Cocaine-mediated supersensitivity of 5-HT2A receptors in hypothalamic paraventricular nucleus is a withdrawal-induced phenomenon

Psilocybin for treating substance use disorders?

INTRODUCTION

Evidence based treatment for Substance use disorders (SUD) includes psychotherapy and pharmacotherapy. However, these are only partially effective. Hallucinogens, such as psilocybin, may represent potential new treatment options for SUD. This review provides a summary of (human) studies on the putative therapeutic effects of psilocybin, and discusses the receptor systems, brain regions and cognitive and emotional processes mediating psilocybin's effects. Psilocybin's chemical structure is similar to that of serotonin. Dysregulations in the serotonin system are associated with alterations in stress hormones, such as cortisol, and mood disorders. After psilocybin administration cortisol levels spike and activate the executive control network, with subsequent increased control over emotional processes, and relief of negative thinking and persistent negative emotions. Preliminary data of ongoing alcohol and smoking addiction studies in humans shows promising effects of psilocybin administration on substance use. Importantly, psilocybin has a low risk of toxicity and dependence and can be used safely under controlled clinical conditions. Areas covered: This paper is a narrative review based on the search terms: psilocybin, substance use disorder, addiction, depression, serotonin. Literature on potential efficacy and mechanisms of action of psilocybin in SUD is discussed. Expert commentary: Recent positive findings with psilocybin need confirmation in well-designed placebo controlled randomized trials employing a large sample size.

Serotonin 5-HT2 receptor interactions with dopamine function: implications for therapeutics in cocaine use disorder

Cocaine exhibits prominent abuse liability, and chronic abuse can result in cocaine use disorder with significant morbidity. Major advances have been made in delineating neurobiological mechanisms of cocaine abuse; however, effective medications to treat cocaine use disorder remain to be discovered. The present review will focus on the role of serotonin (5-HT; 5-hydroxytryptamine) neurotransmission in the neuropharmacology of cocaine and related abused stimulants. Extensive research suggests that the primary contribution of 5-HT to cocaine addiction is a consequence of interactions with dopamine (DA) neurotransmission. The literature on the neurobiological and behavioral effects of cocaine is well developed, so the focus of the review will be on cocaine with inferences made about other monoamine uptake inhibitors and releasers based on mechanistic considerations. 5-HT receptors are widely expressed throughout the brain, and several different 5-HT receptor subtypes have been implicated in mediating the effects of endogenous 5-HT on DA. However, the 5-HT2A and 5-HT2C receptors in particular have been implicated as likely candidates for mediating the influence of 5-HT in cocaine abuse as well as to traits (e.g., impulsivity) that contribute to the development of cocaine use disorder and relapse in humans. Lastly, new approaches are proposed to guide targeted development of serotonergic ligands for the treatment of cocaine use disorder.

Cocaine potentiates multiple 5-HT2A receptor signaling pathways and is associated with decreased phosphorylation of 5-HT2A receptors in vivo

Cocaine addiction is a chronic relapsing disorder in which the underlying mechanisms are not well understood. Here, we used Sprague-Dawley rats injected with either saline (1 ml/kg) or cocaine (15 mg/kg) for 7 days (b.i.d, i.p) to study the effect of cocaine on several components of 5-HT2A receptor signaling in prefrontal cortex (PFCx). We detected enhanced activation of 5-HT2A receptor-mediated phospholipase C beta (PLCβ) and extracellular regulated kinase 1/2 activity in PFCx of cocaine-treated rats. Although we were unable to detect changes in the protein levels of several proteins associated with 5-HT2A receptor signaling such as caveolin-1, postsynaptic density protein 95, β-arrestin 2, etc., we found a significant reduction in the phosphorylation status of cortical 5-HT2A receptors. This phenomenon was associated with reduced levels of G-protein receptor kinase 5 (GRK5), but not GRK2 or RSK2, proteins. Our results suggest that decreased phosphorylation of 5-HT2A receptors could mediate, at least in part, the cocaine-induced potentiation of multiple 5-HT2A receptor signaling pathways in rat PFCx. As discussed in this manuscript, we hypothesize that preventing these neuroadaptations in 5-HT2A receptor signaling may alleviate some of the aversive withdrawal-associated symptoms that contribute to relapse to cocaine abuse.

