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C. Dugast: Spinal Nociceptive Facilitation from the DRt
Material and methods with a specific delay relative to the inducing stimulation
pulse. According to current classification, fast (0-0.004
Experiments were performed in accordance with the and 0.004-0.04 s) responses were typically mediated by
European Communities Council Directive (86/609/EEC) the low-threshold A”$ and A* afferents, respectively,
and the guidelines of the International Association for the while slow ones (0.04-0.2 s) involved high-threshold C
Study of Pain. Male Sprague-Dawley rats were anaesthe-
tised with urethane. Their skull was mounted in a stereo- afferents (Gasser and Erlanger, 1927; Burgess and Perl,
1973). Repetitive electrical stimulation at 0.2 Hz could
taxic frame and drilled above the DRt in order to lower a also evoke more delayed responses currently referred
Hamilton syringe containing glutamate dissolved in saline as post-discharge (0.2-5.0 s).
solution. A laminectomy was performed to expose lumbar Injection of glutamate (10 mM) into the DRt dramatically
spinal segments. The ipsilateral sciatic nerve was careful-
ly dissected free in order to receive a bipolar electrode. increased the post discharge of WDR neurons whereas A-
and C-fibre mediated responses remained unaffected
The stimulation applied was a continuous train at 0.2 Hz throughout the recording (Fig. 1A). Surprisingly, glutamate-
composed of rectangular current pulses (one every 5 s) induced effects appeared about 400 s after the injection,
of noxious intensity. Single-unit activities were detected and developed slowly. Afterwards, despite a slight de-
through conventional electronics and send to a computer
for display, storage and off-line analysis. Only neurons in crease, the post-discharge remained at a higher level than
before injection; an effect that persisted during the entire
the dorsal horn ipsilateral to the DRt, and responding to experimental period and that was observed only in the
electrical noxious stimuli, were considered. Evoked re- presence of peripheral stimulation. In contrast, vehicle
sponses were analysed on the entire subsequent 5-s (NaCl 0.9% containing eosin 0.5%) did not induce any
inter-pulse interval and discriminated according to their
delay of occurrence relative to the inducing pulse. modifications of any of the evoked responses (Fig. 1B).

Results Discussion
Noxious electrical stimulation of the sciatic nerve Our study was aimed at elucidating the neuronal
induced responses of spinal WDR neurons appearing basis of the putative pronociceptive influence of the DRt

Figura 1. Effect of glutamate (A) and vehicle (B) injections into the ipsilateral DRt on the responses of deep dorsal horn
WDR neurons to noxious electrical stimulation. Glutamate DRt activation induced a delayed increase of the post-discharge DOR
without affecting primary-afferent mediated responses. Each point represents the mean ± SEM of 5 to 10 experiments.
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