Chapter 23: Opioid Analgesics and Antagonists

Opioid Analgesics and Antagonists defines pain as a complex sensory and emotional signal of potential tissue injury and explains the biological distinction between somatic, visceral, and neuropathic pain. The text details the neural architecture of pain transmission, highlighting the roles of primary afferent A-delta and C fibers and the dual pathways that process physical sensation in the sensory cortex versus emotional suffering in the limbic system. Crucial mechanisms like the gate-control theory and the descending inhibitory pathways involving the periaqueductal gray are explored, showing how neurotransmitters like serotonin and norepinephrine modulate spinal pain signals. The discussion centers on the three major families of endogenous peptides—enkephalins, endorphins, and dynorphins—and their interaction with mu, delta, and kappa G-protein coupled receptors. Pharmacological agents are categorized into strong agonists such as morphine and fentanyl, moderate agonists like codeine, and innovative biased agonists such as oliceridine, which aims to provide relief with fewer side effects by prioritizing specific intracellular signaling pathways. Additionally, the chapter covers mixed agonist-antagonists and pure antagonists like naloxone, used for reversing life-threatening overdoses. Key therapeutic considerations include the physiological management of respiratory depression, the development of tolerance and physical dependence, and the specialized strategies used to treat chronic, terminal, and neuropathic pain using coanalgesics like antidepressants and antiepileptics. By understanding the pharmacokinetics and adverse effect profiles of these substances, healthcare providers can better navigate the balance between effective analgesia and the risks of long-term opioid use.