While our understanding of the mechanisms of psychosis continues to evolve beyond the dopamine hypothesis, the key role of dopamine in psychosis and its treatment has not faded.1 Over time, the dopamine hypothesis of schizophrenia has evolved from focusing on dopamine hyperactivity to specifying the regional abnormalities in the brain with subcortical hyperdopaminergia and prefrontal hypodopaminergia.2 Despite this divergence in dopaminergic function, antipsychotic medications that block dopamine D2 receptors (D2R) remain central to treating psychotic symptoms and preventing relapse.3,4 Notably, antipsychotics block both presynaptic and postsynaptic receptors affecting the regulation of dopamine synthesis and release in the brain.5,6
Chronic dopamine D2R blockade with antipsychotics induces adaptive changes that can contribute to both acute and chronic adverse effects. In this article, we discuss these changes, and steps clinicians can take to minimize their occurrence.
Dopamine D2R: A primer
There are 5 types of dopamine receptors, numbered D1 through D5, but there are only 2 families of dopamine receptors: the D1 family (D1 and D5), and the D2 family (D2, D3, and D4). All dopamine receptors are G protein–coupled, but the D2 family of receptors generally increases protein kinase A (PKA) as the second messenger, whereas the D1 family increases cyclic adenosine monophosphate (cAMP) as the second messenger.5 There are 2 distinct variants of the D2R of 2 different lengths made from the same gene (DRD2) via posttranslational modification. The long isoform of D2R (D2L) has an additional 29 amino acids compared to the short isoform (D2S).7 Additional evidence points to a third splice variant called D2Longer that arises from aberrant RNA splicing and contains 2 more amino acids than D2L; its relevance is not known.8
The D2L isoform is the primary postsynaptic receptor, expressed more in the striatum and nucleus accumbens (NAc) targeted by dopaminergic afferents. The D2S isoform, however, is predominantly presynaptic, more densely expressed on cell bodies and projection axons of the dopaminergic neurons of the midbrain and hypothalamus.9 Each isoform contributes differentially to the therapeutic and adverse effects of antipsychotics, and evidence from animal studies suggests that D2L is the main variant responsible for drug-induced parkinsonism.10 The D2S acts as the principal autoreceptor for the dopaminergic system.5,11,12
Autoreceptors regulate dopamine transmission. Dopamine itself and D2R agonists are reported to have higher affinity and potency with D2S. Activation of these autoreceptors is a negative feedback mechanism that decreases dopamine release. Similarly, when they are blocked (such as with use of an antipsychotic), there is an increase in dopamine release. Additionally, these autoreceptors modulate several key processes:
- neuronal firing rate by activating potassium conductance
- dopamine synthesis by downregulating the expression of tyrosine hydroxylase (TH) enzyme (the rate-limiting step)
- exocytotic release of dopamine and other neurotransmitters
- dopamine reuptake via increasing the activity of the dopamine transporter (DAT).12
Consequences of antipsychotic D2R blockade
Most antipsychotics begin to produce a therapeutic antipsychotic effect at 65% to 75% occupancy of the D2Rs.3 This level also produces an optimal balance between clinical efficacy and a lower incidence of adverse effects.3 A higher D2R occupancy by both first-generation (FGA) and second-generation (SGA) pure antagonist antipsychotics can lead to parkinsonism.
Parkinsonism is associated with the subsequent appearance of one of the most distressing consequences of long-term antipsychotic treatment, tardive dyskinesia (TD).13 TD is an iatrogenic, usually late-onset syndrome consisting of persistent, involuntary, and repetitive movements. It classically involves the highly innervated striated muscles of the tongue, mouth, face, and fingers, though it can also involve the trunk and extremities.14 It occurs secondary to chronic exposure to dopamine receptor–blocking agents, including dopaminergic antiemetics.15 The prevalence of TD is higher in patients treated long-term with FGAs (30.0% to 32.4%) than in those treated with SGAs (13.1% to 20.7%) due to serotonin 5HT2A blockade that results in increased dopamine release in the basal ganglia.16
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