Choosing the right drug: status and future of endocannabinoid research for the prevention of drug-seeking reinstatement

Negative allosteric modulation of CB1 cannabinoid receptor signalling decreases intravenous morphine self-administration and relapse in mice

The endocannabinoid system interacts with the reward system to modulate responsiveness to natural reinforcers, as well as drugs of abuse. Previous preclinical studies suggested that direct blockade of CB1 cannabinoid receptors (CB1R) could be leveraged as a potential pharmacological approach to treat substance use disorder, but this strategy failed during clinical trials due to severe psychiatric side effects. Alternative strategies have emerged to circumvent the side effects of direct CB1 binding through the development of allosteric modulators. We hypothesized that negative allosteric modulation of CB1R signalling would reduce the reinforcing properties of morphine and decrease behaviours associated with opioid misuse. By employing intravenous self-administration in mice, we studied the effects of GAT358, a functionally-biased CB1R negative allosteric modulator (NAM), on morphine intake, relapse-like behaviour and motivation to work for morphine infusions. GAT358 reduced morphine infusion intake during the maintenance phase of morphine self-administration under a fixed ratio 1 schedule of reinforcement. GAT358 also decreased morphine-seeking behaviour after forced abstinence. Moreover, GAT358 dose dependently decreased the motivation to obtain morphine infusions under a progressive ratio schedule of reinforcement. Strikingly, GAT358 did not affect the motivation to work for food rewards in an identical progressive ratio task, suggesting that the effect of GAT358 in decreasing opioid self-administration was reward specific. Furthermore, GAT58 did not produce motor ataxia in the rotarod test. Our results suggest that CB1R NAMs reduced the reinforcing properties of morphine and could represent a viable therapeutic route to safely decrease misuse of opioids.

Negative allosteric modulation of CB1 cannabinoid receptor signaling decreases intravenous morphine self-administration and relapse in mice

The endocannabinoid system interacts with the reward system to modulate responsiveness to natural reinforcers, as well as drugs of abuse. Previous preclinical studies suggested that direct blockade of CB1 cannabinoid receptors (CB1R) could be leveraged as a potential pharmacological approach to treat substance use disorder, but this strategy failed during clinical trials due to severe psychiatric side effects. Alternative strategies have emerged to circumvent the side effects of direct CB1 binding through the development of allosteric modulators. We hypothesized that pharmacological inhibition of CB1R signaling through negative allosteric modulation (NAM) would reduce the reinforcing properties of morphine and decrease opioid addictive behaviors. By employing i.v. self-administration in mice, we studied the effects of the CB1-biased NAM GAT358 on morphine intake, relapse-like behavior, and motivation to work for morphine infusions. Our data revealed that GAT358 reduced morphine infusion intake during the maintenance phase of morphine self-administration under fixed ratio 1 schedule of reinforcement. GAT358 decreased morphine-seeking behavior after forced abstinence. Moreover, GAT358 dose-dependently decreased the motivation to obtain morphine infusions in a progressive ratio schedule of reinforcement. Strikingly, GAT358 did not affect the motivation to work for food rewards in an identical progressive ratio task, suggesting that the effect of GAT358 in decreasing opioid self-administration is reward specific. Furthermore, GAT58 did not produce motor ataxia in the rota-rod test. Our results suggest that CB1R NAMs reduced the reinforcing properties of morphine and could represent a viable therapeutic route to safely decrease opioid-addicted behaviors.

Mobilization of endocannabinoids by midbrain dopamine neurons is required for the encoding of reward prediction

Brain levels of the endocannabinoid 2-arachidonoylglycerol (2-AG) shape motivated behavior and nucleus accumbens (NAc) dopamine release. However, it is not clear whether mobilization of 2-AG specifically from midbrain dopamine neurons is necessary for dopaminergic responses to external stimuli predicting forthcoming reward. Here, we use a viral-genetic strategy to prevent the expression of the 2-AG-synthesizing enzyme diacylglycerol lipase α (DGLα) from ventral tegmental area (VTA) dopamine cells in adult mice. We find that DGLα deletion from VTA dopamine neurons prevents depolarization-induced suppression of excitation (DSE), a form of 2-AG-mediated synaptic plasticity, in dopamine neurons. DGLα deletion also decreases effortful, cue-driven reward-seeking but has no effect on non-cued or low-effort operant tasks and other behaviors. Moreover, dopamine recording in the NAc reveals that deletion of DGLα impairs the transfer of accumbal dopamine signaling from a reward to its earliest predictors. These results demonstrate that 2-AG mobilization from VTA dopamine neurons is a necessary step for the generation of dopamine-based predictive associations that are required to direct and energize reward-oriented behavior.

Repeated footshock stress induces an escalation of cocaine self-administration in male and female rats: Role of the cannabinoid receptor 1

