Sex Differences in Neurotransmitter Systems

Sex differences in behaviors and other centrally regulated processes have inspired research on structural differences in the brain that might underlie these functional differences. Structural sex differences have been found at almost every level in the brain (1-3). The manner in which such structural differences contribute to functional sex differences is only clear in some cases, notably in sex differences found in the spinal cord and lower brainstem, which contain motomeurons that innervate sexually dimorphic muscles. For example, rats have perineal muscles that are only present in males. These muscles contribute to erection, ejaculation and the deposition of a copulatory plug. They are innervated by motomeurons in the spinal dorsolateral nucleus and the nucleus of the bulbocavemosus, which contain about three times as many motomeurons in males as in females (4). It is more difficult to understand how structural sex differences at