Functional characterisation of the human cloned 5-HT7 receptor (long form); antagonist profile of SB-258719

1. The functional profile of the long form of the human cloned 5-HT₇ receptor (designated h5-HT_7(a)) was investigated using a number of 5-HT receptor agonists and antagonists and compared with its binding profile. Receptor function was measured using adenylyl cyclase activity in washed membranes from HEK293 cells stably expressing the recombinant h5-HT_7(a) receptor.
2. The receptor binding profile, determined by competition with [³H]-5-CT, was consistent with that previously reported for the h5-HT_7(a) receptor. The selective 5-HT₇ receptor antagonist SB-258719 ((R)-3,N-Dimethyl-N-[1-methyl-3-(4-methylpiperidin-1-yl)propyl]benzene sulfonamide) displayed high affinity (pK_i 7.5) for the receptor.
3. In the adenylyl cyclase functional assay, 5-CT and 8-OH-DPAT were both full agonists compared to 5-HT and the rank order of potency for agonists (5-CT>5-HT>8-OH-DPAT) was the same in functional and binding studies.
4. Risperidone, methiothepin, mesulergine, clozapine, olanzapine, ketanserin and SB-258719 antagonised surmountably 5-CT-stimulated adenylyl cyclase activity. Schild analysis of the antagonism by SB-258719 gave a pA₂ of 7.2±0.2 and slope not significantly different from 1, consistent with competitive antagonism.
5. The same antagonists also inhibited basal adenylyl cyclase activity with a rank order of potency in agreement with those for antagonist potency and binding affinity. Both SB-258719 and mesulergine displayed apparent partial inverse agonist profiles compared to the other antagonists tested. These inhibitory effects of antagonists appear to be 5-HT₇ receptor-mediated and to reflect inverse agonism.
6. It is concluded that in this expression system, the h5-HT_7(a) receptor shows the expected binding and functional profile and displays constitutive activity, revealing inverse agonist activity for a range of antagonists.