Gonadal steroids reduce the density of axospinous synapses in the developing rat arcuate nucleus: an electron microscopy analysis.

The developing brain is exquisitely sensitive to gonadal steroid hormones, which permanently differentiate the neural substrate during a critical developmental period. One of the more striking sexual dimorphisms in the adult rat brain is synaptic patterning in the arcuate nucleus (ARC); females have twice the number of axospinous synapses as males (Matsumoto and Arai [1980] Brain Res. 190:238-242). Previously, we have demonstrated that a similar dimorphism in spine densities on ARC dendrites is present as early as early as postnatal day 2 (PN2) in Golgi-impregnated rat brains (Mong et al. [1999] J. Neurosci. 19:1464-1472). Males have 37% fewer dendritic spines than females. Moreover, these spine densities are sensitive to changes in the hormonal milieu such that males castrated on the day of birth have a significant increase in spine density, whereas females masculinized at birth by gonadal steroid exposure have a decreased dendritic spine density. One of the limitations of the Golgi technique is the inability to confirm the presence of synapses. The current study used quantitative electron microscopy and demonstrated that testosterone exposure dramatically reduced axospinous synapses in the ARC by PN 2. Males had 54% fewer and masculinized females had 77% fewer axospinous synapses than females (P < 0.05 and P < 0.01, respectively). We previously reported that gonadal steroids induce coincident changes in neuronal and astrocyte morphology in the neonatal ARC (Mong et al., 1999), and here confirm that these changes include an altered synaptic pattern that is strikingly similar to that observed in the adult (Matsumoto and Arai, 1980).