Neurotransmitter Types Case File

Neurotransmitter Types Case File

EUGENE C.TOY, MD, RAHUL JANDIAL, MD, PhD, EVAN YALE SNYDER, MD, PhD, MARTIN T. PAUKERT, MD

CASE 7

A 53-year-old male presents to his family doctor complaining of tremors in his hands that he first noticed about 1 year ago. Since then, the tremors have also involved his legs. He complains of feeling stiffness throughout his body. As he speaks, his expression does not change much and he does not blink very often. He does not have any significant history of head trauma. He denies any recreational drug use or metal exposures. His only medications are a daily aspirin, an antihypertensive medication, and a cholesterol-lowering drug.

On physical examination, he is alert and attentive. There is an obvious 3-4 Hz resting tremor in his hands which disappears with movement. He has a slow shuffling gait with minimal arm swing. He has cogwheel rigidity to passive movement of his extremities. The muscle stretch reflexes are within normal limits and there is no clonus.

After careful consideration, the patient is diagnosed with Parkinsonism, a disease of dopaminergic transmission.

• To what class of neurotransmitters does dopamine (DA) belong?
• From what precursor is DA synthesized?
• By what two enzymes is DA degraded?

ANSWERS TO CASE 7: NEUROTRANSMITTER TYPES

Summary: A 53-year-old male with a resting tremor, shuffling gait, and cogwheel rigidity to passive movement of his extremities is diagnosed with Parkinsonism.

• Class of neurotransmitter: DA belongs to the catecholamine class of neurotransmitters.
• Precursor of DA: DA is synthesized from the precursor L-dopa.
• Degradation enzymes: DA is degraded by the enzymes catechol-Omethyl transferase (COMT) and monoamine oxidase (MAO).

CLINICAL CORRELATION

Parkinsonism is relatively common with approximately 1% of the population greater than 65 years of age in North America afflicted by the disease. It may be idiopathic or secondary to other known conditions, such as following viral epidemics, as in encephalitis lethargica or von Economo encephalitis; repeated head trauma; drug use, such as antipsychotics, phenothiazines, or MPTP (1-methyl-4- phenyl-1, 2, 3, 6-tetrahydropyridine); or poisonings, such as carbon monoxide or

manganese toxicities. The idiopathic form is known as Parkinson disease, while the others are known as secondary Parkinsonism.

The classic triad of symptoms consists of the pill-rolling tremor of 3–5 Hz, cogwheel rigidity, and bradykinesia. Other symptoms include masked facies, postural instability, and a festinating shuffle. Diagnosis is based upon the history and clinical features. If in the early stages of the disease the diagnosis is in question, repeated evaluations at a later stage are warranted. The most difficult aspect of the diagnosis is to distinguish the idiopathic form from the secondary form. A positive response to the administration of levodopa helps to confirm the diagnosis. The most consistent finding on postmortem examination in both Parkinson disease and secondary Parkinsonism is the loss of pigmented cells in the substantia nigra and in other pigmented nuclei. This corresponds to the loss of the dopamine-producing cells in the substantia nigra pars compacta (SNpc) and subsequent gliosis. Eosinophilic intraneuronal hyaline inclusions called Lewy bodies are found in the remaining cells of the SNpc. The constellation of clinical findings is caused by the loss of DA in the neostriatum, which includes the caudate nucleus, putamen, and globus pallidus. The net result of the striatal pathway is the increased inhibition of the thalamus by the globus pallidus interna (GPi), thus preventing the thalamus from exciting regions of the supplementary motor cortex. Currently, there is no known treatment to stop the progression of this disease. Medical management is aimed at augmenting or replacing the effects of DA in the striatal pathway. Medications include in levodopa-carbidopa, which replaces the loss of DA in the central nervous system, and Selegiline, a monoamine oxidase inhibitor which prevents the metabolic degradation of DA.

