SIMULTANEOUS MEASUREMENT OF [H-3] NORADRENALINE RELEASE AND NEUROGENIC CONTRACTION UNDER IDENTICAL CONDITIONS, TO DETERMINE THE PREJUNCTIONAL INHIBITORY EFFECTS OF SKF-99101H AND BRL-56905 IN DOG SAPHENOUS-VEIN

Using a tissue bath system which allowed the simultaneous measurement of electrically-induced [H-3]noradrenaline release and neurogenic cont raction under identical conditions, we investigated the prejunctional inhibitory activity of the selective 5-HT1D/1B receptor agonists BRL 5 6905 3-amino-6-carboxamido-1,2,3,4-tetrahydrocarbazole) and SKF 99101H (3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate), com pared to sumatriptan and 5-HT. Transmural electrical stimulation (2 Hz ) of dog saphenous vein induced consistent increases in [H-3]noradrena line release as well as reproducible contractile responses (<10% decre ase over four stimulation periods). BRL 56905, SKF 99101H, sumatriptan and 5-HT (60 nM - 6 mu M) inhibited electrically-evoked [H-3]noradren aline release and neurogenic contractile responses in dog saphenous ve in. However, despite being measured under identical conditions, the in hibition of [H-3]noradrenaline release was consistently greater than t he inhibition of neurogenic contraction induced by a particular concen tration of agonist, suggesting that neurogenic contractile responses i n dog saphenous vein result from the combined release of noradrenaline and other non-noradrenergic neurotransmitters. Under the present assa y conditions, since the agonists produced only small (BRL 56905, sumat riptan and 5-HT) or marginal (SKF 99101H) contractile responses, it is unlikely that this is the cause of the discrepancy observed between i nhibition of release and inhibition of contraction. The inhibitory eff ects of BRL 56905, sumatriptan and 5-HT were blocked by the 5-HT1D/1B receptor antagonist methiothepin, consistent with the involvement of c anine ca-5-HT1D/1B receptors in inhibiting neurotransmitter release an d subsequent smooth muscle contraction in dog saphenous vein. The pres ent results show that the novel 5-HT1D/1B receptor agonists BRL 56905 and SKF 99101H are at least as potent as sumatriptan and 5-HT, at acti vating prejunctional inhibitory ca-5-HT1D/1B heteroreceptors on sympat hetic axon terminals in dog saphenous vein. In addition, when measured simultaneously in the same tissue preparation, [H-3]noradrenaline rel ease was inhibited to a much greater extent than neurogenic contractio n by any particular agonist.