Interaction of Dopamine D1 and NMDA Receptors Mediates Acute Clozapine Potentiation of Glutamate EPSPs in Rat Prefrontal Cortex

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Abstract
The atypical antipsychotic drug clozapine effectively alleviates both negative and positive symptoms of schizophrenia via unclear cellular mechanisms. Clozapine may modulate both glutamatergic and dopaminergic transmission in the prefrontal cortex (PFC) to achieve part of its therapeutic actions. Using whole cell patch-clamp techniques, current-clamp recordings in layers V–VI pyramidal neurons from rat PFC slices showed that stimulation of local afferents (in 2 μM bicuculline) evoked mixed [AMPA/kainate andN-methyl-d-aspartate (NMDA) receptors] glutamate receptor-mediated excitatory postsynaptic potentials (EPSPs). Clozapine (1 μM) potentiated polysynaptically mediated evoked EPSPs (VHold = −65 mV), or reversed EPSPs (rEPSP, VHold = +20 mV) for >30 min. The potentiated EPSPs or rEPSPs were attenuated by elevating [Ca2+]O(7 mM), by application of NMDA receptor antagonist 2-amino5-phosphonovaleric acid (50 μM), or by pretreatment with dopamine D1/D5 receptor antagonist SCH23390 (1 μM) but could be further enhanced by a dopamine reuptake inhibitor bupropion (1 μM). Clozapine had no significant effect on pharmacologically isolated evoked NMDA-rEPSP or AMPA-rEPSPs but increased spontaneous EPSPs without changing the steady-state resting membrane potential. Under voltage clamp, clozapine (1 μM) enhanced the frequency, and the number of low-amplitude (5–10 pA) AMPA receptor-mediated spontaneous EPSCs, while there was no such changes with the mini-EPSCs (in 1 μM TTX). Taken together these data suggest that acute clozapine can increase spike-dependent presynaptic release of glutamate and dopamine. The glutamate stimulates distal dendritic AMPA receptors to increase spontaneous EPSCs and enabled a voltage-dependent activation of neuronal NMDA receptors. The dopamine released stimulates postsynaptic D1 receptor to modulate a lasting potentiation of the NMDA receptor component of the glutamatergic synaptic responses in the PFC neuronal network. This sequence of early synaptic events induced by acute clozapine may comprise part of the activity that leads to later cognitive improvement in schizophrenia.