NEUROPLASTIC CHANGES IN THE HYPOTHALAMIC ARCUATE NUCLEUS - THE ESTRADIOL EFFECT IS ACCOMPANIED BY INCREASED EXOENDOCYTOTIC ACTIVITY OF NEURONAL MEMBRANES

1. In the rat hypothalamic arcuate nucleus, estradiol induces coordina ted changes in the number of axosomatic synapses, the amount of glial ensheathing, and the ultrastructure of the membrane of neuronal somas. In the present study we used conventional electron microscopy and fre eze-fracture to examine cellular mechanisms responsible for the estrad iol-induced chages at the membrane level. 2. In freeze-fracture replic as taken 10-60 min and 24 hr after injection of 17 beta-estradiol to a dult ovariectomized females, it was found that there was a rapid incre ase in the number of exoendocytotic images that reached a plateau by 3 0 min. 3. In thin sections from animals injected 24 hr earlier we demo nstrated a significant increase in coated vesicles in the periphery of the neurons and coated pits in the perikaryal membranes and decreased axosomatic synapses. 4. We conclude that these morphological alterati ons are signaling estrogen-induced transport and/or turnover of perika ryal membrane constituents and extracellular components which may affe ct interneuronal and neuroglial interactions.