Response characteristics of nucleus submedius neurons to colo-rectal distension in the rat

The effects of colorectal distension (CRD) were examined on neurons located in and around the nucleus submedius (Sm) in the medial thalamus of urethane-anesthetized rats. A total of 66 units (49 in the Sm and 17 in immediately surrounding regions) responding to cutaneous pinch were tested to examine their responsiveness to the CRD. All the neurons that responded to cutaneous stimulation were nociceptive specific (NS) neurons. Based on their responses to the CRD the Sm neurons were classified into three types as follows: 23 (47%) of 49 neurons in the Sm and three (18%) of 17 neurons near the Sm had tonic excitatory responses with long-lasting after-discharges (type I); nine (18%) Sm neurons and four (24%) peri-Sm neurons were tonically excited but had no after-discharge (type II); and seven (14%) Sm neurons were inhibited (type III). Ten (20%) Sm neurons and 10 (59%) peri-Sm neurons did not respond to CRD. All the excitatory and inhibitory responses to CRD increased with increasing CRD pressure. Simultaneous application of CRD and cutaneous pinch did not produce a reduced response (nocigenic inhibition). These results demonstrate that most of the Sm neurons receive convergent viscerosomatic inputs from the colon and/or rectum and from the skin, suggesting that the Sm may participate in visceral nociception. (C) 1997 Elsevier Science Ireland Ltd.