GAD67-positive puncta: contributors to learning-dependent plasticity in the barrel cortex of adult mice

Mitigation of aging-related plasticity decline through taurine supplementation and environmental enrichment

Aging-related biochemical changes in nerve cells lead to dysfunctional synapses and disrupted neuronal circuits, ultimately affecting vital processes such as brain plasticity, learning, and memory. The imbalance between excitation and inhibition in synaptic function during aging contributes to cognitive impairment, emphasizing the importance of compensatory mechanisms. Fear conditioning-related plasticity of the somatosensory barrel cortex, relying on the proper functioning and extensive up regulation of the GABAergic system, in particular interneurons containing somatostatin, is compromised in aging (one-year-old) mice. The present research explores two potential interventions, taurine supplementation, and environmental enrichment, revealing their effectiveness in supporting learning-induced plasticity in the aging mouse brain. They do not act through a mechanism normalizing the Glutamate/GABA balance that is disrupted in aging. Still, they allow for increased somatostatin levels, an effect observed in young animals after learning. These findings highlight the potential of lifestyle interventions and diet supplementation to mitigate age-related cognitive decline by promoting experience-dependent plasticity.

Basolateral amygdala activation enhances object recognition memory by inhibiting anterior insular cortex activity

Noradrenergic activation of the basolateral amygdala (BLA) by emotional arousal enhances different forms of recognition memory via functional interactions with the insular cortex (IC). Human neuroimaging studies have revealed that the anterior IC (aIC), as part of the salience network, is dynamically regulated during arousing situations. Emotional stimulation first rapidly increases aIC activity but suppresses it in a delayed fashion. Here, we investigated in male Sprague-Dawley rats whether the BLA influence on recognition memory is associated with an increase or suppression of aIC activity during the postlearning consolidation period. We first employed anterograde and retrograde viral tracing and found that the BLA sends dense monosynaptic projections to the aIC. Memory-enhancing norepinephrine administration into the BLA following an object training experience suppressed aIC activity 1 h later, as determined by a reduced expression of the phosphorylated form of the transcription factor cAMP response element-binding (pCREB) protein and neuronal activity marker c-Fos. In contrast, the number of perisomatic γ-aminobutyric acid (GABA)ergic inhibitory synapses per pCREB-positive neuron was significantly increased, suggesting a dynamic up-regulation of GABAergic tone. In support of this possibility, pharmacological inhibition of aIC activity with a GABAergic agonist during consolidation enhanced object recognition memory. Norepinephrine administration into the BLA did not affect neuronal activity within the posterior IC, which receives sparse innervation from the BLA. The evidence that noradrenergic activation of the BLA enhances the consolidation of object recognition memory via a mechanism involving a suppression of aIC activity provides insight into the broader brain network dynamics underlying emotional regulation of memory.

Glucocorticoid enhancement of recognition memory via basolateral amygdala-driven facilitation of prelimbic cortex interactions

Extensive evidence indicates that the basolateral amygdala (BLA) interacts with other brain regions in mediating stress hormone and emotional arousal effects on memory consolidation. Brain activation studies have shown that arousing conditions lead to the activation of large-scale neural networks and several functional connections between brain regions beyond the BLA. Whether such distal interactions on memory consolidation also depend on BLA activity is not as yet known. We investigated, in male Sprague-Dawley rats, whether BLA activity enables prelimbic cortex (PrL) interactions with the anterior insular cortex (aIC) and dorsal hippocampus (dHPC) in regulating glucocorticoid effects on different components of object recognition memory. The glucocorticoid receptor (GR) agonist RU 28362 administered into the PrL, but not infralimbic cortex, immediately after object recognition training enhanced 24-hour memory of both the identity and location of the object via functional interactions with the aIC and dHPC, respectively. Importantly, posttraining inactivation of the BLA by the noradrenergic antagonist propranolol abolished the effect of GR agonist administration into the PrL on memory enhancement of both the identity and location of the object. BLA inactivation by propranolol also blocked the effect of GR agonist administration into the PrL on inducing changes in neuronal activity within the aIC and dHPC during the postlearning consolidation period as well as on structural changes in spine morphology assessed 24 hours later. These findings provide evidence that BLA noradrenergic activity enables functional interactions between the PrL and the aIC and dHPC in regulating stress hormone and emotional arousal effects on memory.

