Allosteric interactions between the binding sites of receptor agonists and guanine nucleotides: a comparative study of the 5-hydroxytryptamine1A and adenosine A1 receptor systems in rat hippocampal membranes.

ABSTRACT

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-hydroxytryptamine1A 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-hydroxytryptamine1A 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 microM) caused an approximately 50% decrease in the Bmax for all drugs without affecting Kd 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-hydroxytryptamine1A 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 Bmax 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.