[Opioids by the perimedullary route: mechanisms of opioid analgesia]

The identification of multiple opioid receptors and opioid peptides in the 1970 was the starting point of an increasing knowledge on opioid physiology and pharmacology. The mechanisms of action of spinally supraspinal levels. At the spinal level, opioids act by a modulation of specific supraspinal effect is the consequence of the migration of opioids, other in the bloodstream or in the cerebrospinal fluid, towards the encephalon. This action involve complex systems of inhibitory control on spinal structures. Opioids act on specific receptors which have a broad distribution in the central nervous system. the classification of opioid receptors is based on the activity of specific ligands: mu (morphine), kappa (ketocyclazocine), sigma (SKF 10047), delta (leu-enkephalin) and epsilon (beta-endorphin). Evidence for the existence of different isoreceptors for each type has been given by experimental studies. At the receptor level, opioid agonists act, hypothetically via the system of adenylcylase, more certainly via a modulation of membrane tonic channels. Thus, opioids modify sodium, calcium and potassium currents, and modify the successives phases of the membrane action potential. The result is an hyperpolarization which is responsible of an inhibition of the release of various neurotransmitters such as P substance.