The space where aging acts: focus on the GABAergic synapse

As it was established that aging is not associated with massive neuronal loss, as was believed in the mid‐20th Century, scientific interest has addressed the influence of aging on particular neuronal subpopulations and their synaptic contacts, which constitute the substrate for neural plasticity. Inhibitory neurons represent the most complex and diverse group of neurons, showing distinct molecular and physiological characteristics and possessing a compelling ability to control the physiology of neural circuits. This review focuses on the aging of GABAergic neurons and synapses. Understanding how aging affects synapses of particular neuronal subpopulations may help explain the heterogeneity of aging‐related effects. We reviewed the literature concerning the effects of aging on the numbers of GABAergic neurons and synapses as well as aging‐related alterations in their presynaptic and postsynaptic components. Finally, we discussed the influence of those changes on the plasticity of the GABAergic system, highlighting our results concerning aging in mouse somatosensory cortex and linking them to plasticity impairments and brain disorders. We posit that aging‐induced impairments of the GABAergic system lead to an inhibitory/excitatory imbalance, thereby decreasing neuron's ability to respond with plastic changes to environmental and cellular challenges, leaving the brain more vulnerable to cognitive decline and damage by synaptopathic diseases.

Activity-dependent axonal plasticity in sensory systems

The rodent whisker-to-barrel cortex pathway is a classic model to study the effects of sensory experience and deprivation on neuronal circuit formation, not only during development but also in the adult. Decades of research have produced a vast body of evidence highlighting the fundamental role of neuronal activity (spontaneous and/or sensory-evoked) for circuit formation and function. In this context, it has become clear that neuronal adaptation and plasticity is not just a function of the neonatal brain, but persists into adulthood, especially after experience-driven modulation of network status. Mechanisms for structural remodeling of the somatodendritic or axonal domain include microscale alterations of neurites or synapses. At the same time, functional alterations at the nanoscale such as expression or activation changes of channels and receptors contribute to the modulation of intrinsic excitability or input-output relationships. However, it remains elusive how these forms of structural and functional plasticity come together to shape neuronal network formation and function. While specifically somatodendritic plasticity has been studied in great detail, the role of axonal plasticity, (e.g. at presynaptic boutons, branches or axonal microdomains), is rather poorly understood. Therefore, this review will only briefly highlight somatodendritic plasticity and instead focus on axonal plasticity. We discuss (i) the role of spontaneous and sensory-evoked plasticity during critical periods, (ii) the assembly of axonal presynaptic sites, (iii) axonal plasticity in the mature brain under baseline and sensory manipulation conditions, and finally (iv) plasticity of electrogenic axonal microdomains, namely the axon initial segment, during development and in the mature CNS.

Spike Timing-Dependent Plasticity in the Mouse Barrel Cortex Is Strongly Modulated by Sensory Learning and Depends on Activity of Matrix Metalloproteinase 9

Experience and learning in adult primary somatosensory cortex are known to affect neuronal circuits by modifying both excitatory and inhibitory transmission. Synaptic plasticity phenomena provide a key substrate for cognitive processes, but precise description of the cellular and molecular correlates of learning is hampered by multiplicity of these mechanisms in various projections and in different types of neurons. Herein, we investigated the impact of associative learning on neuronal plasticity in distinct types of postsynaptic neurons by checking the impact of classical conditioning (pairing whisker stroking with tail shock) on the spike timing-dependent plasticity (t-LTP and t-LTD) in the layer IV to II/III vertical pathway of the mouse barrel cortex. Learning in this paradigm practically prevented t-LTP measured in pyramidal neurons but had no effect on t-LTD. Since classical conditioning is known to affect inhibition in the barrel cortex, we examined its effect on tonic GABAergic currents and found a strong downregulation of these currents in the layer II/III interneurons but not in pyramidal cells. Matrix metalloproteinases emerged as crucial players in synaptic plasticity and learning. We report that the blockade of MMP-9 (but not MMP-3) abolished t-LTP having no effect on t-LTD. Moreover, associative learning resulted in an upregulation of gelatinolytic activity within the "trained" barrel. We conclude that LTP induced by spike timing-dependent plasticity (STDP) paradigm is strongly correlated with associative learning and critically depends on the activity of MMP-9.

