Acupuncture and diffuse noxious inhibitory controls: naloxone-reversible depression of activities of trigeminal convergent neurons

Abstract

Recordings were made from convergent neurons in trigeminal nucleus caudalis of the rat. These neurons could be activated by both innocuous and noxious mechanical stimuli applied to their excitatory receptive fields on the ipsilateral part of the muzzle. Percutaneous application of suprathreshold, 2 ms square-wave electrical stimuli to the centre of the excitatory field resulted in responses to A- and C-fibres being observed. The effects on these responses of manual acupuncture performed by a traditional Chinese acupuncturist at the "Zusanli" point on the right hindlimb were compared with the effects induced by acupuncture applied at a non-acupoint, next to "Zusanli". In addition, the effects of acupuncture were compared with the inhibitory effects evoked by noxious thermal stimulation of the left hindlimb on the responses of the same neurons. This last type of inhibition has been described previously by our group and termed diffuse noxious inhibitory controls. Acupuncture, either applied at "Zusanli" or at a non-acupoint and noxious thermal stimulation induced similar strong inhibitory effects on the C-fibre-evoked responses of trigeminal convergent neurons (77.9 +/- 4.4%; 72.5 +/- 4.6% and 78.5 +/- 3.6% inhibition, respectively) and these inhibitions were followed by long-lasting aftereffects. In addition, both the acupuncture- and noxious thermal stimulation-evoked inhibitions were significantly reduced by systemic naloxone (0.4 mg/kg, i.v.). Since the antinociceptive effects elicited by acupuncture (i) had a similar magnitude and time-course to those evoked by noxious thermal stimulation, (ii) exhibited a lack of topographical specificity and (iii) involved an opioidergic link, we would suggest that, at least in our experimental conditions, acupuncture manoeuvres trigger the neuronal mechanisms involved in diffuse noxious inhibitory controls.