Opioid Receptors

Three classical opioid receptor types, m, S, and K,have been studied extensively; the N/OFQ receptor system is still being defined. Highly selective ligands that allowed for type-specific labeling of the three classical opioid receptors (e.g., DAMGO for m, DPDPE for S, and U-50,488 and U-69,593 for k) made possible the definition of ligand-binding characteristics of each of the receptor types and the determination of anatomical distribution of the receptors using autoradiographic techniques. Each major opioid receptor has a unique anatomical distribution in brain, spinal cord, and the periphery.

Receptor-selective antagonists and agonists have aided the study of the biological functions of opioid receptors. Commonly used antagonists are cyclic analogs of somatostatin such as CTOP as m-receptor antagonists, a derivative of naloxone called naltrindole as a S-receptor antagonist, and a bivalent derivative of naltrexone called nor-binaltorphimine (nor-BNI) as a k-receptor antagonist. In general, functional studies using selective agonists and antagonists have revealed substantial parallels between m and S receptors and dramatic contrasts between jl/S and k receptors. In vivo infusions of selective antagonists and agonists also were used to establish the receptor types involved in mediating various opioid effects (Table 21-2).

Most of the clinically used opioids are relatively selective for m receptors, reflecting their similarity to morphine (Tables 21-3 and 21-4). However, drugs that are relatively selective at standard doses may interact with additional receptor subtypes when given at sufficiently high doses, leading to possible changes in their pharmacological profile. This is especially true as doses are escalated to overcome tolerance. Some drugs, particularly mixed agonist-antagonist agents, interact with more than one receptor class at usual clinical doses and may act as an agonist at one receptor and an antagonist at another.

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