Ganglionic nAChRs and high-affinity nicotinic binding sites are not equivalent

Area of Science:

• Neuroscience
• Pharmacology
• Molecular Biology

Background:

• High-affinity binding sites for nicotine and acetylcholine (ACh) have been observed in the vertebrate brain.
• These sites were hypothesized to represent desensitized ganglionic (C6) nicotinic acetylcholine receptors (nAChRs).

Purpose of the Study:

• To investigate whether the pheochromocytoma cell line PC12, expressing C6 nAChRs, harbors high-affinity [3H]ACh-binding sites.
• To determine the relationship between high-affinity nicotinic binding sites and C6 ganglionic receptors.

Main Methods:

• Radioligand binding assays using [3H]ACh to quantify binding sites in PC12 cells.
• Measurement of carbamylcholine-stimulated Na+ flux to assess receptor desensitization.
• Comparison of binding affinities and desensitization concentrations for nicotine and ACh.

Main Results:

• The high-affinity nicotinic [3H]ACh-binding site was found to be absent in PC12 cells.
• Concentrations of nicotine or ACh required for receptor desensitization were significantly higher (at least two orders of magnitude) than those used in binding experiments.

Conclusions:

• High-affinity nicotinic binding sites are not equivalent to C6 ganglionic receptors.
• The study challenges the proposed identity of high-affinity binding sites as C6 nAChRs.