Expression of the weaver gene in dopamine-containing neural systems is dose-dependent and affects both striatal and nonstriatal regions

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Abstract
In an earlier report we presented evidence pointing to a differential effect of the mutant gene weaver on the dopamine-containing fiber systems innervating the striatum. In mice homozygous for the weaver mutation, there is a severe loss of dopamine in the caudoputamen, the main target of the nigrostriatal system. By contrast, dopamine is entirely conserved in the nucleus accumbens, a target of the mesolimbic system, and is moderately affected in the olfactory tubercle. The present study shows that these defects in dopamine are gene dose- dependent, that they are established by the end of the first month of life, and that the losses are permanent and not progressive. As in homozygous weavers, the greatest defects in striatal dopamine in heterozygous weavers occur in the dorsolateral caudoputamen and the lateral olfactory tubercle. The abnormalities in the striatal dopamine content of weaver mice are not accompanied by abnormalities in the turnover of dopamine, judging from measurements of the dopamine metabolite dihydroxyphenylacetic acid. Norepinephrine content is also normal in each striatal region. No deficits in striatal dopamine occur in mice homozygous for the mutant genes staggerer and Purkinje cell degeneration, which, like the weaver mutation, result in ataxia and cerebellar pathology. A survey of nonstriatal regions in the weaver mice showed that the effects of the weaver gene on the dopamine- containing innervation of the forebrain are not confined to striatal targets but also extend to the septum and the hypothalamus. By contrast, dopamine in the frontal cortex, the amygdala, the olfactory bulb, and the retina is entirely spared. The pattern and extent of loss of dopamine in the weaver forebrain is thus region- and system- specific. In confirmation of our initial findings, a ca. 30% depletion of dopamine occurs in the weaver midbrain, the region containing the cell bodies of origin of the mesostriatal dopamine systems. A comparison of histofluorescent sections through weaver and control midbrains revealed a reduction of catecholamine-containing neurons in the pars compacta of the weaver animals. These results point to a subpopulation of dopamine-containing neurons as primary targets of the weaver gene or as being closely associated with such primary targets. As a gene-dose effect has also been shown for the cerebellar granule cell loss in the weaver, the mutant gene must have at least 2 cellular targets. We suggest that the cerebellar and mesostriatal pathologies may be linked by a common molecular mechanism.