Cannabinoid-induced enhanced interaction and protein levels of serotonin 5-HT(2A) and dopamine D₂ receptors in rat prefrontal cortex

The synthetic cannabinoids menace: a review of health risks and toxicity

Synthetic cannabinoids (SCs) are chemically classified as psychoactive substances that target the endocannabinoid system in many body organs. SCs can initiate pathophysiological changes in many tissues which can be severe enough to damage the normal functionality of our body systems. The majority of SCs-related side effects are mediated by activating Cannabinoid Receptor 1 (CB1R) and Cannabinoid Receptor 2 (CB2R). The activation of these receptors can enkindle many downstream signalling pathways, including oxidative stress, inflammation, and apoptosis that ultimately can produce deleterious changes in many organs. Besides activating the cannabinoid receptors, SCs can act on non-cannabinoid targets, such as the orphan G protein receptors GPR55 and GPR18, the Peroxisome Proliferator-activated Receptors (PPARs), and the Transient receptor potential vanilloid 1 (TRPV1), which are broadly expressed in the brain and the heart and their activation mediates many pharmacological effects of SCs. In this review, we shed light on the multisystem complications found in SCs abusers, particularly discussing their neurologic, cardiovascular, renal, and hepatic effects, as well as highlighting the mechanisms that intermediate SCs-related pharmacological and toxicological consequences to provide comprehensive understanding of their short and long-term systemic effects.

CB2 receptor in the CNS: from immune and neuronal modulation to behavior

Despite low levels of cannabinoid receptor type 2 (CB2R) expression in the central nervous system in human and rodents, a growing body of evidence shows CB2R involvement in many processes at the behavioral level, through both immune and neuronal modulations. Recent in vitro and in vivo evidence have highlighted the complex role of CB2R under physiological and inflammatory conditions. Under neuroinflammatory states, its activation seems to protect the brain and its functions, making it a promising target in a wide range of neurological disorders. Here, we provide a complete and updated overview of CB2R function in the central nervous system of rodents, spanning from modulation of immune function in microglia but also in other cell types, to behavior and neuronal activity, in both physiological and neuroinflammatory contexts.

Synthetic Cannabinoids: A Pharmacological and Toxicological Overview

Synthetic cannabinoids (SCs) are a chemically diverse group of new psychoactive substances (NPSs) that target the endocannabinoid system, triggering a plethora of actions (e.g., elevated mood sensation, relaxation, appetite stimulation) that resemble, but are more intense than, those induced by cannabis. Although some of these effects have been explored for therapeutic applications, anticipated stronger psychoactive effects than cannabis and reduced risk perception have increased the recreational use of SCs, which have dominated the NPS market in the United States and Europe over the past decade. However, rising SC-related intoxications and deaths represent a major public health concern and embody a major challenge for policy makers. Here, we review the pharmacology and toxicology of SCs. A thorough characterization of SCs' pharmacodynamics and toxicodynamics is important to better understand the main mechanisms underlying acute and chronic effects of SCs, interpret the clinical/pathological findings related to SC use, and improve SC risk awareness.

Cannabis use selectively modulates circulating biomarkers in the blood of schizophrenia patients

Cannabis use disorder is frequent in schizophrenia patients, and it is associated with an earlier age of onset and poor schizophrenia prognosis. Serotonin 2A receptors (5-HT2AR) have been involved in psychosis and, like Akt kinase, are known to be modulated by THC. Likewise, endocannabinoid system dysregulation has been suggested in schizophrenia. The presence of these molecules in blood makes them interesting targets, as they can be evaluated in patients by a minimally invasive technique. The aim of the present study was to evaluate 5-HT2AR protein expression and the Akt functional status in platelet homogenates of subjects diagnosed with schizophrenia, cannabis use disorder, or both conditions, compared with age- and sex-matched control subjects. Additionally, endocannabinoids and pro-inflammatory interleukin-6 (IL-6) levels were also measured in the plasma of these subjects. Results showed that both platelet 5-HT2AR and the active phospho (Ser473)Akt protein expression were significantly increased in schizophrenia subjects, whereas patients with a dual diagnosis of schizophrenia and cannabis use disorder did not show significant changes. Similarly, plasma concentrations of anandamide and other lipid mediators such as PEA and DEA, as well as the pro-inflammatory IL-6, were significantly increased in schizophrenia, but not in dual subjects. Results demonstrate that schizophrenia subjects show different circulating markers pattern depending on the associated diagnosis of cannabis use disorder, supporting the hypothesis that there could be different underlying mechanisms that may explain clinical differences among these groups. Moreover, they provide the first preliminary evidence of peripherally measurable molecules of interest for bigger prospective studies in these subpopulations.

