13.1 : 阿片类受体：概述

Opioid receptors respond to endogenous or synthetic opioids. They are distributed across peripheral and central nervous systems and nonneuronal cells like macrophages and astrocytes.

Based on ligand specificity, they are classified as μ, δ, and κ receptors.

The μ and κ receptors modulate nociceptive responses, while δ receptors interact peripherally with enkephalins. All these responses contribute to analgesia.

Opioid receptors belong to the rhodopsin family of GPCRs.

They feature an extracellular N-terminus, seven transmembrane helices, and three extra- and intracellular loops ending in a long carboxy tail.

Opioid receptor signaling is mediated by G-protein and receptor coupling, triggering an intracellular cascade. This results in adenylyl cyclase inhibition, membrane hyperpolarization, and decreased neurotransmitter release.

Drugs acting on opioid receptors, or opioids can be full or partial agonists or antagonists based on binding-site selectivity. For instance, morphine, a naturally occurring alkaloid,  is a full μ opioid receptor agonist.