Differential effects of topiramate on prefrontal glutamatergic transmission when combined with raclopride or clozapine.

Treatment with topiramate may improve negative symptoms in schizophrenia when added to typical antipsychotic drugs (APDs) but not to clozapine. Both dopaminergic and glutamatergic transmissions in the medial prefrontal cortex (mPFC) are facilitated by atypical, but not typical, APDs, which is thought to improve negative symptoms and cognitive dysfunction in schizophrenia. Our previous results show that topiramate increases prefrontal dopamine (DA) outflow when added to the D(2/3) receptorantagonist raclopride. Here, using intracellular recording in vitro, we investigated the effects of topiramate on glutamatergic neurotransmission in the rat mPFC, both when given alone and in combination with raclopride or clozapine. Neither topiramate nor raclopride alone had any effect on N-methyl-D-aspartate (NMDA)-induced currents in pyramidal cells of the mPFC. However, the combination of topiramate and raclopride facilitated the NMDA-induced currents, and this effect was blocked by the D1 receptor antagonist SCH23390. Topiramate also facilitated the effect of a submaximal, but inhibited the effect of a maximal, concentration of clozapine on these currents. The effect of combined topiramate and a submaximal concentration of clozapine could be blocked by SCH23390. In addition, combined topiramate and raclopride facilitated excitatory postsynaptic potentials. In contrast, topiramate inhibited clozapine's facilitating effect on these potentials. These data may help explain the improvement of negative symptoms when topiramate is used as adjunctive therapy in schizophrenic patients receiving typical APDs, but they may also shed light on the observed deterioration of symptoms when topiramate is added to full dose clozapine.

Inhibition of the glycine transporter GlyT-1 potentiates the effect of risperidone, but not clozapine, on glutamatergic transmission in the rat medial prefrontal cortex.

Clinical studies suggest that the efficacy of the atypical antipsychotic drug (APD) risperidone (but not clozapine) can be augmented by adjunctive treatment with agonists at the glycine site of the N-methyl-D-aspartate (NMDA) receptor. By using intracellular recording, we have investigated the effect of the glycine transporter-1 (GlyT-1) inhibitor N [3-(4'-fluorophenyl)-3-(4'phenylphenylphenoxy) propyl] sarcosine (NFPS) on NMDA-induced currents in pyramidal cells of the medial prefrontal cortex (mPFC), both when given alone and in combination with either risperidone or clozapine. Both risperidone and clozapine enhanced the NMDA-induced currents. The concentration-response curves were biphasic, and the maximal effect of clozapine on the NMDA-induced currents was significantly larger than the maximal effect of risperidone. NFPS also significantly potentiated the NMDA-induced currents, when given alone. Moreover, NFPS (1 microM) augmented the effect of both the maximal (20 nM), and a submaximal (10 nM), concentration of risperidone. In contrast, NFPS did not potentiate either the effect of the maximal (100 nM) or a submaximal (80 nM) concentration of clozapine on the NMDA-induced currents. These data may explain the beneficial clinical results of using glycine reuptake antagonists as adjuvant treatment to risperidone. Our findings also suggest that risperidone and clozapine may affect NMDA receptor-mediated neurotransmission differently in the mPFC.

Combined alpha2 and D2/3 receptor blockade enhances cortical glutamatergic transmission and reverses cognitive impairment in the rat.

The alpha(2) adrenoceptor antagonist idazoxan enhances antipsychotic efficacy of classical dopamine D(2) antagonists in treatment-resistant schizophrenia. The mechanisms are not fully understood, but we have previously shown that the combination of idazoxan with the D(2/3) receptor antagonist raclopride, similarly to clozapine but not classical antipsychotic drugs, augments dopamine efflux in the prefrontal cortex, and also generates an enhanced suppression of the conditioned avoidance response. We have now investigated the effects of clozapine, raclopride, idazoxan and the combination of raclopride and idazoxan on (i) electrically evoked excitatory post-synaptic potentials and currents in pyramidal cells of the rat medial prefrontal cortex, using intracellular electrophysiological recording in vitro, (ii) the impaired cognitive function induced by the selective N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, using the 8-arm radial maze test, (iii) the in-vivo D2, alpha(2A) and alpha(2C) receptor occupancies of these pharmacological treatments, using ex-vivo autoradiography. Whereas neither idazoxan nor raclopride alone had any effect, the combination exerted the same facilitation of glutamatergic transmission in rat prefrontal pyramidal neurons as clozapine, and this effect was found to be mediated by dopamine acting at D(1) receptors. Similarly to clozapine, the combination of idazoxan and raclopride also completely reversed the working-memory impairment in rats induced by MK-801. Moreover, these effects of the two treatment regimes were obtained at similar occupancies at D(2), alpha(2A) and alpha(2C) receptors respectively. Our results provide novel neurobiological and behavioural support for a pro-cognitive effect of adjunctive use of idazoxan with antipsychotic drugs that lack appreciable alpha(2) adrenoceptor-blocking properties, and define presynaptic alpha(2) adrenoceptors as major targets in antipsychotic drug development.

