The Relationship Between the Stimulation of Dopamine Synthesis and Release Produced by Amphetamine and High Potassium in Striatal Slices

Abstract

The effects of amphetamine (amph) and high K⁺ on the synthesis and release of dopamine (DA) were compared in striatal slices. Both agents stimulated DA synthesis as well as release. For both agents, Ca²+ was required for the initiation of synthesis stimulation as well as for the maintenance of this stimulation. The addition of EGTA to medium containing slices that were already stimulated by 1.0μm-amph or 55 mm-K⁺ markedly reduced the stimulation of DA synthesis. Although it has been reported that high K⁺ activates soluble tyrosine hydroxylase (TH), neither high K¹ nor amph appeared to increase the affinity of the synthetic cofactor, 6-MPH₄, or decrease the affinity of the catechol, DA, for TH. This finding was supported by the observation that the inhibitory effect of L-DOPA on DA synthesis in slices, in which synthesis was stimulated by either agent, was not decreased. Although both 1.0μM-amph and 55 mm-K® stimulated the release of [³H]DA from striatal slices, the release produced by K⁺ was Ca²+-dependent, whereas release produced by amph did not occur at any concentration tested. Studies on pH requirements for both synthesis and release also confirmed a similarity between amph and K¹ in stimulating synthesis but not in stimulating release. These results suggest that depolarizing agents, such as high K¹, couple synthesis and release of DA by a Ca²+-dependent mechanism. In contrast, the simultaneous stimulation of synthesis and release by amph is not regulated by Ca²+.