Elsevier

Schizophrenia Research

Volume 97, Issues 1–3, December 2007, Pages 254-263
Schizophrenia Research

Alterations of hippocampal and prefrontal GABAergic interneurons in an animal model of psychosis induced by NMDA receptor antagonism

https://doi.org/10.1016/j.schres.2007.05.005Get rights and content

Abstract

Some behavioral symptoms and neuropathological features of schizophrenia, like alterations of local GABAergic interneurons, could be emulated in an animal model of psychosis based on prolonged low-dose exposure to N-methyl-d-aspartate (NMDA) receptor antagonists, e.g. MK-801. Employing this model, we examined distinct subpopulations of GABAergic interneurons within the hippocampus and prefrontal cortex. Compared to saline control, animals receiving MK-801 exhibited a decreased density of hippocampal parvalbumin-positive interneurons. A co-administration of the antipsychotic drug haloperidol ameliorated this effect of MK-801 on PV+ interneurons in the hippocampus, but led to a marked reduction of PV immunoreactivity in the prefrontal cortex, when comparing with saline, MK-801 or haloperidol treatment alone. Neither calretinin immunoreactivity nor nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase staining, representing neuronal nitric oxide synthase activity mostly detectable in interneurons, was altered by either treatment. With special reference to the hippocampus, these data show that a prolonged application of low-dose NMDA receptor antagonist could, in part, mimic some neuropathologic findings in human schizophrenia, thus strengthening the idea that (sub-) chronic NMDA receptor antagonism in animals is a viable approach in mimicking aspects of schizophrenia. Moreover, this study provides further evidence for regional differences in the response of GABAergic interneurons to NMDA receptor antagonism and antipsychotic treatment.

Introduction

In the mammalian brain, the hippocampus and the prefrontal cortex are implicated in learning and memory formation. Within these regions, local inhibitory interneurons play a crucial role in controlling the temporospatial activity patterns of major excitatory neurons, a function supposed to be of fundamental importance for proper information processing (Benes and Berretta, 2001, Grunze et al., 1996). Gamma-amino-butyric-acid (GABA)-ergic interneurons comprise a variety of different subpopulations that can be identified by their neurochemical profile (DeFelipe, 1997). Interneurons expressing the calcium binding protein parvalbumin (PV) provide a powerful inhibitory influence on excitatory, glutamatergic principal neurons (DeFelipe, 1997, Miles et al., 1996, Cobb et al., 1995). Of note, this distinct subset of interneurons seems to be most markedly affected in schizophrenics. Reduced densities were observed in the hippocampus (Zhang and Reynolds, 2002, Torrey et al., 2005). Conflicting results were obtained in the prefrontal cortex, where some investigators reported lower PV+ cell counts (Beasley and Reynolds, 1997, Reynolds et al., 2001, Reynolds and Beasley, 2001), whereas others noted no change (Tooney and Chahl, 2004, Woo et al., 1997). Another region having received attention in ultrastructural studies was the anteroventral thalamic nucleus, which is functionally connected to the prefrontal cortex by PV+ projecting neurons, which proved to be decreased in schizophrenic subjects (Danos et al., 1998). In contrast to PV+ neurons, interneurons immunoreactive for the calcium binding protein calretinin (CR) seem to be spared from substantial modifications in schizophrenia (Beasley and Reynolds, 1997, Reynolds et al., 2001, Reynolds and Beasley, 2001, Tooney and Chahl, 2004, Zhang and Reynolds, 2002).

