Astrocyte oxidative metabolism and metabolite trafficking after fluid percussion brain injury in adult rats

Abstract

Despite various lines of evidence pointing to the compartmentation of metabolism within the brain, few studies have reported the effect of a traumatic brain injury (TBI) on neuronal and astrocyte compartments and/or metabolic trafficking between these cells. In this study we used ex vivo ¹³C NMR spectroscopy following an infusion of [1-¹³C] glucose and [1,2-¹³C₂] acetate to study oxidative metabolism in neurons and astrocytes of sham-operated and fluid percussion brain injured (FPI) rats at 1, 5, and 14 days post-surgery. FPI resulted in a decrease in the ¹³C glucose enrichment of glutamate in neurons in the injured hemisphere at day 1. In contrast, enrichment of glutamine in astrocytes from acetate was not significantly decreased at day 1. At day 5 the ¹³C enrichment of glutamate and glutamine from glucose in the injured hemisphere of FPI rats did not differ from sham levels, but glutamine derived from acetate metabolism in astrocytes was significantly increased. The ¹³C glucose enrichment of the C3 position of glutamate (C3) in neurons was significantly decreased ipsilateral to FPI at day 14, whereas the enrichment of glutamine in astrocytes had returned to sham levels at this time point. These findings indicate that the oxidative metabolism of glucose is reduced to a greater extent in neurons compared to astrocytes following a FPI. The increased utilization of acetate to synthesize glutamine, and the acetate enrichment of glutamate via the glutamate-glutamine cycle, suggests an integral protective role for astrocytes in maintaining metabolic function following TBI-induced impairments in glucose metabolism.

FIG. 1.

Diagrammatic representation of the metabolism of [1-¹³C] glucose in neurons and astrocytes, and [1,2-¹³C₂] acetate in astrocytes. The metabolism of [1-¹³C] glucose and entry of pyruvate into the TCA cycle via pyruvate dehydrogenase (PDH) is indicated by the solid black circles. The entry of pyruvate into the TCA cycle via pyruvate carboxylase (PC) is indicated by the solid grey circles. The metabolism of [1,2-¹³C₂] acetate is indicated by the hatched circles. See text for details (α-KG, α-ketoglutarate; LDH, lactate dehydrogenase; Glu, glutamate; Gln, glutamine).

FIG. 2.

Representative ex vivo ¹³C nuclear magnetic resonance (NMR) spectra from extracts of the left/injured cortex of sham (A), day 1 (B), day 5 (C), and day 14 (D) post-FPI groups. Spectra were acquired from the left/injured hemisphere and scaled to the internal reference (not shown; Glu, glutamate; Gln, glutamine; Lac, lactate; FPI, fluid percussion injury).

FIG. 3.

The acetate:glucose utilization ratio for the synthesis of glutamate and glutamine in the left/injured (A), and right (B) hemisphere, of sham animals and the day 1, day 5, and day 14 post-FPI groups. There is a significant increase in the acetate:glucose utilization ratio for glutamine at day 1 post-FPI in the left/injured hemisphere compared to both the sham and day 14 post-FPI groups. Values are expressed as mean ± standard deviation (FPI, fluid percussion injury). *p < 0.05 compared to sham-controls.

FIG. 4.

The contribution of glucose to the synthesis of glutamate and glutamine in the left/injured (A) and right (B) hemispheres, of sham animals and those in the day 1, day 5, and day 14 post-FPI groups. There was a significant decrease in the contribution of glucose for glutamate synthesis at day 1 post-FPI in the left/injured hemisphere compared to both the sham and day 5 post-FPI groups. In contrast, FPI did not alter the contribution of glucose to the synthesis of glutamine. Values are expressed as mean ± standard deviation (FPI, fluid percussion injury). *p < 0.05 compared to sham-controls.