Abstract
The spinal substantia gelatinosa (SG) plays a pivotal role in modulating nociceptive transmission through dorsal root ganglion (DRG) neurons from the periphery. TRP channels such as TRPV1 and TRPA1 channels expressed in the SG are involved in the regulation of the nociceptive transmission. On the other hand, the TRP channels located in the peripheral terminals of the DRG neurons are activated by nociceptive stimuli given to the periphery and also by plant-derived chemicals, which generates a membrane depolarization. The chemicals also activate the TRP channels in the SG. In this review, we introduce how synaptic transmissions in the SG neurons are affected by various plant-derived chemicals and suggest that the peripheral and central TRP channels may differ in property from each other.
Highlights
Nociceptive stimuli given to the periphery generate a membrane depolarization in the peripheral terminals of primary-afferent, fine myelinated Aδ and unmyelinated C, fibers, resulting in the production of action potentials (APs)
Resiniferatoxin (RTX; Figure 5A) is an ultrapotent TRP vanilloid-1 (TRPV1) agonist, a capsaicin analog isolated from the dried latex of the cactus-like plant, Euphorbia resinifera [59,60,61]. [3H]RTX is widely used to examine the distribution of TRPV1 channels in the nervous system [62,63]
RTX-induced inward current was blocked by CNQX (10 μM) and APV (50 μM) treatment (Figure 6D and 6E), indicating that the inward current resulted from activation of non-NMDA and NMDA receptors by L-glutamate released from the primary-afferent terminals after TRPV1 activation
Summary
Nociceptive stimuli given to the periphery generate a membrane depolarization in the peripheral terminals of primary-afferent, fine myelinated Aδ and unmyelinated C, fibers, resulting in the production of action potentials (APs). Endogenous analgesics such as opioids, nociceptin, serotonin, norepinephrine, adenosine, and galanin reduce the release of L-glutamate from the central terminals of primary-afferent fibers onto the SG neurons through the activation of their receptors, resulting in diminishing the excitability of the neurons (for example see [9,10,11,12,13,14]; for review see [15]). TRP channels, which are synthesized in the cell body of dorsal root ganglion (DRG) neuron, are transferred to the peripheral and central terminals of the neuron by axonal transport (Figure 1). Stimuli (such as temperature and chemicals) given to the periphery activate the peripheral TRP channel, resulting in membrane depolarization, which in turn generates action potential that transfers the stimulus information to the central terminal of the DRG neuron. L-glutamate onto spinal substantia gelatinosa (SG) neurons, which play a pivotal role in modulating nociceptive transmission and is involved in this modulation
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