Monoamine oxidase A inhibition by toxic concentrations of metaxalone.

Context: Serotonin toxicity is a reported complication associated with both therapeutic use and overdose of metaxalone while on therapeutic doses of serotonergic drugs such as serotonin reuptake inhibitors. Monoamine oxidase A (MAO-A) inhibition by metaxalone has been proposed as the etiology of this toxicity. Metaxalone concentrations reported with cases of serotonin toxicity range from 31 to 61 mcg/ml (140-276 µM). We investigated the effect of metaxalone on MAO-A activity using an in vitro model.Methods: Metaxalone at concentrations ranging from 1.56 to 400 µM were incubated with a proprietary MAO substrate and recombinant human MAO-A for 1 h. After that, an esterase and luciferase were added and luminescence measured. Clorgyline, a known MAO-A inhibitor, was used as a positive control. Luminescence was measured using a Biotek Synergy HT microplate reader.Results: Metaxalone demonstrated significant dose-related inhibition of MAO-A activity. Four-parameter logistic regression analysis demonstrated a strong dose-response relationship at increasing concentrations.Conclusions: Our in vitro model shows that at toxic concentrations similar to those reported in case reports metaxalone shows significant MAO-A inhibition. Clinicians should be aware of this mechanism and understand the potentially lethal interactions metaxalone can have when prescribed with other serotonergic drugs and consider this as a potential cause of serotonin toxicity, especially in overdose scenarios.

Inhibitor structure-guided design and synthesis of near-infrared fluorescent probes for monoamine oxidase A (MAO-A) and its application in living cells and in vivo.

A series of near-infrared fluorescent probes based on inhibitor (clorgyline) structure-guided design were synthesized for the specific detection of MAO-A in cells and in vivo. Moreover, we have successfully used the high specificity of one of the probes to visualize MAO-A activity in zebrafish and tumor tissue for the first time.

Risperidone attenuates the increase of extracellular nitric oxide and glutamate levels in serotonin syndrome animal models.

Serotonin (5-hydroxytryptamine; 5-HT) syndrome is a potentially life-threatening neurotoxic condition provoked by pharmacologically induced excess serotonergic activity. Several studies report that nitric oxide (NO) and glutamate play a role in psychostimulant-induced hyperthermia related to neurotoxicity. In the present study, the involvement of NO and glutamate, as well as the effect of risperidone, a potent 5-HT(2A) and D(2) (and a less potent D(1)) receptor antagonist, were investigated in animal models of 5-HT syndrome. Two 5-HT syndrome animal models were utilized. The first model was induced by administration of tranylcypromine, a nonselective monoamine oxidase (MAO) inhibitor, and fluoxetine, a selective 5-HT reuptake inhibitor. The second model was induced by the administration of clorgyline, an MAO-A inhibitor, and 5-hydroxy-l-tryptophan, a precursor of 5-HT. Changes in the level of NO metabolites and glutamate in the anterior hypothalamus were measured using microdialysis. In both models, NO metabolite levels significantly increased, and this increase was significantly attenuated by risperidone pretreatment. Extracellular levels of glutamate were increased only in the tranylcypromine and fluoxetine model, and this increase was significantly attenuated by risperidone pretreatment. These results indicate that NO and glutamate may be involved in the development of 5-HT syndrome and that risperidone may be effective against neurotransmitter abnormalities in 5-HT syndrome.

Assessment of 5-hydroxytryptamine efflux in rat brain during a mild, moderate and severe serotonin-toxicity syndrome.

