Associative Memory Formation Increases the Observation of Dendritic Spines in the Hippocampus

TLDR

Dendritic spines, the sites of synaptic contact, can be modified by experience and are thought to contribute to memory formation. The study examined whether acquiring new memories alters dendritic spine density. Adult male rats underwent trace eyeblink conditioning, with unpaired and naive controls, and spine density was assessed 24 h later using Golgi staining. Trace conditioning increased spine density on basal dendrites of CA1 pyramidal cells, an effect blocked by an NMDA antagonist and also seen with delay conditioning, indicating that associative memory formation elevates dendritic spine availability specifically in hippocampal basal dendrites.

Abstract

Dendritic spines are sources of synaptic contact that can be altered by experience and, as such, may be involved in memories for that experience. Here we tested whether the acquisition of new memories is associated with changes in the density of dendritic spines. Adult male rats were trained using the trace eyeblink conditioning paradigm, an associative learning task that requires the hippocampus for acquisition. Additional groups were exposed to the same number of stimuli presented in an explicitly unpaired manner or were naive. Twenty-four hours later, the density of dendritic spines was measured using Golgi impregnation. Trace conditioning was associated with an increase in the density of dendritic spines on the pyramidal cells of area CA1 of the hippocampus, an effect that was prevented by blocking acquisition of the learned response with a competitive NMDA receptor antagonist. Training with delay conditioning, a similar task that does not require the hippocampus, also produced an increase in spine density. The learning-induced increase in dendritic spine density was specific to basal dendrites of pyramidal cells in the CA1 region of the hippocampus. Changes did not occur on their apical dendrites or on cells in the dentate gyrus or somatosensory cortex. These results suggest that the formation and expression of associative memories increase the availability of dendritic spines and the potential for synaptic contact.