INS seminars 56

Abstract:

Patients with damage to the hippocampal system have great difficulty forming new long-term declarative memories, but still recall their remote memories, while neocortical damage impairs remote memories. This paper presents direct evidence of the establishment of artificial visuoauditory memory traces in the rat auditory cortex and the participation of the entorhinal cortex in the establishment and retrieval of these memory traces. We produced an association between an artificial cortical activation and a visual stimulus with classical conditioning. The memory traces were physiologically visualized from auditory neuronal responses to the visual stimulus after conditioning and behaviorally confirmed with a memory recall experiment. Formation of a new artificial visuoauditory memory in the auditory cortex with classical conditioning was bilaterally blocked when the entorhinal cortex was unilaterally temporarily inactivated, but easily achieved if the entorhinal cortex was not inactivated. We found that cortical projection neurons in the perirhinal and entorhinal cortices were cholecystokinin (CCK) immunopositive. CCK application in the auditory cortex of anesthetized rats enabled the cortical neurons to respond to a previously ineffective tone stimulus, after the tone had been paired with the best-frequency tone stimulus. It also enabled the cortical neurons to respond to a light stimulus after the stimulus had been paired with a strong noise-burst stimulus. Further, in-vivo intracellular recordings in the auditory cortex showed that synaptic strength was potentiated after 2-trial pairing of presynaptic and postsynaptic coactivities in the presence of CCK. An application of a CCK receptor antagonist in the auditory cortex prevented the formation of the visuoauditory associative memory by classical conditioning in behaving rats, similarly to the entorhinal cortex inactivation experiment. We conclude that the hippocampal system exerts its influence on cortical neuroplasticity through the action of CCK. Similar to the WRITE-ENABLED switch for memory writing in a computer, CCK switches the memory writing in the neocortex.