that some neurons release more than one type of neurotransmitter
(neuropeptides; see Chapter 7), but for the moment we will stay with
the one neurotransmitter per neuron narrative.
Glutamate and GABA have a close chemical relationship. GABA is
made from glutamic acid by the action of an enzyme called glutamic
acid decarboxylase (GAD).
0 oO OoHO Ao enzyme HN gee
NH»
Glutamic acid GABAAll cells contain glutamic acid, as it is one of the amino acid build-
ing blocks for proteins. However, GAD is present only in GABAergic
neurons, where it catalyzes the conversion of glutamic acid to GABA.
(An enzyme is a protein that facilitates a particular chemical reaction,
in this case chopping off a carboxylic acid group in glutamic acid to
form GABA.) The gene coding the GAD enzyme is present in the DNA
of all human cells, but only GABAergic neurons transcribe the gene to
make a functional enzyme.