Edith Graef McGeer - Society for Neuroscience

346 Patrick L. McGeer and Edith GraefMcGeer
Mapping of GABA Neurons by GABA Transaminase
Eugene Roberts and coworkers eliminated any last doubts about GABA
being a neurotransmitter when they identified neurons and nerve endings
containing its synthetic enzyme glutamic acid decarboxylase (GAD).
The immunohistochemical methodology they employed was sensitive
enough to reveal certain groups of neurons, but it was not a practical way
to do a complete mapping of the brain. We wished to develop a simple
method for doing so and thus developed the pharmacohistochemical
method for GABA-T GABA-T, being a transaminase, would react with
formazan dyes to yield visible products, but its localization was not
restricted to neurons. Astrocytes, being responsible for picking up
neuronally released GABA, contained large quantities. Steve Vincent and
Hiro Kimura overcame this problem by applying a strategy previously
developed by Larry Butcher for acetylcholinesterase. An irreversible
GABA-T inhibitor was given to a rat in vivo. Sacrifice was then timed so
that recovery of GABA-T would occur in neurons but not in the less active
glial cells. Ethanolamine-0-sulfate was used as the specific and irreversible
inhibitor of GABA-T Toshi Nagai and Masa Araki, two of Hiro
Kimura's successors from Japan, then applied the technique to the first
complete mapping of GABA neurons, i.e., those containing high levels of
GABA-T. Many of the neurons were types already known to be interneurons,
but others were associated with key pathways, particularly in the
basal ganglia.
Glutamate Pathways
Physiologists had long been aware that glutamate and aspartate are
strongly excitatory, and it was suspected that they played major roles as
excitatory neurotransmitters in brain. However, proving this suspicion
and localizing them to specific pathways was difficult because they are
amino acids with numerous nontransmitter roles. Glutamate, for example,
is incorporated into proteins and peptides, is involved in fatty acid synthesis,
and contributes to the regulation of ammonia and the control of
osmotic or anionic balance. It serves as a precursor for GABA and for various
Krebs cycle intermediates, and it is a constituent of important cofactors
such as glutathione and folic acid. How then could neurotransmitter
glutamate be distinguished? Our first attempt was to use high-affinity,
sodium-dependent uptake in synaptosomal fractions from key areas as a
possible measure of glutamatergic nerve endings. Spencer had suggested
that the massive corticostriatal path used glutamate on the grounds that
excitation of striatal cells by stimulation of this pathway was antagonized
by diethyl glutamate. Working with Uschi Scherer and Vijendra Singh,
we found that surgical lesions of the corticostriatal path in rats caused
a significant 40% reduction in glutamate uptake in the synaptosomal