3-mercaptopropionic acid administration increases the affinity of [3H]quinuclidinyl benzilate binding to membranes of the striatum and cerebellum.

It is known that quinuclidinyl benzilate (QNB) binds specifically and with high affinity to the cholinergic muscarinic receptor and that behaves as a potent antagonist of this receptor. We have analysed L-[3H]QNB binding to rat CNS membranes after the administration of the convulsant 3-mercaptopropionic acid (MP) (150 mg.kg-1, i.p.). The studies were done in rats killed at two stages: during and after seizures. No changes in [3H]QNB binding to hippocampus and cerebral cortex membranes were found. [3H]QNB binding increased about 40 and 80% in striatum and cerebellum membranes, respectively. The changes were observed both in seizure and postseizures states. The study was extended to the assay of [3H]QNB binding kinetic constants in the anatomical areas modified by the convulsant. The analysis of the saturation curves indicated an increase in the binding affinity but no change in the number of binding sites. Hill number values were near the unit suggesting a non-cooperative interaction between the ligand and the receptor, and the labelling of a homogeneous population of receptor sites. The results suggest the participation of some cholinergic pathways in the development and maintenance of MP-induced seizures.