Interactions of D-amphetamine with the active site of monoamine oxidase-A

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, “general chemicals,” natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10–15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Effects of amphetamine on subcellular distribution of dopamine and DOPAC

Amphetamine effects on distribution of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and amphetamine in vesicular, cytosolic, and extracellular compartments associated with a striatal varicosity were estimated through use of a computer simulation model. In addition, contribution to overall effects of amphetamine by each of five actions--transport by dopamine transporter (DAT), transport by vesicular monoamine transporter, stimulation of reverse transport, inhibition of monoamine oxidase (MAO), and slowing of dopamine cell firing rate--were evaluated. Amphetamine enters a varicosity almost entirely by DAT and accumulates to very high levels within the varicosity. Both reverse transport by DAT and passive diffusion contribute to continual amphetamine egress across the plasma membrane. Amphetamine enters storage vesicles by both transport and diffusion. The transport portion competes with dopamine storage, resulting in redistribution of approximately half of dopamine from vesicles to cytosol. The high concentration of amphetamine in the cytosol inhibits MAO, protecting cytosolic dopamine. A very small fraction of cytosolic dopamine is moved to extracellular compartment via reverse transport by DAT. The amount of dopamine moved by reverse transport is limited because of competition by very high cytosolic levels of amphetamine. In the presence of amphetamine, rate of dopamine transfer to extracellular compartment is less than control; however, high levels of extracellular dopamine are maintained because amphetamine occupies the DAT, thus limiting dopamine reuptake. Simulation output from a model using exchange-diffusion mechanism of reverse transport does not match all published data that were simulated, suggesting that inward transport of a substrate is not required to initiate reverse transport.

Cerebral microglia mediate sleep/wake and neuroinflammatory effects of methamphetamine

Methamphetamine and modafinil exert their wake-promoting effects by elevating monoaminergic tone. The severity of hypersomnolence that occurs subsequent to induced wakefulness differs between these two agents. Microglia detects and modulates CNS reactions to agents such as D-methamphetamine that induce cellular stress. We therefore hypothesized that changes in the sleep/wake cycle that occur subsequent to administration of D-methamphetamine are modulated by cerebral microglia. In CD11b-herpes thymidine kinase transgenic mice (CD11b-TK(mt-30)), activation of the inducible transgene by intracerebroventricular (icv) ganciclovir results in toxicity to CD11b-positive cells (i.e. microglia), thereby reducing cerebral microglial cell counts. CD11b-TK(mt-30)and wild type mice were subjected to chronic icv ganciclovir or vehicle administration with subcutaneous mini-osmotic pumps. D-methamphetamine (1 and 2 mg/kg), modafinil (30 and 100 mg/kg) and vehicle were administered intraperitoneally to these animals. In CD11b-TK(mt-30) mice, but not wild type, icv infusion of ganciclovir reduced the duration of wake produced by D-methamphetamine at 2 mg/kg by nearly 1h. Nitric oxide synthase (NOS) activity, studied ex vivo, and NOS expression were elevated in CD11b-positive cerebral microglia from wild type mice acutely exposed to d-methamphetamine. Additionally, CD11b-positive microglia, but not other cerebral cell populations, exhibited changes in sleep-regulatory cytokine expression in response to d-METH. Finally, CD11b-positive microglia exposed to d-methamphetamine in vitro exhibited increased NOS activity relative to pharmacologically-naïve cells. CD11b-positive microglia from the brains of neuronal NOS (nNOS)-knockout mice failed to exhibit this effect. We propose that the effects of D-METH on sleep/wake cycles are mediated in part by actions on microglia, including possibly nNOS activity and cytokine synthesis.