Differential regulation of tyrosine hydroxylase in the basal ganglia of mice lacking the dopamine transporter

Abstract

Mice lacking the dopamine transporter (DAT) display biochemical and behavioural dopaminergic hyperactivity despite dramatic alteration in dopamine homeostasis. In order to determine the anatomical and functional integrity of the dopaminergic system, we examined the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis as well as DOPA decarboxylase and vesicular monoamine transporter. TH-positive neurons in the substantia nigra were only slightly decreased (-27.6 +/- 4.5%), which can not account for the dramatic decreases in the levels of TH and dopamine that we previously observed in the striatum. TH mRNA levels were decreased by 25% in the ventral midbrain with no modification in the ratio of TH mRNA levels per cell. However, TH protein levels were decreased by 90% in the striatum and 35% in the ventral midbrain. In the striatum, many dopaminergic projections had no detectable TH, while few projections maintained regular labelling as demonstrated using electron microscopy. DOPA decarboxylase levels were not modified and vesicular transporter levels were decreased by only 28.7% which suggests that the loss of TH labelling in the striatum is not due to loss of TH projections. Interestingly, we also observed sporadic TH-positive cell bodies using immunohistochemistry and in situ hybridization in the striatum of homozygote mice, and to some extent that of wild-type animals, which raises interesting possibilities as to their potential contribution to the dopamine hyperactivity and volume transmission previously reported in these animals. In conjunction with our previous findings, these results highlight the complex regulatory mechanisms controlling TH expression at the level of mRNA, protein, activity and distribution. The paradoxical hyperdopaminergia in the DAT KO mice despite a marked decrease in TH and dopamine levels suggests a parallel to Parkinson's disease implying that blockade of DAT may be beneficial in this condition.

Norepinephrine Transporter–Deficient Mice Exhibit Excessive Tachycardia and Elevated Blood Pressure With Wakefulness and Activity

“…In a similar model of transporter deficiency, TH mRNA, protein, activity, and distribution in NET Ϫ/Ϫ mice were independently regulated in a tissue-specific manner, which demonstrates the need for further investigation of the mechanisms by which NET controls noradrenergic transmission. 34 MAP was elevated immediately after surgery in both genotypes and decreased in a linear fashion thereafter until reaching a plateau at 9 days. The average shift in MAP after transition from rest to activity was equivalent in both genotypes and is in agreement with related studies.…”

Glutamatergic modulation of hyperactivity in mice lacking the dopamine transporter

“…These reductions in dopamine tissue levels could not be explained as a consequence of abnormal development or degeneration of dopamine neurons because quantitation of TH-positive neurons in the substantia nigra was not markedly different from that in WT mice. In addition, markers of dopaminergic terminals such as L-aromatic amino acid decarboxylase (AADC) and the neuronal vesicular monoamine transporter (VMAT2) levels were not significantly decreased (20,23). These findings strongly suggest that depletion of the dopamine storage pools and decreased dopamine release in DAT-KO mice are directly caused by the absence of inward transport of dopamine through the DAT.…”

“…This finding indicates that both dopamine synthesis and turnover are extremely high in the mutant animals. However, the striatal protein levels of TH, the rate-limiting enzyme in the synthesis of dopamine, were reduced by more than 90% of control levels (19,23). This apparent paradox may be explained by the disinhibition of TH, which under normal conditions is subject to tonic inhibition by both intraneuronal and extraneuronal dopamine (3).…”

Paraquat neurotoxicity is mediated by the dopamine transporter and organic cation transporter-3

“…5D) in contrast to their wild-type littermates. Because DAT −/− mice have been reported to display modest reductions of TH in the midbrain (26) and because our in vitro data suggest that cytosolic DA can modulate PQ toxicity, we assessed whether such alterations were present in this hypomorphic DAT model. HPLC analyses of the midbrain demonstrate that DA levels (Fig.…”