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 prepulse 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.
Karmacharya, R.*, S.K. Lynn*, S. Demarco, A. Ortiz, X. Wang, M.Y. Lundy, Z. Xie, B.M. Cohen, G.M. Miller, and E.A. Buttner. 2011. Behavioral effects of clozapine: Involvement of trace amine pathways in C. elegans and M. musculus. Brain Research 1393:91-99.
* These authors contributed equally to the study.
3-Iodothyronamine (T1AM) is a metabolite of thyroid hor- mone. It is an agonist at trace amine-associated receptor 1 (TAAR1), a recently identified receptor involved in mono- aminergic regulation and a potential novel therapeutic tar- get. Here, T1AM was studied using rhesus monkey TAAR1 and/or human dopamine transporter (DAT) co- transfected cells, and wild-type (WT) and TAAR1 knock- out (KO) mice. The IC50 of T1AM competition for binding of the DAT-specific radio-ligand [3H]CFT was highly similar in DAT cells, WT striatal synaptosomes and KO striatal synaptosomes (0.72–0.81 lM). T1AM inhibition of 10 nM [3H]dopamine uptake (IC50: WT, 1.4 6 0.5 lM; KO, 1.2 6 0.4 lM) or 50 nM [3H]serotonin uptake (IC50: WT, 4.5 6 0.6 lM; KO, 4.7 6 1.1 lM) in WT and KO synaptosomes was also highly similar. Unlike other TAAR1 agonists that are DAT substrates, TAAR1 signaling in response to T1AM was not enhanced in the presence of DAT as determined by CRE-luciferase assay. In vivo, T1AM induced robust hypothermia in WT and KO mice equivalently and dose dependently (maximum change degrees Celsius: 50 mg/ kg at 60 min: WT 26.0 6 0.4, KO 25.6 6 1.0; and 25 mg/kg at 30 min: WT 22.7 6 0.4, KO 23.0 6 0.2). Other TAAR1 agonists including beta–phenylethylamine (b-PEA), MDMA (3,4-methylenedioxymethamphetamine) and meth- amphetamine also induced significant, time-dependent thermoregulatory responses that were alike in WT and KO mice. Therefore, TAAR1 co-expression does not alter T1AM binding to DAT in vitro nor T1AM inhibition of [3H]monoamine uptake ex vivo, and TAAR1 agonist- induced thermoregulatory responses are TAAR1-inde- pendent. Accordingly, TAAR1-directed compounds will likely not affect thermoregulation nor are they likely to be cryogens.
Panas, HN, LJ Lynch, EJ Vallender, Z Xie, G Chen, SK Lynn, TS Scanlan, and GM Miller. 2010. Normal thermoregulatory responses to 3-iodothyronamine, trace amines and amphetamine-like psychostimulants in trace amine associated receptor 1 knockout mice. Journal of Neuroscience Research 88:1962-2969.