Because HDAC inhibition preserved axon function during ischemia by preserving mitochondria in optic nerves from young mice (Baltan et al. 2011a), we reasoned that protection of aging axon function during ischemia by HDAC inhibition should similarly preserve aging mitochondria. Axonal mitochondria in MONs can be selectively imaged in the mito-CFP mouse in which a neuron-specific Thy-1 promoter drives expression of mitochondrial-targeted CFP ((Misgeld et al. 2007), Fig. 4a). Therefore, we performed some experiments on 12-month-old Thy-1 mito CFP mice to fluorescently identify of mitochondrial shape, size, and location using SAHA (suberoylanilide hydroxamic acid), a Class I- and II-HDAC blocker. Axonal mitochondria in 12-month-old MONs appeared more abundant based on the increased levels of CFP fluorescence with longer and thicker mitochondria compared with young MONs (Fig. 4a, Control, inset). Interestingly, the reduction in CFP pixel intensity induced by OGD was substantially less (64.8 ± 2.5%, n = 4, p < 0.001, one-way anova) than that observed in 2-month-old mice [19.5 ± 0.5%, (Baltan et al. 2011a)], suggesting that the decline in CFP fluorescence observed in 2-month-old MON was not generally attributable to bleaching of the fluorophore in response to oxidative stress. In addition, OGD resulted in the formation of longer, thicker mitochondria in aging MONs contributing to enhanced CFP fluorescence after OGD (Fig. 4a, OGD, yellow arrows). As expected, the extent of irreversible injury detected after OGD was greater in MONs from 12-month-old Thy-1 mito mice (Fig. 3a, 6.3 ± 1.2%, n = 4), albeit slightly more than axon recovery observed from age-matched Swiss Webster mice (0 ± 0.5%, n = 5, Fig. 2) as previously reported (Baltan et al. 2011a). Conditions that ameliorated axon injury in older MONs, such as blockade of AMPA/kainate receptors with NBQX (Baltan et al. 2008) significantly preserved CFP signal (Fig. 4a, 80.0 ± 2.6%, n = 2, p < 0.05, one-way anova) and improved axon function recovery in older MONs (Fig. 3, 73.2 ± 1.8, n = 4, p < 0.001, one-way anova). Similarly, SAHA (5 μM) pre-treatment preserved CFP pixel intensity (Fig. 4a, 82.6 ± 2.4%, n = 6, p < 0.001, one-way anova) and axon function after OGD (Fig. 3, 85.6 ± 4.7%, n = 6, p < 0.001, one-way anova). Consistent with preservation of mitochondria, SAHA and NBQX prevented loss of the CAP area in the 12-month-old MONs during OGD. An identical and substantial proportion of axons remained functional during OGD with SAHA (42.0 ± 10.1%, n = 6) and NBQX [40.5 ± 1.9%, (n = 4), p = 0.90, Student's t-test] (Fig. 3a). The ischemic injury in older animals is predominantly mediated by glutamate-mediated excitotoxicity (Baltan et al. 2008). Similar amount of protection conferred by AMPA/kainate receptor blockade and HDAC inhibition on CFP pixel intensity and axon function in older animals suggests that HDAC inhibition blocks the excitotoxic pathway during WM ischemic injury.