Suppressive effect of clozapine but not haloperidol on the increases of neuropeptide-degrading enzymes and glial cells in MK-801-treated rat brain regions
Introduction
Noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonists, including ketamine, phencyclidine, and dizocilpine (MK-801) produce cognitive and perceptual disturbances characteristic of schizophrenia-like psychosis (Javitt and Zukin, 1991, Malhotra et al., 1996, Andine et al., 1999). Besides psychotic symptomatology, MK-801 injures neurons, activates astrocytes and microglia, and up-regulates heat shock protein expression, immediately early genes and cathepsin D activity in posterior cingulate/retrosplenial cortices (PC/RSC) (Gass et al., 1993, Olney and Farber, 1995, Hetman et al., 1997, Fujimura et al., 2000). Increasing evidence indicates that NMDA antagonist-evoked psychosis and neuropathological features are prevented by atypical antipsychotic drugs such as clozapine but not typical antipsychotic drugs such as haloperidol (Bakshi et al., 1994, Farber et al., 1996, Fujimura et al., 2000, Gaisler-Salomon and Weiner, 2003). However, the mechanisms underlying the protective efficacy of clozapine as compared with haloperidol remain unclear.
Alterations of protease activities and proteolytic reactions often play critical roles in both physiological and pathological circumstances. Among peptidases, prolyl oligopeptidase (POP) (post-proline cleaving enzyme, prolyl endopeptidase; EC 3.4.21.26) and endopeptidase 24.15 (EP 24.15) (thimet oligopeptidase, PZ-peptidase; EC 3.4.24.15) are widely distributed in the central and peripheral tissues, and are thought to be involved in the metabolism of many physiologically active peptides such as substance P (SP), somatostatin, neurotensin, and bradykinin important for learning and memory (Kato et al., 1980, Toide et al., 1995, Shrimpton et al., 1997, Waters and Davis, 1997). Our recent studies have shown that systemic treatment with MK-801 increases the neuropeptidases and lysosomal enzyme in rat brain regions in a dose-dependent manner and the activation of glial proteins might be related with the pathogenesis of MK-801-induced neurotoxicity (Ahmed et al., 2003). Consistent with our recent findings (Ahmed et al., 2003), the altered levels of neuropeptides in the cortical regions of post-mortem brain tissues from schizophrenic patients have been reported (Iritani, 2001, De Wied and Sigling, 2002) suggesting the possible regulatory role of neuropeptide-degrading enzymes in disease progression of schizophrenia. Since many of the neuropeptide-degrading enzymes are derived from both astrocytes and microglia in the brain (Koshiya et al., 1984, Koshiya et al., 1985, Bayer et al., 1999, Ahmed et al., 2003), glial cells might play an important role in peptidergic transmission as well (Ahmed et al., 2003, Yamamoto et al., 2003). The correlation between the changes of neuropeptide-degrading enzyme levels and glial cell activation in schizophrenia is not well understood. In the present study, therefore, we confirmed our recent neuropeptidase data (Ahmed et al., 2003) and further examined the suppressive effect of clozapine versus haloperidol on the activation of glial cells and the changes in the level of neuropeptide-degrading enzymes in the brain regions of MK-801-treated schizophrenia-like model rat.
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Animals and drug administration
Female Wistar rats (10–12 weeks) (Sankyo Laboratory, Tokyo, Japan) were housed under standard environmental conditions (12 h light/dark cycle, 22 °C) and allowed free access to food and water for two weeks prior to the treatment. Adult female rats were used in all experiments because they are more sensitive than male to the neurotoxic effects induced by MK-801 (Andine et al., 1999). Moreover, MK-801-induced neurotoxic effects are more reproducible in female compared to male rodents (Auer, 1996).
Effects of clozapine and haloperidol on MK-801-induced changes of neuropeptide-degrading enzymes
We determined POP and EP 24.15/16 activities in the PC/RSC, hippocampus, frontal cortex, and striatum after drug treatments (Fig. 1, Fig. 2). Both POP (38%) (Fig. 1A) and EP 24.15/16 (22%) (Fig. 2A) significantly increased in the PC/RSC three days after the treatment with MK-801, whereas no change was detected in other brain regions examined (Figs. 1B–D and 2B–D). Pretreatment with clozapine significantly attenuated the increase of the enzyme activities, whereas haloperidol did not show any
Discussion
In the present study, we showed that systemic treatment with MK-801, a noncompetitive NMDA receptor antagonist, increased neuropeptide-degrading enzyme activities in rat PC/RSC region, which were accompanied with the activation of glial cells, and pretreatment with clozapine suppressed both the activation of glial cells and the increase of the neuropeptide-degrading enzyme activities.
In the current investigation, we found a significant increase of POP activity in the PC/RSC following MK-801
Acknowledgements
We thank Novartis Pharma Ltd. for donating clozapine, and thank professor Michiyuki Yamada of Yokohama City University for the helpful discussion.
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