CHEMOSENSITIVITY OF FINE AFFERENTS FROM RAT SKIN INVITRO

Properties of sensory receptors with slowly conducting nerve fibers (< 10 m/s) were studied using a rat skin-saphenous nerve in vitro preparation where receptive fields of identified single units can be isolated and superfused at the corium side with defined chemical solutions. With mechanical search stimuli, 150 slowly adapting units were identified, 88% C-fibers, and the remainder, Aδ-fibers. The majority of these units (65%) were categorized as mechano-heat sensitive ('polymodal') with controlled radiant heat stimulation. The remaining units were classified as low- or high-threshold mechanoreceptors according to their von Frey thresholds. Bradykinin (BK), in concentrations of 10-8 to 10-4 M, was repeatedly applied for 1 min at 10-min intervals. Fifty-six percent of the polymodal C-fibers responded to BK (up to 10-5 M), in contrast to 17% of the heat-insensitive units (P < 0.01). No correlation between BK sensitivity and conduction velocity or von Frey threshold was found. The Bk 'threshold concentrations' to excite C- and Aδ-fibers were about equally distributed over a range from 10-8 to 10-5 M. There was a large interindividual variability in pattern and magnitude of the response to BK. Intraindividually, a marked tachyphylaxis upon repeated BK stimulation was observed. In fibers with a slow development of tachyphylaxis, the effects of conditioning application of different chemicals on BK responsiveness were studied. Norepinephrine in 10-7 M concentration did not produce a significant effect, whereas 10-5 M and 10-4 M seemed to increase the BK responses. Prostaglandin E2 (10-6 M) caused a weak sensitization to BK on average (n.s.), but serotonin (10-6 M) was clearly effective (P < 0.05). The strongest sensitization to BK (P = 0.01) resulted from conditioning heat stimulation, which also uncovered a responsiveness in some units initially insensitive to BK. In some experiments the calcium concentration in the superfusate of receptive fields was lowered to 0.3 mM, which induced ongoing activity in C-fibers and markedly increased the BK responses in two polymodal units tested. Increasing the calcium concentration to 3.0 mM reversed these effects. After completing the BK test protocol, polymodal C-fibers were exposed to other chemicals. All BK-sensitive units tested were also excited by capsaicin (10-5 M), 75% by histamine (10-4 M) and 43% by serotonin (10-5 M). Among the BK-insensitive units, only one of seven fibers was excited by histamine, none by serotonin, and only three of six units by capsaicin. Heat thresholds of C-fibers treated with and sensitive to BK were significantly lower than those of BK-insensitive or untreated units. Irrespective of any chemical treatments, the von Frey thresholds of all units remained essentially unchanged throughout the experiments.