5-CT stimulation of adenylyl cyclase activity in guinea-pig hippocampus: evidence for involvement of 5-HT7 and 5-HT1A receptors.
1. A number of compounds, including the selective 5-HT7 receptor antagonist SB-258719, were investigated for their effect on [3H]-5-carboxamidotryptamine (5-CT) radioligand binding and 5-CT-stimulated adenylyl cyclase activity in guinea-pig hippocampal membranes, in order to confirm the presence of functionally coupled 5-HT7 receptors in this tissue. 2. The [3H]-5-CT radioligand binding profile was consistent with binding predominantly to 5-HT7 receptors. The affinity of SB-258719 (pKi 7.2+/-0.1) was similar to its reported human 5-HT7 receptor affinity. 3. In the adenylyl cyclase functional assay, 5-CT was a potent and full agonist compared to 5-HT, whereas 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) was a partial agonist (intrinsic activity 0.4+/-0.1). The rank order of potency for agonists (5-CT>5-HT approximately 8-OH-DPAT) was consistent with activation of 5-HT7 receptors. SB-258719 (5 microM) and methiothepin (1 microM) surmountably antagonized the response to 5-CT, consistent with competitive antagonism. The pKB for SB-258719 (7.2+/-0.1) was in good agreement with its reported antagonist potency at the human cloned 5-HT7 receptor. 4. In the functional assay, WAY-100635 (100 nM) and cyanopindolol (1 microM) induced a biphasic 5-CT response curve, consistent with selective antagonism of a component of the response to 5-CT. The estimated pKB values for WAY-100635 and cyanopindolol (9.6 and 8.4 respectively) were in good agreement with their reported 5-HT1A receptor affinities. 5. The data are consistent with the presence of 5-HT7 receptors in guinea-pig hippocampus which are positively coupled to adenylyl cyclase. In addition, 5-HT7 receptor-mediated stimulation of adenylyl cyclase activity in this tissue appears to be augmented by a mechanism involving 5-HT1A receptor activation.
Thomas
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Department of Neuroscience Research, SmithKline Beecham Pharmaceuticals, New Frontiers Science Park, Third Avenue, Harlow, Essex, CM19 5AW. David _R_Thomas@sbphrd.com
Middlemiss
D N
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Taylor
S G
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Nelson
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P
Brown
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AM
D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D001667:Binding, Competitive; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D006168:Guinea Pigs; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism; D008297:Male; D011985:Receptors, Serotonin / Q000378:metabolism*; D044263:Receptors, Serotonin, 5-HT1; D012701:Serotonin / Q000031:analogs & derivatives* / Q000494:pharmacology; D012702:Serotonin Antagonists / Q000378:metabolism / Q000494:pharmacology; D017366:Serotonin Receptor Agonists / Q000378:metabolism / Q000494:pharmacology*
D011985:Receptors, Serotonin; D044263:Receptors, Serotonin, 5-HT1; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; C104425:serotonin 7 receptor; D012701:Serotonin; C047087:5-carboxamidotryptamine; D000262:Adenylyl Cyclases