SIGMA-1 AND SIGMA-2 SITES IN RAT-BRAIN - COMPARISON OF REGIONAL, ONTOGENIC, AND SUBCELLULAR PATTERNS

Radioligand binding assay conditions were established for the selectiv e labeling of sigma-1 and sigma-2 sites in membrane homogenates of rat brain. Selective sigma-1 assays were conducted using 5 nM (+)[H-3]SKF -10,047 in the presence of 300 nM dizocilpine (MK-801). Selective sigm a-2 assays were conducted using 5 nM [H-3]DTG in the presence of 1 mu M (+)SKF-10,047. Distributions of sigma-1 and sigma-2 binding among br ain regions were found to differ. While the brain stem yields the high est level of sigma-1 binding, it yields among the lowest levels of sig ma-2 binding. The reverse is true in hippocampal membranes. Different ontogenetic patterns were also observed. Sigma-2 binding decreases sub stantially during brain development, whereas sigma-1 binding does not vary significantly. Patterns of distribution among subcellular fractio ns of rat brain homogenates were found to be similar. Both sigma-1 and sigma-2 sites are most enriched in microsomal fractions, and neither is enriched in synaptosomal or mitochondrial fractions. The present re sults suggest that sigma-1 and sigma-2 sites are distinct entities; th ey do not appear to be located on a common macromolecule, and they do not represent two different affinity states of a single type of bindin g site. While the precise subcellular locations of sigma-1 and sigma-2 sites remain to be determined, we conclude that localization of eithe r type of binding site to synaptic regions of plasma membrane or to mi tochondria is highly unlikely. (C) 1994 Wiley-Liss, Inc.