APPROACH TO NEUROTRANSMITTER TYPES

Objectives

1. Know the characteristics of a neurotransmitter.
2. Identify the small-molecule neurotransmitters and the neuroactive peptides
3. Know how and where neurotransmitters are produced.
4. Know how neurotransmitters are inactivated and cleared from the synapse.

Definitions

Acetylcholine: A small-molecule neurotransmitter synthesized from choline and acetyl coenzyme A (acetyl CoA).

Glutamate: The most common excitatory neurotransmitter in the central nervous system.

Gamma aminobutyric acid (GABA): An inhibitory small-molecule neurotransmitter synthesized from glutamate.

Glycine: The major inhibitory neurotransmitter in the spinal cord.

Catecholamines: A group of small-molecule neurotransmitters consisting of DA, norepinephrine (NE), and epinephrine (Epi) formed from the metabolism of tyrosine.

DISCUSSION

Neurotransmitters are the means by which a neuron signals other neurons and cells. Some are produced in the soma of the neuron by the free ribosomes and the rough endoplasmic reticulum, packaged in vesicles, modified by the Golgi apparatus, and transported down the axon to the presynaptic terminal. Other neurotransmitters can be produced by enzymes in the cytoplasm and concentrated in synaptic vesicles. The vesicles are stored in the terminal and await the signal for release into the synaptic cleft where the neurotransmitter can diffuse across to the postsynaptic membrane, bind to receptors, and affect a change in the cell. There are specific mechanisms in place to remove neurotransmitters from the synaptic cleft.

In the nervous system, there are two main types of neurotransmitters, smallmolecule neurotransmitters, such as acetylcholine (ACh), glutamate, and gamma-aminobutyric acid (GABA), and neuroactive peptides, such as enkephalin and substance P. We shall present some representatives of the smallmolecule neurotransmitters and neuroactive peptides in the following paragraphs.

Small-molecule transmitters are charged molecules that are derived from the metabolism of carbohydrates. Most of these neurotransmitters are amino acids or their derivatives. The precursors to the neurotransmitters are enzymatically altered in the cytosol and concentrated into synaptic vesicles for storage. As in all biosynthetic pathways, there is generally one enzyme that regulates the production of the neurotransmitter and functions as the rate-limiting step for its production.

ACh is the one small-molecule neurotransmitter that is not a derivative of an amino acid. It is synthesized from choline, which is found in diet, and acetyl coenzyme A (acetyl CoA) by the enzyme choline acetyltransferase. ACh is then packaged into vesicles via a transporter protein, which exchange H+ ions for ACh. Once released into the synaptic cleft, Ach is hydrolyzed by acetylcholinesterase into acetate and choline. ACh is found in the motor neurons of the spinal cord, where it is released at the neuromuscular junction, and in all preganglionic terminals in the autonomic nervous system and in the postganglionic terminals of the parasympathetic nervous system. It is also found widely in many synapses of the brain.

Glutamate is the main excitatory neurotransmitter in the central nervous system and is synthesized from α-ketoglutarate, an intermediary of the tricarboxylic acid cycle. It binds to several different receptor types and acts on both inotropic and metabotropic receptors. Glutamate is cleared from the synaptic cleft by glial cells, which then convert it to glutamine by glutamine synthase. Glutamine diffuses across the plasma membrane, is synthesized back into glutamate in the presynaptic terminal, and is then repackaged into vesicles.

GABA and glycine are important inhibitory neurotransmitters in the central nervous system. GABA is synthesized from glutamate by glutamic acid decarboxylase with the help of cofactor pyridoxal phosphate. Glycine is likely synthesized from serine and is the major inhibitory neurotransmitter in the spinal cord. Both neurotransmitters bind to receptors that lead to the opening Cl_ channels in the postsynaptic neuron. GABA activity in the synapse is terminated by reuptake into presynaptic nerve terminals and surrounding glial cells. The energy needed to drive GABA reuptake is provided by the movement of Na+ down its concentration gradient. While GABA taken back into nerve terminals is available for reutilization, GABA in glia is converted to glutamine which is then eventually used to reform GABA via a number of metabolic steps. Glycine activity in the synaptic cleft is terminated by reabsorption into the presynaptic cleft via active transport.

