Effect of varying the ionic concentration of a microdialysis perfusate on basal striatal dopamine levels in awake rats.

In vivo microdialysis was used to study the effects of Ca2+, Mg2+, and K+ ion concentrations on basal extracellular (EC) levels of striatal DA and metabolites in awake rats on the second day (48 h) after implantation of a microdialysis probe. Basal EC striatal dopamine (DA) levels were markedly (90%) and reversibly reduced by removal and subsequent replacement of Ca2+ ions from the microdialysis perfusate. This implies that the EC DA in this preparation is primarily of synaptic origin. The addition and subsequent removal of 1.7 mM MgCl2 to the Mg2(+)-free perfusate produced a reversible decrease (20%) in basal EC DA levels. This decrease may reflect a competitive interaction between Ca2+ and Mg2+ in the process of vesicular release. Basal EC DA levels were also reduced (27%) by decreasing the K+ concentration of the perfusate from 4 mM to 3 mM. However, after restoring the K+ concentration to 4 mM, EC DA levels were slow to return to pretreatment levels. Basal EC 3,4-dihydroxyphenylacetic acid and homovanillic acid levels exhibited a parallel but diminished response to each manipulation of the ionic concentration of the perfusate. This study demonstrates that small variations in the concentrations of Ca2+, Mg2+, and K+ in the perfusate employed in microdialysis preparations will affect basal EC striatal DA and metabolite levels.