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The activity of soluble invertase, and the variation in glucose, fructose and sucrose contents in onion bulbs (Allium cepa) during long-term storage at 10 degrees C and 20 degrees C were investigated. Invertase activity increased progressively after 8 weeks to 0.084 and 0.092 nkat/g fresh weight (FW), then sharply to 0.29 and 0.35 nkat/g FW at 20 degrees C and 10 degrees C, respectively, and remained high during 5 weeks. Then, activity decreased abruptly to 0.039 and 0.041 nkat/g, and remained low during the last 8 weeks and close to that observed initially. Glucose increased to 17.73 and 14.62 mg/g FW after 4 weeks at 20 degrees C and 10 degrees C, respectively, then decreased sharply between week 5 and week 7 to 4.13 and 4.91 mg/g FW, respectively, and remained rather stable ranging from 9 and 10 mg/g FW at both temperatures. Fructose showed a similar pattern and was 14.8 and 21.68 mg/g FW at 20 degrees C and 10 degrees C, respectively. Between week 10 and week 24, fructose ranged from 5 and 6 mg/g FW, and from 6 and 7 mg/g FW at 20 degrees C and 10 degrees C, respectively. Sucrose increased to 19.63 and 14.43 mg/g FW at 20 degrees C and 10 degrees C, respectively, decreased during 3 weeks, and then increased randomly from 5.69 to 9.42 mg/g FW at 20 degrees C, but remained in a steady state at 10 degrees C ranging 5.03 +/- 0.78 mg/g FW. During the last 6 weeks, the sucrose content was higher at 20 degrees C than at 10 degrees C. The fructose-glucose ratio varied during the first 8 weeks but remained at a steady level during the last 16 weeks. The (glucose+fructose)/sucrose ratio increased randomly at 10 degrees C, whereas at 20 degrees C the ratio increased during 10 weeks then decreased progressively during the final 14 weeks.

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To address the role of brain gangliosides in synaptic plasticity, the synthetic ceramide analog, 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) was used to manipulate the biosynthesis of gangliosides in cultured cortical neurons. Spontaneous synchronized oscillatory activity of intracellular Ca2+ between the neurons, which represents synapse formation, was suppressed by the depletion of endogenous gangliosides by D-threo-PDMP, an inhibitor of glucosylceramide synthase. The decreased functional synapse formation was normalized by supplementation of GQ1b but not by the other gangliosides, suggesting that de novo synthesis of ganglioside GQ1b is essential for the synaptic activity (Mizutani A. et al., Biochem. Biophys. Res. Commun. 222, 494-498, 1996). On the other hand, the enantiomer of the inhibitor, L-threo-PDMP, could elevate cellular levels of glycosphingolipids including gangliosides. This paper presents our recent findings on the neurotrophic actions of L-threo-PDMP in vitro and in vivo. We found that L-PDMP could up-regulate neurite outgrowth, functional synapse formation and ganglioside biosynthesis through activating GM3, GD3 and GQ1b synthases. Simultaneously, the activity of p42 mitogen-activated protein kinase was also facilitated by L-PDMP. To evaluate the efficacy of this drug on long term memory, rats were trained for 2 weeks using an 8-arm radial maze task, and then forebrain ischemia was induced by 4-vessel occlusion (for 10 min x 2 with a 60 min interval). Repeated treatment of L-threo-PDMP (40 mg/kg, i.p. for 6 days, twice a day) starting 24 h after the ischemia, improved the deficit of the well-learned spatial memory, demonstrating the potential therapeutic use of the ceramide analog for treatment of neurodegenerative disorders.

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