Final answer:
Chemical antagonists or noncompetitive inhibitors decrease the activity of neurotransmitters by binding to alternative sites on enzymes or receptors, altering their function without competing with natural ligands for the same binding site.
Step-by-step explanation:
Chemical Antagonists (Non-Receptor Antagonists)
Antagonists are drugs that interact with the body to decrease the activity of particular neurotransmitters, which are chemicals that transmit signals across a synapse from one neuron to another. Non-receptor antagonists, or noncompetitive inhibitors, operate by hindering the action of neurotransmitters without competing for the same binding site as the endogenous ligand. For instance, they might act by altering neurotransmitter synthesis or by binding to a different site on the enzyme or receptor, which then alters its functional shape.
Noncompetitive inhibitors like toxic metals (Pb²⁺, Hg²⁺, and Ag⁺) can bind to an enzyme or receptor at a site distinct from the active site, leading to a change in the enzyme's structure and function. This change may result in the inhibition of enzyme activity, even if the normal substrate concentration is increased. In some cases, however, the effect of the noncompetitive inhibitor can be reversed by other chemical agents that remove the inhibitor.
Psychoactive drugs can function as either agonists or antagonists. In the context of treating diseases such as Parkinson's, which involves low levels of dopamine, agonists are used to mimic the effects of dopamine and increase neurotransmitter activity, whereas antagonists would be used to decrease neurotransmitter activity when there is an excess.