Effects of hypoglycemia on functional magnetic resonance imaging response to median nerve stimulation in the rat brain

The authors studied the effects of a standardized mild-moderate hypoglycemi c stimulus (glucose clamp) on brain functional magnetic resonance imaging ( fMRI) responses to median nerve stimulation in anesthetized rats. In the ba seline period (plasma glucose 6.6 +/- 0.3 mmol/L), the MR signal changes in duced by median nerve activation were determined within a fixed region of t he somatosensory cortex from preinfusion activation maps. Subsequently, ins ulin and a variable glucose infusion were administered to decrease plasma g lucose. The goal was to produce a stable hypoglycemic plateau (2.8 +/- 0.2 mmol/L) for 30 minutes. Thereafter, plasma glucose was restored to euglycem ic levels (6.0 +/- 0.3 mmol/L). In the early phase of insulin infusion (15 to 30 minutes), before hypoglycemia was reached (4.7 +/- 0.3 mmol/L), the a ctivation signal was unchanged. However, once the hypoglycemic plateau was achieved, the activation signal was significantly decreased to 57 +/- 6% of the preinfusion value. Control regions in the brain that were not activate d showed no significant changes in MR signal intensity. Upon return to eugl ycemia, the activation signal change increased to within 10% of the origina l level. No significant activation changes were noted during euglycemic hyp erinsulinemic clamp experiments. The authors concluded that fMRI can detect alterations in cerebral function because of insulin-induced hypoglycemia. The signal changes observed in fMRI activation experiments were sensitive t o blood glucose levels and might reflect increases in brain metabolism that are limited by substrate deprivation during hypoglycemia.