Elsevier

Neuropharmacology

Volume 62, Issue 3, March 2012, Pages 1422-1431
Neuropharmacology

Repeated phencyclidine administration alters glutamate release and decreases GABA markers in the prefrontal cortex of rats

https://doi.org/10.1016/j.neuropharm.2011.01.008Get rights and content

Abstract

Repeated phencyclidine (PCP) administration induces cognitive disruptions resembling those seen in schizophrenia. Alterations in glutamate transmission and γ-aminobutyric acid (GABA) function in the prefrontal cortex (PFC) have been implicated in these PCP-induced deficits, as well as in cognitive symptoms of schizophrenia. PCP-induced cognitive deficits are reversed by chronic treatment with the atypical antipsychotic clozapine in rats. We investigated the effects of a single injection vs. repeated administration of PCP on glutamate levels in the PFC using in vivo microdialysis. Furthermore, we examined how these PCP regimens affect GABA neuronal markers in the PFC. Finally, we investigated the effects of clozapine on disruptions in glutamate levels and GABA neuronal markers induced by repeated PCP administration. Acute PCP administration (2 mg/kg) increased extracellular PFC glutamate; this increase appeared blunted, but was not eliminated, after repeated PCP pretreatment. PCP administration also strongly decreased levels of parvalbumin and glutamic acid decarboxylase-67 (two markers of GABA function) in the PFC, an effect that was maintained after a 10 day drug-free washout period and unaltered by the resumption of repeated PCP injections. All of the observed PCP effects were attenuated by chronic treatment with clozapine, an atypical antipsychotic that has partial effectiveness on cognitive impairment in schizophrenia. These findings suggest that abnormal cortical glutamate transmission, possibly driven by pathological changes in GABA function in parvalbumin-positive fast-spiking interneurons, may underlie some of the cognitive deficits in schizophrenia. A better understanding of glutamate and GABA dysregulation in schizophrenia may uncover new treatment targets for schizophrenia-related cognitive dysfunction.

This article is part of a Special Issue entitled ‘Schizophrenia’.

Highlights

► Repeated PCP administration increased glutamate release in the PFC of rats. ► The repeated PCP-induced increase in PFC glutamate was blunted compared with that seen after a single PCP injection. ► Repeated PCP administration decreased markers of GABA function in the PFC. ► Clozapine attenuated the PCP-induced changes in glutamate and GABA function.

Introduction

Glutamate abnormalities reported in schizophrenia patients (Tamminga, 1998) are proposed to be involved in schizophrenia pathology (Olney and Farber, 1995), including cognitive deficits that are core features of the disorder (Elvevag and Goldberg, 2000). N-methyl-d-aspartate (NMDA) glutamate receptor antagonists induce a psychosis-like state in healthy humans that mimics most major symptoms of schizophrenia (Javitt and Zukin, 1991), including cognitive impairment (Krystal et al., 1994). Acute NMDA receptor antagonist administration increases extracellular glutamate levels in the prefrontal cortex (PFC; Moghaddam et al., 1997), a brain region critically involved in cognition (Fuster, 1973, Robbins, 1996) and implicated in cognitive deficits in schizophrenia (Weinberger et al., 1986). Thus, excessive glutamate transmission, possibly through non-NMDA glutamate receptors, has been hypothesized to underlie schizophrenia pathology, including cognitive deficits (Mathé et al., 1998, Moghaddam et al., 1997).

The increase in PFC glutamate efflux induced by NMDA receptor antagonists may be attributable to blockade of excitatory NMDA receptors located on inhibitory γ-aminobutyric acid (GABA) interneurons, leading to disinhibition of glutamate release from neurons targeted by those interneurons (Adams and Moghaddam, 1998, Farber et al., 1998, Olney and Farber, 1995). NMDA receptor antagonist treatment has been shown to decrease markers of GABA function both in vitro and in vivo (Behrens et al., 2007, Cochran et al., 2003, Kinney et al., 2006, Morrow et al., 2007, Paulson et al., 2003). Two important markers that are reduced after NMDA receptor antagonist exposure are parvalbumin, a calcium binding protein located within a subpopulation of fast-spiking GABAergic interneurons centrally involved in information processing in the brain (Bartos et al., 2007, Cardin et al., 2009, Sohal et al., 2009), and glutamic acid decarboxylase-67 (GAD67), the main isoform of the synthesizing enzyme for GABA in the brain (Asada et al., 1997). The reductions in these two markers occur in the same neuronal population, the parvalbumin-positive fast-spiking inhibitory interneurons. Interestingly, disturbances in GABA systems, including reductions in parvalbumin and GAD67 in the PFC, have also been found in schizophrenia patients (Benes and Berretta, 2001, Guidotti et al., 2000, Hashimoto et al., 2003, Lewis et al., 2004, Olney and Farber, 1995). Disruption of GABA function in parvalbumin-positive fast-spiking interneurons may thus drive the glutamate disinhibition and cognitive impairment induced by NMDA receptor antagonists, as well as glutamate dysregulation and cognitive deficits in schizophrenia.