Somatostatin-Expressing Inhibitory Interneurons in Cortical Circuits

Cortical inhibitory neurons exhibit remarkable diversity in their morphology, connectivity, and synaptic properties. Here, we review the function of somatostatin-expressing (SOM) inhibitory interneurons, focusing largely on sensory cortex. SOM neurons also comprise a number of subpopulations that can be distinguished by their morphology, input and output connectivity, laminar location, firing properties, and expression of molecular markers. Several of these classes of SOM neurons show unique dynamics and characteristics, such as facilitating synapses, specific axonal projections, intralaminar input, and top-down modulation, which suggest possible computational roles. SOM cells can be differentially modulated by behavioral state depending on their class, sensory system, and behavioral paradigm. The functional effects of such modulation have been studied with optogenetic manipulation of SOM cells, which produces effects on learning and memory, task performance, and the integration of cortical activity. Different classes of SOM cells participate in distinct disinhibitory circuits with different inhibitory partners and in different cortical layers. Through these disinhibitory circuits, SOM cells help encode the behavioral relevance of sensory stimuli by regulating the activity of cortical neurons based on subcortical and intracortical modulatory input. Associative learning leads to long-term changes in the strength of connectivity of SOM cells with other neurons, often influencing the strength of inhibitory input they receive. Thus despite their heterogeneity and variability across cortical areas, current evidence shows that SOM neurons perform unique neural computations, forming not only distinct molecular but also functional subclasses of cortical inhibitory interneurons.

Learning-Dependent Plasticity of the Barrel Cortex Is Impaired by Restricting GABA-Ergic Transmission

Experience-induced plastic changes in the cerebral cortex are accompanied by alterations in excitatory and inhibitory transmission. Increased excitatory drive, necessary for plasticity, precedes the occurrence of plastic change, while decreased inhibitory signaling often facilitates plasticity. However, an increase of inhibitory interactions was noted in some instances of experience-dependent changes. We previously reported an increase in the number of inhibitory markers in the barrel cortex of mice after fear conditioning engaging vibrissae, observed concurrently with enlargement of the cortical representational area of the row of vibrissae receiving conditioned stimulus (CS). We also observed that an increase of GABA level accompanied the conditioning. Here, to find whether unaltered GABAergic signaling is necessary for learning-dependent rewiring in the murine barrel cortex, we locally decreased GABA production in the barrel cortex or reduced transmission through GABAA receptors (GABAARs) at the time of the conditioning. Injections of 3-mercaptopropionic acid (3-MPA), an inhibitor of glutamic acid decarboxylase (GAD), into the barrel cortex prevented learning-induced enlargement of the conditioned vibrissae representation. A similar effect was observed after injection of gabazine, an antagonist of GABAARs. At the behavioral level, consistent conditioned response (cessation of head movements in response to CS) was impaired. These results show that appropriate functioning of the GABAergic system is required for both manifestation of functional cortical representation plasticity and for the development of a conditioned response.

Increases in the numerical density of GAT-1 positive puncta in the barrel cortex of adult mice after fear conditioning

Three days of fear conditioning that combines tactile stimulation of a row of facial vibrissae (conditioned stimulus, CS) with a tail shock (unconditioned stimulus, UCS) expands the representation of “trained” vibrissae, which can be demonstrated by labeling with 2-deoxyglucose in layer IV of the barrel cortex. We have also shown that functional reorganization of the primary somatosensory cortex (S1) increases GABAergic markers in the hollows of “trained” barrels of the adult mouse. This study investigated how whisker-shock conditioning (CS+UCS) affected the expression of puncta of a high-affinity GABA plasma membrane transporter GAT-1 in the barrel cortex of mice 24 h after associative learning paradigm. We found that whisker-shock conditioning (CS+UCS) led to increase expression of neuronal and astroglial GAT-1 puncta in the “trained” row compared to controls: Pseudoconditioned, CS-only, UCS-only and Naïve animals. These findings suggest that fear conditioning specifically induces activation of systems regulating cellular levels of the inhibitory neurotransmitter GABA.

Interneurons containing somatostatin are affected by learning-induced cortical plasticity

The maintenance of neural circuit stability is a dynamic process that requires the plasticity of many cellular and synaptic components. By changing the excitatory/inhibitory balance, inhibitory GABAergic plasticity can regulate excitability, and contribute to neural circuit function and refinement in learning and memory. Increased inhibitory GABAergic neurotransmission has been shown in brain structures involved in the learning process. Previously, we showed that classical conditioning in which tactile stimulation of one row of vibrissae (conditioned stimulus, CS) was paired with a tail shock (unconditioned stimulus, UCS) in adult mice results in the increased density of GABAergic interneurons and increased expression of glutamic acid decarboxylase (GAD)-67 in barrels of the "trained" row cortical representation. In inhibitory neurons of the rat cortex GAD co-localizes with several proteins and peptides. We found previously that the density of the parvalbumin (GAD+/Prv+)-containing subpopulation is not changed after conditioning. In the present study, we examined GABAergic somatostatin (Som)-, calbindin (CB)- and calretinin (CR)-positive interneurons in the cortical representation of "trained" vibrissae after training. Cells showing double immunostaining for GAD/Som, GAD/CR and GAD/CB were counted in the barrels representing vibrissae activated during the training and in control, untouched rows. We found a substantial increase of GAD/Som-containing cells in the trained row representation. No changes in the density of GAD/CR or GAD/CB neurons were observed. These results suggest that Som-containing interneurons are involved in learning-induced changes in the inhibitory cortical network.

