Homeostasis established by coordination of subcellular compartment plasticity improves spike encoding

Associative memory neurons of encoding multi-modal signals are recruited by neuroligin-3-mediated new synapse formation

The joint storage and reciprocal retrieval of learnt associated signals are presumably encoded by associative memory cells. In the accumulation and enrichment of memory contents in lifespan, a signal often becomes a core signal associatively shared for other signals. One specific group of associative memory neurons that encode this core signal likely interconnects multiple groups of associative memory neurons that encode these other signals for their joint storage and reciprocal retrieval. We have examined this hypothesis in a mouse model of associative learning by pairing the whisker tactile signal sequentially with the olfactory signal, the gustatory signal, and the tail-heating signal. Mice experienced this associative learning show the whisker fluctuation induced by olfactory, gustatory, and tail-heating signals, or the other way around, that is, memories to multi-modal associated signals featured by their reciprocal retrievals. Barrel cortical neurons in these mice become able to encode olfactory, gustatory, and tail-heating signals alongside the whisker signal. Barrel cortical neurons interconnect piriform, S1-Tr, and gustatory cortical neurons. With the barrel cortex as the hub, the indirect activation occurs among piriform, gustatory, and S1-Tr cortices for the second-order associative memory. These associative memory neurons recruited to encode multi-modal signals in the barrel cortex for associative memory are downregulated by neuroligin-3 knockdown. Thus, associative memory neurons can be recruited as the core cellular substrate to memorize multiple associated signals for the first-order and the second-order of associative memories by neuroligin-3-mediated synapse formation, which constitutes neuronal substrates of cognitive activities in the field of memoriology.

Associative memory neurons of encoding multi-modal signals are recruited by neuroligin-3-mediated new synapse formation

The joint storage and reciprocal retrieval of learnt associated signals are presumably encoded by associative memory cells. In the accumulation and enrichment of memory contents in lifespan, a signal often becomes a core signal associatively shared for other signals. One specific group of associative memory neurons that encode this core signal likely interconnects multiple groups of associative memory neurons that encode these other signals for their joint storage and reciprocal retrieval. We have examined this hypothesis in a mouse model of associative learning by pairing the whisker tactile signal sequentially with the olfactory signal, the gustatory signal, and the tail-heating signal. Mice experienced this associative learning show the whisker fluctuation induced by olfactory, gustatory, and tail-heating signals, or the other way around, that is, memories to multi-modal associated signals featured by their reciprocal retrievals. Barrel cortical neurons in these mice become able to encode olfactory, gustatory, and tail-heating signals alongside the whisker signal. Barrel cortical neurons interconnect piriform, S1-Tr, and gustatory cortical neurons. With the barrel cortex as the hub, the indirect activation occurs among piriform, gustatory, and S1-Tr cortices for the second-order associative memory. These associative memory neurons recruited to encode multi-modal signals in the barrel cortex for associative memory are downregulated by neuroligin-3 knockdown. Thus, associative memory neurons can be recruited as the core cellular substrate to memorize multiple associated signals for the first-order and the second-order of associative memories by neuroligin-3-mediated synapse formation, which constitutes neuronal substrates of cognitive activities in the field of memoriology.

The nematode (Ascaris suum) intestine is a location of synergistic anthelmintic effects of Cry5B and levamisole

A novel group of biocidal compounds are the Crystal 3D (Cry) and Cytolytic (Cyt) proteins produced by Bacillus thuringiensis (Bt). Some Bt Cry proteins have a selective nematocidal activity, with Cry5B being the most studied. Cry5B kills nematode parasites by binding selectively to membrane glycosphingolipids, then forming pores in the cell membranes of the intestine leading to damage. Cry5B selectively targets multiple species of nematodes from different clades and has no effect against mammalian hosts. Levamisole is a cholinomimetic anthelmintic that acts by selectively opening L-subtype nicotinic acetylcholine receptor ion-channels (L-AChRs) that have been found on muscles of nematodes. A synergistic nematocidal interaction between levamisole and Cry5B has been described previously, but the location, mechanism and time-course of this synergism is not known. In this study we follow the timeline of the effects of levamisole and Cry5B on the Ca2+ levels in enterocyte cells from the intestine of Ascaris suum using fluorescence imaging. The peak Ca2+ responses to levamisole were observed after approximately 10 minutes while the peak responses to activated Cry5B were observed after approximately 80 minutes. When levamisole and Cry5B were applied simultaneously, we observed that the responses to Cry5B were bigger and occurred sooner than when it was applied by itself. It is proposed that there is an irreversible cytoplasmic Ca2+ overload that leads to necrotic cell-death in the enterocyte that is induced by levamisole opening Ca2+ permeable L-subtype nAChRs and the development of Ca2+ permeable Cry5B toxin pores in enterocyte plasma membranes. The effects of levamisole potentiate and speed the actions of Cry5B.

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.

Anthelmintic resistance and homeostatic plasticity (Brugia malayi)

Homeostatic plasticity refers to the capacity of excitable cells to regulate their activity to make compensatory adjustments to long-lasting stimulation. It is found across the spectrum of vertebrate and invertebrate species and is driven by changes in cytosolic calcium; it has not been explored in parasitic nematodes when treated with therapeutic drugs. Here we have studied the adaptation of Brugia malayi to exposure to the anthelmintic, levamisole that activates muscle AChR ion-channels. We found three phases of the Brugia malayi motility responses as they adapted to levamisole: an initial spastic paralysis; a flaccid paralysis that follows; and finally, a recovery of motility with loss of sensitivity to levamisole at 4 h. Motility, calcium-imaging, patch-clamp and molecular experiments showed the muscle AChRs are dynamic with mechanisms that adjust their subtype composition and sensitivity to levamisole. This homeostatic plasticity allows the parasite to adapt resisting the anthelmintic.

