Modulation Of TMP On The Responses Mediated By P2X Receptors In Rat Dorsal Root Ganglion Neurons

2,3,5,6-tetramethylpyrazine (TMP) is one of the alkaloids contained in the traditional Chinese herbal medicine, Ligusticum wallichii Franchat (Chuan Xiong), and which posses the antagonistic action to calcium. TMP currently used in China for the treatment of cardiovascular disease and cerebrovascular disease in clinic. It was also found to power the function of heart, cut down blood pressure and inhibit platelet aggregation. Our previous works (2004) showed that TMP attenuated the acute nociception induced by P2X receptor agonists in rat paw. However, the mechanism of TMP action is still not clear, and require study further. Dorsal root ganglion (DRG) neurons not only are the pathway of transmitting sensory information from periphery to centre but also are the modulation site of primary sensory information. DRG neurons play a crucial role to analysis mechanism of medicine on primary sensory nerve. AIM The purpose of the present study was to explore the modulation of tetramethylpyrazine (TMP) on the responses mediated by P2X receptors in freshly isolated DRG neurons. METHODS Using whole-cell patch-clamp technique to study the effects of TMP on P2X receptor agonists-activated currents in freshly isolated DRG neurons. RESULTS The majority of the neurons examined(94.4%, 102/108)were sensitive to ATP in the concentration range from 1 to 1000 μmol·L-1 with a concentration-dependent inward current. The ATP-activated currents showed a rapid desensitization or a slow desensitization, which were blocked by TNP-ATP (a selective antagonist of P2X receptors). It is possible that the activation of P2X receptor induces the currents. TMP preapplied could inhibit ATP-activated currents (IATP) markedly; this effect was concentration-dependent. In the 102 ATP sensitive neurons, preapplication of 0.1, 0.3, 0.6, 1.0, 3.0 and 10 mmol·L-1 TMP inhibited ATP(100μmol·L-1)-activated currents by 7.99±1.56%(n=7), 19.20±2.97%(n=8), 35.82±4.62%(n=7), 41.76±5.20%(n=8), 38.46±7.30%(n=6) and 28.52±6.73%(n=7) in the majority of the neurons examined(89.2%,91/102), respectively. TMP (1mmol·L-1) reduced α,β-meATP (10μmol·L-1 )-activated current. The concentration-response curves for ATP with and without preapplication of TMP show that: (1) preapplication of TMP (1 mmol·L-1) shifted the concentration-response curve of IATP downward markedly; (2) the maximal amplitude of IATP with TMP preapplication decreased by 41.3%; (3) the EC50 values of the two curves were very close (82 vs. 75μmol·L-1); (4) the threshold value remained unchanged. TMP (1 mmol·L-1) did not alter the reversal potential (0 mV) of ATP-activated current. TMP(1 mmol·L-1) significantly inhibited ATP(100μmol·L-1)-activated currents potentiated by PGE2(100 μmol·L-1)or SP(0.1 μmol·L-1). Intracellular application of 10μmol·L-1 H89 (which is an inhibitor of PKA) reduced the inhibitory effect of TMP on ATP(100μmol·L-1)-activated current. CONCLUTION TMP can inhibit the currents activated by P2X receptors. The mechanism of TMP action may be the allosteric regulation via acting on PKA system to inhibit P2X receptor agonists-activated currents in rat DRG neurons.