The rodent brain expresses StAR, TSPO, P450scc and the steroidogenic enzymes necessary to produce oestradiol from cholesterol2. Several of these enzymes have also been identified in the human brain143. The steroids produced in the nervous system, known as neurosteroids1, act through various mechanisms, including the direct modulation of the channel properties of synaptic neurotransmitter receptors, such as the NMDA receptor and the GABAA receptor, by the progesterone metabolite 3α,5α tetrahydroprogesterone (also known as allopregnanolone)144,145. Some neurosteroids, such as oestradiol, regulate neural function, behaviour, cognition and affect, and have neuroprotective actions144,146. The term neuroactive steroid is also currently used in the literature to refer to steroids with activity in the nervous system, independently of whether they are peripheral hormones, neurosteroids or synthetic steroids144.