Local Homeostatic Regulation of the Spectral Radius of Echo-State Networks

Recurrent cortical networks provide reservoirs of states that are thought to play a crucial role for sequential information processing in the brain. However, classical reservoir computing requires manual adjustments of global network parameters, particularly of the spectral radius of the recurrent synaptic weight matrix. It is hence not clear if the spectral radius is accessible to biological neural networks. Using random matrix theory, we show that the spectral radius is related to local properties of the neuronal dynamics whenever the overall dynamical state is only weakly correlated. This result allows us to introduce two local homeostatic synaptic scaling mechanisms, termed flow control and variance control, that implicitly drive the spectral radius toward the desired value. For both mechanisms the spectral radius is autonomously adapted while the network receives and processes inputs under working conditions. We demonstrate the effectiveness of the two adaptation mechanisms under different external input protocols. Moreover, we evaluated the network performance after adaptation by training the network to perform a time-delayed XOR operation on binary sequences. As our main result, we found that flow control reliably regulates the spectral radius for different types of input statistics. Precise tuning is however negatively affected when interneural correlations are substantial. Furthermore, we found a consistent task performance over a wide range of input strengths/variances. Variance control did however not yield the desired spectral radii with the same precision, being less consistent across different input strengths. Given the effectiveness and remarkably simple mathematical form of flow control, we conclude that self-consistent local control of the spectral radius via an implicit adaptation scheme is an interesting and biological plausible alternative to conventional methods using set point homeostatic feedback controls of neural firing.

microRNA-15b contributes to depression-like behavior in mice by affecting synaptic protein levels and function in the nucleus accumbens

Major depression is a prevalent affective disorder characterized by recurrent low mood. It presumably results from stress-induced deteriorations of molecular networks and synaptic functions in brain reward circuits of genetically-susceptible individuals through epigenetic processes. Epigenetic regulator microRNA-15b inhibits neuronal progenitor proliferation and is up-regulated in the medial prefrontal cortex of mice that demonstrate depression-like behavior, indicating the contribution of microRNA-15 to major depression. Using a mouse model of major depression induced by chronic unpredictable mild stress (CUMS), here we examined the effects of microRNA-15b on synapses and synaptic proteins in the nucleus accumbens of these mice. The application of a microRNA-15b antagomir into the nucleus accumbens significantly reduced the incidence of CUMS-induced depression and reversed the attenuations of excitatory synapse and syntaxin-binding protein 3 (STXBP3A)/vesicle-associated protein 1 (VAMP1) expression. In contrast, the injection of a microRNA-15b analog into the nucleus accumbens induced depression-like behavior as well as attenuated excitatory synapses and STXBP3A/VAMP1 expression similar to the down-regulation of these processes induced by the CUMS. We conclude that microRNA-15b-5p may play a critical role in chronic stress-induced depression by decreasing synaptic proteins, innervations, and activities in the nucleus accumbens. We propose that the treatment of anti-microRNA-15b-5p may convert stress-induced depression into resilience.

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.

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.

PKC and CaMK-II inhibitions coordinately rescue ischemia-induced GABAergic neuron dysfunction

Cerebral ischemia leads to neuronal death for stroke, in which the imbalance between glutamatergic neurons and GABAergic neurons toward neural excitotoxicity is presumably involved. GABAergic neurons are vulnerable to pathological factors and impaired in an early stage of ischemia. The rescue of GABAergic neurons is expected to be the strategy to reserve ischemic neuronal impairment. As protein kinase C (PKC) and calmodulin-dependent protein kinase II (CaMK-II) are activated during ischemia, we have investigated whether the inhibitions of these kinases rescue the ischemic impairment of cortical GABAergic neurons. The functions of GABAergic neurons were analyzed by whole-cell recording in the cortical slices during ischemia and in presence of 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (CaMK-II inhibitor) and chelerythrine chloride (PKC inhibitor). Our results indicate that PKC inhibitor or CaMK-II inhibitor partially prevents ischemia-induced functional deficits of cortical GABAergic neurons. Moreover, the combination of PKC and CaMK-II inhibitors synergistically reverses this ischemia-induced deficit of GABAergic neurons. One of potential therapeutic strategies for ischemic stroke may be to rescue the ischemia-induced deficit of cortical GABAergic neurons by inhibiting PKC and CaMK-II.

Activity-induced spontaneous spikes in GABAergic neurons suppress seizure discharges: an implication of computational modeling

Background

Epilepsy, a prevalent neurological disorder, appears self-termination. The endogenous mechanism for seizure self-termination remains to be addressed in order to develop new strategies for epilepsy treatment. We aim to examine the role of activity-induced spontaneous spikes at GABAergic neurons as an endogenous mechanism in the seizure self-termination.

Methods and Results

Neuronal spikes were induced by depolarization pulses at cortical GABAergic neurons from temporal lobe epilepsy patients and mice, in which some of these neurons fired activity-induced spontaneous spikes. Neural networks including excitatory and inhibitory neurons were computationally constructed, and their functional properties were based on our studies from whole-cell recordings. With the changes in the portion and excitability of inhibitory neurons that generated activity-induced spontaneous spike, the efficacies to suppress synchronous seizure activity were analyzed, such as its onset time, decay slope and spike frequency. The increases in the proportion and excitability of inhibitory neurons that generated activity-induced spontaneous spikes effectively suppressed seizure activity in neural networks. These factors synergistically strengthened the efficacy of seizure activity suppression.

