Haloperidol-sensitive (+)[ 3H]SKF-10,047 binding sites (σ sites) exhibit a unique distribution in rat brain subcellular fractions

Abstract

The distribution of haloperidol-sensitive (+)[ 3H]N-allylnormetazocine ((+)[ 3H]SKF-10,047) binding sites (σ sites) in subcellular fractions of rat brain homogenates was extensively characterized. In synaptosomal fractions, enriched in choline acetyltransferase activity, σ sites were present in lower concentrations than in whole brain homogenates. On the other hand, microsomal and myelin fractions were found to be enriched in σ sites. A similar pattern of enrichment was seen for 5′-nucleotidase activity, a general plasma membrane marker. However, subsequent experiments in which microsomes were subfractionated on linear sucrose gradients led to the recovery of σ sites over a significantly lower density range than 5′-nucleotidase activity or ATP-stimulated [ 3H]ouabain binding, an additional plasma membrane marker. In addition, previously reported distributions of a number of other subcellular markers, including those for endoplasmic reticulum, were found to contrast with the observed distribution of σ sites. It is concluded that rat brain σ sites are not concentrated at synaptic regions of plasma membrane. However, the possibility that σ sites are localized to specialized areas of nonsynaptic plasma membrane cannot be excluded.