Insulin has been demonstrated to be neuroprotective in brain and
spinal cord ischemia. The mechanism of neuroprotection may involve alterations in metabolism,
protein synthesis or uptake of
GABA by astrocytes. Conversely,
hyperglycemia increases the extent of neurologic damage observed during
ischemia/reperfusion. Diabetic patients are 2-4 times more likely to suffer a
stroke as normoglycemic patients and they also have worsened neurologic outcome. Determining if
insulin, which many diabetics already use as
therapy, can be neuroprotective, would be a possible means of alleviating the detrimental outcome from diabetic
stroke. This study looked at the relationship between topically administered
insulin (1 mIU
insulin/ml and 100 mIU
insulin/ml) during a four vessel occlusion model of global
ischemia and the release of
amino acids, especially
glutamate, from the cortex in
streptozotocin (STZ)-treated rats. The rats were utilized either 5-7 days (ASTZ) or 4-6 weeks (CSTZ) after a single STZ injection. In the ASTZ animals both doses of
insulin increased the amount of the excitotoxic
amino acids,
aspartate and
glutamate, released during reperfusion and the higher dose also increased the levels of
taurine and
GABA during reperfusion. In the CSTZ animals, both doses of
insulin increased the amount of excitotoxic
amino acids during reperfusion and the lower dose increased
GABA levels released during reperfusion. The differences between the ACTZ and CSTZ animals may be due to metabolic differences in the utilization of
glucose.
Insulin may act as a
neuroprotectant by increasing extracellular
GABA resulting in neuroinhibition.