Whether or not you agree with the chemical imbalance theory, there has been research showing differences between the brains of depressed and non-depressed people.
www.thevisualmd.com/health_centers/neurological_health/depression/depression_changes_the_brain
Brain neurons may decrease in size and density.
The number of glial support cells may lessen. Glial support cells are vital for communication between neurons.
Certain parts of the brain associated with emotion, memory, and learning, particularly the prefrontal cortex and the hippocampus, can shrink in size. This might explain some of the emotional changes observed in people with depression.
Due to the loss of brain tissue, cavities in the brain called ventricles can become larger.
The corpus callosum, which connects the two hemispheres of the brain, can become either bigger or smaller in size.
Regions of the brain that may be affected by depression include the hypothalamus, hippocampus, anterior cingulate gyrus, amygdala, and all parts of the limbic system.
Other areas that may be affected include the thalamus, which functions as a sort of gateway for the filtering of sensory information; the prefrontal cortex, which is implicated in personality expression and moderating correct social behavior; and the corpus callosum, which connects the two hemispheres of the brain.