Electrical low-frequency stimulation (LFS) of cutaneous afferents elicits long-term depression (LTD) of craniofacial nociception and pain (Suppl Clin Neurophysiol 58: 195–208, 2006). In-vitro studies indicate that LFS exclusively induces LTD at the first nociceptive synapse without any effect on convergent input onto the same secondary sensory neuron (homosynaptic). A recent study in man reports on LTD of craniofacial nociception after contralateral LFS suggesting heterosynaptic effects as well (Exp Brain Res 170: 414–22, 2006). The human experimental study addresses spatial organization of LTD within one side of the face.

Forty experimental sessions were conducted in 10 healthy volunteers (22 to 29 yr). Trigeminal nociceptive processing was investigated by the blink reflex (BR) recorded from orbicularis oculi muscles. The BR was evoked by electrical test stimulation (500 ms) applied to the supraorbital nerve area above the eyebrow. BR series consisted of 10 stimuli each (0.1 Hz) and were repeated every 8 minutes. Noxious LFS (1 Hz, 1200 pulses) was applied to supraorbital (sLFS), infraorbital (iLFS), or mental (mLFS) nerve areas. Electrical test stimulation and conditioning LFS were applied to the same side of the face (n=5 right, n=5 left) by a concentric electrode with an intensity of 3times pain threshold. Volunteers were asked to rate the test simulus intensity according to a verbal rating scale (0 to 100). Each volunteer participated in 4 sessions with sLFS, iLFS, mLFS, or without LFS (Control) in a balanced order. BR series were continued for one hour after conditioning.

Pain ratings due to electrical test stimulation (supraorbital) were differently affected in experimental sessions (p<0.01). A median reduction of pain ratings by -19.6% after sLFS was significantly stronger than in all other sessions. After sLFS, BR integral decreased by -34.712.2% significantly stronger than in all other experimental conditions (p<0.001). Comparison of relative changes of electrical pain thresholds during experimental sessions revealed significantly stronger increase in sLFS as compared to iLFS, mLFS, and Control (p<0.001).

Trigeminal nociception (BR) and pain (rating) were inhibited by homotopic LFS at the forehead but not after heterotopic LFS in infraorbital and mental nerve areas. In contrast to a recent study on contralateral effects of sLFS on BR the present study does not indicate any suppressive effects of heterotopic LFS within one side of the face. Direct trigeminal projections of primary sensory neurons to contralateral secondary sensory neurons may mediate LTD after contralateral LFS (Somatosens Mot Res 7: 153–83, 1990). Intersubnuclear connections within the spinal trigeminal nucleus on one side seem to restrict LFS effects within the stimulated site (Somatosens Mot Res 7: 399–420, 1990). Data support a homotopic spatial organization of ipsilateral trigeminal LTD in man.