We report a significant reduction in the expression of BDNF, trkB-TK+ and GAD67 mRNA in several temporal regions of individuals with schizophrenia and mood disorders. The most striking reductions in mRNA were found in the DG, CA4 subregions of the hippocampus and in layer II of the EC. Brain-derived neurotrophic factor mRNA was significantly reduced in CA4 (hilar region) in the bipolar disorder group, and both the major depression and bipolar disorder groups had reduced levels of trkB-TK+ mRNA in layer II of the EC. Reductions in trkB-TK+ mRNA expression were also observed in CA4 in the schizophrenia group. The schizophrenia and major depression groups also showed reductions in GAD67 mRNA expression in CA4. We found that GAD67 mRNA expression was also lower in the DG in the schizophrenia group. Thus, in the hippocampus, the abnormalities in schizophrenia occur mainly in trkB-TK+ and GAD67 mRNAs and to a lesser extent in BDNF mRNA, whereas in bipolar disorder the abnormalities are most prominent in BDNF and trkB-TK+ mRNAs. These 2 psychotic disorders appear to share some neurotrophin pathology, especially in CA4, yet they also have a somewhat different pattern of abnormalities with imprecise overlap.
The results from this study corroborate those of earlier studies that describe abnormalities in the expression of cortical BDNF and trkB-TK+ mRNA in individuals with schizophrenia while also extending these findings to a third independent cohort and additional brain regions.20,21
Studies of BDNF protein levels in the hippocampus of patients with schizophrenia have yielded inconsistent results;35
however, a recent study using this same cohort reported no abnormalities in proBDNF protein levels in the hippocampus of the schizophrenia group.36,37
Likewise, we find only a limited decrease in BDNF mRNA, restricted to CA4 in the hippocampus in the schizophrenia group. Whereas there was a reduction in BDNF mRNA in CA4 in the schizophrenia group, it did not reach a level of significance after Bonferroni testing; however, the level of BDNF in CA4 in the schizophrenia group was not significantly different from that in the bipolar disorder group, suggesting that the BDNF mRNA reduction is similar in these 2 disorders (). Thus, the failure to detect a significant change in schizophrenia likely reflects the increased variability in BDNF expression in the schizophrenia group, the relatively low number of individuals with schizophrenia and the conservative statistical significance criteria rather than a true biological difference. Nevertheless, the abnormalities in BDNF mRNA and protein expression36
tend to be more pronounced in bipolar disorder. In contrast, we found significant deficits in trkB-TK+ mRNA levels in the hippocampus of the schizophrenia group, but the protein levels for trkB-TK+ did not appear to decline in schizophrenia.36
Reductions in trkB-TK+ mRNA levels were also observed within layer II of the EC in individuals with bipolar disorder and major depression but not schizophrenia. Layer II consists primarily of densely packed stellate, interneuronal and medium–large pyramidal cells,38
which project to the dentate gyrus and CA3. Qualitative microscopic analyses of morphologic abnormalities as well as quantitative measurements of neuronal density revealed no significant differences between the schizophrenia and control groups,39,40
which supports our result showing no significant differences in BDNF, trkB-TK+ or GAD67
mRNA expression between individuals with schizophrenia and unaffected controls in this region. However, a study by Pantazopoulos and colleagues41
reported a decrease in parvalbumin-immunoreactive neurons within the superficial layers of the EC in individuals with bipolar disorder but not those with schizophrenia. This calcium-binding protein delineates inhibitory interneurons within the cortex and has revealed GABAergic abnormalities in the dorso-lateral prefrontal cortex (DLPFC) and hippocampus in the major psychiatric disorders.18,42,43
Since trkB-TK+ is also associated with inhibitory neurons, the expressional deficits of trkB-TK+ mRNA in layer II could be a result of a loss of GABAergic neuronal density within this superficial layer of the EC in individuals with bipolar disorder or depression. However, additional studies will be necessary to directly correlate the parvalbumin deficit with trkB-TK+ expression.
