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

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 microM bicuculline) evoked mixed [AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors] glutamate receptor-mediated excitatory postsynaptic potentials (EPSPs). Clozapine (1 microM) potentiated polysynaptically mediated evoked EPSPs (V(Hold) = -65 mV), or reversed EPSPs (rEPSP, V(Hold) = +20 mV) for >30 min. The potentiated EPSPs or rEPSPs were attenuated by elevating [Ca(2+)](O) (7 mM), by application of NMDA receptor antagonist 2-amino5-phosphonovaleric acid (50 microM), or by pretreatment with dopamine D1/D5 receptor antagonist SCH23390 (1 microM) but could be further enhanced by a dopamine reuptake inhibitor bupropion (1 microM). 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 microM) 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 microM 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.

Dopamine D1 and D4 Receptor Subtypes Differentially Modulate Recurrent Excitatory Synapses in Prefrontal Cortical Pyramidal Neurons

“…DA D2/D4 receptors and mRNAs are reported to be abnormally elevated in frontal cortex of schizophrenia patients (Tallerico et al, 2001, also see Introduction) and the atypical antipsychotic drug, clozapine, has a high affinity for blockade of D4 receptors (Van Tol et al, 1991). Similar polysynaptic activity was reported following acute application of clozapine to prefrontal cortical pyramidal neurons (Chen and Yang, 2001), which may be due to enhanced release of DA and glutamate (Daly and Moghaddam, 1993) thereby leading to a D1 potentiation of NMDA receptor functions (see Introduction) and the late NMDA-mediated polysynaptic responses as observed here by D4 receptor blockade. D4 receptor proteins present in high levels in cortical layer II/III or V/VI are associated with both calbindin-and parvalbumin-containing GABA interneurons (Mezljak et al, 1996;Le Moine and Gaspar, 1998;Wedzony et al, 2000).…”

“…Furthermore, stimulation of the corpus callosum with increasing stimulus current strength resulted in a current-dependent increase in a short-latency EPSP and spike discharge, at an average latency of 3.470.1 and 6.170.2 ms, respectively ( Figure 2a). Since this component increased monotonically with increasing stimulus strength, exhibited constant latency, and could follow high-frequency (eg 20 Hz) stimulation (Figure 2a2), this was presumed to be a monosynaptically evoked potential (Berry and Pentreath, 1976;Chen and Yang, 2001), which we defined as a Type I neuron response. In contrast, a subset of neurons recorded with both techniques that were identified as layer V projection neurons ( Figure 1c1; Figure 2c2) had axons that emitted extensive collaterals that ramified across all laminae, for example, ascending to superficial layers ( Figure 1c1).…”

“…These neurons responded to stimulation of the corpus callosum with complex evoked spikes, consisting of both early and late components that gave rise to spike discharge at an average latency of 7.670.2 and 28.470.5 ms, respectively. Since the late components did not follow high-frequency stimulation (see Figure 7a2) and did not show a smooth increase in amplitude with increasing stimulation current ( Figure 2c1) and gave rise to spike discharge at variable onset latencies, these were presumed to be a combination of monosynaptic/multisynaptic origin (Berry and Pentreath, 1976;Chen and Yang, 2001). This type of neuron was classified as a Type II response pattern.…”

“…Functional studies have revealed that the D1 receptor family is involved in working memory and other cognitive tasks (Williams and Goldman-Rakic, 1995;Goldman-Rakic and Selemon, 1997). However, physiological studies of the D2 receptor family have been hampered by the lack of a ligand selective for the various receptor subtypes, in particular the D4 receptor subtype, one of the proposed target sites of antipsychotic drugs (Arvanov and Wang, 1997;Chen and Yang, 2001). …”

“…High concentrations of DA or its agonists (20-30 mM; Seamans et al, 2001a;Zheng et al, 1999) were also tested to verify that any observed lack of effect on membrane excitability or synaptic responses was not due to inadequate receptor activation by the lower (ie 10 mM) doses. These afferent-evoked excitatory synaptic responses and their modulation by DA were analyzed in terms of their mono-vs polysynaptic components (Berry and Pentreath, 1976;Chen and Yang, 2001) as well as their axonal morphology in 350 mm thick brain slices. The specific DA receptor subtype involved in the responses was examined by use of selective DA agonists and antagonists, that is, D1/D5 receptor agonists (SKF 38393 and SKF 81297), D2/D4 receptor agonists (quinpirole) or D4 receptor selective agonist (PD168077; Glase et al, 1997) and D4 receptor antagonists (PD101387 and U101958; Merchant et al, 1996, and L745870;Patel et al, 1997).…”