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Opiate states of memory: receptor mechanisms
L A Bruins Slot1, F C Colpaert
1Centre de Recherche Pierre Fabre, F 81106 Castres Cedex, France.
Abstract:
The present studies characterized the receptor mechanisms of morphine-induced states of memory. Morphine (5 mg/kg) produced a state in which rats could learn and retrieve an operant response; retrieval was impaired, however, when the rats were tested in the normal state. Conversely, rats that were trained in the normal state failed to retrieve the response in the morphine state. In either case the mnesic state was dose dependent, commencing at morphine doses as low as 0.8 mg/kg. In rats trained with 5 mg/kg of morphine, retrieval was fully adequate when tested with this same dose but not when tested with either lower or higher doses. Naloxone, but not naltrindole, antagonized the morphine-induced state; heroin and (-)-cyclazocine, but not U50,488H, (+)-cyclazocine and SNC80, produced a state in which retrieval occurred at least partially. Time-effect studies in which injections were made from 0 to 240 min before the sessions indicated that the retrieval in saline-to-morphine and morphine-to-saline conditions occurred along different time courses; a theory of opiate signal transduction suggests that these temporal profiles result from morphine producing two bi-directional mnesic states that may differ as much as the analgesia and hyperalgesia that morphine also induces. It appears that a particular magnitude of mu opiate receptor activation produces a state to which a memory trace can be confined in a highly selective manner. The normal and this particular morphine state are only some of the many mutually inaccessible and molecularly definable states of memory that are likely to exist, thus challenging the unitary concept of an individual organism's memory.
Insights
Morphine creates distinct memory states, affecting learning and retrieval differently depending on the drug
Area of Science:
- Neuroscience
- Pharmacology
- Memory Research
Background:
- Opioid drugs like morphine are known to affect cognitive functions, including memory.
- The precise mechanisms by which opioids influence memory formation and retrieval remain incompletely understood.
- Previous research suggests that drug-induced states may interact with memory processes.
Purpose of the Study:
- To investigate the receptor mechanisms underlying morphine-induced memory states.
- To determine if learning and retrieval are state-dependent under morphine influence.
- To explore the dose-dependency and receptor specificity of these morphine-induced memory effects.
Main Methods:
- Rats were trained and tested on an operant response task under different conditions: normal state, morphine state, and state transitions.
- Various doses of morphine and other opioid receptor ligands (naloxone, naltrindole, heroin, cyclazocine, SNC80) were administered.
- Time-course studies were conducted to examine the temporal dynamics of morphine's effects on memory retrieval.
Main Results:
- Morphine (5 mg/kg) induced a specific memory state where learning and retrieval occurred, but retrieval failed if tested in the normal state, and vice versa.
- These morphine-induced memory effects were dose-dependent, observable as low as 0.8 mg/kg.
- Naloxone antagonized the morphine state, while heroin and (-)-cyclazocine partially induced a retrieval state. Specific receptor activation magnitude appeared critical for memory confinement.
Conclusions:
- Morphine can induce unique, state-dependent memory engrams that are mutually inaccessible.
- These findings challenge the traditional unitary concept of memory, suggesting the existence of multiple, molecularly defined memory states.
- Opiate receptor activation magnitude is crucial for the selective confinement of memory traces within specific states.