CALCIUM DEPENDENCY AND TETRODOTOXIN SENSITIVITY OF NEOSTRIATAL DOPAMINE RELEASE IN 5-DAY-OLD AND ADULT-RATS AS MEASURED BY IN-VIVO MICRODIALYSIS

The calcium dependency and tetrodotoxin sensitivity of extracellular d opamine levels were assessed by microdialysis in the neostriatum of 5- day-old rat pups and were compared with those obtained in adult rats. The removal of calcium from the dialysate reduced spontaneous levels o f extracellular dopamine to 20% of normal in the 5-day-old pups and to 10% of normal in the adults. Calcium-free dialysate also decreased po tassium-evoked dopamine release to similar to 20% of baseline in both ages. Furthermore, the addition of tetrodotoxin to the dialysate decre ased spontaneous levels of extracellular dopamine to 10% of baseline i n both ages. The effects of calcium removal and the addition of tetrod otoxin on extracellular levels of the dopamine metabolite 3,4-dihydrox yphenylacetic acid were less pronounced. The results of this study dem onstrate that extracellular levels of dopamine sampled by microdialysi s in rats as young as 5 days of age are both calcium dependent and tet rodotoxin sensitive; thus, they are derived from neuronal activity and not from injury caused by acute implantation of the probe. Other age- related differences support the hypothesis that dopamine release and t urnover is greater in immature rats and may represent a form of compen sation for incomplete dopamine nerve terminal ingrowth.