Effects of gonadal steroids on the in vivo binding of [I]α-bungarotoxin to the suprachiasmatic nucleus

The radioligand ¹²⁵I]α-bungarotoxin (α-BTX) has been used to test receptor binding to putative nicotinic cholinergic receptors in the hypothalamus. Using light microscopic autoradiography following third ventricular infusion of the radioligand we have previously demonstrated that in normally cycling rats and in normal males, the suprachiasmatic nucleus (SCN) consistently binds the α-neurotoxin. In chronically (5 weeks) oophorectomized female, binding of ¹²⁵I]α-BTX to the SCN is markedly diminished. The present series of experiments were designed to test the effects of gonadal steroids on the binding of ¹²⁵I]α-BTX to the SCN. We first tested whether or not estradiol administered to ovariectomized females could duplicate the presence of the ovary. In females ovariectomized and immediately provided with a constant dose of estradiol-17β (E₂)—1.0 cm silastic capsules for 5 weeks, (n= 4), the binding of the neurotoxin to the SCN was maintained. In females ovariectomized for 3 weeks and replaced with E₂ for 2 weeks (n= 4), the binding of ¹²⁵I]α-BTX to the SCN was restored. In chronically (4 weeks) ovariectomized females receivingf E₂ for 6 dyas (n= 2), the binding of the neurotoxin was partially restored. We next tested the effect of chronic (5 weeks) castration (n= 100) and observed that binding of ¹²⁵I]α-BTX to the SCN of castrated males was like that of intact male controls (n= 6). We also tested the effect of neonatal adrogenization (1.25 mg testosterone proprionate, neonatal days 2 and 5) with subsequent adult oophorectomy (day 83) and observed that neonatally androgenized females with intact ovaries (n= 4) demonstrated ¹²⁵I]α-BTX binding to the SCN like that of non-adrogenized females, while neonatally androgenized females without ovaries (n= 4) had decreased binding of the neurotoxin to the SCN like that of oophorectomized adult females. These data demonstrate that: (1) the administration of estradiol can maintain and restore ¹²⁵I]α-BTX labeling of the SCN in ovariectomized rats; (2) binding of ¹²⁵I]α-BTX to the SCN is different in castrated males and ovariectomized females; and (3) the difference in α-BTX binding to the SCN receptor in adult castrated male and female rats may be dependent upon the presence of gonadal steroids at times other than the ‘critical’ postnatal period.