Neurobiology: March 14, 2000

Lecture notes:

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Now that we've discussed how neurotransmitter is released, I'd like to talk about what a neurotransmitter actually is and how one can establish that a candidate substance is, in fact, the transmitter at a given synapse. In the broadest sense, a neurotransmitter is a chemical substance that's released by a neuron at a synapse and carries a message from the neuron to a target cell. So it's essentially a messenger molecule. As the title of this lecture implies, there are two classes of chemical messengers released by neurons. These include both "small molecule" chemical messengers and larger peptidergic messengers. We'll be focusing on the small molecule transmitters today. As well as the obvious criterion of size, as a general rule, small molecule transmitters differ from peptide messengers in four respects:

Criteria by which one defines a substance as a neurotransmitter at a specific synapse:

There are four generally accepted criteria for accepting a substance as a neurotransmitter at a given synapse:

1) IT IS SYNTHESIZED IN THE PRESYNAPTIC NEURON (its presence in the nerve terminal can be demonstrated, synthetic enzymes can be identified)

2) IT IS RELEASED FROM THE NERVE TERMINAL UPON STIMULATION (in quantities sufficient to exert its post-synaptic response)

3) WHEN THE SUBSTANCE IS APPLIED EXOGENOUSLY IT MIMICS THE ENDOGENOUS TRANSMITTER BOTH PHYSIOLOGICALLY AND PHARMACOLOGICALLY (this is frequently called "identity of action")

4) SPECIFIC MECHANISMS EXIST FOR REMOVING THE SUBSTANCE FROM THE SYNAPTIC CLEFT (debatable for neuropeptides... concept of neuromodulator)

At peripheral synapses, like the neuromuscular junction, it's fairly straightforward to satisfy these criteria and identify a given substance as a neurotransmitter. You already know that acetylcholine has been established as the neurotransmitter at vertebrate neuromuscular junctions; similarly, norepinephrine has been definitively identified as the transmitter used by nearly all post-ganglionic sympathetic neurons,

while GABA has been identified as the inhibitory neurotransmitter at crustacean neuromuscular junctions.

In the CNS, it's been much more difficult to satisfy these criteria: much less transmitter is released, and there are usually so many neurons intermingled with each other and interacting with each other that it becomes very difficult to stimulate a particular synapse in isolation. However there's been enough data that's highly suggestive of a number of molecules as acting as neurotransmitters. There are about nine generally accepted small molecule neurotransmitters, and a few other more-or-less generally accepted ones. Most of these are amino acids or their derivatives, although there are a few exceptions.

(Acetylcholine synthesis and degradation)

Acetylcholine is not an amino acid or an amino acid derivative. Its synthesized by the enzyme choline acetyltransferase, which, as the name implies transfers an acetyl group from acetyl coenzymeA to choline.

 As you know, acetylcholine is inactivated by the enzyme acetylcholinesterase, which cleaves the ester linkage to choline and acetate. Acetylcholinesterase is present both in the nerve terminal cytoplasm and extracellularly, in the synaptic cleft. When acetylcholine is broken down by extracellular cholinesterase, the choline is transported back into the nerve terminal, where it is recycled back into acetylcholine. This reuptake of choline back into the nerve terminal is important in terms of replenishing the supply of acetylcholine, because neurons cannot synthesize choline themselves.

hemicholinium inhibits: presynaptic decrease in quantal size

dietary choline (lecithin) affects brain ACh

ATP often found in synaptic vesicles and can be detected in the extracellular fluid upon neuronal stimulation. ATP, and its close relative adenosine, can also act as neurotransmitters and they are another exception to the amino acid or derivative composition of most small molecule transmitters. Interestingly enough, while ATP usually acts on post-synaptic target cells -- the way we ordinarily think of a neurotransmitter working -- adenosine often acts on autoreceptors present on the terminal of the neuron that released it.

Potential role in synaptic depression

The biogenic amines are a group of small molecule neurotransmitters derived from amino acids. The catecholamines are all derivatives of the amino acid tyrosine:

Catecholamine Biosynthesis:
 
 

Catecholamine supplies are controlled by the activity of tyrosine hydroxylase, which is the rate limiting enzyme for catecholamine biosynthesis.

