Neuropharmacologic drugs are those that alter processes controlled by either the central nervous system or the peripheral nervous system.

Review of Neuron Elicited Responses:

Neurons influence postsynaptic cells (which could be other neurons, muscle cells or secretory gland cells).  The way this happens is very simple: the action potential is conducted along the neuron's axon, then the neurotransmitter is released from the axon terminal.  Finally the transmitter binds to receptors on the postsynaptic cell.  The binding of the transmitter is what makes things happen.  The steps listed above are known as axonal conduction and synaptic transmission.

Neuropharmacologic Agents:

A drug can alter either axonal conduction, or synaptic transmission.  Most neuropharmacologic agents produce their effects by altering synaptic transmission.  

Drugs that alter axonal conduction:

• Not very selective (because axons are all the same, so if a drug influences axon conduction, it will influence all axon conductions)

• Limited indications (anesthetics)

Drugs that alter synaptic transmission

• Highly selective (because synapses are not all the same.  At different sites, different transmitters are employed and different types of receptors)

• Numerous applications

Receptors

• Neurotransmitter specific.

• The impact of a drug on a neuronally regulated process is dependent upon the ability of that drug to directly or indirectly influence receptor activity on target cells.

Steps in Synaptic Transmission

• Synthesis of the transmitter.

• Storage of transmitter until time for release.

• Release - triggered by the arrival of the action potential at the axon terminal

• Receptor Binding (reversible)

• Termination - dissociation of transmitter from receptors and removal of free transmitter from the synaptic gap.  This removal can take place by: reuptake, enzymatic degradation or diffusion.

Effects of Drugs on the Steps of Synaptic Transmission: A drug can enhance receptor activation or reduce receptor activation.  Activation = an effect on receptor function equivalent to that produced by the natural neurotransmitter at a particular synapse.

• Synthesis of Transmitter:

  • Drug Can:  

    • Increase transmitter synthesis (increase receptor activation)

    • Decrease transmitter synthesis (decrease receptor activation)

    • Cause synthesis of more effective transmitters (increase receptor activation)

• Storage of Transmitter

  • Drug Can:

    • Reduce transmitter storage (decrease receptor activation)

• Release of Transmitter

  • Drug Can:

    • Promote transmitter release (increase receptor activation)

    • Inhibit transmitter release (decrease receptor activation)

• Binding to Receptor

  • Drug Can:

    • Stimulate receptor directly (Increase receptor activation)

    • Enhance receptor's response to natural transmitter (increase receptor activation)

    • Block receptor from being activated by other agents (decrease receptor activation)

• Termination of Transmission

  • Drug Can:

    • Block reuptake of transmitter (increase receptor activation)

    • Prevent breakdown of transmitter (increase receptor activation)

Understanding how PNS drugs work requires three types of information:

1. The type or types of receptors through which the drug acts.

2. The normal response to activation of those receptros

3. What the drug in question does to receptor function (increase or decrease)

Back to Pharmacology Index

[Home] [Free Gift] [Study Tips] [Class Notes] [About Me]

[Awards] [Links & Rings] [Site Map] [Graphics] [Online Tests]