The answer is Serotonin and norepinephrine. Serotonin is an inhibitory neurotransmitter that has been found to be closely involved in emotion and mood. While norepinephrine, in 1946, a Swedish biologist by the name of Ulf von Euler discovered norepinephrine (formerly called noradrenalin). It is strongly linked with bringing our nervous systems into a high alert. It is dominant in the sympathetic nervous system, and it increases our heart rate and our blood pressure. Our adrenal glands release it into the blood stream, along with its close relative epinephrine (aka adrenalin). It is also important for forming memories. The antidepressant he must be taking is serotonin–norepinephrine reuptake inhibitors (SNRIs). These are a class of antidepressant drugs used in the treatment of major depressive disorder (MDD) and other mood disorders.