Recurrent Moderate Hypoglycemia Suppresses Brain-Derived Neurotrophic Factor Expression in the Prefrontal Cortex and Impairs Sensorimotor Gating in the Posthypoglycemic Period in Young Rats

Abstract

Recurrent hypoglycemia is common in infants and children. In developing rat models, recurrent moderate hypoglycemia leads to neuronal injury in the medial prefrontal cortex. To understand the effects beyond neuronal injury, 3-week-old male rats were subjected to 5 episodes of moderate hypoglycemia (blood glucose concentration, approx. 30 mg/dl for 90 min) once daily from postnatal day 24 to 28. Neuronal injury was determined using Fluoro-Jade B histochemistry on postnatal day 29. The effects on brain-derived neurotrophic factor (BDNF) and its cognate receptor, tyrosine kinase receptor B (TrkB) expression, which is critical for prefrontal cortex development, were determined on postnatal day 29 and at adulthood. The effects on prefrontal cortex-mediated function were determined by assessing the prepulse inhibition of the acoustic startle reflex on postnatal day 29 and 2 weeks later, and by testing for fear-potentiated startle at adulthood. Recurrent hypoglycemia led to neuronal injury confined primarily to the medial prefrontal cortex. BDNF/TrkB expression in the prefrontal cortex was suppressed on postnatal day 29 and was accompanied by lower prepulse inhibition, suggesting impaired sensorimotor gating. Following the cessation of recurrent hypoglycemia, the prepulse inhibition had recovered at 2 weeks. BDNF/TrkB expression in the prefrontal cortex had normalized and fear-potentiated startle was intact at adulthood. Recurrent moderate hypoglycemia during development has significant adverse effects on the prefrontal cortex in the posthypoglycemic period.

Figure 1

Neuronal injury after recurrent hypoglycemia in young rats. Three-week-old male rats were subjected to insulin-induced moderate hypoglycemia, once daily from postnatal day 24 to 28. Littermates in the Control group were injected with equal volume of normal saline. Coronal brain sections of the prefrontal cortex collected 24 hours after the last hypoglycemia episode from representative rats from the Control group (A) and Recurrent Hypoglycemia group (B) and stained for injured cells using Fluoro-Jade B histochemistry demonstrate Fluoro-Jade B positive cells in the medial prefrontal cortex in the Recurrent Hypoglycemia group (arrows in B). Bar = 100 μm.

Figure 2

Acute and long-term effects of recurrent hypoglycemia on brain-derived neurotrophic factor (BDNF) and tyrosine kinase B (TrkB) receptor mRNA and protein expressions in the prefrontal cortex of young rats. Three-week-old male rats were subjected to insulin-induced moderate hypoglycemia, once daily from postnatal day 24 to 28. Littermates in the Control group were injected with equal volume of normal saline. BDNF and TrkB transcript and protein expression in the prefrontal cortex was determined 24 hours post-hypoglycemia (A and B) and at adulthood (C and D). Black bars = Control group, white bars = Recurrent Hypoglycemia group. Values are mean ± SEM normalized to the Control group; n = 6–10 for mRNA expression and n = 4–6 for protein expression. *p<0.05 vs. Control group (unpaired t tests). Abbreviations: TrkB(_FL), full-length isoform of TrkB; TrkB(_TR), truncated isoform of TrkB.

Figure 3

Acute and long-term effects of recurrent hypoglycemia on prepulse inhibition of acoustic stimulus in young rats. Three-week-old male rats were subjected to insulin-induced moderate hypoglycemia, once daily from postnatal day 24 to 28. Littermates in the Control group were injected with equal volume of normal saline. Prepulse inhibition was tested 24 hours (on P29) and two weeks (on P42) post-hypoglycemia. Black bars = Control group, white bars = Recurrent Hypoglycemia group. Values are mean ± SEM; n = 16. There are significant main effects of Group, Age at test and Group × Age interaction (p<0.05 for each; ANOVA). *p<0.05 vs. P29 Control group; ^§p<0.05 vs. P42 Control group and P42 Recurrent Hypoglycemia group, both (Tukey’s HSD tests). **p<0.05; P29 vs. P42 (ANOVA). Abbreviation: P, postnatal day.