Changes in nerve growth factor and preprotachykinin messenger RNA levels in the iris and trigeminal ganglion in diabetic rats; effects of treatment with insulin or nerve growth factor.
This study was designed to explore effects of experimental diabetes mellitus on expression of substance P in the trigeminal ganglion and of nerve growth factor (NGF) in the iris. Rats with streptozotocin-diabetes showed an increase in NGF mRNA in the iris (P < 0.01) which was corrected by insulin treatment. There was also an increase in the levels of mRNA for the substance P precursor, preprotachykinin (PPT), in the trigeminal ganglion of these animals, despite there being no change in GAP-43 mRNA. Since expression of both of these peptides is sensitive to NGF in vitro, we examined the effect of treatment of diabetic rats with NGF at three different doses (0.2, 0.5 and 1.0 mg/kg body weight). There was a dose-dependent increase in both gamma-PPT and GAP-43 mRNA in the trigeminal ganglia of NGF-treated diabetic rats. The findings indicate that increased NGF may be responsible for raised substance P levels in the iris.