Alcohol depletes 5ht. Alcohol causes whatever serotonin is available in the brain to be released. This is why you feel good initially. Antidepressants help the brain better utilize what serotonin exists, so after drinking you have basically depleted all the serotonin and can get acutely depressed. It can take up to a week to replenish the serotonin after one episode of drinking (only 4 or more drinks).