EXPRESSION OF SODIUM-CHANNELS WITH DIFFERENT SAXITOXIN AFFINITY DURING RAT FOREBRAIN DEVELOPMENT

This work characterizes the development of the saxitoxin (STX)-sensiti ve Na+ channels from rat whole forebrain between embryonic day 15 (E15 ) and postnatal day 90 (P90), both with binding studies and with singl e channel studies. The Na+ channel total mRNA and the individual mRNAs encoding Na+ channels I, II and III were also determined. The total S TX binding rose about 40-fold from E15 to reach a plateau at P30 and i ts temporal course correlated with the expression of Na+ channel total mRNA. Low affinity and high-affinity STX binding sites, predominant i n embryonic and postnatal forebrains, respectively, were found. The si ngle channel studies of batrachotoxin-modified channels also revealed two main populations. In E15 only low-affinity channels (K-D = 32.7 nM ; 200 mM NaCl) and in P30 only high affinity ones (K-D = 1.6 nM) were present. At P0 channels with intermediate affinity (K-D range 3-34 nM) were observed. The increase in affinity was due to a gradual increase in the STX association rate.