Altered striatal amino acid neurotransmitter release monitored using microdialysis in R6/1 Huntington transgenic mice

Huntington's disease is an autosomal dominant disease which presents with striatal and cortical degeneration causing involuntary movements, dementia and emotional changes. We employed 16-week-old transgenic Huntington mice (R6/1 line developed by Bates and coworkers) that express exon 1 of the mutant human Huntington gene with 115 CAG triplet repeats. At this age, R6/1 mice do not exhibit an overt neurological phenotype nor any striatal neuronal loss. Using microdialysis, we monitored basal and intrastriatal N-methyl D-aspartate (NMDA, 100 microM, 15 min)- and KCl (100 mM, 15 min)-induced increases in local aspartate, glutamate and GABA release in halothane-anaesthetized transgenic mice and wild-type controls. Basal striatal dialysate glutamate levels were reduced by 42% in R6/1 mice whilst aspartate and GABA levels did not differ from those observed in control mice. Intrastriatal NMDA was associated with significantly greater aspartate (at 15 min) and GABA (at 30 min) levels in the R6/1 mice compared to controls, whilst glutamate release rapidly increased to the same extent in both groups. Intrastriatal KCl was associated with enhanced increases (30 min) in local aspartate and glutamate release in the R6/1 mice above those observed in controls whilst the rapid increase (15 min) in GABA release was similar in both groups. The results provide compelling evidence for specific alterations in both basal, as well as NMDA- and KCl-induced, release of striatal amino acid neurotransmitters in this transgenic model of Huntington's disease, even in the absence of manifest neurodegeneration

Evidence for Dysfunction of the Nigrostriatal Pathway in the R6/1 Line of Transgenic Huntington's Disease Mice.

The present multidisciplinary study examined nigrostriatal dopamine and striatal amino acid transmission in the R6/1 line of transgenic Huntington's disease (HD) mice expressing exon 1 of the HD gene with 115 CAG repeats. Although the number of tyrosine hydroxylase-positive neurons was not reduced and nigrostriatal connectivity remained intact in 16-week-old R6/1 mice, the size of tyrosine hydroxylase-positive neurons in the substantia nigra was reduced by 15%, and approximately 30% of these cells exhibited aggregated huntingtin. In addition, using in vivo microdialysis, we found that basal extracellular striatal dopamine levels were reduced by 70% in R6/1 mice compared to their wild-type littermates. Intrastriatal perfusion with malonate in R6/1 mice resulted in a short-lasting, attenuated increase in local dopamine release compared to wild-type mice. Furthermore, the size of the malonate-induced striatal lesion was 80% smaller in these animals. Taken together, these findings suggest that a functional deficit in nigrostriatal dopamine transmission may contribute to the behavioral phenotype and the resistance to malonate-induced neurotoxicity characteristic of R6/1 HD mice

Changes in striatal procedural memory coding correlate with learning deficits in a mouse model of Huntington disease

In hereditary neurodegenerative Huntington disease (HD), early cognitive impairments before motor deficits have been hypothesized to result from dysfunction in the striatum and cortex before degeneration. To test this hypothesis, we examined the firing properties of single cells and local field activity in the striatum and cortex of pre–motor-symptomatic R6/1 transgenic mice while they were engaged in a procedural learning task, the performance on which typically depends on the integrity of striatum and basal ganglia. Here, we report that a dramatically diminished recruitment of the vulnerable striatal projection cells, but not local interneurons, of R6/1 mice in coding for the task, compared with WT littermates, is associated with severe deficits in procedural learning. In addition, both the striatum and cortex in these mice showed a unique oscillation at high γ-frequency. These data provide crucial information on the in vivo cellular processes in the corticostriatal pathway through which the HD mutation exerts its effects on cognitive abilities in early HD

Partial resistance to malonate-induced striatal cell death in transgenic mouse models of Huntington's disease is dependent on age and CAG repeat length

Transgenic Huntington's disease (HD) mice, expressing exon 1 of the HD gene with an expanded CAG repeat, are totally resistant to striatal lesion induced by excessive NMDA receptor activation. We now show that striatal lesions induced by the mitochondrial toxin malonate are reduced by 70-80% in transgenic HD mice compared with wild-type littermate controls. This occurred in 6- and 12-week-old HD mice with 150 CAG repeats (line R6/2) and in 18-week-old, but not 6-week-old, HD mice with 115 CAG repeats (line R6/1). Therefore, we show for the first time that the resistance to neurotoxin in transgenic HD mice is dependent on both the CAG repeat length and the age of the mice. Importantly, most HD patients develop symptoms in adulthood and exhibit an inverse relationship between CAG repeat length and age of onset. Transgenic mice expressing a normal CAG repeat (18 CAG) were not resistant to malonate. Although endogenous glutamate release has been implicated in malonate-induced cell death, glutamate release from striatal synaptosomes was not decreased in HD mice. Malonate-induced striatal cell death was reduced by 50-60% in wild-type mice when they were treated with either the NMDA receptor antagonist MK-801 or the caspase inhibitor zVAD-fmk. These two compounds did not reduce lesion size in transgenic R6/1 mice. This might suggest that NMDA receptor- and caspase-mediated cell death pathways are inhibited and that the limited malonate-induced cell death still occurring in HD mice is independent of these pathways. There were no changes in striatal levels of the two anti cell death proteins Bcl-X(L) and X-linked inhibitor of apoptosis protein (XIAP), before or after the lesion in transgenic HD mice. We propose that mutant huntingtin causes a sublethal grade of metabolic stress which is CAG repeat length-dependent and results in up-regulation over time of cellular defense mechanisms against impaired energy metabolism and excitotoxicity

Norepinephrine in the Medial Pre-frontal Cortex Supports Accumbens Shell Responses to a Novel Palatable Food in Food-Restricted Mice Only

Previous findings from this laboratory demonstrate: (1) that different classes of addictive drugs require intact norepinephrine (NE) transmission in the medial pre Frontal Cortex (mpFC) to promote conditioned place preference and to increase dopamine (DA) tone in the nucleus accumbens shell (NAc Shell); (2) that only food-restricted mice require intact NE transmission in the mpFC to develop conditioned preference for a context associated with milk chocolate; and (3) that food-restricted mice show a significantly larger increase of mpFC NE outflow then free fed mice when experiencing the palatable food for the first time. In the present study we tested the hypothesis that only the high levels of frontal cortical NE elicited by the natural reward in food restricted mice stimulate mesoaccumbens DA transmission. To this aim we investigated the ability of a first experience with milk chocolate to increase DA outflow in the accumbens Shell and c-fos expression in striatal and limbic areas of food-restricted and ad-libitum fed mice. Moreover, we tested the effects of a selective depletion of frontal cortical NE on both responses in either feeding group. Only in food-restricted mice milk chocolate induced an increase of DA outflow beyond baseline in the accumbens Shell and a c-fos expression larger than that promoted by a novel inedible object in the nucleus accumbens. Moreover, depletion of frontal cortical NE selectively prevented both the increase of DA outflow and the large expression of c-fos promoted by milk chocolate in the NAc Shell of food-restricted mice. These findings support the conclusion that in food-restricted mice a novel palatable food activates the motivational circuit engaged by addictive drugs and support the development of noradrenergic pharmacology of motivational disturbance