Reduction in striatal D2 dopamine receptor mRNA and binding following AF64A lesions

Localization of dopamine receptors and associated mRNA in transplants of human fetal striatal tissue in rodents with experimental Huntington's disease

Huntington's Disease (HD) is characterized by deficits in motor and cognitive functions. This neurodegenerative disease shows an extensive loss of medium-sized spiny projection neurons (GABAergic) within the neostriatum. With the loss of these neurons, there is a concomitant loss of associated receptors, such as those for GABA, glutamate, and dopamine. In the present study, we have addressed the question of whether dopamine receptors are re-established in the lesioned rodent striatum following the transplantation of human striatal cells. Human striatal cell suspension or saline (transplant controls) was injected into the striatum of rats previously lesioned with quinolinic acid (QA). Three nine months following transplantation, the animals were sacrificed and the brains were processed for receptor autoradiography and in situ hybridization of dopamine D1 and D2 receptor subtypes. Our results demonstrate that animals transplanted with human striatal cells show a significant increase in D1 receptors following transplantation when compared to the lesion area in control animals, while D1 receptor mRNA remains unchanged. In contrast to D1 receptor binding, D2 receptor levels are not increased in the lesioned and transplanted area of the striatum when compared to controls; however, D2 receptor mRNA levels are significantly increased. These results demonstrate that at the times the animals were examined, D1 and D2 receptors were differentially regulated. Our results further indicate that human striatal primordium will survive following transplantation and will express D1 receptors and D2 receptor mRNA that are depleted in the QA lesioned rodent striatum. This study compliments and extends previous findings on human striatal cell transplantation in rodent models of HD.

Time dependent changes in DA uptake sites, D1 and D2 receptor binding and mRNA after 6-OHDA lesions of the medial forebrain bundle in the rat brain

Quantitative receptor autoradiography and in situ hybridization techniques were used to examine the temporal pattern of changes in dopamine uptake sites, D1 and D2 receptors and their transcripts in the striata of animals lesioned with 6-hydroxydopamine. Animals were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4, 6, 8, and 16 weeks postlesion. Degeneration of the nigrostriatal pathway induced a significant loss of dopamine uptake sites in the ipsilateral caudate putamen of all lesioned animals. D1 receptor binding was significantly increased in the caudate putamen on the lesioned side from 1 week to 16 weeks postlesion, whereas the expression of D1 receptor mRNA did not show any change during this period. There was a significant upregulation of D2 receptor binding as well as D2 mRNA from 2 weeks to 8 weeks postlesion. However, at 16 weeks postlesion, D2 receptor binding continued to increase, whereas the mRNA appeared to compensate. These studies show that a different regulatory mechanism may exist between these two DA receptor subtypes. D1 receptor changes occur at the post-transcriptional or translational level, whereas D2 alterations occur by both transcriptional and translational processes. These studies also indicate that the postsynaptic supersensitivity observed in D1 receptors may not be accompanied by a corresponding increase in D1 receptor mRNA.