Subcellular distribution of dopamine in substantia nigra of the rat brain: effects of gamma-butyrolactone and destruction of noradrenergic afferents suggest formation of particles from dendrites.

Abstract— The subcellular distribution of dopamine (DA) in substantia nigra from individual male rats was studied with a fractionation procedure on microscale. After differential centrifugation the distribution of DA coincided with that of noradrenaline (NA) which can serve as a marker for synaptosomes in this area. The proportion of DA/NA concentrations was about 1–2 in most fractions. Sixty per cent of nigral DA was found in P₂ (17,000 g). When P, was layered on a continuous density gradient, DA and NA peaked at the density of 1.0–1.2 M-sucrose. Since DA-containing particles covered a relatively broad range on this gradient, particles between 0.7 and 1.3 M-sucrose were collected with a discontinuous density gradient. Sixty per cent of DA from P₂, was found in this subfraction. The particles containing DA could have been derived from dendrites or axon collaterals of nigrostriatal neurones or represent precursor DA in noradrenergic (NA) terminals. The role of collaterals was investigated by comparing the effect of γ-butyrolactone (GBL, 750 mg/kg, 1 h) on DA concentrations in subcellular fractions from substantia nigra and caudate-putamen. In caudate-putamen, GBL produced a marked increase of DA in total homogenates and subcellular fractions except P₃, whereas DA concentrations remained unchanged in all fractions from substantia nigra. This speaks against a contribution from DA terminals. The proportion of DA contained as precursor in NA terminals was analysed after destruction of the NA input to substantia nigra by two methods. A single injection of 6-hydroxydopamine into the IV ventricle decreased nigral NA by 5574, DA only by 17%. Unilateral electrolytic lesions in the pontine tegmentum affected NA concentrations in homogenates and fraction P₂ of the ipsilateral substantia nigra to a much greater extent than DA. From the results obtained with the two approaches, it is estimated that precursor DA in particulate fractions does not exceed 10%. Our observations indicate that dendrites of the DA neurones in substantia nigra can form particles which behave like synaptosomes on density gradients centrifugation; they may be termed ‘dendrosomes’. According to the proportion of DA found in the particulate fractions at least 4040% of nigral DA appear to be localised in dendrites.