Pharmacodynamics
The main action of an antipsychotic (regardless of typical or atypical) is to decrease the action of dopamine and/or epinephrine and norepinephrine levels in the brain. Because of the risk of developing pre-existing Parkinson's disease as a result of decreased dopaminergic levels in the brain, risperidone possesses effects similar to those of haloperidol. It is a highly potent dopaminergic antagonist for the treatment of schizophrenia and bipolar disorder. However, the risk of extrapyramidal symptoms are decreased only slightly because of the antagonism of the serotonin and α2 adrenergic receptors. Risperidone acts on the following receptors:
Dopamine receptors - The main targets of this drug are the D1 and D2 family receptors. Risperidone may induce the "Parkinsonian gait" and tremors associated with diminished dopaminergic activity in the substantia nigra.
Serotonin receptors - anxiolytic and antiaggressive properties, but may cause an inability to achieve orgasm. It may also increase appetite, leading to significant weight gain. Decreased extrapyramidal symptoms are also seen in patients. Side effects may include depression and anxiety disorders.
Alpha α1 adrenergic receptors - Its diminished action of epinephrine and norepinephrine accounts for its sedation, anxiolytic effects, and smooth muscle relaxation.
Alpha α2 adrenergic receptors - decreased extrapyramidal symptoms; increased sympathetic tone is associated with the blocked action of the adrenergic α2 receptors. Also, it possesses antiaggressive properties.
Histamine H1 receptors - effects on these receptors accounts for its sedation and inability to maintain vigilance. This may also lead to weight gain.
Though this medication possesses similar effects to other typical and atypical antipsychotics, it does not possess an affinity for the muscarinic acetylcholine receptors. In many respects, this medication can be useful as an "acetylcholine release-promoter" similar to gastrointestinal drugs such as metoclopramide and cisapride.
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