Another group of small-molecule neurotransmitters are the catecholamines, consisting of DA, NE, and Epi. They are all synthesized from the amino acid tyrosine from a common pathway. Tyrosine is first converted into L-dihydroxyphenylalanine (L-dopa) by tyrosine hydroxylase. This is the ratelimiting step for the production of both DA and NE. L-dopa is then decarboxylated to form DA, which is important in the nigrostriatal pathways in the brain. DA can be converted to NE in the synaptic vesicle by dopamine hydroxylase. NE acts as an important neurotransmitter in the autonomic nervous system and is also found in large concentrations in brain in the locus ceruleus. NE can be methylated by phenylethanolamine-N-transferase to form Epi in the adrenal medulla.

The neuroactive peptides are produced by ribosomes on the endoplasmic reticulum of the cell body and, following modifications, are transported down the axon to the terminal. Several different peptides can be encoded by a single mRNA. They are produced as a large precursor protein called polyproteins in the membrane-limited organelles of the neuron. These larger proteins are cleaved to form the neuroactive peptides, which are removed by both diffusion and breakdown by extracellular proteases. Since polyproteins can encode different neurotransmitters, this processing of the polyprotein is crucial in determining which of the neuroactive peptides are ultimately released by the neuron. Neuroactive peptides have long-lasting effects because they all work through G protein–coupled receptors.

Figure 7-1. Comparison of the biochemical events at cholinergic endings with those at noradrenergic endings.

Neuroactive peptides differ from the small-molecule neurotransmitters in several ways. First, because they rely on protein synthesis and modification, neuroactive peptides can only be produced in the cell body. Small-molecule neurotransmitters rely on enzymes found throughout the cytosol and are predominantly manufactured at the presynaptic terminal. They are also taken up and concentrated within the synaptic vesicles, unlike the neuroactive peptides which are packaged into vesicles by the Golgi apparatus. Because of the different processing steps, the type of synaptic vesicles also differs between the two classes. The vesicles for small-molecule neurotransmitters can be recycled quickly at the nerve terminal following exocytosis to produce more synaptic vesicles. The membrane that constitutes the vesicles for neuroactive peptides come from the Golgi apparatus and are transported from the cell body in a more time-consuming fashion.

Despite these differences, neuroactive peptides and small-molecule neurotransmitters often coexist within the same neuron. They can be released together to function synergistically on the postsynaptic cell. Additionally, several different neuroactive peptides processed from a single polyprotein can be released into the synaptic cleft.

Following release, the neurotransmitters must be removed from the synaptic cleft to prevent desensitization of the postsynaptic receptors and to allow future transmissions to occur. As learned previously, enzymes in the synaptic cleft degrade and inactivate certain neurotransmitters, such as acetylcholine. Neuroactive peptides are cleared more slowly from the synapse by simple diffusion. Most neurotransmitters, however, are taken up by the neuron to terminate their action. Transporter proteins in the neuron often rely on the electrochemical gradient for the active reuptake of the neurotransmitter.

COMPREHENSION QUESTIONS

[7.1] A 62-year-old man presents to your office complaining of tremor and difficulty with movements. He is noted to have masked facies and a shuffling gate. Based on clinical presentation and additional studies, you diagnose him with Parkinson disease. Which molecule is the immediate precursor in the synthetic pathway leading to the neurotransmitter involved in this disease?

A. L-dopa

B. NE

C. Tryptophan

D. Tyrosine

[7.2] A 41-year-old woman presents to your office complaining of chronic widespread pain, particularly in “trigger points” throughout her body. She is ultimately diagnosed with fibromyalgia, a disorder that is associated with elevated levels of the neurotransmitter “Substance P.” In which of the following locations is substance P synthesized?

