Synaptic concentration of dopamine in rat striatal slices in relationship to [3H]raclopride binding to the dopamine D2 receptor

The in vivo binding of dopamine (DA) radioligands to D2 receptors can be affected by competition with endogenous dopamine. In the present study, we used a brain slice preparation that provides more controlled conditions than in vivo preparations in order to examine the relationship between synaptic DA and the binding of [3H]raclopride to D2 receptors. We also estimated the synaptic DA concentration in rat striatal slices by determining the changes in [3H]raclopride binding. To correlate the changes in [3H]raclopride binding with the concentration of synaptic DA, the kinetic parameters were determined. [3H]Raclopride reached equilibrium binding conditions within two hours. The Ki value for DA in inhibiting [3H]raclopride binding was about 2.2 nM. The increase in synaptic DA evoked by electrical stimulation decreased the striatal binding of [3H]raclopride in a frequency-dependent manner. Increases in the DA concentration evoked by amphetamine (AMPH) or cocaine decreased [3H]raclopride binding by 74% or 20%, respectively, corresponding to increases in the synaptic DA concentrations of 1.6 nM or 0.6 nM, respectively. Pargyline also decreased [3H]raclopride binding by 36% corresponding at a concentration of 1.2 nM. In contrast, the depletion of synaptic DA by alpha-methyl-p-tyrosine (alpha-MpT) increased the specific binding of [3H]raclopride by 43% when the DA concentration was decreased to 0.7 nM. The changes in the DA concentration at the synapse were responsible for the changes in the striatal binding of [3H]raclopride. The values calculated in this study may therefore approximate the changes in the synaptic DA concentration in rat striatal slices following manipulation.