PSYC62H3 Chapter Notes - Chapter 10: Mesolimbic Pathway, Constipation, Analgesic
Chapter 10: Opioids
• Opioids: psychoactive substances that elicit pharmacological effects by acting on opioid
receptors in the CNS and other parts of the body
• Among the most effective pain-relieving medications today
• Narcotic
o “Sleep inducing”
o Sedative effects of drugs and generally serves as a synonym for opioids
• Serve as major drug of abuse
• Easy to access
• Prescription opioid drugs → hydrocodone (Vicodin) and oxycodone
o Popular b/c they produce powerful reinforcing effects and are easy to obtain
• Can also obtain opioids by drinking over-the-counter cough syrups that contain the
opioid dextromethorphan
Opioids: Natural and Synthetic
• Produced naturally in environment or synthetically in lab
• Natural opioids found in opium exudate from poppy plants → Papaver somniferum
• Naturally occurring opioids: opioids found within opium
o Primary ones are morphine and codeine
• All other opioids are synthetically produced
• Semisynthetic opioids: synthesized from morphine or codeine
• Fully synthetic opioids: not derived from morphine, codeine, or any other naturally
occurring opioids
• Both produced for medicinal purposes
Pharmacokinetic Properties and Opioid Abuse
• Users administer opioids through inhalation, oral administration, or intravenous
administration
• To achieve reinforcing effects users prefer intravenous injection or inhalation for rapid
speed of onset
• Intravenous route also allows rapid relief from pain
• Opioids permeate BBB and enter brain
• Opioids vary in lipid solubility
• Opioids primarily metabolized in liver
o Produces metabolites that have biological effects
o For codeine the cytochrome P450 enzyme CYP2D6 converts appro 5% of
codeine to morphine
o Opioid metabolites are key indicators of opioid use → detectable in urine
samples
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• Elimination rates of opioids varies from drug to drug
• Heroin cessation programs prescribe patients methadone
o Levels peak in the body several hours after administration and remain at
biologically active levels 24 hours after administration
o Functions to prevent heroin withdrawal symptoms from occurring
Opioid Drug Interactions with the Endogenous Opioid System
• Endogenous system includes opioid NTs and receptors
• Endogenous opioids = neuropeptide NTs
o Amino acids that link to make up each neuropeptide is part of a larger chain of
amino acids → propeptide
o Beta-endorphin, metenkephalin, leu-enkephalin, dynorphin A, dynorphin B,
and neoendorphin
▪ Derive from 3 propeptides → proopiomelanocortin, proenkephalin, and
prodynorphin
o Most of the pharmacological effects associated with opioids derived through
activating the mu and delta receptors
• Opioid receptors are g-protein-coupled metabotropic receptors
o Reduce metabolic activity within neurons → these neurons that have opioid
receptors tend to have reduced neurotransmission after administration of an
opioid receptor agonist
o G-protein-coupled inwardly rectifying K+ channel (GIRK)
▪ Known inhibitory mechanism
▪ Influx of K+ when a neuron is hyperpolarized and diminished K+ influx
when neuron is depolarized
• Adaptation occurs to opioids during repeated administration
o Involves reduction in sodium-potassium pump activity
Opioid Drugs; Classification by Receptor Action
• Most opioid drugs used act on mu opioid receptors
• Pure opioid receptor agonists
o Produce full agonist actions at mu opioid receptors
o Example: fentanyl and morphine
• Partial opioid receptor agonists
o Produce partial agonist actions at mu opioid receptors
o Smaller magnitude of pharmacological effects compared to pure receptors
• Pure opioid receptor antagonists
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Document Summary
Chapter 10: opioids: opioids: psychoactive substances that elicit pharmacological effects by acting on opioid receptors in the cns and other parts of the body, among the most effective pain-relieving medications today, narcotic. Pharmacokinetic properties and opioid abuse: users administer opioids through inhalation, oral administration, or intravenous administration, to achieve reinforcing effects users prefer intravenous injection or inhalation for rapid speed of onset. Influx of k+ when a neuron is hyperpolarized and diminished k+ influx when neuron is depolarized: adaptation occurs to opioids during repeated administration. Leading to greater dopamine release in nucleus accumbens. Opioids produce reinforcing effects through activating mu opioid receptors in nucleus accumbens: opioids also closely interact with pain signaling pathways to the brain, opioid receptors found outside the ns as well. Immune, cardiovascular, respiratory, and digestive systems: chronic administration of opioid agonist suppresses immune system functioning by reducing the reproduction of immune system cells.
