Our data indicated that patients with MDD whose course of illness was characterized by multiple relapses or a chronic evolution presented lower levels of Glx and NAA than healthy controls, especially in the right medial temporal region. When patients with greater illness burden were compared with patients with a more benign course of MDD (i.e., first episode, later onset and shorter duration), differences emerged in both hemispheres for Cho concentrations, whereas patients with treatment-resistant/chronic illness displayed the highest levels. Changes of Glx and Cho peaks correlated in the opposite direction with a longer duration of illness, and higher Cho concentrations were also consistently associated with more previous depressive episodes.
Selected VOIs were situated in a medial temporal region that mainly included the head of the hippocampus. Converging evidence from clinical and preclinical studies underlines the central role of this highly stress-sensitive brain structure on MDD. Volumetric hippocampal reductions have been replicated in depressed patients, and they are preferentially observed in patients with an earlier onset of illness, recurrent evolution or treatment refractoriness,5
as confirmed by our present results.
On the other hand, scant literature on hippocampus neuropathological analyses of patients with MDD reveals increases in the mean densities of neurons and glia contrasting with reductions in neuronal soma size27
and diminished levels of neurotrophic and astrocyte viability markers, which have been putatively related to dysfunctional adult hippocampal neurogenesis.7
An earlier study observed neuronal loss in CA1 and CA4 regions, but the phenomenon, although convincing, was only moderate.6
However, reductions in neuropil seem to be prominent27
and were the most probable explanation for the cumulative hippocampal shrinkage.28
Whatever the deficiencies in the cellular integrity and neuropil, these would be likely to underlie the abnormalities in spectroscopic resonances reported herein. It has to be stressed that metabolic alterations found in our patients were evident even when we controlled for differences in total hippocampal volumes and VOI tissue composition.
Milne and colleagues14
did not find defects in Glx concentration in a sample of patients with recurrent MDD compared with healthy controls. In contrast, Michael and colleagues12
reported lower Glx levels in the hippocampus/amygdala of severely ill patients with treatment-resistant depression compared with healthy controls; the levels recovered after successful electroconvulsive therapy. Diminished levels of Glx were also described in a sample mainly composed of untreated patients with a first depressive episode.13
It has been assumed that Glx signal reflects the status of glia and pyramidal glutamatergic neurons.9
In the present study, the pattern of reduced Glx concentrations seen in the patients with treatment-resistant/chronic and remitted-recurrent depression was not apparent in patients presenting for a first episode, suggesting that long-lasting depressive history may have an important influence on the glial integrity and glutamatergic metabolism of the hippocampus. The presence of diminished Glx levels in patients with remitted-recurrent illness in our sample extends the previously reported findings, emphasizing the possibility that these abnormalities can remain within the hippocampus after resolution of a depressive episode.
A quite similar pattern of lower concentrations in chronic and recurrent forms of MDD was observed for NAA in our sample. N
-acetylaspartate has been described as a sensitive marker of neuronal functionality/viability,10
and defects on that signal could reflect damage or loss of neurons, reductions of interneuronal neuropil, neuronal or axonal metabolic dysfunction or some combination of these processes.29
Reductions of NAA have been observed in patients with bipolar30
and pediatric depression,31
but there is no consistent agreement for altered NAA levels in the hippocampus in previous MRS studies of adult unipolar depression.8
The lack of control for VOI tissue composition in some of these studies might represent a serious limitation, which we attempted to correct herein. At least 2 independent groups reported increases of NAA concentrations after successful response to either electroconvulsive or pharmacological therapy.12,13
These findings suggest that antidepressant treatments could elicit neurotrophic effects, with a positive role in restoring neuronal integrity being therefore relevant for clinical recovery. Strikingly, 2 preclinical reports revealed decreased NAA concentrations associated with reductions of hippocampal volume and neurogenesis in chronically stressed animals, changes that were prevented with antidepressant treatment.32,33
A recent postmortem study in which quantification of metabolites was performed directly on brain tissue reported NAA decrements in different subcortical structures, including the hippocampus and amygdala, in patients with long-term, severe MDD.29
Although in the present study NAA did not correlate with any of the explored clinical variables, reduction of its levels in the right hippocampus was conspicuous, reaching up to 17% and 21% among patients with remitted-recurrent and treatment-resistant/chronic depression, respectively. These findings support the notion that hippocampal neuronal damage could be present in depression when the burden of illness is prominent, even in asymptomatic states.
The increased Cho signal seen bilaterally in patients with treatment-resistant/chronic depression compared with healthy controls and patients with first-episode depression represents one of the most intriguing findings. The Cho resonance mainly reflects changes in the concentrations of phosphocholine and glycerophosphocholine, a precursor and a degradation product, respectively, of membrane phospholipids. For this reason, it is considered a potential biomarker for the status of membrane metabolism.11
Our results are in agreement with those of previous studies that found increased Cho/creatine ratios in a sample of patients with treatment-resistant MDD15
and higher absolute Cho concentrations in patients with extensive past illness (but not in those with a first episode) than age-matched healthy controls.14
We also describe a robust association between higher Cho levels and longer duration of illness and more recurrences. This association provides preliminary evidence that increases in Cho concentrations might reveal an augmented membrane turnover related to past illness burden.