Conclusion

Our study supports a notion that activity-induced spontaneous spikes in GABAergic neurons may be an endogenous mechanism for seizure self-termination. A potential therapeutic strategy for epilepsy is to upregulate the cortical inhibitory neurons that generate activity-induced spontaneous spikes.

GABAergic neurons in nucleus accumbens are correlated to resilience and vulnerability to chronic stress for major depression

Background

Major depression, persistent low mood, is one of common psychiatric diseases. Chronic stressful life is believed to be a major risk factor that leads to dysfunctions of the limbic system. However, a large number of the individuals with experiencing chronic stress do not suffer from major depression, called as resilience. Endogenous mechanisms underlying neuronal invulnerability to chronic stress versus major depression are largely unknown. As GABAergic neurons are vulnerable to chronic stress and their impairments is associated with major depression, we have examined whether the invulnerability of GABAergic neurons in the limbic system is involved in resilience.

Results

GABAergic neurons in the nucleus accumbens from depression-like mice induced by chronic unpredictable mild stress appear the decreases in their GABA release, spiking capability and excitatory input reception, compared with those in resilience mice. The levels of decarboxylase and vesicular GABA transporters decrease in depression-like mice, but not resilience.

Materials and Methods

Mice were treated by chronic unpredictable mild stress for three weeks. Depression-like behaviors or resilience was confirmed by seeing whether their behaviors change significantly in sucrose preference, Y-maze and forced swimming tests. Mice from controls as well as depression and resilience in response to chronic unpredictable mild stress were studied in terms of GABAergic neuron activity in the nucleus accumbens by cell electrophysiology and protein chemistry.

Conclusions

The impairment of GABAergic neurons in the nucleus accumbens is associated with major depression. The invulnerability of GABAergic neurons to chronic stress may be one of cellular mechanisms for the resilience to chronic stress.

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.

More sensitivity of cortical GABAergic neurons than glutamatergic neurons in response to acidosis

Acidosis impairs brain functions. Neuron-specific mechanisms underlying acidosis-induced brain dysfunction remain elusive. We studied the sensitivity of cortical GABAergic neurons and glutamatergic neurons to acidosis by whole-cell recording in brain slices. The acidification to the neurons was induced by perfusing artificial cerebral spinal fluid with lower pH. This acidification impairs excitability and synaptic transmission in the glutamatergic and GABAergic neurons. Acidosis impairs spiking capacity in the GABAergic neurons more than in the glutamatergic neurons. Acidosis also strengthens glutamatergic synaptic transmission and attenuates GABAergic synaptic transmission on the GABAergic neurons more than the glutamatergic neurons, which results in the functional impairment of these GABAergic neurons. This acidosis-induced dysfunction predominantly in the cortical GABAergic neurons drives the homeostasis of neuronal networks toward overexcitation and exacerbates neuronal impairment.

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.

Evidence for frequency-dependent cortical plasticity in the human brain

Frequency-dependent plasticity (FDP) describes adaptation at the synapse in response to stimulation at different frequencies. Its consequence on the structure and function of cortical networks is unknown. We tested whether cortical "resonance," favorable stimulation frequencies at which the sensory cortices respond maximally, influenced the impact of FDP on perception, functional topography, and connectivity of the primary somatosensory cortex using psychophysics and functional imaging (fMRI). We costimulated two digits on the hand synchronously at, above, or below the resonance frequency of the somatosensory cortex, and tested subjects' accuracy and speed on tactile localization before and after costimulation. More errors and slower response times followed costimulation at above- or below-resonance, respectively. Response times were faster after at-resonance costimulation. In the fMRI, the cortical representations of the two digits costimulated above-resonance shifted closer, potentially accounting for the poorer performance. Costimulation at-resonance did not shift the digit regions, but increased the functional coupling between them, potentially accounting for the improved response time. To relate these results to synaptic plasticity, we simulated a network of oscillators incorporating Hebbian learning. Two neighboring patches embedded in a cortical sheet, mimicking the two digit regions, were costimulated at different frequencies. Network activation outside the stimulated patches was greatest at above-resonance frequencies, reproducing the spread of digit representations seen with fMRI. Connection strengths within the patches increased following at-resonance costimulation, reproducing the increased fMRI connectivity. We show that FDP extends to the cortical level and is influenced by cortical resonance.

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.

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

Associative memory is essential for cognition, in which associative memory cells and their plasticity presumably play important roles. The mechanism underlying associative memory extinction vs. maintenance remains unclear, which we have studied in a mouse model of cross-modal associative learning. Paired whisker and olfaction stimulations lead to a full establishment of odorant-induced whisker motion in training day 10, which almost disappears if paired stimulations are not given in a week, and then recovers after paired stimulation for an additional day. In mice that show associative memory, extinction and recovery, we have analyzed the dynamical plasticity of glutamatergic neurons in layers II–III of the barrel cortex and layers IV–V of the motor cortex. Compared with control mice, the rate of evoked spikes as well as the amplitude and frequency of excitatory postsynaptic currents increase, whereas the amplitude and frequency of inhibitory postsynaptic currents (IPSC) decrease at training day 10 in associative memory mice. Without paired training for a week, these plastic changes are persistent in the barrel cortex and decayed in the motor cortex. If paired training is given for an additional day to revoke associative memory, neuronal plasticity recovers in the motor cortex. Our study indicates persistent neuronal plasticity in the barrel cortex for cross-modal memory maintenance as well as the dynamical change of neuronal plasticity in the motor cortex for memory retrieval and extinction. In other words, the sensory cortices are essential for long-term memory while the behavior-related cortices with the inability of memory retrieval are correlated to memory extinction.

