Low-frequency conditioning electrical stimulation evokes long-term facilitation of human nociceptive withdrawal reflex

J.A. BIURRUN MANRESA; C.D. MØRCH; O.K. ANDERSEN

Chicago

Conferencia; Neuroscience 2009; 2009

Long-term changes in spinal nociception after conditioning electrical stimulation to the dorsum of the foot were assessed in 13 healthy volunteers. In three separate sessions, two conditioning paradigms were tested: high- and low-frequency stimulation (HFS and LFS), delivered through a concentric electrode (a central cathode with 16 stainless-steel pins of 0.2 mm diameter, and a common ring anode around the cathode), designed to induce high current density. A third session without conditioning stimulation was used as a control. HFS stimulation consisted of 5 trains of 100 pulses, 2 ms pulse width, delivered at 100 Hz for 1 s and repeated at 10 s intervals. LFS stimulation consisted of a single train of 1000 pulses, 2 ms pulse width, delivered at 1 Hz, i.e. with a total duration of 16.7 min. The stimulation intensity in both paradigms was set to 10 times the detection threshold for a single 2 ms pulse. Perceptual intensity ratings to mechanical stimuli and blood flow changes were assessed in the conditioned area and closely around it. The nociceptive withdrawal reflex (NWR) was elicited distal to the conditioning electrode but within the same innervation area (superficial peroneal nerve). The reflex was elicited by randomized, graded stimulation intensities ranging from 0.5 to 1.5 times the baseline reflex threshold, and recorded in biceps femoris. Following low-frequency stimulation, an intensity-independent long-lasting facilitation of the NWR was observed, with a significant increase in the reflex size (average of 31 ± 4%, p < 0.001), the number of detected reflexes (average increase of 22 ± 10%, p < 0.01), and the blood flow in the conditioned area (increase of 50 ± 11% right after conditioning, p < 0.001). Perceptual ratings to mechanical stimuli and NWR test stimuli were not able to reflect this facilitation. HFS had no significant effect on the NWR, blood flow or perceptual measurements. These findings suggest that activity-dependent spinal sensitization can be elicited using conditioning electrical stimulation with a stimulation frequency that lies within the physiological firing range of primary afferents, and that it can be objectively assessed in humans using the NWR.