Molecular and Pharmacological Characterization of Serotonin 5-HT.sub.2[alpha] and 5-HT.sub.7 Receptors in the Salivary Glands of the Blowfly Calliphora vicina

Secretion in blowfly (Calliphora vicina) salivary glands is stimulated by the biogenic amine serotonin (5-hydroxytryptamine, 5-HT), which activates both inositol 1,4,5-trisphosphate (InsP.sub.3 )/Ca.sup.2+ and cyclic adenosine 3',5'-monophosphate (cAMP) signalling pathways in the secretory cells. In order to characterize the signal-inducing 5-HT receptors, we cloned two cDNAs (Cv5-ht2[alpha], Cv5-ht7) that share high similarity with mammalian 5-HT.sub.2 and 5-HT.sub.7 receptor genes, respectively. RT-PCR demonstrated that both receptors are expressed in the salivary glands and brain. Stimulation of Cv5-ht2[alpha]-transfected mammalian cells with 5-HT elevates cytosolic [Ca.sup.2+ ] in a dose-dependent manner (EC.sub.50 = 24 nM). In Cv5-ht7-transfected cells, 5-HT produces a dose-dependent increase in [cAMP].sub.i (EC.sub.50 = 4 nM). We studied the pharmacological profile for both receptors. Substances that appear to act as specific ligands of either Cv5-HT.sub.2[alpha] or Cv5-HT.sub.7 in the heterologous expression system were also tested in intact blowfly salivary gland preparations. We observed that 5-methoxytryptamine (100 nM) activates only the Cv5-HT.sub.2[alpha] receptor, 5-carboxamidotryptamine (300 nM) activates only the Cv5-HT.sub.7 receptor, and clozapine (1 [micro]M) antagonizes the effects of 5-HT via Cv5-HT.sub.7 in blowfly salivary glands, providing means for the selective activation of each of the two 5-HT receptor subtypes. This study represents the first comprehensive molecular and pharmacological characterization of two 5-HT receptors in the blowfly and permits the analysis of the physiological role of these receptors, even when co-expressed in cells, and of the modes of interaction between the Ca.sup.2+ - and cAMP-signalling cascades.