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

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Abstract
Purified neurotoxin II of the scorpion A. australis (ScTx) binds specifically to the Na+-ionophore-associated, voltage-sensitive, receptor sites of excitable cells. Binding studies were conducted 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/.mu.m2). 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 .mu.m2. 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 to brain cells grown in culture is consistently correlated with the presence of tetanus toxin binding cells. In cultures of central CNS glia without neurons, only nonspecific low-level ScTx binding was detected. The high-affinity scorpion toxin receptors may be used as quantitative markers of neuronal differentation.