Nardil exerts its antidepressant and anti-anxiety effects in the brain by boosting the expression of numerous important neurotransmitters. MAO inhibitors, such as phenelzine, block the breakdown of monoamine neurotransmitters such as serotonin, melatonin, epinephrine, and norepinephrine. By increasing their concentration inside neurons, these chemicals are better able to signal those cells that are affected by depression or anxiety. Nardil has been shown to increase the levels of serotonin and dopamine within the brains of animals given the drug. This proof of concept study showed that the rate at which the drug was administered into the bloodstream of rats changed the nature of their activity during testing sessions. Animals who were given a dose of nardil shortly before they were tested had significantly less active than control animals who were given a placebo. It is thought that this effect may be due to the drug making them feel happier because they behave more like normal animals instead of losing their interest in moving about. The researchers also noted that the drug had an impact on animal behavior when given after testing had occurred. Rats who had received nardil several days earlier showed a reduction in their overall activity level compared to controls. They behaved more like depressed or anxious animals and did not explore their environment as much. These results show that nardil can have an acute effect on mood and behavior and suggest that it could be used to treat certain types of anxiety or depression.