Abstract
Activation of indirect pathway medium spiny neurons (MSNs) via promotion of cAMP production is the principal mechanism of action of current antipsychotics with dopamine D(2) receptor antagonism. TAK-063 [1-[2-fluoro-4-(1H-pyrazol-1-yl)phenyl]-5-methoxy-3-(1-phenyl-1H-pyrazol-5-yl)pyridazin-4(1H)-one] is a novel phosphodiesterase 10A inhibitor that activates both direct and indirect pathway MSNs through increasing both cAMP and cGMP levels by inhibition of their degradation. The activation of indirect pathway MSNs through the distinct mechanism of action of these drugs raises the possibility of augmented pharmacological effects by combination therapy. In this study, we evaluated the potential of combination therapy with TAK-063 and current antipsychotics, such as haloperidol or olanzapine after oral administration. Combined treatment with TAK-063 and either haloperidol or olanzapine produced a significant increase in phosphorylation of glutamate receptor subunit 1 in the rat striatum. An electrophysiological study using rat corticostriatal slices showed that TAK-063 enhanced N-methyl-(D) -aspartic acid receptor-mediated synaptic responses in both direct and indirect pathway MSNs to a similar extent. Further evaluation using pathway-specific markers revealed that coadministration of TAK-063 with haloperidol or olanzapine additively activated the indirect pathway, but not the direct pathway. Combined treatment with TAK-063 and either haloperidol or olanzapine at subeffective doses produced significant effects on methamphetamine- or MK-801-induced hyperactivity in rats and MK-801-induced deficits in prepulse inhibition in mice. TAK-063 at 0.1 mg/kg did not affect plasma prolactin levels and cataleptic response from antipsychotics in rats. Thus, TAK-063 may produce augmented antipsychotic-like activities in combination with antipsychotics without effects on plasma prolactin levels and cataleptic responses in rodents.