Behavioral effects of clozapine: Involvement of trace amine pathways in C. elegans and M. musculus

Abstract

Clozapine is an antipsychotic medication with superior efficacy in treatment-refractory schizophrenia. The molecular basis of clozapine’s therapeutic profile is not well understood. We studied behavioral effects of clozapine in Caenorhabditis elegans to identify novel pathways that modulate clozapine’s biological effects. Clozapine stimulated egg laying in C. elegans in a dose-dependent manner. This effect was clozapine-specific, as it was not observed with exposure to a typical antipsychotic, haloperidol or an atypical antipsychotic, olanzapine. A candidate gene screen of biogenic amine neurotransmitter systems identified signaling pathways that mediate this clozapine-specific effect on egg laying. Specifically, we found that clozapine-induced increase in egg laying requires tyramine biosynthesis. To test the implications of this finding across species, we explored whether trace amine systems modulate clozapine’s behavioral effects in mammals by studying trace amine-associated receptor 1 (TAAR1) knockout mice. Clozapine increased pre-pulse inhibition (PPI) in wild-type mice. This increase in PPI was abrogated in TAAR1 knockout mice, implicating TAAR1 in clozapine-induced PPI enhancement. In transfected mammalian cell lines, we found no TAAR activation by antipsychotics, suggesting that modulation of trace amine signaling in mice does not occur directly at the receptor itself. In summary, we report a heretofore-unknown role for trace amine systems in clozapine-mediated effects across two species: C. elegans and mice.

Crossing the Worm-Brain Barrier by Using <b><i>Caenorhabditis elegans</i></b> to Explore Fundamentals of Human Psychiatric Illness

“…For example, studies of clozapine revealed that it binds to the trace amine-associated receptor TAAR1, which was then implicated in clozapine enhancement of PPI [56]. Moreover, two independent groups simultaneously identified the IIS pathway in C. elegans as a physiologically relevant target of a wide variety of antipsychotic drugs [57,58].…”

Clozapine Modulates Glucosylceramide, Clears Aggregated Proteins, and Enhances ATG8/LC3 in Caenorhabditis elegans

“…To address differences among worm strains, we adjusted concentrations of the drug, for example, for some clozapine-sensitive mutants, the drug concentrations were lowered. These procedures follow observations published in previous manuscripts (Karmacharya et al, 2009(Karmacharya et al, , 2011.…”

“…Clozapine delays the development of C. elegans and, depending on the drug concentration used, can cause lethality. It also alters worm behavior, including decreasing the rate of pharyngeal pumping, inducing faster egg laying, and increasing the speed of locomotion (Donohoe et al, 2006;Karmacharya et al, 2011). The insulin/PKT pathway (which is altered in schizophrenia) has a role in clozapine-induced development delay/lethality, but it does not fully explain the effects of clozapine (Karmacharya et al, 2009).…”

Methods for Studying the Mechanisms of Action of Antipsychotic Drugs in <em>Caenorhabditis elegans</em>

“…APD exposure during adulthood produces behavioral phenotypes. For example, clozapine exposure inhibits locomotion and pharyngeal pumping and enhances egg laying 1,2,7 . APD-induced developmental delay and lethality are powerful phenotypes for large-scale chemical genetic screens.…”

“…However, our laboratory has conducted candidate gene screens and a genome-wide RNAi screen for suppressors of APDinduced developmental delay and lethality and has successfully recovered genes which likely encode direct targets, including dopamine, insulin, and nicotinic acetylcholine receptors 2,8 . Genetic screens based on APD-induced behaviors in the adult have also led to the identification of novel APD targets, and we are now validating targets from both developmental and behavioral screens in mammals 7 . Thus, an invertebrate chemical genetic approach to discover novel molecular mechanisms of action of APDs appears to be feasible 5,8 .…”