Dopamine supersensitivity psychosis (DSP) is a term that describes the clinical iatrogenic phenomenon that might be observed with long-term antipsychotic treatment. DSP is suggested to be strongly associated with treatment failure/resistance in schizophrenia.17,18 Manifestations of DSP include development of antipsychotic drug tolerance that undermines treatment efficacy, rebound psychosis during or after treatment discontinuation, and the presence of TD. Like TD, it may be reversed temporarily by increasing the dose of the antipsychotic.18
DSP and (more extensively) TD are commonly hypothesized to result from the postsynaptic dopamine receptor supersensitivity that develops because of chronic D2Rs blockade by antipsychotics. Neostriatal dopamine receptor supersensitivity is believed to lead to TD, while mesolimbic supersensitivity leads to DSP.19 Supersensitivity has traditionally been believed to be due to upregulation of postsynaptic D2R number and sensitivity.20,21 However, both TD and DSP are more likely a consequence of a host of compensatory neurobiological adaptations across the synapse that include:
- postsynaptic increase in the number of D2Rs that amplifies the dopamine signal
- an increased number of synapses, dendritic spines, and perforated synapses (seen in animal models), all of which lead to a potentiated dopamine signal
- presynaptic changes with higher levels of dopamine released into the synapse via an increase in quantal size as postsynaptic D2Rs blockade results in more dopamine becoming available in the synapse for recycling via the dopamine transporter
- increased dopamine turnover due to presynaptic D2S autoreceptor blockade.22
So if giving a D2R blocking agent for a long time increases the dopamine signal, at least in some patients, what can the clinician do to treat the psychosis, and not cause changes in the brain that could lead to TD or DSP?
Partial agonist antipsychotics and biased agonism of D2Rs
One approach to try to avoid the compensatory changes to dopamine blockade might be to use a D2R partial agonist.18,23 For example, aripiprazole is a partial agonist at the D2R commonly used to manage schizophrenia and bipolar disorder. It possesses greater affinity at the D2R compared with the serotonin 2A (5-hydroxytryptamine, 5HT2A) serotonin receptor. Unlike full antagonists, aripiprazole requires exceptionally high D2 receptor occupancy (approximately 90%) to be at a clinically effective antipsychotic dose.24,25 This is a general requirement for all D2R partial agonists.26
A partial agonist generally has to possess greater affinity to the receptor than the neurotransmitter with which it is competing. Aripiprazole has more than twice the affinity to D2R than dopamine. Other partial agonists have similarly high, or higher, D2R affinity. Effective antipsychotic partial agonists stimulate the D2Rs at approximately 30% ± 10% the maximal signal achieved with dopamine. This is essentially equivalent to having approximately 70% receptor occupancy with a full antagonist, except it is built into how the molecule works. Having this low-grade partial activation of D2Rs creates multiple receptor-mediated actions:
- reduction of cAMP accumulation
- antagonism to guanosine 5’-0-(3-thio) triphosphate (GTPgamma S) binding with relatively less recruitment of beta-arrestin 2 (these diverging effects on G protein are the definition of biased agonism)
- antagonism of G protein activation of K+ channels (GIRK) activity
- agonism for the inhibition of TH.
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