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The excitotoxic hypothesis of Huntington's disease pathogenesis suggests that selective striatal neuronal loss results from excessive activation of striatal excitatory amino acid receptors. Using a microdialysis probe mated to an Alzet 2002 mini-osmotic pump three different concentrations of quinolinic acid or vehicle were administered to the striata of rats over a 3-week period. Animals that received a total of 3.3 μmol of quinolinic acid had significant striatal atrophy that could be attributed to two distinct areas of neuronal loss. First, an area of necrosis surrounding the probe was marked by inflammatory infiltrate and a lack of neurons. In the second region, surrounding the necrotic area, there was a significant reduction in nissl-stained cells, with relative sparing of NADPH-diaphorase-staining neurons. In addition, there was a reduction in cytochrome oxidase staining throughout both of the areas of cell loss. Beyond the area of cell loss, the striatum appeared normal in all respects. The striata of animals that received 880 nmol quinolinic acid appeared identical to those that received vehicle. The striata of animals that received 8.8 μmol quinolinic acid showed severe nonselective atrophy of the striatum and some surrounding structures. We conclude that dialytic delivery of 3.3 μmol quinolinic acid produces an area of neuronal destruction that resembles the selective neuronal loss seen in Huntington's disease. This selective neurodegeneration produced by chronic exposure to quinolinic acid simulates more closely the course of Huntington's disease than previously described methods.

Metabotropic glutamate receptors are important mediators of excitatory amino acid neurotransmission in the striatum. Two-color immunofluorescence histochemistry and immunohistochemistry in combination with retrograde tract-tracing techniques were used to examine the distribution of metabotropic glutamate receptor subtypes 1a and 5 (mGluR1a and mGluR5) among identified subpopulations of striatal projection neurons and interneurons. The majority of striatopallidal and striatonigral neurons were double-labeled for both mGluR1a or mGluR5. Approximately 60% to 70% of either striatonigral or striatopallidal neurons expressed mGluR1a- or mGluR5-like immunoreactivity. The percentage of double-labeled striatopallidal or striatonigral projection neurons did not differ among striatal quadrants. Striatal interneurons expressing parvalbumin or somatostatin or choline acetyltransferase exhibited varying degrees of expression of mGluR1a or mGluR5. Virtually all (94%) parvalbumin-immunoreactive striatal neurons expressed mGluR1a-like immunoreactivity with a majority (79%) of these neurons expressing mGluR5-like immunoreactivity. A high percentage (89%) of striatal choline acetyltransferase-immunoreactive neurons were double-labeled for mGluR1a-like immunoreactivity. Approximately 65% of striatal choline acetyltransferase-immunoreactive neurons expressed mGluR5-like immunoreactivity. A majority (65%) of somatostatin-immunoreactive striatal interneurons expressed mGluR1a-like immunoreactivity with a slightly lower percentage (55%) expressing mGluR5-like immunoreactivity. These findings indicate considerable heterogeneity among striatal projection and interneurons with respect to mGluR1a and mGluR5 expression. There may be subpopulations of striatonigral and striatopallidal projection neurons. These results are consistent as well with prior data indicating subpopulations of the different classes of striatal interneurons.

Prior work has shown that activation of metabotropic glutamate receptors can induce burst firing and a form of NMDA receptor independent long term potentiation in lateral septal slice preparations. To study this phenomenon in vivo we used the expression of immediate early gene products as markers for increased neuronal activity following intraseptal injection of the metabotropic agonist 1 S ,3 R -ACPD. Intraseptal injection of 1 S , 3 R ACPD induced the expression of Fos-like, Jun 13-like and Krox24-like immunoreactivity in lateral septal neurons in a dose-dependent fashion. Immediate early gene product expression peaked at 4 to 6 h post-injection and then declined to baseline. Immediate early gene expression was diminished by co-injection of l -AP3 and was not elicited by intraseptal injection of l -AP4, cysteine sulfinic acid or DHPG. Immediate early gene expression was not diminished by chronic lithium treatment but was diminished by chronic treatment with the phospholipase A 2 inhibitor quinacrine. Co-injection of the phospholipase AZ inhibitor NDGA partially suppressed the induction of immediate early gene expression. Metabotropic glutamate receptors regulate lateral septal neuron excitability in vivo and some of their effects may be mediated by activation of phospholipase A 2 . Alternatively, arachidonic acid may play a permissive role it the effects of metabotropic glutamate receptors on lateral septal neurons.

