Effects of ovariectomy and estrogen replacement on dye coupling among rat supraoptic nucleus neurons

Among magnocellular neurosecretory neurons (MNCs), the frequency of dye coupling, and thus also of electrotonic coupling, is reduced in male rats following castration. Testosterone replacement prevented this reduction suggesting a modulatory role for gonadal steroids. To determine whether gonadal steroids in females influenced coupling incidence. Lucifer yellow CH injections were made in MNCs in slices taken from ovariectomized rats, either untreated or implanted with capsules containing estradiol-17β or estradiol-17α, or from sham operated rats. In groups without biologically active estradiol, incidence of dye coupling was increased by 138%–169% over those with normal plasma levels, as measured by radioimmunoassay. We conclude that estradiol and testosterone have opposite effects on coupling frequency among MNCs and that the facilitatory effects of testosterone in males are unlikely to be via its aromatization to estrogen.