[Participation of opioids and opioid receptors in antinociception at various levels of the nervous system].

At the cerebral level, studies employing several experimental approaches point to an essential role of beta-endorphin in analgesia, induced by electrical stimulation of the periaqueductal grey of the midbrain. Tolerance and cross-tolerance studies suggest that mu-opioid receptors mediate this effect. The significance of sigma- and, in particular, kappa-opioid receptors in cerebral pain modulation is less well documented. At the spinal level, nociception is relayed in the dorsal horn, where opioid peptides as well as all types of opioid receptors are abundant. mu-opioid receptor-mediated anti-nociceptive processes appear to be most important in this region, but sigma-opioid receptors may also be involved. In addition, a role of kappa-opioid receptors can be demonstrated under certain conditions. Recent experiments indicate that opioids may also modulate nociception in the periphery, in particular in inflamed tissue.