Immunomodulatory Role of CB2 Receptors in Emotional and Cognitive Disorders

Emotional behavior, memory, and learning have been associated with alterations in the immune system in neuropsychiatric and neurodegenerative diseases. In recent years, several studies pointed out the involvement of the cannabinoid receptor 2 (CB2r) in the immune system and the regulation of inflammation. This receptor is widely distributed in different tissues and organs with higher expression in spleen and immune system cells. However, CB2r has also been detected in several brain areas and different brain cell types, such as neurons and glia. These findings suggest that CB2r may closely relate the immune system and the brain circuits regulating inflammation, mood, and cognitive functions. Therefore, we review the studies that may help elucidate the molecular bases of CB2r in regulating inflammation in different brain cells and its role in the pathophysiology of psychiatric and neurodegenerative disorders.

Cannabinoid 2 receptors regulate dopamine 2 receptor expression by a beta-arrestin 2 and GRK5-dependent mechanism in neuronal cells

We have previously reported that the repeated exposure to cannabinoids upregulates and enhances the activity of serotonin 2A (5-HT_2A) and dopamine 2 (D₂) receptors and facilitates the formation of D₂-5-HT_2A receptor heterodimers in the rat prefrontal cortex and two neuronal cell lines. Because the repeated exposure to cannabinoids has been associated with adverse neuropsychiatric disorders, this study investigated the mechanisms that underly the cannabinoid-mediated regulation of D₂ receptor expression in a neuronal cell model, CLU213 cells. We initially tested the effects of repeated exposure (72 h) to a non-selective cannabinoid agonist (1 nM CP55940), a selective CB₁ receptor agonist (15 nM ACEA), or a selective CB₂ receptor drug (1 nM GP1a) on the expression of postsynaptic D₂ (D₂L) receptors in CLU213 cells. Repeated CP55940, GP1a, or ACEA treatments significantly increased D₂L receptor protein levels (99 % ± 7%, 30 % ± 7%, and 39 % ± 5% increases compared with control levels, respectively). Repeated exposure to both GP1a and ACEA increased D₂L receptor protein levels by 73 % ± 8%. Interestingly, CP55940 and GP1a, but not ACEA, upregulated D₂ mRNA. Using cells that were stably transfected with short-hairpin RNA (shRNA) lentiviral particles targeting CB₂ receptors, G protein-coupled receptor kinase 5 (GRK5), and β-arrestin 2, we found that CB₂ receptors regulated D₂ expression through a mechanism that is dependent on GRK5, β-arrestin 2, and extracellular signal-related kinase (ERK)1/2. We also found that repeated exposure to either ACEA or GP1a selectively stimulated the protein and mRNA expression of GRK proteins. ACEA significantly upregulated GRK2 proteins, whereas GP1a upregulated GRK5 protein expression. Our results identified mechanisms associated with the upregulation of D₂ receptors in neuronal cells after the repeated exposure to cannabinoids. These data can shed light on the mechanisms that can be targeted to prevent potential adverse effects, while simultaneously determining the therapeutic benefits of cannabinoids.

Cannabis sativa: Interdisciplinary Strategies and Avenues for Medical and Commercial Progression Outside of CBD and THC

Cannabis sativa (Cannabis) is one of the world’s most well-known, yet maligned plant species. However, significant recent research is starting to unveil the potential of Cannabis to produce secondary compounds that may offer a suite of medical benefits, elevating this unique plant species from its illicit narcotic status into a genuine biopharmaceutical. This review summarises the lengthy history of Cannabis and details the molecular pathways that underpin the production of key secondary metabolites that may confer medical efficacy. We also provide an up-to-date summary of the molecular targets and potential of the relatively unknown minor compounds offered by the Cannabis plant. Furthermore, we detail the recent advances in plant science, as well as synthetic biology, and the pharmacology surrounding Cannabis. Given the relative infancy of Cannabis research, we go on to highlight the parallels to previous research conducted in another medically relevant and versatile plant, Papaver somniferum (opium poppy), as an indicator of the possible future direction of Cannabis plant biology. Overall, this review highlights the future directions of cannabis research outside of the medical biology aspects of its well-characterised constituents and explores additional avenues for the potential improvement of the medical potential of the Cannabis plant.