Mechanisms underlying rule learning-induced enhancement of excitatory and inhibitory synaptic transmission

Training rats to perform rapidly and efficiently in an olfactory discrimination task results in robust enhancement of excitatory and inhibitory synaptic connectivity in the rat piriform cortex, which is maintained for days after training. To explore the mechanisms by which such synaptic enhancement occurs, we recorded spontaneous miniature excitatory and inhibitory synaptic events in identified piriform cortex neurons from odor-trained, pseudo-trained, and naive rats. We show that olfactory discrimination learning induces profound enhancement in the averaged amplitude of AMPA receptor-mediated miniature synaptic events in piriform cortex pyramidal neurons. Such physiological modifications are apparent at least 4 days after learning completion and outlast learning-induced modifications in the number of spines on these neurons. Also, the averaged amplitude of GABAA receptor-mediated miniature inhibitory synaptic events was significantly enhanced following odor discrimination training. For both excitatory and inhibitory transmission, an increase in miniature postsynaptic current amplitude was evident in most of the recorded neurons; however, some neurons showed an exceptionally great increase in the amplitude of miniature events. For both excitatory and inhibitory transmission, the frequency of spontaneous synaptic events was not modified after learning. These results suggest that olfactory discrimination learning-induced enhancement of synaptic transmission in cortical neurons is mediated by postsynaptic modulation of AMPA receptor-dependent currents and balanced by long-lasting modulation of postsynaptic GABAA receptor-mediated currents.

Agmatinase, an inactivator of the putative endogenous antidepressant agmatine, is strongly upregulated in hippocampal interneurons of subjects with mood disorders

The diamine agmatine may serve as a precursor in polyamine synthesis. In addition, agmatine may also act as a neurotransmitter, binding to imidazoline receptors. Behaviorally, agmatine exerts antidepressant-like effects. The enzyme agmatinase degrades agmatine. The gene coding for human agmatinase is located on chromosome 1p36, a gene locus which has been linked to bipolar disorder and major depression, but the enzyme has not yet been studied in the context of neuropsychiatric diseases. We analyzed agmatinase protein expression in postmortem hippocampi of individuals with affective disorders. Data from eleven patients with mood disorders (unipolar and bipolar depression) and twelve matched control cases were compared by immunocytochemical and morphometrical analysis. Agmatinase protein was detected in a subset of hippocampal interneurons. The protein was localized to perikarya, neurites and putative nerve endings contacting hippocampal pyramidal neurons and dentate gyrus granule cells. The number and the numerical density of agmatinase-immunopositive cell bodies were strongly elevated in depressive patients. In addition, a significantly increased density of agmatinase-immunoreactive punctate profiles was observed in the CA(4) region in unipolar and bipolar depression. The reported increased expression of agmatinase suggests a functional relevance of the enzyme in the pathophysiology of human affective disorders. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Retrieval of conditioned fear activates the basolateral and intercalated nucleus of amygdala

The amygdala is one of the crucial brain structures for conditioned fear, in which conditioned stimuli are received by the basolateral nucleus of the amygdala (BLA), inducing a fear reaction via the central nucleus of the amygdala (CeA). Whereas BLA sends glutamatergic projections into CeA, the intercalated nucleus of the amygdala (ITC) sends GABAergic projections into CeA, which is doubly regulated by BLA and ITC. In the present study, we investigated the characteristics of the neural cells activated by retrieval of conditioned fear in BLA and ITC using immunohistochemistry, in situ hybridization, and Western blot analysis of transcription factors and neural cell markers. Because most conditioned fear-induced c-Fos-positive cells in BLA were glutaminase positive and 67-kDa isomer of glutamic acid decarboxylase (GAD67) negative, these cells are speculated to be glutamatergic. Seventy-eight percent of the phosphorylated CREB (pCREB)-positive cells were glutaminase double positive and 13% of the pCREB-positive cells were GAD67 double positive, indicating that many of the conditioned fear-induced pCREB-positive cells in BLA were glutamatergic, but at least some of the pCREB-positive cells were GABAergic. These results suggested that CREB phosphorylation was increased both in glutamatergic and in GABAergic neurons, but c-Fos expression was increased mainly in glutamatergic neurons in BLA. CREB phosphorylation but not c-Fos expression in ITC was specifically increased by retrieval of conditioned fear. It is therefore speculated that ITC GABAergic neurons were activated by retrieval of conditioned fear and that transcription factors other than c-Fos were relevant to the activation.

