Evidence for Effects of Insulin on Sensory Processing in Humans

Systemic insulin passes the blood-brain barrier and insulin receptors have been detected in various brain regions. Yet, the biological significance of insulin acting on the brain remains rather unclear. Reports of different awareness of hypoglycemic symptoms during hypoglycemia induced by human insulin (HI) and porcine insulin (PI) suggest a modulatory influence of insulin on sensory processing. In a double-blind, within-subject, crossover comparison, we recorded visual-evoked potentials (VEP) in 30 healthy men during euglycemia and after 20 or 50 min of constant hypoglycemia of 2.66 mM (47.9 mg/dl) induced by HI and PI. Blood glucose and serum insulin levels were identical in both sessions. Hypoglycemia reduced amplitudes of the VEP components P1 and N2 and increased latencies of N1, P1, and N2. However, hypoglycemia-induced changes in VEP amplitudes and latencies were significantly stronger during PI and HI infusion: P1-N2 difference amplitude decreased from (mean ± SE) 11.9 ± 0.9 to 10.7 ± 0.8 μV during HI and from 12.4 ± 0.9 to 8.7 ± 0.7 μV during PI infusion (P < 0.002). P1 latency increased from 112.0 ± 3.2 to 118.8 ± 3.2 ms during HI and from 114.0 ± 3.3 to 126.3 ± 4.6 ms during PI infusion (P < 0.05). Differences between the effects of the insulins were consistently apparent after 20 min of hypoglycemia, which indicates a short-term action of the hormone. The results add to those of a foregoing study demonstrating differential effects of HI- and PI-induced hypoglycemia on auditory evoked potentials. The changes in sensory processing during hypoglycemia, depending on the type of insulin, suggest a direct modulation of these brain functions by insulin.