CHARACTERIZATION OF MECHANOSENSITIVE PELVIC NERVE AFFERENT-FIBERS INNERVATING THE COLON OF THE RAT

1. Single-unit activity was recorded from S, sacral dorsal,root afferent fibers in the anesthetized rat. A total of 364 afferent fibers were identified by electrical stimulation of the pelvic nerve and subsequently tested for response to colorectal distension (CRD) and urinary bladder distension (UBD). Sixty-seven percent (n = 244) of the fibers were unmyelinated C-fibers and 33% (n = 120) were thinly myelinated AG-fibers. 2. In three initial experiments, 35 fibers were identified by pelvic nerve stimulation and tested for response only to CRD; none of these fibers responded to CRD. In 20 subsequent experiments, 329 pelvic nerve afferent fibers were tested for response to CRD and UBD. Thirty-four percent (n = 112) of the 329 fibers were unresponsive to noxious CRD (80 mmHg) or to UBD (slow filling less than or equal to 100 mmHg), 44% (n = 146) responded to UBD, 16% (n = 53) responded to CRD, and 6% (n = 18) responded to mechanical stimulation of the anal mucosa. 3. Of the total of 53 pelvic nerve afferent fibers that responded to CRD, 43 (81%) were C-fibers (mean: 1.5 m/s) and 10 (19%) were A delta-fibers (mean: 4.7 m/s). Fifteen of the CRD-sensitive fibers had no resting activity, whereas 38 fibers exhibited some resting activity (mean: 2.6 imp/s). 4. Reproducibility of responses to repeated CRD (80 mmHg, 30s, 10 trials at 4-min intervals) was tested in 17 fibers. In 16, responses to repeated distension were reproducible without evidence of facilitation or inhibition of subsequent responses. One fiber gave greater responses during the 9th and 10th trials. 5. Responses to graded CRD were studied in 44 fibers. All fibers exhibited monotonic, increasing stimulus-response functions (80 mmHg of distension. Thresholds for response of the 44 fibers were determined after extrapolation of the least-squares linear-regression line to the ordinate and varied between 0 and 40 mmHg. Two populations of pelvic nerve afferent fibers in the colon were apparent: low threshold (LT) afferent fibers had a mean threshold of 2.9 mmHg (range: 0-10 mmHg; n = 34) and high threshold (HT) afferent fibers had a mean threshold of 32.6 mmHg (range: 28.0-40.0 mmHg; n = 10). 6. Chemosensitivity to bradykinin (BK) was tested in nine LT fibers. Seven fibers responded to BK (0.1 to 100 mu g/kg ia) and two fibers did not respond up to 100 mu g/kg of BK. Responses to BK tested in three fibers were dose dependent. BK produced responses in the presence of loperamide-induced smooth muscle paralysis, suggesting that responses to BK represent a direct action at the fibers' receptive endings. D-Arg-[Hyp(3),Thi(5),D-TiC7,Oic(8)]-BK, a B-2- receptor antagonist, blocked responses of the fibers to BK. 7. A total of 18 pelvic nerve afferent fibers were identified in the perianal mucosa. These fibers responded with a burst to light stroking of mucosal receptive fields. Seventeen of these 18 fibers were unresponsive to CRD; 1 fiber exhibited a transient burst of discharge at the onset of phasic CRD. The majority (77%) of the mucosal afferent fibers, in contrast to colonic afferent fibers, were myelinated with a mean conduction velocity of 13.6 m/s (n = 14). Four fibers (23%) were unmyelinated C-fibers with a mean conduction velocity of 2.0 m/s.