The serotonin paradox: drug-receptor interaction in rat vas deferens.

The contractile effect of serotonin was studied in rat vas deferens, in comparison with that of noradrenaline and tyramine, after reserpine treatment, surgical denervation, and transplantation to the colon. In reserpinized animals the effect of 5HT resembled that of tyramine, since it was strikingly reduced, in spite of a small residual effect, showing that in normal preparations the effects of 5HT and tyramine are predominantly due to the release of endogenous noradrenaline. However, in denervated or transplanted vas deferens, in which the effect of tyramine is also abolished, the effect of 5HT was potentiated. It is suggested that after chronic, long lasting depletion of endogenous noradrenaline, there are alternate mechanisms that are generated to improve the contractile effect of 5HT, but not of tyramine. The nature of these mechanisms is still unknown.