Evidence for a D1 dopamine receptor in the salivary glands of Amblyomma americanum (L.).

Abstract

Adenylate cyclase from the salivary gland of the lone star tick, Amblyomma americanum, is stimulated by several derivatives of phenylethylamine; dopamine, noradrenaline, adrenaline and isoproterenol. Octopamine and iota-DOPA had no effect on basal adenylate cyclase activity. Dopamine had the highest potency and the lowest Ka, 0.4 micron; followed by adrenaline and noradrenaline, 23 micron; and isoproterenol, 0.15mM. When either noradrenaline or adrenaline was added to dopamine, each at its maximally effective concentration, no additional stimulation of gland cyclase activity was noted. Furthermore, adrenaline and noradrenaline together did not equal the potency of dopamine alone. The most potent inhibitors of gland cyclase activity were the dopamine receptor antagonists. The phenothiazine drugs (thioridazine, chlorpromazine, and fluphenazine) were more effective inhibitors of cyclase activity than the butyrophenone drug (haloperidol). The inhibition constants (Ki) for the phenothiazine drugs were: 60nM for thioridazine, 1.9 micron for chlorpromazine and 2.3 micron for fluphenazine. The inhibition of adenylate cyclase activity was found to be specific for the (+) enantiomer of butaclamol, a stereospecific dopamine receptor antagonist. Our findings suggest that the lone star tick salivary gland adenylate cyclase has a D1 type dopamine receptor.