Endocannabinoid-serotonin systems interaction in health and disease

Endocannabinoid (eCB) and serotonin (5-HT) neuromodulatory systems work both independently and together to finely orchestrate neuronal activity throughout the brain to strongly sculpt behavioral functions. Surprising parallelism between the behavioral effects of 5-HT and eCB activity has been widely reported, including the regulation of emotional states, stress homeostasis, cognitive functions, food intake and sleep. The distribution pattern of the 5-HT system and the eCB molecular elements in the brain display a strong overlap and several studies report a functional interplay and even a tight interdependence between eCB/5-HT signaling. In this review, we examine the available evidence of the interaction between the eCB and 5-HT systems. We first introduce the eCB system, then we describe the eCB/5-HT crosstalk at the neuronal and synaptic levels. Finally, we explore the potential eCB/5-HT interaction at the behavioral level with the implication for psychiatric and neurological disorders. The precise elucidation of how this neuromodulatory interaction dynamically regulates biological functions may lead to the development of more targeted therapeutic strategies for the treatment of depressive and anxiety disorders, psychosis and epilepsy.

The neuropharmacology of cannabinoid receptor ligands in central signaling pathways

The endocannabinoid system is a complex neuronal system involved in a number of biological functions, like attention, anxiety, mood, memory, appetite, reward, and immune responses. It is at the centre of scientific interest, which is driven by therapeutic promise of certain cannabinoid ligands and the changing legalization of herbal cannabis in many countries. The endocannabinoid system is a modulatory system, with endocannabinoids as retrograde neurotransmitters rather than direct neurotransmitters. Neuropharmacology of cannabinoid ligands in the brain can therefore be understood in terms of their modulatory actions through other neurotransmitter systems. The CB₁ receptor is chiefly responsible for effects of endocannabinoids and analogous ligands in the brain. An overview of the neuropharmacology of several cannabinoid receptor ligands, including endocannabinoids, herbal cannabis and synthetic cannabinoid receptor ligands is given in this review. Their mechanism of action at the endocannabinoid system is described, mainly in the brain. In addition, effects of cannabinoid ligands on other neurotransmitter systems will also be described, such as dopamine, serotonin, glutamate, noradrenaline, opioid, and GABA. In light of this, therapeutic potential and adverse effects of cannabinoid receptor ligands will also be discussed.

Elevated fatty acid amide hydrolase in the prefrontal cortex of borderline personality disorder: a [11C]CURB positron emission tomography study

Amygdala-prefrontal cortex (PFC) functional impairments have been linked to emotion dysregulation and aggression in borderline personality disorder (BPD). Fatty acid amide hydrolase (FAAH), the major catabolic enzyme for the endocannabinoid anandamide, has been proposed as a key regulator of the amygdala-PFC circuit that subserves emotion regulation. We tested the hypothesis that FAAH levels measured with [¹¹C]CURB positron emission tomography in amygdala and PFC would be elevated in BPD and would relate to hostility and aggression. Twenty BPD patients and 20 healthy controls underwent FAAH genotyping (rs324420) and scanning with [¹¹C]CURB. BPD patients were medication-free and were not experiencing a current major depressive episode. Regional differences in [¹¹C]CURB binding were assessed using multivariate analysis of covariance with PFC and amygdala [¹¹C]CURB binding as dependent variables, diagnosis as a fixed factor, and sex and genotype as covariates. [¹¹C]CURB binding was marginally elevated across the PFC and amygdala in BPD (p = 0.08). In a priori selected PFC, but not amygdala, [¹¹C]CURB binding was significantly higher in BPD (11.0%, p = 0.035 versus 10.6%, p = 0.29). PFC and amygdala [¹¹C]CURB binding was positively correlated with measures of hostility in BPD (r > 0.4; p < 0.04). This study is the first to provide preliminary evidence of elevated PFC FAAH binding in any psychiatric condition. Findings are consistent with the model that lower endocannabinoid tone could perturb PFC circuitry that regulates emotion and aggression. Replication of these findings could encourage testing of FAAH inhibitors as innovative treatments for BPD.

Effect of cannabinoid-serotonin interactions in the regulation of neuropeptide Y1 receptors expression in rats: the role of CB1 and 5-HT2C receptor

