Altered brain neurotransmitter receptors in transgenic mice expressing a portion of an abnormal human huntington disease gene

Abstract

Loss of neurotransmitter receptors, especially glutamate and dopamine receptors, is one of the pathologic hallmarks of brains of patients with Huntington disease (HD). Transgenic mice that express exon 1 of an abnormal human HD gene (line R6/2) develop neurologic symptoms at 9-11 weeks of age through an unknown mechanism. Analysis of glutamate receptors (GluRs) in symptomatic 12-week-old R6/2 mice revealed decreases compared with age-matched littermate controls in the type 1 metabotropic GluR (mGluR1), mGluR2, mGluR3, but not the mGluR5 subtype of G protein-linked mGluR, as determined by [3H]glutamate receptor binding, protein immunoblotting, and in situ hybridization. Ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate receptors were also decreased, while N-methyl-D-aspartic acid receptors were not different compared with controls. Other neurotransmitter receptors known to be affected in HD were also decreased in R6/2 mice, including dopamine and muscarinic cholinergic, but not gamma-aminobutyric acid receptors. D1-like and D2-like dopamine receptor binding was drastically reduced to one-third of control in the brains of 8- and 12-week-old R6/2 mice. In situ hybridization indicated that mGluR and D1 dopamine receptor mRNA were altered as early as 4 weeks of age, long prior to the onset of clinical symptoms. Thus, altered expression of neurotransmitter receptors precedes clinical symptoms in R6/2 mice and may contribute to subsequent pathology.

Figure 1

Receptor binding autoradiography of normal and R6/2 mice brains. Representative coronal sections of control (A, C, and E) and transgenic R6/2 mouse brain hemispheres (B, D, and F) subjected to [³H]AMPA binding (A and B), [³H]QNB binding for muscarinic acetylcholine receptors (C and D), or [³H]SCH-23390 binding for D₁-like receptors (E and F). With all ligands, the R6/2 mice have lower levels of binding, but the extent of receptor binding decrease varies greatly, with D₁ DARs most affected, mAChRs somewhat less affected, and AMPA subtype of ionotropic GluR only slightly decreased.

Figure 2

In situ hybridization and immunofluorescence for mGluRs in control (A–D) and transgenic R6/2 (E–H) mice. In situ hybridization for mGluR2 mRNA in 4-week-old (A and E) and 12-week-old mice (B and F). mGluR2 mRNA hybridization signal appears dark on this film image. Note decreased labeling of cortical neurons in 12-week-old R6/2 mice; some decrease in cortical signal can be noticed in 4-week-old animals. (C and G): Fluorescent immunohistochemistry using an antibody that recognizes mGluR2 and mGluR3; signal appears bright in this image. Bar = 20 μm. Decreased mGluR2/3 immunoreactivity is apparent in the cortex of R6/2 mice. (D and H): In situ hybridization for mGluR5 mRNA.