Electrophysiological characterization of facilitated spinal withdrawal reflex to repetitive electrical stimuli and its modulation by central glutamate receptor in spinal anesthetized rats.

The present study is aimed to systematically investigate wind-up and after-discharge of the spinal withdrawal reflex assessed by recording single motor unit (SMU) electromyographic (EMG) response to different intensities [0.5-1.5xreflex threshold (T)] of repetitive [frequencies (0.5-200 Hz)] transcutaneous electrical stimuli for 5 s. The role of central glutamate receptors in modulation of the withdrawal reflex facilitation was observed and evaluated in order to explore the potential central mechanism. Stimulus intensities below reflex threshold, such as 0.8xT, but not 0.5xT, could by repetition elicit and facilitate withdrawal reflex. The facilitation (wind-up and after-discharge) of the withdrawal reflex is a result of central integration and is increased significantly for increasing stimulus intensity and frequency. Electrical stimuli at 3-5 Hz for 5 s are appropriate to elicit wind-up. In contrast, 10-20 Hz frequencies of electrical stimuli are adequate to evoke the after-discharge. For pharmacological intervention, suprathreshold (1.5xT) repeated (5 Hz) electrically evoked facilitated reflex (wind-up) were apparently depressed by intrathecal (i.t.) administration of MK-801 as well as CNQX (40 nmol/10 microl, respectively). However, wind-up of spinal reflexes evoked by subthreshold (0.8xT) electrical stimuli could only be depressed by the treatment with CNQX, not MK-801. The after-discharge of the withdrawal reflex elicited by 20 Hz electrical stimulation with either 0.8xT or 1.5xT intensity was depressed by i.t. treatment with CNQX. I.t. application of MK-801 only depressed 0.8xT the intensity of electrically evoked after-discharge. In conclusion, for the first time, the present study clearly demonstrates that, following the wind-up phase, the spinal withdrawal reflex pathways continue to fire spontaneously in a stimulus frequency- and intensity-dependent way (temporal and/or spatial summation). This inherited memory and the central non-N-methyl-d-aspartate (non-NMDA) receptor, but not the NMDA receptor, mainly involving pharmacological mechanisms, may play an important role in pathological conditions with spontaneous nociceptive firing. Furthermore, the after-discharge of the spinal reflex may be an important indicator for studies on central sensitization in many pathological pain conditions.