Ondansetron, a selective 5-HT3 antagonist, antagonizes methamphetamine-induced anorexia in mice

Pharmacological Treatments for Methamphetamine Use Disorder: Current Status and Future Targets

The illicit use of the psychostimulant methamphetamine (METH) is a major concern, with overdose deaths increasing substantially since the mid-2010s. One challenge to treating METH use disorder (MUD), as with other psychostimulant use disorders, is that there are no available pharmacotherapies that can reduce cravings and help individuals achieve abstinence. The purpose of the current review is to discuss the molecular targets that have been tested in assays measuring the physiological, the cognitive, and the reinforcing effects of METH in both animals and humans. Several drugs show promise as potential pharmacotherapies for MUD when tested in animals, but fail to produce long-term changes in METH use in dependent individuals (eg, modafinil, antipsychotic medications, baclofen). However, these drugs, plus medications like atomoxetine and varenicline, may be better served as treatments to ameliorate the psychotomimetic effects of METH or to reverse METH-induced cognitive deficits. Preclinical studies show that vesicular monoamine transporter 2 inhibitors, metabotropic glutamate receptor ligands, and trace amine-associated receptor agonists are efficacious in attenuating the reinforcing effects of METH; however, clinical studies are needed to determine if these drugs effectively treat MUD. In addition to screening these compounds in individuals with MUD, potential future directions include increased emphasis on sex differences in preclinical studies and utilization of pharmacogenetic approaches to determine if genetic variances are predictive of treatment outcomes. These future directions can help lead to better interventions for treating MUD.

Chronic methamphetamine exposure exerts few effects on the iTat mouse model of HIV, but blocks Tat expression-induced slowed reward retrieval

Human immunodeficiency virus (HIV) continues to infect millions worldwide, negatively impacting neurobehavioral function. Further understanding of the combined effects of HIV and methamphetamine use is crucial, as methamphetamine use is prevalent in people with HIV. The HIV-associated protein Tat may contribute to cognitive dysfunction, modeled preclinically in mice using doxycycline (DOX)-inducible Tat expression (iTat). Tat may exert its effects on cognitive function via disruption of the dopamine transporter, similar to the action of methamphetamine. Additionally, Tat and methamphetamine both decrease interneuron populations, including those expressing calbindin. It is important to understand the combined effects of Tat and methamphetamine in preclinical models of HIV infection. Here, we used iTat transgenic mice and a chronic binge regimen of methamphetamine exposure to determine their combined impact on reward learning and motivation. We also measured calbindin expression in behavior-relevant brain regions. Before induction with DOX, iTat mice exhibited no differences in behavior. Chronic methamphetamine exposure before Tat induction impaired initial reward learning but did not affect motivation. Furthermore, DOX-induced Tat expression did not alter behavior, but slowed latencies to retrieve rewards. This effect of Tat, however, was not observed in methamphetamine-treated mice, indicative of a potential protective effect. Finally, Tat expression was associated with an increase in calbindin-expressing cells in the VTA, while methamphetamine exposure did not alter calbindin numbers. These findings may indicate a protective role of methamphetamine in HIV neuropathology, which in turn may help in our understanding of why people with HIV use methamphetamine at disproportionately higher rates.

Effects of chronic methamphetamine on psychomotor and cognitive functions and dopamine signaling in the brain

Methamphetamine (MA) studies in animals usually involve acute, binge, or short-term exposure to the drug. However, addicts take substantial amounts of MA for extended periods of time. Here we wished to study the effects of MA exposure on brain and behavior, using an animal model analogous to this pattern of MA intake. MA doses, 4 and 8mg/kg/day, were based on previously reported average daily freely available MA self-administration levels. We examined the effects of 16 week MA treatment on psychomotor and cognitive function in the rat using open field and novel object recognition tests and we studied the adaptations of the dopaminergic system, using in vitro and in vivo receptor imaging. We show that chronic MA treatment, at doses that correspond to the average daily freely available self-administration levels in the rat, disorganizes open field activity, impairs alert exploratory behavior and anxiety-like state, and downregulates dopamine transporter in the striatum. Under these treatment conditions, dopamine terminal functional integrity in the nucleus accumbens is also affected. In addition, lower dopamine D1 receptor binding density, and, to a smaller degree, lower dopamine D2 receptor binding density were observed. Potential mechanisms related to these alterations are discussed.

Peripheral and spinal 5-HT receptors participate in cholestatic itch and antinociception induced by bile duct ligation in rats

Although 5-HT has been implicated in cholestatic itch and antinociception, two common phenomena in patients with cholestatic disease, the roles of 5-HT receptor subtypes are unclear. Herein, we investigated the roles of 5-HT receptors in itch and antinociception associated with cholestasis, which was induced by common bile duct ligation (BDL) in rats. 5-HT-induced enhanced scratching and antinociception to mechanical and heat stimuli were demonstrated in BDL rats. 5-HT level in the skin and spinal cord was significantly increased in BDL rats. Quantitative RT-PCR analysis showed 5-HT_1B, 5-HT_1D, 5-HT_2A, 5-HT_3A, 5-HT_5B, 5-HT₆, and 5-HT₇ were up-regulated in peripheral nervous system and 5-HT_1A, 5-HT_1F, 5-HT_2B, and 5-HT_3A were down-regulated in the spinal cord of BDL rats. Intradermal 5-HT₂, 5-HT₃, and 5-HT₇ receptor agonists induced scratching in BDL rats, whereas 5-HT₃ agonist did not induce scratching in sham rats. 5-HT_1A, 5-HT₂, 5-HT₃, and 5-HT₇ agonists or antagonists suppressed itch in BDL rats. 5-HT_1A agonist attenuated, but 5-HT_1A antagonist enhanced antinociception in BDL rats. 5-HT₂ and 5-HT₃ agonists or antagonists attenuated antinociception in BDL rats. Our data suggested peripheral and central 5-HT system dynamically participated in itch and antinociception under cholestasis condition and targeting 5-HT receptors may be an effective treatment for cholestatic itch.

