Testosterone-sensitive neurones respond to oestradiol but not to dihydrotestosterone.

Testosterone is aromatized to oestradiol in the brain, and it has been suggested that its effects on the brain are produced by this metabolite. Oestradiol does indeed restore sexual behaviour in castrated rats, but is is much more effective if combined with a second metabolite, dihydrotestosterone. This may be because the two hormones have a synergistic effect on the brain; or because the central effect of oestradiol synergises with the peripheral effects of dihydrotestosterone (which include stimulation of penile development). Evidence that the corticomedial amygdala (CMA) and the preoptic area of the rat brain are both involved in the control of sexual behaviour led us to examine the neurones of the stria terminalis that connect the two. These neurones are hormone-sensitive; their absolute refractory period is lengthened by castration and reduced again by testosterone. So we have investigated the aromatization hypothesis further by examining the effect of oestradiol and dihydrotestosterone on these testosterone-sensitive neurones. Oestradiol has exactly the same effect as testosterone; dihydrotestosterone has no effect, whether it is given alone or in combination with oestradiol.