Early-phase of learning enhances communication between brain hemispheres

Effect of Associative Learning on Memory Spine Formation in Mouse Barrel Cortex

Associative fear learning, in which stimulation of whiskers is paired with mild electric shock to the tail, modifies the barrel cortex, the functional representation of sensory receptors involved in the conditioning, by inducing formation of new inhibitory synapses on single-synapse spines of the cognate barrel hollows and thus producing double-synapse spines. In the barrel cortex of conditioned, pseudoconditioned, and untreated mice, we analyzed the number and morphological features of dendritic spines at various maturation and stability levels: sER-free spines, spines containing smooth endoplasmic reticulum (sER), and spines containing spine apparatus. Using stereological analysis of serial sections examined by transmission electron microscopy, we found that the density of double-synapse spines containing spine apparatus was significantly increased in the conditioned mice. Learning also induced enhancement of the postsynaptic density area of inhibitory synapses as well as increase in the number of polyribosomes in such spines. In single-synapse spines, the effects of conditioning were less pronounced and included increase in the number of polyribosomes in sER-free spines. The results suggest that fear learning differentially affects single- and double-synapse spines in the barrel cortex: it promotes maturation and stabilization of double-synapse spines, which might possibly contribute to permanent memory formation, and upregulates protein synthesis in single-synapse spines.

Correlated activation of the thalamocortical network in a simple learning paradigm

The thalamocortical loop is a key player in sensory processing. We examined the functional interactions among its elements, expressed as cross-correlations between metabolic activity of the barrel cortex, somatosensory thalamic nuclei and posterior parietal cortex, in classical conditioning. In the training stimulation of vibrissae in mice was paired with a tail shock. [14C]-2-Deoxyglucose brain mapping was performed during the first and the final sessions of conditioning (conditioned stimulus+unconditioned stimulus; CS+UCS), in groups that received only the stimulation of vibrissae (conditioned stimulus; CS-only) and in nonstimulated controls (NS). In the CS-only group, the CS evoked the correlated activity of the examined structures during the first session, but in the third session these structures did not act in a correlated manner. Conversely, in the CS+UCS condition correlations among the thalamocortical loop structures activities became stronger during the course of the training. Particularly, the posterior parietal cortex, which controls voluntary deployment of attention, together with the barrel cortex becomes involved in the network of structures with the correlated activity. The results suggest a predominant role for bottom-up processing in the somatosensory pathway at the beginning of conditioning followed by top-down processing. This is consistent with the idea that the thalamocortical loop plays a crucial role in attentional processes.

Dynamic changes of rodent somatosensory barrel cortex are correlated with learning a novel conditioned stimulus

The rodent somatosensory barrel cortex (S1bf) has proved a valuable model for studying neural plasticity in vivo. It has been observed that sensory deprivation or conditioning reorganizes sensory-driven activity within S1bf. These observations suggest a role for S1bf in somatosensory learning. This study evaluated the hypothesis that the response properties of extracellularly recorded neurons in S1bf would change as subjects learned to respond to stimulation of S1bf. Intracortical microstimulation (ICMS) of S1bf was used as a means for bypassing feedforward drive from the sensory periphery, midbrain, and thalamus while exciting local cortical networks. To separate the learning of this conditioned stimulus-conditioned response (CS-CR) from other elements of the task, we employed a cross-modal transfer schedule. Long-Evans rats were initially trained to respond to an auditory stimulus. All subjects were then implanted in both S1bfs with chronic microwire arrays for recording neural activity and delivering ICMS. Next, this association was transferred to ICMS of one hemisphere's S1bf. S1bf responded to ICMS with a brief increase in firing rate followed by a longer reduction in activity. We observed that the duration of reduced activity elicited by ICMS increased as the subjects began to respond correctly more often than expected by chance, and the magnitude of the initial positive response increased as they consolidated this CS-CR. Subsequent ICMS of the opposite S1bf revealed that this CS-CR did not generalize across hemispheres. These results suggest that a mechanism involving a single hemisphere's S1bf tunes cortical responses in concert with changes in rodent behavior during somatosensory learning.