A distorted distribution pattern of another subtype of neurons, characterized by its nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase reactivity, was noted within the prefrontal cortex of patients with psychosis (Akbarian et al., 1996). NADPH-diaphorase is a nitric oxide synthase (NOS) capable of generating nitric oxide (NO) (Dawson and Snyder, 1994, Hope and Vincent, 1989, Hope et al., 1991), a diffusible messenger fulfilling various functions within the brain, ranging from regulation of cerebral blood flow to modulation of synaptic plasticity and memory formation (Dawson and Snyder, 1994). However, an excessive production of NO contributes to neurotoxicity mainly through the formation of peroxynitrite when reacting with superoxide anions from different sources. Ample evidence suggests an involvement of the nitric oxide system in schizophrenia (Akbarian et al., 1996, Baba et al., 2004, Xing et al., 2002, Yanik et al., 2003), but its implication in the etiopathogenesis of this condition is still a matter of debate.

Many of these findings, including the altered PV+ and NADPH-d+ neuron densities, could be recently mimicked in animal models of psychosis based on chronic N-methyl-d-aspartate (NMDA) receptor antagonism (Keilhoff et al., 2004a, Rujescu et al., 2006). Uncompetitive NMDA receptor antagonists, such as phencyclidine (PCP), MK-801 or ketamine can induce a transient and reversible psychotic state with the full range of positive and negative psychotic symptoms in healthy volunteers and exacerbate these symptoms in schizophrenic patients (Lahti et al., 2001). In animals, repeated low-dose application of NMDA receptor antagonists evoke several behavioral and neurochemical changes resembling those observed in human psychosis (Rujescu et al., 2006; see Jentsch and Roth, 1999 for review).

In the present study we sought to investigate the influence of chronic low-dose MK-801, which is a more selective and potent uncompetitive NMDA receptor antagonist than PCP or ketamine (Wong et al., 1986), on PV+, CR+ and NADPH-d+ neurons in the hippocampus and prefrontal cortex of rats at the transition from prepuberty to adolescence. This time-frame and treatment strategy was chosen on the basis of preceding studies (own unpublished material; Rujescu et al., 2006) having shown a high susceptibility to the effects of NMDA-R antagonism. Since prolonged antipsychotic treatment might provide a potential confounding factor in human post mortem studies, we evaluated whether repeated exposure to haloperidol alone or in combination with MK-801 modulate the aforementioned neurochemical markers.

Section snippets

Animal treatment

All manipulations were performed in accordance with the current version of the German and US Laws for the Protection of Animals. Maximum care was taken to minimize animal suffering.

48 juvenile, male Long Evans rats (Janvier Breeding Centre, Le Genest Saint Isle, France), aged 35 ± 1 days and weighting 121–148 g at the beginning of treatment, were matched according to their body weight and housed in groups of four in an environmentally controlled facility (T = 23 ± 1 °C, fixed 12/12 h light/dark

Results

Semi-quantitative neuronal counts are shown in Table 1.

Hippocampal PV immunoreactivity

The finding of lower hippocampal PV+ neuron counts in human schizophrenia (Zhang and Reynolds, 2002, Torrey et al., 2005) could be recently reproduced in animal models of psychosis based on chronic NMDA receptor antagonism (Keilhoff et al., 2004a, Rujescu et al., 2006, Schroeder et al., 2000). In line with these reports, MK-801 significantly decreased the PV immunoreactivity within the dentate gyrus and the CA1 region in the current study. This decline was completely abolished in animals

Concluding remarks

Although the alterations induced by repetitive NMDA antagonist applications are far from modeling a multifaceted condition like schizophrenia completely, we were able to model a schizophrenia-related GABA-neuron pathology in animals. The similarity between the results obtained in post mortem studies, specifically on hippocampal pathology, and the present data, supports the contention that repeated administration of NMDA receptor antagonists to rodents might represent a complementary tool in

Role of funding source

No funding was received.

Contributors

All individuals included as authors of the paper have contributed substantially to the scientific process leading up to the writing of the paper. Just Genius was involved in the conception, design of experiments, performance and analysis of experiments, interpretation of the data, writing of the paper. Isabella Braun was involved in the performance of experiments, interpretation of the data and the writing of the paper. Heinz Grunze, Dan Rujescu and Hans-Jürgen Möller provided the critical

Conflict of interest

None declared.

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