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.
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.
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∼8-OH-DPAT) was consistent with activation of 5-HT7 receptors. SB-258719 (5 μM) and methiothepin (1 μM) 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.
In the functional assay, WAY-100635 (100 nM) and cyanopindolol (1 μM) 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.
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.
British Journal of Pharmacology (1999) 128, 158–164; doi:10.1038/sj.bjp.0702759