The 1988 Merck Frosst Award. The role of ascending and descending noradrenergic and serotonergic pathways in opioid and non-opioid antinociception as revealed by lesion studies

Both ascending and descending noradrenergic and serotonergic pathways have been implicated in mechanisms of antinociception produced by systemic administration of morphine and the non-opioid drugs, baclofen and clonidine. These agents affect the turnover and release of noradrenaline and 5-hydroxytryptamine in various brain regions and the spinal cord, and alter neuronal activity in regions from which ascending and descending aminergic pathways originate. The role of specific pathways in morphine analgesia has been examined by applying electrolytic lesions to discrete brain regions. However, this technique is limited because lesions are nonselective for a particular neuronal population. More recent studies have used microinjection of the neurotoxins 6-hydroxydopamine and 5,7-dihydroxytryptamine to lesion specific noradrenergic and serotonergic pathways, respectively. Although more selective, this approach may be limited by the development of receptor supersensitivity or other mechanisms of compensation, as certain changes seen soon after microinjection (days) are no longer apparent at later intervals (weeks). Systemic drug administration reveals drug actions at predominant but not clearly identified sites of action. The role of a particular aminergic pathway can be revealed most clearly by combining microinjection of drugs into discrete brain sites with neurotoxin-induced lesions, and examining the effects of such lesions at a range of time intervals. A differential role of a particular pathway may become apparent following systemic or intracerebral administration.