α-Bungarotoxin (α-BTX) has been used to label receptor binding sites on neural membranes. α-BTX fractions 2.2 and 3.3 were purified from Bungarus multicinctus by the method of Ravdin and Berg (1979) and were iodinated. There was no difference between these two fractions in their binding affinity or specificity of binding with hypothalamic synaptosomes. [125I]α-BTX 2.2S, 3.3 and commercially obtained [125I]α-BTX were injected into the third ventricle of ovariectomized female rats (n = 22), normally cycling rats (n = 12) or normal male rats (n = 4) and autoradiographic examination performed. Saline injected hypothalami (n = 4) or hypothalami injected with unlabeled α-BTX in combination with radioligand (n = 2) did not show silver grain accumulation in the hypothalamus. Examination of serial sections from animals injected with each [125I]α-BTX showed that the supraoptic, periventricular, arcuate, premamillary and mamillary nuclei were consistently labeled. While the suprachiasmatic nucleus (SCN) in the intact females and males both showed high densities of α-BTX binding, the SCN in ovariectomized females showed little or no α-BTX binding. Thus, the labeling of the SCN in females without ovaries and ovarian hormones was markedly different from that of the intact males and females. Labeling patterns in castrate and intact animals may contribute to our understanding of gonadal steroid regulation of hypothalamic function. © 1982.
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