CELLULAR COMPOSITION OF THE ADULT-RAT ANTERIOR-PITUITARY IS INFLUENCED BY THE NEONATAL SEX STEROID ENVIRONMENT

Growth hormone (GH) and prolactin (PRL) secretion differ significantly between adult males and females and this is due, at least in part, to the postpubertal hormone environment which affects GH and PRL gene ex pression, as well as somatotrope and lactotrope proliferation. However , the role of the neonatal steroid environment in this phenomenon is l ess well understood. We have used in situ hybridization to determine t he number of GH and PRL mRNA containing cells, as well as the level of expression of these two hormones and of the pituitary transcription f actor 1 (Pit-1). Neonatally castrated male rats that had been exposed to testosterone during the neonatal period, adulthood or during both p eriods, males castrated as adults, normal adult males and normal proes trous females were used. Orchidectomy of adult rats had no effect on t he number of somatotropes or lactotropes, but significantly reduced GH and PRL mRNA levels. Neonatal castration significantly reduced the pe rcentage of somatotropes and increased that of lactotropes in the adul t male. In addition, GH and Pit-1 mRNA levels were reduced significant ly, but PRL mRNA levels were not modified, Treatment of neonatally cas trated males with testosterone during the neonatal period significantl y increased the percentage of somatotropes and decreased the percentag e of lactotropes compared to vehicle-treated animals. It also increase d GH and Pit-1 mRNA levels, but did not affect PRL mRNA levels. Adult testosterone treatment significantly increased the percentage of both somatotropes and lactotropes, as well as GH, PRL and Pit-1 mRNA levels . Treatment of neonatally castrated males with testosterone during bot h the neonatal and adult periods returned the percentage of somatotrop es and lactotropes, as well as GH, PRL and Pit-1 mRNA levels, to that of the intact male. These results suggest that, although the postpuber tal steroid environment is important in determining anterior pituitary hormone synthesis and cellular composition, the neonatal steroid envi ronment also plays an important role in this phenomenon.