Forebrain ischemia triggers GABAergic system degeneration in substantia nigra at chronic stages in rats

The long-term consequences of forebrain ischemia include delayed Parkinson's syndrome. This study revealed delayed neurodegeneration in the substantia nigra 8 weeks after 12.5 minutes of global ischemia in rat brain. Following neuronal loss of 30-40% in central and dorsolateral striatum at day 3, neuronal damage in the substantia nigra (SN) was assessed at 4-8 weeks using immunohistochemistry for glutamate decarboxylase 67 (GAD67), vesicular GABA transporter (VGAT), and calretinin (CR). At day 56, the optical density of GAD67-, but not VGAT-, immunoreactivity in substantia nigra pars reticulata (SNR)-significantly decreased. CR-neurons concentrated in substantia nigra pars compacta (SNC) were reduced by 27% from day 3 (n = 5) to day 56 (n = 7, ANOVA, p < .01). Movement coordination was impaired at day 56, as evaluated using beam-walking test (time-to-traverse 5.6 ± 1.2 sec versus 11.8 ± 5.4 sec; sham versus ischemia, p < .05, n = 5, and 7, resp.). Our results demonstrate delayed impairment of the GABAergic system components in SN and associated with movement deficits after global ischemia.

Sensory learning differentially affects GABAergic tonic currents in excitatory neurons and fast spiking interneurons in layer 4 of mouse barrel cortex

Pairing tactile stimulation of whiskers with a tail shock is known to result in expansion of cortical representation of stimulated vibrissae and in the increase in synaptic GABAergic transmission. However, the impact of such sensory learning in classical conditioning paradigm on GABAergic tonic currents has not been addressed. To this end, we performed whole cell patch-clamp slice recordings of tonic currents from neurons (excitatory regular spiking, regular spiking nonpyramidal, and fast spiking interneurons) of layer 4 of the barrel cortex from naive and trained mice. Interestingly, endogenous tonic GABAergic currents measured from the excitatory neurons in the cortical representation of "trained" vibrissae were larger than in the "naïve" or pseudoconditioned ones. On the contrary, sensory learning markedly reduced tonic currents in the fast spiking interneurons but not in regular spiking nonpyramidal neurons. Changes of tonic currents were accompanied by changes in the input resistances-decrease in regular spiking and increase in fast spiking neurons, respectively. Applications of nipecotic acid, a GABA uptake blocker, enhanced the tonic currents, but the impact of the sensory learning remained qualitatively the same as in the case of the tonic currents. Similar to endogenous tonic currents, sensory learning enhanced currents induced by THIP (superagonist for delta subunit-containing GABA(A) receptors) in regular spiking neurons, whereas the opposite was observed for the fast spiking interneurons. In conclusion, our data show that the sensory learning strongly affects the GABAergic tonic currents in a cell-specific manner and suggest that the underlying mechanism involves regulation of expression of delta subunit-containing GABA(A) receptors.

Rapid, learning-induced inhibitory synaptogenesis in murine barrel field

The structure of neurons changes during development and in response to injury or alteration in sensory experience. Changes occur in the number, shape, and dimensions of dendritic spines together with their synapses. However, precise data on these changes in response to learning are sparse. Here, we show using quantitative transmission electron microscopy that a simple form of learning involving mystacial vibrissae results in approximately 70% increase in the density of inhibitory synapses on spines of neurons located in layer IV barrels that represent the stimulated vibrissae. The spines contain one asymmetrical (excitatory) and one symmetrical (inhibitory) synapse (double-synapse spines), and their density increases threefold as a result of learning with no apparent change in the density of asymmetrical synapses. This effect seems to be specific for learning because pseudoconditioning (in which the conditioned and unconditioned stimuli are delivered at random) does not lead to the enhancement of symmetrical synapses but instead results in an upregulation of asymmetrical synapses on spines. Symmetrical synapses of cells located in barrels receiving the conditioned stimulus also show a greater concentration of GABA in their presynaptic terminals. These results indicate that the immediate effect of classical conditioning in the "conditioned" barrels is rapid, pronounced, and inhibitory.

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

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