Spontaneous l-glutamate release enhancement in rat substantia gelatinosa neurons by (−)-carvone and (+)-carvone which activate different types of TRP channel

Transient receptor potential (TRP) channels in the spinal dorsal horn lamina II (substantia gelatinosa; SG), which are involved in the modulation of nociceptive transmission, have not yet been fully examined in property. Activation of the TRP channels by various plant-derived chemicals results in an increase in the spontaneous release of l-glutamate onto the SG neurons. We examined the effects of a monoterpene ketone (−)-carvone (contained in spearmint) and its stereoisomer (+)-carvone (in caraway) on glutamatergic spontaneous excitatory transmission in SG neurons of adult rat spinal cord slices by using the whole-cell patch-clamp technique. (−)-Carvone and (+)-carvone increased the frequency of spontaneous excitatory postsynaptic current (sEPSC) in a reversible and concentration-dependent manner with a small increase in its amplitude. Half-maximal effective concentrations of (−)-carvone and (+)-carvone in increasing sEPSC frequency were 0.70 mM and 0.72 mM, respectively. The (−)-carvone but not (+)-carvone activity was inhibited by a TRPV1 antagonist capsazepine. On the other hand, the (+)-carvone but not (−)-carvone activity was inhibited by a TRPA1 antagonist HC-030031. These results indicate that (−)-carvone and (+)-carvone activate TRPV1 and TRPA1 channels, respectively, resulting in an increase in spontaneous l-glutamate release onto SG neurons, with almost the same efficacy. Such a difference in TRP activation between the stereoisomers may serve to know the properties of TRP channels in the SG.