*3.1. Cannabinoid Receptors*

The effects of phytocannabinoids on humans and other mammalians are partly mediated by the Gi/o protein-coupled CB1 (encoded by the CNR1 gene) and CB2 (encoded by the CNR2 gene) cannabinoid receptors that consist of seven transmembrane domains [124–126].

The stimulation of these receptors leads to the inhibition of adenylyl cyclase with consequent reduction in the intracellular cAMP levels, activation of potassium channels, activation of mitogen-activated protein kinases (MAPKs) such as the extracellular signalregulated kinase (ERK) and c-Jun N-terminal kinase (JNK), as well as activation of the phosphoinositide-3 kinase (PI3K)/Akt signaling pathways and the mammalian target of rapamycin (mTOR) [126–134].

The CB1 and CB2 receptors also recognize the endogenous arachidonic acid-derived endocannabinoids, such as *N*-Arachidonoylethanolamine (anandamide; AEA) and 2-arach idonoylglycerol (2-AG) [134–136]. Both CB1 and CB2 are expressed in various cells in the brain and in peripheral tissues [137]. CB1 is especially expressed at high levels in the neocortex, hippocampus, basal ganglia, cerebellum, and brainstem, but it is also found in peripheral nerve terminals and some tissues, such as the vascular endothelium, spleen, testis, and eye [137]. CB2 is predominantly found in cells of the immune system, and in the central nervous system, it is primarily localized to microglia and tissue macrophages [137].

The CB1 receptor regulates the balance between excitatory and inhibitory neuronal activity. The psychoactive effect is believed to be mediated through the CB1 receptor in the brain, whereas the immunomodulatory effects are anticipated to be mediated via the CB2 receptor expressed on immune cells [138,139]. In addition, CB1 signaling affects metabolism and is involved in maintaining whole body energy homeostasis by increasing appetite and stimulating feeding [140]. Many efforts have been made to develop CB2 specific agonists at an attempt to achieve anti-inflammatory actions without psychotropic adverse effects [13,141–143]. The sesquiterpene (E)-β-caryophyllene produced by *Cannabis* as well as other plants, including oregano (*Origanum vulgare* L.), cinnamon (*Cinnamomum* spp.), and black pepper (*Piper nigrum* L.), was found to bind selectively to the CB2 receptor and exert anti-inflammatory activities [144–147].

Other cannabinoid receptors include transient receptor potential vanilloid 1 (TRPV1), the G-protein-coupled receptors GPR18 and GPR55, and peroxisome proliferator activated receptors (PPARs) [126,134,148–152]. The anti-nociceptive effect of *Cannabis sativa* extracts was found to be mediated by the binding of CBD to TRPV1 [153]. A study by Ibrahim et al. [154] showed that activation of the CB2 receptor by its agonist AM1241 stimulated the release of beta-endorphin from keratinocytes, which, in turn, acted on neuronal μopoid receptors to inhibit nociception. The *Cannabis sativa* extract containing multiple cannabinoids, terpenes, and flavonoids had stronger anti-nociceptive effect than a single cannabinoid given alone [153], suggesting an "entourage" effect of the various *Cannabis*containing compounds [74].

The CB1 and CB2 can form receptor heteromers [155]. The activity of the receptor heteromer is affected by the agonists and antagonists that bind to each of them. A CB1 antagonist can block the effect of a CB2 agonist and vice versa; a CB2 antagonist can block the effect of a CB1 receptor agonist [155]. CB1 has also been shown to form heteromers with dopamine and adenosine receptors [156–158], AT1 angiotensin receptor [159], <sup>μ</sup>1-opoid receptor [160,161], and OX1 orexin A receptor [162]. The many interacting partners put CB1 signaling under strict regulation.

#### *3.2. Pharmacological Effects of Selected Phytocannabinoids*
