Prefrontal cortical GABAergic dysfunction contributes to age-related working memory impairment

Abstract

Working memory functions supported by the prefrontal cortex decline in normal aging. Disruption of corticolimbic GABAergic inhibitory circuits can impair working memory in young subjects; however, relatively little is known regarding how aging impacts prefrontal cortical GABAergic signaling and whether such changes contribute to cognitive deficits. The current study used a rat model to evaluate the effects of aging on expression of prefrontal GABAergic synaptic proteins in relation to working memory decline, and to test whether pharmacological manipulations of prefrontal GABAergic signaling can improve working memory abilities in aged subjects. Results indicate that in aged medial prefrontal cortex (mPFC), expression of the vesicular GABA transporter VGAT was unchanged; however, there was a significant increase in expression of the GABA synthesizing enzyme GAD67, and a significant decrease in the primary neuronal GABA transporter GAT-1 and in both subunits of the GABA(B) receptor (GABA(B)R). Expression of VGAT, GAD67, and GAT-1 was not associated with working memory ability. In contrast, among aged rats, GABA(B)R expression was significantly and negatively associated with working memory performance, such that lower GABA(B)R expression predicted better working memory. Subsequent experiments showed that systemic administration of a GABA(B)R antagonist, CGP55845, dose-dependently enhanced working memory in aged rats. This enhancing effect of systemic CGP55845 was reproduced by direct intra-mPFC administration. Together, these data suggest that age-related dysregulation of GABAergic signaling in prefrontal cortex may play a causal role in impaired working memory and that targeting GABA(B)Rs may provide therapeutic benefit for age-related impairments in executive functions.

Keywords: CGP55845; GABA(B) receptor; aging; executive function; inhibition; prefrontal cortex.

Figure 1.

Working memory is impaired in aged Fischer 344 rats. A, A schematic of the delayed response task used to assess working memory ability. There are three phases to this task. In the sample phase, rats are presented with either a left or right lever. After the rat presses the extended lever, the delay phase begins. In the delay phase, both levers are retracted for a variable time period ranging from 0 to 24 s, during which the rat must nosepoke into the food trough to initiate the choice phase. In the choice phase, both levers are presented and the rat must press the same lever presented in the sample phase to obtain a food reward. B, Young (n = 6) and aged (n = 12) rats performance on the delayed response task. Aged rats displayed significantly less accurate performance relative to young and were disproportionately impaired at long delays. C, Individual young and aged rats plotted by mean long delay (average of choice accuracy at 18–24 s) on the delayed response task. This measure was used as an index of individual working memory ability. Error bars represent ± SEM. See Results for statistical analysis.

Figure 2.

Age-related changes in GABAergic signaling protein expression and relationship with working memory ability. A, Representative immunoreactive bands from young (n = 6) and aged (n = 12) mPFC homogenates following incubation with antibodies to the GABA(B)R subunits R1 and R2, the GABA transporter GAT-1, the GABA synthesizing enzyme GAD67, the vesicular GABA transporter VGAT and loading control GAPDH. B, In the aged mPFC, GABA(B)R1a expression was significantly reduced compared with young; however, as shown in the scatter plot of individual aged rats, there was no significant relationship between GABA(B)R1a expression and delayed response performance. C, Expression of GABA(B)R1b was significantly reduced in aged mPFC compared with young. The scatter plot of individual aged rats shows that, in contrast to GABA(B)R1a, GABA(B) R1b expression was significantly and inversely associated with delayed response performance such that lower expression was associated with better working memory ability. D, GABA(B)R2 expression was also significantly reduced in aged mPFC compared with young. Like the R1b isoform, the scatter plot of protein expression of individual aged rats shows that GABA(B)R2 expression in the mPFC is significantly and inversely related to working memory performance such that those aged rats with the lowest levels of GABA(B)R2 exhibited better working memory. E, Expression of GAT-1 was significantly decreased in aged mPFC compared with young. F, In contrast to all other signaling proteins examined, expression of GAD67, was significantly elevated in aged mPFC compared with young. Neither GAT-1 nor GAD67 expression in mPFC was reliably related to working memory performance. G, Notably, expression of VGAT did not differ between young and aged mPFC, suggesting that aging does not produce a robust loss of inhibitory terminals in this brain region. Error bars represent ± SEM. See Results for statistical analyses. Asterisks indicate significant differences (p < 0.05).

Figure 3.

Systemic GABA(B) receptor antagonist administration significantly improves working memory performance in aged rats. A, Baseline performance on the delayed response task of young (n = 10) and aged (n = 13) rats before drug administration. Aged rats displayed significantly less accurate performance relative to young and were disproportionately impaired at long delays. B, Systemic injection of the selective GABA(B)R antagonist CGP55845 (shaded circles) significantly improved performance of aged rats in the delayed response task over vehicle conditions (open circles). Note that at the highest dose administered (black circles), accuracy of aged rats was restored to a level on a par with young baseline performance (open triangles in A). C, In contrast to drug effects observed in aged rats, systemic injection of CGP55845 (shaded circles) significantly impaired working memory performance in young rats compared with vehicle conditions (open circles). Error bars represent ± SEM. See Results for statistical analyses.

Figure 4.

Intra-mPFC GABA(B) receptor antagonist administration improves working memory performance in aged rats. A, shows bilateral cannula placements in mPFC for each rat included in this experiment (schematic illustrations modified from Paxinos and Watson, 2005). B, Baseline delayed response task performance of young (n = 8) and aged (n = 10) rats before drug administration. Aged rats displayed significantly less accurate performance relative to young. C, Direct mPFC administration of the selective GABA(B)R antagonist CGP55845 (shaded circles) significantly enhanced working memory performance in aged rats over vehicle conditions (open circles). Note that administration of the 0.6 μmol dose (dark gray circles), improved accuracy of aged rats to a level on par with young baseline performance (open triangles in B). D, In contrast to drug effects observed in aged rats, direct mPFC administration of CGP55845 (shaded circles) did not significantly affect working memory performance of young rats compared with vehicle conditions (open circles). Error bars represent ± SEM. See Results for statistical analyses.