The distribution of haloperidol-sensitive (+)[H-3]N-allylnormetazocine ((+)[H-3]SKF-10,047) binding sites (sigma-sites) in subcellular fractions of rat brain homogenates was extensively characterized. In synaptosomal fractions, enriched in choline acetyltransferase activity, sigma-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 sigma-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 sigma-sites over a significantly lower density range than 5'-nucleotidase activity or ATP-stimulated [H-3]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 sigma-sites. It is concluded that rat brain sigma-sites are not concentrated at synaptic regions of plasma membrane. However, the possibility that sigma-sites are localized to specialized areas of nonsynaptic plasma membrane cannot be excluded.
