Actions of enkephalin, mu and partial agonist analgesics on acetylcholine turnover in rat brain

The actions of μ agonists, partial agonists and enkephalin analogues on the turnover rate of acetylcholine (TR_Ach in several brain areas have been studied. The μ agonists and enkephalin analogues all suppressed TR_Ach in the parietal cortex and hippocampus but not in the striatum or frontal cortex. The partial agonists (κ agonists, μ antagonists), however, were unique in that they did not alter hippocampal TR_ACh, suggesting that the septal hippocampal cholinergic neurons do not possess κ receptors and/or are not regulated by neurons bearing κ receptors. These data along with those of Yaksh and Rudy (Pain4: 299–359, 1978) indicate that κ receptor mediated analgesia may involve different neural substrates from those of μ agonists and supports the concept of multiple opiate receptors within the CNS.