Associative Memory Extinction Is Accompanied by Decayed Plasticity at Motor Cortical Neurons and Persistent Plasticity at Sensory Cortical Neurons

Neuroligin-3-Mediated Synapse Formation Strengthens Interactions between Hippocampus and Barrel Cortex in Associative Memory

Memory traces are believed to be broadly allocated in cerebral cortices and the hippocampus. Mutual synapse innervations among these brain areas are presumably formed in associative memory. In the present study, we have used neuronal tracing by pAAV-carried fluorescent proteins and neuroligin-3 mRNA knockdown by shRNAs to examine the role of neuroligin-3-mediated synapse formation in the interconnection between primary associative memory cells in the sensory cortices and secondary associative memory cells in the hippocampus during the acquisition and memory of associated signals. Our studies show that mutual synapse innervations between the barrel cortex and the hippocampal CA3 region emerge and are upregulated after the memories of associated whisker and odor signals come into view. These synapse interconnections are downregulated by a knockdown of neuroligin-3-mediated synapse linkages. New synapse interconnections and the strengthening of these interconnections appear to endorse the belief in an interaction between the hippocampus and sensory cortices for memory consolidation.

The Framework and Memristive Circuit Design for Multisensory Mutual Associative Memory Networks

In this work, we propose a multisensory mutual associative memory networks framework and memristive circuit to mimic the ability of the biological brain to make associations of information received simultaneously. The circuit inspired by neural mechanisms of associative memory cells mainly consists of three modules: 1) the storage neurons module, which encodes external multimodal information into the firing rate of spikes; 2) the synapse module, which uses the nonvolatility memristor to achieve weight adjustment and associative learning; and 3) the retrieval neuron module, which feeds the retrieval signal output from each sensory pathway to other sensory pathways, so that achieve mutual association and retrieval between multiple modalities. Different from other one-to-one or many-to-one unidirectional associative memory work, this circuit achieves bidirectional association from multiple modalities to multiple modalities. In addition, we simulate the acquisition, extinction, recovery, transmission, and consolidation properties of associative memory. The circuit is applied to cross-modal association of image and audio recognition results, and episodic memory is simulated, where multiple images in a scene are intramodal associated. With power and area analysis, the circuit is validated as hardware-friendly. Further research to extend this work into large-scale associative memory networks, combined with visual-auditory-tactile-gustatory sensory sensors, is promising for application in intelligent robotic platforms to facilitate the development of neuromorphic systems and brain-like intelligence.

The interconnection and function of associative memory neurons are upregulated for memory strengthening

Memories associated to signals have been proven to rely on the recruitment of associative memory neurons that are featured by mutual synapse innervations among cross-modal cortices. Whether the consolidation of associative memory is endorsed by the upregulation of associative memory neurons in an intramodal cortex remains to be examined. The function and interconnection of associative memory neurons were investigated by in vivo electrophysiology and adeno-associated virus-mediated neural tracing in those mice that experienced associative learning by pairing the whisker tactile signal and the olfactory signal. Our results show that odorant-induced whisker motion as a type of associative memory is coupled with the enhancement of whisking-induced whisker motion. In addition to some barrel cortical neurons encoding both whisker and olfactory signals, i.e., their recruitment as associative memory neurons, the synapse interconnection and spike-encoding capacity of associative memory neurons within the barrel cortex are upregulated. These upregulated alternations were partially observed in the activity-induced sensitization. In summary, associative memory is mechanistically based on the recruitment of associative memory neurons and the upregulation of their interactions in intramodal cortices.

