Evidence for sigma‐1‐like receptors in isolated rat liver mitochondrial membranes

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Abstract
Sigma (σ) receptors have generated a great deal of interest on the basis of their possible roles in various pathologies, including cytoprotection. Although the exact function of sigma-1 (σ1) receptors is not yet known, their role in the regulation of intracellular Ca2+ levels and sterol biosynthesis, functions that could be assigned to mitochondria, are the only mechanisms described. Using preparations of purified rat liver and brain mitochondria we demonstrate herein the presence of σ-like binding sites. [3H](+)-pentazocine, a σ1 radioligand was used to label these sites. In the liver, [3H](+)-pentazocine labelled one class of binding sites with high affinity (Kd=3 nM), similar to that observed in liver microsomes and synaptic membranes. These sites were located on the outer mitochondrial membranes and displayed high affinity for other σ1 ligands namely, haloperidol, ifenprodil, carbetapentane or 1,3-di(2-tolyl)guanidine (DTG). The presence of σ1 receptors on liver mitochondria was confirmed using double fluorescence immunostaining. [3H](+)-pentazocine binding sites were also found on brain mitochondria but they appeared pharmacologically distinct to the liver ones as [3H](+)-pentazocine and typical σ1 ligands displayed lower affinities for these sites. Nevertheless, [3H](+)-pentazocine binding on both liver and brain mitochondria was modulated by progesterone, a putative endogenous ligand for σ receptors. Our data demonstrates the presence of [3H](+)-pentazocine binding sites with pharmacological characteristics identical to σ1 receptors on rat liver mitochondrial membranes. The pharmacological significance of these sites and their role on mitochondrial function remain unknown.

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