Protein kinase C activity is associated with prefrontal cortical decline in aging

Abstract

Aging is associated with deficiencies in the prefrontal cortex, including working memory impairment and compromised integrity of neuronal dendrites. Although protein kinase C (PKC) is implicated in structural plasticity, and overactivation of PKC results in working memory impairments in young animals, the role of PKC in prefrontal cortical impairments in the aged has not been examined. This study provides the first evidence that PKC activity is associated with prefrontal cortical dysfunction in aging. Pharmacological inhibition of PKC with chelerythrine rescued working memory impairments in aged rats and enhanced working memory in aged rhesus monkeys. Improvement correlated with age, with older monkeys demonstrating a greater degree of improvement following PKC inhibition. Furthermore, PKC activity within the prefrontal cortex was inversely correlated with the length of basal dendrites of prefrontal cortical neurons, as well as with working memory performance in aged rats. Together these findings indicate that PKC is dysregulated in aged animals and that PKC inhibitors may be useful in the treatment of cognitive deficits in the elderly.

Figure 1

Pearson’s tests revealed a significant positive correlation between age of the monkey (years) and cognitive improvement following 0.3 μg/kg CHEL (r = 0.777, p = 0.008, A). There was not a significant relationship between age of the monkey and performance following vehicle treatment (r = 0.044, p = 0.904, B). All monkeys were maintained at a baseline of 60-73% correct prior to treatment administration (gray bar).

Figure 2

Prefrontal PKC activation predicted working memory impairment in aged rats. PKC activation was determined by evaluating PKC concentration in the membrane fraction of the medial prefrontal cortex of cognitively characterized rats using ELISA. PKC activation inversely correlated with mean working memory performance over the last 3 test sessions in aged rats, with higher membrane-bound PKC concentration predicting poorer performance (r = -0.780, p = 0.013, A). Performance was not predicted by PKC activation in young adult rats (r = -0.015, p = 0.977, B).

Figure 3

Basal dendritic length correlated with PKC activation and tended to predict working memory performance in aged rats. Apical and basal dendritic length of layer III prefrontal cortical pyramidal neurons was compared in young adult (N = 4 rats, n = 25 neurons) and aged (N = 9 rats, n = 93 neurons) rats. Sholl analysis was used to evaluate quantity of dendritic material in serial bins increasing in increments of 30 μm in radial distance. No significant differences in apical (A) or basal (B) dendritic length between aged (gray lines) and young adult (black lines) rats were observed (values represent means ± SEM.). Measures of total dendritic length were obtained by summating across the Sholl bins and were used for correlations within the aged (C-F). Working memory performance over the last 3 cognitive characterization sessions was not significantly correlated with apical dendritic length (r = -0.068, p = 0.862, C) in aged rats. Basal dendritic length was more strongly related to working memory performance than apical dendritic length. Longer basal dendrites tended to predict better performance, although this correlation did not reach statistical significance (r = 0.495, p = 0.176, D). PKC activation was not significantly correlated with apical dendritic length (r = -0.280, p = 0.466, E). PKC activation inversely correlated with basal dendritic length (r = -0.687, p = 0.041, F). All correlations were performed using Pearson’s test.

Figure 4

Dendritic spine density of layer III prefrontal cortical pyramidal neurons was reduced in aged relative to young adult rats. Representative reconstructions of dendritic segments are illustrated for young adult (N = 3 rats, n = 4 neurons/rat, A) and aged (N = 5 rats, n = 4 neurons/rat, B) rats. Spine density was evaluated in apical (C) and basal (D) dendrites 50 μm and 100 μm from the center of the soma for each neuron. Spine density was significantly reduced in aged relative to young adult rats in both proximal and distal portions of apical dendrites (proximal: p = 0.021; distal: p = 0.023, C) and in proximal portions of basal dendrites (p = 0.023, D). Spine measures were collapsed across regions of the apical and basal dendrite (“Total”). Total spine density was significantly reduced in apical dendrites of aged relative to young rats (p = 0.036, C). Total spine density differences did not reach statistical significance for basal dendrites (p = 0.064, D). Values represent means ± SEM. *, p < 0.05.