SYNAPTIC CONCENTRATION OF DOPAMINE IN THE MOUSE STRIATUM IN RELATIONSHIP TO THE KINETIC-PROPERTIES OF THE DOPAMINE-RECEPTORS AND UPTAKE MECHANISM

The concentration of dopamine (DA) in the synaptic cleft in the mouse striatum in vivo was estimated from the competition between the synaptic DA and the H-3-labelled DA D2 receptor agonists N-n-propylnorapomorphine (NPA) or N,N-diethyl-N'-](3-alpha,4a-alpha,10-beta)-1,2,3,4,4-alpha,5,10,10-alpha-octahydro-7-hydroxyl-1-propyl-3-benzo (g) quinolinyl]sulfamide (Sandoz 205-501) injected intravenously in tracer doses. Knowing the inhibitor constant for DA in inhibiting the binding of these receptor agonists in vitro, attempts were made to calculate the changes in the synaptic DA concentration from the changes in the in vivo binding of the receptor agonists evoked by various pharmacological agents. Inhibiting the firing of the dopaminergic neurons by gamma-butyrolactone (GBL) increased the binding of the receptor agonists corresponding to a decrease in the synaptic DA concentration of 55 +/- 2 nM in the experiments with [H-3]Sandoz 205-501 and 48 +/- 3 nM in the experiments with tracer doses of [H-3]NPA. These values may therefore approximate the normal DA concentration in the synaptic cleft in the mouse striatum. With this technique it was also possible to determine the synaptic concentration of NPA by its competition with [H-3]Sandoz 205-501 for the DA D2 receptors in the striatum of GBL-treated mice in vivo. To compare the estimated synaptic concentration of DA with the affinity of DA to D1 and D2 receptors and to the DA transporter in the mouse striatum the kinetic parameters were determined at 37-degrees-C in vitro. The K(i) value for DA in inhibiting the binding of ligands to the agonist sites of the D1 receptors (30-40 nM) was about one-third of that of the D2 receptors (80-120 nM). The K(m) value of the initial uptake of [H-3]DA into synaptosomes from the mouse striatum was 209 +/- 20 nM (n = 3) and the V(max) was 7.00 +/- 1.29 nmol/g tissue/min. Comparison of the affinities of DA for the D1 and D2 receptors with the synaptic DA concentrations shows that a considerable proportion of the receptors is occupied by DA under these conditions, whereas the uptake sites are occto a lesser degree.