Associative memory cells: Formation, function and perspective

Associative learning and memory are common activities in life, and their cellular infrastructures constitute the basis of cognitive processes. Although neuronal plasticity emerges after memory formation, basic units and their working principles for the storage and retrieval of associated signals remain to be revealed. Current reports indicate that associative memory cells, through their mutual synapse innervations among the co-activated sensory cortices, are recruited to fulfill the integration, storage and retrieval of multiple associated signals, and serve associative thinking and logical reasoning. In this review, we aim to summarize associative memory cells in their formation, features and functional impacts.

Associative memory cells: Formation, function and perspective

Associative learning and memory are common activities in life, and their cellular infrastructures constitute the basis of cognitive processes. Although neuronal plasticity emerges after memory formation, basic units and their working principles for the storage and retrieval of associated signals remain to be revealed. Current reports indicate that associative memory cells, through their mutual synapse innervations among the co-activated sensory cortices, are recruited to fulfill the integration, storage and retrieval of multiple associated signals, and serve associative thinking and logical reasoning. In this review, we aim to summarize associative memory cells in their formation, features and functional impacts.

Ischemia deteriorates spike encoding at cortical GABAergic neurons and cerebellar Purkinje cells by increasing the intracellular Ca2

GABAergic neurons play a critical role in the central nervous system, such as well-organized behaviors. The ischemic cell death is presumably initiated by neuronal excitotoxicity resulted from the dysfunction of GABAergic neurons. It is not clear how ischemia influences different types of GABAergic neurons and whether intracellular Ca²⁺ plays a key role in the ischemic excitotoxicity. We have investigated this issue at cortical GABAergic neurons and cerebellar Purkinje cells by whole-cell recording in mouse brain slices, and the roles of intracellular Ca²⁺ are examined by BABTA infusion. Compare with the data from a group of control, ischemia causes by lowering purfusion rate lowers spike encoding at cortical GABAergic neurons and enhances encoding ability at cerebellar Purkinje cells. These differential effects of ischemia on spike encoding are mechanistically associated with the changes in the refractory periods and threshold potentials of sequential spikes. These ischemia-induced dysfunction of spike encoding at two types of GABAergic cells are prevented by BABTA infusion. Therefore, the ischemia destabilizes the spike encoding of GABAergic cells via raising intracellular Ca²⁺. Our findings indicate that ischemia preferentially causes the dysfunction of spike encoding at GABAergic neurons by the up-regulation of intracellular Ca²⁺ level, which leads to neuronal excitotoxicity.

Associations of Unilateral Whisker and Olfactory Signals Induce Synapse Formation and Memory Cell Recruitment in Bilateral Barrel Cortices: Cellular Mechanism for Unilateral Training Toward Bilateral Memory

Somatosensory signals and operative skills learned by unilateral limbs can be retrieved bilaterally. In terms of cellular mechanism underlying this unilateral learning toward bilateral memory, we hypothesized that associative memory cells in bilateral cortices and synapse innervations between them were produced. In the examination of this hypothesis, we have observed that paired unilateral whisker and odor stimulations led to odorant-induced whisker motions in bilateral sides, which were attenuated by inhibiting the activity of barrel cortices. In the mice that showed bilateral cross-modal responses, the neurons in both sides of barrel cortices became to encode this new odor signal alongside the innate whisker signal. Axon projections and synapse formations from the barrel cortex, which was co-activated with the piriform cortex, toward its contralateral barrel cortex (CBC) were upregulated. Glutamatergic synaptic transmission in bilateral barrel cortices was upregulated and GABAergic synaptic transmission was downregulated. The associative activations of the sensory cortices facilitate new axon projection, glutamatergic synapse formation and GABAergic synapse downregulation, which drive the neurons to be recruited as associative memory cells in the bilateral cortices. Our data reveal the productions of associative memory cells and synapse innervations in bilateral sensory cortices for unilateral training toward bilateral memory.

Coordinated Plasticity between Barrel Cortical Glutamatergic and GABAergic Neurons during Associative Memory

Neural plasticity is associated with memory formation. The coordinated refinement and interaction between cortical glutamatergic and GABAergic neurons remain elusive in associative memory, which we examine in a mouse model of associative learning. In the mice that show odorant-induced whisker motion after pairing whisker and odor stimulations, the barrel cortical glutamatergic and GABAergic neurons are recruited to encode the newly learnt odor signal 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. The analyses by high throughput sequencing show that certain microRNAs related to regulating synapses and neurons are involved in this cross-modal reflex. Thus, the coactivation of the sensory cortices through epigenetic processes recruits their glutamatergic and GABAergic neurons to be the associative memory cells as well as drive their coordinated refinements toward the optimal state for the storage of the associated signals.

Glucocorticoid Induces Incoordination between Glutamatergic and GABAergic Neurons in the Amygdala

BACKGROUND

Stressful life leads to mood disorders. Chronic mild stress is presumably major etiology for depression, and acute severe stress leads to anxiety. These stressful situations may impair hypothalamus-pituitary-adrenal axis and in turn induce synapse dysfunction. However, it remains elusive how the stress hormones mess up subcellular compartments and interactions between excitatory and inhibitory neurons, which we have investigated in mouse amygdala, a structure related to emotional states.

METHODS AND RESULTS

Dexamethasone was chronically given by intraperitoneal injection once a day for one week or was acutely washed into the brain slices. The neuronal spikes and synaptic transmission were recorded by whole-cell patching in amygdala neurons of brain slices. The chronic or acute administration of dexamethasone downregulates glutamate release as well as upregulates GABA release and GABAergic neuron spiking. The chronic administration of dexamethasone also enhances the responsiveness of GABA receptors.

