Effects of electrical stimulation of thalamic nucleus submedius and periaqueductal gray on the visceral nociceptive responses of spinal dorsal horn neurons in the rat

Electrical stimulation of the nucleus submedius (Sm) has been shown to suppress the viscerosomatic reflex (VSR), which is evoked by colorectal distension (CRD). We have examined the effects of focal electrical stimulation (0.3 ms, 50 Hz, 100 microA, 10 s) of the Sm and the periaqueductal gray (PAG) on the excitatory responses evoked by CRD in spinal dorsal horn neurons within the L6-S1 region in the urethane-anesthetized Wistar rats. Extracellular recordings were made from 32 spinal excitatory CRD responses. All of these neurons were convergent neurons with cutaneous receptive fields. The majority of the neurons (27/32) were wide dynamic range (WDR) neurons (responding to noxious and non-noxious cutaneous stimuli) while the remaining five neurons were nociceptive specific (NS) neurons (responding only to noxious cutaneous stimuli). The effects of electrical stimulation applied to 28 sites within the Sm were assessed for spinal neurons. Electrical stimulation in seven sites within the Sm (25%) inhibited the CRD excitatory response of dorsal horn neurons, while in two sites (7%) the same stimulation yielded facilitation. Electrical stimulation in the majority of the sites in the Sm (19/28, 68%) did not affect spinal excitatory CRD responses. On the other hand, electrical stimulation of the PAG clearly inhibited 20 of 22 (90%) CRD excitatory responses. These results suggest that the majority of Sm neurons may suppress VSR activity at a supraspinal reflex center rather than via a descending inhibition of spinal visceral nociceptive transmission, as is the case for the PAG.