Homotopic long-term depression of trigeminal pain and blink reflex within one side of the human face.

Objective: Electrical low-frequency stimulation (LFS) of cutaneous afferents elicits long-term depression (LTD) of craniofacial nociception and pain. The study addresses spatial organization of LTD within one side of the face.

Methods: The blink reflex was evoked by electrical test stimulation in the supraorbital nerve area before and after conditioning LFS in 10 healthy volunteers. Noxious LFS (1Hz) was applied to skin afferents of supraorbital (sLFS), infraorbital (iLFS), or mental (mLFS) nerves. All stimuli were applied to the same side of the face with intensities of three times pain threshold. Volunteers rated the test stimulus intensity. Each volunteer participated in four sessions with sLFS, iLFS, mLFS, or without LFS (control).

Results: Pain ratings of electrical test stimulation were differently affected in experimental sessions (p<0.01) with strongest reduction by -19.6% after sLFS. The reflex integral decreased by -34.7% significantly stronger after sLFS than in all other experimental conditions (p<0.001). Electrical pain thresholds increased significantly stronger in sLFS than in iLFS, mLFS, and control (p<0.001).

Conclusions: Trigeminal nociception and pain were inhibited by homotopic LFS at the forehead but not after heterotopic LFS of infraorbital and mental nerve skin afferents.

Significance: Homotopic organization of ipsilateral trigeminal LTD in man may have implications for future neuromodulatory treatment of chronic craniofacial pain.