CONCLUSION

The upregulation of GABAergic neurons and the downregulation of glutamatergic neurons by glucocorticoid impair their balance in the amygdala, which leads to emotional disorders during stress.

Impaired GABA synthesis, uptake and release are associated with depression-like behaviors induced by chronic mild stress

Major depression is a prevalent emotion disorder. Chronic stressful life in genetically susceptible individuals is presumably a major etiology that leads to neuron and synapse atrophy in the limbic system. Molecular mechanisms underlying the pathological changes remain elusive. Mice were treated by chronic unpredictable mild stress (CUMS) until they demonstrated depression-like behavior. GABA release in the medial prefrontal cortex was evaluated by cell electrophysiology and imaging. Molecular profiles related to GABA synthesis and uptake were investigated by the high-throughput sequencings of microRNAs and mRNAs as well as western blot analysis in this cortical area. In CUMS-induced depression mice, there appear the decreases in the innervation and function of GABAergic axons and in the levels of mRNAs and proteins of glutamate decarboxylase-67, vesicular GABA transporter and GABA transporter-3. miRNA-15b-5p, miRNA-144-3p, miRNA-582-5p and miRNA-879-5p that directly downregulate such mRNAs increase in this cortex. Our results suggest that chronic mild stress impairs GABA release and uptake by upregulating miRNAs and downregulating mRNAs and proteins, which may constitute the subcellular and molecular mechanisms for the lowered GABA tone in major depression.

Molecular Mechanism for Stress-Induced Depression Assessed by Sequencing miRNA and mRNA in Medial Prefrontal Cortex

Background

Major depression is a prevalent mood disorder. Chronic stress is presumably main etiology that leads to the neuron and synapse atrophies in the limbic system. However, the intermediate molecules from stresses to neuronal atrophy remain elusive, which we have studied in the medial prefrontal cortices from depression mice.

Methods and Results

The mice were treated by the chronic unpredictable mild stress (CUMS) until they expressed depression-like behaviors confirmed by the tests of sucrose preference, forced swimming and Y-maze. High-throughput sequencings of microRNA and mRNA in the medial prefrontal cortices were performed in CUMS-induced depression mice versus control mice to demonstrate the molecular profiles of major depression. In the medial prefrontal cortices of depression-like mice, the levels of mRNAs that translated the proteins for the GABAergic synapses, dopaminergic synapses, myelination, synaptic vesicle cycle and neuronal growth were downregulated. miRNAs of regulating these mRNAs are upregulated.

Conclusion

The deteriorations of GABAergic and dopaminergic synapses as well as axonal growth are associated with CUMS-induced depression.

Incoordination among Subcellular Compartments Is Associated with Depression-Like Behavior Induced by Chronic Mild Stress

BACKGROUND

Major depressive disorder is characterized as persistent low mood. A chronically stressful life in genetically susceptible individuals is presumably the major etiology that leads to dysfunctions of monoamine and hypothalamus-pituitary-adrenal axis. These pathogenic factors cause neuron atrophy in the limbic system for major depressive disorder. Cell-specific pathophysiology is unclear, so we investigated prelimbic cortical GABAergic neurons and their interaction with glutamatergic neurons in depression-like mice.

METHODS

Mice were treated with chronic unpredictable mild stress for 3 weeks until they expressed depression-like behaviors confirmed by sucrose preference, Y-maze, and forced swimming tests. The structures and functions of GABAergic and glutamatergic units in prelimbic cortices were studied by cell imaging and electrophysiology in chronic unpredictable mild stress-induced depression mice vs controls.

RESULTS

In depression-like mice, prelimbic cortical GABAergic neurons show incoordination among the subcellular compartments, such as decreased excitability and synaptic outputs as well as increased reception from excitatory inputs. GABAergic synapses on glutamatergic cells demonstrate decreased presynaptic innervation and increased postsynaptic responsiveness.

CONCLUSIONS

Chronic unpredictable mild stress-induced incoordination in prelimbic cortical GABAergic and glutamatergic neurons dysregulates their target neurons, which may be the pathological basis for depressive mood. The rebalance of compatibility among subcellular compartments would be an ideal strategy to treat neural disorders.

Acidosis-Induced Dysfunction of Cortical GABAergic Neurons through Astrocyte-Related Excitotoxicity

BACKGROUND

Acidosis impairs cognitions and behaviors presumably by acidification-induced changes in neuronal metabolism. Cortical GABAergic neurons are vulnerable to pathological factors and their injury leads to brain dysfunction. How acidosis induces GABAergic neuron injury remains elusive. As the glia cells and neurons interact each other, we intend to examine the role of the astrocytes in acidosis-induced GABAergic neuron injury.

RESULTS

Experiments were done at GABAergic cells and astrocytes in mouse cortical slices. To identify astrocytic involvement in acidosis-induced impairment, we induced the acidification in single GABAergic neuron by infusing proton intracellularly or in both neurons and astrocytes by using proton extracellularly. Compared the effects of intracellular acidification and extracellular acidification on GABAergic neurons, we found that their active intrinsic properties and synaptic outputs appeared more severely impaired in extracellular acidosis than intracellular acidosis. Meanwhile, extracellular acidosis deteriorated glutamate transporter currents on the astrocytes and upregulated excitatory synaptic transmission on the GABAergic neurons. Moreover, the antagonists of glutamate NMDA-/AMPA-receptors partially reverse extracellular acidosis-induced injury in the GABAergic neurons.

CONCLUSION

Our studies suggest that acidosis leads to the dysfunction of cortical GABAergic neurons by astrocyte-mediated excitotoxicity, in addition to their metabolic changes as indicated previously.

