Limbic System & Depression
I've added the following text to "The Limbic System: Seat of Consciousness and Emotions in the Human Brain" by Vexen Crabtree (1998):

Over-activation of the limbic system can result in depression as well as religious phenomenon. In the case of depression, symptoms can be removed by medicating with drugs that affect certain parts of the limbic system, in particular through quantitively altering the action of certain neurotransmitters.

“Because of its relevance to the so-called vegetative symptoms of depression, such as disturbances in appetite and sleep, the hypothalamic - pituitary - adrenocortical axis is thought to be overactive in depression. Various findings support this proposition.”
"Abnormal Psychology" by Davison & Neale, p242


On the same subject but on a different page, I've also added the following to "The Soul and Emotions (The Biological Basis of Our Emotions): Depression and Mood Disorders" by Vexen Crabtree (1999):


Serotonin and norepinephrine are both related to our mood, and their flow causes changes in mood. So, antidepressants such as tricyclics and monoamine oxidase inhibitors increase the effect of certain neurotransmitters, and thereby raise the mood. Panic and mood disorders are inheritable6. This means, before a person is even conscious, they are predisposed to a neurotransmitter dysfunction that causes certain moods and even some mental disorders. When these symptoms become serious, depression can be lifted by the correct medication. All this shows that if there is a soul, it is unnecessary for the feelings of happiness, sadness, depress, etc, and that these things can even be beyond our control at all.

“Three major categories of antidepressant drugs:
Tricyclics, such as imipramine (Tofranil) and amitriptyline (Elavil);
Selective serotonin reuptake inhibitors, such as fluoxetine (Prozac); and
Monoamine oxidase (MAO) inhibitors, such as Parnate. [...]
Tricyclic drugs are a group of antidepressant medications [that] prevent some of the reuptake of both norepinephrine and serotonin by the presynaptic neuron after it has fired, leaving more of the neurotransmitter in the synapse so that transmission of the next nerve impulse is made easier. [...]

All three types of drugs are believed to work by facilitating neural transmission. [...] People with the mood swings of bipolar disorder are often helped by carefully monitored dosages of lithium, an element, taken in a salt form, lithium carbonate.”
"Abnormal Psychology" by Davison & Neale, p240, 246-7