Prior work has shown that intrastriatal injection of the metabotropic glutamate receptor agonist 1S, 3R-ACPD results in pronounced contralateral rotation, and the basis for this effect is thought to be increased activity of dopaminergic nigrostriatal neurons. We tested this hypothesis by determining the expression of Fos-like immunoreactivity after intrastriatal injection of 1S, 3R-ACPD. Intense Fos-like immunoreactivity was noted in the globus pallidus, entopeduncular nucleus, subthalamic nucleus and substantia nigra pars reticula. Ablation of the subthalamic nucleus 10 days prior to intrastriatal injection of 1S, 3R-ACPD abolished rotational behaviour but not Fos-like immunoreactivity in the globus pallidus, entopeduncular nucleus and substantia nigra. Intrasubthalamic injection of 1S, 3R-ACPD produced marked contralateral rotation and a pattern of Fos-like immunoreactivity similar to that seen after intrastriatal 1S, 3R-ACPD injection. These results suggest that stimulation of striatal metabotropic glutamate receptors inhibits striatal projection neuron activity, while stimulation of subthalamic metabotropic glutamate receptors increases subthalamic nucleus activity. Increased subthalamic nucleus activity is necessary and sufficient for the expression of rotational behavior. These results also suggest that metabotropic glutamate receptor antagonists may be useful in the treatment of Parkinson's disease.

We have used RNA in situ hybridization to study the regional expression of the Huntington's disease gene (HD) and its rat homologue in brain and selected nonneural tissues. The HD transcript was expressed throughout the brain in both rat and human, especially in the neurons of the dentate gyrus and pyramidal neurons of the hippocampal formation, cerebellar granule cell layer, cerebellar Purkinje cells and pontine nuclei. Other brain areas expressed lower levels of the HD transcript without pronounced regional differences. Neuronal expression predominated over glial expression in all regions. HD mRNA was also expressed in colon, liver, pancreas and testes. The regional specificity of neuropathology in HD, which is most prominent in the basal ganglia, thus cannot be accounted for by the pattern of expression of HD.

A new and convenient myelin stain is described. Paraformaldehyde fixed tissue is serially immersed in a nitroblue tetrazolium solution and then in a diaminobenzidine solution. The result is distinct blue staining of myelinated fiber tracts. This technique has advantages over presently used myelin stains.

Excitatory amino acids (EAA) such as glutamate and aspartate are probably the neurotransmitters of a majority of mammalian neurons. Only a few previous studies have been concerned with the distribution of the subtypes of EAA receptor binding in the primate brain. We examined NMDA- and quisqualate-sensitive [3H]glutamate binding using quantitative autoradiography in monkey brain (Macaca fascicularis). The two types of binding were differentially distributed. NMDA-sensitive binding was most dense in dentate gyrus of hippocampus, stratum pyramidale of hippocampus, and outer layers of cerebral cortex. Quisqualate-sensitive binding was most dense in dentate gyrus of hippocampus, inner and outer layers of cerebral cortex, and molecular layer of cerebellum. In caudate nucleus and putamen, quisqualate- and NMDA-sensitive binding sites were nearly equal in density. However, in globus pallidus, substantia nigra, and subthalamic nucleus, quisqualate-sensitive binding was several-fold greater than NMDA-sensitive binding. In thalamus, r3H]glutamate binding was generally low for both subtypes of binding except for the anterior ventral, lateral dorsal, and pulvinar nuclei. In the brainstem, low levels of binding were found, and strikingly the red nucleus and pons, which are thought to receive glutamatergic projections, had approximately 1/20 the binding observed in cerebral cortex. These results demonstrate that NMDA- and quisqualate-sensitive [3H]glutamate binding are observed in all regions of primate brain, but that in some regions one subtype predominates over the other. In addition, certain areas thought to receive glutamatergic projections have low levels of both types of binding.