Site- and modality-specific modulation of experimental muscle pain in humans

The neurophysiological mechanisms involved in diffuse noxious inhibitory co ntrols (DNIC) have been investigated extensively, but information is lackin g about the effect of different stimulus modalities and somatic locations o n the effectiveness of DNIC. This study is the first to examine the hypoalg esic effects on a deep, tonic and painful test stimulus (TS) of both painfu l and non-painful conditioning stimuli (CS) applied to different sites of t he body. Two separate experiments were performed using painful electrical s timulation of the left anterior tibialis muscle as the TS. In the first exp eriment (n = 9), injection of 5% hypertonic saline was used as a painful CS into one of four muscles: anterior tibialis of each leg and brachioradiali s of each arm. In the second experiment (n = 5), a non-painful vibratory st imulus was used as the CS at the same four sites. Compared with TS alone, t he perceived pain intensity of the TS increased (4.5 +/- 1.8%; P = 0.019) i n combination with the painful CS applied to the same muscle (ipsilateral h omotopic site), but decreased (-25.3 +/- 1.4%; P < 0.001) in combination wi th non-painful CS at the same site. Both painful and non-painful CS applied at the three heterotopic sites caused significant and site-dependent decre ases in the perceived pain intensity of the TS (range 15%-37%; P < 0.05). W e conclude that a hypoalgesic DNIC-like effect on muscle pain is not produc ed exclusively by painful stimuli, and that the valence and magnitude of th e modulation depend on the nature of the CS and its location relative to th e applied TS. (C) 1999 Elsevier Science B.V. All rights reserved.