Neuropeptide Y (NPY) is involved in a diversity of critical functions such as circadian rhythms, energy homeostasis, and appetite regulation in the hypothalamus. It has identified as a crucial participant in adjusting energy intake and energy storage as fat via central neuropeptide Y1 receptor (NPY1R), leading to obesity and metabolic disorders. The present study was expected to investigate the interaction between 2-AG (CB1R agonist), m-CPP (5HT2CR agonist), SB-242084 (5HT2CR antagonist), and SR-141716A (CB1R antagonist) by mediating through the NPY1R for treating or preventing obesity, metabolic disorders, and other abnormalities. The expression level of NPY1R mRNA has studied on the rat brain by real-time quantitative PCR assay. Based on our findings, intracerebroventricular (ICV) injection of combined 2-AG (1 μg) + m-CPP (2.5 μg) has antagonistic interaction in the expression of the NPY1R gene (P < 0.001). Moreover, the ICV co-injection of SB-242084 (3 μg) + SR-141716A (1 μg) has antagonistic interaction in the NPY1R gene expression (P < 0.001). Co-administration of 2-AG (1 μg) + SB-242084 (3 μg) amplified NPY1R gene expression (P < 0.001), while the ICV co-injection of m-CPP (2.5 μg) + SR-141716A (1 μg) decreased NPY1R gene expression in the hypothalamus (P < 0.001). These results revealed the interference in cannabinoid and serotonergic systems via CB1 and 5HT2C receptors in the expression of NPY1R mRNA in the hypothalamic area of rats.

AP-1 controls the p11-dependent antidepressant response

Selective serotonin reuptake inhibitors (SSRIs) are the most widely prescribed drugs for mood disorders. While the mechanism of SSRI action is still unknown, SSRIs are thought to exert therapeutic effects by elevating extracellular serotonin levels in the brain, and remodel the structural and functional alterations dysregulated during depression. To determine their precise mode of action, we tested whether such neuroadaptive processes are modulated by regulation of specific gene expression programs. Here we identify a transcriptional program regulated by activator protein-1 (AP-1) complex, formed by c-Fos and c-Jun that is selectively activated prior to the onset of the chronic SSRI response. The AP-1 transcriptional program modulates the expression of key neuronal remodeling genes, including S100a10 (p11), linking neuronal plasticity to the antidepressant response. We find that AP-1 function is required for the antidepressant effect in vivo. Furthermore, we demonstrate how neurochemical pathways of BDNF and FGF2, through the MAPK, PI3K, and JNK cascades, regulate AP-1 function to mediate the beneficial effects of the antidepressant response. Here we put forth a sequential molecular network to track the antidepressant response and provide a new avenue that could be used to accelerate or potentiate antidepressant responses by triggering neuroplasticity.

Chronic cannabis promotes pro-hallucinogenic signaling of 5-HT2A receptors through Akt/mTOR pathway

Long-term use of potent cannabis during adolescence increases the risk of developing schizophrenia later in life, but to date, the mechanisms involved remain unknown. Several findings suggest that the functional selectivity of serotonin 2A receptor (5-HT2AR) through inhibitory G-proteins is involved in the molecular mechanisms responsible for psychotic symptoms. Moreover, this receptor is dysregulated in the frontal cortex of schizophrenia patients. In this context, studies involving cannabis exposure and 5-HT2AR are scarce. Here, we tested in mice the effect of an early chronic Δ9-tetrahydrocannabinol (THC) exposure on cortical 5-HT2AR expression, as well as on its in vivo and in vitro functionality. Long-term exposure to THC induced a pro-hallucinogenic molecular conformation of the 5-HT2AR and exacerbated schizophrenia-like responses, such as prepulse inhibition disruption. Supersensitive coupling of 5-HT2AR toward inhibitory Gαi1-, Gαi3-, Gαo-, and Gαz-proteins after chronic THC exposure was observed, without changes in the canonical Gαq/11-protein pathway. In addition, we found that inhibition of Akt/mTOR pathway by rapamycin blocks the changes in 5-HT2AR signaling pattern and the supersensitivity to schizophrenia-like effects induced by chronic THC. The present study provides the first evidence of a mechanistic explanation for the relationship between chronic cannabis exposure in early life and increased risk of developing psychosis-like behaviors in adulthood.

Pro-psychotic effects of synthetic cannabinoids: interactions with central dopamine, serotonin, and glutamate systems

An association between marijuana use and schizophrenia has been noted for decades, and the recent emergence of high-efficacy synthetic cannabinoids (SCBs) as drugs of abuse has lead to a growing number of clinical reports of persistent psychotic effects in users of these substances. The mechanisms underlying SCB-elicited pro-psychotic effects is unknown, but given the ubiquitous neuromodulatory functions of the endocannabinoid system, it seems likely that agonist actions at cannabinoid type-1 receptors (CB1Rs) might modulate the functions of other neurotransmitter systems known to be involved in schizophrenia. The present review surveys what is currently known about the interactions of CB1Rs with dopamine, serotonin, and glutamate systems, because all three of those neurotransmitters are well-established in the pathophysiology of schizophrenia and psychosis. Identification of molecular mechanisms underlying the pro-psychotic effects of SCB drugs of abuse may establish certain classes of these substances as particularly dangerous, guiding regulations to control availability of these drugs. Likewise, an understanding of the pharmacological interactions which lead to schizophrenia and psychosis subsequent to SCB exposure might guide the development of novel therapies to treat afflicted users.

