11.2 In Vitro Pharmacology of Cannabidiol
and the Modulation of Neuronal Excitability
11.2.1 Actions of Cannabidiol in the Endocannabinoid
System
CBD has been demonstrated to interact with numerous biomolecular targets and to
possess several potential mechanisms for antiepileptic activity. However, unlike
D^9 -THC and several other phytocannabinoids, CBD has relatively low affinity
(Ki>1lM) for the orthosteric binding sites on the CB1 (Thomas et al. 1998 ) and
CB2 receptors (Bisogno et al.2001b). In a recent review of CBD pharmacology
(McPartland et al. 2015 ), the authors calculated a pooled mean affinity of CBD at
CB1 receptors ofKi= 3245±803 nM using data from 1 human, 3 mouse and 11
rat studies (noting that species differences were not statistically significant). In a
similar approach, they calculated an average CB2 affinity for CBD of
Ki= 3612±1382nM, and noted that CBD had a similar affinity at transient
receptor potential (TRP) V1 receptors. Interestingly, Wiley et al. (Wiley et al. 2002 )
synthesized and tested both receptors binding and in vivo pharmacological effects
of phenols and resorcinols that resemble CBD, and several analogs displayed rel-
atively high affinity at both the CB1 and CB2 receptor and were potent in in vivo
assays of cannabinoid activity. Thus the structural attributes and binding modalities
precluding high affinity binding of CBD to orthosteric sites on the CB1 and CB2
receptors are still unclear.
Cannabidiol does appear to modulate CB1 receptor signaling in a more complex
or allosteric fashion at clinically relevant doses. For example, CBD antagonized
CP55, 940- or WIN55212-2-induced GTP-g-S binding in mouse brain membrane
preparations with a pooled meanKB= 88.5±18.46nM, nearly 40 times lower
than theKiof CBD in binding assays (Thomas et al. 2007 ; McPartland et al. 2015 ).
In one study, CBD produced a downward and rightward shift in the GTP-g-S
binding stimulation curve of CP55, 940 (Petitet et al. 1998 ). Studies in HEK293A
cells heterologously expressing CB1 receptors and in the STHdhQ7/Q7 cell model
of striatal neurons endogenously expressing CB1 receptors showed that CBD also
reduced the efficacy and potency of 2-arachidonylglycerol andD^9 -THC on PLCb3-
and ERK1/2-dependent signaling (Laprairie et al. 2015 ). By reducingb-arrestin 2
recruitment, CBD also diminished CB1 receptor internalization in vitro at
nanomolar concentrations where other CB1 receptor-dependent effects on signaling
were not observed (Laprairie et al. 2014 ). Using these data and an operational
model for allosteric modulation, the authors concluded that CBD acted as a neg-
ative allosteric modulator to reduce the binding of THC and 2-AG [a, co-operativity
coefficient for ligand binding 1.0 (0.37)], and also reduced the efficacy of the
orthosteric ligand [b, co-operativity coefficient for ligand efficacy1 (0.44)].
This behavior, combined with the discrepancy between affinity and activity, sug-
gests that CBD can produce complex allosteric modulation of endogenous
cannabinoid and phytocannabinoid signaling at CB1 and CB2 receptors. Additional
252 B.F. Thomas