Lateral Fluid Percussion Injury Causes Sex-Specific Deficits in Anterograde but Not Retrograde Memory

Cognitive impairment is a common symptom after traumatic brain injury (TBI). Memory, in particular, is often disrupted during chronic post-injury recovery. To understand the sex-specific effects of brain injury on retrograde and anterograde memory, we examined paired associate learning (PAL), spatial learning and memory, and fear memory after lateral fluid percussion TBI. We hypothesized that male and female mice would display unique memory deficits after TBI. PAL task acquisition was initiated via touchscreen operant conditioning 22 weeks before sham injury or TBI. Post-injury PAL testing occurred 7 weeks post-injury. Barnes maze and fear conditioning were completed at 14- and 15-weeks post-injury, respectively. Contrary to our expectations, behavioral outcomes were not primarily influenced by TBI. Instead, sex-specific differences were observed in all tasks which exposed task-specific trends in male TBI mice. Male mice took longer to complete the PAL task, but this was not affected by TBI and did not compromise the ability to make a correct choice. Latency to reach the goal box decreased across testing days in Barnes maze, but male TBI mice lagged in improvement compared to all other groups. Use of two learning indices revealed that male TBI mice were deficient in transferring information from 1 day to the next. Finally, acquisition and contextual retention of fear memory were similar between all groups. Cued retention of the tone-shock pairing was influenced by both injury and sex. Male sham mice displayed the strongest cued retention of fear memory, evidenced by increased freezing behavior across the test trial. In contrast, male TBI mice displayed reduced freezing behavior with repetitive tone exposure. An inverse relationship in freezing behavior to tone exposure was detected between female sham and TBI mice, although the difference was not as striking. Together, these studies show that retrograde memory is intact after lateral TBI. However, male mice are more vulnerable to post-injury anterograde memory deficits. These behaviors were not associated with gross pathological change near the site injury or in subcortical brain regions associated with memory formation. Future studies that incorporate pre- and post-injury behavioral analysis will be integral in defining sex-specific memory impairment after TBI.

[Fluoxetine enhances combined learning and memory abilities of mice by promoting neural cell functions in the barrel cortex]

OBJECTIVE

To study the behavioral characteristics of memory maintenance and regression in a mouse model of combined learning and memory training with fluoxetine treatment and explore the neural basis for learning and memory in the barrel area of the brain.

METHODS

Twenty-six 16-day-old C57 mice were randomized into two equal groups and were given daily intraperitoneal injection of saline (control) or fluoxetine. The mice were subjected to stimulation of the right whiskers using a multi-sensory stimulation simulator and were given simultaneously olfactory stimulation with butyl acetate. In the initial 10 days of the experiment, the mice were given corresponding drug treatment followed by whisker and olfactory stimulations on a daily basis; from day 11 to day 17, only the drugs were administered without the stimulations; on day 18, both the drugs and stimulations were administered. The daily performance of the mice was recorded and analyzed. In the field potential experiment, the left barrel cortex of the mouse brain was selected to record the frequency of field potential signals in response to whisker stimulation.

RESULTS

In the behavioral test, the mice treated with fluoxetine showed greater increments of the frequency and angle of whisker deflection than the control mice (P < 0.01). Compared with the peak levels that occurred on the 10th day, the swing angle and frequency of the whisker deflection decreased on the 17th day decreased in both groups, and the reduction was more obvious in the control group (P < 0.05). During the training on the 18th day, the whisker movement of the mice increased rapidly to the peak level and showed significant differences between the two groups (P < 0.05). In the field potential experiment on the 10th and 17th day, the frequencies of field potential signal in response to whisker stimulation was significantly higher in fluoxetine group than in the control group (P < 0.05).

CONCLUSIONS

Combined training of the mice results in the formation of combined memory. Fluoxetine can enhance combined learning and memory abilities and prolong such memories in mice by promoting the function of the barrel cortex cells.

Coactivations of barrel and piriform cortices induce their mutual synapse innervations and recruit associative memory cells

After associative learning, a signal induces the recall of its associated signal, or the other way around. This reciprocal retrieval of associated signals is essential for associative thinking and logical reasoning. For the cellular mechanism underlying this associative memory, we hypothesized that the formation of synapse innervations among coactivated sensory cortices and the recruitment of associative memory cells were involved in the integrative storage and reciprocal retrieval of associated signals. Our study indicated that the paired whisker and olfaction stimulations led to an odorant-induced whisker motion and a whisker-induced olfaction response, a reciprocal form of associative memory retrieval. In mice that showed the reciprocal retrieval of associated signals, their barrel and piriform cortical neurons became mutually innervated through their axon projection and new synapse formation. These piriform and barrel cortical neurons gained the ability to encode both whisker and olfaction signals based on synapse innervations from the innate input and the newly formed input. Therefore, the associated activation of sensory cortices by pairing input signals initiates their mutual synapse innervations, and the neurons innervated by new and innate synapses are recruited to be associative memory cells that encode these associated signals. Mutual synapse innervations among sensory cortices to recruit associative memory cells may compose the primary foundation for the integrative storage and reciprocal retrieval of associated signals. Our study also reveals that new synapses onto the neurons enable these neurons to encode memories to new specific signals.