Repeated Exposure to the "Spice" Cannabinoid JWH-018 Induces Tolerance and Enhances Responsiveness to 5-HT1A Receptor Stimulation in Male Rats

Naphthalen-1-yl-(1-pentylindol-3-yl)methanone (JWH-018) is a synthetic compound found in psychoactive "spice" products that activates cannabinoid receptors. Preclinical evidence suggests that exposure to synthetic cannabinoids increases 5-HT_2A/2C receptor function in the brain, an effect which might contribute to psychotic symptoms. Here, we hypothesized that repeated exposures to JWH-018 would enhance behavioral responsiveness to the 5-HT_2A/2C receptor agonist DOI. Male Sprague-Dawley rats fitted with subcutaneously (sc) temperature transponders received daily injections of JWH-018 (1.0 mg/kg, sc) or its vehicle for seven consecutive days. Body temperature and catalepsy scores were determined at 1, 2, and 4 h post-injection each day. At 1 and 7 days after the final repeated treatment, rats received a challenge injection of either DOI (0.1 mg/kg, sc) or the 5-HT_1A receptor agonist 8-OH-DPAT (0.3 mg/kg, sc), then temperature and behavioral responses were assessed. Behaviors induced by DOI included wet dog shakes and back muscle contractions (i.e., skin jerks), while behaviors induced by 8-OH-DPAT included ambulation, forepaw treading, and flat body posture. On the first day of repeated treatment, JWH-018 produced robust hypothermia and catalepsy which lasted up to 4 h, and these effects were significantly blunted by day 7 of treatment. Repeated exposure to JWH-018 did not affect behaviors induced by DOI, but behavioral and hypothermic responses induced by 8-OH-DPAT were significantly augmented 1 day after cessation of JWH-018 treatment. Collectively, our findings show that repeated treatment with JWH-018 produces tolerance to its hypothermic and cataleptic effects, which is accompanied by transient enhancement of 5-HT_1A receptor sensitivity in vivo.

RNA-seq analysis of amygdala tissue reveals characteristic expression profiles in schizophrenia

The amygdala brain region has been implicated in the pathophysiology of schizophrenia through emotion processing. However, transcriptome messages in the amygdala of schizophrenia patients have not been well studied. We used RNA sequencing to investigate gene-expression profiling in the amygdala tissues, and identified 569 upregulated and 192 downregulated genes from 22 schizophrenia patients and 24 non-psychiatric controls. Gene functional enrichment analysis demonstrated that the downregulated genes were enriched in pathways such as 'synaptic transmission' and 'behavior', whereas the upregulated genes were significantly over-represented in gene ontology pathways such as 'immune response' and 'blood vessel development'. Co-expression-based gene network analysis identified seven modules including four modules significantly associated with 'synaptic transmission', 'blood vessel development' or 'immune responses'. Taken together, our study provides novel insights into the molecular mechanism of schizophrenia, suggesting that precision-tailored therapeutic approaches aimed at normalizing the expression/function of specific gene networks could be a promising option in schizophrenia.

Modulation of Synaptic Plasticity in the Cortex Needs to Understand All the Players

The prefrontal cortex (PFC) is involved in cognitive tasks such as working memory, decision making, risk assessment and regulation of attention. These functions performed by the PFC are supposed to rely on rhythmic electrical activity generated by neuronal network oscillations determined by a precise balance between excitation and inhibition balance (E/I balance) resulting from the coordinated activities of recurrent excitation and feedback and feedforward inhibition. Functional alterations in PFC functions have been associated with cognitive deficits in several pathologies such as major depression, anxiety and schizophrenia. These pathological situations are correlated with alterations of different neurotransmitter systems (i.e., serotonin (5-HT), dopamine (DA), acetylcholine…) that result in alterations of the E/I balance. The aim of this review article is to cover the basic aspects of the regulation of the E/I balance as well as to highlight the importance of the complementarity role of several neurotransmitters in the modulation of the plasticity of excitatory and inhibitory synapses. We illustrate our purpose by recent findings that demonstrate that 5-HT and DA cooperate to regulate the plasticity of excitatory and inhibitory synapses targeting layer 5 pyramidal neurons (L5PyNs) of the PFC and to fine tune the E/I balance. Using a method based on the decomposition of the synaptic conductance into its excitatory and inhibitory components, we show that concomitant activation of D1-like receptors (D1Rs) and 5-HT_1ARs, through a modulation of NMDA receptors, favors long term potentiation (LTP) of both excitation and inhibition and consequently does not modify the E/I balance. We also demonstrate that activation of D2-receptors requires functional 5-HT_1ARs to shift the E-I balance towards more inhibition and to favor long term depression (LTD) of excitatory synapses through the activation of glycogen synthase kinase 3β (GSK3β). This cooperation between different neurotransmitters is particularly relevant in view of pathological situations in which alterations of one neurotransmitter system will also have consequences on the regulation of synaptic efficacy by other neurotransmitters. This opens up new perspectives in the development of therapeutic strategies for the pharmacological treatment of neuronal disorders.

