Conditioning and pseudoconditioning differently change intrinsic excitability of inhibitory interneurons in the neocortex

Many studies indicate a broad role of various classes of GABAergic interneurons in the processes related to learning. However, little is known about how the learning process affects intrinsic excitability of specific classes of interneurons in the neocortex. To determine this, we employed a simple model of conditional learning in mice where vibrissae stimulation was used as a conditioned stimulus and a tail shock as an unconditioned one. In vitro whole-cell patch-clamp recordings showed an increase in intrinsic excitability of low-threshold spiking somatostatin-expressing interneurons (SST-INs) in layer 4 (L4) of the somatosensory (barrel) cortex after the conditioning paradigm. In contrast, pseudoconditioning reduced intrinsic excitability of SST-LTS, parvalbumin-expressing interneurons (PV-INs), and vasoactive intestinal polypeptide-expressing interneurons (VIP-INs) with accommodating pattern in L4 of the barrel cortex. In general, increased intrinsic excitability was accompanied by narrowing of action potentials (APs), whereas decreased intrinsic excitability coincided with AP broadening. Altogether, these results show that both conditioning and pseudoconditioning lead to plastic changes in intrinsic excitability of GABAergic interneurons in a cell-specific manner. In this way, changes in intrinsic excitability can be perceived as a common mechanism of learning-induced plasticity in the GABAergic system.

Astrocytic β-catenin signaling via TCF7L2 regulates synapse development and social behavior

The Wnt/β-catenin pathway contains multiple high-confidence risk genes that are linked to neurodevelopmental disorders, including autism spectrum disorder. However, its ubiquitous roles across brain cell types and developmental stages have made it challenging to define its impact on neural circuit development and behavior. Here, we show that TCF7L2, which is a key transcriptional effector of the Wnt/β-catenin pathway, plays a cell-autonomous role in postnatal astrocyte maturation and impacts adult social behavior. TCF7L2 was the dominant Wnt effector that was expressed in both mouse and human astrocytes, with a peak during astrocyte maturation. The conditional knockout of Tcf7l2 in postnatal astrocytes led to an enlargement of astrocytes with defective tiling and gap junction coupling. These mice also exhibited an increase in the number of cortical excitatory and inhibitory synapses and a marked increase in social interaction by adulthood. These data reveal an astrocytic role for developmental Wnt/β-catenin signaling in restricting excitatory synapse numbers and regulating adult social behavior.

GABAAR-mediated tonic inhibition differentially modulates intrinsic excitability of VIP- and SST- expressing interneurons in layers 2/3 of the somatosensory cortex

Extrasynaptic GABAA receptors (GABAARs) mediating tonic inhibition are thought to play an important role in the regulation of neuronal excitability. However, little is known about a cell type-specific tonic inhibition in molecularly distinctive types of GABAergic interneurons in the mammalian neocortex. Here, we used whole-cell patch-clamp techniques in brain slices prepared from transgenic mice expressing red fluorescent protein (TdTomato) in vasoactive intestinal polypeptide- or somatostatin- positive interneurons (VIP-INs and SST-INs, respectively) to investigate tonic and phasic GABAAR-mediated inhibition as well as effects of GABAA inhibition on intrinsic excitability of these interneurons in layers 2/3 (L2/3) of the somatosensory (barrel) cortex. We found that tonic inhibition was stronger in VIP-INs compared to SST-INs. Contrary to the literature data, tonic inhibition in SST-INs was comparable to pyramidal (Pyr) neurons. Next, tonic inhibition in both interneuron types was dependent on the activity of delta subunit-containing GABAARs. Finally, the GABAAR activity decreased intrinsic excitability of VIP-INs but not SST-INs. Altogether, our data indicate that GABAAR-mediated inhibition modulates neocortical interneurons in a type-specific manner. In contrast to L2/3 VIP-INs, intrinsic excitability of L2/3 SST-INs is immune to the GABAAR-mediated inhibition.

Somatostatin-expressing interneurons modulate neocortical network through GABAb receptors in a synapse-specific manner

The firing activity of somatostatin-expressing inhibitory neurons (SST-INs) can suppress network activity via both GABAa and GABAb receptors (Rs). Although SST-INs do not receive GABAaR input from other SST-INs, it is possible that SST-IN-released GABA could suppress the activity of SST-INs themselves via GABAbRs, providing a negative feedback loop. Here we characterized the influence of GABAbR modulation on SST-IN activity in layer 2/3 of the somatosensory cortex in mice. We compared this to the effects of GABAbR activation on parvalbumin-expressing interneurons (PV-INs). Using in vitro whole-cell patch clamp recordings, pharmacological and optogenetic manipulations, we found that the firing activity of SST-INs suppresses excitatory drive to themselves via presynaptic GABAbRs. Postsynaptic GABAbRs did not influence SST-IN spontaneous activity or intrinsic excitability. Although GABAbRs at pre- and postsynaptic inputs to PV-INs are modestly activated during cortical network activity in vitro, the spontaneous firing of SST-INs was not the source of GABA driving this GABAbR activation. Thus, SST-IN firing regulates excitatory synaptic strength through presynaptic GABAbRs at connections between pyramidal neurons (Pyr-Pyr) and synapses between pyramidal neurons and SST-INs (Pyr-SST), but not Pyr-PV and PV-Pyr synapses. Our study indicates that two main types of neocortical inhibitory interneurons are differentially modulated by SST-IN-mediated GABA release.

