Monoamines as Neurotransmitters

A variety of chemicals in the CNS function as neurotransmitters. Among these are the monoamines,a chemical family that includes dopamine,norepinephrine,and serotonin. Although these molecules have similar mechanisms of action, they are used by different neurons for different functions.

The regulatory molecules epinephrine, norepinephrine, dopamine, and serotonin are in the chemical family known as monoamines. Serotonin is derived from the amino acid tryptophan. Epinephrine, norepinephrine, and dopamine are derived from the amino acid tyrosine and form a subfamily of monoamines called the catecholamines (see fig. 9.8, p. 229). Epinephrine (also called adrenaline) is a hormone secreted by the adrenal gland, not a neurotransmitter, while the closely related norepinephrine functions both as a hormone and a neurotransmitter.

Like ACh, monoamine neurotransmitters are released by exocytosis from presynaptic vesicles, diffuse across the synaptic

Chapter Seven cleft, and interact with specific receptor proteins in the membrane of the postsynaptic cell. The stimulatory effects of these monoamines, like those of ACh, must be quickly inhibited so as to maintain proper neural control. The inhibition of monoamine action is due to (1) reuptake of monoamines into the presynaptic neuron endings, (2) enzymatic degradation of monoamines in the presynaptic neuron endings by monoamine oxidase (MAO), and (3) the enzymatic degradation of catecholamines in the postsynaptic neuron by catechol-O-methyltransferase (COMT). This process is illustrated in figure 7.27.

Monoamine oxidase (MAO) inhibitors are drugs that block monoamine oxidase, the enzyme in presyn-aptic endings that breaks down catecholamines and serotonin after they have been taken up from the synaptic cleft. These drugs thus promote transmission at synapses that use monoamines as neurotransmitters. Such drugs have proven useful in the treatment of clinical depression, suggesting that a deficiency in monoamine transmission contributes to that disorder. An MAO inhibitor is also used to treat Parkinson's disease, because it increases the ability of dopamine to function as a neurotransmitter.

Presynaptic neuron ending

Ca2+

Action potentials

Action potentials

Ca2+

Dopa i

Dopamine

Tyrosine .1

Dopa i

Dopamine

Inactivated by MAO

/Norepinephrine m

Receptor

Reuptake (most)

Norepinephrine

Circulation at

Postsynaptic cell

Inactivated by COMT -^-Inactive products

■ Figure 7.27 The production, release, and reuptake of catecholamine neurotransmitters. The transmitters combine with receptor proteins in the postsynaptic membrane. (COMT = catechol-O-methyltransferase; MAO = monoamine oxidase.)

Fox: Human Physiology, I 7. The Nervous System: I Text Eighth Edition Neurons and Synapses

The Nervous System: Neurons and Synapses

Clinical Investigation Clues

Remember that Sandra was taking an MAO inhibitor, and that her blood levels of this drug were not unduly high. Why was Sandra taking an MAO inhibitor drug? Why might the paramedics suspect that she might have a neuromuscular disorder?

The monoamine neurotransmitters do not directly cause opening of ion channels in the postsynaptic membrane. Instead, these neurotransmitters act by means of an intermediate regulator, known as a second messenger. In the case of some synapses that use catecholamines for synaptic transmission, this second messenger is a compound known as cyclic adenosine monophosphate (cAMP). Although other synapses can use other second messengers, only the function of cAMP as a second messenger will be considered here. Other second-messenger systems are discussed in conjunction with hormone action in chapter 11.

Binding of norepinephrine, for example, with its receptor in the postsynaptic membrane stimulates the dissociation of the G-protein alpha subunit from the others in its complex (fig. 7.28). This subunit diffuses in the membrane until it binds to an enzyme known as adenylate cyclase (also called adenylyl cyclase). This enzyme converts ATP to cyclic AMP (cAMP) and pyrophosphate (two inorganic phosphates) within the postsynaptic cell cyto-

© The McGraw-Hill Companies, 2003

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Responses

  • Marva Blankenship
    What regulatory molecules are neurotransmitters in the chemical family known as monoamines?
    8 years ago
  • VALENTIN ANDERSON
    Are mao on the postsynaptic membrane?
    8 years ago
  • dana
    What is monoamine physiologic effect in humans?
    8 years ago
  • robel
    What enzyme that breaks down catecholamine neurotransmitters in the synapse?
    8 years ago
  • negassi
    Why are neurotransmitters inactivated by mao?
    8 years ago
  • marcia pierce
    Why is monoamine oxidase related to physiology?
    7 years ago
  • Markus
    What is the enzyme that breaks down extra neurotransmitters in the synapse?
    7 years ago
  • Ciro
    What enzyme breaks down asparate in the synspse?
    6 years ago
  • Seppo
    What breaks down serotonin in the synaptic cleft?
    6 years ago
  • manuel
    What is monoamines role in the nervous system?
    1 year ago
  • aisha
    How are the five monoamines known to be neurotransmitters similar and different?
    11 months ago
  • lennox
    How adrenaline is related to depression monoamines?
    11 months ago
  • generosa
    HOW IS DOPAMINE INACTIVATED?
    6 months ago
  • Monica
    What do monoamines neurotransmitter do?
    6 months ago
  • bertha burrows
    What do monoanimes affect in a person?
    6 months ago
  • aedan
    What monoamines act as neurotransmitters in the central nervous system?
    6 months ago
  • saverio
    How to increase the availability of monoamines in the synapse?
    1 month ago

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