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

Abstract

Thymopoietin, a 48–49-amino acid polypeptide present in the thymus gland, was investigated as a potential ligand for the neuronal nicotinic α-bungarotoxin binding site in rat brain. Binding of [ 125I]α-bungarotoxin to whole rat brain sections was inhibited by thymopoietin in a concentration-dependent manner with an ic 50 of30.0 ± 8.2nM as compared to1.1 ± 0.3nM for α-bungarotoxin. However, at concentrations of thymopoietin of up to 1 μM, [ 3H]nicotine binding to high affinity sites was not inhibited. Thysplenin, a polypeptide with considerable homology to thymopoietin did not affect [ 125I]α-bungarotoxin binding. These results suggest that thymopoietin selectively interacts with the nicotinic α-bungarotoxin binding site labelled by [ 125I]α-bungarotoxin rather than the neuronal nicotinic receptor(s) labelled by [ 3H]nicotine. Autoradiographic studies revealed that 1 μM thymopoietin almost completely inhibited [ 125I]α-bungarotoxin binding in all brain regions. Computer-assisted image analysis of displacement curves was performed on various brain areas rich in α-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]α-bungarotoxin binding with similar potency in all these regions, suggesting that it interacted at the same site in the different brain areas. The ic 50 values averaged over the six regions were24.6 ± 2.8nM for thymopoietin and1.2 ± 0.2nM for α-bungarotoxin. These results show that thymopoietin specifically interacted with the α-bungarotoxin site with a similar potency in different brain regions. It is suggested that thymopoietin represents a selective ligand for α-bungarotoxin binding sites in brain.