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The release of dopamine (DA) from striatal terminals appears to be a rather complex and misunderstood phenomenon. It has been observed from many years that this release is only partially dependent on calcium ions in the extracellular space (Arbilla and Langer, 1978; Raiteri et al., 1979; Schwartz et al, 1980; Okuma et al., 1983; Woodward et al., 1988). However, it is now well known that the synthesis of DA is modulated by means of alterations of tyrosine hydroxylase (TH) activity under the control of calcium-dependent phosphorylating mechanisms (Fung and Uretsky, 1982; El Mestikawy et al., 1983; Albert et al., 1984; Chowdhury and Fillenz, 1988; Zigmond et al. 1989). In addition, the synthesis of DA is the basis of its release (Costa et al., 1972; Roth et al., 1973; Schwarz et al., 1980) probably through an increase of both the vesicular and the cytosolic stores. Thus, in the striatum calcium is clearly related to the extracellular DA concentration both by its action on synthesis and release.

Positron emission tomography (PET) is a visualization technique which offers the feasibility to realize “in vivio” quantitative autoradiographic measurements. This method yields, in a non invasive way, an image (which can be quantified) of a previously administered radioligand in any transverse section of the body.