AN IN-VITRO STUDY OF THE PROPERTIES OF SINGLE VAGAL AFFERENTS INNERVATING GUINEA-PIG AIRWAYS

1. A novel preparation of the trachea and main bronchi with attached v agus nerve from the guinea-pig maintained in vitro was used to study t he properties of single vagal afferent nerve fibres with identified re ceptive fields. 2. Recordings were made from twenty-eight C fibres wit h a mean conduction velocity of 0.9+/-0.1 m s-1 and twenty-four Adelta fibres with a mean conduction velocity of 8.4+/-1.3 m s-1. Receptive fields for C and Adelta fibres were of small diameter, distributed thr oughout the trachea and right bronchus and possessed very low mechanic al thresholds of 2.2+/-0.4 and 1.1+/-0.3 mN respectively. 3. The chemo sensitivity of isolated afferents was studied by applying drugs direct ly onto identified receptive fields. Adelta fibres were insensitive to capsaicin (up to 3 muM), bradykinin (3 muM), histamine (10 muM) and 5 -hydroxytryptamine (5-HT; 10 muM) applied for up to 1 min. Histamine ( 10 muM), 5-HT (10 muM) and m-chlorophenylbiguanide (10 muM) were also ineffective in exciting C fibres. 4. Capsaicin, at concentrations rang ing from 30 nM to 3 muM, evoked a sustained firing of all C fibres tes ted when applied for a period of 30 s directly onto receptive fields. Bradykinin (0.1-1 muM) also potently excited C fibres in a concentrati on-related manner. The effect of bradykinin appeared to be mediated by a B2 receptor since it was not mimicked by the selective B1 receptor agonist [des-Arg9]-bradykinin (3 muM) and was abolished by prior appli cation of the selective B2 receptor antagonist D-Arg-[Hyp3,Thi5,D-Tic7 ,Oic8]-bradykinin (HOE 140; 0.1 muM). HOE 140 was without effect again st capsaicin-evoked discharge of C fibres. 5. Capsaicin- and bradykini n-evoked discharge of C fibres was present to a similar degree in prep arations pretreated with ibuprofen (1 muM), indicating that it was not dependent on, or influenced by, endogenous prostaglandin production. 6. These data demonstrate that single vagal afferents may be studied i n vitro and provide the first examination of the properties of sensory fibres innervating guinea-pig airways. C and Adelta fibres both exhib it low threshold mechanical sensitivity but show marked differences in terms of their chemosensitivity.