Depression, both mild and severe, is often treated with medications which balance the brain's chemistry, affecting neurotransmitters such as serotonin and norepinephrine. At the same time, clinical practice recognizes that episodes of depression are often caused by traumatic life events, such as abuse, stress, or grief.
In his book, Listening to Prozac, Peter Kramer describes one possible model which may explain how traumatic life events might be related to brain chemistry, using research on epilepsy which posits a process called kindling.
What is Kindling?
Kindling is like tolerance: it is an acquired response to a repeated stimulus. However, an organism which develops a tolerance becomes less affected by the stimulus, while kindling creates a negative response with less and less stimulus over time. For example, rats whose brains are stimulated with a mild electric current will at first have no response. After repeated exposures, the animal will eventually have a seizure. As the neural pathways become sensitized, the seizures will become worse and worse, and eventually the animal will have seizures with no external stimulation at all. The kindled seizures are a kind of learned sensitivity.
This research has also shown that medication can be an effective treatment for kindled seizures, and that different medications are effective early and late in the process.
Kindling and Depression
This kindling model can potentially explain the way that some mood disorders progress, and provide a way of explaining how external events may cause biochemical changes. Obviously, life events are not the same a electric shocks, but a traumatic experience can stimulate biochemical responses. Stress, for example, produces epinephrine and cortisol, both of which affect mood, sleep, and appetite: all factors in depression.
Indeed, rats subjected to repeated stress produce higher and higher levels of of a substance in the brain which stimulates the release of cortisol. This repeated stress can actually cause permanent changes to the brain's "hard-wiring", including cell death. Antidepressants can block some of these changes.
A long-term study of girls who have suffered childhood sexual abuse shows that humans, too, may experience similar changes in brain chemistry as a result of repeated stress.
Consequences of the Kindling Model
This model, then, may provide one way of thinking about the relationship between neurochemistry and life problems. It may not be necessary to choose between explanations for depression which emphasize personal experience, and those which emphasize brain chemistry.
The kindling model may also affect the way that drugs are used to treat depression. If depression is in fact a kind of progressive condition, patients who have experienced it once may be at increased risk of another episode, information which may be useful for clinicians. It may also be the case that different treatments are effective early and late in the life of a patient with long-term or recurring depression.
