Effects Of (+)-carvone And (-)-carvone On Glutamatergic Spontaneous Excitatory Transmission In Adult Rat Substantia Gelatinosa Neurons

Objective:Spinal lamina II (substantia gelatinosa; SG) neurons play an important role inmodulating nociceptive transmission to the central nervous system (CNS). Transientreceptor potential (TRP) channels are thought to play a role in regulating nociceptivetransmission to SG neurons from the periphery; the properties of the channels havenot yet been fully examined. It remains to be unveiled whether the TRP channels inthe CNS are different in property from those involved in receiving nociceptive stimuliin the periphery. Carvone is a monoterpene that is the main components of essentialoils (EO) extracted from aromatic plants and possess antinociceptive andanticonvulsant activities. The effect of a stereoisomer of (-)-carvone,(+)-carvone, onthe transmission was also investigated, because they are known to have actionsdifferent from each other in the nervous system. In dorsal root ganglia (DRG) neuronsand TRPV1-expressing HEK293cells,(–)-carvone has an ability to induce increasesin cytosolic calcium concentration through TRPV1activation. We examined theeffect of an antinociceptive monoterpene (-)-carvone, which activates TRPV1channels in the cell body of primary-afferent neuron, and (+)-carvone onglutamatergic spontaneous excitatory transmission in the SG neurons by using thewhole-cell patch-clamp technique. We also addressed whether they can activate TRPchannels.Methods:Male adult6-8weeks old Sprague-Dawley rats were anesthetized with urethane(1.2g/kg, intraperitoneal), and then lumbosacral laminectomy was performed. Thelumbosacral spinal cord (L1-S3) was mounted by a vibrating microslicer and then a600-700μm thick transverse slice was cut.(+)-Carvone,(-)-carvone, TRPV1channelantagonist capsazepine and TRPA1channel antagonist HC-030031were applied bysuperfusion with a change in solutions in the recording chamber. The blind whole-cellpatch-clamp technique was applied to the SG neurons to record sEPSCs. Results:(+)-Carvone and (-)-carvone increased the frequency of spontaneous excitatorypostsynaptic current (sEPSC) in a reversible and concentration-dependent mannerwith a small increase in its amplitude. Half-maximal effective concentrations for(+)-carvone and (-)-carvone in increasing sEPSC frequency were0.48mM and0.43mM, respectively. In some neurons, the sEPSC frequency increase was accompaniedby an inward current at a holding potential of-70mV. The effects of (-)-carvone butnot (+)-carvone were inhibited by capsazepine (10μM). On the contrary, the effectsof (+)-carvone but not (-)-carvone were inhibited by HC-030031(50μM).Conclusion:These results indicate that (+)-carvone and (-)-carvone activate TRPA1andTRPV1channels, respectively, resulting in an increase in the spontaneous release ofL-glutamate onto SG neurons. In DRG neurons,(–)-carvone has an ability to induceincreases in cytosolic calcium concentration through TRPV1activation. This resultsuggests that (-)-carvone may be the same type of TRP channel so that in the soma ofDRG neurons in the SG.(+)-Carvone and (-)-carvone activate TRPA1and TRPV1channels in the SG, respectively. This result may serve to know the chemicalstructures of central TRP channels.