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)gu anidine (DTG) binding to mouse whole brain,cortex, cerebellum and spin al cord membranes were investigated in radioreceptor assays. [H-3](+)- Pentazocine bound to a single, high affinity site (K-d = 1.2-1.6 nM) w ith increasing density along the neuraxis from the cortex (B-max = 543 fmol/mg protein) to the spinal cord (B-max = 886 fmol/mg protein). Ho t saturation studies resolved the presence of one binding site for [H- 3]DTG showing no tissue variations in terms of density (B-max = 1075-1 264 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 t issues. Different pharmacological profiles were demonstrated for the s igma(2) sites in mouse whole brain compared to mouse spinal cord. Howe ver, competition studies indicated that the whole brain and spinal [H- 3](+)-pentazocine labeled sigma(1) binding sites exhibited similar pha rmacological properties. The density of [H-3](+)-pentazocine labeled s igma(1) population was found not to match that of [H-3]DTG labeled sig ma(1) site throughout the mouse central nervous system. The presence o f low affinity [H-3]DTG labeled sites was demonstrated in cold saturat ion experiments. Equilibrium binding data for the low affinity [H-3]DT G binding site resulted in an increasing density (B-max = 1973-11369 f mol/mg protein) with a decreasing affinity (K-d = 242-943 nM) in mouse cortex through the spinal cord.