Abstract
Associative fear learning, resulting from whisker stimulation paired with application of a mild electric shock to the tail in a classical conditioning paradigm, changes the motor behavior of mice and modifies the cortical functional representation of sensory receptors involved in the conditioning. It also induces the formation of new inhibitory synapses on double-synapse spines of the cognate barrel hollows. We studied density and distribution of polyribosomes, the putative structural markers of enhanced synaptic activation, following conditioning. By analyzing serial sections of the barrel cortex by electron microscopy and stereology, we found that the density of polyribosomes was significantly increased in dendrites of the barrel activated during conditioning. The results revealed fear learning-induced increase in the density of polyribosomes associated with both excitatory and inhibitory synapses located on dendritic spines (in both single- and double-synapse spines) and only with the inhibitory synapses located on dendritic shafts. This effect was accompanied by a significant increase in the postsynaptic density area of the excitatory synapses on single-synapse spines and of the inhibitory synapses on double-synapse spines containing polyribosomes. The present results show that associative fear learning not only induces inhibitory synaptogenesis, as demonstrated in the previous studies, but also stimulates local protein synthesis and produces modifications of the synapses that indicate their potentiation.
Highlights
Sensory experience and learning can induce changes in the connections between neurons
This part of somatosensory cortex offers an excellent model to study the mechanisms of learning and memory because of its clearly defined structure, and the ease of inducing plasticity via a fear learning paradigm [15,16]. 2-deoxyglucose autoradiography revealed enlargement of functional cortical representation of the whiskers stimulated during conditioning [16]
We identified the synapses in B2 barrel of mouse somatosensory cortex and divided them according to their location into synapses on dendritic shafts and on dendritic spines
Summary
Sensory experience and learning can induce changes in the connections between neurons. Shortlasting classical conditioning, in which stimulation of a row of whiskers is paired with the administration of a mild electric shock to the tail, produces plastic modification in layer IV of the barrel cortex This part of somatosensory cortex offers an excellent model to study the mechanisms of learning and memory because of its clearly defined structure, and the ease of inducing plasticity via a fear learning paradigm [15,16]. The learninginduced plastic change of cortical representation is associated with a number of alterations in the inhibitory neurotransmission It is paralleled by a rapid increase of GAD67 mRNA, and increased density of GAD and GABA immunoreactive cells in the barrels of the row receiving input from the stimulated whiskers [18,19]. Increase in mRNA and protein of NMDA2A subunit was found in the barrels receiving the conditioned stimulus [22]
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