The Effect By Cineole Of Spontaneous Excitatory Transmission In Rat Spinal Substantia Gelatinosa Neurons

Objective: The transient receptor potential (TRP) channel is a kind of transme-mbrane protein that exist in the nervous system widely, and a kind of nonselectivecation channel. There are seven subfamilies in TRP channels, and many of themregulate the sensation of pain. There are abundant of classical neurotransmitter and itsreceptors in substantia gelatinosa which is the important region to regulate thesensation of pain. Although TRP channels play a role in regulating nociceptivetransmission in spinal substantia gelatinosa (SG) neurons, the properties of the TRPchannels have not yet been examined fully. The central TRP channels are activated byvarious plant-derived chemicals such as capsaicin and isothiocyanate. In many kinds ofessential oil there is cineole which has analgesic effect, but the mechanism is not clearyet. The present study examined the effects of1,4-cineole and1,8-cineole onglutamatergic spontaneous excitatory postsynaptic currents (sEPSC) in the SG neuronsof spinal cord slices by using the whole-cell patch-clamp technique, and investigatewhether they can activate TRP channels.Methods: Male adult Sprague-Dawley rats (6-8weeks old) were anesthetizedwith urethane (1.2g/kg, intraperitoneal), and then lumbosacral laminectomy wasperformed. The lumbosacral spinal cord (L1-S3) was removed and mounted on avibrating microslicer and then a650-700μm thick transverse slice was cut. The slicewas placed on nylon mesh in the recording chamber, superfused Krebs, one hour laterthe blind whole-cell patch-clamp technique was applied to the SG neurons of the slicesto record the sEPSC.1,4-cineole,1,8-cineole, menthol, capsaicin, capsazepine andHC-030031were applied by superfusion with a change in solutions in the recordingchamber.Results: Bath-applied1,8-cineole for3min in a range of0.2-7mM increased thefrequency of sEPSC in a dose-dependent manner with a half-maximal effectiveconcentration value of2.5mM (Hill coefficient=2.0) with no significant increase in itsamplitude and unchanged holding currents at a holding potential of-70mV.1,4-Cineole, a minor component of plant extracts containing1,8-cineole, superfused for3min in a range of0.1-0.7mM increased sEPSC frequency (EC50=0.18mM, Hillcoefficient=7.9) effectively more than1,8-cineole with a small increase in itsamplitude. The sEPSC frequency increase produced by cineole was reversible, theeffect of twice was similar with a20min interval. The excitatory transmissionenhancement produced by cineole was unaffected by a TRPV1antagonist capsazepine;the cineole effect was not occluded by capsaicin. On the other hand, the facilitatoryeffect of cineole was inhibited by a TRPA1antagonist HC-030031. The sEPSCfrequency increase produced by1,8-cineole was much greater than that expected fromTRPM8activation by1,8-cineole compared to (-) menthol. Cineole produced aninward current in some of the neurons examined.Conclusion: It is concluded that cineole enhances the spontaneous release ofL-glutamate onto SG neurons by activating TRPA1but not TRPV1channel; this actionof1,4-cineole is much larger than that of1,8-cineole. This result could serve to knowthe properties of the central TRPA1channels that are therapeutic targets for pain andother disorders.