Differences in the allosteric modulation by phenytoin of the binding properties of the σ1 ligands [3H](+)-pentazocine and [3H]NE-100

• Published in: Synapse (New York, N.Y.) N.Y.), 2006-03, Vol.59 (3), p.152-161
• Main Authors: Cobos, Enrique J., Lucena, Gema, Baeyens, José M., Del Pozo, Esperanza
• Format: Article
• Language: English
• Subjects: [3H](+)-pentazocine; [3H]NE-100; allosteric modulation; binding assays; phenytoin; σ1 ligands; σ1 receptors
• ISSN: 0887-4476; 1098-2396
• DOI: 10.1002/syn.20230

The present study evaluated the effects of phenytoin (DPH) on the binding to synaptosomal fraction membranes from guinea pig brain of the prototypic sigma1 (σ1) receptor agonist [3H](+)‐pentazocine and the putative σ1 antagonist [3H]NE‐100. Equilibrium and binding kinetics studies were done. The order of affinity of 12 σ1 ligands for binding sites labeled with [3H](+)‐pentazocine correlated well with their order of affinity for sites labeled with [3H]NE‐100, suggesting that both radioligands label the same receptor. Phenytoin increased the binding of [3H](+)‐pentazocine, enhancing its affinity (KD value) for σ1 receptors and decreasing its dissociation rate from these receptors. The maximal number of receptors (Bmax value) labeled with [3H](+)‐pentazocine was not changed. In contrast, phenytoin decreased the specific binding and maximal number of receptors labeled with [3H]NE‐100, and increased its dissociation rate from σ1 receptors. The affinity of this radioligand for σ1 receptors was not modified. In conclusion, phenytoin behaved as a positive allosteric modulator on the binding of [3H](+)‐pentazocine, whereas it negatively modulated the binding of [3H]NE‐100. These results add evidence in favor of the use of phenytoin in vitro to distinguish between agonists and antagonists of σ1 receptors. Synapse 59:152–161, 2006. © 2005 Wiley‐Liss, Inc.