The medullary dorsal reticular nucleus enhances the responsiveness of spinal nociceptive neurons to peripheral stimulation in the rat

Single-unit spinal recordings combined with application of glutamate into the medullary dorsal reticular nucleus were used to assess the action of this nucleus upon deep dorsal horn neurons in rats. Injection of high glutamate concentrations (10 and 100 mm) induced a dramatic and long-lasting increase of the responses of wide-dynamic range neurons to electrical stimulation of the sciatic nerve in the noxious range, without affecting ongoing discharges. Post-stimulus time histograms revealed that this increase concerned the post-discharge, but not A- or C-fibre-mediated responses, which remained unchanged independently of the stimulation frequency applied. The onset of the glutamate-induced response enhancement occurred with a concentration-dependent time delay and developed slowly until its maximum. These data indicate that the medullary dorsal reticular nucleus exerts a facilitating action upon deep dorsal horn wide-dynamic range neurons by enhancing their capacity to respond to peripheral stimulation through prolongation of their discharge. This action is accompanied by the strengthening of wind-up of deep dorsal horn wide-dynamic range neurons, hence providing a plausible substrate for chronic pain states. These results are in agreement with previous behavioural studies suggesting a pronociceptive role for the dorsal reticular nucleus [Almeida et al . (1996) Brain Res. Bull ., 39 , 7-15; Almeida et al . (1999) Eur. J. Neurosci ., 11 , 110-122], and support the involvement of a reverberating circuit, previously described in morphological studies [Almeida et al . (1993) Neuroscience , 55 , 1093-1106; Almeida et al . (2000) Eur. J. Pain , 4 , 373-387], which probably operates only at a certain threshold of activation.