Transmitter release from peripheral sensory neurons

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The peripheral sensory nerves specialized for detection of noxious stimuli, temperature and itch have their peripheral terminals in the skin, muscle, bone and viscera, their cell bodies in the dorsal root ganglion (DRG), and their central terminals in the spinal dorsal horn. The stimuli are carried by AD and C-fibers of small diameter sensory neurons, many of which express the noxious heat-sensitive receptor, TRPV1, a Ca2+ permeable cation channel. TRPV1 in the peripheral sensory terminals of DRG neurons serves as a major transducer of multiple noxious stimuli, including noxious thermal stimuli above 43oC. Despite the well- established presence of TRPV1 on the central synaptic terminals of DRG neurons in the dorsal horn, its physiological relevance there has not been clearly established. Recent findings imply that under certain conditions, TRPV1 can be expected to become active at core body temperature (~37oC), well below TRPVI's normal 43oC temperature threshold for activation. The first condition involves the interaction between temperature and voltage. At resting membrane potentials, TRPV1 shows measurable, tonic activation at 37oC. Subsequent depolarization leads to substantial increases in TRPV1 activation (Voets et al, 2004). However, most investigations of TRPV1 function in central terminals have been carried out at room temperature rather than body temperature. Second, activation of PLC-coupled receptors leads to a dramatic reduction in TRPVI's temperature threshold of activation leading to its activation even at room temperature (Chuang et al, 2001). We propose to test the hypothesis that at physiological or body temperatures, and under the conditions of either receptor mediated PLC activation or physiological membrane depolarization mediated by action potentials and ionotropic receptor transmitters, TRPV1 activation in central sensory terminals is substantial and that this activation will lead to modulation of synaptic transmission at DRG-spinal cord synapses. These studies focus on a critical part of the pain pathway; the intersection of the peripheral sensory fiber with the central nervous systems and they will provide insight into new regulatory mechanisms in that pathway.