Ablation of the locus coeruleus increases oxidative stress in tg-2576 transgenic but not wild-type mice

Abstract

Mice transgenic for production of excessive or mutant forms of beta-amyloid differ from patients with Alzheimer's disease in the degree of inflammation, oxidative damage, and alteration of intermediary metabolism, as well as the paucity or absence of neuronal atrophy and cognitive impairment. Previous observers have suggested that differences in inflammatory response reflect a discrepancy in the state of the locus coeruleus (LC), loss of which is an early change in Alzheimer's disease but which is preserved in the transgenic mice. In this paper, we extend these observations by examining the effects of the LC on markers of oxidative stress and intermediary metabolism. We compare four groups: wild-type or Tg2576 Aβ transgenic mice injected with DSP4 or vehicle. Of greatest interest were metabolites different between ablated and intact transgenics, but not between ablated and intact wild-type animals. The Tg2576_DSP4 mice were distinguished from the other three groups by oxidative stress and altered energy metabolism. These observations provide further support for the hypothesis that Tg2576 Aβ transgenic mice with this ablation may be a more congruent model of Alzheimer's disease than are transgenics with an intact LC.

Figure 1

Effects of treatment with DSP4 on monoamine neurotransmitter levels. DSP4 (50 mg/kg in PBS, N = 5) or PBS (vehicle, N = 5) was administered intraperitoneally to C57BL/6SJL mice on days 1 and 7. Mice were sacrificed 7 days following the second injection (day 14). Brain was harvested and frontal cortex dissected for analysis of monoamine concentrations, demonstrating a 68% reduction of NE levels but no effect on serotonin.

Figure 2

Principal components analysis. From an autofit model that yielded 7 principal components, a combination of PC3 and PC6 gives a separation of the wild type and transgenic samples of brain metabolites. This figure shows the scores plot, derived by principal components analysis, as a combination of Principal Component (PC) 3 on the x-axis and PC 6 on the y-axis. Each transgenic animal is represented by a red circle, each wild type by a purple triangle. Each point is labeled either with VEH for vehicle treatment or DSP for drug treatment followed by the number assigned to an individual mouse after assignment to a treatment group, but before any experimental procedure. The clustering observed in this plot indicates separation of the wild-type and transgenic samples based on the global profile of all of the brain metabolites detected in our analysis.

Figure 3

Glutathione metabolism. Glutathione metabolism and gamma-glutamyl-dipeptide formation are altered in the brains of the Tg2576_DSP4 group. For each metabolite, the distribution of values within each of the four cohorts is represented as a Whisker plot, with the relative normalized intensity as the abscissa. Mean values represented by the black arrowheads, median values by blue arrowheads. Detailed statistical comparisons of oxidized glutathione, reduced glutathione, and  γ-glutamine-leucine are given in Table 1. Glutamic acid was omitted from the table even though the P-value for the comparison of ablated transgenics to ablated wild-types was  .0109, the Q-value exceeded our threshold at a value of 0.154.

Figure 4

Ascorbic acid, another marker of oxidative stress. Box plots for levels of ascorbic acid in the brain, displaying the distribution of values within each of the four cohorts represented as a Whisker plot, with the relative normalized intensity as the abscissa. Mean values represented by the black arrowheads, median values by blue arrowheads. Detailed statistical comparisons of ascorbate in the four cohorts are given in Table 1.

Figure 5

Carbohydrate metabolism. Box plots demonstrating selective depressions in the brains of glucose-6-phosphate (an early intermediary of anaerobic glycolysis as well as the pentose monophosphate shunt), lactate the end product of anaerobic glycolysis, and D-arabitol, a polyol with antioxidant properties which is thought to be a product of the pentose phosphate pathway. For each metabolite, the distribution of values within each of the four cohorts is represented as a Whisker plot, with the relative normalized intensity as the abscissa. Mean values represented by the black arrowheads, median values by blue arrowheads. Detailed statistical comparisons of each of the pictured metabolites in the four cohorts are given in Table 1.

Figure 6

Lipids: Octadecanoic (stearic) acid and cholesterol. Box plots describing the modest elevations of octadecanoic acid and cholesterol in the brains of transgenic animals in which the locus coeruleus had been ablated. For each metabolite, the distribution of values within each of the four cohorts is represented as a Whisker plot, with the relative normalized intensity as the abscissa. Mean values represented by the black arrowheads, median values by blue arrowheads. Detailed statistical comparisons of both of these metabolites in the four cohorts are given in Table 1.