FosGFP expression does not capture a sensory learning-related engram in superficial layers of mouse barrel cortex

Immediate-early gene (IEG) expression has been used to identify small neural ensembles linked to a particular experience, based on the principle that a selective subset of activated neurons will encode specific memories or behavioral responses. The majority of these studies have focused on "engrams" in higher-order brain areas where more abstract or convergent sensory information is represented, such as the hippocampus, prefrontal cortex, or amygdala. In primary sensory cortex, IEG expression can label neurons that are responsive to specific sensory stimuli, but experience-dependent shaping of neural ensembles marked by IEG expression has not been demonstrated. Here, we use a fosGFP transgenic mouse to longitudinally monitor in vivo expression of the activity-dependent gene c-fos in superficial layers (L2/3) of primary somatosensory cortex (S1) during a whisker-dependent learning task. We find that sensory association training does not detectably alter fosGFP expression in L2/3 neurons. Although training broadly enhances thalamocortical synaptic strength in pyramidal neurons, we find that synapses onto fosGFP+ neurons are not selectively increased by training; rather, synaptic strengthening is concentrated in fosGFP- neurons. Taken together, these data indicate that expression of the IEG reporter fosGFP does not facilitate identification of a learning-specific engram in L2/3 in barrel cortex during whisker-dependent sensory association learning.

Precisely Timed Nicotinic Activation Drives SST Inhibition in Neocortical Circuits

Sleep, waking, locomotion, and attention are associated with cell-type-specific changes in neocortical activity. The effect of brain state on circuit output requires understanding of how neuromodulators influence specific neuronal classes and their synapses, with normal patterns of neuromodulator release from endogenous sources. We investigated the state-dependent modulation of a ubiquitous feedforward inhibitory motif in mouse sensory cortex, local pyramidal (Pyr) inputs onto somatostatin (SST)-expressing interneurons. Paired whole-cell recordings in acute brain slices and in vivo showed that Pyr-to-SST synapses are remarkably weak, with failure rates approaching 80%. Pharmacological screening revealed that cholinergic agonists uniquely enhance synaptic efficacy. Brief, optogenetically gated acetylcholine release dramatically enhanced Pyr-to-SST input, via nicotinic receptors and presynaptic PKA signaling. Importantly, endogenous acetylcholine release preferentially activated nicotinic, not muscarinic, receptors, thus differentiating drug effects from endogenous neurotransmission. Brain state- and synapse-specific unmasking of synapses may be a powerful way to functionally rewire cortical circuits dependent on behavioral demands.

POm Thalamocortical Input Drives Layer-Specific Microcircuits in Somatosensory Cortex

Higher-order thalamic nuclei, such as the posterior medial nucleus (POm) in the somatosensory system or the pulvinar in the visual system, densely innervate the cortex and can influence perception and plasticity. To systematically evaluate how higher-order thalamic nuclei can drive cortical circuits, we investigated cell-type selective responses to POm stimulation in mouse primary somatosensory (barrel) cortex, using genetically targeted whole-cell recordings in acute brain slices. We find that ChR2-evoked thalamic input selectively targets specific cell types in the neocortex, revealing layer-specific modules for the summation and processing of POm input. Evoked activity in pyramidal neurons from deep layers is fast and synchronized by rapid feedforward inhibition from GABAergic parvalbumin-expressing neurons, and activity in superficial layers is weaker and prolonged, facilitated by slow inhibition from GABAergic neurons expressing the 5HT3a receptor. Somatostatin-expressing GABAergic neurons do not receive direct input in either layer and their spontaneous activity is suppressed during POm stimulation. This novel pattern of weak, delayed, thalamus-evoked inhibition in layer 2 suggests a longer integration window for incoming sensory information and may facilitate stimulus detection and plasticity in superficial pyramidal neurons.