Serotonin (5-hydroxytryptamine; 5-HT)-toxicity syndrome, an iatrogenic brain disorder induced by excessive efflux of 5-HT, has received much attention because of increasing incidents of serotonergic antidepressants. However, the neural mechanism by which extracellular 5-HT is elevated to a toxic level for the syndrome remains to be determined. The goal of the present study was to test the hypothesis that extracellular 5-HT is composed of two component effluxes responsible for distinct aspects of the syndrome. The first set of experiments was to characterize the syndrome by measuring changes in neuromuscular signs, body-core temperature and mortality rate. Our results indicate that the syndrome severity can be categorized into mild, moderate and severe levels. The second set of experiments was to determine a threshold of extracellular 5-HT for induction of each level of the syndrome. Our results demonstrate that there were an 11-fold increase in the mild syndrome and an over 55-fold increase in the severe syndrome. In the last series of experiments, the excessive increases in 5-HT were pharmacologically separated into primary and secondary component effluxes with the 5-HT2A receptor antagonists cyproheptadine and ketanserin and NMDA receptor antagonist (+)-MK-801. Our results suggest that the primary component efflux was caused by direct drug effects on 5-HT biosynthetic and metabolic pathways and secondary efflux ascribed to indirect drug effect on a positive-feedback circuit involving 5-HT2A and NMDA receptors. In summary, the primary efflux could be an initial cause for the induction of the syndrome while the secondary efflux might involve deterioration of the syndrome.

Effects of co-administration of a selective serotonin reuptake inhibitor and monoamine oxidase inhibitors on 5-HT-related behavior in rats.

5-hydroxytryptamine (5-HT) syndrome is a dangerous condition of 5-HT excess that can occur in the case of co-administration of a monoamine oxidase (MAO) inhibitor and a serotonin reuptake inhibitor (SSRI). The goal of the present study was to investigate the effects of acute administration of MAO inhibitors and subchronic administration of fluvoxamine on 5-HT-related behaviors (head shaking and 5-HT syndrome) in rats treated with 5-hydroxytryptophan (5-HTP). Administration of the non-selective MAO inhibitor, pargyline, and the selective MAO-A inhibitor, clorgyline, resulted in 5-HT syndrome in 5-HTP-treated rats, and subchronic co-administration of fluvoxamine intensified the syndrome. However, administration of the selective MAO-B inhibitor, selegiline, did not induce 5-HT syndrome with or without subchronic fluvoxamine co-administration. These data suggest that non-selective MAO and selective MAO-A inhibitors can induce 5-HT syndrome in humans when co-administered with SSRI. Further, the risk of 5-HT syndrome may be lower with the selective MAO-B inhibitor, selegiline.

Diazepam and chlormethiazole attenuate the development of hyperthermia in an animal model of the serotonin syndrome.

The serotonin (5-HT) syndrome is the most serious toxic interaction of antidepressants, but no pharmacotherapy has yet been established. In the present study, we created an animal model of the 5-HT syndrome by intraperitoneally injecting rats with clorgyline (2 mg/kg) and 5-hydroxy-L-tryptophan (5-HTP) (100 mg/kg) and evaluated the effectiveness of potent 5-HT(2A) receptor antagonists and GABA-enhancing drugs, including diazepam and chlormethiazole. The rectal temperature of the rats was measured, and the noradrenaline (NA) and 5-HT levels in the anterior hypothalamus were measured by microdialysis. In the group pre-treated with saline, the rectal temperature increased to more than 40 degrees C, and all of the animals died within 90 min after administration. Pre-treatment with potent 5-HT(2A) receptor antagonists prevented the development of hyperthermia and death in the rats. Pre-treatment with diazepam, 10 and 20mg/kg, and chlormethiazole, 50 and 100mg/kg, attenuated the development of hyperthermia. Although neither of these drugs completely prevented the rats from dying, they prolonged their survival time. Regardless of the type of therapeutic agents, the concentration of 5-HT increased to about 1100-fold the pre-administration level. The NA levels in the saline group increased to about 16-fold the pre-administration levels, but the increase was significantly lower in the rats that survived as a result of drug therapy. These results suggest that GABA-mimetic drugs may be effective against the 5-HT syndrome, although they have a somewhat weaker effect than the potent 5-HT(2A) receptor blockers, and that not only is 5-HT activity increased in the brain in the 5-HT syndrome, but the NA system is also enhanced.

