Acupuncture-evoked responses of subnucleus reticularis dorsalis neurons in the rat medulla.

Recordings were made from neurons in subnucleus reticularis dorsalis of the rat. Two populations of neurons could be distinguished: those with total nociceptive convergence which were driven by activating A delta- and C-fibers from any part of the body and those with partial nociceptive convergence which were driven by activating A delta-fibers from any part of the body or C-fibers from some, mainly contralateral, regions. The effects on subnucleus reticularis dorsalis neurons of manual acupuncture, performed by a traditional Chinese acupuncturist at the "Renzhong", "Sousanli", "Changqiang", and "Zusanli" acupoints and at a non-acupoint next to "Zusanli", were studied. Acupuncture stimulation for 30 s at the acupoints or the non-acupoint strongly excited all the total nociceptive convergence neurons tested; these neurons responded with a discharge of rapid onset which was often followed by after-discharges lasting for approximately 30-60 s. The majority but not all of the partial nociceptive convergence neurons responded to 30 s of acupuncture stimulation at the acupoints or the non-acupoint. This was especially the case when the stimulus was applied to contralateral or midline parts of the body. The potency of acupuncture as a means of activating subnucleus reticularis dorsalis neurons varied significantly with the area of the body being stimulated such that: contralateral greater than midline greater than ipsilateral areas. The levels of induced activity were of similar magnitude to those evoked by noxious mechanical stimuli applied under identical experimental conditions. No differences were found between the capacities to activate subnucleus reticularis dorsalis neurons of the "Zusanli" point and the adjacent non-acupoint, no matter whether these were stimulated ipsi- or contralaterally; this suggests a lack of topographical specificity in the activation of these neurons. Since subnucleus reticularis dorsalis neurons are activated exclusively or preferentially by noxious inputs, it is concluded that the signals elicited by manual acupuncture travel through pathways responsible for the transmission of nociceptive information. Since acupuncture, a manoeuvre which is known to elicit widespread extrasegmental antinociceptive effects, activates subnucleus reticularis dorsalis neurons which, anatomically, send dense projections to the dorsal horn at all levels of the spinal cord, we would suggest that this structure may be involved not only in signalling pain but also in modulating pain by means of spino-reticulo-spinal feed-back mechanisms.