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The Journal of Neuroscience, December 1, 1999, 19(23):10520-10529
Opiate States of Memory: Receptor Mechanisms
L. A. Bruins
Slot and
F. C.
Colpaert
Centre de Recherche Pierre Fabre, F 81106 Castres Cedex, France
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 µ 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.
Key words:
memory; opiates; learning; retrieval; encoding; mnesic
state
Copyright © 1999 Society for Neuroscience 0270-6474/99/192310520-10$05.00/0
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