Young schizophrenia patients with minimal previous antipsychotic exposure had reduced NAA and increased ratio of Gln/Glu in the AC, but not in the adjacent frontal white matter or thalamus. In the patient group, these metabolic abnormalities were inversely correlated to each other. Group differences in NAA and Gln/Glu were not related to the subject’s socioeconomic or educational levels, or to baseline symptom ratings in the patients.
Few other 1
H-MRS studies have measured Glu, Gln or both combined, in schizophrenia/healthy volunteer comparisons. These were mainly cross-sectional in design and used the single-voxel 1
H-MRS at high field strengths and short TE. Our study is most comparable with the investigation by Theberge et al
because we implemented their STEAM single-voxel sequence in a similar Varian 4 T scanner. They found increased Gln and normal NAA in the AC and thalamus in antipsychotic-naive schizophrenia (mean age = 21 years; mean illness duration = 20 months). An important difference is that we examined a larger AC voxel (8 cm3
compared with 1.5 cm3
; Theberge et al
) that encompassed bilateral AC; this could have resulted in higher signal to noise, and better sensitivity to detect NAA reductions. In addition, our patients were somewhat older (mean age = 27) and perhaps glutamate-related NAA reductions had progressed further. Finally, most of our patients had some, although minimal, antipsychotic exposure. However, several studies in animals have not found effects of antipsychotics on NAA10,21,22
or on Glu or Gln10
in these regions.
We failed to document AC Gln elevations but found increased Gln/Glu. This is consistent with a recent investigation of spinal cerebrospinal fluid in drugnaïve schizophrenia patients.9
In this study Glu and Gln, measured with high-performance liquid chromatography, were normal (Gln was 468.6 ± 146 µm
in patients vs 405.6 ± 108 µm
in controls; Glu was 4.5 ± 1.77 µm
in patients vs 4.73 ± 1.29 mm
in controls). However, the Gln/Glu ratio was significantly higher in the patient group. Interestingly, in our study of NMDAR blockade with ketamine in healthy volunteers, in addition to the reported elevation of Gln,6
we also found marginally increased Gln/Glu in AC (8-cm3
voxel) using the same technique as in the current study (ratios mean increase = 0.12, s.d. = 0.19; t
(8) = 1.83, P
= 0.1). Gln is the principal metabolite of synaptic Glu23
and increased turnover of glutamatergic neurotransmission could potentially result in a greater shift toward Gln with corresponding Glu reduction. Consistently, somatosensory activation in rats resulted in increased Gln and reduced Glu assessed with 1
H-MRS at 11.7T, suggestive of ‘…augmented Glu release from glutamatergic neurons and subsequent uptake by high-affinity Glu transporters on the surrounding glial cells and conversion of Glu into Gln by Gln synthetase.’24
Hence, in clinical 1
H-MRS studies, Gln/Glu ratio may be a more sensitive measure of glutamatergic release than Gln or Glu concentrations. However, 1
H-MRS only provides static metabolite measures. Future studies of glutamate–glutamine cycle in schizophrenia with dynamic 13
C-MRS would be informative.
A few other studies have examined glutamatergic metabolism and NAA in different disease stages in schizophrenia. Tibbo et al
at 3T (TE = 20), found increased Glu plus Gln and normal NAA in AC in adolescents at risk for schizophrenia (age = 16). Tebartz et al
at 2T (TE = 30), found increased Glu and normal NAA in a dorsolateral prefrontal and medial temporal voxels in acutely ill, medicated, schizophrenia subjects (age = 28 years; illness duration = 5.3 years). In chronically ill patients (age = 42 years; length of illness = 15 years), Theberge et al
found elevated Gln in the thalamus but Glu plus Gln reduction in AC; NAA was normal in both regions. Finally, Chang et al
studied bilateral white matter prefrontal, temporal and occipital regions (total of 6 voxels) at 4T (TE = 30ms) in older schizophrenia (age = 66 years; illness duration = 43 years). They found elevated Glu plus Gln in bilateral prefrontal and left occipital white matter and reduced NAA in bilateral prefrontal and temporal regions. Hence, most of these short TE studies report elevations in glutamatergic indices with normal NAA. Inconsistencies across this literature are likely due to differences in populations, regions of interest studied, as well as spectroscopic technique.
In contrast to the few investigations of glutamate, many studies have measured NAA in schizophrenia/healthy control comparisons, mostly in large single voxels at 1.5 T and longer TEs. These were recently summarized in a meta-analysis (64 studies: 1256 patients, 1209 healthy controls; Steen et al
). Important findings included consistent NAA reductions in schizophrenia in combined gray and white matter tissue in prefrontal and medial temporal regions, but also found in the AC, thalamus and cerebellum. There was no evidence of lower NAA in chronic compared with early schizophrenia, but the great majority of studies included chronically ill patients (only four ‘first episode’ schizophrenia studies are included in this analysis). Our results are consistent with this broad literature and suggest that at short TE (20 ms), AC NAA reductions are apparent early in the illness.