A portion of inhibitory neurons in human temporal lobe epilepsy are functionally upregulated: an endogenous mechanism for seizure termination

MAIN PROBLEM

Epilepsy is one of the more common neurological disorders. The medication is often ineffective to the patients suffering from intractable temporal lobe epilepsy (TLE). As their seizures are usually self-terminated, the elucidation of the mechanism underlying endogenous seizure termination will help to find a new strategy for epilepsy treatment. We aim to examine the role of inhibitory interneurons in endogenous seizure termination in TLE patients.

METHODS

Whole-cell recordings were conducted on inhibitory interneurons in seizure-onset cortices of intractable TLE patients and the temporal lobe cortices of nonseizure individuals. The intrinsic property of the inhibitory interneurons and the strength of their GABAergic synaptic outputs were measured. The quantitative data were introduced into the computer-simulated neuronal networks to figure out a role of these inhibitory units in the seizure termination.

RESULTS

In addition to functional downregulation, a portion of inhibitory interneurons in seizure-onset cortices were upregulated in encoding the spikes and controlling their postsynaptic neurons. A patch-like upregulation of inhibitory neurons in the local network facilitated seizure termination. The upregulations of both inhibitory neurons and their output synapses synergistically shortened seizure duration, attenuated seizure strength, and terminated seizure propagation.

CONCLUSION

Automatic seizure termination is likely due to the fact that a portion of the inhibitory neurons and synapses are upregulated in the seizure-onset cortices. This mechanism may create novel therapeutic strategies to treat intractable epilepsy, such as the simultaneous upregulation of cortical inhibitory neurons and their output synapses.

Protein kinase C is essential for kainate-induced anxiety-related behavior and glutamatergic synapse upregulation in prelimbic cortex

AIM

Anxiety is one of common mood disorders, in which the deficit of serotonergic and GABAergic synaptic functions in the amygdala and prefrontal cortex is believed to be involved. The pathological changes at the glutamatergic synapses and neurons in these brain regions as well as their underlying mechanisms remain elusive, which we aim to investigate.

METHODS

An agonist of kainate-type glutamate receptors, kainic acid, was applied to induce anxiety-related behaviors. The morphology and functions of glutamatergic synapses in the prelimbic region of mouse prefrontal cortex were analyzed using cellular imaging and electrophysiology.

RESULTS

After kainate-induced anxiety is onset, the signal transmission at the glutamatergic synapses is upregulated, and the dendritic spine heads are enlarged. In terms of the molecular mechanisms, the upregulated synaptic plasticity is associated with the expression of more protein kinase C (PKC) in the dendritic spines. Chelerythrine, a PKC inhibitor, reverses kainate-induced anxiety and anxiety-related glutamatergic synapse upregulation.

CONCLUSION

The activation of glutamatergic kainate-type receptors leads to anxiety-related behaviors and glutamatergic synapse upregulation through protein kinase C in the prelimbic region of the mouse prefrontal cortex.

Frequency-dependent reliability of spike propagation is function of axonal voltage-gated sodium channels in cerebellar Purkinje cells

The spike propagation on nerve axons, like synaptic transmission, is essential to ensure neuronal communication. The secure propagation of sequential spikes toward axonal terminals has been challenged in the neurons with a high firing rate, such as cerebellar Purkinje cells. The shortfall of spike propagation makes some digital spikes disappearing at axonal terminals, such that the elucidation of the mechanisms underlying spike propagation reliability is crucial to find the strategy of preventing loss of neuronal codes. As the spike propagation failure is influenced by the membrane potentials, this process is likely caused by altering the functional status of voltage-gated sodium channels (VGSC). We examined this hypothesis in Purkinje cells by using pair-recordings at their somata and axonal blebs in cerebellar slices. The reliability of spike propagation was deteriorated by elevating spike frequency. The frequency-dependent reliability of spike propagation was attenuated by inactivating VGSCs and improved by removing their inactivation. Thus, the functional status of axonal VGSCs influences the reliability of spike propagation.

Voltage-independent sodium channels emerge for an expression of activity-induced spontaneous spikes in GABAergic neurons

BACKGROUND

Cerebral overexcitation needs inhibitory neurons be functionally upregulated to rebalance excitation vs. inhibition. For example, the intensive activities of GABAergic neurons induce spontaneous spikes, i.e., activity-induced spontaneous spikes (AISS). The mechanisms underlying AISS onset remain unclear. We investigated the roles of sodium channels in AISS induction and expression at hippocampal GABAergic neurons by electrophysiological approach.

RESULTS

AISS expression includes additional spike capability above evoked spikes, and the full spikes in AISS comprise early phase (spikelets) and late phase, implying the emergence of new spikelet component. Compared with the late phase, the early phase is characterized as voltage-independent onset, less voltage-dependent upstroke and sensitivity to TTX. AISS expression and induction are independent of membrane potential changes. Therefore, AISS's spikelets express based on voltage-independent sodium channels. In terms of AISS induction, the facilitation of voltage-gated sodium channel (VGSC) activation accelerates AISS onset, or vice versa.

CONCLUSION

AISS expression in GABAergic neurons is triggered by the spikelets based on the functional emergence of voltage-independent sodium channels, which is driven by intensive VGSCs' activities.

Input-dependent subcellular localization of spike initiation between soma and axon at cortical pyramidal neurons

Background

Action potentials can be initiated at various subcellular compartments, such as axonal hillock, soma and dendrite. Mechanisms and physiological impacts for this relocation remain elusive, which may rely on input signal patterns and intrinsic properties in these subcellular compartments. We examined this hypothesis at the soma and axon of cortical pyramidal neurons by analyzing their spike capability and voltage-gated sodium channel dynamics in response to different input signals.

