Antagonism of conditioned salivation in conscious dogs by antihypertensive drugs.

Methyldopate (methyldopa (ethyl ester)), carbidopa, clonidine, and ST-91 were evaluated for their effects on conditioned salivation in unanesthetized dogs. Clonidine produced dose-dependent inhibition of salivation 20 min after an intravenous injection. At equivalent and larger doses, ST-91, a clonidine analog which does not penetrate the blood-brain barrier, was ineffective in inhibiting conditioned salivation, suggesting that central rather than peripheral mechanisms are involved in clonidine-induced inhibition of salivation. Methyldopate also produced a dose-dependent inhibition of salivation in dogs. The mechanism involved in methyldopa-induced inhibition of salivation may involve both central and peripheral mechanisms because carbidopa, an inhibitor (like methyldopa) of peripheral aromatic decarboxylase (EC 4.1.1.28), significantly inhibited salivation.