Ontogeny of sexual difference in alpha-bungarotoxin binding capacity in the mouse amygdala

We have shown a sex difference in the nucleus amygdaloideus medialis posterior (NAMP) of the mouse with respect to the binding capacity for alpha-bungarotoxin (alpha-BGT) under various steroid-hormonal environments. The present study describes histochemically the postnatal development of alpha-BGT binding capacity in the NAMP of the intact male and female mouse, and characterizes biochemically the toxin binding component at different developmental stages. Light microscopic autoradiography using radio-iodinated alpha-BGT revealed characteristic patterns of development of alpha-BGT binding capacity in the NAMP of both sexes. On the day of birth, the autoradiographic grain density for [125]alpha-BGT binding sites was low in the NAMP and no sex difference was detected. During the next 4 days the grain density increased in each sex, but the density in the female increased to a lesser extent than in the male, resulting in a marked sex difference at 4 days after birth. The grain density in each sex was maximal at 7-10 days and then declined toward the adult level by 28 days of age. The density in the male exceeded that in the female throughout postnatal life except for the day of birth. The biochemical filtration assay on the tissue homogenates provided evidence suggesting that alpha-BGT binding sites in the posterior corticomedial amygdaloid region including the NAMP are sexually different in number, but not in the binding affinity, at an early postnatal age as well as in adulthood. These results indicate the importance of the early postnatal days for the sexual differentiation of alpha-BGT binding sites in the NAMP.