Exploring the pathophysiology underlying clozapine-induced enhancement of glutamatergic transmission through L-glutamate and D-serine release associated with pannexin1 hemichannels.

Abstract

Background and purpose: Clozapine, an approved antipsychotic for treatment-resistant schizophrenia (TRS), enhances glutamatergic transmission by increasing exocytosis and non-exocytosis glutamate release; however, its full action remained to be clarified.

Experimental approach: This study determined the effects of chronic administration of clozapine on tripartite-synaptic glutamatergic transmission associated with N-methyl-D-aspartate (NMDA)/glutamate receptors (NMDARs) by using microdialysis, cultured astrocytes and capillary immunoblotting.

Key results: Chronic clozapine administration dose- and time-dependently increased the basal release of L-glutamate and D-serine from astrocytes in the prefrontal cortex. A therapeutically relevant concentration of clozapine increased the expression of phosphorylated-Src, but not pannexin1, whereas a supratherapeutic concentration of clozapine increased the expression of both pannexin1 and phosphorylated-Src. Clozapine-induced increase of L-glutamate and D-serine release was inhibited by inhibitors of Src kinase and pannexin1 hemichannels, but not by inhibitors of NMDAR and connexin43-hemichannels. Clozapine enhanced synthesis of L-β-aminoisobutyrate (L-BAIBA) also increased astroglial release of L-glutamate and D-serine through pannexin1 hemichannels and increased expression of pannexin1 and phosphorylated Src. Activation of pannexin1-hemichannels by chronic administration of clozapine and L-BAIBA was regulated by Src kinase, which was inhibited by an inhibitor of group-III metabotropic glutamate receptors (III-mGluR). L-BAIBA enhances III-mGluR, although clozapine did not directly affect III-mGluR activity.

Conclusions and implications: Enhanced glutamatergic transmission by chronic administration of therapeutically relevant concentrations of clozapine involves increased L-BAIBA signalling, which activates Src signalling via III-mGluR agonistic action, without affecting pannexin1 expression. Activation of pannexin1-hemichannel activity induced by Src signalling and III-mGluR may play an important role in the effectiveness of clozapine in TRS.