Thymopoietin, a polypeptide ligand for the alpha-bungarotoxin binding site in brain: an autoradiographic study

Thymopoietin, a 48-49-amino acid polypeptide present in the thymus gland, was investigated as a potential ligand for the neuronal nicotinic alpha-bungarotoxin binding site in rat brain. Binding of [125I]alpha-bungarotoxin to whole rat brain sections was inhibited by thymopoietin in a concentration-dependent manner with an IC50 of 30.0 +/- 8.2 nM as compared to 1.1 +/- 0.3 nM for alpha-bungarotoxin. However, at concentrations of thymopoietin of up to 1 microM, [3H]nicotine binding to high affinity sites was not inhibited. Thysplenin, a polypeptide with considerable homology to thymopoietin did not affect [125I]alpha-bungarotoxin binding. These results suggest that thymopoietin selectively interacts with the nicotinic alpha-bungarotoxin binding site labelled by [125I]alpha-bungarotoxin rather than the neuronal nicotinic receptor(s) labelled by [3H]nicotine. Autoradiographic studies revealed that 1 microM thymopoietin almost completely inhibited [125I]alpha-bungarotoxin binding in all brain regions. Computer-assisted image analysis of displacement curves was performed on various brain areas rich in alpha-bungarotoxin binding, such as the dorsal endopiriform nucleus, fields 1 and 2 of Ammon's horn, the polymorph cell layer of the dentate gyrus and cortical layers 4 and 5. Thymopoietin inhibited [125I]alpha-bungarotoxin binding with similar potency in all these regions, suggesting that it interacted at the same site in the different brain areas. The IC50 values averaged over the six regions were 24.6 +/- 2.8 nM for thymopoietin and 1.2 +/- 0.2 nM for alpha-bungarotoxin. These results show that thymopoietin specifically interacted with the alpha-bungarotoxin site with a similar potency in different brain regions. It is suggested that thymopoietin represents a selective ligand for alpha-bungarotoxin binding sites in brain.