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

Abstract

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 <1mM 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 <3mM 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.

Age-related metabolic fatigue during low glucose conditions in rat hippocampus

“…Exposure to 2.5 mM glucose buffer resulted in an acute suppression of fEPSP amplitudes in all 4 age groups (Fig. 1B and C), but evoked potentials were partially maintained (Sadgrove et al, 2007). Hippocampal slices of rats from different age groups showed a similar reduction in fEPSP amplitude after 40 minutes of 2.5 mM glucose (about 80%, Fig.…”

“…During the low-glucose condition with intact oxygen availability, the NAD + /NADH redox ratio increased significantly, because of oxidation of available mitochondrial NADH to NAD + in the electron transport chain and limited substrate availability for NADH regeneration (Bryan and Jobsis, 1986; Sadgrove et al, 2007). Interestingly, we have found that the shift of NAD + /NADH toward oxidation was similar in slices of young and aging rats despite the lower oxygen utilization in aging rats compared with younger rats during moderate glucose deprivation.…”

“…Based on our previous study (Sadgrove et al, 2007), we simulated conditions of moderate glucose deprivation by lowering glucose concentration in the buffer from 10 mM to 2.5 mM. Euglycemic plasma glucose concentration of 7–8 mM correlates with a brain extracellular glucose concentration ranging between 1 and 2.5 mM in humans (Abi-Saab et al, 2002; Gruetter et al, 1992; Magnoni et al, 2003) as well as in rodents (McNay et al, 2000; Silver and Erecinska, 1994), resulting in a linear 3:1 ratio between plasma and tissue glucose concentrations (Magnoni et al, 2003).…”

Pyruvate incubation enhances glycogen stores and sustains neuronal function during subsequent glucose deprivation

“…Transverse hippocampal slices (400 μm) were prepared from male rats as previously described (Galeffi et al, 2007, Sadgrove et al, 2007) and were kept in oxygenated (95 % O 2 /5% CO 2 ) ACSF (in mM: NaCl 124, KCl 3.0, NaH 2 PO 4 1.25, NaHCO 3 24, CaCl 2 2.0, MgSO 4 2.0, and dextrose 10, pH 7.4) at room temperature (22°C) for 1 hr for recovery. The slices were then transferred to oxygenated ACSF with and without 10 mM pyruvate (with changes in NaCl concentration to maintain osmolality) and incubated at 32°C for 2 - 6 hr.…”

Hypoglycemia-induced alterations in hippocampal intrinsic rhythms: Decreased inhibition, increased excitation, seizures and spreading depression

“…The reasons for this difference in regional cellular survival in the hippocampus are not well understood. Functionally, in vitro hypoglycemic experiments showed that the CA1 region is susceptible to synaptic failure (Abdelmalik et al, 2007;Crepel et al, 1992;Fan et al, 1988;Sadgrove et al, 2007), an effect exacerbated by the presence of seizures (Abdelmalik et al, 2007).…”