Association Study of Serotonin 3 Receptor Subunit Gene Variants in Antipsychotic-Induced Weight Gain

BACKGROUND

Schizophrenia (SCZ) is a chronic severe neuropsychiatric disorder, where pharmacological treatment has been hindered by adverse effects, including antipsychotic-induced weight gain (AIWG) and related complications. Genetic studies have been exploring the appetite regulation and energy homeostasis pathways in AIWG with some promising leads. The serotonin system has been shown to participate in these pathways.

METHODS

In the current study, we examined single nucleotide polymorphisms across the serotonin receptor genes HTR3A and HTR3B. Prospective weight change was assessed for a total of 149 SCZ patients of European ancestry.

RESULTS

We did not find the tested HTR3A or HTR3B gene markers to be associated with AIWG in our sample.

CONCLUSION

Our preliminary findings suggest that these receptors may not play a major role in predicting AIWG.

Functional variants of the serotonin receptor type 3A and B gene are associated with eating disorders

OBJECTIVE

As a key player in modulating both human physiological and behavioural functions including anxiety, perception and in particular appetite, serotonin (5-hydroxytryptamine, 5-HT) is likely to be involved in the aetiology of eating disorders. Studies showing serotonin receptor type 3 (5-HT3) receptors to mediate food intake depression (anorexic response) have triggered our interest in investigating the putative role of variants in the 5-HT3 receptor genes, HTR3A and HTR3B, in the susceptibility to anorexia nervosa (AN) and bulimia nervosa (BN).

METHODS

Two hundred and sixty-five patients with AN and 91 patients with BN as well as 191 healthy controls served as a pilot study group for mutational analysis by direct sequencing. Variants showing a significant association were subsequently genotyped in an independent Spanish cohort of 78 patients with AN and 119 patients with BN as well as 331 healthy controls for replication purposes.

RESULTS

In the pilot study, we found the coding HTR3B variant, p.Y129S, (rs1176744, P = 0.004, odds ratio = 2.06) to be associated with the restrictive subtype of AN. The association was confirmed in the Spanish study group (P = 0.034, odds ratio = 2.26).

CONCLUSION

Our study provides first evidence for an involvement of 5-HT3 variants in the aetiopathology of eating disorders in humans.

Changes in leptin, ghrelin, growth hormone and neuropeptide-Y after an acute model of MDMA and methamphetamine exposure in rats

Club drug abuse is a growing problem in the United States. Beyond addiction and toxicity are endocrine effects which are not well characterized. Specifically, the changes in appetite following exposure to drugs of abuse are an interesting but poorly understood phenomenon. Serum hormones such as leptin, ghrelin, growth hormone (GH), and neuropeptide-Y (NP-Y) are known to affect appetite, but have not been studied extensively with drugs of abuse. In this work, we examine the effects of club drugs 3,4-methylenedioxymethamphetamine (MDMA) (ecstasy) and methamphetamine (METH) (doses of 5, 20 and 40 mg/kg) on serum concentrations of these hormones in adult male Sprague-Dawley rats 6, 12, 24 and 48 hours after drug administration. In a dose-dependent manner, MDMA was shown to cause transient significant decreases in serum leptin and GH followed by a base line recovery after 24 hours. Conversely, serum ghrelin increased and normalized after 24 hours. Interestingly, serum NP-Y showed a steady decrease in both treatment of MDMA and METH at different time points and dosages. In humans, abuse of these drugs reduces eating. As evident from these data, acute administration of METH and MDMA had significant effects on different serum hormone levels involved in appetite regulation. Future studies should be performed to see how chronic, low dose drug administration would affect hormone levels and try to answer questions about the physiological mechanisms involved in the anorexic paradigm observed in drug use.

The neuronal 5-HT3 receptor network after 20 years of research — Evolving concepts in management of pain and inflammation

The 5-HT3 receptor is a pentameric ligand-gated cation channel which is found in the central and peripheral nervous system and on extraneuronal locations like lymphocytes, monocytes and fetal tissue. Five monomer subtypes, the 5-HT(3A-E) subunits, have been identified which show differences in the amino-terminal and the transmembrane region. The functional relevance of different receptor compositions is not yet clarified. 5-HT3 receptors are located predominantly in CNS regions that are involved in the integration of the vomiting reflex, pain processing, the reward system and anxiety control. The preferential localization on nerve endings is consistent with a physiological role of 5-HT3 receptors in the control of neurotransmitter release such as dopamine, cholecystokinin, glutamate, acetylcholine, GABA, substance P, or serotonin itself. 5-HT3-receptor agonists cause unpleasant effects like nausea and anxiety, and no clinical use has been considered. In contrast, the introduction of 5-HT3-receptor antagonists for chemotherapy-induced vomiting was extremely successful. After development of other gastrointestinal indications like postoperative vomiting and diarrhea-predominant irritable bowel syndrome recent research focuses on rheumatological indications such as fibromyalgia, rheumatoid arthritis and tendinopathies. Positive effects have also been observed for pain syndromes such as chronic neuropathic pain and migraine. These effects seem to be related to substance P-mediated inflammation and hyperalgesia. Furthermore, antiinflammatory and immunomodulatory properties have been observed for 5-HT3-receptor antagonists which might explain promising findings in systemic sclerosis and other immunological conditions. For all of these innovative indications the optimal dosing schedule is a crucial issue, since a bell-shaped dose-response curve has been observed repeatedly for 5-HT3-receptor antagonists, particularly in CNS effects.