Effects of relative hypoglycemia on LTP and NADH imaging in rat hippocampal slices

Cognitive and neuronal impairment in diabetes may be associated with iatrogenic hypoglycemia, particularly at low serum glucose levels (<3 mM). To evaluate cellular impairment, we assessed acute hippocampal slice functioning during decreased ambient glucose, by monitoring evoked field excitatory post-synaptic potentials (fEPSP), and slice nicotinamide adenine dinucleotide (NADH) fluorescence. The effects of lowered glucose levels (60 min) were analyzed by examining the induction and maintenance of long-term potentiation (LTP), and NADH metabolic imaging in the CA1 region. The basal fEPSP response was reduced by lowered ambient glucose, an effect that was reversible in 2.5 mM glucose, partially reversible in 1.25 mM glucose and irreversible in 0 mM glucose, after 25 min recovery. LTP induction and maintenance declined during glucose restriction, demonstrating reversibly failed maintenance in 5 mM and 2.5 mM ambient glucose, and absent induction in 1.25 mM glucose. Peak NADH levels observed during train-induced biphasic transients were significantly reduced during 1.25 mM and 2.5 mM glucose. Significant functional compromise in our slice model occurred at 2.5 mM ambient glucose, equivalent to <1 mM tissue glucose in the slice center, due to diffusion limitations and active glucose utilization. This tissue glucose level correlates with human observations of a serum threshold of <3 mM for cognitive impairment, since brain tissue glucose is approximately one third of serum levels. The physiological effects of hypoglycemia in our slice model, assessed through fEPSP, LTP, and NADH responses, replicate closely the in vivo situation, confirming the usefulness of this model in assessing consequences of relative hypoglycemia.