<Li> reward (minor role) </Li> <P> Understanding the effects of drugs on neurotransmitters comprises a significant portion of research initiatives in the field of neuroscience . Most neuroscientists involved in this field of research believe that such efforts may further advance our understanding of the circuits responsible for various neurological diseases and disorders, as well as ways to effectively treat and someday possibly prevent or cure such illnesses . </P> <P> Drugs can influence behavior by altering neurotransmitter activity . For instance, drugs can decrease the rate of synthesis of neurotransmitters by affecting the synthetic enzyme (s) for that neurotransmitter . When neurotransmitter syntheses are blocked, the amount of neurotransmitters available for release becomes substantially lower, resulting in a decrease in neurotransmitter activity . Some drugs block or stimulate the release of specific neurotransmitters . Alternatively, drugs can prevent neurotransmitter storage in synaptic vesicles by causing the synaptic vesicle membranes to leak . Drugs that prevent a neurotransmitter from binding to its receptor are called receptor antagonists . For example, drugs used to treat patients with schizophrenia such as haloperidol, chlorpromazine, and clozapine are antagonists at receptors in the brain for dopamine . Other drugs act by binding to a receptor and mimicking the normal neurotransmitter . Such drugs are called receptor agonists . An example of a receptor agonist is Valium, a benzodiazepine that mimics effects of the endogenous neurotransmitter gamma - aminobutyric acid (GABA) to decrease anxiety . Other drugs interfere with the deactivation of a neurotransmitter after it has been released, thereby prolonging the action of a neurotransmitter . This can be accomplished by blocking re-uptake or inhibiting degradative enzymes . Lastly, drugs can also prevent an action potential from occurring, blocking neuronal activity throughout the central and peripheral nervous system . Drugs such as tetrodotoxin that block neural activity are typically lethal . </P> <P> Drugs targeting the neurotransmitter of major systems affect the whole system, which can explain the complexity of action of some drugs . Cocaine, for example, blocks the re-uptake of dopamine back into the presynaptic neuron, leaving the neurotransmitter molecules in the synaptic gap for an extended period of time . Since the dopamine remains in the synapse longer, the neurotransmitter continues to bind to the receptors on the postsynaptic neuron, eliciting a pleasurable emotional response . Physical addiction to cocaine may result from prolonged exposure to excess dopamine in the synapses, which leads to the downregulation of some post-synaptic receptors . After the effects of the drug wear off, an individual can become depressed due to decreased probability of the neurotransmitter binding to a receptor . Fluoxetine is a selective serotonin re-uptake inhibitor (SSRI), which blocks re-uptake of serotonin by the presynaptic cell which increases the amount of serotonin present at the synapse and furthermore allows it to remain there longer, providing potential for the effect of naturally released serotonin . AMPT prevents the conversion of tyrosine to L - DOPA, the precursor to dopamine; reserpine prevents dopamine storage within vesicles; and deprenyl inhibits monoamine oxidase (MAO) - B and thus increases dopamine levels . </P>

Which category of drugs includes many substances that are chemically similar to neurotransmitters