Brain endorphins: possible role in long-term memory

Relationship between morphine and etonitazene-induced working memory impairment and analgesia

An 8-arm radial maze task was used to assess the possible role of the opiate system in the spatial memory of the rat. Increasing doses of etonitazene (0.005-0.06 mg/kg i.p.) and morphine (2.5-100 mg/kg i.p.) significantly impaired performance in the working memory components of the task. For both drugs this impairment was linearly related to the log of the administered dose, and the log-dose relationships were parallel. The regression lines calculated for each parameter for both drugs were parallel thus allowing us to calculate the potency: etonitazene proved to about 1000 times more potent than morphine in terms of correct arm entries, the number of errors and the total time taken to complete the task. Moreover, the progressive cognitive impairment produced by both opiates was closely related to an increase in analgesic effect. Pretreatment with naloxone (5 mg/kg i.p.) completely antagonised the disruptive effect of the opiates on working memory. The importance of the mu subtype opiate receptor in cognitive processes is discussed.

Immunocytochemical localization of enkephalin in the cat visual cortex

The localization of enkephalin-immunoreactivity in the cat visual cortex (area 17) was analyzed by using immunohistochemical methods with a monoclonal antibody directed against enkephalin. The majority of the immunoreactive product was localized in neuronal processes. The density of immunopositive fibers was greatest in layer VI, with moderate staining in layers I, II, III and V, and the least dense staining in layer IV. Layer IVab neurons showed a striking concentration of immunopositive puncta around their cell bodies. Immunopositive neurons were scarcely present in the visual cortex. They were found in all cortical layers, but mostly in layer VI. The immunopositive neurons were non-pyramidal, mostly multipolar in shape and occasionally bipolar. The results provide anatomical evidence that enkephalin may have modulatory effects on visual cortical neurons.

Neural plasticity in vivo: opioid sensitivity of memory develops gradually after a septal lesion

Neuronal plasticity can manifest itself in alterations in the sensitivity of memory to the effects of drugs. After the production of a brain lesion, the memory processing of a passive-avoidance task in mice gradually becomes sensitive to the effect of morphine, i.e., an improvement in retention performance is seen after 6 weeks, but not after 1 or 2 weeks. The results presented demonstrate that, even if they lead to no discernible changes in behaviour, plastic processes can still be detected by means of behavioural tests.