Post-stress changes in BDNF and Bcl-2 immunoreactivities in hippocampal neurons: effect of chronic administration of olanzapine

Abstract

In the present study, we used a repeated restraint stress animal model to observe the changes in the expression of brain-derived neurotrophic factor (BDNF) and B cell lymphoma protein-2 (Bcl-2) in hippocampal neurons of rats, monitored the time course of the expression over 3 weeks post-stress period, and examined the effects of the chronic administration of olanzapine on the time course. Olanzapine is an atypical antipsychotic drug that has been shown to be neuroprotective in previous in vitro studies. We found: (1) the repeated restraint stress decreases the levels of expression of BDNF and Bcl-2 in hippocampal neurons; (2) the stress-induced decreases spontaneously recover to their pre-stress levels in 3 weeks after the last stress exposure; (3) administration of olanzapine for 1 week returns the expression of Bcl-2 to its pre-stress level, and the administration for 3 weeks causes an excessive expression of BDNF in hippocampal neurons. In the context of the lower levels of BDNF and Bcl-2, and structural brain abnormalities observed in patients with schizophrenia, our findings suggest that BDNF and Bcl-2 may be involved in the pathophysiology of schizophrenia and in the therapeutic action of atypical antipsychotic drugs.

Introduction

A body of human studies suggests the involvement of hippocampus in the pathophysiology of psychiatric disorders such as schizophrenia and major depression. Patients with major depression have been shown, by magnetic resonance imaging (MRI) technique, to appear reduced hippocampal volume compared with matched normal controls [9], [46], [47], [60]. Histopathological studies of schizophrenic patients have found a variety of qualitative and quantitative hippocampal abnormalities including a decrease in area or whole volume of the hippocampus, a decrease in the number of hippocampal neurons, reduced neuronal size and density, and pyramidal cell disarray [3], [13], [72]. The decrease in hippocampal volume of schizophrenics has been confirmed in MRI studies by independent investigators [20], [25], [43], [67].

Animal studies have shown that the hippocampus undergoes a wide range of cellular changes under stressful conditions. In primates, pronounced neuronal degeneration was observed in the hippocampi of ulcerated vervet monkeys subjected to sustained social stress [55]. In rodents, repeated daily restraint stress for 21 days caused apical dendrites of CA3 pyramidal neurons to atrophy [35], [61], [63]. The same apical dendritic atrophy of hippocampal CA3 pyramidal neurons was also observed in subordinate tree shrews [19], [36]. Another common observation is that psychosocial stress decreases the neurogenesis in the dentate gyrus of developing and adult animals [15], [24], [51]. At the molecular level, stressful stimuli affect the expression of monoaminergic neurotransmitters and neurotrophins such as brain-derived neurotrophic factor (BDNF) and their receptors in the hippocampus [8], [12], [37], [41], [42], [45], [54], [58]. These cellular and molecular changes in response to stress have been explained and discussed in context of the pathophysiology of psychiatric disorders [1], [10], [56].

In the present study, we used a repeated restraint stress animal model to observe the possible changes in the expression of BDNF and Bcl-2 in rat hippocampal neurons, monitored the time course of the expression over 3 weeks post-stress period, and examined the effects of the chronic administration of olanzapine on the time course. Both BDNF and Bcl-2 are neuroprotective proteins; changed levels of their expression have been reported in patients with schizophrenics and major depression [28], [29], [31]. Olanzapine, as an atypical antipsychotic drug, has been used to treat patients with schizophrenia. Similar to other atypical antipsychotic drugs, this drug is more effective than typical antipsychotics in treating refractory schizophrenia [30]. At the same time, it ameliorates some of the negative and positive symptoms of schizophrenia [33]. It can reduce relapse, improve certain cognitive functions, alleviate mood symptoms associated with schizophrenia, and can reduce the likelihood of suicidal behavior [34]. More recently, we demonstrated that atypical antipsychotic drugs clozapine, olanzapine, quetiapine, and risperidone protect PC12 cells from death induced by hydrogen peroxide, serum withdrawal, β-amyloid, or MPP⁺[5], [44], [64], [65]. In in vivo studies, chronic administration of olanzapine up-regulated the expression of BDNF and Bcl-2 mRNA in rat hippocampus [4], [6] and prevented the down-regulation of Bcl-2 produced by a neurotoxic dose of methamphetamine [26]. We hypothesize that repeated restraint stress may change the expression of BDNF and Bcl-2 in the hippocampus and that post-stress administration of olanzapine may affect the post-stress time course of the expression of these two neuroprotective proteins in the hippocampus.