NA-TERMINAL( CHANNEL CHANGES IN THE GROWTH CONE AND DEVELOPING NERVE)

Previous saxitoxin binding studies indicated two forms of the sodium c hannel in the fetal rat brain; a low-affinity precursor located in an internal membrane compartment, present exclusively in growth cones and a high-affinity mature form present in the plasmalemma of growth cone s and characteristic of synapses. This raises the questions (1) of the presence or absence of the beta 2 subunit in these channel forms and (2) of the developmental regulation of the beta 2 subunit. Antibodies against the alpha and beta 2 channel subunits were used to probe Weste rn blots of subcellular fractions from rat brains at embryonic day 18 (E18), pups at postnatal (P) days 7-25, and adults, as well as purifie d sodium channels from adult brain. In both synaptosomes and the purif ied sodium channel the beta 2 antibody recognized the expected band at 38 kDa under reducing conditions. However, in contrast to the alpha s ubunit, this band was absent at E18 and became apparent only from P7 o nwards. At the earlier time intervals a very prominent immunoreactive band of unknown identity was evident at 260-300 kDa, which declined in intensity concomitant with the appearance of the 38 kDa beta 2 band. These data indicate that beta 2 subunits are regulated independently f rom alpha subunits, are absent in differentiating neurons, and hence a re not necessary for insertion of the sodium channel into the plasmale mma, at least during early development of the neuron.