The molecular modes of action of antipsychotic drugs are poorly understood beyond their effects at the dopamine D2 receptor. Previous studies have placed Akt signaling downstream of D2 dopamine receptors, and recent data have suggested an association between psychotic illnesses and defective Akt signaling. To characterize the effect of antipsychotic drugs on the Akt pathway, we used the model organism C. elegans, a simple system where the Akt/forkhead box O transcription factor (FOXO) pathway has been well characterized. All major classes of antipsychotic drugs increased signaling through the insulin/Akt/FOXO pathway, whereas four other drugs that are known to affect the central nervous system did not. The antipsychotic drugs inhibited dauer formation, dauer recovery, and shortened lifespan, three biological processes affected by Akt signaling. Genetic analysis showed that AKT-1 and the insulin and insulin-like growth factor receptor, DAF-2, were required for the antipsychotic drugs to increase signaling. Serotonin synthesis was partially involved, whereas the mitogen activated protein kinase (MAPK), SEK-1 is a MAP kinase kinase (MAPKK), and calcineurin were not involved. This is the first example of a common but specific molecular effect produced by all presently known antipsychotic drugs in any biological system. Because untreated schizophrenics have been reported to have low levels of Akt signaling, increased Akt signaling might contribute to the therapeutic actions of antipsychotic drugs.

Chronic administration of antipsychotic drugs produces adaptive responses at the cellular and molecular levels that may be responsible for both the main therapeutic effects and rebound psychosis, which is often observed upon discontinuation of these drugs. Here we show that some antipsychotic drugs produce significant functional changes in serotonergic neurons that directly impact feeding behavior in the model organism, Caenorhabditis elegans. In particular, antipsychotic drugs acutely suppress pharyngeal pumping, which is regulated by serotonin from the NSM neurons. By contrast, withdrawal from food and drug is accompanied by a striking recovery and overshoot in the rate of pharyngeal pumping. This rebound response is absent or diminished in mutant strains that lack tryptophan hydroxylase (TPH-1) or the serotonin receptors SER-7 and SER-1, and is blocked by serotonin antagonists, which implicates serotonergic mechanisms in this adaptive response. Consistent with this, continuous drug exposure stimulates an increase in serotonin and the number of varicosities along the NSM processes. Cyclosporin A and calcineurin mutant strains mimic the effects of the antipsychotic drugs and reveal a potential role for the calmodulin-calcineurin signaling pathway in the response of serotonergic neurons. Similar molecular and cellular changes may contribute to the long-term adaptive response to antipsychotic drugs in patients.

Antipsychotic drugs produce acute behavioral effects through antagonism of dopamine and serotonin receptors, and long-term adaptive responses that are not well understood. The goal of the study presented here was to use Caenorhabditis elegans to investigate the molecular mechanism or mechanisms that contribute to adaptive responses produced by antipsychotic drugs. First-generation antipsychotics, trifluoperazine and fluphenazine, and second-generation drugs, clozapine and olanzapine, increased the expression of tryptophan hydroxylase-1::green fluorescent protein (TPH-1::GFP) and serotonin in the ADF neurons of C. elegans. This response was absent or diminished in mutant strains lacking the transient receptor potential vanilloid channel (TRPV; osm-9) or calcium/calmodulin-dependent protein kinase II (CaMKII; unc-43). The role of calcium signaling was further implicated by the finding that a selective antagonist of calmodulin and a calcineurin inhibitor also enhanced TPH-1::GFP expression. The ADF neurons modulate foraging behavior (turns/reversals off food) through serotonin production. We found that short-term exposure to the antipsychotic drugs altered the frequency of turns/reversals off food. This response was mediated through dopamine and serotonin receptors and was abolished in serotonin-deficient mutants (tph-1) and strains lacking the SER-1 and MOD-1 serotonin receptors. Consistent with the increase in serotonin in the ADF neurons induced by the drugs, drug withdrawal after 24-hr treatment was accompanied by a rebound in the number of turns/reversals, which demonstrates behavioral adaptation in serotonergic systems. Characterization of the cellular, molecular, and behavioral adaptations to continuous exposure to antipsychotic drugs may provide insight into the long-term clinical effects of these medications.

Numerous studies have implicated low levels of signaling in the Akt network with psychotic illnesses, and a growing body of literature has shown that all classes of antipsychotic drugs increase Akt signaling. The most clinically effective antipsychotic drug is clozapine. With Caenorhabditis elegans as a model system, this study demonstrates that clozapine is unique among antipsychotic drugs because it requires β-arrestin and serum and glucocorticoid-inducible kinase (SGK) in addition to Akt to suppress the nuclear localization of DAF-16 (Forkhead box O [FOXO]). Lithium, a mood stabilizer often used to treat psychosis, also requires β-arrestin and SGK to suppress the nuclear localization of DAF-16.

Antipsychotic drugs are increasingly being prescribed for children and adolescents, and are used in pregnant women without a clear demonstration of safety in these populations. Global effects of these drugs on neurodevelopment (e.g., decreased brain size) have been reported in rats, but detailed knowledge about neuronal effects and mechanisms of action are lacking. Here we report on the evaluation of a comprehensive panel of antipsychotic drugs in a model organism (Caenorhabditis elegans) that is widely used to study neuronal development. Specifically, we examined the effects of the drugs on neuronal migration and axonal outgrowth in mechanosensory neurons visualized with green fluorescent protein expressed from the mec-3 promoter. Clozapine, fluphenazine, and haloperidol produced deficits in the development and migration of ALM neurons and axonal outgrowth in PLM neurons. The defects included failure of neuroblasts to migrate to the proper location, and excessive growth of axons past their normal termination point, together with abnormal morphological features of the processes. Although the antipsychotic drugs are potent antagonists of dopamine and serotonin receptors, the neurodevelopmental deficits were not rescued by coincubation with serotonin or the dopaminergic agonist, quinpirole. Other antipsychotic drugs, risperidone, aripiprazole, quetiapine, trifluoperazine and olanzapine, also produced modest, but detectable, effects on neuronal development. This is the first report that antipsychotic drugs interfere with neuronal migration and axonal outgrowth in a developing nervous system.