Modification of Synaptic Transmission by Drugs and Disease

The great majority of drugs that act on the nervous system do so by altering synaptic mechanisms and thus synaptic effectiveness. All the synaptic mechanisms labeled in Figure 8-33 are vulnerable.

The long-term effects of drugs are sometimes difficult to predict because the imbalances produced by the initial drug action are soon counteracted by feedback mechanisms that normally regulate the processes. For example, if a drug interferes with the action of a neurotransmitter by inhibiting the rate-limiting enzyme in its synthetic pathway, the neurons may respond by increasing the rate of precursor transport into the axon terminals to maximize the use of any enzyme that is available.

Recall from Chapter 7 that drugs that bind to a receptor and produce a response similar to the normal activation of that receptor are called agonists, and drugs that bind to the receptor but are unable to activate it are antagonists. By occupying the receptors, antagonists prevent binding of the normal neurotrans-mitter when it is released at the synapse. Specific agonists and antagonists can affect receptors on both pre- and postsynaptic membranes.

Diseases can also affect synaptic mechanisms. For example, the toxin produced by the bacillus Clostridium tetani (tetanus toxin) is a protease that destroys certain proteins in the synaptic-vesicle docking mechanism of neurons that provide inhibitory synaptic input to the neurons supplying skeletal muscles. The toxin of the Clostridium botulinum bacilli and the venom of the black widow spider also affect neurotransmitter

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Direction of action potential transmission

TABLE 8-6 Factors that Determine Synaptic Effectiveness

Presynaptic neuron

Direction of action potential transmission

Presynaptic neuron

Synthesizing enzyme

Neurotransmitter precursors

Empty vesicle

Presynaptic Control Systems

Postsynaptic neuron

FIGURE 8-33

Drug actions at synapses: (A) Increase leakage of neurotransmitter from vesicle to cytoplasm, exposing it to enzyme breakdown, (B) increase transmitter release, (C) block transmitter release, (D) inhibit transmitter synthesis, (E) block transmitter reuptake, (F) block enzymes that metabolize transmitter, (G) bind to receptor to block (antagonist) or mimic (agonist) transmitter action, and (H) inhibit or facilitate second-messenger activity. [Q]

Redrawn from DRUGS AND THE BRAIN by Solomon H. Snyder. Copyright © 1986 by Scientific American Books, Inc. Reprinted by permission of W. H. Freeman and Company.

Synthesizing enzyme

Neurotransmitter precursors

Empty vesicle

Postsynaptic neuron

FIGURE 8-33

Drug actions at synapses: (A) Increase leakage of neurotransmitter from vesicle to cytoplasm, exposing it to enzyme breakdown, (B) increase transmitter release, (C) block transmitter release, (D) inhibit transmitter synthesis, (E) block transmitter reuptake, (F) block enzymes that metabolize transmitter, (G) bind to receptor to block (antagonist) or mimic (agonist) transmitter action, and (H) inhibit or facilitate second-messenger activity. [Q]

Redrawn from DRUGS AND THE BRAIN by Solomon H. Snyder. Copyright © 1986 by Scientific American Books, Inc. Reprinted by permission of W. H. Freeman and Company.

release from synaptic vesicles by interfering with docking proteins, but they act on axon terminals of neurons different from those affected by tetanus toxin.

Table 8-6 summarizes the factors that determine synaptic effectiveness.

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Responses

  • Silvio
    Which drugs and diseases modify the synaptic transmission?
    6 years ago
  • Zahra
    How does disease affect synaptic transmission?
    6 years ago
  • tobold
    What are drug effects on synaptic function?
    6 years ago
  • Procopio
    How drugs affect Synaptic Transmission on an article?
    5 years ago
  • Rosetta
    How drugs affect brain activity by interfering with synaptic transmission?
    5 years ago
  • uffo
    What is a known drug effect on synaptic function?
    5 years ago
  • steven
    What are the drug effect on synaptic funtion?
    5 years ago
  • roddy
    How are reuptake and enzymes related to synaptic function?
    5 years ago

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