5-HT1A receptor agonist-antagonist binding affinity difference as a measure of intrinsic activity in recombinant and native tissue systems

1 It has been reported that radiolabelled agonist:antagonist binding affinity ratios can predict functional efficacy at several different receptors. This study investigates whether this prediction is true for recombinant and native tissue 5-HT1A receptors. 2 Saturation studies using [H-3]-8-OH-DPAT and [H-3]-MPPF revealed a single, high affinity site (K(D)similar to 1 nM) in HEK293 cells expressing human 5-HT1A receptors and rat cortex. In recombinant cells, [H-3]-MPPF labelled 3-4 fold more sites than [H-3]-8-OH-DPAT suggesting the presence of more than one affinity state of the receptor. [H-3]-Spiperone labelled a single, lower affinity site in HEK293 cells expressing h5-HT1A receptors but did not bind to native tissue 5-HT1A receptors. These data suggest that, in transfected HEK293 cells, human 5-HT1A receptors exist in different affinity states but in native rat cortical tissue the majority of receptors appear to exist in the high agonist affinity state. 3 Receptor agonists inhibited [H-3]-MPPF binding from recombinant 5-HT1A receptors in a biphasic manner, whereas antagonists and partial agonists gave monophasic inhibition curves. All compounds displaced [H-3]-8-OH-DPAT and [H-3]-spiperone binding in a monophasic manner. In rat cortex, all compounds displaced [H-3]-MPPF and [H-3]-8-On-DPAT in a monophasic manner. 4 Functional evaluation of compounds, using [S-35]-GTP gamma S binding, produced a range of intrinsic activities from full agonism, displayed by 5-HT and 5-CT to inverse agonism displayed by spiperone. 5 [H-3]-8-OH-DPAT: [H-3]-MPPF pK(i) difference correlated well with functional intrinsic activity (r = 0.86) as did [H-3]-8-OH-DPAT:[H-3]-spiperone pK(i) difference with functional intrinsic activity (r = 0.96). 6 Thus agonist:antagonist binding affinity differences may be used to predict functional efficacy at human 5-HT1A receptors expressed in HEK293 cells where both high and low agonist affinity stales are present but not at native rat cortical 5-HT1A receptors in which only the high agonist affinity state was detectable.