microRNA and mRNA profiles in the amygdala are relevant to fear memory induced by physical or psychological stress

Anxiety is presumably driven by fear memory. Molecular profiles in the amygdala of mice with fear memory induced by psychological and physical stresses remain to be elucidated. Fear memory in mice was induced by a paradigm of social defeat. Physical and psychological stresses (PPS) to an intruder were given by attacks from an aggressive resident. Psychological stress (PS) to an observer was given by the witnessing of aggressor attacks. Amygdala tissues from these mice showing fear memory and anxiety vs. tissues from control mice were harvested to analyze mRNA and microRNA profiles by high-throughput sequencing. In the amygdala of intruders and observers with fear memory, the genes encoding 5-HTR1b, 5-HTR2a, DAR2, AChRM3, and IP3R1 are upregulated, whereas genes encoding GPγ11, GPγ13, GPγT2, RasC3, and P450 are downregulated, indicating that these molecules are involved in fear memory induced by physical/psychological stresses. In the comparison of intruders with observers, the upregulation of genes encoding 5-HTR6, GPγ8, P2R7, NFκ2, CREB3/1, and Itgα9 as well as the downregulation of genes encoding DAR5, 5-HTR1a, and HSP1a are involved in fear memory induced by physical stress. The upregulation of genes encoding DAR1, 5-HTR5a and SSR2/3 as well as the downregulation of AdRα1, CREB3/1, GPγ13 and GPγ8 are involved in fear memory induced by psychological stress. Results obtained by sequencing mRNA and microRNA profiles are consistent with results of quantitative RT-PCR analysis and dual-luciferase reporter assays performed for validation. In conclusion, fear memories and anxiety induced by PPS vs. PS are caused by the imbalanced regulation of different synapses and signaling pathways in the amygdala.NEW & NOTEWORTHY The current study identifies the molecular mechanism underlying fear memory and anxiety induced by psychological stress vs. physical stress, in which the imbalanced expression of microRNA-regulated mRNAs relevant to dopaminergic, adrenergic, and serotonergic synapses in the amygdala plays an important role. This result reveals different molecular profiles for psychological and physical stresses.

Searching basic units in memory traces: associative memory cells

The acquisition of associated signals is commonly seen in life. The integrative storage of these exogenous and endogenous signals is essential for cognition, emotion and behaviors. In terms of basic units of memory traces or engrams, associative memory cells are recruited in the brain during learning, cognition and emotional reactions. The recruitment and refinement of associative memory cells facilitate the retrieval of memory-relevant events and the learning of reorganized unitary signals that have been acquired. The recruitment of associative memory cells is fulfilled by generating mutual synapse innervations among them in coactivated brain regions. Their axons innervate downstream neurons convergently and divergently to recruit secondary associative memory cells. Mutual synapse innervations among associative memory cells confer the integrative storage and reciprocal retrieval of associated signals. Their convergent synapse innervations to secondary associative memory cells endorse integrative cognition. Their divergent innervations to secondary associative memory cells grant multiple applications of associated signals. Associative memory cells in memory traces are defined to be nerve cells that are able to encode multiple learned signals and receive synapse innervations carrying these signals. An impairment in the recruitment and refinement of associative memory cells will lead to the memory deficit associated with neurological diseases and psychological disorders. This review presents a comprehensive diagram for the recruitment and refinement of associative memory cells for memory-relevant events in a lifetime.

