Dextroamphetamine - Pharmacology

Pharmacology

Scientific findings have established that dextroamphetamine administration increases the activity of the phosphoinositol cycle via an indirect release of dopamine and Norepinephrine. These results are the first time that this has been confirmed in humans. Because dextroamphetamine is a substrate analog at monoamine transporters, at all doses, dextroamphetamine prevents the re-uptake of these neurotransmitters by competing with endogenous monoamines for uptake. Transporter inhibition causes monoamines to remain in the synaptic cleft for a prolonged period (amphetamine inhibits monoamine reuptake in rats with a norepinephrine to dopamine ratio (NE:DA) of 1:1 and a norepinephrine to 5-hydroxytryptamine ratio (NE:5-HT) of about 100:1).

At higher doses, when the concentration of dextroamphetamine is sufficient, the drug can trigger direct release of norepinephrine and dopamine from the cytoplasmic transmitter pool, that is, dextroamphetamine will cause norepinephrine and dopamine efflux via transporter proteins, functionally reversing transporter action, which triggers a cascading release of catecholamines. This inversion leads to a release of large amounts of these neurotransmitters from the cytoplasm of the presynaptic neuron into the synapse, causing increased stimulation of post-synaptic receptors, inducing euphoria. Dextroamphetamine releases monoamines in rats with selectivity ratios of about NE:DA = 1:3.5 and NE:5-HT = 1:250, meaning that NE and DA are readily released, but release of 5-HT occurs at a 1/4 ration than of NE:DA.

Dextroamphetamine increases dopamine release in the prefrontal cortex; activation of the dopamine-2 receptors inhibits glutamate release in the prefrontal cortex. Activation of the dopamine-1 receptors in the prefrontal cortex, however, results in elevated glutamate levels in the nucleus accumbens. An increase of the glutamate levels in the nucleus accumbens is the reason that dextroamphetamine has an ability to increase locomotor activity in rats. Serotonin also plays a role in dextroamphetamine's effect on glutamate levels; however, at therapeutic doses, dextroamphetamine has minuscule effect on the serotonin transporter (SERT).