Intro
Agonistic Drug Effects:
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Antagonist Drug Effects:
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Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
6. The final class of agonist drugs bind to the receptor sites of the postsynaptic membrane and activate or increase the effect of a neurotransmitter. Direct agonists have an affinity for the neurotransmitter's primary receptor site. The binding that results is called competitive binding. Indirect agonists bind to an alternative site. This process is called noncompetitive binding. That is, the indirect agonist does not compete with the neurotransmitter at its receptor site, but is able to facilitate the neurotransmitter's action by binding elsewhere. For example, the benzodiazepines (anti-anxiety agents) bind to a portion of the GABAA receptor. The changes brought about by this reaction increase the binding of GABA to its receptor site. These anti-anxiety drugs increase inhibitory effects of GABA by stimulating the opening of chloride ion channels. Chloride influx induces hyperpolarization of the postsynaptic membrane. Nicotine and muscarine are examples of direct agonists for acetylcholine. Indeed, acetylcholine receptors are identified based on the affinity of each of these agonists. One will find reference to muscarinic and nicotinic cholinergic receptors whenever this neurotransmitter is considered.