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

The effects of muscarinic agonist, oxotremorine (0.3 mg/kg), and antagonist, scopolamine (0.5 mg/kg), on in vivo 3H]raclopride (RAC) and 3H]N-methylspiperone (NMSP) binding were investigated. Following tracer administration to control or pretreated mice, binding potentials, and the rate constants k3 and k4 were determined by kinetic analysis. Oxotremorine resulted in a 70% increase in striatal RAC binding potential compared with controls. RAC and NMSP showed almost identical decreases in k3 (40%), whereas k4 for RAC was unexpectedly decreased by 64%. Scopolamine resulted in no significant changes in RAC or NMSP binding. These results, in combination with previous data obtained in reserpinized mice, show that 1) competition by endogenous ligand may not be the only factor influencing the magnitude of apparent in vivo receptor binding, and 2) interneuronal communication may be partly mediated by changes in the rates of ligand-receptor binding.