Cane Toad Skin Extract-Induced Upregulation and Increased Interaction of Serotonin 2A and D2 Receptors via Gq/11 Signaling Pathway in CLU213 Cells

Recent evidences show that activation of serotonin 2A receptors (5-HT_2A R) by agonists is significant in improving therapeutic activity of disease conditions, such as obsessive-compulsive disorder (OCD). Though the exact molecular mechanism is still not well understood, it is thought to involve agonist-driven, enhanced expression of 5-HT_2A R in certain areas of brain, such as the pre-frontal cortex (PFC). Several other reports have also demonstrated association of OCD with lower dopamine receptor (D₂ R) availability, primarily in the striatum of the brain along with dysfunction of 5-HT_2A R-D₂ R heteromer regulation. We thus hypothesized that compound(s) interacting with this molecular mechanism could be developed as drugs for long-term beneficial effects against OCD. In the present study, we have obtained experimental evidence in cultured neuronal cells (CLU213) that aqueous extract (AE, 50 μg/mL, P < 0.05) of the Australian cane toad skin significantly increased the levels of 5-HT_2A R and D₂ R protein and mRNA expression. AE was also found to enhance the interaction between 5-HT_2A R and D₂ R and formation of expression of 5-HT_2A R-D₂ R heteromer using co-immunoprecipitation and Western blot. Further investigation showed the involvement of classical signaling pathway (G_q/11 -PLCβ) along with c-FOS transcription factor preferentially in 5-HT_2A -mediated agonist activation. These results obtained demonstrated that AE upregulates 5-HT_2A R by a mechanism that appears to involve G_q/11 -PLCβ signaling pathway and c-FOS transcription factor activation. We indicate this enhanced 5-HT_2A R and D₂ R expression and their interaction to induce increased 5-HT_2A R-D₂ R heteromer formation by exposure to AE might provide a molecular mechanism to develop potential novel drug candidates to ameliorate OCD symptoms. J. Cell. Biochem. 118: 979-993, 2017. © 2016 Wiley Periodicals, Inc.

The central cannabinoid receptor type-2 (CB2) and chronic pain

Cannabinoid receptor type-2 (CB2, CB2 receptor or CB2-R) mediates analgesia via two mechanisms. CB2 receptors contained in peripheral immune tissue mediate analgesia by altering cytokine profiles, and thus have little adverse effects on central nervous systems (CNSs). CB2 is also expressed in the neurons and glial cells of the CNS. This neuronal expression may also contribute to pain attenuation. The CB2 receptor has been proposed as a potential target in treating chronic pain of several etiologies.

Ultrastructural evidence for synaptic contacts between cortical noradrenergic afferents and endocannabinoid-synthesizing post-synaptic neurons

Endocannabinoids (eCBs) are involved in a myriad of physiological processes that are mediated through the activation of cannabinoid receptors, which are ubiquitously distributed within the nervous system. One neurochemical target at which cannabinoids interact to have global effects on behavior is brain noradrenergic circuitry. We, and others, have previously shown that CB type 1 receptors (CB1r) are positioned to pre-synaptically modulate norepinephrine (NE) release in the rat frontal cortex (FC). Diacylglycerol lipase (DGL) is a key enzyme in the biosynthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG). While DGL-α is expressed in the FC in the rat brain, it is not known whether noradrenergic afferents target neurons expressing synthesizing enzymes for the endocannabinoid, 2-AG. In the present study, we employed high-resolution neuroanatomical approaches to better define cellular sites for interactions between noradrenergic afferents and FC neurons expressing DGL-α. Immunofluorescence microscopy showed close appositions between processes containing the norepinephrine transporter (NET) or dopamine-β-hydroxylase (DβH) and cortical neurons expressing DGL-α-immunoreactivity. Ultrastructural analysis using immunogold-silver labeling for DGL-α and immunoperoxidase labeling for NET or DβH confirmed that NET-labeled axon terminals were directly apposed to FC somata and dendritic processes that exhibited DGL-α-immunoreactivity. Finally, tissue sections were processed for immunohistochemical detection of DGL-α, CB1r and DβH. Triple label immunofluorescence revealed that CB1r and DβH were co-localized in common cellular profiles and these were in close association with DGL-α. Taken together, these data provide anatomical evidence for direct synaptic associations between noradrenergic afferents and cortical neurons exhibiting endocannabinoid synthesizing machinery.

