Neurobiology of cannabinoid receptor signaling

The endocannabinoid system (ECS) is a highly versatile signaling system within the nervous system. Despite its widespread localization, its functions within the context of distinct neural processes are very well discernable and specific. This is remarkable, and the question remains as to how such specificity is achieved. One key player in the ECS is the cannabinoid type 1 receptor (CB1), a G protein–coupled receptor characterized by the complexity of its cell-specific expression, cellular and subcellular localization, and its adaptable regulation of intracellular signaling cascades. CB1 receptors are involved in different synaptic and cellular plasticity processes and in the brain’s bioenergetics in a context-specific manner. CB2 receptors are also important in several processes in neurons, glial cells, and immune cells of the brain. As polymorphisms in ECS components, as well as external impacts such as stress and metabolic challenges, can both lead to dysregulated ECS activity and subsequently to possible neuropsychiatric disorders, pharmacological intervention targeting the ECS is a promising therapeutic approach. Understanding the neurobiology of cannabinoid receptor signaling in depth will aid optimal design of therapeutic interventions, minimizing unwanted side effects.

The endocannabinoid system in modulating fear, anxiety, and stress

The endocannabinoid system is widely expressed in the limbic system, prefrontal cortical areas, and brain structures regulating neuroendocrine stress responses, which explains the key role of this system in the control of emotions. In this review, we update recent advances on the function of the endocannabinoid system in determining the value of fear-evoking stimuli and promoting appropriate behavioral responses for stress resilience. We also review the alterations in the activity of the endocannabinoid system during fear, stress, and anxiety, and the pathophysiological role of each component of this system in the control of these protective emotional responses that also trigger pathological emotional disorders. In spite of all the evidence, we have not yet taken advantage of the therapeutic implications of this important role of the endocannabinoid system, and possible future strategies to improve the treatment of these emotional disorders are discussed.

The costs and benefits of cannabis control policies

As is the case for most drugs, cannabis use has costs and benefits, and so do the policies that attempt to minimize the first and maximize the second. This article summarizes what we know about the harmful effects of recreational cannabis use and the benefits of medical cannabis use under the policy of prohibition that prevailed in developed countries until 2012. It outlines three broad ways in which cannabis prohibition may be relaxed, namely, the depenalization of personal possession and use, the legalization of medical use, and the legalization of adult recreational use. It reviews evidence to date on the impacts of each of these forms of liberalization on the costs and benefits of cannabis use. It makes some plausible conjectures about the future impacts of the commercialization of cannabis using experience from the commercialization of the alcohol, tobacco, and gambling industries. Cannabis policy entails unavoidable trade-offs between competing social values in the face of considerable uncertainty about the effects that more liberal cannabis policies will have on cannabis use and its consequences for better or worse.

History of cannabis and the endocannabinoid system

This article retraces the story of cannabis from the earliest contacts of humans with the plant to its subsequent global expansion, its medicinal uses, and the discovery of the endocannabinoid system in the 20th century. Cannabis was attested to around 12 000 years ago near the Altai Mountains in Central Asia, and since then, cannabis seeds have accompanied the migration of nomadic peoples. Records of the medicinal use of cannabis appear before the Common Era in China, Egypt, and Greece (Herodotus), and later in the Roman empire (Pliny the Elder, Dioscorides, Galen). In the 19th century, orientalists like Silvestre de Sacy, and Western physicians coming into contact with Muslim and Indian cultures, like O’Shaughnessy and Moreau de Tours, introduced the medicinal use of cannabis into Europe. The structure of the main psychoactive phytocannabinoid, tetrahydrocannabinol (THC), was determined in Israel by Mechoulam and Gaoni in 1964. This discovery opened the gate for many of the subsequent developments in the field of endocannabinoid system (ECS) research. The advances in the scientific knowledge of the ECS place the debate on cannabis liberalization in a new context.