In contrast with the treatment-resistant/chronic group, our sample of patients with a first depressive episode displayed the lowest concentrations of Cho. Block and colleagues13
also reported marginally diminished baseline Cho peaks in untreated patients with a first episode of depression. Interestingly, they reported that low baseline Cho levels and a subsequent increase after 8 weeks of treatment were associated with good response. An earlier study16
had already described lower Cho concentrations that increased after a successful course of electroconvulsive therapy in patients with current, severe depression. Therefore, low Cho levels represent a marker of good response to treatment, but increments of such levels seem to be necessary for recovery from depression. However, in light of our results, excessive levels of Cho would mark resistance to treatment. From a neurobiological perspective, increases of Cho would reflect changes in membrane turnover, precipitated by either antidepressant treatment or endogenous mechanisms, as expression of the brain’s efforts to restore abnormal neural functioning. These changes, however, might end up being insufficient and even pathologically maintained in chronic stages of depression. In fact, it would be possible that increases in Cho peak occur at the expense of membrane precursors (as phosphocholine) during initial stages of treatment and at the expense of degradation products (as glycerophosphocholine) in patients with chronic depression that is unresponsive to treatment, although current MRS techniques do not allow this distinction to be made.
Abnormalities of Glx, NAA and Cho signal described herein within the hippocampus are in accordance with those reported in the vmPFC in a previous study of our group composed of patients in different stages of MDD.18
Alterations in vmPFC cellular neurochemistry were also consistently related to illness-course variables. The well-established structural and functional connection between the hippocampus, parahippocampal gyrus and prefrontal areas — all strongly linked with the physiopathology of mood disorders1
— may explain the similarity of metabolite alterations observed in our 2 studies. In fact, these areas, particularly the hippocampus, are known to play an important role in the response to stress, being extremely sensitive to its potential neurotoxic effects under chronic stress conditions. Glial and neuronal pathology and remodelling processes — as reflected by reductions of Glx and NAA and by increases in Cho, respectively — in both the vmPFC and medial temporal region of patients with MDD would then be considered a phenomenon associated with the burden of the disease.
Although other studies have reflected metabolite changes associated with treatment response or symptom severity, we did not obtain similar results. Our sample included a variety of patients, some of them entirely comparable in terms of age at onset of illness and illness duration but extremely different in terms of symptomatic state (treatment-resistant/chronic v. remitted-recurrent), who finally displayed notable similarities on metabolite abnormalities. This is a preliminary cross-sectional study designed to explore the role of past illness burden on brain metabolites, but we do not rule out the possibility of an association between metabolite resonances and symptoms from alternative approaches (e.g., analysis of specific groups of patients, longitudinal designs).
There are some methodological limitations of this study that need to be mentioned. First, the hippocampus is a complex anatomic region that often entails spectral quality–related problems. However, MRS data were acquired using a 3 T magnetic resonance scanner, which provides a higher signal-to-noise ratio and better separation among metabolite peaks than commonly used 1.5 or 2 T scanners. Second, the present study was spatially limited to the anterior region of the hippocampus, and it remains to be clarified whether our results could be extrapolated to the whole structure. Nonetheless, we report data for both hemispheres in contrast to many previous works that have centred exclusively on the left temporal region.12–15
Third, a systematic bias in voxel placement/selection could have given rise to different tissue composition of the VOIs among the groups and accounted for differences in the measured spectra. To overcome this potential limitation, we controlled for VOI tissue segmentation in the main analyses. Fourth, the sample comprised a fairly large number of individuals, including healthy controls and a representative, well-characterized group of outpatients in different stages of illness, but the cross-sectional nature of the study should lead us to be cautious in interpreting the findings. Longitudinal studies are warranted to provide definitive evidence of the role of illness burden on brain metabolites in patients with MDD. Finally, the lack of a drug washout period represents an additional limitation. Given that the hypothesis to be tested required the inclusion of severely ill outpatients, there were inherent ethical challenges (e.g., patients’ conditions could worsen without medication). Moreover a washout period that was not long enough might have yielded metabolic changes related more to psychotropic withdrawal than with the illness, per se. To address these challenges, we decided to maintain medication in all patients and recruited an age- and sex-matched group of patients treated for a first episode as well as a non-treated healthy control group. The presence of treatment did not entail substantial metabolic differences between these latter groups. But, as might have been expected, patients with treatment-resistant/chronic illness more often received combined treatments (more than 1 antidepressant and/or atypical antipsychotics). For their part, patients with remitted-recurrent or first-episode depression received an almost equivalent antidepressant regimen, although the former group had a longer history of treatment. Nevertheless, the direction of the metabolic changes observed in our sample fit in with previous studies based on medication-free patients.12,13
Altogether, these comments suggest that the abnormalities described in the present study cannot be merely attributable to the treatment itself.