Practical Aspects of Radioligand Binding

Experimental strategies for studying G protein-coupled receptor homo- and heteromerization with radioligand binding and signal transduction methods

Before the molecular biology era, functional experiments on isolated organs and radioligand binding and biochemical experiments on animal tissues were widely used to characterize G protein-coupled receptors (GPCRs). The introduction of recombinant cell lines expressing a single GPCR type has been a big step forward for studying both drug-receptor interactions and signal transduction. Before the introduction of the concept of receptor oligomerization, all data generated were attributed to the interaction of drugs with receptor monomers. Now, considerable data must be reinterpreted in light of receptor homo- and heteromerization. In this chapter, we will review some of the methods used to study radioligand binding and signal transduction modifications induced by GPCR homo- and heteromerization.

An engineered cocaine hydrolase blunts and reverses cardiovascular responses to cocaine in rats

There is increasing evidence that human plasma butyrylcholinesterase can lower the toxicity of cocaine overdose. Recently, with structure-based protein engineering, we converted this enzyme into a more efficient cocaine hydrolase (CocE). When tested in rats, CocE shortened cocaine's plasma half-life and decreased drug accumulation in heart and brain. Here, we have investigated the potential of CocE to antagonize cardiovascular responses to cocaine. Anesthetized rats were instrumented for continuous recording of blood pressure from the femoral artery. Cocaine (7 mg/kg i.v.) caused blood pressure to rise within 30 s by 25 to 37 mmHg, but pressure returned to baseline within 60 s. These transient pressor responses were prolonged up to 5 min when vagal reflexes were blocked with atropine (1 mg/kg). Under such conditions, pretreatment with CocE (3 mg/kg i.v.) reduced cocaine's pressor effect, whereas delayed treatment with CocE rapidly restored normal mean blood pressure. CocE had no hemodynamic effects in control animals not treated with cocaine. The finding that CocE can oppose pre-established physiologic actions of cocaine suggests that similar or improved hydrolases might help rescue patients from the life-threatening toxicity of drug overdose.