Differential effects of atypical and typical antipsychotic drugs on N-methyl-D-aspartate- and electrically evoked responses in the pyramidal cells of the rat medial prefrontal cortex.

In the present study, we have demonstrated that atypical antipsychotic drugs (APDs, e.g., clozapine, olanzapine, risperidone, and quetiapine) and atypical APD candidates (e.g., M100907 and Y-931) share a common property in facilitating responses evoked by electrical stimulation of the forceps minor and by N-methyl-D-aspartate (NMDA), but not (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), in pyramidal cells of the medial prefrontal cortex (mPFC). The concentrations of these drugs to exert their action are in a clinically relevant range. Although haloperidol has shown a considerably smaller potentiation of NMDA-evoked current at 50 and 100 nM, it consistently depressed the AMPA-induced current. Chlorpromazine and loxapine failed to modulate significantly NMDA- or AMPA-induced current in the pyramidal cells. Moreover, haloperidol and loxapine demonstrated depression of excitatory postsynaptic currents, whereas chlorpromazine did not show any effect. These findings combined indicate that atypical, but not typical, APDs augment glutamatergic neurotransmission in pyramidal cells of the mPFC. We propose that the beneficial effect of atypical APDs in cognitive dysfunction and negative symptoms in schizophrenia is due to their ability to enhance glutamatergic neurotransmission in the PFC and functionally related limbic structures. Our results further suggest the possible use of glutamatergic neurotransmission in the mPFC as a model for screening and studying the action of potential atypical APDs.

Calcium/calmodulin-dependent kinase II is involved in the facilitating effect of clozapine on NMDA- and electrically evoked responses in the medial prefrontal cortical pyramidal cells.

Using the method of intracellular recording in in vitro brain slices, we investigated whether calcium/calmodulin-dependent kinase II (CaMKII) is involved in the facilitating action produced by the atypical antipsychotic drug (APD) clozapine on N-methyl-D-aspartate (NMDA)-induced inward currents and electrically evoked excitatory postsynaptic currents (EPSCs) in pyramidal cells of the medial prefrontal cortex (mPFC). The CaMKII inhibitor, KN-93 (N-[2-(N-(4-Chlorocinnamyl)-N-methylaminomethyl)phenyl]-N-[2-hydroxyethyl]-4-methoxybenzenesulfonamide), but not the inactive isomer, KN-92 (2-[N-(4-Methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine, phosphate), blocked clozapine's augmenting effect on NMDA-evoked responses in pyramidal cells of the rat mPFC. KN-93 also inhibited the facilitatory effect of clozapine on electrically evoked responses in the pyramidal cells, while KN-92 did not show any effect. Similarly, the calmodulin antagonist W-7 (N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide) inhibited the augmenting effect of clozapine on NMDA- and electrically evoked responses in the pyramidal cells. To further test the role of CaMKII in mediating the augmenting action of clozapine, we performed experiments in alpha-CaMKII mutant and wild-type mice. In contrast to results in pyramidal cells from rats or wild-type mice, clozapine was not able to potentiate NMDA-induced currents in the mPFC pyramidal cells from the CaMKII mutant mouse. Both KN-93 and W-7, but not KN-92, inhibited the augmenting action of clozapine in the pyramidal cells of wild-type mice. Taken together, these results suggest that the facilitating action of clozapine on the NMDA- and electrically evoked responses in pyramidal cells of the mPFC requires activation of CaMKII enzyme.