Results

Electrophysiological recordings were simultaneously conducted at the somata and axons of identical pyramidal neurons in the cortical slices. The somata dominantly produced sequential spikes in response to long-time steady depolarization pulse, and the axons produced more spikes in response to fluctuated pulse. Compared with the axons, the somata possessed lower spike threshold and shorter refractory periods in response to long-time steady depolarization, and somatic voltage-gated sodium channels demonstrated less inactivation and easier reactivation in response to steady depolarization. Based on local VGSC dynamics, computational simulated spike initiation locations were consistent with those from the experiments. In terms of physiological impact, this input-dependent plasticity of spike initiation location made neuronal encoding to be efficient.

Conclusions

Long-time steady depolarization primarily induces somatic spikes and short-time pulses induce axonal spikes. The input signal patterns influence spike initiations at the axon or soma of cortical pyramidal neurons through modulating local voltage-gated sodium channel dynamics.

Essential role of axonal VGSC inactivation in time-dependent deceleration and unreliability of spike propagation at cerebellar Purkinje cells

BACKGROUND

The output of the neuronal digital spikes is fulfilled by axonal propagation and synaptic transmission to influence postsynaptic cells. Similar to synaptic transmission, spike propagation on the axon is not secure, especially in cerebellar Purkinje cells whose spiking rate is high. The characteristics, mechanisms and physiological impacts of propagation deceleration and infidelity remain elusive. The spike propagation is presumably initiated by local currents that raise membrane potential to the threshold of activating voltage-gated sodium channels (VGSC).

RESULTS

We have investigated the natures of spike propagation and the role of VGSCs in this process by recording spikes simultaneously on the somata and axonal terminals of Purkinje cells in cerebellar slices. The velocity and fidelity of spike propagation decreased during long-lasting spikes, to which the velocity change was more sensitive than fidelity change. These time-dependent deceleration and infidelity of spike propagation were improved by facilitating axonal VGSC reactivation, and worsen by intensifying VGSC inactivation.

CONCLUSION

Our studies indicate that the functional status of axonal VGSCs is essential to influencing the velocity and fidelity of spike propagation.

Cortical GABAergic neurons are more severely impaired by alkalosis than acidosis

Background

Acid–base imbalance in various metabolic disturbances leads to human brain dysfunction. Compared with acidosis, the patients suffered from alkalosis demonstrate more severe neurological signs that are difficultly corrected. We hypothesize a causative process that the nerve cells in the brain are more vulnerable to alkalosis than acidosis.

Methods

The vulnerability of GABAergic neurons to alkalosis versus acidosis was compared by analyzing their functional changes in response to the extracellular high pH and low pH. The neuronal and synaptic functions were recorded by whole-cell recordings in the cortical slices.

Results

The elevation or attenuation of extracellular pH impaired these GABAergic neurons in terms of their capability to produce spikes, their responsiveness to excitatory synaptic inputs and their outputs via inhibitory synapses. Importantly, the dysfunction of these active properties appeared severer in alkalosis than acidosis.

Conclusions

The severer impairment of cortical GABAergic neurons in alkalosis patients leads to more critical neural excitotoxicity, so that alkalosis-induced brain dysfunction is difficultly corrected, compared to acidosis. The vulnerability of cortical GABAergic neurons to high pH is likely a basis of severe clinical outcomes in alkalosis versus acidosis.

Upregulation of excitatory neurons and downregulation of inhibitory neurons in barrel cortex are associated with loss of whisker inputs

Loss of a sensory input causes the hypersensitivity in other modalities. In addition to cross-modal plasticity, the sensory cortices without receiving inputs undergo the plastic changes. It is not clear how the different types of neurons and synapses in the sensory cortex coordinately change after input deficits in order to prevent loss of their functions and to be used for other modalities. We studied this subject in the barrel cortices from whiskers-trimmed mice vs. controls. After whisker trimming for a week, the intrinsic properties of pyramidal neurons and the transmission of excitatory synapses were upregulated in the barrel cortex, but inhibitory neurons and GABAergic synapses were downregulated. The morphological analyses indicated that the number of processes and spines in pyramidal neurons increased, whereas the processes of GABAergic neurons decreased in the barrel cortex. The upregulation of excitatory neurons and the downregulation of inhibitory neurons boost the activity of network neurons in the barrel cortex to be high levels, which prevent the loss of their functions and enhances their sensitivity to sensory inputs. These changes may prepare for attracting the innervations from sensory cortices and/or peripheral nerves for other modalities during cross-modal plasticity.

The functional upregulation of piriform cortex is associated with cross-modal plasticity in loss of whisker tactile inputs

BACKGROUND

Cross-modal plasticity is characterized as the hypersensitivity of remaining modalities after a sensory function is lost in rodents, which ensures their awareness to environmental changes. Cellular and molecular mechanisms underlying cross-modal sensory plasticity remain unclear. We aim to study the role of different types of neurons in cross-modal plasticity.

METHODOLOGY/PRINCIPAL FINDINGS

In addition to behavioral tasks in mice, whole-cell recordings at the excitatory and inhibitory neurons, and their two-photon imaging, were conducted in piriform cortex. We produced a mouse model of cross-modal sensory plasticity that olfactory function was upregulated by trimming whiskers to deprive their sensory inputs. In the meantime of olfactory hypersensitivity, pyramidal neurons and excitatory synapses were functionally upregulated, as well as GABAergic cells and inhibitory synapses were downregulated in piriform cortex from the mice of cross-modal sensory plasticity, compared with controls. A crosswire connection between barrel cortex and piriform cortex was established in cross-modal plasticity.

