Effects of 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) and quipazine on heroin self-administration in rhesus monkeys

Evaluation of potential punishing effects of 2,5-dimethoxy-4-methylamphetamine (DOM) in rhesus monkeys responding under a choice procedure

OBJECTIVES

There has been substantial and growing interest in the therapeutic utility of drugs acting at serotonin 2A subtype (5-HT2A) receptors, increasing the need for characterization of potential beneficial and adverse effects of such compounds. Although numerous studies have evaluated the possible rewarding and reinforcing effects of 5-HT2A receptor agonists, there have been relatively few studies on potential aversive effects.

METHODS

The current study investigated punishing effects of 2,5-dimethoxy-4-methylamphetamine (DOM) in four rhesus monkeys responding under a choice procedure in which responding on one lever delivered a sucrose pellet alone and responding on the other lever delivered a sucrose pellet plus an intravenous infusion of a range of doses of fentanyl (0.1-3.2 µg/kg/infusion), histamine (3.2-100 µg/kg/infusion), or DOM (3.2-100 µg/kg/infusion).

RESULTS

When fentanyl was available, responding for a pellet plus an infusion increased dose dependently in all subjects, indicating a positive reinforcing effect of fentanyl. When histamine was available, responding for a pellet plus an infusion decreased in three of four subjects, indicating a punishing effect of histamine. Whether available before or after histamine, DOM did not systematically alter choice across the range of doses tested.

CONCLUSION

These results suggest that the 5-HT2A receptor agonist DOM has neither positive reinforcing nor punishing effects under a choice procedure that is sensitive to both processes.

Effects of psychedelic, DOI, on nucleus accumbens dopamine signaling to predictable rewards and cues in rats

Psychedelics produce lasting therapeutic responses in neuropsychiatric diseases suggesting they may disrupt entrenched associations and catalyze learning. Here, we examine psychedelic 5-HT2A/2C agonist, DOI, effects on dopamine signaling in the nucleus accumbens (NAc) core, a region extensively linked to reward learning, motivation, and drug-seeking. We measure phasic dopamine transients following acute DOI administration in rats during well learned Pavlovian tasks in which sequential cues predict rewards. We find that DOI (0.0-1.2 mg/kg, i.p.) increases dopamine signals, photometrically measured using GRABDA optical sensor, to rewards and proximal reward cues, but not to the distal cues that predict these events. We determine that the elevated dopamine produced by DOI to reward cues occurs independently of DOI-induced changes in reward value. The increased dopamine associated with predictable reward cues and rewards supports DOI-induced increases in prediction error signaling. These findings lay a foundation for developing psychedelic strategies aimed at engaging error-driven learning mechanisms to disrupt entrenched associations or produce new associations.

Psilocybin reduces heroin seeking behavior and modulates inflammatory gene expression in the nucleus accumbens and prefrontal cortex of male rats

Preclinical and human studies indicate psilocybin may reduce perseverant maladaptive behaviors, including nicotine and alcohol seeking. Such studies in the opioid field are lacking, though opioids are involved in more >50% of overdose deaths. Psilocybin is an agonist at the serotonin 2A receptor (5-HT2AR), a well-documented target for modulation of drug seeking, and evidence suggests 5-HT2AR agonists may dampen motivation for opioids. We sought to investigate the therapeutic efficacy of psilocybin in mediating cessation of opioid use and maintenance of long-lasting abstinence from opioid seeking behavior in a rat model of heroin self-administration (SA). Psilocybin or 5-HT2AR antagonists ketanserin and volinanserin were administered systemically to rats prior to SA of 0.075 mg/kg/infusion of heroin, or relapse following forced abstinence. Psilocybin did not alter heroin taking, but a single exposure to 3.0 mg/kg psilocybin 4-24 hours prior to a relapse test blunted cue-induced heroin seeking. Conversely, 5-HT2AR antagonists exacerbated heroin relapse. To begin to elucidate mechanisms of psilocybin, drug-naïve rats received psilocybin and/or ketanserin, and tissue was collected from the prefrontal cortex (PFC), a region critical for drug seeking and responsive to psilocybin, 24 hours later for RNA-sequencing. 3.0 mg/kg psilocybin regulated ~2-fold more genes in the PFC than 1.0 mg/kg, including genes involved in the cytoskeleton and cytokine signaling. Ketanserin blocked >90% of psilocybin-regulated genes, including the IL-17a cytokine receptor, Il17ra. Psychedelic compounds have reported anti-inflammatory properties, and therefore we performed a gene expression array to measure chemokine/cytokine molecules in the PFC of animals that displayed psilocybin-mediated inhibition of heroin seeking. Psilocybin regulated 4 genes, including Il17a, and a subset of genes correlated with relapse behavior. Selective inhibition of PFC IL-17a was sufficient to reduce heroin relapse. We conclude that psilocybin reduces heroin relapse and highlight IL-17a signaling as a potential downstream pathway of psilocybin that also reduces heroin seeking.

