ISCHEMIA-SENSITIVE SYMPATHETIC AFFERENTS INNERVATING THE GASTROINTESTINAL-TRACT FUNCTION AS NOCICEPTORS IN CATS

1. Activation of sympathetic visceral afferents during mesenteric isch aemia induces visceral pain and reflexly excites the cardiovascular sy stem. The present study investigated the differential responses of isc haemically sensitive and insensitive sympathetic C fibre afferents to graded distension of the gastrointestinal tract. 2. Single-unit activi ty of C fibre afferents innervating the stomach, duodenum and jejunum was recorded from the right thoracic sympathetic chain of anaesthetize d cats. Ischaemically sensitive and insensitive C fibre afferents were identified according to their response to 5-20 min of ischaemia. The functional characteristics of the stimulus-response relationships of a fferents were determined by distension of a balloon placed in the corr esponding segment of gastrointestinal tract. 3. The results show that ischaemically insensitive C fibre afferents had a lower threshold in r esponse to distension (13 +/- 5 mmHg, n = 10). The discharge frequency of these afferents was saturated within a low pressure range of diste nsion. However, ischaemically sensitive C fibre afferents had a high t hreshold (86 +/- 12 mmHg, n = 10) and a larger peak response to mechan ical distension in the noxious range (60-180 mmHg). There were no diff erences between ischaemically sensitive and insensitive C fibre affere nts with regard to their resting activity or responses to bradykinin ( IO mu g I.A.). 4. This study demonstrates that the gastrointestinal sy stem is innervated by low and high threshold sympathetic C fibre affer ents, the latter having the distinct ability to encode nociceptive inf ormation such as excessive distension and ischaemia.