Tuesday, July 3, 2007

Neurophysiological functions for Vitamin B1

11.6.1 Introduction
There is increasing evidence that vitamin B1, specifi -
cally thiamin triphosphate, is somehow involved in
nerve membrane function. This property appears
to be independent of the known coenzyme role of
TPP. The evidence is substantiated by the fi nding that
thiamin triphosphate, which accounts for 1% of total
thiamin in rat brain, makes up 90% of total thiamin in
the electric organ of the electric eel (Bettendorff et al.,
1987). In the lamb, vitamin B1 deprivation for 4 weeks
led to a 20% depletion of total thiamin in the brain,
with a similar percentage loss of free thiamin, thiamin
monophosphate and TPP. There was, however, no
appreciable fall in thiamin triphosphate (Thornber
et al., 1980).
Most of the vitamin B1 present in the brain and
peripheral nerves is in the coenzyme form, TPP. The
1% or so of thiamin triphosphate present in whole
brain is largely concentrated in the membrane fraction
(Matsuda & Cooper, 1981). Fluorescence microscopy
shows that the vitamin is localized in the
membranes of peripheral nerves rather than in the
axoplasm (Tanaka & Cooper, 1968). A complete set
of enzymes catalysing the interconversion of thiamin
and its phosphate esters has been isolated and purifi ed
from nervous tissue (Fox & Duppel, 1975).
As discussed in the following, vitamin B1 may play
a direct role in nerve conduction or it may be implicated
in nerve transmission.

