Presynaptic Regulation of Dopamine Release in Striatal Compartments and Functional Heterogeneity of the Matrix

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The regulation of dopamine (DA) release from nerve terminals of the nigrostriatal DA neurones depends not only on nerve activity but also on presynaptic processes. DA itself inhibits its own release by acting on DA autoreceptors (D2 and D3 types) present on DA nerve terminals. Other transmitters located in striatal afferent fibers (glutamate), striatal interneurones (acetylcholine-ACh, somatostatine) or collaterals of the medium-sized spiny neurones which innervate either the substantia nigra and the entopeduncular nucleus or the external globus pallidus (GABA, opioid peptides, tachykinines) can facilitate or reduce the release of DA (Chesselet, 1984; Cheramy et al, 1986; Kemel et al, 1989; Krebs et al., 1989; Gauchy et al., 1991). These effects are either direct or indirect mediated by receptors located on DA neurones or on neurones in contact with DA nerve terminals. Due to the heterogeneity of the striatum, local circuits involved in the presynaptic control of DA release could differ from one area to another and this may have functional significance. The striatum is divided indeed in two main compartments, the striosomes (or patches) and the matrix which can be distinguished by several biochemical markers and their afferent or efferent neurones (Graybiel, 1990; Gerfen, 1992). In addition, the matrix itself is heterogeneous since some efferent cells are grouped in clusters (matrisomes) (Desban et al, 1989; Graybiel, 1990). Moreover, although some nigral DA cells project to both striatal compartments, a group of nigral DA cells located in the so-called denso-cellular zone in the cat and in both the ventral part of the pars compacta and the pars reticulata in the rat innervate exclusively the striosomes while other DA cells located in the A8 and AIO DA cell groups project only to the matrix (Gerfen et al., 1987; Jimenez-Castellanos and Graybiel, 1987). Therefore, DA neurones innervating either the striosomes or the matrix could exhibit different properties and be submitted to different presynaptic regulations of DA release. In fact, DA cells which innervate exclusively the striosomes mature first during ontogenesis (Graybiel, 1990), while those responsible for the matrix innervation exhibit a faster turnover rate of DA, contain the calcium calmodulin binding protein and are more sensitive to the neurotoxic effect of MPTP (Gerfen, 1985; Graybiel et al., 1987; Turner et al, 1988; Moratalla et al, 1992).