vagal afferents and its impact on food intake.
The vagus nerve relays information from peripheral organs to the brain; providing critical controls of food intake, digestive function, and the development of obesity. Recently we have discovered a new form of neuronal communication that fundamentally changes how the vagus communicates with the brain and involves the capsaicin receptor TRPV1 and other thermosensitive TRP channels. Experiments in this project will directly determine the contribution of TRPV1 and other important cellular mechanisms underlying information transfer between vagal afferent neurons and the nucleus of the solitary tract in the brainstem. Results from these studies will provide novel therapeutic targets for the control of obesity and its associated diseases.
