DISCREPANCIES IN CHARACTERIZATION OF SIGMA-SITES IN THE MOUSE CENTRAL-NERVOUS-SYSTEM

The characteristics of [H-3](+)-pentazocine and [H-3]1,3-di(2-tolyl)guanidine (DTG) binding to mouse whole brain,cortex, cerebellum and spinal cord membranes were investigated in radioreceptor assays. [H-3](+)-Pentazocine bound to a single, high affinity site (K-d = 1.2-1.6 nM) with increasing density along the neuraxis from the cortex (B-max = 543 fmol/mg protein) to the spinal cord (B-max = 886 fmol/mg protein). Hot saturation studies resolved the presence of one binding site for [H-3]DTG showing no tissue variations in terms of density (B-max = 1075-1264 fmol/mg protein) or affinity (K-d = 16.6-22.3 nM). Incubation with 100 nM (+)-pentazocine revealed two classes of high affinity [H-3]DTG labeled binding sites corresponding to sigma(1) and sigma(2) subtypes. A preponderance of sigma(2) sites was revealed in all investigated tissues. Different pharmacological profiles were demonstrated for the sigma(2) sites in mouse whole brain compared to mouse spinal cord. However, competition studies indicated that the whole brain and spinal [H-3](+)-pentazocine labeled sigma(1) binding sites exhibited similar pharmacological properties. The density of [H-3](+)-pentazocine labeled sigma(1) population was found not to match that of [H-3]DTG labeled sigma(1) site throughout the mouse central nervous system. The presence of low affinity [H-3]DTG labeled sites was demonstrated in cold saturation experiments. Equilibrium binding data for the low affinity [H-3]DTG binding site resulted in an increasing density (B-max = 1973-11369 fmol/mg protein) with a decreasing affinity (K-d = 242-943 nM) in mouse cortex through the spinal cord.