We are aware of only four other longitudinal early schizophrenia studies that assessed metabolic changes in the context of antipsychotic drug exposure. Three of these were at 1.5 T and focused on NAA. Choe et al
found low frontal NAA/Cre at baseline with no further reductions after treatment with typical and atypical agents (follow-up 1–6 months). Fannon et al
reported reduced medial temporal NAA/Cre at baseline, which was no longer statistically different from healthy subjects after 3 months of atypical antipsychotic treatment. Although our preliminary report suggested progressive frontal reductions,30
follow-up of the whole sample did not detect NAA reductions in patients randomized to haloperidol or quetiapine for a mean follow-up of 9 months.31
Finally, Theberge et al
detected reductions in thalamic glutamine after 30 months of treatment, but no changes in NAA. Hence, the absence of NAA changes with treatment in this study is generally consistent with this literature. However, longer follow-up periods may be necessary to detect changes in other metabolites like glutamine.
Reduced NAA may result from actual loss of neurons or reductions in their size (soma and/or processes) relative to non-neuronal tissue. The postmortem literature does not support classic neurodegenerative changes in schizophrenia with neuronal loss or gliosis.33
However disease-related reductions in synaptic spines34
could result in decreased NAA. A recent study in a model of early human immunodeficiency virus brain infection in monkeys documented that 1
H-MRS-measured NAA decrements were related to reductions in synaptophysin, a marker of synaptic integrity.35
Furthermore, subchronic NMDAR blockade with phencyclidine in rats resulted in a 41% reduction in prefrontal spine synapses36
and of temporal NAA.4
Finally, increased mRNA expression of the astrocytic glutamate transporter37
and elevations of phosphate-activated glutaminase, a neuronal enzyme that converts glutamine to glutamate,38
were both detected in the prefrontal cortex of schizophrenia subjects, consistent with increased turnover of synaptic glutamate. Hence, our findings of increased Gln/Glu correlated with lower NAA early in the illness, and are consistent with a process of glutamate-related dendritic toxicity as suggested by the NMDAR hypofunction model of schizophrenia.
Some study limitations should be considered. First, sample size was small and hence replication of the AC findings in larger groups is necessary. In addition, it is possible that our negative findings for frontal white matter and thalamus represent type-II statistical errors. Likewise, the lack of neurometabolic change with follow-up should be interpreted cautiously because of low power. Second, spatial coverage was restricted to only three regions, thus neurometabolic changes in other locations may have been missed. Proton echo-planar spectroscopic imaging techniques at 4T and short TE with broader spatial coverage are being implemented by our group. Third, at the TE of 20 ms, significant contributions from various macro-molecule (MM) resonances will be present, which may be coupled to each other. We did not measure MM resonances in patients or controls but estimated them using common prior knowledge in our fitting model in which only the amplitude is allowed to vary independently. If MM shapes and contributions (and/or their relationships) differ in schizophrenia from control subjects, this could potentially affect our Gln, Glu and NAA findings. Fourth, the voxels for the thalamus and frontal white matter were smaller than the AC voxel, and consequently had lower signal-to-noise ratio (SNR). This reduced SNR might contribute to reduced reliability of our measures, leading us to miss possible differences in these regions due to type-II error. Fifth, most patients had been treated with antipsychotic medications at baseline. Rodent studies suggest that even acute antipsychotic exposure can modulate glutamatergic response to NMDA blockade.39
Hence, our baseline Gln/Glu findings could still be the results of antipsychotic medication. Sixth, we segmented cerebrospinal fluid and partial-volume corrected the metabolite concentrations, but were unable to segment the gray and white matter tissue in each voxel. As NAA concentration is slightly higher in gray than in white matter,40
differences in voxel tissue proportion between the groups could potentially account for the reduced NAA in the AC in the schizophrenia group. Finally, our use of endogenous water as a concentration reference might be problematic if there were substantial alterations in water MR visibility, perhaps due to water dysregulation. However, this would have likely lead to group differences in the same direction across all metabolites studied. Pfefferbaum et al
reported prolonged water T2 in schizophrenia. We minimized this possible effect by using a short echo time of 20ms so that any associated alteration of T2 is unlikely to have a substantial effect. Similarly, our use of a 2000-ms recycle time results in minimal saturation effects. Still, differences in water T1 relaxation in voxels with larger water content (such as the AC in the schizophrenia group) could also confound metabolite concentrations. However, analyses of the metabolite data as Cre ratios produced the same baseline results (see Supplementary data
). Future studies measuring T1 and T2 in addition to neurometabolites would be challenging but potentially informative.
In summary, these results suggest that NAA reductions and correlated elevations of Gln/Glu in the AC are present early in schizophrenia. The neurometabolic findings are consistent with reductions in neuropil secondary to glutamate-mediated toxicity as predicted by the NMDAR hypofunction model.1
Direct evidence of such a process supports the study of ‘neuroprotective’ glutamate modulating agents, which may prevent cognitive dysfunction and the poor functional outcomes that are still the norm in schizophrenia.