Gonadal steroids promote glial differentiation and alter neuronal morphology in the developing hypothalamus in a regionally specific manner

One of the more striking sexual dimorphisms in the adult brain is the synap tic patterning in some hypothalamic nuclei. In the arcuate nucleus (ARC) ma les have twice the number of axosomatic and one-half the number of axodendr itic spine synapses as females. The opposite pattern is observed in the imm ediately adjacent ventromedial nucleus (VMN). In both cases, early exposure to testosterone dictates adult dimorphism, but the exact timing, mechanism , and site of steroid action remain unknown. Astrocytes also exhibit sexual dimorphisms, and their role in mediating neuronal morphology is becoming i ncreasingly evident. Using Golgi-Cox impregnation to examine neuronal morph ology and glial fibrillary acidic protein immunoreactivity (GFAP-IR) to cha racterize astrocytic morphology, we compared structural differences in dend rites and astrocytes from the ARC and VMN in postnatal day 2 rat pups from four hormonally different groups. Consistent with previous observations, te stosterone exposure induced a rapid and dramatic stellation response in ARC astrocytes, Coincident with this change in astrocytic morphology was a 37% reduction in the density of dendritic spines on ARC neurons, In contrast, astrocytes in the VMN were poorly differentiated and did not respond to tes tosterone exposure, nor were there any changes in neuronal dendrite spine d ensity. However, VMN neurons exposed to testosterone had almost double the number of branches compared with that in controls. These data suggest that the degree of maturation and the differentiation of hypothalamic astrocytes in vivo are correlated with the ability of neurons to sprout branches or s pines in response to steroid hormones and may underlie regionally specific differences in synaptic patterning.