Cannabidiol has been found to act as an antagonist of GPR55, a G protein-coupled receptor and putative cannabinoid receptor that is expressed in the caudate nucleus and putamen in the brain. It has also been found to act as an inverse agonist of GPR3, GPR6, and GPR12. Although currently classified as orphan receptors, these receptors are most closely related phylogenetically to the cannabinoid receptors. In addition to orphan receptors, CBD has been shown to act as a serotonin 5-HT1A receptor partial agonist, and this action may be involved in its antidepressant, anxiolytic, and neuroprotective effects. It is an allosteric modulator of the μ- and δ-opioid receptors as well. The pharmacological effects of CBD have additionally been attributed to PPARγ agonism and intracellular calcium release.