Involvement Of NMDA Receptor In Modulatory Role Of Glia And Zn～(2+) On The Spinal LTP

It has been well established that NMDA receptors involved in the induction of long-term potentiation (LTP) of spinal cord neurons, which might contribute to the central sensitization in the spinal cord. Recent studies have suggested that glia play a more active role in synaptic function than previously thought. There is also evidence indicating involvement of Zn2+ in modulation of the neuropathic and inflammation pain. LTP and long-term depression (LTD) of the spinal cord have been introduced as models to study plastic changes in spinal pain-sensitive neurons. The present work studied the possible roles of glial cells and NMDA receptors, which could be modulated by Zn2+, on the process of spinal central sensitization by using the model of spinal LTP and whole cell patch clamp technique. The field potential by electrical stimulation of the sciatic nerve was recorded in the rat spinal cord. The stimulation at intensity enough to excite C fibers of the sciatic nerve induced the field potential including A responses and C responses which were triggered by excitation of A and C afferent fibers. The latencies of A and C responses were 10 ms and 150 ms, respectively. The amplitude of C responses reflected the degree of nociception. Tetanicstimulation of the sciatic nerve induced the long-term potentiation (LTP) of C-fiber-evoked field potentials. The selective inhibition of glial metabolism with intrathecal injection of fluorocitrate (FC, 1 nmol) failed to influence the baseline amplitude of C-fiber-evoked field potentials, but resulted in LTD of C-fiber-evoked field potentials in the spinal dorsal horn. The LTD was reversed by the second tetanic stimulation after 3 hrs. The effects of FC were abolished by intrathecal administration of kynurenic acid (50 nmol) or 2-amino-5-phosphonovaleric acid (AP-5, 0.5 nmol), but not by 6,7-dinitroquinoxaline-2,3-dione (DNQX, 5 nmol), picrotoxin (5 nmol) or strychnine (1 nmol). Glutamate (100 nmol) given half an hour before tetanic stimulation also induced similar form of LTD. Taken together, the present data suggest that spinal glia may play an active role in the central sensitization of nociceptive neurons in the spinal cord, and the glutamate concentration and the state of NMDA receptors may help determine the direction of synaptic gain changes in the spinal cord. Compared with the control side (contralateral to tetanic stimulation), the GFAP expression in the ipsilateral superfical dorsal horn has no difference either 5 minuters or 24 hours after the spinal LTP establishment. Topical administration of Zinc chloride (15 (M) to the spinal cord 15 min before tetanic stimulation completely blocked the induction of LTP, but not the baseline C responses. When ZnCl2 was given 2 h after induction of LTP, there was no significant effect.Chelation of Zn2+ by CaEDTA (500 (M) did not influenced LTP. Co-administration of ZnCl2 (15 (M) and NMDA (5 (M) significantly attenuated C fiber-evoked potentials, which was prevented by the NMDA receptor antagonist AP-5 (100 (M). These results showed that Zn2+ might contribute to the modulation of the formation, but not the maintenance, of spinal LTP. NMDA receptors might be involved in Zn2+-induced modulation of spinal LTP. In whole-cell patch clamp recording, NMDA (1mM) induced inward currents in 33% of tested dorsal root ganglion neurons. Zn2+ reversibly inhibited the inward current induced by NMDA (1mM) in concentration (IC50=2.832μM) and non-voltage dependent manner. The fact that Zn2+ inhibited NMDA-induced current in DRG neuron suggested presynaptic glutamate release might be modulated by Zn2+ in negative feed-back manner. Therefore, the presynaptic mechanism may be, at least partially, involved in modulation of NMDA receptor by Zn2+. The present study suggested that the state of NMDA receptors may determine the plasticity of nociception transmission, although the detail mechanisms remain to be investigated.