Amino Acid Transmitters

I. Background

A. Transmitters discussed to date account for only ~12-15% of all synapses in brain. Most are located in ascending projection systems and most are linked to slow, long-lasting postsynaptic responses. While ACh in ganglionic synapses and in somatic motor nerves elicits fast-acting responses via nicotinic receptors, such Ach responses are unusual in the brain.
B. Two main groups of amino acids serve transmitter function
1. Dicarboxylic amino acids, glutamate and aspartate; have pronounced excitatory effects on brain; are ionized at physiologic pH--anions
2. Omega monocarboxylic, neutral group, GABA and glycine; have pronounced inhibitory effects on brain.
3. Are nonessential amino acids, produced in the brain

II. Localization

A. Only GABA neurons have been unequivocally identified. An antibody has been raised against the synthesizing enzyme for GABA and new methods of conjugating GABA to serum albumin have been used to identify GABA-containing cells.
B. Glutamate has highest concentration of any amino acid in brain; it is also present in blood.
It has many roles in brain function: production of TCA cycle intermediates, detoxification of ammonia, precursor to neurotransmitter as well as neurotransmitter. No specific transmitter-related enzyme to which an antibody can be raised. Some studies looking at distribution of glutamate conjugated to serum albumin, but glutamate is present in every cell.
C. Distribution profiles based on electrophysiologic evidence as well as anatomical evidence
D. GABA
1. Is present in interneurons in many regions: cortex, cerebellum, hippocampus, spinal cord.
2. Is also transmitter in several projection neurons: striatal nigral, Purkinje cells to deep cerebellar nuclei and vestibular nuclei, posterior hypothalamus to cortex, amygdala to BNST
3. In periphery, is found in enteric nervous system E. Glycine
1. Is primarily a transmitter in spinal cord and brain stem; distribution profile seems to have been preserved across evolution. F. Glutamate/aspartate
1. Considered to be transmitters in excitatory projection pathways.
Corticospinal, primary sensory fibers, corticostriatal.

III. Synthesis

A. Synthesis of amino acid transmitters is closely linked to TCA intermediates.
B. Synthesis of GABA has specific enzyme linked to production of GABA as a transmitter
1. GAD (glutamic acid decarboxylase) decarboxylates glutamic acid to form GABA. Requires pyridoxal phosphate. Two forms of enzyme identified in mammalian tissues: GAD65 and GAD67. Two separate genes. GABA synthesis may be regulated by pyridoxal phosphate binding to GAD65. Unbound enzyme is apoenzyme; bound enzyme is holoenzyme.
2. GAD is localized in the cytoplasm of nerve endings but its synthesis is a shunt off of the TCA cycle.
3. GABA is enzymatically degraded by GABA-a-KG transminase and a molecule of glutamate is produced; however, glutamate is not necessarily formed in GABA-releasing neurons. Glutamate may be formed in glial cells where it must be converted to glutamine by glutamine synthetase. Glutamine then is taken up by neurons and glutamate is produced by action of glutaminase, which is not present in glial cells.
Transaminases transfer the amino component from amino acids to a-keto acids, mostly to a-ketoglutarate. C. Synthesis of glutamate
1. Most important source of glutamate as a transmitter may be synthesis from glutamine by glutaminase; glutamine cycle
2. Could be directly from a-KG by glutamic dehydrogenase
3. Produced in many transaminase reaction: aspartic-oxoloacetate T, alanine-pyruvate T D. Synthesis of glycine
Not understood

IV. Vesicular Storage and release

A. Release from slices and synaptosome preparations has been shown to be Ca++ dependent. B. Difficult to prove that source of amino acids released by depolarization was a vesicular pool
C. Recent evidence shows that isolated vesicles take up glutamate and GABA by an energy-dependent transport system unique from plasma membrane transporters.
D. Generally accepted that release of amino acids is exocytotic.

V. Inactivation of released amino acid transmitters

1. High affinity carrier systems are critically important to regulating levels of amino acid transmitters in synaptic cleft. Carriers not necessarily localized to presynaptic nerve terminal but reside on glial cells also.
2. Several transporters have been identified for both glutamate and GABA. For glutamate, the different transporters appear to be localized to select cell populations; one is present in neurons and one is localized specifically to astrocytes. A third transporter is found on Bergmann glia in the cerebellum. At the moment it is not clear whether one of the two GABA transporters cloned is selectively localized to glial cells.
3. Regulating synaptic levels of these fast-acting transmitters is critical
Glutamate is also an 'excitotoxin'; that is, it has been linked to degeneration of neurons associated with ischemia, etc.
GABA is major inhibitory substance and its regulation is critical to appropriate patterning of neural responses.
4. GABA transporter has been cloned. The movement of a single molecule of GABA is associated with a gain of a single positive charge. The movement of GABA is linked to the inward movement of 2Na+ and 1Cl- ions. There appear to be multiple classes of GABA transporters.
5. Glutamate is an anion at physiologic pH, and is cotransported with 2-3 Na+, while a single K+ ion is transported in the opposite direction. Glutamate transport appears to result in the movement of one charge per molecule of glutamate.
6. Regulation of extracellular amino acid transmitter levels by manipulation of their transport may prove to be a potential mechanism for therapeutic management of CNS disorders.
7. GABA and glutamate may be taken up into each other's presynaptic nerve endings.
8. High affinity uptake may indicate that a particular amino acid has a transmitter role in a particular region, but cannot be used as a marker for neurons, since uptake sites are not limited to neurons.

VI. Regulation of transmitter availability

1. Since synthesis is related to so many other mechanisms, regulation of synthesis may not be point of regulation.
2. High affinity uptake has been shown in several preparations to be under regulation by second messenger-linked systems.
3. Amino Acid receptor function has been shown to be modifiable by many endogenous substances as well as second messenger systems.

VII. Amino acids as transmitters

A. Acceptance of any one amino acid as a transmitter must come from finding good correlation from several lines of evidence.
B. Glycine
In spinal cord and brain stem, glycine levels, glycine uptake and receptor density (strychnine binding) correlate highly, whereas in the cortex and cerebellum, uptake and receptors seem to be minimal.
C. Glutamate levels and uptake correlate in many regions, but there is poor correlation in brainstem and cerebellum.

Last modified: 9/15/97