Inhibitory effects evoked from the rostral ventrolateral medulla are selective for the nociceptive responses of spinal dorsal horn neurons.

The aim of the present study was to determine whether or not descending control of spinal dorsal horn neuronal responsiveness following neuronal activation at pressor sites in the rostral ventrolateral medulla is selective for nociceptive information. Extracellular single-unit activity was recorded from 49 dorsal horn neurons in the lower lumbar spinal cord of anaesthetized rats. The 30 Class 2 neurons selected for investigation responded to noxious (pinch and radiant heat) and non-noxious (prod, stroke and/or brush) stimulation within their cutaneous receptive fields on the ipsilateral hindpaw. The excitatory amino acid, DL-homocysteic acid, was microinjected into either the rostral or the caudal rostral ventrolateral medulla at sites that evoked increases in arterial blood pressure. Effects of neuronal activation at these sites were then tested on the responses of Class 2 neurons to noxious and non-noxious stimulation within their excitatory receptive fields. The noxious pinch and radiant heat responses of Class 2 neurons were depressed, respectively to 13+/-3.8% (n=23) and to 16+/-3.7% (n=18) of control, following stimulation at sites in the rostral rostral ventrolateral medulla. In contrast, the low-threshold (prod) responses of eight Class 2 neurons tested were not depressed following neuronal activation at the same sites. When tested, control injections of the inhibitory amino acid, GABA, at the same sites in the rostral rostral ventrolateral medulla had no significant effects on neuronal activity. Neither intravenous administration of noradrenaline (to mimic the pressor responses evoked by DL-homocysteic acid microinjections in the rostral ventrolateral medulla) nor activation at pressor sites in the caudal rostral ventrolateral medulla had any significant effect on neuronal responsiveness. With regard to sensory processing in the spinal cord, these data suggest that descending inhibitory control that originates from neurons in pressor regions of the rostral rostral ventrolateral medulla is highly selective for nociceptive inputs to Class 2 neurons. These data are discussed in relation to the role of the rostral ventrolateral medulla in executing the changes in autonomic and sensory functions that are co-ordinated by higher centres in the CNS.