Colonic nociception via nucleus submedius is modulated by pontine centres in the rat

The rat thalamic nucleus submedius responds to noxious pressure stimuli in the colon. Some neurons in and near Barrington's nucleus, a pontine center related to bladder function, also respond to colon distension. We hypothesized that colonic nociception may be relayed via Barrington's nucleus to the nucleus submedius. Experiments were carried out in 22 urethane-anesthetized male rats. Noxious stimuli were applied to the toes using standardized clips and to the colon by inflation of the balloon to 80 mmHg for 30 s using a barostat. The brain was exposed to allow recording from the nucleus submedius with a monopolar tungsten electrode and the activity of rectus muscle was assessed via silver wire electrodes. A glass pipette was inserted into Barrington's nucleus for injection of 5 mM CoCl2, a temporary neural blocker. The site of CoCl2 injection was confirmed by the presence of FluoroGold which was incorporated into the CoCl2 solution. We recorded 51 units in submedius that were excited by noxious toe pinch, 4 were inhibited. Colon distension to 80 mmHg produced visceromotor responses, excited 23 units in submedius and inhibited 13 units. Injection of CoCl2 into the region of Barrington's nucleus blocked the response to colon distension in 10 of 12 Sm units tested, but had no influence on the accompanying visceromotor response. These data point to a previously unrecognized relationship between Barrington's nucleus and submedius that may subserve colon nociception.