Receptor reserve at striatal dopamine receptors modulating the release of [3H]dopamine

Electrically stimulated release of [3H]dopamine [( 3H]DA) in slices of rat striatum was dose dependently inhibited by apomorphine (67% maximal inhibition) with an EC50 of 17 nM. DA receptor inactivation with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ; 0.5 and 2 mg/kg) dose dependently shifted the ED50 for apomorphine to the right and reduced the maximal inhibition obtained. Analysis of the results yielded a hyperbolic plot of receptor occupancy vs. response, indicating that a large receptor reserve (50-60%) for apomorphine exists at the DA autoreceptor in rat striatum mediating inhibition of transmitter release.