σRecognition sites in brain and peripheral tissues Characterization and effects of cytochrome P450 inhibitors

Abstract

Binding to σ sites in subcellular fractions of brain and in crude homogenates from peripheral tissues of the guinea pig was characterized with the [ 3H]ligands (+)pentazocine and di(2-tolyl)guanidine (DTG). The inhibitory effects of representative σ compounds and cytochrome P450 inhibitors were evaluated in guinea pig tissues, and the effects of cytochrome P450 induction σ binding in the rat were investigated. For both ligands, the majority of sites were localized to the microsomal fractions. The K D values for [ 3H](+)pentazocine- or [ 3H]DTG-labeled by sites in guinea pig liver and testes were 2-fold lower than those in brain and heart. The number of sites labeled by [ 3H](+)pentazocine varied, with an order of liver>testes>brain>heart. In contrast, the B MAX values for [ 3H]DTG-defined σ sites were greatest in testes, followed by liver, brain and heart. The rank order of potency for representative σ and P450 compounds was similar in brain, liver and testes for both [ 3H]ligands, and the potency of selective compounds to displace σ binding in guinea pig liver failed to correlate with their abilities to inhibit cytochrome P450IID1 activity in human liver. Following induction of cytochrome P450IIB1 with phenobarbital or cytochrome P450IA1 with β-naphthoflavone, neither the affinity nor the number of σ sites was altered in rat brain or liver. These results suggest that σ sites in the periphery are similar to those in the brain, and that the σ binding site is not identical with cytochrome P450IIB1, P450IA1 or P450IID1.