Zero net flux estimates of septal extracellular glucose levels and the effects of glucose on septal extracellular GABA levels

Abstract

Although hippocampal infusions of glucose enhance memory, we have found repeatedly that septal glucose infusions impair memory when gamma-aminobutyric acid (GABA) receptors are activated. For instance, hippocampal glucose infusions reverse the memory-impairing effects of co-infusions of the GABA agonist muscimol, whereas septal glucose infusions exacerbate memory deficits produced by muscimol. One potential explanation for these deleterious effects of glucose in the septum is that there are higher levels of endogenous extracellular fluid glucose concentrations in the septum than in the hippocampus. Another hypothesis is that septal glucose infusions impair memory by increasing septal GABA synthesis or release, which is possible because elevating glucose increases GABA levels in other brain regions. To test these hypotheses, Experiment 1 quantified extracellular fluid glucose levels in the septum and hippocampus using zero net flux in vivo microdialysis procedures in conscious, freely moving rats. Experiment 2 determined whether septal infusions of glucose would increase GABA concentrations in dialysates obtained from the septum. The results of Experiment 1 indicated that extracellular fluid glucose levels in the hippocampus and septum are comparable. The results of Experiment 2 showed that co-infusions of glucose with muscimol, at doses that did not affect memory on their own, decreased percent alternation memory scores. However, none of the infusions significantly affected GABA levels. Collectively, these findings suggest that the memory-impairing effects of septal infusions of glucose are not likely due to regional differences in basal extracellular fluid glucose concentrations and are not mediated via an increase in septal GABA release.

Fig. 1

Schematic illustration of coronal sections of the rat brain showing the approximate locations of (A) septal and (B) hippocampal dialysis probe and infusion sites. Atlas plates were adapted from Paxinos and Watson (1998).

Fig. 2

Glucose gain or loss to the brain as a function of perfusate concentration (0–3 mM) in the septum or hippocampus of rats using dual-probe microdialysis procedures. The point of zero net flux is the estimated extracellular fluid concentration of glucose. The solid (hippocampus) and dashed (septum) lines are the line of best fit for the point of zero net flux determination. The insert shows the mean (±S.E.M.) estimated glucose concentrations based on the individual regression analyses of each rat. The extracellular fluid glucose concentration in the septum (1.22±.25 mM) and the hippocampus (0.96±.16 mM) do not differ significantly (P>.05).

Fig. 3

Glucose gain or loss to the brain as a function of perfusate concentration (0–3 mM) when the septum and hippocampus are sampled separately rather than simultaneously. The point of zero net flux is the estimated extracellular fluid concentration of glucose. The solid (hippocampus) and dashed (septum) lines are the line of best fit to the data for the point of zero net flux determination. The insert shows the mean (±S.E.M.) estimated glucose concentrations based on the individual regression analyses of each rat. The estimated extracellular fluid glucose concentration in the septum (1.39±.36 mM) and hippocampus (1.27±.13 mM) did not significantly differ (P>.05).

Fig. 4

Schematic illustration of coronal sections of the rat brain showing the approximate locations of septum dialysis probe and infusion sites. Atlas plates were adapted from Paxinos and Watson (1998).

Fig. 5

A. Septal infusions of muscimol or glucose alone did not significantly decrease mean (±S.E.M.) percent alternation scores (P>.05 vs. control). Septal infusions of glucose with muscimol significantly decreased percent alternation scores (*P<.05 vs. PBS control). B. Septal drug treatments did not significantly affect the number of arms entered in the maze (P>.05).

Fig. 6

Septal drug infusions did not significantly affect mean (±S.E.M.) extracellular fluid GABA levels (P>.05).

Fig. 7

Septal perfusions of 6.6 mM glucose did not significantly affect mean (±S.E.M.) extracellular fluid GABA levels (P>.05) whereas septal perfusions of 50 mM potassium did (P<.05). Inset shows the pooled mean (±S.E.M.) extracellular fluid GABA values for the baseline samples (i.e., samples 1–3) and the extracellular fluid GABA values for the two post-baseline samples in which glucose or potassium were elevated (i.e., samples 4–5). Septal perfusion of glucose (6.6 mM) did not significantly affect mean septal extracellular fluid GABA levels (P>.05 vs. baseline). The mean septal extracellular fluid GABA levels in the samples in which potassium was elevated were significantly higher than the extracellular fluid GABA values of the baseline samples (P<.05).