Ch 7 - depressive and bipolar disorder: biological view. Now activity of two neurotransmitter chemicals ( norepinephrine has been strongly linked to unipolar depression. Antidepressant drugs increase the level of these chemicals. It has to be the interaction of the two neurotransmitters and not just one alone. Reserpine and other medications for high blood pressure often caused depression. Serotonin may be a neuromodulator , a chemical whose primary function is to increase or decrease the activity of other key neurotransmitters. Body"s endocrine system may play a role in unipolar depression. Endocrine glands throughout the body release hormones, chemicals that in turn spur body organs into action. People who unipolar depression have been found to have abnormally high levels of cortisol , one of the hormones released by the adrenal glands during times of stress. Melatonin (dracula hormone) - only released in the dark that is connected.