Diabetes-induced alterations in calcium homeostasis in sensory neurones of streptozotocin-diabetic rats are restricted to lumbar ganglia and are prevented by neurotrophin-3

Aims/hypothesis: In diabetic sensory polyneuropathy the earliest and most severe pathophysiology occurs in neurones with the longest axons. The aim of this study was to characterise a diabetes-induced neuro-degenerative marker that was selective for sensory neurones with the longest axons. We studied alterations in calcium homeostasis since this occurs in other neurodegenerative diseases. Methods: Sensory neurones were cultured from control and streptozotocin-diabetic rats, treated with or without human recombinant neurotrophin-3 (hrNT-3), and neurones from L4-L6 dorsal root ganglia (DRG) which exhibit the longest axons in vivo were compared with those from C5-L3 DRG. Fluorescent video-imaging was used to measure cytoplasmic calcium dynamics. Results: Streptozotocin diabetes of 8 to 14 weeks, induced an increase in resting internal Ca²⁺ concentration ([Ca²⁺]_i), from 67 ± 7 nmol/l in small neurones and 79 ± 9 nmol/l in big neurones obtained from control animals to 214 ± 19 nmol/l in small neurones and 273 ± 30 nmol/l in big neurones after 14 weeks of diabetes (p < 0.05) in L4-L6 DRG cultures. Neurones from C5-L3 ganglia and non-neuronal cells were not affected. Treatment of 14-week streptozotocin-diabetic rats with subcutaneous injection of 5 mg/kg NT-3 normalised the increase in resting [Ca²⁺]_i. The amplitudes induced by depolarisation, caffeine and ATP [Ca²⁺]_i responses were reduced in small (< 30μm diameter) but not big (> 35μm diameter) neurones of L4-L6 DRG from streptozotocin-diabetic animals; the C5-L3 DRG were not similarly affected and the changes in the L4-L6 DRG were corrected by NT-3 treatment. Conclusions/interpretation: Altered calcium homeostasis could be an early molecular marker linked to the onset of diabetic sensory neuropathy. This neurodegenerative index can be corrected by NT-3 therapy and should encourage further work aimed at understanding the mechanistic basis of these observations.