Site of action of antinociceptive acupuncture like nerve stimulation in the spinal rat as visualized by the 14C-2-deoxyglucose method

Aim: It has been claimed that supraspinal structures are necessary for
the depression of nocifensive behaviour elicited by acupuncture or acupuncturelike (AL) stimulation. However, nocifensive reflexes in the spinal cat
were inhibited for long periods by a mechanism releasing opieids on AL conditioning stimulation of groups II-III muscle afferents (Sj~lund and Eriksson, Neurosci.Lett. S3:264:1979). We now present autoradiographic measurements of regional "nervous" activity with the 14C2deoxyglucose method
(Sokoloff et al.,J.Neurochem.28:897,1977),indicating that this inhibition
of nociceptive activity occurs in the outer dorsal horn of spinal rats.
Methods: Wistar rats were decerebrated and spinalized at C1 under halothane. After curarization, noxious stimuli were given to one hindpaw as
bradykinin intracut, or as radiant heat, and 25uC 14C2deoxyglucose was administered. In two test groups, 20min of AL stimulation (needles in ipsilateral sciatic nerve; lOOHz train, duration 70ms, repetition rate 2Hz; intensity 4x twitch threshold) or morphine (img/kg iv) preeeeded the nociceptive stimulation. The autoradiograms were scanned along lines covering
laminae I-VI of both sides of the lumbar cord.
Results: In controls the activity increased ipsilateral to the noxious
stimuli in all the laminae, notably l-III. In rats receiving AL stimulation
an increased activity was seen only in the ipsilateral laminae II-IIIwhereas no side to side differences we~e seen in morphine treated rats.
Conclusions: Inhibition of nociceptive activity in the rat dorsal horn by
opioids seems to occur already in lamina I. The activity in laminae II-III
after AL stimulation might be due to interneurones normally responsible for
this inhibition. (Supported by the Swedish MRC, proj.no 05658).