Hazards of Using Masking Protocols When Performing Ligand Binding Assays: Lessons From the Sigma-1 and Sigma-2...

Area of Science:

• Pharmacology
• Neuroscience
• Biochemistry

Background:

• Sigma-1 and sigma-2 receptors are key therapeutic targets.
• Quantifying sigma-2 receptors often involves masking sigma-1 receptors using selective agents in radioligand binding assays.
• The molecular identity of sigma-2 receptors is recently elucidated, necessitating accurate quantification methods.

Purpose of the Study:

• To assess the suitability of current masking agents and protocols for quantifying sigma-2 receptors.
• To identify potential interferences and inaccuracies in existing sigma-2 receptor binding assays.
• To propose alternative strategies for reliable sigma-2 receptor characterization.

Main Methods:

• Radioligand binding assays were employed to evaluate sigma-1 receptor masking agents, specifically (+) pentazocine and dextrallorphan.
• Saturation and competition binding isotherm assays were utilized to quantify sigma-2 receptor parameters.
• The affinity of masking agents and radioligands like [³H] di-O-tolyl guanidine (DTG) for both sigma-1 and sigma-2 receptors was determined.

Main Results:

• The sigma-1 selective agent (+) pentazocine, while having low affinity for sigma-2 receptors, can compete with [³H] DTG at commonly used masking concentrations, affecting DTG's apparent affinity.
• [³H] DTG can displace (+) pentazocine from sigma-1 receptors, leading to incomplete masking and potential overestimation of sigma-2 receptor numbers.
• Dextrallorphan also fails to fully mask sigma-1 receptors in the presence of [³H] DTG, similarly risking overestimation of sigma-2 receptors.

Conclusions:

• Current masking protocols using (+) pentazocine or dextrallorphan are unsuitable for accurate sigma-2 receptor quantification due to receptor unmasking and competitive binding.
• These masking agents can lead to significant overestimation of sigma-2 receptor density when sigma-1 receptors are present.
• Researchers should utilize cell lines lacking sigma-1 receptors (e.g., MCF7) without masking agents or employ advanced binding analysis for reliable sigma-2 receptor studies.