Long-term depression of the human blink reflex.

Long-term depression (LTD) of synaptic transmission is reliably induced by low-frequency stimulation (LFS) of presynaptic nerve fibers in vitro. Many experiments suggest that LTD of basal transmission is not readily induced either in awake or anesthetized animals in vivo. In order to fill the gap between the in vitro cell studies and the in vivo situation, the effect of LFS on trigeminal somatosensory processing in healthy volunteers was investigated. Excitability of trigeminal sensory neurons was tested by applying the blink reflex (BR) elicited by electric stimulation of supraorbital nerve afferents. LFS of these afferents induced a significant reduction of the BR integral, a significant increase of the BR onset latency and a significant decrease of the stimulus intensity ratings. This depressive effect on the BR lasted for at least 1 h after the end of LFS. Thus, this study documented for the first time a long-term depression of trigeminal somatosensory processing in healthy volunteers.