The female brain appears to be twice as vulnerable as the male brain to many stress-related disorders, including depression and PTSD. Studies have revealed a discovery in rat brains that may help to explain this male-female difference. Researchers, including Rita Valentino PhD, a NIMH grantee, used antibodies and an electron microscope to see how receptors for Corticotropin releasing factor (CRF), which acts as both a hormone and a neurotransmitter, responded in the brains of male versus female rats. The brains were studied unstressed and after exposure to a stressful swim. In the male brain under stress, many of the hormone's receptors retreated inside the cell, making the brain less stress reactive. In the female brain, the hormone’s receptors remained on the surface of the cell, resulting in a lower threshold for stress-induced activation of the brain’s alarm system. According to Valentino, this study is the first to uncover sex differences at the level of receptor signaling.