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Early testosterone exposure also influenced the rats' sensitivity to morphine-induced analgesia when they were adults. The males deprived of testosterone as neonates were less sensitive to morphine than normal males (i.e., more like "normal" females), and the females given testosterone as neonates were more sensitive to morphine than control females (i.e., more like "normal" males). Interestingly, the effects of neonatal testosterone deprivation in males could be reversed in some tests simply by exposing the males to testosterone for two weeks during adulthood. In contrast, females exposed to testosterone soon after birth and then again in adulthood showed no further effect from the later exposure.

"Thus, it can be concluded that testosterone exposure before birth or on the first day after birth is sufficient to enable males to regain their 'male phenotype' as long as they are exposed to testosterone in adulthood," says Craft, "whereas exposing females to testosterone soon after birth is insufficient to completely change them into the male phenotype in terms of their sensitivity to pain and to morphine-induced analgesia."

Craft and her colleagues are now attempting to replicate their findings in other strains of rat. "Overall, our current results suggest that adult females' greater sensitivity to pain and lesser sensitivity to the pain-killing effects of morphine will be difficult to ameliorate by simple hormone treatment--with testosterone, for example--in adulthood," says Craft. "Instead, these sex differences would appear to be determined very early in development."

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Contact: Leah Ariniello
dawn@sfn.org
202-462-6688
Society for Neuroscience
24-Oct-2004