A Randomized and Controlled Crossover Study Investigating the Improvement of Walking and Posture Functions in Chronic Stroke Patients Using HAL Exoskeleton - The HALESTRO Study (HAL-Exoskeleton STROke Study)

Background: The exoskeleton HAL (hybrid assistive limb) has proven to improve walking functions in spinal cord injury and chronic stroke patients when using it for body-weight supported treadmill training (BWSTT). Compared with other robotic devices, it offers the possibility to initiate movements actively. Previous studies on stroke patients did not compare HAL-BWSTT with conventional physiotherapy (CPT). Therefore, we performed a crossover clinical trial comparing CPT and HAL-BWSTT in chronic stroke patients with hemiparesis, the HALESTRO study. Our hypothesis was that HAL-training would have greater effects on walking and posture functions compared to a mixed-approach CPT.

Methods: A total of 18 chronic stroke patients participated in this study. Treatment consisted of 30 CPT sessions and of 30 sessions of BWSTT with a double leg type HAL exoskeleton successively in a randomized, crossover study design. Primary outcome parameters were walking time and speed in 10-meter walk test (10MWT), time in timed-up-and-go test (TUG) and distance in 6-min walk test (6MWT). Secondary outcome parameters were the functional ambulatory categories (FAC) and the Berg-Balance Scale (BBS). Data were assessed at baseline, at crossover and at the end of the study, all without using and wearing HAL.

Results: Our study demonstrate neither a significant difference in walking parameters nor in functional and balance parameters. When HAL-BWSTT was applied to naïve patients, it led to an improvement in walking parameters and in balance abilities. Pooling all data, we could show a significant effect in 10MWT, 6MWT, FAC and BBS, both therapies sequentially applied over 12 weeks. Thereby, FAC improve from dependent to independent category (3 to 4). One patient dropped out of the study due to intensive fatigue after each training session.

Conclusion: HAL-BWSTT and mixed-approach CPT were effective therapies in chronic stroke patients. However, compared with CPT, HAL training with 30 sessions over 6 weeks was not more effective. The combination of both therapies led to an improvement of walking and balance functions. Robotic rehabilitation of walking disorders alone still lacks the proof of superiority in chronic stroke. Robotic treatment therapies and classical CPT rehabilitation concepts should be applied in an individualized therapy program.

Cell-specific plasticity associated with integrative memory of triple sensory signals in the barrel cortex

Neuronal plasticity occurs in associative memory. Associative memory cells are recruited for the integration and storage of associated signals. The coordinated refinements and interactions of associative memory cells including glutamatergic and GABAergic neurons remain elusive, which we have examined in a mouse model of associative learning. Paired olfaction, tail and whisker stimulations lead to odorant-induced and tail-induced whisker motions alongside whisker-induced whisker motion. In mice that show this cross-modal associative memory, barrel cortical glutamatergic and GABAergic neurons are recruited to encode the newly learned odor and tail signals alongside the innate whisker signal. These glutamatergic neurons are functionally upregulated, and GABAergic neurons are refined in a homeostatic manner. The mutual innervations between these glutamatergic and GABAergic neurons are upregulated. Therefore, the co-activations of sensory cortices by pairing the input signals recruit their glutamatergic and GABAergic neurons to be associative memory cells, which undergo coordinated refinement among glutamatergic and GABAergic neurons as well as homeostatic plasticity among subcellular compartments in order to drive these cells toward the optimal state for the integrative storage of associated signals.

Barrel Cortical Neuron Integrates Triple Associated Signals for Their Memory Through Receiving Epigenetic-Mediated New Synapse Innervations

Associative learning is common way for information acquisition. Associative memory is essential to logical reasoning and associative thinking. The storages of multiple associated signals in individual neurons facilitate their integration, expand memory volume, and strengthen cognition ability. Associative memory cells that encode multiple signals have been reported, however, the mechanisms underlying their recruitment and working principle remain to be addressed. We have examined the recruitment of associative memory cells that integrate and store triple sensory signals as well as the potential mechanism of their recruitment. Paired mouse whisker, olfaction, and tail stimulations lead to odorant-induced motion and tail-induced whisker motion. In mice of expressing this cross-modal response, their barrel cortical neurons become to encode odor and tail signals alongside whisker signal. These barrel cortical neurons receive new synapse innervations from piriform and S1-tail cortical neurons. The emergence of cross-modal responses as well as the recruitments of new synapse innervations and associative memory cells in the barrel cortex need miRNA-324 and miRNA-133a, which downregulate Ttbk1 and Tet3. The co-activations of sensory cortices recruit their mutual synapse innervations and associative memory cells that integrate and store multiple associated signals through epigenetic-mediated process.

