The inhibition of the G-protein coupled cannabinoid 1 (CB1) receptor of mammalian brain by phthalate esters in vitro

This research investigates in vitro interactions of phthalates with G protein-coupled cannabinoid 1 (CB1) receptors. Diesters, n-butylbenzylphthalate (nBBP), di-n-hexylphthalate (DnHP), di-n-butylphthalate (DnBP), di-2-ethylhexylphthalate (DEHP), di-isooctylphthalate (DiOP) and di-n-octylphthalate (DnOP) inhibited specific binding of CB1 receptor agonist [3H]CP-55940 to mouse brain membranes at micromolar concentrations. Monoesters, mono-2-ethylhexylphthalate (M2EHP) and mono-isohexylphthalate (MiHP) had low activity and mono-n-butylphthalate (MnBP) was inactive. Inhibition of CB1 receptor agonist-stimulated binding of [35S]GTPγS to the G-protein demonstrated that phthalates functionally impair this complex. DnBP, nBBP and DEHP also inhibited binding of [3H]SR141716A, whereas inhibition by MiHP was comparatively weak and MnBP was ineffective. DnBP and nBBP reduced the association rate of [3H]SR141716A with CB1 receptors and rapidly enhanced the rate of [3H]SR141716A dissociation, the latter being consistent with an allosteric mechanism for inhibition. Thus, phthalates act as relatively low affinity antagonists of CB1 receptors and cause inhibition of cannabinoid agonist-dependent activation of the G-protein in vitro.