Brain ischaemia is often transient, reflecting the temporary closing o f a blood
vessel by vasospasm, or the temporary block o f a vessel by a clot which is then removed
by endogenous mechanisms. W hether an anoxic depolarisation occurs in this situation,
and is followed by massive glutamate release and neuronal death, will be determined by
the energy reserves present to maintain ATP levels in the brain. In Chapter 5 I carry out
experiments to assess where these reserves lie. In this section I review the aspects o f
brain energy production needed to understand the experiments in Chapter 5.
1.4.1 Review o f normal metabolism
Under non-pathological situations cells produce energy via the sequential
processes o f glycolysis (anaerobic metabolism) and oxidative phosphorylation (aerobic
metabolism), as summarised in Figure 1.4. During glycolysis 1 glucose molecule is
converted into 2 pyruvate molecules, and 2 ATP and 2 NADH molecules are produced;
this takes place in the cell cytoplasm. Pyruvate produced by glycolysis enters
the citric acid cycle or tricarboxylic acid cycle. Metabolism o f acetyl CoA leads to the
production o f 3 NADH and 1 FADH2 per cycle, along with 1 GTP (yielding 8 NADH, 2
FADH2 and 2 GTP per 2 pyruvate molecules). The GTP is interconvertible to ATP.
NADH and FADH2 enter the electron transport chain, for which they provide the
electrons and some o f the protons. Protons are pumped through the complexes o f the
electron transport chain into the intermembrane space, setting up a proton gradient
across the inner mitochondrial membrane. Protons can only move back into the
mitochondrial matrix by passing through ATP synthase; in doing so they enable ADP +
Pi to be converted into ATP. Electrons pass down the electron transport chain to
Complex IV where they combine with ‘/2O2 and 2H^ to form water, and in doing so
remove protons from the mitochondrial matrix and maintain the proton gradient
(summarised in Voet and Voet, 1990). The citric acid cycle followed by oxidative
phosphorylation provides an additional 29 ATP molecules to the 2 produced by
glycolysis (taking account o f H^ leak across the mitochondrial membrane: Rolfe and
Brown, 1997); thus, oxygen is necessary in order to liberate the majority o f energy from
glucose. In most o f the body fatty acids and amino acids can also be utilised for ATP
production, but under normal conditions the brain lacks the necessary enzymes for this
(they are present at very low levels; Yang et al., 1987), so it is dependent on a constant
supply o f glucose.
It has been suggested that there is a metabolic compartmentation in the brain,
with glycolysis occurring mainly in glia, and oxidative phosphorylation occurring
m ainly in neurons (Pellerin et al., 1998; Sibson et al., 1998; Magistretti & Pellerin,
1999). Pyruvate produced by glycolysis in glia is hypothesised to be converted to
lactate, and exported to neurons, where it is converted back to pyruvate and used to
power mitochondria. M any aspects o f this hypothesis have been criticised (Chih et al.,
although there is evidence that neurons may use lactate as a substrate in some conditions
(Schurr et al., 1988; Izumi et al., 1994, 1997; Saitoh et al., 1994; Cater et al., 2001)
there is only weak evidence that this occurs normally.
1.4.2 The effects of depriving the brain of energy
Cells need ATP in order to function. ATP is required to maintain ion gradients
across membranes (generated by the Na^-K^-ATPase, Ca^^-ATPase and H^-ATPase), to
phosphorylate proteins and to allow cells to grow and repair. The requirements o f a
particular cell type for ATP vary according to its function. In the brain neurons use
more energy than astrocytes (Attwell and Laughlin, 2001), and the Na^-K^-ATPase is
thought to use 75% o f the ATP produced in the grey matter (reviewed by Ames, 1992;
Rolfe and Brown, 1997). It is essential for neurons to have a constant supply o f ATP in
order to maintain their transmembrane Na"^ and gradients, and hence to remain
excitable. If the ion gradients dissipate, then neurons can no longer fire action
potentials, making them non-functional. During ischaemia, when the oxygen and
glucose supply to the brain are cut off, there is a rapid fall in ATP levels (to 20% after
2.5 minutes; Folbergrova et al., 1997). The consequence o f this is that all ATP-
dependent processes within cells will be reduced in rate. This leads to the redistribution
o f ions across the extracellular membrane, with the [Na"^] increasing inside cells while
the [K^] increases in the extracellular space (Figure 1.1). One o f the major
consequences o f the alteration in ion gradients is the reversal o f neurotransmitter
transporters, which will be considered in more detail in the next section.
Figure 1.4 illustrates some o f the temporary sources o f ATP that are available to
neurons to prevent the run-down o f ion gradients during transient ischaemia. ATP is
addition, glycogen stores (largely in glia) provide a reserve supply o f glucose. The
relative importance o f these different stores is assessed in Chapter 5.