Opioids and opioid receptors mediating antinociception at various levels of the neuraxis.

One of the central issues in present experimental pain research is to establish the identity, location, and mechanism of action of various opioids (opioid peptides and alkaloids) and multiple opioid receptors in the modulation of nociceptive processes. 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 delta- and, in particular, chi-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 antinociceptive processes appear to be most important in this region, but delta-opioid receptors may also be involved. In addition, a role of chi-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. The identification of opioid receptors and their endogenous ligands, the opioid peptides, marked the beginning of a new era in pain research. The differentiation of several types of opioid receptors and the subsequent characterization of a series of opioid peptides illustrate the striking complexity of opioid systems. The implications of this multiplicity for neurobiology in general and for the understanding of pain mechanisms in particular are presently not fully understood. In this presentation some aspects of opioidergic pain control at various levels of the neuraxis will be discussed.