CNS glycogen is located in astrocytes, but its function is unclear. Glycogen metabolism supports mouse optic nerve (MON) function during aglycaemia (Brown et al., 2003), and supports function during high intensity stimulus, when ambient glucose is insufficient to meet the elevated energy demands (Brown et al., 2003). Isofagomine inhibits mouse brain glycogen phosphorylase activity (Waagepetersen et al., 2000), thus we used isofagomine to evaluate functional importance of glycogen metabolism in supporting MON axon conduction. Adult male Swiss Webster mice were killed by decapitation under deep CO₂ anaesthesia. MONs were placed in a superfusion chamber at 37°C. Axon function was assessed as CAP area, with glycogen content measured biochemically. Data presented as means S.E.M., and one-way ANOVA employed to determine significance. MONs perfused in 10 or 30 mM glucose, respectively, had glycogen content of 6.01 ±

1. pmoles per µg protein (n = 3), or 7.91 ± 0.64 pmoles per µg protein (n = 3; p ⟨ 0.05), respectively. Isofagomine (400 µM) increased glycogen content of MONs perfused in 10 or 30 mM glucose, respectively, to 9.73 ± 0.75 pmoles per µg protein (n = 4; p ⟨ 0.05), or 11.31 ± 1.28 pmoles per µg protein, respectively (n = 4; n.s.). The robust CAPs seen in MONs bathed in 10 mM glucose disappeared after 15.9 ± 0.4 mins (n = 6) after aglycaemia onset. Introduction of glucose free aCSF in the presence of isofagomine shortened CAP failure to 11.2 ± 0.7 mins (n = 4, p ⟨ 0.05). MONs bathed in 30 mM glucose aCSF, in the absence or presence of 400 µM isofagomine, respectively, displayed onset to CAP failure after

2. ± 2.2 mins (n = 4) or 13.5 ± 1.2 mins, respectively (n = 4, p ⟨ 0.001) after onset of aglycaemia.In MONs in 10 mM glucose, CAP increased then decreased during a 4 min period of 100 Hz stimulation, and fell below baseline after 3 mins (n = 4). MONs pre-treated (4 hrs) with 400 µM isofagomine showed an initial increase and then a rapid, marked decline in the CAP area below baseline after 1 min (n = 4). The results show isofagomine significantly increased MON glycogen content, but during subsequent aglycaemia latency to CAP failure declined, showing the importance of glycogen metabolism under such conditions. Additionally, inhibiting glycogen metabolism accelerated CAP failure during high intensity stimulus, implying glycogen metabolism supports axon function during intense activity in normoglycaemic ambient glucose.