Numerous studies have shown that BDNF and/or trkB-TK+ signalling may also be associated with depressive behaviour. Levels of BDNF and trkB-TK+ mRNA were not changed in the hippocampus of the major depression group, although there was a decrease in trkB-TK+ mRNA levels in layer II of the EC in individuals with depression. However, chronic treatment with antidepressants increases the expression of BDNF and trkB-TK+ in the hippocampus of the rat brain, and hippocampal BDNF immunoreactivity is elevated in clinically depressed patients treated with antidepressants.44
Moreover, a recent study has shown that a selective deletion of BDNF within the DG in mice results in a reduction of anti-depressant efficacy, suggesting that BDNF is essential for mediating the beneficial effects of antidepressants.45
In conjunction with these findings, we also found that compared with patients taking antidepressants at the time of death, patients free of antidepressants at the time of death expressed significantly less BDNF and trkB-TK+ mRNA in the DG and CA4 regions, where this antidepressant effect has also been found.44
This indicates that antidepressants were more likely to obscure abnormal BDNF/trkB-TK+ mRNA expression than to cause the reduction observed in this study.
Brain-derived neurotrophic factor trkB-TK+ signalling appears to regulate both excitatory and inhibitory impulses within the hippocampal formation.23,25
Multiple studies have shown that BDNF and trkB-TK+ mRNA expression levels are increased by glutamate activity,23,46–49
and that trkB-TK+ is located, although not exclusively, in GABAergic neurons,23,50
which contain the GABA-synthesizing enzyme GAD67
. This suggests that BDNF/trkB-TK+ signalling participates in regulating the balance between excitation and inhibition within the hippocampus. We found that GAD67
mRNA levels were significantly reduced in the DG in the schizophrenia group and in CA4 in the schizophrenia and major depression groups. These reductions in GAD67
expression may have implications for neuroimaging studies that show an increase in hippocampal activity during the retrieval of words,10,51
in individuals with schizophrenia. Together these results indicate that the abnormal activity seen in individuals with schizophrenia may be due to a decrease in the inhibitory tone mandatory for normal hippocampal function. Several studies have shown that there is a reduction in the density of at least a subset of the hippocampal GABAergic interneurons,17,18,56
as well as a reduction in GAD67
mRNA in the hippocampus.57,58
Thus the decrease of GAD67
mRNA observed in this study could also be due to a reduction in the density of GABAergic neurons.
Previous studies have indicated that abnormal signalling in the BDNF/trkB-TK+ pathway may lead to abnormalities in the GABAergic neurons.20,23–26
mRNA levels were correlated with both BDNF and trkB-TK+ in almost all hippocampal areas examined, the correlations were much stronger with trkB-TK+. However, given the abnormal trkB-TK+ expression, for example, in layer II of the EC in the bipolar disorder and major depression groups, one may expect to see abnormalities of GAD67
mRNA expression in the EC or downstream in the DG or CA3 in the bipolar disorder and depression groups. In contrast to what was expected, there were no GAD67
mRNA abnormalities in the EC, and the GAD67
mRNA abnormalities in the DG actually occured in the schizophrenia group. Thus, although there may be an underlying connection between the reductions in GAD67
expression and BDNF/trkB-TK+ signalling in the hippocampus of people with mental illness, the relation is complex and cannot be definitively resolved with the current data.
The limitations encountered in this study are those that are common to all human postmortem brain research and require accounting for various demographic and clinical variables that may influence molecular preservation in the tissue. We covaried for each variable that significantly correlated with BDNF, trkB-TK+ or GAD67 mRNA expression levels in the analysis. In addition, there are limitations to film-based densitometric measurement, which does not allow us to determine if a decrease in mRNA signal in a given area is due to less mRNA per cell, fewer expressing cells or fewer cells altogether. Nevertheless, film-based densitometric measurement is well established as an acceptable and well validated method in postmortem neuropathology research. Film-based quantification is an efficient way to assess a large number of samples. Thus, we were able to survey a larger surface area of anatomically defined tissue in a larger number of sections (n = 120 in this cohort) than would have been possible at the cellular level. In addition, to account for possible limitations due to the sensitivity of the film-based method of quantification, we used standards to calibrate the radioactivity, which is a widely accepted practice.