Na+ channel changes in the growth cone and developing nerve terminal

Area of Science:

• Neuroscience
• Molecular Biology
• Developmental Biology

Background:

• Two forms of sodium channels, differing in saxitoxin affinity, exist in fetal rat brains.
• A low-affinity precursor is in internal compartments of growth cones, while a high-affinity mature form is in the plasmalemma.
• The role and developmental regulation of the beta2 subunit in these sodium channel forms remain unclear.

Purpose of the Study:

• To investigate the presence and developmental timing of the beta2 subunit in rat brain sodium channels.
• To determine if the beta2 subunit is associated with specific sodium channel forms.
• To understand the developmental regulation of the beta2 subunit in relation to the alpha subunit.

Main Methods:

• Western blot analysis using antibodies against alpha and beta2 sodium channel subunits.
• Subcellular fractions from rat brains at embryonic day 18 (E18), postnatal days 7-25 (P7-P25), and adult stages were analyzed.
• Purified sodium channels from adult rat brain were also examined.

Main Results:

• The beta2 subunit (38 kDa) was detected in synaptosomes and purified sodium channels from P7 onwards, but was absent at E18.
• A high molecular weight band (260-300 kDa) recognized by the beta2 antibody was prominent at E18 and decreased as the 38 kDa band appeared.
• Alpha subunits were present at E18, indicating independent regulation from beta2 subunits.

Conclusions:

• Beta2 subunits are developmentally regulated independently of alpha subunits.
• Beta2 subunits are absent in early differentiating neurons and not required for initial sodium channel insertion into the plasmalemma.
• The high molecular weight band at E18 may represent an precursor or assembly intermediate of the beta2 subunit.