Receptor Level Dissection of Common Versus Discrete Vesicle Release Pathways from Primary Vagal Afferent Terminals

Abstract

Primary vagal afferent neurons form strong excitatory glutamatergic synapses onto second-order neurons in the nucleus of the solitary tract and initiate key autonomic and homeostatic reflex pathways. Vesicle fusion and release from vagal central terminals occurs via three distinct pathways including synchronous, asynchronous, and spontaneous. While synchronous and asynchronous release mechanisms require presynaptic action-potential depolarization; spontaneous release is ongoing and independent of action potentials. Evidence suggests these forms of release can be controlled independently indicating discrete and potentially separate vesicle pools and release sites. However, the presence and activation of transient receptor potential (TRP) channels (including TRPV1) at the central terminals dramatically increases the rate of spontaneous vesicle release and eventually diminish action-potential driven synchronous release; consistent with a common releasable pool. Confounding this interpretation is the observation that TRP channel activation also diminishes voltage activated sodium channel signaling. As such the decrease in synchronous release may be a result of action potential failure, rather than depleted vesicle pools. Given that synchronous and spontaneously release glutamate activates both AMPA and NMDA receptors postsynaptically we utilized ligand dependent post-synaptic NMDA receptor blockade (MK-801) to overcome this confound. We found that treatment with MK-801 during exclusively spontaneous release blocked the NMDA component of spontaneous events and subsequent action-potential driven synchronous release. This result demonstrates that glutamate released via spontaneous vesicle release is activating the same population of NMDA receptors as glutamate released synchronously. This finding is consistent with a common release site for synchronous and spontaneous release at vagal afferent terminals.