Thymopoietin, a thymic polypeptide, potently interacts at muscle and neuronal nicotinic alpha-bungarotoxin receptors

Abstract

Current studies suggest that several distinct populations of nicotinic acetylcholine (ACh) receptors exist. One of these is the muscle-type nicotinic receptors with which neuromuscular nicotinic receptor ligands and the snake toxin alpha-bungarotoxin interact. alpha-Bungarotoxin potently binds to these nicotinic receptors and blocks their function, two characteristics that have made the alpha-toxin a very useful probe for the characterization of these sites. In neuronal tissues, several populations of nicotinic receptors have been identified which, although they share a nicotinic pharmacology, have unique characteristics. The alpha-bungarotoxin-insensitive neuronal nicotinic receptors, which may be involved in mediating neuronal excitability, bind nicotinic agonists with high affinity but do not interact with alpha-bungarotoxin. Subtypes of these alpha-toxin-insensitive receptors appear to exist, as evidenced by findings that some are inhibited by neuronal bungarotoxin whereas others are not. In addition to the alpha-bungarotoxin-insensitive sites, alpha-bungarotoxin-sensitive neuronal nicotinic receptors are also present in neuronal tissues. These latter receptors bind alpha-bungarotoxin with high affinity and nicotinic agonists with an affinity in the microM range. The function of the nicotinic alpha-bungarotoxin receptors are as yet uncertain. Thymopoietin, a polypeptide linked to immune function, appears to interact specifically with nicotinic receptor populations that bind alpha-bungarotoxin. Thus, in muscle tissue where alpha-bungarotoxin both binds to the receptor and blocks activity, thymopoietin also potently binds to the receptor and inhibits nicotinic receptors-mediated function. In neuronal tissues, thymopoietin interacts only with the nicotinic alpha-bungarotoxin site and not the alpha-bungarotoxin-insensitive neuronal nicotinic receptor population. These observations that thymopoietin potently and specifically interacts with nicotinic alpha-bungarotoxin-sensitive receptors in neuronal and muscle tissue, together with findings that thymopoietin is an endogenously occurring agent, could suggest that this immune-related polypeptide represents a ligand for the alpha-bungarotoxin receptors. The function of thymopoietin at the alpha-bungarotoxin receptor is as yet uncertain; however, a potential trophic, as well as other roles are suggested.