Cannabinoid-induced upregulation of serotonin 2A receptors in the hypothalamic paraventricular nucleus and anxiety-like behaviors in rats

Recent behavioral reports suggest that repeated exposure to cannabis and synthetic cannabinoid agonists is linked with mental disorders associated with dysfunction of serotonin 2A (5-HT2A) receptor neurotransmission such as anxiety and depression. Here, we studied the effect of a nonselective cannabinoid agonist, CP55940, on the activity of 5-HT2A receptors in hypothalamic paraventricular nucleus (PVN). We detected that repeated exposure to CP55940 enhanced the prolactin and corticosterone neuroendocrine responses mediated by 5-HT2A receptors and increased the membrane-associated levels of 5-HT2A receptors in PVN. Importantly, we also detected increased anxiety-like behaviors in CP55940 treated rats compared to controls. The data presented here suggest that the mechanisms mediating the cannabinoid-induced upregulation of 5-HT2A receptors would be brain-region specific, as we were unable to detect a CP55940-induced upregulation of 5-HT2A mRNA. Our results might provide insight into the molecular mechanism by which repeated exposure to cannabinoids could be associated with the pathophysiology of neuropsychiatric disorders.

G-protein receptor kinase 5 regulates the cannabinoid receptor 2-induced up-regulation of serotonin 2A receptors

We have recently reported that cannabinoid agonists can up-regulate and enhance the activity of serotonin 2A (5-HT2A) receptors in the prefrontal cortex (PFCx). Increased expression and activity of cortical 5-HT2A receptors has been associated with neuropsychiatric disorders, such as anxiety and schizophrenia. Here we report that repeated CP55940 exposure selectively up-regulates GRK5 proteins in rat PFCx and in a neuronal cell culture model. We sought to examine the mechanism underlying the regulation of GRK5 and to identify the role of GRK5 in the cannabinoid agonist-induced up-regulation and enhanced activity of 5-HT2A receptors. Interestingly, we found that cannabinoid agonist-induced up-regulation of GRK5 involves CB2 receptors, β-arrestin 2, and ERK1/2 signaling because treatment with CB2 shRNA lentiviral particles, β-arrestin 2 shRNA lentiviral particles, or ERK1/2 inhibitor prevented the cannabinoid agonist-induced up-regulation of GRK5. Most importantly, we found that GRK5 shRNA lentiviral particle treatment prevented the cannabinoid agonist-induced up-regulation and enhanced 5-HT2A receptor-mediated calcium release. Repeated cannabinoid exposure was also associated with enhanced phosphorylation of CB2 receptors and increased interaction between β-arrestin 2 and ERK1/2. These latter phenomena were also significantly inhibited by GRK5 shRNA lentiviral treatment. Our results suggest that sustained activation of CB2 receptors, which up-regulates 5-HT2A receptor signaling, enhances GRK5 expression; the phosphorylation of CB2 receptors; and the β-arrestin 2/ERK interactions. These data could provide a rationale for some of the adverse effects associated with repeated cannabinoid agonist exposure.

Cannabinoid receptor agonists upregulate and enhance serotonin 2A (5-HT(2A)) receptor activity via ERK1/2 signaling

Recent behavioral studies suggest that nonselective agonists of cannabinoid receptors may regulate serotonin 2A (5-HT(2A)) receptor neurotransmission. Two cannabinoids receptors are found in brain, CB1 and CB2 receptors, but the molecular mechanism by which cannabinoid receptors would regulate 5-HT(2A) receptor neurotransmission remains unknown. Interestingly, we have recently found that certain cannabinoid receptor agonists can specifically upregulate 5-HT(2A) receptors. Here, we present experimental evidence that rats treated with a nonselective cannabinoid receptor agonist (CP 55,940, 50 µg/kg, 7 days) showed increases in 5-HT(2A) receptor protein levels, 5-HT(2A) receptor mRNA levels, and 5-HT(2A) receptor-mediated phospholipase C beta (PLCβ) activity in prefrontal cortex (PFCx). Similar effects were found in neuronal cultured cells treated with CP 55,940 but these effects were prevented by selective CB2, but not selective CB1, receptor antagonists. CB2 receptors couple to the extracellular kinase (ERK) signaling pathway by Gα(i/o) class of G-proteins. Noteworthy, GP 1a (selective CB2 receptor agonist) produced a strong upregulation of 5-HT(2A) receptor mRNA and protein, an effect that was prevented by selective CB2 receptor antagonists and by an ERK1/2 inhibitor, PD 198306. In summary, our results identified a strong cannabinoid-induced upregulation of 5-HT(2A) receptor signaling in rat PFCx. Our cultured cell studies suggest that selective CB2 receptor agonists upregulate 5-HT(2A) receptor signaling by activation of the ERK1/2 signaling pathway. Activity of cortical 5-HT(2A) receptors has been associated with several physiological functions and neuropsychiatric disorders such as stress response, anxiety and depression, and schizophrenia. Therefore, these results may provide a molecular mechanism by which activation of cannabinoid receptors might be relevant to the pathophysiology of some cognitive and mood disorders in humans.