Associative memory cells and their working principle in the brain

The acquisition, integration and storage of exogenous associated signals are termed as associative learning and memory. The consequences and processes of associative thinking and logical reasoning based on these stored exogenous signals can be memorized as endogenous signals, which are essential for decision making, intention, and planning. Associative memory cells recruited in these primary and secondary associative memories are presumably the foundation for the brain to fulfill cognition events and emotional reactions in life, though the plasticity of synaptic connectivity and neuronal activity has been believed to be involved in learning and memory. Current reports indicate that associative memory cells are recruited by their mutual synapse innervations among co-activated brain regions to fulfill the integration, storage and retrieval of associated signals. The activation of these associative memory cells initiates information recall in the mind, and the successful activation of their downstream neurons endorses memory presentations through behaviors and emotion reactions. In this review, we aim to draw a comprehensive diagram for associative memory cells, working principle and modulation, as well as propose their roles in cognition, emotion and behaviors.

Synapse Innervation and Associative Memory Cell Are Recruited for Integrative Storage of Whisker and Odor Signals in the Barrel Cortex through miRNA-Mediated Processes

Associative learning is a common way for information acquisition, and the integrative storage of multiple associated signals is essential for associative thinking and logical reasoning. In terms of the cellular mechanism for associative memory, our studies by behavioral task and cellular imaging demonstrate that paired whisker and odor stimulations lead to odorant-induced whisker motion and associative memory cell recruitment in the barrel cortex (BC), which is driven presumably by synapse innervation from co-activated sensory cortices. To confirm these associative memory cells and synapse innervations essential for associative memory and to examine their potential mechanisms, we studied a causal relationship between epigenetic process and memory cell/synapse recruitment by manipulating miRNAs and observing the changes from the recruitments of associative memory cells and synapse innervations to associative memory. Anti-miRNA-324 and anti-miRNA-133a in the BC significantly downregulate new synapse innervation, associative memory cell recruitment and odorant-induced whisker motion, where Tau-tubulin kinase-1 expression is increased. Therefore, the upregulated miRNA-324 in associative learning knocks down Ttbk1-mediated Tau phosphorylation and microtubule depolymerization, which drives the balance between polymerization and depolymerization toward the axon prolongation and spine stabilization to initiate new synapse innervations and to recruit associative memory cells.

Piriform cortical glutamatergic and GABAergic neurons express coordinated plasticity for whisker-induced odor recall

Neural plasticity occurs in learning and memory. Coordinated plasticity at glutamatergic and GABAergic neurons during memory formation remains elusive, which we investigate in a mouse model of associative learning by cellular imaging and electrophysiology. Paired odor and whisker stimulations lead to whisker-induced olfaction response. In mice that express this cross-modal memory, the neurons in the piriform cortex are recruited to encode newly acquired whisker signal alongside innate odor signal, and their response patterns to these associated signals are different. There are emerged synaptic innervations from barrel cortical neurons to piriform cortical neurons from these mice. These results indicate the recruitment of associative memory cells in the piriform cortex after associative memory. In terms of the structural and functional plasticity at these associative memory cells in the piriform cortex, glutamatergic neurons and synapses are upregulated, GABAergic neurons and synapses are downregulated as well as their mutual innervations are refined in the coordinated manner. Therefore, the associated activations of sensory cortices triggered by their input signals induce the formation of their mutual synapse innervations, the recruitment of associative memory cells and the coordinated plasticity between the GABAergic and glutamatergic neurons, which work for associative memory cells to encode cross-modal associated signals in their integration, associative storage and distinguishable retrieval.

