Long-term depression of the human masseter inhibitory reflex

Long-term depression (LTD) of synaptic transmission is reliably induced by low-frequency stimulation (LFS) of nociceptive afferents in vitro. LTD can only exceptionally be induced in anesthetized animals. In order to fill the gap between the in vitro cell studies and the in vivo situation, the effects of LFS on the masseter inhibitory reflex (MIR) were investigated in man. Noxious LFS of mental nerve afferents caused a significant depression of the early MIR1 and the late MIR2 components. Whereas the onset latency (+25%), the duration (-42%) and the integral (+68%) of the MIR2 were strongly modulated after LFS, only the integral (+35%) of the MIR1 significantly changed. The results document a long-term depression of trigeminal somatosensory processing in man. The different effects of LFS on the two components of the MIR may point to central mechanisms of LTD. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.