Effects of streptozotocin-induced diabetes on acetylcholine metabolism in rat brain

The main objective of this study was to determine whether uncontrolled hyperglycemia, as a consequence of diabetes, altered the metabolism of acetylcholine (ACh) in rat brain. To accomplish this, rats received injections of streptozotocin (STZ, 60 mg/kg, i.v.) or vehicle, and were maintained for up to 7 weeks after the injections. Various indices of ACh metabolism were determined in striatum and hippocampus, two brain regions densely innervated by cholinergic neurons. STZ induced diabetes in 96% of the rats injected, as evidenced by glucose spillage into the urine within 48 hours. Serum glucose levels increased to 326% of control values by 1 week and remained at this level for the duration of the study. The steady-state concentrations of ACh and choline, determined in brain tissue from animals killed by head-focused microwave irradiation, did not differ between the control and STZ-injected groups. However, the synthesis and release of neurotransmitter by striatal slices, measured in vitro, decreased in a time-dependent manner. Although the basal release of ACh was unaltered at 1 week, neurotransmitter release decreased significantly by 21% at 5 weeks and by 26% at 7 weeks. The release of ACh evoked by incubation with 35 mM KCl was inhibited significantly by 20% at all time points studied. ACh synthesis by slices incubated under basal conditions decreased by 13% and 27% at 5- and 7-weeks, respectively, the latter significantly less than controls. Synthesis by striatal slices incubated with 35 mM KCl was inhibited by 17% at 7 weeks. Although the synthesis and release of ACh by hippocampal slices from diabetic animals tended to be less than controls, these alterations were not statistically significant. Investigations into the mechanism(s) mediating the deficit in ACh synthesis exhibited by striatal slices indicated that it did not involve alterations in precursor choline availability, nor could it be attributed to alterations in the activities of the synthetic or hydrolytic enzymes choline acetyltransferase or acetylcholinesterase; rather, the decreased turnover of ACh may be secondary to other STZ-induced, hyperglycemia-mediated neurochemical alterations.