title: Characterization of a serotonin transporter and an adenylate cyclase-linked serotonin receptor in the cestode Hymenolepis diminuta.
abstract: [3H]5-HT exhibited specific binding in membrane preparations of Hymenolepis diminuta. The specific binding was saturable, reversible and temperature dependent. A non-linear Scatchard plot was obtained in a concentration range of 11 nM - 1000 nM [3H]5-HT, which could be resolved into sites having apparent dissociation constants (KD) of 0.10 microM and 6.25 microM for the high-affinity and low-affinity components, respectively. The latter could be selectively eliminated by binding [3H]5-HT to H. diminuta membranes in the presence of 10(-3) M nitroimipramine. Drug displacement studies, using 0.20 microM and 2.0 microM [3H]5-HT, revealed that while low-affinity [3H]5-HT binding was displaced by unlabelled 5-HT and inhibitors of 5-HT uptake, high affinity [3H]5-HT binding was affected only by tryptamine derivatives and, to a lesser extent, methysergide. In addition, high-affinity binding was stimulated by MgCl2 while low-affinity binding showed sodium-dependency. The data implicate the low-affinity site as a putative 5-HT transporter and the high-affinity site as a putative 5-HT 1 receptor. Exposure of H. diminuta membranes to 5-HT resulted in a 3-4 fold stimulation of cAMP levels. The EC 50 for the 5-HT-induced activation of adenylate cyclase (0.76 microM) was of the same order of magnitude as the apparent KD for high-affinity binding. Furthermore, the order of drug potency for the elevation of cAMP levels by 5-HT agonists and reversal by 5-HT antagonists was identical to the order of drug potency for the inhibition of high-affinity binding, suggesting linkage of the putative 5-HT 1 receptor to adenylate cyclase in H. diminuta.
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D000242:Cyclic AMP / Q000096:biosynthesis; D006926:Hymenolepis / Q000378:metabolism*; D007099:Imipramine / Q000031:analogs & derivatives / Q000378:metabolism; D007700:Kinetics; D008566:Membranes / Q000378:metabolism; D011985:Receptors, Serotonin / Q000187:drug effects / Q000378:metabolism*; D012701:Serotonin / Q000378:metabolism*; D013696:Temperature
chemical_list: D011985:Receptors, Serotonin; D012701:Serotonin; C030721:2-nitroimipramine; D000242:Cyclic AMP; D000262:Adenylyl Cyclases; D007099:Imipramine