The schizophrenia-like state evoked by NMDA receptor antagonists in humans is present only during intoxication and dissipates after clearance of the drug from the body (Adler et al., 1998, Cho et al., 2005, Krystal et al., 1994, Krystal et al., 2003, Malhotra et al., 1996, Meltzer et al., 1972, Pradhan, 1984, Rosenbaum et al., 1959). However, a single administration of an NMDA receptor antagonist, such as phencyclidine (PCP), induces severe nonspecific behavioral disruptions (for review, see Amitai and Markou, 2010a, Jentsch and Roth, 1999), which often preclude the precise quantification of cognitive deficits that are likely induced by the first PCP administration. Repeated administration of PCP allows tolerance to develop to the initial nonspecific behavioral disruptions induced by PCP (Melnick et al., 2002). Upon administration of acute re-challenges with PCP, selective cognitive deficits can be observed (Podhorna and Didriksen, 2005). We therefore developed a repeated PCP administration regimen in which two initial PCP injections are followed by two drug-free weeks, after which several additional daily PCP injections are administered. This PCP regimen has been shown to induce robust, significant, and selective cognitive deficits with relevance to schizophrenia in the 5-choice serial reaction time task (Amitai et al., 2007, Amitai and Markou, 2009a, Amitai and Markou, 2009b, Amitai and Markou, 2010b), as well as impulsive-like response disinhibition in the intracranial self-stimulation procedure (Amitai et al., 2009). These PCP-induced deficits were sensitive to partial attenuation by chronic treatment with the atypical antipsychotic clozapine (Amitai et al., 2007, Amitai et al., 2009). By examining the potential of an atypical antipsychotic medication to prevent the disruptive effects of a re-challenge with a psychotomimetic drug, this experimental design parallels the prevention of recurrence of a psychotic episode in schizophrenia by antipsychotic treatment.

Repeated PCP administration alters the effects of PCP on neurotransmitter levels compared with a single acute administration. For example, while a single acute administration of PCP strongly increases dopamine levels in the PFC, repeated PCP administration decreases baseline PFC dopamine levels in the drug-free state and blunts the increase in PFC dopamine release immediately after acute re-challenges with PCP (Jentsch et al., 1998). However, little is known about the effects of repeated PCP exposure on glutamate function. To address this question, we investigated the effects of repeated PCP administration on glutamate in the PFC using in vivo microdialysis combined with the repeated PCP administration regimen that we developed previously (Amitai et al., 2007) and compared the resulting glutamate levels with the effects of a single PCP injection. Furthermore, we treated an independent group of rats with the repeated PCP regimen, harvested their brains at different time points during treatment, and assessed levels of the GABA markers parvalbumin and GAD67 in the PFC using immunohistochemistry. Finally, we assessed the effects of clozapine, an atypical antipsychotic that partially ameliorates cognitive deficits in schizophrenia, on the changes in PFC glutamate efflux and GABA markers induced by repeated PCP administration.

Section snippets

Subjects

Eighty male Wistar rats (Charles River Laboratories, Wilmington, MA) were housed two per cage on a 12 h/12 h reverse light/dark cycle (lights off at 8:00 am). All behavioral testing was conducted during the animals’ dark cycle. Rats were allowed to reach a body weight of at least 300 g before being restricted to 20 g of food per day and before initiating drug treatment. Food restriction was introduced to match as closely as possible the experimental conditions of previous studies in which

Experiment 1: effects of single or repeated PCP administration and chronic clozapine on extracellular glutamate in the PFC

Baseline glutamate levels (Single PCP group: 2.58 ± 0.65 μM; Vehicle/Repeated PCP group: 3.50 ± 0.72 μM; Clozapine/Repeated PCP group: 4.59 ± 0.70 μM; Clozapine/Saline group: 2.92 ± 0.43 μM) did not differ significantly among groups. ANOVA of the effects of PCP/saline administration detected significant main effects of both Time Point (F8,216 = 14.95, p < 0.0001) and Drug Treatment (F3,216 = 10.23, p = 0.0001), as well as a Drug Treatment × Time Point interaction (F24,216 = 6.41, p < 0.0001).

Discussion

Confirming previous studies (Abekawa et al., 2003, Abekawa et al., 2006, Abekawa et al., 2007, Adams and Moghaddam, 1998, Adams and Moghaddam, 2001), acute PCP exposure significantly elevated extracellular glutamate levels in the PFC of rats. This PCP-induced glutamate efflux was observed after a single PCP injection as well as after repeated PCP administration.

Interestingly, the increase in PFC glutamate in response to PCP appeared blunted after repeated PCP exposure compared to that seen

Acknowledgments

The authors would like to thank Mrs. Jessica Benedict for outstanding technical assistance, Mr. Michael Arends for excellent editorial assistance, and Dr. Daniel Hoyer from Novartis Pharma AG for providing us with clozapine.

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