Dopamine–glutamate interactions in the striatum: behaviourally relevant modification of excitotoxicity by dopamine receptor-mediated mechanisms
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The two most important afferent projections to the striatum contain glutamate and dopamine, respectively. Excitotoxic damage resulting from excessive stimulation of the N-methyl-D-aspartate subtype of glutamate receptor has been implicated in pathophysiology of ischaemic stroke, hypoglycaemic brain damage and Huntington's disease. We studied the ability of the dopamine system to modify the anatomical, neurochemical and behavioural consequences of glutamatergic toxicity in the striatum. In a first set of experiments, the specific N-methyl-D-aspartate receptor agonist quinolinate was injected unilaterally into the striatum of rats pretreated with one of (i) intraperitoneal (i.p.) saline (controls); (ii) i.p. haloperidol, a D2 dopamine receptor agonist; or (iii) 6-hydroxydopamine lesion of the ipsilateral nigrostriatal tract. Quinolinate-induced striatal damage, as assessed by morphometric and neurochemical criteria, was significantly attenuated in the animals with 6-hydroxydopamine lesions and in those pretreated with haloperidol, compared with saline-pretreated controls. There were no significant differences between the 6-OHDA and haloperidol groups. In a second set of experiments, animals received (i) bilateral intrastriatal quinolinate plus perioperative i.p. saline; (ii) bilateral intrastriatal quinolinate plus i.p. haloperidol; or (iii) bilateral intrastriatal saline. Again, the quinolinate-lesioned animals treated with perioperative haloperidol had significantly less striatal damage than the bilateral quinolinate rats. Behavioural assessment in the Morris Water Maze showed the bilateral quinolinate+haloperidol group to be significantly less impaired on a spatial acquisition task than the bilateral quinolinate animals. Measures of spontaneous daytime motor activity showed significant differences in average speed and rest time between the bilateral quinolinate+haloperidol rats and the bilateral quinolinate group. The performance of the bilateral quinolinate+haloperidol group was not significantly different from that of controls on any of the behavioural tasks. These results indicate an important role for D2 dopamine receptor-mediated mechanisms in striatal excitotoxicity. Since the excitotoxic process involves the same fundamental signalling mechanism that is involved in normal glutamatergic transmission, these findings imply an ability of D2 receptor blockade to modify glutamate signalling in the striatum. These results may have implications for treatment strategies in ischaemic stroke, hypoglycaemic brain damage and schizophrenia.
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