A second important dopaminergic system arising in the midbrain is the mesocorticolimbic pathway, which is implicated in the pathogenesis of addictive behaviors including drug reward. Its key components include the midbrain ventral tegmental area (VTA), medial forebrain bundle, and nucleus accumbens. The cholinergic pathway originating in the nucleus basalis of Meynert plays a role in memory function in Alzheimer's disease.
Addictive drugs
share the property of
increasing dopamine release in the nucleus accumbens. Amphetamine
increases intracellular release of dopamine from vesicles and reverses the transport of dopamine through the dopamine transporters. Patients prone to
addiction show increased activation of the nucleus accumbens after the administration
of amphetamine. Cocaine binds to dopamine transporters and inhibits dopamine
reuptake. Ethanol
inhibits inhibitory neurons in the VTA, leading to increased dopamine release
in the nucleus accumbens. Opioids also disinhibit these dopaminergic neurons by
binding to receptors expressed by -aminobutyric acid (GABA)- containing
interneurons in the VTA. Nicotine
increases dopamine release by activating nicotinic acetylcholine receptors on cell bodies
and nerve terminals of dopaminergic VTA neurons.
Tetrahydrocannabinol, the
active ingredient of
cannabis, also increases dopamine levels in the nucleus accumbens. The blockade of dopamine in the
nucleus accumbens can terminate the rewarding effects of addictive drugs.
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