Activity strengths of cortical glutamatergic and GABAergic neurons are correlated with transgenerational inheritance of learning ability

The capabilities of learning and memory in parents are presumably transmitted to their offsprings, in which genetic codes and epigenetic regulations are thought as molecular bases. As neural plasticity occurs during memory formation as cellular mechanism, we aim to examine the correlation of activity strengths at cortical glutamatergic and GABAergic neurons to the transgenerational inheritance of learning ability. In a mouse model of associative learning, paired whisker and odor stimulations led to odorant-induced whisker motion, whose onset appeared fast (high learning efficiency, HLE) or slow (low learning efficiency, LLE). HLE male and female mice, HLE female and LLE male mice as well as HLE male and LLE female mice were cross-mated to have their first generation of offsprings, filials (F1). The onset of odorant-induced whisker motion appeared a sequence of high-to-low efficiency in three groups of F1 mice that were from HLE male and female mice, HLE female and LLE male mice as well as HLE male and LLE female mice. Activities related to glutamatergic neurons in barrel cortices appeared a sequence of high-to-low strength in these F1 mice from HLE male and female mice, HLE female and LLE male mice as well as HLE male and LLE female mice. Activities related to GABAergic neurons in barrel cortices appeared a sequence of low-to-high strength in these F1 mice from HLE male and female mice, HLE female and LLE male mice as well as HLE male and LLE female mice. Neuronal activity strength was linearly correlated to learning efficiency among three groups. Thus, the coordinated activities at glutamatergic and GABAergic neurons may constitute the cellular basis for the transgenerational inheritance of learning ability.

Functional compatibility between Purkinje cell axon branches and their target neurons in the cerebellum

A neuron sprouts an axon, and its branches to innervate many target neurons that are divergent in their functions. In order to efficiently regulate the diversified cells, the axon branches should differentiate functionally to be compatible with their target neurons, i.e., a function compatibility between presynaptic and postsynaptic partners. We have examined this hypothesis by using electrophysiological method in the cerebellum, in which the main axon of Purkinje cell projected to deep nucleus cells and the recurrent axons innervated the adjacent Purkinje cells. The fidelity of spike propagation is superior in the recurrent branches than the main axon. The capabilities of encoding spikes and processing GABAergic inputs are advanced in Purkinje cells versus deep nucleus cells. The functional differences among Purkinje's axonal branches and their postsynaptic neurons are preset by the variable dynamics of their voltage-gated sodium channels. In addition, activity strengths between presynaptic and postsynaptic partners are proportionally correlated, i.e., active axonal branches innervate active target neurons, or vice versa. The physiological impact of the functional compatibility is to make the neurons in their circuits to be activated appropriately. In conclusion, each cerebellar Purkinje cell sprouts the differentiated axon branches to be compatible with the diversified target cells in their functions, in order to construct the homeostatic and efficient units for their coordinated activity in neural circuits.

Coordinated Plasticity among Glutamatergic and GABAergic Neurons and Synapses in the Barrel Cortex Is Correlated to Learning Efficiency

Functional plasticity at cortical synapses and neurons is presumably associated with learning and memory. Additionally, coordinated refinement between glutamatergic and GABAergic neurons occurs in associative memory. If these assumptions are present, neuronal plasticity strength and learning efficiency should be correlated. We have examined whether neuronal plasticity strength and learning efficiency are quantitatively correlated in a mouse model of associative learning. Paired whisker and odor stimulations in mice induce odorant-induced whisker motions. The fully establishment of this associative memory appears fast and slow, which are termed as high learning efficiency and low learning efficiency, respectively. In the study of cellular mechanisms underlying this differential learning efficiency, we have compared the strength of neuronal plasticity in the barrel cortices that store associative signals from the mice with high vs. low learning efficiencies. Our results indicate that the levels of learning efficiency are linearly correlated with the upregulated strengths of excitatory synaptic transmission on glutamatergic neurons and their excitability, as well as the downregulated strengths of GABAergic neurons' excitability, their excitatory synaptic inputs and inhibitory synaptic outputs in layers II~III of barrel cortices. The correlations between learning efficiency in associative memory formation and coordinated plasticity at cortical glutamatergic and GABAergic neurons support the notion that the plasticity of associative memory cells is a basis for memory strength.