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 N-methyl-D-aspartate receptors is implicated in the spinal and trigeminal processing of nociceptive information conveyed by convergent (wide dynamic range) neurons and particularly in C-fiber-evoked responses elicited by repetitive and high-intensity electrical stimulation of the neuronal receptive field. In this study, the effects of intrathecal NMDA application on the electrically evoked nociceptive responses of trigeminal subnucleus caudalis convergent neurons have been investigated. The total C-fiber-evoked activity triggered by 30 successive stimuli was divided into two components: the C-fiber input response and the 'wind-up' response. Application of 0.1 microg (in 50 microl) of NMDA evoked a bi-directional effect on the total 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 application for nine neurons located in superficial laminae II and III. In contrast, a significant decrease in the total C-fiber-evoked activity was observed 5-25 min after the NMDA application for 10 neurons located more deeply, in lamina V. The NMDA-induced modifications seen in the total C-fiber-evoked activity were likely a reflection of a significant increase or decrease in neuronal activity evoked by the C-fiber input rather than wind-up of the responses since the latter was not significantly modified by the NMDA application. These results provide evidence for a possible inhibitory role for NMDA-dependent interneurons of the superficial laminae of the medullary dorsal horn on the nociceptive activity of deep convergent neurons.