μ-opioid receptor agonists and psychedelics: pharmacological opportunities and challenges

Opioid misuse and opioid-involved overdose deaths are a massive public health problem involving the intertwined misuse of prescription opioids for pain management with the emergence of extremely potent fentanyl derivatives, sold as standalone products or adulterants in counterfeit prescription opioids or heroin. The incidence of repeated opioid overdose events indicates a problematic use pattern consistent with the development of the medical condition of opioid use disorder (OUD). Prescription and illicit opioids reduce pain perception by activating µ-opioid receptors (MOR) localized to the central nervous system (CNS). Dysregulation of meso-corticolimbic circuitry that subserves reward and adaptive behaviors is fundamentally involved in the progressive behavioral changes that promote and are consequent to OUD. Although opioid-induced analgesia and the rewarding effects of abused opioids are primarily mediated through MOR activation, serotonin (5-HT) is an important contributor to the pharmacology of opioid abused drugs (including heroin and prescription opioids) and OUD. There is a recent resurgence of interest into psychedelic compounds that act primarily through the 5-HT2A receptor (5-HT 2A R) as a new frontier in combatting such diseases (e.g., depression, anxiety, and substance use disorders). Emerging data suggest that the MOR and 5-HT2AR crosstalk at the cellular level and within key nodes of OUD circuitry, highlighting a major opportunity for novel pharmacological intervention for OUD. There is an important gap in the preclinical profiling of psychedelic 5-HT2AR agonists in OUD models. Further, as these molecules carry risks, additional analyses of the profiles of non-hallucinogenic 5-HT2AR agonists and/or 5-HT2AR positive allosteric modulators may provide a new pathway for 5-HT2AR therapeutics. In this review, we discuss the opportunities and challenges associated with utilizing 5-HT2AR agonists as therapeutics for OUD.

Tabernanthalog Reduces Motivation for Heroin and Alcohol in a Polydrug Use Model

Background

The potential use of psychedelic drugs as therapeutics for neuropsychiatric disorders has been limited by their hallucinogenic properties. To overcome this limitation, we developed and characterized tabernanthalog (TBG), a novel analogue of the indole alkaloids ibogaine and 5-methoxy-N,N-dimethyltryptamine with reduced cardiac arrhythmogenic risk and a lack of classical psychedelic drugs-induced sensory alterations. We previously demonstrated that TBG has therapeutic efficacy in a preclinical model of opioid use disorder (OUD) in rats and in a binge model of alcohol drinking in mice. Alcohol is commonly co-used in ∼35-50% of individuals with OUD, and yet, preclinical models that recapitulate this comorbidity are lacking.

Methodology

Here we employed a polydrug model of heroin and alcohol couse to screen the therapeutic efficacy of TBG on metrics of both opioid and alcohol seeking. We first exposed rats to alcohol (or control sucrose-fade solution) in the home-cage (HC), using a two-bottle binge protocol, over a period of 1 month. Rats were then split into two groups that underwent self-administration training for either intravenous heroin or oral alcohol, so that we could assess the impact of HC alcohol exposure on the self-administration of each substance separately. Thereafter, rats began self-administering both heroin and alcohol in the same sessions. Finally, we tested the effects of TBG on break points for heroin and alcohol in a progressive ratio test, where the number of lever presses required to obtain a single reward increased exponentially.

Results and Conclusion

TBG effectively reduced motivation for heroin and alcohol in this test, indicating its efficacy is preserved in animals with a history of heroin and alcohol polydrug use.

