The in vivo modulation of dopamine synthesis by calcium ions: influences on the calcium independent release

To investigate the contribution of the dopamine (DA) synthesis to both the calcium-dependent and the carrier-mediated, mechanisms of DA release in the striatum, anaesthetized rats were locally superfused in the striatum with a push‐pull cannula supplied with an artificial CSF containing tritiated tyrosine. DA, dihydroxyphenylacetic acid (DOPAC) and their respective specific activity were measured in eCuent and used to evaluate changes in the DA synthesizing rate. Excluding calcium ions from the CSF only partially reduced spontaneous DA release (70%) still leaving a possible carrier-mediated DA release. This eAect was not additive with a local superfusion with 0.1 mM a-methyl-p-tyrosine, a blocker of DA synthesis, suggesting that synthesis could already be reduced by calcium-free superfusion. Local superfusion with 100 mM cadmium in the presence or not of calcium ions, increased the DA release (220 and 350%, respectively), simultaneously reducing DA synthesis. Local application of 1 mM calcium ionophore (A23187) was without eAect on the basal release of DA but enhanced DA synthesis and increased the amphetamine-evoked and carrier-mediated amine release. We conclude that DA synthesis can be a modulatory process of the firing-independent and carrier-mediated amine release while it weakly aAects the classical calcium-dependent release. # 1999 Elsevier Science Ltd. All rights reserved.