Opioid pharmacology

Endogenous and exogenous opioid bind to specific receptors. There are four different types of opioid receptors: mu, kappa, delta and nociceptin. All of them are membrane receptors coupled to protein G. Opioid drugs are classified, taking into account its affinity and efficacy for receptors, in four classes: pure agonists, agonist-antagonists, partial agonists and antagonists. The main pharmacological effects induced by agonists are sedation, euphoria, analgesia, nausea and vomiting, miosis, cough suppression, respiratory depression, truncal rigidity, constipation, face flushing and pruritus, urinary retention, and dependence (tolerance, withdrawal). The upregulation of the AMPc pathway seem responsible for tolerance and withdrawal symptoms. Opioide agonist induce long-lasting neural adaptations that are related to the synthesis of some transcription factors as CREB and ΔFosB, that seem relevant in relapse. Pharmacokinetic, drug interactions and therapeutic indications are reviewed.