Major depressive disorder (MDD) is characterized by depressed mood, low self-esteem, anhedonia, and disrupted sleeping, eating, and cognition, and has now been recognized to have an overall impact on global illness that is projected to be second only to ischemic heart disease in social and economical burden by 2020 (Greden, 2001; Murray and Lopez, 1997). In the United States, about 14 million people, or about 10% of the population, suffer from depression at any point in time (Kessler et al., 2003). Because approximately one third of MDD patients do not respond to traditional pharmacological medications such as monoamine reuptake inhibitors, there is a major unmet need for the development of novel, more efficacious therapeutic agents.