Stress is a significant contributor to the development and progression of substance use disorders (SUDs) and is problematic as it is unavoidable in daily life. Therefore, it is important to understand the neurobiological mechanisms that underlie the influence of stress on drug use. We have previously developed a model to examine the contribution of stress to drug-related behavior by administering a stressor, electric footshock stress, daily at the time of cocaine self-administration in rats resulting in an escalation of cocaine intake. This stress-induced escalation of cocaine intake involves neurobiological mediators of stress and reward such as cannabinoid signaling. However, all of this work has been conducted in male rats. Here we test the hypothesis that repeated daily stress can produce an escalation of cocaine in both male and female rats. We further hypothesize that cannabinoid receptor 1 (CB1R) signaling is recruited by repeated stress to influence cocaine intake in both male and female rats. Male and female Sprague-Dawley rats self-administered cocaine (0.5 mg/kg/inf, i.v.) during a modified short-access paradigm wherein the 2-hr access was separated into 4-30 min self-administration blocks separated by 4-5 min drug free period. Footshock stress produced a significant escalation of cocaine intake similarly in both male and female rats. Female stress-escalated rats did display greater time-out non-reinforced responding and greater "front-loading" behavior. In males, systemic administration of a CB1R inverse agonist/antagonist Rimonabant only attenuated cocaine intake in rats with a history of combined repeated stress and cocaine self-administration. However, in females, Rimonabant attenuated cocaine intake in the no stress control group but only at the highest dose of Rimonabant (3 mg/kg, i.p.) suggesting that females show a greater sensitivity to CB1R antagonism. However, female rats with a history of stress showed even greater sensitivity to CB1R antagonism as both doses of Rimonabant (1, 3 mg/kg) attenuated cocaine intake in stress-escalated rats similar to males. Altogether these data demonstrate that stress can produce significant changes in cocaine self-administration and suggests that repeated stress at the time of cocaine self-administration recruits CB1Rs to regulate cocaine-taking behavior across sexes.

Design, synthesis, and pharmacological profiling of cannabinoid 1 receptor allosteric modulators: Preclinical efficacy of C2-group GAT211 congeners for reducing intraocular pressure

Allosteric modulators of cannabinoid 1 receptor (CB1R) show translational promise over orthosteric ligands due to their potential to elicit therapeutic benefit without cannabimimetic side effects. The prototypic 2-phenylindole CB1R allosteric modulator, GAT211 (1), demonstrates preclinical efficacy in various disease models. The limited systematic structure-activity relationship (SAR) data at the C2 position of the indole ring within GAT211 invites the opportunity for further modifications to improve GAT211's pharmacological profile while serving to amplify and variegate this library of therapeutically attractive agents. These considerations prompted this focused SAR study in which we substituted the GAT211 C2-phenyl ring with heteroaromatic substituents. The synthesized GAT211 analogs were then evaluated in vitro as CB1R allosteric modulators in cAMP and β-arrestin2 assays with CP55,940 as the orthosteric ligand. Furan and thiophene rings (15c-f and 15m) were the best-tolerated substituents at the C2 position of GAT211 for engagement with human CB1R (hCB1R). The SAR around the novel ligands reported allowed direct experimental characterization of the interaction profile of that pharmacophore with its binding domain in functional, human CB1R, thus offering guidance for accessing subsequent-generation hCB1R allosteric modulators as potential therapeutics. The most potent analog, 15d, markedly promoted orthosteric ligand binding to hCB1R. Pharmacological profiling in the GTPγS and mouse vas deferens assays demonstrated that 15d behaves as a CB1R agonist-positive allosteric modulator (ago-PAM), as confirmed electrophysiologically in autoptic neurons. In vivo, 15d was efficacious as a topical agent that significantly reduced intraocular pressure (IOP) in the ocular normotensive murine model of glaucoma. Since elevated IOP is a decisive risk factor for glaucoma and attendant vision loss, our data support the proposition that the 2-phenylindole class of CB1R ago-PAMs has therapeutic potential for glaucoma and other diseases where potentiation of CB1R signaling may be therapeutic.

Repurposing Peroxisome Proliferator-Activated Receptor Agonists in Neurological and Psychiatric Disorders

Common pathophysiological mechanisms have emerged for different neurological and neuropsychiatric conditions. In particular, mechanisms of oxidative stress, immuno-inflammation, and altered metabolic pathways converge and cause neuronal and non-neuronal maladaptative phenomena, which underlie multifaceted brain disorders. The peroxisome proliferator-activated receptors (PPARs) are nuclear receptors modulating, among others, anti-inflammatory and neuroprotective genes in diverse tissues. Both endogenous and synthetic PPAR agonists are approved treatments for metabolic and systemic disorders, such as diabetes, fatty liver disease, and dyslipidemia(s), showing high tolerability and safety profiles. Considering that some PPAR-acting drugs permeate through the blood-brain barrier, the possibility to extend their scope from the periphery to central nervous system has gained interest in recent years. Here, we review preclinical and clinical evidence that PPARs possibly exert a neuroprotective role, thereby providing a rationale for repurposing PPAR-targeting drugs to counteract several diseases affecting the central nervous system.

Discovery of a Biased Allosteric Modulator for Cannabinoid 1 Receptor: Preclinical Anti-Glaucoma Efficacy

We apply the magic methyl effect to improve the potency/efficacy of GAT211, the prototypic 2-phenylindole-based cannabinoid type-1 receptor (CB1R) agonist-positive allosteric modulator (ago-PAM). Introducing a methyl group at the α-position of nitro group generated two diastereomers, the greater potency and efficacy of erythro, (±)-9 vs threo, (±)-10 constitutes the first demonstration of diastereoselective CB1R-allosteric modulator interaction. Of the (±)-9 enantiomers, (-)-(S,R)-13 evidenced improved potency over GAT211 as a CB1R ago-PAM, whereas (+)-(R,S)-14 was a CB1R allosteric agonist biased toward G protein- vs β-arrestin1/2-dependent signaling. (-)-(S,R)-13 and (+)-(R,S)-14 were devoid of undesirable side effects (triad test), and (+)-(R,S)-14 reduced intraocular pressure with an unprecedentedly long duration of action in a murine glaucoma model. (-)-(S,R)-13 docked into both a CB1R extracellular PAM and intracellular allosteric-agonist site(s), whereas (+)-(R,S)-14 preferentially engaged only the latter. Exploiting G-protein biased CB1R-allosteric modulation can offer safer therapeutic candidates for glaucoma and, potentially, other diseases.