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PSYC62H3 (313)

chapter notes 5

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Suzanne Erb

Chapter 5 NEUROACTIVE LIGANGS AND THE NERVOUS SYSTEM - Specific neuroactive ligands are highly localized in some areas of the brain, while others are distributed widely throughout the nervous system. - By interacting with the natural neuroactive ligands in various areas of the brain, drugs can shift normal psychological activities carried out by the brain into abnormal ones and in some cases, can serve to normalize abnormal psychological activities. Neurotransmitters, Neuromodulators, and Neurohormones - Neurotransmitters are commonly viewed as chemical that are located in specific regions of neurons, are released under specific stimulation, act on a specific set of receptors, and induce short-duration changes in membrane potential. - Neurohormones, these are chemicals that are synthesized in one area of the nervous system, are released into circulation, travel to some site that is distant from the release site, and then produce some effect on the brain or body. - There are chmicals that have some of the characteristics of neurotransmitters or neurohormones, but by themselves have no intrinsic activity except in the presence of other synaptic activity =Neuromodulators because they modify response to other transmitter presynaptically or postsynaptically while not showing any direct shifts in membrane potential or conductance when tested for actions on their own. Eg. A neuromodulator may induce a change in the binding of a neurotransmitter to its receptor, or it can act through secondary messengers to modulate neural responses to a neurotransmitter. - A chemical that appears to play the role of a neurotransmitter in one area of the nervous system may play the role of a neuromodulator or neurohormone somewhere else in the body. - Hundreds of chemicals in the nerbous system have eeb indentified as either being or having the potential to be neurotransmitter. - They range in structural complexity from molecules of two atoms eg NO and CO - The major chemical classes of transmitters are amines, amino acides, and peptides. - With respect to the CNS, most of these chemicals have yet to meet several criteria that would definitely indicate their roles as neurotransmitters. In order for a chemical to be designated as a neurotransmitter: 1. It should be found in presynaptic neurons 2. Enzymes necessary for its synthesis must also be present in the neuron 3. There should a mechanism for terminating its action 4. Its direct application to the postsynaptic neuron should be equivalent to stimulation of the presynaptic neuron www.notesolution.com5. When the presynaptic neuron is stimulated, the synaptic cleft should contain the neurotransmitter 6. Drugs interfering with the synthesis or reaction at the postsynaptic membrane should block the effects of the presnaptic neuronal stimulation and 7. Drugs blocking the action of the inactivating enzyme should prolong the transmitters action - Though most of the chemicals satisfy one or more of these criteria only Ach and NE in the peripheral nervous system satisfy all of them - Virtually every drug that alters psychological function does so by interacting with one or more neurotransmitter systems in the brain. - Drugs have been shown to alter the synthesis, storage, release, enzymatic inactivation, and reuptake of the neurotransmitter. - Many drugs either mimic (agonists) or block (antagonists) specific neurotransmitters at their receptors, both presynaptic and postsynaptic. - If the drug mimics the neurotransmitter at its postsynaptic receptor, it will enhance the neurotransmitters ability to alter activity of the postsynaptic cell. - If the drug mimics the action of the neurotransmitter at its auto-receptors and inhibits the release of the neurotransmitter, it will reduce the neurotransmitters ability to alter the activity of the postsynaptic cell. - If a drug mimics a neurotransmitter at both its auto-receptor and its postsynaptic receptors, but has a greater affinity for its auto-receptora situation that is nor uncommon. The drug may reduce the neurotransmitters action at postsynaptic receptor when administered in low doses (because activation of the auto-receptors decrease the amount of endogenous neurotransmitter available for activating the postsynaptic receptor), but it may activate the transmitters postsynaptic receptor when administered in higher doses. In such cases, even though there is less endogenous transmitter available for the receptor, the drug takes its place so that there is more postsynaptic receptor activation that might occur normally with the transmitter alone. - One of the primary problems we currently have with respect to drugs is their relative lack of speficity, which can result in dependence and side effects that may, in the long run, be worse for patients than the condition they took the drug to counteract. A psychoactive drug can alter any number of the processes involved in the communication system. It can: 1. Increase or decrease the rate of synthesizing one of more neurotransmitters 2. Increase or decrease the amount of neurotransmitter released 3. Enhance or prevent the storage of neurotransmitter 4. Increase or decrease a neurotransmitters rate of metabolic breakdown. 5. Bind to the presynaptic or postsynaptic receptors for the neurotransmitter and, depending on whether or not the drug activates the receptor, it can accentuate or blunt the neurotransmitters effect 6. Reduce or enhance the neurotransmitters reuptake. www.notesolution.com
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