Pharmacology
A member of the family of phenethylamines, methamphetamine is chiral, with two isomers, levorotatory and dextrorotatory. The levorotatory form, called levomethamphetamine, is an over-the-counter drug used in inhalers for nasal decongestion. Levomethamphetamine (a levoamphetamine derivative) does not possess any significant central nervous system activity or addictive properties. This article deals only with the dextrorotatory form, called dextromethamphetamine, and the racemic form.
Methamphetamine is a potent central nervous system stimulant that affects neurochemical mechanisms responsible for regulating heart rate, body temperature, blood pressure, appetite, attention, mood and emotional responses associated with alertness or alarming conditions. The acute physical effects of the drug closely resemble the physiological and psychological effects of an epinephrine-provoked fight-or-flight response, including increased heart rate and blood pressure, vasoconstriction (constriction of the arterial walls), bronchodilation, and hyperglycemia (increased blood sugar). Users experience an increase in focus, increased mental alertness, and the elimination of fatigue, as well as a decrease in appetite. It is known to produce central effects similar to the other stimulants, but at smaller doses, with fewer peripheral effects. Methamphetamine's fat solubility also allows it to enter the brain faster than other stimulants, where it is more stable against degradation by monoamine oxidase (MAO).
The methyl group is responsible for the potentiation of effects as compared to the related compound amphetamine, rendering the substance more lipid-soluble, enhancing transport across the blood–brain barrier, and more stable against enzymatic degradation by monoamine oxidase (MAO). Methamphetamine causes the norepinephrine, dopamine, and serotonin (5HT) transporters to reverse their direction of flow. This inversion leads to a release of these transmitters from the vesicles to the cytoplasm and from the cytoplasm to the synapse (releasing monoamines in rats with ratios of about NE:DA = 1:2, NE: 5HT = 1:60), causing increased stimulation of post-synaptic receptors. Methamphetamine also indirectly prevents the reuptake of these neurotransmitters, causing them to remain in the synaptic cleft for a prolonged period (inhibiting monoamine reuptake in rats with ratios of about: NE:DA = 1:2.35, NE:5HT = 1:44.5). Methamphetamine also interacts with TAAR1 to trigger phosphorylation of PKA and PKC, ultimately resulting in the internalization of dopamine transporters. The presynaptic cell is less able to effectively remove dopamine from the synapse. The binding of methamphetamine to TAAR1 also activates adenylyl cyclase, which allows for increased intracellular cAMP. Taken together, the binding of methamphetamine to TAAR1 results in a massive efflux of neurogenic monoamines with a sustained synaptic presence.
Methamphetamine is a potent neurotoxin, shown to cause dopaminergic degeneration. High doses of methamphetamine produce losses in several markers of brain dopamine and serotonin neurons. Dopamine and serotonin concentrations, dopamine and 5HT uptake sites, and tyrosine and tryptophan hydroxylase activities are reduced after the administration of methamphetamine. It has been proposed that dopamine plays a role in methamphetamine-induced neurotoxicity, because experiments that reduce dopamine production or block the release of dopamine decrease the toxic effects of methamphetamine administration. When dopamine breaks down, it produces reactive oxygen species such as hydrogen peroxide. It is likely that the approximate twelvefold increase in dopamine levels and subsequent oxidative stress that occurs after taking methamphetamine mediates its neurotoxicity. The lab of David Sulzer and colleagues at Columbia University developed a technique known as "intracellular patch electrochemistry" to measure concentrations of dopamine in the cytosol, and found massive increases following methamphetamine, leading to the "cytosolic dopamine hypothesis" of neurotoxicity, in which dopamine oxidation, particularly close to synaptic vesicles, produce oxidative stress that in turn leads to exacerbation of autophagy that can destroy axons and dendrites.
Recent research published in the Journal of Pharmacology And Experimental Therapeutics (2007) indicates that methamphetamine binds to and activates a G protein-coupled receptor called TAAR1. TAARs are a newly discovered receptor family whose members are activated by a number of amphetamine-like molecules called trace amines, thyronamines, and certain volatile odorants.
It has been demonstrated that a high core temperature is correlated with an increase in the neurotoxic effects of methamphetamine.
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