Bilateral plasticity of Vibrissae SII representation induced by classical conditioning in mice

The somatosensory cortex in mice contains primary (SI) and secondary (SII) areas, differing in somatotopic precision, topographic organization, and function. The role of SII in somatosensory processing is still poorly understood. SII is activated bilaterally during attentional tasks and is considered to play a role in tactile memory and sensorimotor integration. We measured the plasticity of SII activation after associative learning based on classical conditioning, in which unilateral stimulation of one row of vibrissae was paired with a tail shock. The training consisted of three daily 10 min sessions, during which 40 pairings were delivered. Cortical activation driven by stimulation of vibrissae was mapped with 2-[(14)C]deoxyglucose (2DG) autoradiography 1 d after the end of conditioning. We reported previously that the conditioning procedure resulted in unilateral enlargement of 2DG-labeled cortical representation of the "trained" row of vibrissae in SI. Here, we measured the width and intensity of the labeled region in SII. We found that both measured parameters in SII increased bilaterally. The increase was observed in cortical layers II/III and IV. Apparently, plasticity in SII is not a simple reflection of changes in SI. It may be attributable to bilateral integrative role of SII, its lesser topographical specificity, and strong involvement in attentional processing.

Correlations between neuron activity in the sensorimotor cortex of the right and left hemispheres in rabbits during a defensive dominant and "animal hypnosis"

A latent focus of excitation with a rhythmic nature (a defensive dominant focus) was created in the CNS of rabbits. The focus was formed by threshold electrocutaneous stimulation of the left forelimb using series of impulses consisting of 15-20 stimuli with interstimulus intervals of 2 sec. The linked activity of cells in the sensorimotor cortex of the right and left hemispheres was analyzed. When cross-correlation histograms of the spike activity of sensorimotor cortex neurons in the left hemisphere were constructed and analyzed in relation to spikes of high and intermediate amplitude recorded in the right hemisphere, the linked activity of 15% and 23% of neuron pairs, respectively, showed predominance of a rhythm equal or close to the stimulation rhythm used to form the dominant focus. When the appearance times of spikes from neurons in the sensorimotor cortex of the right hemisphere were analyzed in relation to spikes of high and intermediate amplitude recorded in the cortex of the left hemisphere, predominance of 2-sec rhythms was seen in the linked activity of only 3% and 10% of neuron pairs, respectively. After induction of "animal hypnosis," differences between the hemispheres in relation to this measure leveled out.

Alpha calcium/calmodulin dependent protein kinase II in learning-dependent plasticity of mouse somatosensory cortex

Calcium/calmodulin dependent protein kinase II (CaMKII), and more specifically its alpha subunit, is widely believed to be fundamental for hippocampal synaptic plasticity. In the cerebral cortex, deprivation-evoked plasticity was shown to depend on alphaCaMKII autophosphorylation abilities. Here we analyzed how learning-induced functional reorganization of cortical representations affected alphaCaMKII in adult Swiss mice. Mice were subjected to short-lasting sensory training in which stimulation of whiskers was paired with tail shock. The pairing results in enlargement of functional representation of vibrissae activated during the training. alphaCaMKII protein and its autophosphorylation level were determined by Western-blotting in somatosensory cortex crude synaptosomal fraction (P2) and postsynaptic protein-enriched, Triton X-100 insoluble fraction (TIF). The first training session resulted in an increase in alphaCaMKII autophosphorylation at autonomy site observed in TIF. A similar increase was also observed after the first session of just whiskers stimulation, which alone does not induce rearrangement of cortical representations. These data indicate that increased autophosphorylation of postsynaptic alphaCaMKII is not a correlate of induction phase of plasticity related reorganization of cortical representation of vibrissae. The increase observed in both experimental groups was transient and did not persist in the maintenance phase of the plastic change. Furthermore, we found that the training caused a delayed upregulation of alphaCaMKII protein level in crude synaptosomal fraction, but not in TIF, and the upregulation was not accompanied by an increase in autophosphorylation level of the kinase. The result indicates alphaCaMKII involvement in the late phase of plastic change and suggests the participation of a presynaptic pool of kinase rather than postsynaptic at this point.

Sensory learning-induced enhancement of inhibitory synaptic transmission in the barrel cortex of the mouse

In adult mice, repetitive pairing of stimulation of mystacial vibrissae with an electrical shock to the tail induces expansion of the cortical representation of stimulated vibrissae accompanied by elevation of the GABAergic markers. Here, we show that this associative learning paradigm results in a selective increase in the frequency of spontaneous inhibitory postsynaptic currents in layer IV excitatory neurons located within the barrel representing stimulated vibrissae, evident 24 h after the end of training. The mean amplitude of spontaneous inhibitory postsynaptic potentials recorded from excitatory neurons was unchanged. Recordings from layer IV excitatory and fast spiking neurons showed that the training induced changes neither in the mean frequency nor it the mean amplitude of spontaneous excitatory postsynaptic currents. On the other hand, the mean amplitude of field potentials evoked by the stimulation of layer VI and recorded in layer IV was significantly reduced. These data indicate that aversive training results in a selective and long-lasting enhancement of GABAergic transmission within the cortical representation of stimulated vibrissae, which may result in a decrease in layer VI-evoked field responses.