The atypical antipsychotic clozapine effectively treats refractory schizophrenia. It also presents serious side-effects, such as agranulocytosis and metabolic syndrome. However, the molecular basis of this drug''s unique therapeutic profile remains unknown. Previously, we characterized clozapine-induced behavioral phenotypes in C. elegans and reported clozapine-specific effects in comparison to the typical antipsychotic haloperidol. The results showed that clozapine enhanced egg laying without the involvement of dopaminergic systems. We also observed that exposure to clozapine decreased the rate of pharyngeal pumping and the rate of locomotion in well-fed worms. We went on to test olanzapine, an atypical antipsychotic that has a structure similar to clozapine but that lacks clozapine''s unique efficacy in treating schizophrenia. Interestingly, we found that olanzapine failed to recapitulate clozapine''s effect on egg laying. By testing a panel of C. elegans mutants with defects in biogenic amine neurotransmitter systems, we aimed to identify novel pathways underlying clozapine-induced behaviors. We found that
tdc-1, a gene required for tyramine synthesis, and
ser-4, which encodes a serotonin-octopamine receptor, mediate the clozapine-induced increase in egg laying. Trace amine signaling also appears to be necessary for clozapine''s inhibition of pharyngeal pumping. Our observations suggest that the clozapine-induced decrease in pharyngeal pumping requires the presence of two distinct tyramine receptors:
tyra-2, a G protein-coupled tyramine receptor, and
sho-1, a tyramine-gated ion channel. We also found that
tbh-1, a gene encoding tyramine beta-hydroxylase for octopamine synthesis, is needed for clozapine to decrease pharyngeal pumping rates. Clozapine''s inhibition of locomotion rate requires both the trace amine system and the biosynthesis of serotonin and dopamine. Interestingly, we found that worms lacking
sho-1 did not demonstrate clozapine-induced reduction in locomotion rate. Similarly, mutations in
bas-1, a gene encoding an aromatic amino acid decaboxylase necessary for serotonin and dopamine synthesis, did not show a change in locomotion rate after clozapine exposure. These results demonstrate that clozapine may be acting through a variety of pathway to produce its wide range of effects. Our observations may shed light on the molecular mechanisms underlying clozapine''s effects in human patients.