Abstract
The cannabinoid CB(1) receptor is abundant in the central nervous system and regulates neuronal transmission and other key physiological processes including those leading to pain, inflammation, memory, and feeding behavior. CB(1) is activated by the endogenous ligands, arachidonoyl ethanolamine and 2-arachidonoyl glycerol, by various synthetic ligands (e.g., CP55940), and by Δ(9)-tetrahydrocannabinol, the psychoactive component of Cannabis sativa. These CB(1) ligands are orthosteric and transduce downstream signals by binding CB(1) and primarily inducing G(i) coupling, but G(s) and β-arrestin coupling are also possible. Recently, allosteric modulators for CB(1) were discovered that bind to topographically distinct sites and can noncompetitively impact the potency and efficacy of orthosteric compounds. These offer the exciting potential for mechanistic analyses and for developing therapeutics. Yet, it is critical to elucidate whether a compound is a positive allosteric modulator or a negative allosteric modulator of orthosteric ligand-induced CB(1) profiles to understand pathway specificity and ameliorate diseases. In this chapter, we present equilibrium and kinetic binding analysis to reveal the impact of allosteric modulators on CB(1). Also described are activities consistent with CB(1) activation (or inactivation) and include cellular internalization of CB(1) and downstream signaling patterns. Since many CB(1) allosteric modulators do not enhance G protein coupling, it is critical to distinguish CB(1) activation and biased signaling patterns via β-arrestin from CB(1) inactivation. These strategies can illuminate pathway specificity and are valuable for the fine-tuning of CB(1) function.