Cytochrome P450 in the brain: neuroendocrine functions

The effectiveness of steroid hormone metabolites as sedatives and anesthetics has been known for many years. More recently, their interaction with neurotransmitter receptors has helped to elucidate their mechanism of action, but their physiological functions and their role in disturbances of behavior, anxiety, and sleep/wakefulness have yet to be elucidated. Until 1981 it was assumed that metabolites of steroid hormones arose from the adrenals and gonads and that their action on neurotransmitter receptors was a mechanism of communication between the brain and the periphery. The evidence that the brain could accumulate steroids independently of the adrenals and gonads in 1981 and later the evidence for the presence of the cholesterol side chain cleavage enzyme (P450scc) in the brain have challenged this concept and stimulated a great deal of interest in the possibility that the brain could be making its own steroids from cholesterol for some as yet undefined purpose. In this review we examine the data pertaining to the role of brain P450 in the synthesis and degradation of neurosteroids. We summarize the data on the presence of P450scc in the brain and try to answer the following questions: (1) Does P450scc in the brain contribute significantly to the synthesis of GABAA receptor active steroids? (2) Can the P450scc in the brain account for the accumulation of pregnenolone in the brain? (3) Is there evidence for special functions of the pregnenolone synthesized in the brain? (4) Is there a role for other forms of brain P450 in neurosteroid action?