Considerable evidence implicates sympathetic neural activity in acupuncture analgesia. However, the exact nature of these effects has not been specified in detail, and many experimental findings are contradictory. This study evaluated in normal conditions the specific sympathetic effects of both manual and electrical acupuncture applied to the same hand (Hoku) point. Thermographic measures of superficial skin temperature were used to assess sympathetic vasomotor tone in the face, hand and foot of 19 normal subjects. Baseline assessment, manual acupuncture and electrical acupuncture were performed in 3 separate sessions in a well controlled, 23 degrees C environment. Superficial skin temperature decreased slowly in the control condition. Both manual and electrical acupuncture produced a generalized long-lasting warming effect, indicating reduced sympathetic activity (sympatholytic effect). In addition, electrical acupuncture induced a localized short-term cooling effect, indicating a transient segmental increase in sympathetic activity (sympathomimetic effect).

Pain responses (pain detection and pain discomfort) to electrical dental stimulation were studied in 16 normal subjects. The repetition of the dental stimuli induced a significant and long-lasting (60 min) decrease in pain sensitivity at both sensory levels (after 60 min of repetitive stimulation, 79% increase in pain detection, P less than 0.0001, 45% increase in pain discomfort, P less than 0.0004). The sensory response decrement through repeated elicitation was not influenced by naloxone administration (1.2 mg i.m.). This study clearly demonstrates the induction of pain sensory decrease through repetitive stimulation which differs from peripheral sensory receptor adaptation, from the inhibitory gating mechanism or from diffuse inhibitory controls activation; its unresponsiveness to naloxone suggests that this phenomenon is not opioid-dependent. A technique has been standardized which will enable the systematic study of pain decrease under sustained nociceptive stimulation in chronic pain patients.