introduction
Dopamine is a catecholamine neurotransmitter found in neurons of both the central and peripheral nervous systems. It is stored in vesicles in axon terminals and released when the neuron is depolarised. Dopamine interacts with specific membrane receptors to produce its effects. These effects are terminated by re-uptake into the presynaptic neuron by a dopamine transporter, or by metabolic inactivation by monoamine oxidase B (MAO-B) or catechol-0-methyltransferase (COMT) (Fig. 1).
Drugs affecting dopamine actions
The sites of action of drugs affecting dopamine transmission are shown in Fig. 1. Many drugs affect dopamine transmission directly by either blocking or stimulating its receptors. For example, antipsychotic drugs are dopamine antagonists, whereas bromocriptine, used to treat hyperprolactinaemia and Parkinson's disease, is a dopamine agonist.
Several drugs of clinical importance act indirectly e.g. levodopa, which is converted to dopamine, or amphetamine, which releases dopamine from terminal stores. Other drugs increase the synaptic concentration of dopamine by blocking its uptake or metabolism. For example, cocaine is a potent inhibitor of the dopamine re-uptake transporter and this may be the basis of its addictive properties. On the other hand, selegiline, a MAO-B inhibitor, elevates dopamine concentrations by inhibiting its breakdown.