The development of astrocytes immunoreactive for glial fibrillary acidic protein in the mediobasal hypothalamus of hypogonadal mice.

Numbers of astrocytes immunoreactive for glial fibrillary acidic protein (GFAP) are markedly increased in the mediobasal hypothalamus (MBH) of adult hypogonadal (hpg) mice. The astrocytosis cannot be reversed by administration of gonadal steroids. To investigate whether the glial changes are established in the perinatal period, when crucial developments occur in rodent brain, we determined the distribution of GFAP-IR astrocytes in normal and hpg mouse brain from birth to adulthood. The period up to 3 weeks of age was characterized by the gradual disappearance of radial glia and the increase in mature astrocytes in some brain regions, for example hippocampus. However, there were no apparent differences in GFAP-IR elements between normal and hpg brains and very few astrocytes in the MBH. From age 1 month, increased numbers of GFAP-IR astrocytes were apparent in the hypothalamus in hpg mice and this difference was more obvious at 2 months. By 4 months of age the characteristic astrocytosis in the MBH had been attained and this did not change in older hpg mice. These observations provide no evidence for upregulation of the GFAP gene during the first 2 postnatal weeks when its transcription is highest and astrocytes are proliferating most rapidly. It is more likely that the astrocyte response in the MBH in hpg mice reflects permanent differences in steroid-induced neuronal connectivity, caused by the hypogonadism.