Na+-channel-associated scorpion toxin receptor sites as probes for neuronal evolution in vivo and in vitro.

Purified neurotoxin II of the scorpion Androctonus australis Hector (ScTx) has previously been shown to bind specifically to the Na+-ionophore-associated, voltage-sensitive receptor sites of excitable cells. We have conducted binding studies, using high-specific-activity 125I-labeled ScTx, to detect and quantify the Na+-channel receptors on cells of the developing fetal mouse brain. In vivo, the onset of detectable specific binding is at 12 fetal days. The rate of receptor appearance is initially slow but increases sharply as of the 16th day of mouse ontogenesis. The mean number of receptors at 12 and 19 days is 120 and 20,000 per cell, respectively (i.e., 0.5 and 80 per square micrometer). When corrected for the fraction of cell population corresponding to putative neuroblasts and neurons, identified by immunofluorescence as tetanus toxin binding cells, these values are, respectively, 1040 and 33,900 ScTx receptors per tetanus toxin binding cell or 4.2 and 136 per square micrometer. At all stages, the toxin binds to a single class of noninteracting sites; Kd = 0.1-0.5 nM. Similar findings in terms of ScTx-receptor properties and quantitative evolution were obtained in vitro. Specific 125I-labeled ScTx binding the presence of tetanus toxin binding cells. In cultures of central nervous system glia without neurons, only nonspecific low-level ScTx binding was detected. These results suggest that the high-affinity scorpion toxin receptors may be used as quantitative markers of neuronal differentiation.