Appearance of [3H]saxitoxin binding sites in developing rat brain.

The binding of [3H]saxitoxin to a crude membrane fraction from rat brains of various ages was determined. We found that at two days before birth, the maximum number of binding sites is 100 fmol/mg of membrane protein. This value increases rapidly for the two weeks following birth, eventually reaching a plateau value of about 800 fmol of toxin sites per mg protein. Since published reports indicate that rat brain myelination occurs primarily between days 15 and 21, we conclude that the development of sodium channels, as measured by [3H]saxitoxin binding, precedes myelination.