Evidence for multiple receptors mediating fluid secretion in salivary glands of ticks

Abstract

Using isolated salivary glands of the ixodid tick Amblyomma hebraeum Koch, we tested the effectiveness of butaclamol and sulpiride in blocking fluid secretion stimulated by a number of agonists. (+)-Butaclamol was a potent inhibitor of dopamine, N-methyldopamine and noradrenaline (K i ≅ 30–60 nM), but was less effective on ergometrine (K i ≅ 310 nM). Tranylcypromine-stimulated fluid secretion in the absence and presence of (+)-butaclamol and (±)-sulpiride suggested that tranylcypromine's action is mediated through two receptors. (±)-Sulpiride, though a rather weak antagonist of ergometrine (K i ≅ 6150 nM), was ineffectual as a dopamine blocker, indicating distinct receptor sites on this epithelium for dopamine and ergometrine. Both (+)-butaclamol and sulpiride reversed the autoinhibition associated with supramaximal levels of dopamine. Sulpiride also abolished spiperone's potentiation of dopamine. Butaclamol, on the other hand, had no such effect on spiperone's potentiation of dopamine. Finally, although the CNS of ticks contains both dopamine and noradrenaline in quantity (≅650 and ≅370 ng · g −1 respectively), the salivary glands contain far more dopamine than noradrenaline (≅85 and ≅6 ng · g −1 respectively). The data support the hypothesis that dopamine is a natural transmitter substance in the tick salivary gland, and that there are distinct receptor sites in the epithelium mediating the actions of catecholamines, ergot alkaloids and butyrophenones. The physiological significance of the ergot alkaloid and butyrophenone sites is not clear.