Allosteric interactions between the binding sites of receptor agonists and guanine nucleotides: A comparative study of the 5-hydroxytryptamine(1A) and adenosine A₁ receptor systems in rat hippocampal membranes

The ternary complex formed between agonist, receptor and guanine nucleotide binding protein and its destabilization by guanine nucleotides (GN) was utilized to study early events in signal transduction, by characterizing the allosteric interactions between agonist and GN binding to the receptor/guanine nucleotide binding protein, G complex for adenosine A₁ and 5-hydroxytryptamine(1A) receptors. The functional interaction between the ternary complex and GTP was examined by assaying adenylyl cyclase activity. Binding of a full adenosine A₁ agonist {[³H]-R-(-)-N⁶-(2- phenylisopropyl)adenosine}, and a full {(±)-[³H]-8- hydroxydipropylaminotetralin} and partial {[³H]-8-[2-[4-(2- methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4.5]-decane-7,9-dione} 5- hydroxytryptamine(1A) agonist was examined in relation to the binding of GN. The amount of ternary complex formed depended upon receptor type and drug relative efficacy. The ratio between the drug's EC₅₀ value (adenylyl cyclase) and dissociation constant (binding) was also receptor and drug relative efficacy dependent. 5'-Guanylylimidodiphosphate (100 μM) caused an approximately 50% decrease in the B(max) for all drugs without affecting K(d) values. 5'-Guanylylimidodiphosphate and guanosine 5'-O-(3-thiotriphosphate) attenuated [³H]-agonist binding in a concentration-dependent and saturable manner, with IC₅₀ values increased 2- to 6-fold with increasing receptor occupancy. IC₅₀ values were approximately one-tenth lower at the 5- hydroxytryptamine(1A) receptor than adenosine A₁ receptor; similar values were obtained for inhibition of (±)-[³H]-8-hydroxydipropylaminotetralin and [³H]-8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4.5]- decane-7,9-dione binding, suggesting an independence of agonist efficacy. We propose that the stabilization of the ternary complex by hormone binding, measured by B(max) values, is related to drug-relative efficacy, thus the amount of ternary complex available for destabilization by GN is greater for the more efficacious agonist. This is translated into greater relative efficacy observed in the maximal inhibition of adenylyl cyclase.