The MAO-B inhibitor deprenyl, but not the MAO-A inhibitor clorgyline, potentiates the neurotoxicity of p-chloroamphetamine.

The effect of co-administration of MAO inhibitors together with a low dose of the neurotoxic amphetamine p-chloroamphetamine (pCA) on neurotoxicity was examined. Neurotoxicity was assessed by measuring decreases in the binding of [3H]cyanoimipramine to serotonin uptake sites using quantitative autoradiography. By itself, a low dose of pCA (2 mg/kg) did not produce any alterations in radioligand binding, measured 7 days after drug administration. However, co-administration of the MAO-B selective inhibitor deprenyl (1 mg/kg) or the non-selective inhibitor pargyline (50 mg/kg) produced significant decreases in radioligand binding. Measurements of the effects of these drugs on body temperature ruled out the possibility that deprenyl and pargyline were increasing neurotoxicity by producing a drug-induced hyperthermia. In contrast to the effects of deprenyl and pargyline, co-administration of the MAO-A selective inhibitor clorgyline (1 mg/kg) did not alter binding. By themselves none of the MAO inhibitors produced neurotoxic effects. There are a number of possible explanations for these results. Administration of deprenyl or pargyline, together with pCA, itself a MAO-A inhibitor, will lead to inhibition of both MAO-A and MAO-B activities. This will likely lead to an enhanced release of dopamine and serotonin compared with the release following administration of pCA alone or pCA together with clorgyline. Elevation of the extracellular levels of either or both of these monoamines could lead to enhanced neurotoxicity. Whatever the mechanism involved, our results show that the co-administration of a type-B MAOI enhances the neurotoxic effects of pCA on serotonin neurons.

Monoamine oxidase B inhibitor enhances experimental carcinogenesis in rat colon induced by azoxymethane.

The effects of prolonged administration of the monoamine oxidase (MAO)-A inhibitor N-methyl-N-propargyl-3-(2,4-dichlorophenoxy)propylamine (clorgyline) and the MAO-B inhibitor N-methyl-N-2-propynyl-benzylamine (pargyline) on the incidence, number and histology of colon tumors induced by azoxymethane (AOM), and on the norepinephrine (NE) concentration in the colon wall and the labeling index of colon mucosa were investigated in Wistar rats. Rats were treated s.c. with 7.4 mg/kg body weight of AOM once a week for 10 weeks, and also s.c. with 5 mg/kg body weight of clorgyline or 50 mg/kg body weight of pargyline in 0.9% NaCl until the end of the experiment. Treatment with pargyline significantly increased the incidence of colon tumors in week 35. However, it did not influence the histological appearance of the colon tumors or the histological types and depth of involvement of colon adenocarcinomas. Furthermore, it significantly increased the NE concentration in the colon wall and the labeling index of the colon mucosa during and after AOM-treatment. In contrast, clorgyline had no influence on the development or histological appearance of colon tumors. These findings indicate that the MAO-B inhibitor, but not the MAO-A inhibitor, enhanced colon carcinogenesis, and that its effect may be related to its effect in increasing the NE concentration in the colon wall and subsequently increasing the proliferation of colon epithelial cells.

Potentiation of tyramine pressor responses in conscious rats by reversible inhibitors of monoamine oxidase.

Intraperitoneal injection of cimoxatone, moclobemide, amiflamine CGP 11305 A (all 5 mg/kg) increased pressor responses to intravenous tyramine (100 micrograms) in conscious, freely moving rats. Area under mean arterial pressure curve increased by 7.01, 4.28, 5.3 and 3.46 fold respectively. Clorgyline (2 mg/kg) increased area under pressor response curve by 10.4 fold. Tyramine responses returned to normal 24 hours after reversible inhibitors but were still potentiated 24 hours after clorgyline.