CONCLUSION/SIGNIFICANCE

An upregulation of pyramidal neurons and a downregulation of GABAergic neurons strengthen the activities of neuronal networks in piriform cortex, which may be responsible for olfactory hypersensitivity after a loss of whisker tactile input. This finding provides the clues for developing therapeutic strategies to promote sensory recovery and substitution.

Upregulation of transmitter release probability improves a conversion of synaptic analogue signals into neuronal digital spikes

Action potentials at the neurons and graded signals at the synapses are primary codes in the brain. In terms of their functional interaction, the studies were focused on the influence of presynaptic spike patterns on synaptic activities. How the synapse dynamics quantitatively regulates the encoding of postsynaptic digital spikes remains unclear. We investigated this question at unitary glutamatergic synapses on cortical GABAergic neurons, especially the quantitative influences of release probability on synapse dynamics and neuronal encoding. Glutamate release probability and synaptic strength are proportionally upregulated by presynaptic sequential spikes. The upregulation of release probability and the efficiency of probability-driven synaptic facilitation are strengthened by elevating presynaptic spike frequency and Ca²⁺. The upregulation of release probability improves spike capacity and timing precision at postsynaptic neuron. These results suggest that the upregulation of presynaptic glutamate release facilitates a conversion of synaptic analogue signals into digital spikes in postsynaptic neurons, i.e., a functional compatibility between presynaptic and postsynaptic partners.

mGluR₁,5 activation improves network asynchrony and GABAergic synapse attenuation in the amygdala: implication for anxiety-like behavior in DBA/2 mice

Anxiety is a prevalent psychological disorder, in which the atypical expression of certain genes and the abnormality of amygdala are involved. Intermediate processes between genetic defects and anxiety, pathophysiological characteristics of neural network, remain unclear. Using behavioral task, two-photon cellular imaging and electrophysiology, we studied the characteristics of neural networks in basolateral amygdala and the influences of metabotropic glutamate receptor (mGluR) on their dynamics in DBA/2 mice showing anxiety-related genetic defects. Amygdala neurons in DBA/2 high anxiety mice express asynchronous activity and diverse excitability, and their GABAergic synapses demonstrate weak transmission, compared to those in low anxiety FVB/N mice. mGluR_1,5 activation improves the anxiety-like behaviors of DBA/2 mice, synchronizes the activity of amygdala neurons and strengthens the transmission of GABAergic synapses. The activity asynchrony of amygdala neurons and the weakness of GABA synaptic transmission are associated with anxiety-like behavior.

Barrel cortical neurons and astrocytes coordinately respond to an increased whisker stimulus frequency

Background

Nerve cells program the brain codes to manage well-organized cognitions and behaviors. It remains unclear how a population of neurons and astrocytes work coordinately to encode their spatial and temporal activity patterns in response to frequency and intensity signals from sensory inputs.

Results

With two-photon imaging and electrophysiology to record cellular functions in the barrel cortex in vivo, we analyzed the activity patterns of neurons and astrocytes in response to whisker stimuli with increasing frequency, an environmental stimulus pattern that rodents experience in the accelerated motion. Compared to the resting state, whisker stimulation caused barrel neurons and astrocytes to be activated more synchronously. An increased stimulus frequency up-regulated the activity strength of neurons and astrocytes as well as coordinated their interaction. The coordination among the barrel neurons and astrocytes was fulfilled by increasing their functional connections.

Conclusions

Our study reveals that the nerve cells in the barrel cortex encode frequency messages in whisker tactile inputs through setting their activity coordination.

An impairment of cortical GABAergic neurons is involved in alkalosis-induced brain dysfunctions

Acid-base imbalance leads to pathological cognition and behaviors in the clinical practices. In the comparison with acidosis, the cellular mechanisms underlying alkalosis-induced brain dysfunction remain unclear. By using electrophysiological approach, we investigated the influences of high extracellular pH environment on cortical GABAergic neurons in terms of their responsiveness to synaptic inputs and their ability to produce action potentials. Artificial cerebral spinal fluid in high pH impairs excitatory synaptic transmission and spike initiation in cortical GABAergic neurons. The alkalosis-induced dysfunction of GABAergic neurons is associated with the decrease of receptor responsiveness and the increases of spike refractory periods and threshold potentials. Our studies reveal that alkalosis impairs cortical GABAergic neurons and subsequently deteriorate brain functions. The molecular targets for alkalosis action include glutamate receptor-channels and voltage-gated sodium channels on GABAergic neurons.

Quantal glutamate release is essential for reliable neuronal encodings in cerebral networks

Background

The neurons and synapses work coordinately to program the brain codes of controlling cognition and behaviors. Spike patterns at the presynaptic neurons regulate synaptic transmission. The quantitative regulations of synapse dynamics in spike encoding at the postsynaptic neurons remain unclear.

Methodology/Principal Findings

With dual whole-cell recordings at synapse-paired cells in mouse cortical slices, we have investigated the regulation of synapse dynamics to neuronal spike encoding at cerebral circuits assembled by pyramidal neurons and GABAergic ones. Our studies at unitary synapses show that postsynaptic responses are constant over time, such as glutamate receptor-channel currents at GABAergic neurons and glutamate transport currents at astrocytes, indicating quantal glutamate release. In terms of its physiological impact, our results demonstrate that the signals integrated from quantal glutamatergic synapses drive spike encoding at GABAergic neurons reliably, which in turn precisely set spike encoding at pyramidal neurons through feedback inhibition.

Conclusion/Significance

Our studies provide the evidences for the quantal glutamate release to drive the spike encodings precisely in cortical circuits, which may be essential for programming the reliable codes in the brain to manage well-organized behaviors.