Both norepinephrine and epinephrine act to inhibit the activity of tyrosine hydroxylase. So if they build up in the nerve terminal, their synthesis is slowed down. On the other hand, if a lot of transmitter is being released, supplies in the nerve terminal drop and tyrosine hydroxylase is free to act at a higher rate. This is called feedback inhibition. Additionally, during stimulation, tyrosine hydroxylase acquires a higher affinity for a cofactor, causing it to be less sensitive to inhibition by what norepinephrine is present.

During prolonged activity, the levels of TH and DbH are actually increased; we'll talk more about the mechanisms involved in these long-term changes when we get to the lectures on regulation of neuronal gene expression.

DOPA, dopamine and Parkinson's disease:

DOPA bypasses TH and can cross the BBB (dopamine can't). DOPA given together with something that will inhibit the conversion to dopamine peripherally.

Serotonin, Histamine, GLUTAMATE, GABA and Glycine

The other biogenic amines are serotonin, which is a derivative of the amino acid tryptophan and histamine, which is derived from histidine.

The three (or four) amino acid neurotransmitters:

Glutamate, which is the major excitatory neurotransmitter in the brain (aspartate is likely one, too, but less well accepted).

GABA, which is the major inhibitory neurotransmitter in the brain and which is synthesized from glutamate by glutamic acid decarboxylase

and Glycine, which is the major inhibitory amino acid in the spinal cord

Glutamic acid decarboxylase (GAD) is the synthetic enzyme for GABA ; Glycine and glutamate don't appear to have specific enzymes to synthesize them in their neurotransmitter role; they are found in all cells. They are found in higher levels in the neurons that utilize them as transmitters. This may reflect sequestration in synaptic vesicles.

 Nitric Oxide (NO) and Carbon Monoxide (CO) can be considered a special class of small molecule transmitters. (Gases released from one neuron to act on another; act locally like SMTs, not stored in vesicles -- they just diffuse across cell membranes -- highly reactive; act inside target cell and are quickly inactivated.)
 
 

Storage:

Small molecule transmitters are generally synthesized by enzymes in the nerve terminal, generally in the cytosol although dopamine-b-hydroxylase is actually located in the synaptic vesicles themselves.

Small molecule transmitters are actively accumulated into synaptic vesicles by specific transporter molecules that use energy derived from an ATP-powered proton pump (which makes the inside of the vesicle acidic and potentially positively charged) transmitter. For catecholamines, the transmitter crosses via a membrane carrier in the neutral form, in exchange for a proton, once inside the vesicle it becomes trapped inside as protonated form.

In some vesicles, there is a membrane chloride channel, which allows chloride to enter and dissipate the potential gradient. In other cases, a potential gradient develops; this potential gradient can drive accumulation of glutamate or aspartate (which are negatively charged).

 Mechanisms for termination of activity in the synaptic cleft:

As you know, the activity of ACh is terminated by a specific enzyme, acetylcholinesterase. Because ACh was the first transmitter to be intensively investigated, people assumed that termination of activity by a degradative enzyme was the way things worked and they spent a lot of time trying to implicate degradative enzymes in the termination of catecholamine activity. While such enzymes do exist: COMT and MAO, they don't appear to play a major role in the termination of synaptic activity of the catecholamines -- although they do play a significant role in neuro/psychopharmacology. Instead the activity of the biogenic amines at the synapse is terminated by rapid and specific uptake mechanisms into either the presynaptic nerve terminal or into neighbouring glia: cocaine

Similarly GABA, glutamate.

Slides

Nicholls JG, Martin AR and Wallace BG (1992) "Cellular and molecular biology of synaptic transmission" in From Neuron to Brain, Sinauer, Sunderland, MA.

Schwartz JH (1992) "Chemical messengers: small molecules and peptides" in Kandel ER, Schwartz JH and Jessell TM (eds) Principles of Neural Science, Elsevier, NY, p 213-224.

Burnstock G (1990) "Overview: Purinergic mechanisms" Annals NY Acad Sciences 603:1-17.

Zigmond RE, Schwarzschild MA and Rittenhouse AR (1989) "Acute regulation of tyrosine hydroxylase by nerve activity and by neurotransmitters via phosphorylation" Annual Review Neurosci 12:415-461.

Snyder SH (1992) "Nitric oxide: First in a new class of neurotransmitters?" Science 257: 494-496.

Johnson RG (1988) "Accumulation of biogenic amines into chromaffin granules: A model of hormone and neurotransmitter transport" Physiol Reviews 68:232-307.