A kinetic model for positive allosteric modulator (PAM)-antagonists for the type 1 cannabinoid receptor (CB1 ).

Abstract

The type 1 cannabinoid receptor (CB1 ) is amongst the most abundant G-protein coupled receptors in brain. Allosteric ligands bind to a different site on receptors than the orthosteric ligand and can have effects that are both unique to the allosteric ligand and/or modulate orthosteric ligand activity. We aim to propose a unified mathematical model to describe the interaction effects of the allosteric ligand ORG27569 (ORG) and the orthosteric agonist CP55940 (CP) on CB1 . A ternary complex model was constructed which incorporated kinetic properties to describe the time-course of effects of ORG and CP reported in the literature: i) enhanced receptor binding of CP, ii) reduced internalisation and iii) time dependent modulation of cAMP. Underlying mechanisms of time-dependent modulation by ORG were evaluated by simulation. A hypothetical transitional state of CP-CB1 -ORG, which can internalise but cannot inhibit cAMP, was shown to be necessary and sufficient to describe the allosteric modulation by ORG, prior to receptors adopting an inactive conformation. The model indicated that the formation of this transitional CP-CB1 -ORG state and final inactive CP-CB1 -ORG state contributes to the enhanced CP binding. The inactive CP-CB1 -ORG cannot internalise or inhibit cAMP, leading to reduced internalisation and cessation of cAMP inhibition. In conclusion, a kinetic mathematical model for CB1 allosteric modulation was developed. However, a standard ternary complex model was not sufficient to capture the data and a hypothetical transitional state was required to describe the allosteric modulation properties of ORG.