Mood disorders range from major depressive disorders to major manic episodes. These disorders are both unipolar and bipolar. One main area of mood disorder research is that of unipolar major depression. Major depression can last just one episode or it can be a disorder, which can last for years with multiple depressive episodes over this extended period. The psychological aspects of depression are well understood but the biological foundations are less understood. As some evidence of this, the DSM-IV manual does not include any neurological information concerning major depression. In this handbook, depression is treated purely as a mental condition without an explanation of the biological aspects of the disorder. On the other hand, there are many psychopharmaceuticals prescribed to people with depression, which suggests that there is more than a cursory acknowledgment of the biological basis of this mental illness. However, this biological focus is mainly a focus on neurotransmitters and not anatomy. Recently, there have been numerous studies conducted to investigate the relationship between brain structure and depression (see Videbech & Ravnkilde, 2004). One of the structures most often studied in connection with depression is the hippocampus, which is a key structure for memory. The purpose of this paper is to investigate whether the hippocampus specifically is negatively impacted in depressed patients.
Frodl et al. (2002) investigated hippocampal changes in patients with first episode major depression. The authors had 30 adult depressed subjects (mean age = 40.3) and 30 matched controls (mean age = 40.6). The mean time of the depressive episode for the depression group was 0.71 years. The researchers collected MR images for all subjects. They compared the hippocampal volumes of the depressed group with the control group with ANCOVAs. Depressed men had significantly smaller left hippocampal volume than did healthy male subjects but right hippocampal volume was not significantly different. Female depressed subjects had significantly larger right hippocampal volume than did their matched controls and left volume did not differ, which implicates differing effects of depression on men and women. There was a significant left-right hippocampal volume disparity in the depressed patients but there was not one in the healthy subjects. Overall, the difference in hippocampal volume was not significant between the depressed and control groups though. There was also no significant correlation between age and hippocampal volume for either group but this finding goes against that of other research (Frodl et al.). On the other hand, between groups there was a significant reduction of hippocampal white matter volume. In other words, both male and female depressed patients had on average a reduction in the hippocampal white matter compared to the control subjects.
The authors concluded that there are likely physiologic gender differences in how males and females react to stress, which would explain why depressed males had smaller hippocampal volume and females did not. They believe this may be an example of the protective effects of estrogen against stress seen in other studies. In any case, there was a tendency for both depressed males and females to have significant left-right hippocampal asymmetry and reduced white matter. They concluded that this represents the beginning of left hippocampus volume loss and disrupted axonal transmission, respectively. The researchers could not conclude, however, that depression caused the volume loss. It may be that the loss came in response to stress or some other factor, which in turn predisposed the depressed subjects to major depression. Alternatively, the depression could have been the catalyst for the reduction (Frodl et al., 2002). Further longitudinal research is needed to uncover the causal relationship between depression and hippocampal volume.