Effects of 2,5-Dimethoxy-4-Methylamphetamine (DOM) and 2-Piperazin-1-yl-Quinoline (Quipazine) on Fentanyl Versus Food Choice in Rhesus Monkeys

There has been increasing interest in the potential therapeutic effects of drugs with agonist properties at serotonin 2A subtype (5-HT2A) receptors (e.g., psychedelics), including treatment of substance use disorders. Studying interactions between 5-HT2A receptor agonists and other drugs is important for understanding potential therapeutic effects as well as adverse interactions. Direct-acting 5-HT2A receptor agonists such as 2,5-dimethoxy-4-methylamphetamine (DOM) and 2-piperazin-1-yl-quinoline (quipazine) enhance some (e.g., antinociceptive) effects of opioids; however, it is unclear whether they alter the abuse-related effects of opioids. This study examined whether DOM and quipazine alter the reinforcing effects of fentanyl in rhesus monkeys (n = 6) responding under a food versus drug choice procedure. Responding on one lever delivered sucrose pellets and responding on the other lever delivered intravenous (i.v.) infusions. In one set of experiments, fentanyl (0.1-3.2 µg/kg/infusion) versus food choice sessions were preceded by noncontingent i.v. pretreatments with DOM (0032-0.32 mg/kg), quipazine (0.32-1.0 mg/kg), naltrexone (0.032 mg/kg), or heroin (0.1 mg/kg). In another set of experiments, fentanyl was available during choice sessions in combination with DOM (0.32-100 µg/kg/infusion) or quipazine (3.2-320 µg/kg/infusion) in varying dose ratios. Naltrexone decreased and heroin increased fentanyl choice, demonstrating sensitivity of responding to pharmacological manipulation. However, whether given as a pretreatment or made available in combination with fentanyl as a mixture, neither DOM nor quipazine significantly altered fentanyl choice. These results suggest that 5-HT2A receptor agonists do not enhance the reinforcing effects of opioids and, thus, will not likely enhance abuse potential. SIGNIFICANCE STATEMENT: Serotonin 2A subtype receptor agonists enhance some (e.g., antinociceptive) effects of opioids, suggesting they could be combined with opioids in some therapeutic contexts such as treating pain. However, it is unclear whether they also enhance adverse effects of opioids, including abuse. Results of this study indicate that serotonin 2A subtype receptor agonists do not reliably enhance opioid self-administration and, thus, are unlikely to enhance the abuse potential of opioids.

Effects of 5-HT2A receptor stimulation on economic demand for fentanyl after intermittent and continuous access self-administration in male rats

The relative value of and motivation for abused drugs often increases with drug experience and differs based on drug availability. Here, we determined how different intake patterns of fentanyl, a μ-opioid agonist, alter economic demand for fentanyl and how 5-HT_2A receptor stimulation affects economic demand for fentanyl. We used a within-session demand threshold procedure to characterize changes in economic demand for fentanyl before and after intermittent or continuous access schedules. We subsequently tested the acute effects of 5-HT_2A receptor stimulation with psychedelic 2,5-dimethoxy-4-iodoamphetamine (DOI) on economic demand for fentanyl. Extended fentanyl experience with both intermittent and continuous schedules increased fentanyl consumption at low cost (Q₀ ), increased total fentanyl consumption, and decreased demand elasticity (α), indicating both schedules elevated motivation to self-administer fentanyl. Overall, the two schedules produced similar alterations in economic demand for fentanyl, although low-cost consumption (Q₀ ) increased more in the continuous access group. Systemic injections of DOI (0.0-0.4 mg/kg, i.p.) dose-dependently produced economic demand changes in the opposite direction produced by fentanyl experience. DOI decreased motivation (increased "α"), decreased Q₀ , and decreased total fentanyl consumption. The selective 5-HT_2A antagonist, M100907 (0.3 mg/kg, i.p.), blocked the effects of DOI, indicating that DOI is acting through 5-HT_2A receptors to alter economic demand for fentanyl. In an economic food demand experiment, DOI (0.4 mg/kg) also increased demand elasticity and reduced food consumption. These results demonstrate that both intermittent and continuous fentanyl experience raise the economic demand for fentanyl, and acute 5-HT_2A receptor activation reduces economic demand for fentanyl and food.

