Neuronal information is transferred by action potential (AP) conduction in nerve fibers and by chemical transmission at neuro-neuronal synapses. Inhibition of AP conduction in nociceptive nerve fibers results in antinociception. Although modulation of the chemical transmission plays an important role in various physiological functions, synapses in the neuronal pathway involved in nociceptive transmission become a target of endogenous and exogenous analgesics. Various compounds derived from plants were found to activate transient receptor potential (TRP) channels to enhance glutamatergic spontaneous excitatory transmission in rat spinal cord lamina II neurons that play a pivotal role in regulating nociceptive transmission from the periphery. This enhancement was due to an increase in the spontaneous release of L-glutamate from nerve terminals. Some of the plant-derived compounds produced a membrane hyperpolarization in rat spinal lamina II neurons without TRP channel activation. Moreover, various plant-derived compounds were found to inhibit AP conduction in the frog sciatic nerve. These activities of plant-derived compounds depended on their chemical structures. This result could serve to develop antinociceptive drugs that are related to plant-derived compounds. This article will review the effects of plant-derived compounds on excitatory transmission in rat spinal lamina II neurons and on nerve AP conduction in the frog sciatic nerve.
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