5-HT(1A) 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-HT(1A) receptors. 2. Saturation studies using (3)H]-8-OH-DPAT and (3)H]-MPPF revealed a single, high affinity site (K(D)approximately 1 nM) in HEK293 cells expressing human 5-HT(1A) receptors and rat cortex. In recombinant cells, (3)H]-MPPF labelled 3 - 4 fold more sites than (3)H]-8-OH-DPAT suggesting the presence of more than one affinity state of the receptor. (3)H]-Spiperone labelled a single, lower affinity site in HEK293 cells expressing h5-HT(1A) receptors but did not bind to native tissue 5-HT(1A) receptors. These data suggest that, in transfected HEK293 cells, human 5-HT(1A) 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 (3)H]-MPPF binding from recombinant 5-HT(1A) receptors in a biphasic manner, whereas antagonists and partial agonists gave monophasic inhibition curves. All compounds displaced (3)H]-8-OH-DPAT and (3)H]-spiperone binding in a monophasic manner. In rat cortex, all compounds displaced (3)H]-MPPF and (3)H]-8-OH-DPAT in a monophasic manner. 4. Functional evaluation of compounds, using (35)S]-GTPgammaS binding, produced a range of intrinsic activities from full agonism, displayed by 5-HT and 5-CT to inverse agonism displayed by spiperone. 5. (3)H]-8-OH-DPAT : (3)H]-MPPF pK(i) difference correlated well with functional intrinsic activity (r=0.86) as did (3)H]-8-OH-DPAT : (3)H]-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-HT(1A) receptors expressed in HEK293 cells where both high and low agonist affinity states are present but not at native rat cortical 5-HT(1A) receptors in which only the high agonist affinity state was detectable.