Impaired production of nerve growth factor in the submandibular gland of diabetic mice

The production of nerve growth factor (NGF) in submandibular glands was examined in two kinds of diabetic mice. In genetically diabetic (C57BL/KsJ db/db) mice, which manifest marked insulin resistance and hyperglycemia, the concentration of NGF in the submandibular gland was less than one-tenth that of the nondiabetic controls. In streptozotocin-induced diabetic C57BL/KsJ mice, which show pancreatic insulitis leading to insulin deficiency and hyperglycemia, the glandular NGF concentration fell in a time-dependent manner to 26% of control level at 5 wk after the streptozotocin injection. A daily administration of insulin to the streptozotocin-induced diabetic mice restored the NGF concentration to almost the control level. The molecular size of NGF (13 kDa) in the glandular extracts of the genetically diabetic (db/db) mice in Western blots was indistinguishable from that of the control mice, but its level was reduced in the glands of the diabetic (db/db) animals. Although plasma NGF concentrations were normally below the sensitivity of the assay (less than 0.80 ng/ml) in both the control and the diabetic (db/db) mice, administration of cyclocytidine, which stimulates NGF release from the submandibular gland into the blood circulation, increased the plasma NGF level to 5.95 ng/ml in the control mice, but it failed to do so in the diabetic (db/db) mice. These findings suggest that, in diabetic mice, NGF production in the submandibular gland and its capacity to release NGF into the circulation are decreased.