11.6.2 Nerve conduction
When Eichenbaum & Cooper (1971) UV-irradiated
electrically stimulated vagus nerves dissected from a
rabbit, action potentials were completely abolished
after 2 hours and never spontaneously reappeared
when the irradiated nerve was immersed in physiological
solution. The irradiation almost completely
destroyed endogenous thiamin in the nerve, as expected.
Immersion of the irradiated nerve in physiological
solution containing 1 mM thiamin restored
the action potentials after about 1 hour. Thiamin is
known to be rapidly taken up by the vagus nerve,
so the delay may be explained by the two enzymatic
steps required to convert thiamin to its triphosphate
TPP. Mg2+
TPP. Mg2+
D-Xylulose-5-phosphate D-Ribose-5-phosphate D-Sedoheptulose-7-phosphate D-Glyceraldehyde-3-phosphate
D-Xylulose-5-phosphate D-Erythrose-4-phosphate D-Fructose-6-phosphate D-Glyceraldehyde-3-phosphate
Fig. 11.6 Two transketolation reactions in the pentose phosphate pathway.
Thiamin (vitamin B1) 281
Sasa et al. (1976) studied the effects of thiamin, thiamin
triphosphate and pyrithiamin on the excitability
of the perfused giant axon of the crayfi sh. A stimulating
current was delivered to the axon every 2 s. The
following treatments were performed sequentially.
Addition of thiamin to the perfusate produced an
increase in the rising rate (dV/dt) of the action potential
as early as 5 min; there was no accompanying
effect on either the resting membrane potential or the
threshold potential. Perfusing the axon with physiological
solution restored dV/dt to control rates in
40 min. Addition of pyrithiamin elicited a decrease of
dV/dt and prolongation of the duration of the action
potential in 60 to 90 min; again the resting membrane
potential and threshold potential were unaffected.
Perfusing with physiological solution did not restore
the action potential within 30 min, but addition of
thiamin did so in 30 min. Addition of thiamin triphosphate
produced a similar effect to thiamin. The
additional fi nding that protein-binding thiamin was
reduced in axons treated with pyrithiamin indicates
a replacement of thiamin located in the membrane.
Since dV/dt of the action potential refl ects the sodium
conductance, the fi nding that thiamin and thiamin
triphosphate increased dV/dt could be construed
as an increase in sodium conductance. Conversely,
the fi nding that pyrithiamin decreased dV/dt could
mean an impairment of sodium conductance. It was
inferred from these experiments that thiamin plays
an essential role in the membrane excitability of the
crayfi sh giant axon.
In a series of experiments summarized by Itokawa
(1977), treatment of perfused spinal cords or sciatic
nerves with a variety of neuroactive agents released
free thiamin and thiamin monophosphate. The nerve
preparations were obtained from either bullfrogs or
rats made defi cient in thiamin by dietary restriction
and injected with radioactive thiamin. The neuroactive
agents also released free thiamin and thiamin
monophosphate from the membrane-myelin fraction
obtained from homogenized rat brain, spinal
cord and sciatic nerves. As the bulk of thiamin in the
membrane fraction consisted of TPP and thiamin
triphosphate, the release phenomenon presumably
involved dephosphorylation. One of the neuroactive
agents, tetrodotoxin, acts as a nerve poison by inhibiting
the early inward current of sodium. Itokawa
(1976) postulated that the propagation of an action
potential involves the shift of TPP or thiamin triphosphate
through dephosphorylation from a specifi c
site in the sodium channel to allow the early inward
current of sodium. Tetrodotoxin, by displacing these
phosphate esters, would occupy the site and prevent
the fl ow of current.
Goldberg et al. (1975), using voltage-clamped squid
giant axons, showed that the magnitudes of both sodium
and potassium conductance were decreased by
treatment with thiamin antimetabolites. However,
there was no appreciable change in the kinetics of the
conductance changes, hinting that the mechanism of
ion channel operation was unaffected. Most probably,
the antimetabolites prevented the operation of
a certain percentage of channels: those channels that
remained open during perfusion with the antimetabolites
functioned normally.
Fox & Duppel (1975) showed that B1 vitamers (thiamin
triphosphate > TPP thiamin) applied internally
to the cut internodes of frog sciatic nerve preparations
prevented the exponential decline of sodium and potassium
currents in the node of Ranvier. Neither thiamin
triphosphate nor TPP was active when applied
to the node externally. Tetrodotoxin did not alter this
property of the thiamin compounds, implying that
the tetrodotoxin-induced release of thiamin from
nerve membranes (shown by other investigators)
is not related to the mechanism by which tetrodotoxin
blocks the sodium channels. Fox and Duppel
reasoned that the thiamin dephosphorylation and
rephosphorylation process is probably not directly
coupled to the excitation process. They suggested that
the thiamin phosphates control the number of functioning
voltage-gated ion channels by stabilizing the
density of negative surface charges at the inner side of
the nerve membrane.
Further evidence for a role of thiamin triphosphate
as a membrane electrical fi eld stabilizer was provided
by Bettendorff et al. (1990). These authors reported
that thiamin triphosphate (1 μM) increased the uptake
of radioactive chloride (36Cl–) by rat brain membrane
vesicles, while TPP, thiamin monophosphate
and thiamin had no signifi cant effect. The opening
of chloride channels to allow an infl ux of negatively
charged chloride ions into the postsynaptic neuron
contributes to neuronal inhibition. Thus, if thiamin
triphosphate controls the opening of chloride channels,
it could act as a membrane stabilizer.
282 Vitamins: their role in the human body
11.6.3 Nerve transmission
One of the characteristics of thiamin-defi ciency
encephalopathy is its predilection for specifi c brain
structures with sparing of neighbouring ones. This
selective vulnerability of certain brain regions to thiamin
defi ciency has been suggested to have a metabolic
Butterworth (1982) reviewed evidence that the
neurological signs of thiamin defi ciency may involve
a defect in synaptic transmission. Changes in
the concentration or metabolism of catecholamines
(noradrenaline and adrenaline), serotonin (5-hydroxytryptamine),
γ-aminobutyric acid (GABA)
and acetylcholine were reported in certain regions
of the brain of pyrithiamin-treated animals. In addition
to these recognized neurotransmitters, changes
were also observed in the dicarboxylic amino acids
glutamic acid and aspartic acid. These amino acids act
like neurotransmitters in that they open sodium and
potassium ionic channels and cause a rapid, powerful,
excitatory response.
The synthesis of GABA, acetylcholine, glutamic
acid and aspartic acid is directly associated with the
metabolism of glucose in the brain. There is evidence
to suggest that a decreased activity of TPP-dependent
pyruvate and α-ketoglutarate dehydrogenases and
transketolase due to vitamin B1 defi ciency may be
ultimately responsible for a block in the synthesis of
one or more of these neurotransmitters.
11.6.4 Subacute necrotizing
Subacute necrotizing encephalomyelopathy (Leigh’s
disease) is a rare recessively inherited degenerative
disease of the central nervous system which generally
becomes symptomatic in the fi rst year of life and
is fatal. Diagnosis is complicated by the wide variety
of symptoms, which include swallowing diffi culties,
abnormal respiration, ataxia, ophthalmoplegia,
hypotonia, convulsions and progressive mental deterioration.
The neuropathology at autopsy shows
characteristic lesions of the brain stem and spinal
cord, notable features being capillary infi ltration and
demyelination of axons. Dietary vitamin B1 defi ciency
is not a factor in the aetiology of Leigh’s disease. Early
reports describing encouraging responses to large
doses of thiamin or thiamin derivatives have not been
borne out by further experience (Blass, 1981).
Infants with Leigh’s disease exhibit a defi ciency
of thiamin triphosphate in the brain. A substance,
possibly a lipoprotein, that inhibits the synthesis
of thiamin triphosphate from TPP via the thiamin
pyrophosphate-ATP phosphotransferase has been
detected in the blood, urine and cerebrospinal fl uid
of these patients (Cooper & Pincus, 1979). This substance
only inhibits the brain enzyme; the phosphotransferase
that catalyses the formation of thiamin
triphosphate in the liver is unaffected. Leigh’s disease
appears therefore to be the result of disordered brain
metabolism, possibly caused by the genetic lack of
some enzyme. The resultant defi ciency of thiamin
triphosphate explains the neurological symptoms and
neuropathological changes.

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