Age of stress exposure modulates the immediate and sustained effects of repeated stress on corticolimbic cannabinoid CB₁ receptor binding in male rats

Chronic stress is known to modulate cannabinoid CB1 receptor binding densities in corticolimbic structures, in a region-dependent manner; however, the ontogeny of these changes and the degree to which they recover following exposure to stress have yet to be determined. To this extent, we examined both the immediate and sustained effects (following a 40-day recovery period) of a repeated restraint stress paradigm (30-min restraint/day for 10 days) on CB1 receptor binding in the prefrontal cortex (PFC), hippocampus and amygdala in both adolescent (stress onset at post-natal day [PND] 35) and adult (stress onset at PND 75) male Sprague-Dawley rats. Consistent with previous reports, we found that repeated stress in adult rats resulted in an increase in CB1 receptor binding in the PFC, a reduction in CB1 receptor binding in the hippocampus and no effect in the amygdala. Interestingly, adolescent rats exposed to repeated restraint stress did not show any change in hippocampal CB1 receptor density, but exhibited an upregulation of CB1 receptor binding in both the PFC and amygdala. In adults, a 40-day recovery period resulted in a normalization of CB1 receptor binding in the PFC, and surprisingly a pronounced upregulation of CB1 receptor binding in the hippocampus, possibly indicative of a rebound effect. Adolescents similarly exhibited this rebound increase in hippocampal CB1 receptor binding, despite a lack in immediate downregulation following repeated restraint. Of particular interest, adolescents exposed to stress were found to have a sustained downregulation of prefrontocortical CB1 receptors in adulthood, which may relate to some of the reported sustained behavioral effects of stress in adolescence. Collectively, these data indicate that the effects of chronic stress on cannabinoid CB1 receptor binding are modulated by the age of stress exposure and period of recovery following the cessation of stress.

Cannabinoid agonists increase the interaction between β-Arrestin 2 and ERK1/2 and upregulate β-Arrestin 2 and 5-HT(2A) receptors

We have recently reported that selective cannabinoid 2 (CB(2)) receptor agonists upregulate 5-HT(2A) receptors by enhancing ERK1/2 signaling in prefrontal cortex (PFCx). Increased activity of cortical 5-HT(2A) receptors has been associated with several neuropsychiatric disorders such as anxiety and schizophrenia. Here we examine the mechanisms involved in this enhanced ERK1/2 activation in rat PFCx and in a neuronal cell model. Sprague-Dawley rats treated with a non-selective cannabinoid agonist (CP55940, 50μg/kg, 7 days, i.p.) showed enhanced co-immunoprecipitation of β-Arrestin 2 and ERK1/2, enhanced pERK protein levels, and enhanced expression of β-Arrestin 2 mRNA and protein levels in PFCx. In a neuronal cell line, we found that selective CB(2) receptor agonists upregulate β-Arrestin 2, an effect that was prevented by selective CB(2) receptor antagonist JTE-907 and CB(2) shRNA lentiviral particles. Additionally, inhibition of clathrin-mediated endocytosis, ERK1/2, and the AP-1 transcription factor also prevented the cannabinoid receptor-induced upregulation of β-Arrestin 2. Our results suggest that sustained activation of CB(2) receptors would enhance β-Arrestin 2 expression possibly contributing to its increased interaction with ERK1/2, thereby driving the upregulation of 5-HT(2A) receptors. The CB(2) receptor-mediated upregulation of β-Arrestin 2 would be mediated, at least in part, by an ERK1/2-dependent activation of AP-1. These data could provide the rationale for some of the adverse effects associated with repeated cannabinoid exposure and shed light on some CB(2) receptor agonists that could represent an alternative therapeutic because of their minimal effect on serotonergic neurotransmission.