A. Cell body

B. Presynaptic terminal

C. Synaptic vesicles

D. Synaptic cleft

[7.3] You are following a patient who has been previously diagnosed with Alzheimer dementia and is undergoing pharmacologic treatment. One of the proposed pathologic mechanisms of this disease is a lack of cholinergic neurotransmission in certain areas of the brain. Some treatments are therefore aimed at increasing cholinergic neurotransmission. By which of the following mechanisms is a drug that increases acetylcholine in the synaptic cleft most likely to act?

A. Inhibition of diffusion out of the synaptic cleft

B. Inhibition of reuptake of acetylcholine into the neuron and surrounding glia

C. Inhibition of acetylcholinesterase-mediated degradation of acetylcholine

D. Increased synthesis and release of acetylcholin

Answers

[7.1] A. The neurotransmitter implicated in the pathogenesis of Parkinson disease is DA, the immediate precursor of which is L-dopa. In the synthetic pathway leading to DA, tyrosine is converted to L-dopa by tyrosine hydroxylase; L-dopa is then converted to DA by dopa decarboxylase. DA can be further processed by dopamine β-hydroxylase to yield NE. Tryptophan is the first step in the synthetic pathway leading to serotonin.

[7.2] A. Substance P is a neurotransmitter that belongs to the neuroactive peptide class and, like all peptides, is synthesized in the cell body on the rough endoplasmic reticulum. After synthesis, these peptides are further processed by the Golgi complex, which also packages them into vesicles. These vesicles are transported down the axon via fast anterograde axonal transport to the presynaptic terminal, where they are secreted into the synaptic cleft when properly stimulated. Many of the small-molecule neurotransmitters (ACh, DA, GABA, etc.) are synthesized in the cytoplasm of the presynaptic terminal and subsequently packaged into secretory vesicles. DA is converted to NE inside the synaptic vesicles. Neurotransmitters are degraded, not synthesized in the synaptic cleft.

[7.3] C. While all of the above are potentially mechanisms by which the acetylcholine levels in the synaptic cleft could be increased, the most likely candidate is inhibition of acetylcholinesterase-mediated degradation. The primary method of removal of acetylcholine from the cleft is enzymatic degradation by acetylcholinesterase. Inhibition of this enzyme, therefore, increases the level of ACh in the cleft. There are numerous drugs that do just this, and they are in fact used for treatment of Alzheimer as well as other disorders. Diffusion from the cleft is the primary means of removal of neuropeptides, and reuptake is the primary means of removal of amino acid neurotransmitters like GABA and glycine. While increasing the synthesis and release of ACh would also increase its concentration in the synaptic cleft, this is a considerably more complicated process than inhibiting acetylcholinesterase, and therefore a less prominent drug target.

NEUROSCIENCE PEARLS ❖ Acetylcholine is synthesized from choline and acetyl coenzyme A (acetyl CoA) by the enzyme choline acetyltransferase. ❖ In the synaptic cleft, acetylcholine is hydrolyzed by acetylcholinesterase into acetate and choline. ❖ In the synaptic cleft, glial cells facilitate the clearance of glutamate by converting it to glutamine, which then diffuses across the plasma membrane where it is converted back to glutamate and then repackaged into vesicles for future release. ❖ Glycine is the major inhibitory neurotransmitter of the spinal cord. ❖ Conversion of tyrosine to L-dopa by tyrosine hydroxylase is the ratelimiting step for DA and NE production.

REFERENCES

Bear MF, Connors B, Paradiso M, eds. Neuroscience: Exploring the Brain. 3rd ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006. 

Kandel K, Schwartz J, Jessell T, eds. Principles of Neural Science. 5th ed. New York: McGraw-Hill; 2000. 

Zigmond MJ, Squire LR, Bloom FE, Landis SC, Roberts JL, eds. Fundamental Neuroscience. 2nd ed. San Diego, CA: Academic Press; 1999.

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