1. Exogenous and endogenous tachykinins facilitate cholinergic nerve-induced bronchoconstriction in guinea-pig. Using a vagally innervated guinea-pig tracheal tube preparation we have investigated the involvement of endogenous capsaicin-sensitive neuropeptides in both pre- and postganglionic cholinergic neurotransmission. The effects of the neutral endopeptidase inhibitor (NEP), phosphoramidon, were investigated in this preparation either alone or in conjunction with sensory neuropeptide depletion by capsaicin pretreatment. The subtype of neurokinin receptor mediating this facilitatory effect of tachykinins has also been examined, by the use of selective tachykinin receptor agonists and a selective NK1 receptor antagonist. 2. Cholinergic contractions of the sealed Krebs filled tracheal tube preparation were recorded as increases in intraluminal pressure and were induced either by (i) pre-ganglionic vagus nerve stimulation (PGS), (ii) stimulation of postganglionic intramural nerves via transmural stimulating electrodes (TMS) in the presence of ganglion-blocking concentrations of hexamethonium and (iii) application of exogenous acetylcholine (ACh). 3. The effect of phosphoramidon, which inhibits the breakdown of tachykinins, was investigated on ACh-, PGS- and TMS-induced contractions. Phosphoramidon (1-10 microM) facilitated contractions of the trachea induced by PGS, in a concentration-dependent manner, but had no effect on contractions of the trachea induced either by TMS or exogenous ACh. 4. The facilitatory effect of phosphoramidon (10 microM) on PGS-induced contractions was abolished by pretreating guinea-pigs with capsaicin 7 +/- 2 days before the in vitro experiments. Capsaicin pretreatment did not significantly alter responses to the spasmogens, ACh or substance P. Depletion of sensory neuropeptides, by capsaicin pretreatment was confirmed by the lack of response to capsaicin (1 microM) in vitro. 5. The facilitatory effect of phosphoramidon (10 microM) on PGS-induced contractions was inhibited by the selective NK1 receptor antagonist, GR71251 (1 microM). When applied to the tissues during nerve stimulation,GR71251 caused a small, but significant, inhibition of PGS-induced contractions during low frequency stimulation. No significant effect of GR71251 on TMS-induced contractions was seen at any frequency. There was no significant effect of the NK1 receptor antagonist on contractions of the trachea induced by exogenous ACh.6. The selective NK1 receptor agonist, GR73632 facilitated contractions of the trachea induced by stimulation of both pre- and postganglionic cholinergic nerves, in a concentration-dependent manner, at concentrations that had no significant effect on basal tone (0.01-0.3 nM). The facilitatory effect ofGR73632 on both PGS- and TMS-induced contractions was antagonized by GR71251 (1 microM). In contrast, neurokinin A (1 - 10 nM), which preferentially stimulates NK2 receptors, facilitated contractions induced by both PGS and TMS, and caused a significant increase in basal tone of the trachea. The selective NK3 receptor agonist, senktide (30-300 mM), had no significant effect on nerve-induced contractions or basal tone of the trachea.7. These results suggest that there is release of endogenous tachykinins during vagus nerve stimulation,which can be depleted by capsaicin pretreatment and, which facilitate cholinergic nerve-induced contractions at the level of the parasympathetic ganglia. Facilitatory tachykinin receptors on the postganglionic nerve terminals can be demonstrated by exogenous agonists but do not appear to be activated by endogenous tachykinins under the stimulation conditions of these studies. These data suggest that NK1,receptors may be involved in mediating this facilitatory response to tachykinins but do not exclude an involvement of NK2 receptors. It appears unlikely, however, that NK3 receptors are involved.