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First published online June 15, 2006
Journal of Experimental Biology 209, 2525-2534 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02274

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Pharmacological characterization of the ergot alkaloid receptor in the salivary gland of the ixodid tick Amblyomma hebraeum

W. Reuben Kaufman^* and Jessi L. Minion

Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada

^* Author for correspondence (e-mail: reuben.kaufman{at}ualberta.ca)

Accepted 18 April 2006

Female ticks of the family Ixodidae osmoregulate by secreting the excess fluid of the blood meal back into the host's circulation via the salivary glands. At least three receptors control salivary fluid secretion in the tick Amblyomma hebraeum: (1) dopamine (DA) stimulates fluid secretion via a DA receptor, (2) ergot alkaloids (ErAs) stimulate fluid secretion via an ErA-sensitive receptor (the natural ligand of which has not been identified), and (3) a GABA receptor potentiates the action of DA and ErAs. Here we present some pharmacological properties of the ErA-sensitive receptor. Of the 11 ErAs we tested, (i) four were complete agonists (approximate concentration eliciting 50% maximum response is given in parentheses): dihydroergotamine (0.02 µmol l^–1), ergonovine (ErN; 0.06 µmol l^–1), methylergonovine (0.1 µmol l^–1) and -ergocriptine (0.9 µmol l^–1); (ii) three were `incomplete agonists' (approximate concentration eliciting 20% maximum response is given in parentheses): ergocorninine (3.5 µmol l^–1), ergocristinine (7.5 µmol l^–1) and ergocristine (10 µmol l^–1); (C) three were partial agonists (approximate concentration eliciting the respective maximum response in parentheses): ergocornine (50% maximum by 1 µmol l^–1), methysergide (28% maximum by 10 µmol l^–1) and bromocriptine (22% maximum by 10 µmol l^–1); and (D) one had no activity up to 1 mmol l^–1: ergothioneine. Bromocriptine and methysergide did not antagonize the action of DA, but were effective competitive antagonists of ErN, with K_is of 0.3 µmol l^–1 and 11 µmol l^–1, respectively. Ergothioneine was not an antagonist at either the DA- or ErA-sensitive receptor. The putative protein kinase C activators, 1-oleoyl-2-acetyl-sn-glycerol (OAG) and 1,2-dioctanoyl-sn-glycerol (DiC₈), neither stimulated salivary fluid secretion nor potentiated the action of DA or ErN. The putative protein kinase C inhibitors, bisindolymaleimide (BIM) and calphostin C did not inhibit the action of DA or ErN, although low concentrations of calphostin C (10 nmol l^–1) appeared to potentiate the action of DA but not ErN. The ion transport inhibitors, furosemide and amiloride (both up to 1 mmol l^–1), had no significant effect on DA-stimulated or ErN-stimulated fluid secretion.

Key words: Amblyomma hebraeum, bromocriptine, dihydroergotamine, ergocornine, ergocorninine, ergocristine, ergocristinine, ergocryptine, ergonovine, ergothioneine, ergot alkaloids, methylergonovine, methysergide, tick salivary gland