EVIDENCE THAT TACHYKININS ARE THE MAIN NANC EXCITATORY NEUROTRANSMITTERS IN THE GUINEA-PIG COMMON BILE-DUCT

1 Application of electrical field stimulation (EFS; trains of 10 Hz, 0 .25 ms pulse width, supramaximal voltage for 60 s) to the guinea-pig i solated common bile duct pretreated with atropine (1 mu M), produced a slowly-developing contraction ('on' response) followed by a quick pha sic 'off' contraction ('off peak' response) and a tonic response ('off late' response), averaging 16+/-2, 73+/-3 and 20+/-4% of the maximal contraction to KCl (80 mM), n=20 each, respectively. Tetrodotoxin (1 m u M; 15 min before) abolished the overall response to EFS (n=8). 2 Nei ther in vitro capsaicin pretreatment (10 mu M for 15 min), nor guaneth idine (3 mu M, 60 min before) affected the excitatory response to EFS (n=5 each), showing that neither primary sensory neurons, nor sympathe tic nerves were involved. N-omega-nitro-L-arginine (L-NOARG, 100 mu M, 60 min before) or naloxone (10 mu M, 30 min before) significantly enh anced the 'on' response (294+/-56 and 205+/-25% increase, respectively ; n=6-8, P<0.01) to EFS. The combined administration of L-NOARG and na loxone produced additive enhancing effects (655+/-90% increase of the 'on' component, n=6, P<0.05). 3 The tachykinin NK2 receptor-selective antagonist MEN 11420 (1 mu M) almost abolished both the 'on' and 'off late' responses (P<0.01; n=5 each) to EFS, and reduced the 'off-peak' contraction by 55+/-8% (n=5, P<0.01). The subsequent administration of the tachykinin NK1 receptor-selective antagonist GR 82334 (1 mu M) an d of the tachykinin NK3 receptor-selective antagonist SR 142801 (30 nM ), in the presence of MEN 11420 (1 mu M), did not produce any further inhibition of the response to EFS (P>0.05; n=5 each). At 3 mu M, GR 82 334 significantly reduced (by 68+/-9%, P<0.05, n=6) the 'on' response to EFS. 4 The contractile 'off peak' response to EFS observed in the p resence of both MEN 11420 and GR 82334 (3 mu M each) was abolished (P< 0.01; n=6) by the administration of the Pt purinoceptor antagonist pyr idoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 mu M). P PADS (30 mu M) selectively blocked (75+/-9 and 50+/-7% inhibition, n=4 each) the contractile responses produced by 100 and 300 mu M ATP. 5 T achykinin-containing nerve fibres were detected by using immunohistoch emical techniques in all parts of the bile duct, being distributed to the muscle layer and lamina propria of mucosa. In the terminal part of the duct (ampulla) some labelled ganglion cells were observed. 6 In c onclusion, this study shows that in the guinea-pig terminal biliary tr act tachykinins, released from intrinsic neuronal elements, are the ma in NANC excitatory neurotransmitters, which act by stimulating tachyki nin NK2 (and possibly NK1) receptors. ATP is also involved as excitato ry neurotransmitter. Nitric oxide and opioids act as inhibitory mediat ors/modulators in this preparation.