Hippocampal Neurogenesis v Area 25
OR, Is Hippocampal Neurogenesis Really Responsible for Antidepressant Treatment Response? [Maybe it's just a convenient marker of neural plasticity...]
How do we resolve the differences between rodent studies that implicate the hippocampus and human studies that implicate the midline prefrontal cortex [Brodmann area 25 aka subgenual cingulate aka sad cingulate] [in major depression]? Of course, the discrepancies might be attributed to neuroanatomical differences between rodent and human brains. Rodents have at most a primitive subgenual anterior cingulate cortex, whereas this region in the primate brain shows extensive connections with subcortical and cortical targets. But other fundamental issues should be kept in mind when jumping from studies of rodent behavior to human psychopathology. Human psychiatric disorders are complicated amalgams of affective, cognitive, and behavioral abnormalities. We might model aspects of one of these dimensions, such as helplessness or memory loss, in rodents; but we are then studying an aspect of the disorder, not the disorder itself. [Insel, 2007]The quote above and the one in the previous post (both by Insel, 2007) were part of a commentary on a new paper in Science (Airan et al., 2007) that did optical imaging of hippocampal slices taken from rats who had been exposed to chronic mild stress (Willner, 2005), an animal model of depression. Here's the abstract of the article by Airan et al. (2007):
The hippocampus, as an integral component of the limbic system, is a focus of depression research, drives other brain regions implicated in depression, and appears to serve as a primary site of action for antidepressants that inhibit pathological hyperactivity. Complicating this picture, however, is evidence suggesting that antidepressants can stimulate hippocampal activity. Antidepressant-induced hippocampal neurogenesis is linked to behavioral responses; moreover, excitatory hippocampal neurons are injured by chronic stress. Animal models have proven useful in identifying molecular and cellular markers relevant to depression but have not identified neurophysiological final common pathways relevant to behavior. Voltage-sensitive dye imaging (VSDI) could allow analysis of disease-related neural activity on millisecond time scales, with micrometer spatial resolution and a scope spanning entire brain networks. We applied VSDI to hippocampal physiology in the chronic mild stress (CMS) model, a well-validated rodent model of core depressive behavioral symptoms.But back to the commentary. Insel is just so quotable! Here he is on the pitfalls of the new fMRI phrenology:
Major depressive disorder, the result of an unfortunate convergence of genetic and environmental factors, is certainly more than the sum of its observable parts. Identifying brain regions correlated with "the parts" will be an important next step for human imaging studies, but the field will need to avoid high-tech phrenology.References
Airan RD, Meltzer LA, Roy M, Gong Y, Chen H, Deisseroth K (2007). High-Speed Imaging Reveals Neurophysiological Links to Behavior in an Animal Model of Depression. Science 317:819-823.
Insel TR. (2007). Shining a Light on Depression. Science 317:757-758.
Willner P. (2005). Chronic Mild Stress (CMS) Revisited: Consistency and Behavioural-Neurobiological Concordance in the Effects of CMS. Neuropsychobiology 52:90-111.
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