Changes in apparent in vivo binding of [H-3]raclopride and [H-3]N-methylspiperone induced by oxotremorine - Short communication

The effects of muscarinic agonist, oxotremorine (0.3 mg/kg), and antagonist , scopolamine (0.5 mg/kg) on in vivo [H-3]raclopride (RAC) and [H-3]N-methy lspiperone (NMSP) binding were investigated. Following tracer administratio n to control or pretreated mice, binding potentials, and the rate constants k(3) and k(4) were determined by kinetic analysis. Oxotremorine resulted i n a 70% increase in striatal RAC binding potential compared with controls. RAC and NMSP showed almost identical decreases in k(3) (40 %), whereas k(4) for RAC was unexpectedly decreased by 64%. Scopolamine resulted in no sign ificant changes in RAC or NMSP binding. These results, in combination with previous data obtained in reserpinized mice, show that 1) competition by en dogenous ligand may not be the only factor influencing the magnitude of app arent in vivo receptor binding, and 2) interneuronal communication may be p artly mediated by changes in the rates of ligand-receptor binding.