Summary, conclusions and implications for further research

4. Discussion, summary and conclusions 143

4. Discussion, summary and conclusions 144

the drug, but no marked differences in PEX14 or CAT expression 1 h after treatment. There was, however, an increase in D-AspOx expression, thereby showing in vivo that MK-801-induced hyperdopaminergia may in turn lead to NMDAR-hypofunction.

Continued treatment with MK-801 over 25, 49 and 73 hrs showed differential effects on the expression on genes coding for dopamine degrading enzymes. A slight increase in gene expression was observed after 49 hrs in MAOA and COMT, while MAOB-levels continuously decreased. This could be interpreted in such a fashion that the increased levels of dopamine can still be degraded by the existing amount of enzymes, whereby increases in enzyme activities are a probable mechanism herefore. Both RT-PCRs and immunolabelings showed no conclusive evidence of peroxisomal proliferation within the aforementioned time frame, although levels of expression and protein abundance for the two antioxidant enzymes catalase and SOD2 increased in the MK-801-treated animals. It can therefore be concluded that antioxidant pathways of the brain are still active and can be adapted to increased ROS production caused by dopamine neurotoxicity, thereby questioning the proposition that oxidative stress is a primary cause of schizophrenia.

In conclusion the preliminary results of the exploratory experiments performed within this thesis suggest that that schizophrenia is a not disorder caused by oxidative stress, a condition in which antioxidant defense mechanisms are relatively hypofunctional, that dopamine hyperfunction and glutamate hypofunction are not separate entities, but may induce and increase each other in vivo and that dopamine induced neurotoxicity plays an important role in the upkeep and exacerbation of the core pathogenic mechanism of schizophrenia on the one hand and on the other via the induction of ROS-production leads to an increase in atypical neurodegeneration which could play an important role in the development and persistence of cognitive symptoms in schizophrenic patients. The results lead to the postulation of an integrative model regarding the etiopathogenesis and upkeep of schizophrenia based on dopamine neurotoxicity and dopamine-glutamate-interactions.

Fig. 4_2: Schematic illustration of the proposed integrative “mesolimbic bottleneck model” of schizophrenia

4. Discussion, summary and conclusions 145

4.3.1 Implications for further research

In order to further examine the verisimilitude of the aforementioned hypothesis (q.v.

Fig. 4_2) the results from this thesis need to be replicated in larger samples. Furthermore the effects of systemic treatment with MK-801 need to be evaluated over longer time spans in order to differentiate between acute and chronic effects of increased activity of the mesolimbic dopamine system. Especially regarding findings of decreased antioxidant defense in patients suffering from schizophrenia, it would be relevant to ascertain if and when the antioxidant defense systems of the brain (which appear to be functional during acute and short-term treatment with MK-801) become hypofunctional when confronted with chronically elevated dopamine levels. In this respect there is also a strong necessity for research into the confounding effects of neuroleptic treatment, since results from this thesis show that haloperidol has a markedly higher potential for (oxidative) damage than dopamine (q.v. section 3.2.2). It is therefore likely that oxidative stress as found in patients is partly caused or at the least exacerbated through treatment with antipsychotic medication, especially typical neuroleptics like haloperidol.

The possibility of palliative treatment of schizophrenia with antioxidant medication is therefore another question worth examining based on the findings of this thesis. The animal research application for this thesis, approved by the regional board for animal protection, incorporates treatment of animals for up to 14 days as well as a second set of experiments in which animals are to be treated with a peroxisome proliferator (rosiglitazone) prior to injections with MK-801. It is therefore planned to expand upon the preliminary research as laid out in this thesis in order to answer some of the aforementioned questions regarding both acute vs. chronic MK-801-treatment as well as palliative treatment with drugs augmenting antioxidant capacity.

Additionally a number of targeted experiments need to be performed using quantitative methods like real-time RT-PCR (qRT-PCR), microarrays, ELISA (enzyme-linked immunosorbent assay) and enzyme-activity assays in larger samples in vivo, in animals and in human patients.

Finally it would be of great interest to examine possible effects of altered dopamine transmission in non-clinical samples. Since, on the genotype level, polymorphisms of candidate genes for schizophrenia that were also analyzed within this thesis, like DAAO (Stefanis et al., 2007) or COMT (Avramopoulos et al., 2002), have also been shown to be

4. Discussion, summary and conclusions 146

associated with the trait of schizotypy, examinations of the genes’ expression patterns could show correlations with reported schizotypy levels, especially when other factors, like self-reported life-events or gene methylation patterns would be taken into account additionally.

Since Stefanis et al. (2004) report that variations in the COMT Val¹⁵⁸Met polymorphism are associated with schizotypy, but not with cognition, the question arises, whether or not atypical neurodegeneration caused by dopamine neurotoxicity might play a role in the transition from subclinical schizotypy to clinical schizophrenia. There is an ongoing dispute on the question, if schizotypy and schizophrenia are represented by the same continuum or if there is a clear dichotomy between the two concepts (conference statement by Gordon Claridge, Oxford). It would therefore be interesting to examine the levels of antioxidant defense, neurotoxicity and atypical neurodegeneration in schizotypy in order to establish, whether these are also continuously distributed and correlated with schizoptypy or whether they make the difference between schizoptypy and schizophrenia in a sense that atypical neurodegeneration only presents in clinical schizophrenia, but is absent in persons with high schizotypy. This would mean that persons high in schizotypy may be prone to develop schizophrenia (e.g. through the experience of negative life-events), but that the latter condition would then involve additional processes leading to manifest damage to the brain.

Should this be the case, the question would arise, what the nature of this discontinuity between schizotypy and clinical schizophrenia on the neurophysiological, neurochemical and neuropathological level would be. It is, however, in the opinion of the author more likely that schizotypy and schizophrenia are presented both phenotypically as well as a neuropsychologically on the same continuum, wherefore the research into the neurophysiological and -chemical correlates of schizoptypy could answer many of the open questions in schizophrenia research, but without the additional confounding influences of many of the variables (e.g. antipsychotic medication) commonly associated with clinical schizophrenia.

References 147