Equilibrium binding characteristics of [3H]thiomuscimol.

The equilibrium binding characteristics of the tritiated GABAA agonist, 5-aminomethyl-3-isothiazolol (thiomuscimol) are described. Using the filtration technique to separate bound- from free-ligand, [3H]thiomuscimol was shown to bind to the GABA(A) receptor site(s) in a saturable manner with a Kd value of 28+/-6.0 nM and a Bmax value of 50+/-4.0 fmol/mg original tissue. In parallel binding experiments, the Kd and Bmax values for [3H]muscimol were determined to be 5.4+/-2.8 nM and 82+/-11 fmol/mg original tissue, respectively. In binding assays using the centrifugation technique, Kd and Bmax values for [3H]thiomuscimol were found to be 116+/-22 nM and 154 13 fmol/mg original tissue, respectively, whereas a Kd value of 16+/-1.8 nM and a Bmax value of 155+/-8.0 fmol/mg original tissue were determined for [3H]muscimol. In comparative inhibition studies using the GABA(A) antagonist SR 95531 and a series of specific GABAA agonists, the binding sites for [3H]thiomuscimol and [3H]muscimol were shown to exhibit similar pharmacological profiles. Autoradiographic studies disclosed similar regional distribution of [3H]thiomuscimol and [3H]muscimol binding sites in rat brain. Highest densities of binding sites were detected in cortex, hippocampus, and cerebellum, whereas low densities were measured in the midbrain structures of rat cortex. In conclusion, the equilibrium GABA(A) receptor binding characteristics of [3H]thiomuscimol are very similar to those of [3H]muscimol.