Monoamine transporter inhibitors and substrates as treatments for stimulant abuse

The acute and chronic effects of abused psychostimulants on monoamine transporters and associated neurobiology have encouraged development of candidate medications that target these transporters. Monoamine transporters, in general, and dopamine transporters, in particular, are critical molecular targets that mediate abuse-related effects of psychostimulants such as cocaine and amphetamine. Moreover, chronic administration of psychostimulants can cause enduring changes in neurobiology reflected in dysregulation of monoamine neurochemistry and behavior. The current review will evaluate evidence for the efficacy of monoamine transporter inhibitors and substrates to reduce abuse-related effects of stimulants in preclinical assays of stimulant self-administration, drug discrimination, and reinstatement. In considering deployment of monoamine transport inhibitors and substrates as agonist-type medications to treat stimulant abuse, the safety and abuse liability of the medications are an obvious concern, and this will also be addressed. Future directions in drug discovery should identify novel medications that retain efficacy to decrease stimulant use but possess lower abuse liability and evaluate the degree to which efficacious medications can attenuate or reverse neurobiological effects of chronic stimulant use.

Cannabinoid 2 receptor- and beta Arrestin 2-dependent upregulation of serotonin 2A receptors

Recent evidence suggests that cannabinoid receptor agonists may regulate serotonin 2A (5-HT(2A)) receptor neurotransmission in the brain, although no molecular mechanism has been identified. Here, we present experimental evidence that sustained treatment with a non-selective cannabinoid agonist (CP55,940) or selective CB2 receptor agonists (JWH133 or GP1a) upregulate 5-HT(2A) receptors in a neuronal cell line. Furthermore, this cannabinoid receptor agonist-induced upregulation of 5-HT(2A) receptors was prevented in cells stably transfected with either CB2 or β-Arrestin 2 shRNA lentiviral particles. Additionally, inhibition of clathrin-mediated endocytosis also prevented the cannabinoid receptor-induced upregulation of 5-HT(2A) receptors. Our results indicate that cannabinoid agonists might upregulate 5-HT(2A) receptors by a mechanism that requires CB2 receptors and β-Arrestin 2 in cells that express both CB2 and 5-HT(2A) receptors. 5-HT(2A) receptors have been associated with several physiological functions and neuropsychiatric disorders such as stress response, anxiety and depression, and schizophrenia. Therefore, these results might provide a molecular mechanism by which activation of cannabinoid receptors might be relevant to some cognitive and mood disorders in humans.

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.

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.

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.

Potential role of cortical 5-HT(2A) receptors in the anxiolytic action of cyamemazine in benzodiazepine withdrawal

The antipsychotic cyamemazine is a potent serotonin 5-HT(2A) receptor (5-HT(2AR)) antagonist. A positron emission tomography (PET) study in human patients showed that therapeutic doses of cyamemazine produced near saturation of 5-HT(2AR) occupancy in the frontal cortex, whereas dopamine D(2) occupancy remained below the level for motor side effects observed with typical antipsychotics. Recently, numerous studies have revealed the involvement of 5-HT(2AR) in the pathophysiology of anxiety and a double-blind, randomized clinical trial showed similar efficacy of cyamemazine and bromazepam in reducing the anxiety associated with benzodiazepine withdrawal. Therefore, we reviewed the above articles about 5-HT(2AR) and anxiety in order to understand better the anxiolytic mechanisms of cyamemazine in benzodiazepine withdrawal. The 5-HT(2AR) is the most abundant serotonin receptor subtype in the cortex. Non-pharmacological studies with antisense oligodeoxynucleotides and genetically modified mice clearly showed that cortical 5-HT(2AR) signaling positively modulates anxiety-like behavior. With a few exceptions, most other studies reviewed here further support this view. Therefore, the anxiolytic efficacy of cyamemazine in benzodiazepine withdrawal can be due to a 5-HT(2AR) antagonistic activity at the cortical level.

Involvement of tissue plasminogen activator in stress responsivity during acute cocaine withdrawal in mice

There is evidence that increased release of corticotropin-releasing factor (CRF) in the central nucleus of the amygdala (CeA) contributes to stress responsivity during cocaine withdrawal (WD). Recent studies suggest that tissue plasminogen activator (tPA) in the CeA is a downstream effector protein for CRF after acute "binge" cocaine administration. The purpose of this study was to determine if tPA modulates cocaine WD-induced stress responsivity. Wild-type (WT) and tPA-deficient (tPA - / - ) mice were subjected to chronic (14 days) "binge" cocaine (45 mg/kg per day) or its acute (1 day) WD. Extracellular tPA activity, CRF mRNA levels, and plasma corticosterone (CORT) levels were measured in tPA - / -  and WT mice. Extracellular tPA activity was reduced by 50% in the CeA and medial amygdala of WT mice after chronic cocaine and returned to basal levels after acute WD. Unlike WT mice, tPA - / -  mice did not display elevated amygdalar CRF mRNA levels during cocaine WD. In comparison to WT mice, tPA - / -  mice showed a blunted plasma CORT response during acute WD. These results demonstrate that tPA activity in the amygdala (Amy) is altered by chronic cocaine exposure, and further suggest an involvement of tPA in modulating amygdalar CRF stress responsive system and hypothalamic-pituitary-adrenal axis in response to acute cocaine WD.

