Synaptic remodelling in arcuate nucleus after injection of estradiol valerate in adult female rats.

Adult female rats were injected with a single dose (20 mg/kg) of estradiol valerate (EV). The number of synapses was evaluated in thin sections of arcuate nucleus fixed 3, 8, 16 and 32 weeks after EV treatment and compared with the values obtained in the arcuate nucleus of uninjected proestrus control rats. By 8 weeks after EV treatment a significant (P less than 0.001) decrease was found in the number of axo-somatic and axo-dendritic synapses on dendritic shafts, but not in the number of axo-dendritic synapses on dendritic spines. However, by 32 weeks postinjection, the number of axo-somatic and axo-dendritic synapses had returned to control values. This transient decrease in the number of synapses was preceded by a massive appearance of neuronal degenerative images by 3 weeks after EV injection. These results are interpreted as reflecting a process of circuitry remodelling in the arcuate nucleus after a neuronal lesion induced by estrogen.