Concentrations of testosterone in neonatal male rats suckled naturally and hand-fed

Concentrations of testosterone were measured daily in plasma of neonatal male rats from the day of birth for 7 days. It was found that a significant (P less than 0.001) increase in mean plasma levels of testosterone occurs on day 2 life, followed by a decrease on day 3. Separation of male rat pups from their mothers on the second day of life for as little as 2 h was associated with a significant (P less than 0.001) fall i plasma testosterone concentration. Hand-feeding the pups with a proprietary human milk substitute (milk-replacer) from birth until the expected time of the testosterone peak resulted in no increase in plasma levels of testosterone; inclusion of an antiserum to LH-releasing hormone (LH-RH) in the milk-replacer decreased the testosterone levels as did removing any supposed endogenous LH-RH in the milk replacer. Addition of a highly potent analogue of LH-RH, either in the presence or absence of the LH-RH antiserum, to the milk-replacer resulted in mean plasma levels of testosterone similar to those in naturally suckled rats. Rats fed with human milk showed an increase in plasma levels of testosterone. It is concluded that the increase in the plasma testosterone concentration found in male rat pups on the second day of life, which may have an important effect on the organization of the brain, is provoked indirectly by LH-RH ingested during suckling.