Background:
The atypical antipsychotic drug clozap-ine is superior to all other drugs in treatment of refractory schizophrenia. This is likely to involve clozapine-inducing long-term neuronal adaptations contingent on drug-receptor activation of intracellular signaling to affect gene transcription. One candidate intracellu-lar signaling pathway is the mitogen-activated protein kinase-extracellular signal-regulated kinase (ERK) cascade. This pathway regulates synaptic proliferation and plasticity, processes impaired in schizophrenia. We have previously reported that although clozapine and haloperidol acutely inhibited ERK activation in cortical neurons, only clozapine stimulated ERK with continued treatment. However, this stimulation was not through the canonical dopamine D2-Gi/o-PKA or the serotonin 5HT2A-Gq-phospholipase C-linked signaling pathways. Thus, we examined alternative signaling pathways that clozapine could mobilize to activate ERK including growth factor receptor systems.
Methods:
Clozapine-induced phosphorylation of ERK1/2 in the absence or presence of growth factor receptor-specific inhibitors was measured in primary murine cortical cultures by Western immunoblotting. Results were normalized against vehicle and total ERK1 and 2 levels.
Results:
The epidermal growth factor (EGF) receptor inhibitor, AG1478, caused significant dose-dependent inhibition of pERK1 (IC50 0.083 μM) and pERK2 (IC50 0.106 mM) in the presence of clozapine, whereas the platelet-derived growth factor receptor inhibitor, tyrphostin, A9 did not.
Conclusions:
This is the first evidence that the effects of clozapine may involve a neuronal signaling system previously not linked to antipsychotic drug action. This presents a novel series of targets for exploration in the development of new therapeutics and insights into the pathology of schizophrenia.
