A comparison of cinobufotenine (the quaternary derivative of 5-HT) and some related compounds with coryneine (the quaternary derivative of dopamine) on the frog rectus, guinea-pig ileum and rat fundus strip preparations.

1 Coryneine is 2.7 times as active as cinobufotenine on the frog rectus but on the guinea-pig ileum cinobufotenine is 1.5 times as active as coryneine. Cinobufotenine is a potent stimulant of parasympathetic ganglia and its effect are competitively antagonized by hexamethonium. 2 The effects of pH on activity relative to a standard whose ionisation is constant (Me4+N or the trimethylammonium analogue of tryptamine) are consistent with the phenate form being weaker than the phenolic form but the changes are smaller than with coryneine because cinobufotenine is a weaker acid. 3 The hydroxyl group makes a large contribution to activity. Cinobufotenine is 9 times as active as the analogue without a hydroxyl on the frog rectus and 12 times as active as it on the ileum. The 5-methoxy analogue is an antagonist on the frog rectus and a very weak partial agonist on the ileum. 4 Cinobufotenine and the quaternary derivative of tryptamine have less than one-thousandth of the activity of 5-hydroxytryptamine on the rat fundus strip.