Lorcaserin decreases the reinforcing effects of heroin, but not food, in rhesus monkeys

Preclinical studies indicate that lorcaserin (Belviq®), an FDA-approved serotonin 2 C receptor agonist, may be a promising medication for the treatment of stimulant addiction. However, few studies have investigated its effects on self-administration of drugs of abuse from other pharmacological classes, including opioids. Here, adult male rhesus macaques (N = 3) responded under a fixed ratio 30 schedule of food (1-g banana-flavored pellets) and IV heroin delivery. Following stable self-administration of a range of heroin doses (0.32-32 μg/kg/inj, IV), the effects of acute pretreatment with lorcaserin (0.32-1.0 mg/kg, IM) on heroin- and food-maintained responding was determined. The ability of repeated treatment with lorcaserin (1.0 mg/kg) to produce sustained decreases in heroin and/or food intake and reinforcing strength were then analyzed using behavioral economic demand procedures. Results show that self-administration of intravenous heroin was dose-dependent, with peak responding maintained, on average, by the unit dose of 3.2 μg/kg/inj. Lorcaserin pretreatment produced a dose-dependent flattening of the dose-response function for heroin self-administration in each subject. On the other hand, lorcaserin did not decrease food-maintained responding. Repeated lorcaserin treatment reduced heroin intake by selectively decreasing its reinforcing strength, as evidenced by a leftward shift in the demand curves for heroin and the absence of comparable changes in the reinforcing strength of food. These results indicate that serotonin 2 C receptor agonists, such as lorcaserin, have behaviorally selective effects on IV self-administration behavior, and deserve further consideration as candidates for the management of opioid use disorder.

Effects of daily delta-9-tetrahydrocannabinol treatment on heroin self-administration in rhesus monkeys

Opioid abuse remains a significant public health problem; together with the greater availability of marijuana in some regions there is an increasing likelihood that opioids and marijuana will be used together. Polydrug abuse is associated with increased toxicity and poorer treatment outcome; thus, a better understanding of the consequences of repeated coadministration of these drugs will facilitate the development of better prevention and treatment strategies. This study examined the effects of daily treatment with the cannabinoid receptor agonist delta-9-tetrahydrocannabinol (Δ-THC) and its discontinuation on self-administration of heroin in rhesus monkeys (n=4) lever-pressing under a fixed-ratio 30 schedule. Heroin self-administration (0.32-32 μg/kg/infusion, intravenously) generated an inverted U-shaped dose-effect curve. Administered acutely, Δ-THC (0.01-0.32 mg/kg, subcutaneously) dose dependently decreased responding for heroin and flattened the self-administration dose-effect curve. Daily treatment with Δ-THC (0.01-0.1 mg/kg/12 h, subcutaneously) either had no effect on or decreased responding for heroin. In addition, daily treatment did not significantly impact extinction of heroin self-administration or resumption of responding for heroin after extinction. Discontinuation of daily Δ-THC treatment did not systematically impact rates of heroin self-administration. These data suggest that repeated administration of a cannabinoid receptor agonist likely does not increase, and possibly decreases, the positive reinforcing effects of a mu opioid receptor agonist.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

Interactions between Δ(9)-tetrahydrocannabinol and heroin: self-administration in rhesus monkeys

The cannabinoid receptor agonist Δ(9)-tetrahydrocannabinol (THC) enhances the antinociceptive effects of µ-opioid receptor agonists, raising the possibility of using a combination of THC and opioids for treating pain. This study examined the effects of noncontingent and contingent administration of THC on intravenous heroin self-administration in rhesus monkeys. Self-administration of different unit doses of heroin (0.0001-0.1 mg/kg/infusion) generated a typical inverted U-shaped dose-response curve. In one experiment (n=4), noncontingent THC (0.1-1.0 mg/kg) dose dependently shifted the heroin dose-response curve downward in three monkeys and slightly leftward in one monkey. In a second experiment (n=4), monkeys could self-administer THC alone (0.0032-0.032 mg/kg/infusion), heroin alone, or a mixture of THC and heroin. THC alone did not maintain responding above that obtained with saline; however, increasing the THC dose with heroin dose dependently decreased the number of infusions received and the rate of responding, as compared with data that were obtained with heroin alone. These results indicate that THC does not significantly enhance the positive reinforcing effects of heroin, further supporting the view that combining cannabinoid and opioid receptor agonists (e.g. for treating pain) does not increase, and might decrease, the abuse liability of individual drugs.