Comparative aspects of neurosteroidogenesis: From fish to mammals

•Neurons and/or glial cells express steroidogenic enzymes.•Nerve cells are capable of synthesizing biologically active steroids.•Neurosteroid biosynthesis occurs in the brain of all classes of vertebrates.

It is now clearly established that the central and peripheral nervous systems have the ability to synthesize de novo steroids referred to as neurosteroids. The major evidence for biosynthesis of neuroactive steroids by nervous tissues is based on the expression of enzymes implicated in the formation of steroids in neural cells. The aim of the present review is to summarize the current knowledge regarding the presence of steroidogenic enzymes in the brain of vertebrates and to highlight the very considerable contribution of Professor Kazuyoshi Tsutsui in this domain. The data indicate that expression of steroid-producing enzymes in the brain appeared early during vertebrate evolution and has been preserved from fish to mammals.