Heterosynaptic long-term depression of craniofacial nociception: divergent effects on pain perception and blink reflex in man.

Noxious low-frequency stimulation (LFS) of presynaptic nerve fibers induces long-term depression (LTD) of synaptic transmission. In vitro studies suggest a sole homosynaptic effect. Consequently, the present study addressed the hypothesis that LTD of craniofacial nociception in man is mediated by a homosynaptic mechanism. Nociceptive supraorbital afferents were excited by electric pulses via a concentric electrode in ten healthy volunteers. The electrically evoked bilateral blink reflex (BR) was recorded from both orbicularis oculi muscles by surface electrodes. The BR was evoked in blocks of ten electric stimuli each (0.1 Hz) with an interblock interval of 8 min. Conditioning noxious LFS (1 Hz, 20 min) was applied via concentric electrode either to the same site as BR test stimuli (ipsilateral) or to the corresponding contralateral forehead area (contralateral). LFS and test stimulus intensities corresponded to about threefold the pain threshold. After three baseline stimulus blocks, either conditioning ipsilateral or contralateral LFS were applied or stimulation was interrupted for 20 min as a control task. Afterwards, test stimulation blocks were continued for 40 min. Each volunteer participated in all three sessions on different days. Noxious LFS induced LTD of the BR independently from the side of conditioning stimulation. Pain perception decreased after ipsilateral LFS but not after contralateral LFS. The bilateral effect of noxious LFS on the BR provides evidence for heterosynaptic LTD based on bilateral projections of supraorbital nerve afferents onto spinal trigeminal nuclei. The divergent effect on pain perception may be due to a preferential contralateral projection of nociceptive afferents onto reflex interneurons but not onto trigeminothalamic projection neurons.

Peripheral nerve stimulation inhibits nociceptive processing: an electrophysiological study in healthy volunteers.

Objectives. Electric peripheral nerve stimulation (PNS) is a neuromodulatory therapy in pain patients. The efficacy of this neurosurgical pain treatment is controversial because its antinociceptive effect in humans has not been objectively proven so far. Materials and Methods. Noxious infrared laser stimulation of the left hand dorsum evoked cortical potentials (LEP) by selective excitation of Aδ-fiber nociceptors in 15 healthy volunteers under control and PNS conditions. LEP were recorded before, during, and after electric Aß-fiber stimulation (PNS) of the left superficial radial nerve. In the control session LEP were recorded without PNS. Laser stimulus intensity ratings, LEP latencies, and amplitudes were statistically analyzed (anova). Results. During PNS, LEP amplitudes (p < 0.001) and laser intensity ratings (p < 0.05) significantly decreased, and LEP latencies significantly increased (p < 0.05). Under control conditions LEP and intensity ratings remained unchanged. Conclusions. The electrophysiologic data provide evidence that electric stimulation of peripheral Aß-fibers reliably suppresses Aδ-fiber nociceptive processing in human volunteers.