Behavioral evidence for the significance of serotoninergic (5-HT) receptors in cocaine addiction

Cocaine addiction has somatic, psychological, psychiatric, socio-economic and legal implications in the developed world. Presently, there is no medication approved for the treatment of cocaine addiction. In recent years, data from the literature (pre-clinical studies and clinical trials) have provided several lines of evidence that serotonin (5-HT) and 5-HT receptors play a modulatory role in the mechanisms of action of cocaine. Here we review the contribution of 5-HT receptor subtypes to cocaine sensitization, discrimination, conditioned place preference, self-administration, reinstatement of seeking behavior and withdrawal symptoms in laboratory animals. Additionally, the consequences of chronic cocaine exposure on particular 5-HT receptor-assigned functions in pre-clinical studies are presented.

Interactions of serotonin (5-HT)2 receptor-targeting ligands and nicotine: locomotor activity studies in rats

Male Wistar rats were used to verify the hypothesis that serotonin (5-HT)(2A) or 5-HT(2C) receptors may control the locomotor effects evoked by nicotine (0.4 mg/kg). The 5-HT(2A) receptor antagonist (M100,907), the 5-HT(2A) receptor agonist (DOI), the 5-HT(2C) receptor antagonist (SB 242,084), and the 5-HT(2C) receptor agonists (Ro 60-0175 and WAY 163,909) were used. M100,907 (0.5-2mg/kg) did not alter, while DOI (1 mg/kg) enhanced the nicotine-induced hyperlocomotion. The effect of DOI was antagonized by M100,907 (1 mg/kg). SB 242,084 (0.25-1 mg/kg) augmented, while Ro 60-0175 (1 and 3 mg/kg) and WAY 163,909 (1.5 mg/kg) decreased the overall effect of acute nicotine; effects of Ro 60-0175 and WAY 163,909 were attenuated by SB 242,084 (0.125 mg/kg). In another set of experiments, M100,907 (2 mg/kg) on Day 10 attenuated, while DOI (0.1-1 mg/kg) enhanced the nicotine-evoked conditioned hyperlocomotion in rats repeatedly (Days 1-5) treated with nicotine in experimental chambers. SB 242,084 (0.125 or 1 mg/kg) did not change, while Ro 60-0175 (1 mg/kg) or WAY 163,909 (1.5 mg/kg) decreased the expression of nicotine-induced conditioned hyperactivity. Only DOI (0.3 and 1 mg/kg) and SB 242,084 (1 mg/kg) enhanced the basal locomotion. The present data indicate that 5-HT(2A) receptors are significant for the expression of nicotine-evoked conditioned hyperactivity. Conversely, 5-HT(2C) receptors play a pivotal role in the acute effects of nicotine. Pharmacological stimulation of 5-HT(2A) receptors enhances the conditioned hyperlocomotion, while activation of 5-HT(2C) receptors decreases both the response to acute nicotine and conditioned hyperactivity.

Blockade of the serotonin 5-HT2A receptor suppresses cue-evoked reinstatement of cocaine-seeking behavior in a rat self-administration model

The serotonin 5-HT2A receptor (5-HT-sub(2A)R) may play a role in reinstatement of drug-seeking. This study investigated the ability of a selective 5-HT-sub(2A)R antagonist to suppress reinstatement evoked by exposure to cues conditioned to cocaine self-administration. Cocaine self-administration (0.75 mg/kg/0.1 mL/6 s infusion; FR 4) was trained in naïve, free-fed rats to allow interpretation of results independent from changes related to food deprivation stress. Pretreatment with the selective 5-HT-sub(2A)R antagonist M100907 (volinanserin) failed to reduce rates of operant responding for cocaine infusions. On the other hand, M100907 (0.001-0.8 mg/kg ip) significantly suppressed the cue-induced reinstatement of cocaine-seeking behavior following extinction; effective M100907 doses did not alter operant responding for cues previously associated with sucrose self-administration. Importantly, a greater magnitude of active lever presses on the initial extinction session (high extinction responders) predicted the maximal susceptibility to M100907-induced suppression of cue-evoked reinstatement. The findings indicate that blockade of the 5-HT-sub(2A)R attenuates the incentive-motivational effects of cocaine-paired cues, particularly in high extinction responders, and suggests that M100907 may afford a therapeutic advance in suppression of cue-evoked craving and/or relapse.