Cannabinoids and the endocannabinoid system in reward processing and addiction: from mechanisms to interventions

The last decades have seen a major gain in understanding the action of cannabinoids and the endocannabinoid system in reward processing and the development of addictive behavior. Cannabis-derived psychoactive compounds such as Δ9-tetrahydrocannabinol and synthetic cannabinoids directly interact with the reward system and thereby have addictive properties. Cannabinoids induce their reinforcing properties by an increase in tonic dopamine levels through a cannabinoid type 1 (CB1 ) receptor–dependent mechanism within the ventral tegmental area. Cues that are conditioned to cannabis smoking can induce drug-seeking responses (ie, craving) by eliciting phasic dopamine events. A dopamine-independent mechanism involved in drug-seeking responses involves an endocannabinoid/glutamate interaction within the corticostriatal part of the reward system. In conclusion, pharmacological blockade of endocannabinoid signaling should lead to a reduction in drug craving and subsequently should reduce relapse behavior in addicted individuals. Indeed, there is increasing preclinical evidence that targeting the endocannabinoid system reduces craving and relapse, and allosteric modulators at CB1 receptors and fatty acid amide hydrolase inhibitors are in clinical development for cannabis use disorder. Cannabidiol, which mainly acts on CB1 and CB2 receptors, is currently being tested in patients with alcohol use disorder and opioid use disorder.

Cannabinoids and their therapeutic applications in mental disorders

Mental disorders represent a significant public health burden worldwide due to their high prevalence, chronically disabling nature, and substantial impact on quality of life. Despite growing knowledge of the pathological mechanisms that underlie the development of these disorders, a high percentage of patients do not respond to first-line clinical treatments; thus, there is a strong need for alternative therapeutic approaches. During the past half-century, after the identification of the endocannabinoid system and its role in multiple physiological processes, both natural and synthetic cannabinoids have attracted considerable interest as putative medications in pathological conditions such as, but not exclusive to, mental disorders. Here, we provide a summary of cannabinoid effects in support of possible therapeutic applications for major depression, bipolar disorder, anxiety, posttraumatic stress disorder, and schizophrenia. Considering this evidence, highlighted benefits and risks of cannabinoid use in the management of these illnesses require further experimental study.

The endocannabinoidome as a substrate for noneuphoric phytocannabinoid action and gut microbiome dysfunction in neuropsychiatric disorders

The endocannabinoid (eCB) system encompasses the eCBs anandamide and 2-arachidonoylglycerol, their anabolic/catabolic enzymes, and the cannabinoid CB1 and CB2 receptors. Its expansion to include several eCB-like lipid mediators, their metabolic enzymes, and their molecular targets, forms the endocannabinoidome (eCBome). This complex signaling system is deeply involved in the onset, progress, and symptoms of major neuropsychiatric disorders and provides a substrate for future therapeutic drugs against these diseases. Such drugs may include not only THC, the major psychotropic component of cannabis, but also other, noneuphoric plant cannabinoids. These compounds, unlike THC, possess a wide therapeutic window, possibly due to their capability of hitting several eCBome and non-eCBome receptors. This is particularly true for cannabidiol, which is one of the most studied cannabinoids and shows promise for the treatment of a wide range of mental and mood disorders. The eCBome plays a role also in the microbiota-gut-brain axis, which is emerging as an important actor in the control of affective and cognitive functions and in their pathological alterations.

Cannabinoids: for better and for worse

The use of cannabis as a drug has undergone a remarkable change of direction: considered as a symbol of countercultures in past decades, it is presently being hailed as a cure for any number of diseases and conditions. Thus, despite concerns about the safety of cannabis and cannabinoids, quite a few drugs that contain cannabinoids have recently been approved by several drug agencies, and the medicinal and recreational use of cannabis has been legalized in various countries and states. The promise of cannabinoids for therapeutic use, as well as potentially detrimental health risks and regulatory issues, will need to be carefully weighed.

Cannabis points to the synaptic pathology of mental disorders: how aberrant synaptic components disrupt the highest psychological functions

Cannabis can elicit an acute psychotic reaction, and its long-term use is a risk factor for schizophrenia. The main active psychoactive ingredient ∆9-tetrahydrocannabinol (Δ9 -THC) activates cannabinoid 1 (CB1) receptors, which are localized to the terminals of glutamate and GABA neurons in the brain. The endogenous cannabinoids are involved in information processing and plasticity at synapses in the hippocampus, basal ganglia, and cerebral cortex. Exogenously applied CB1 receptor agonists disrupt neuronal dynamics and synaptic plasticity, resulting in cognitive deficits and impairment of the highest psychological functions. Various other pro-psychotic drugs, such as ketamine and methamphetamine, exert their effects in the same microdomain of synaptic spines as Δ9 -THC. Additionally, many of the most robust findings in psychiatric genetics include components that localize to dendritic spines and have important roles in information processing and plasticity.