GABA--Gamma-aminobutyric acid
Gamma-aminobutyric acid (GABA) is a neurotransmitter in widely
divergent species. In humans, GABA acts
at inhibitory synapses
in the brain and spinal cord. As in the other
contexts where GABA acts as a transmitter, the inhibition results from a hyperpolarization of
the synaptic transmembrane potential
of the inhibited neuron, which is elicited by the
binding of GABA molecules to specific receptors in the
plasma membrane of both
pre- and post-synaptic cells. This binding opens ion channels to allow either
the flow of chloride or potassium ions into or out of
the cell. In insects GABA mediates muscle activation
at synapses between nerves and muscle cells and also
the stimulation of certain glands. So far three general classes
of GABA receptor are known, more than one of which is often represented in the
same organisms. These include both
so-called ionotropic receptors,
which are ion channels themselves, and metabotropic receptors,
which are G protein-coupled
receptors that open ion channels via intermediaries (G proteins).
With
regard to the human brain, it has been asserted that GABA signals interfere with
the registration and consolidation stages of memory formation. This is thought
to be possible because the GABA system is found in the hippocampus, an area of brain
which has been implicated in memory formation.
Organisms synthesize GABA from glutamate using the enzyme L-glutamic acid decarboxylase and pyridoxal phosphate as a cofactor.
Three types of GABA receptors:
Drugs which affect GABA receptors:
- alcohol
- bicuculline
- benzodiazepines and barbiturates
- baclofen
- carbamazepine, phenytoin, valproate
- gamma-hydroxybutyrate (GHB)
- picrotoxin
- propofol
- zolpidem, zopiclone