Steroid-induced developmental plasticity in hypothalamic astrocytes: implications for synaptic patterning.

We have previously demonstrated that astrocytes in the developing arcuate nucleus of the rat hypothalamus exhibit a sexually dimorphic morphology as a result of differential exposure to gonadal steroids. Testosterone via its aromatized byproduct, estrogen, induces arcuate astrocytes to undergo differentiation during the first few days of life. These differentiated astrocytes exhibit a stellate morphology. Coincident with the steroid-induced increase in astrocyte differentiation is a reduction of dendritic spines on arcuate neurons. As a result, the arcuate nucleus of males has fewer axodendritic spine synapses than females and this dimorphism is retained throughout life. In the immediately adjacent ventromedial nucleus, neonatal astrocytes are immature and unresponsive to steroids. Neurons in this region show no change in dendritic spines in the first few days of life but do exhibit increased dendritic branching as a result of testosterone exposure. These findings illustrate the importance of distinct populations of astrocytes in restricted brain regions and their potential importance to the establishment of regionally specific synaptic patterning. Conflicting reports leave the site of steroid-mediated astrocyte responsiveness in the arcuate nucleus unresolved: Are gonadal steroids acting directly on astrocytes or are steroid-concentrating neurons mediating astrocytic responsiveness? In this review, we discuss the current understanding of astrocyte-neuron interactions and the possible mechanisms for steroid-mediated, astrocyte-directed synaptic patterning in the developing hypothalamus.