Smoking and depression

Cigarette smoking is a significant factor in depression. Central to the effect of nicotine is the stimulation of adrenal hormone,  including cortisol, secretion. Elevated cortisol levels are a well-recognized feature of depression. One of the key effects of cortisol on mood is related to activating an enzyme (tryptophan oxygenase). When activated, this enzyme results in less tryptophan being delivered to the brain. Since the level of serotonin in the brain is dependent upon how much tryptophan is delivered to the brain, cortisol dramatically reduces the level

of serotonin and melatonin. In addition, cortisol also "down regulates" serotonin receptors in the brain, making them less sensitive to the serotonin that is available. Smoking also leads to a relative vitamin C deficiency, as the vitamin C is utilized to detoxify the cigarette smoke. Low levels of vitamin C in the brain can result in depression and hysteria.