Superficial and deep convergent nociceptive neurons are differentially affected by N-methyl-D-aspartate applied on the brainstem surface of the rat medullary dorsal horn

The activation of V-methyl-D-aspartate receptors is implicated in the spina l and trigeminal processing of nociceptive information conveyed by converge nt (wide dynamic range) neurons and particularly in C-fiber-evoked response s elicited by repetitive and high-intensity electrical stimulation of the n euronal receptive field. In this study, the effects of intrathecal NMDA app lication on the electrically evoked nociceptive responses of trigeminal sub nucleus caudalis convergent neurons have been investigated. The total C-fib er-evoked activity triggered by 30 successive stimuli was divided into two components: the C-fiber input response and the 'wind-up' response. Applicat ion of 0.1 mug (in 50 mul) of NMDA evoked a bi-directional effect on the to tal C-fiber-evoked activity of 19 neurons tested. A significant increase in the total C-fiber-evoked activity was observed 15-25 min after the NMDA ap plication for nine neurons located in superficial laminae II and III. In co ntrast. a significant decrease in the total C-fiber-evoked activity was obs erved 5-25 min after the NMDA application for 10 neurons located more deepl y, in lamina V. The NMDA-induced modifications seen in the total C-fiber-ev oked activity were likely a reflection of a significant increase or decreas e in neuronal activity evoked by the C-fiber input rather than wind-up of t he responses since the latter was not significantly modified by the NMDA ap plication. These results provide evidence for a possible inhibitory role for NMDA-depe ndent interneurons of the superficial laminae of the medullary dorsal horn on the nociceptive activity of deep convergent neurons. (C) 2001 IBRO. Publ ished by Elsevier Science Ltd. All rights reserved.