Somatostatin and Somatostatin-Containing Neurons in Shaping Neuronal Activity and Plasticity

Since its discovery over four decades ago, somatostatin (SOM) receives growing scientific and clinical interest. Being localized in the nervous system in a subset of interneurons somatostatin acts as a neurotransmitter or neuromodulator and its role in the fine-tuning of neuronal activity and involvement in synaptic plasticity and memory formation are widely recognized in the recent literature. Combining transgenic animals with electrophysiological, anatomical and molecular methods allowed to characterize several subpopulations of somatostatin-containing interneurons possessing specific anatomical and physiological features engaged in controlling the output of cortical excitatory neurons. Special characteristic and connectivity of somatostatin-containing neurons set them up as significant players in shaping activity and plasticity of the nervous system. However, somatostatin is not just a marker of particular interneuronal subpopulation. Somatostatin itself acts pre- and postsynaptically, modulating excitability and neuronal responses. In the present review, we combine the knowledge regarding somatostatin and somatostatin-containing interneurons, trying to incorporate it into the current view concerning the role of the somatostatinergic system in cortical plasticity.

Neocortical somatostatin neurons reversibly silence excitatory transmission via GABAb receptors

BACKGROUND

Understanding the dynamic range for excitatory transmission is a critical component of building a functional circuit diagram for the mammalian brain. Excitatory synaptic transmission is typically studied under optimized conditions, when background activity in the network is low. The range of synaptic function in the presence of inhibitory and excitatory activity within the neocortical circuit is unknown.

RESULTS

Paired-cell recordings from pyramidal neurons in acute brain slices of mouse somatosensory cortex show that excitatory synaptic transmission is markedly suppressed during spontaneous network activity: EPSP amplitudes are 2-fold smaller and failure rates are greater than 50%. This suppression is mediated by tonic activation of presynaptic GABAb receptors gated by the spontaneous activity of somatostatin-expressing (Sst) interneurons. Optogenetic suppression of Sst neuron firing was sufficient to enhance EPSP amplitude and reduce failure rates, effects that were fully reversible and occluded by GABAb antagonists.

CONCLUSIONS

These data indicate that Sst interneurons can rapidly and reversibly silence excitatory synaptic connections through the regulation of presynaptic release. This is an unanticipated role for Sst interneurons, which have been assigned a role only in fast GABAa-mediated inhibition. Because Sst interneuron activity has been shown to be regulated by sensory and motor input, these results suggest a mechanism by which functional connectivity and synaptic plasticity could be gated in a state-dependent manner.

Experience-dependent regulation of presynaptic NMDARs enhances neurotransmitter release at neocortical synapses

Sensory experience can selectively alter excitatory synaptic strength at neocortical synapses. The rapid increase in synaptic strength induced by selective whisker stimulation (single-row experience/SRE, where all but one row of whiskers has been removed from the mouse face) is due, at least in part, to the trafficking of AMPA receptors (AMPARs) to the post-synaptic membrane, and is developmentally regulated. How enhanced sensory experience can alter presynaptic release properties in the developing neocortex has not been investigated. Using paired-pulse stimulation at layer 4-2/3 synapses in acute brain slices, we found that presynaptic release probability progressively increases in the spared-whisker barrel column over the first 24 h of SRE. Enhanced release probability can be at least partly attributed to presynaptic NMDA receptors (NMDARs). We find that the influence of presynaptic NMDARs in enhancing EPSC amplitude markedly increases during SRE. This occurs at the same time when recently potentiated synapses become highly susceptible to a NMDAR-dependent form of synaptic depression, during the labile phase of plasticity. Thus, these data show that augmented sensory stimulation can enhance release probability at layer 4-2/3 synapses and enhance the function of presynaptic NMDARs. Because presynaptic NMDARs have been linked to synaptic depression at layer 4-2/3 synapses, we propose that SRE-dependent up-regulation of presynaptic NMDARs is responsible for enhanced synaptic depression during the labile stage of plasticity.

Differential wiring of layer 2/3 neurons drives sparse and reliable firing during neocortical development

Sensory information is transmitted with high fidelity across multiple synapses until it reaches the neocortex. There, individual neurons exhibit enormous variability in responses. The source of this diversity in output has been debated. Using transgenic mice expressing the green fluorescent protein coupled to the activity-dependent gene c-fos, we identified neurons with a history of elevated activity in vivo. Focusing on layer 4 to layer 2/3 connections, a site of strong excitatory drive at an initial stage of cortical processing, we find that fluorescently tagged neurons receive significantly greater excitatory and reduced inhibitory input compared with neighboring, unlabeled cells. Differential wiring of layer 2/3 neurons arises early in development and requires sensory input to be established. Stronger connection strength is not associated with evidence for recent synaptic plasticity, suggesting that these more active ensembles may not be generated over short time scales. Paired recordings show fosGFP+ neurons spike at lower stimulus thresholds than neighboring, fosGFP- neurons. These data indicate that differences in circuit construction can underlie response heterogeneity amongst neocortical neurons.

The brain-specific Beta4 subunit downregulates BK channel cell surface expression

The large-conductance K⁺ channel (BK channel) can control neural excitability, and enhanced channel currents facilitate high firing rates in cortical neurons. The brain-specific auxiliary subunit β4 alters channel Ca⁺⁺- and voltage-sensitivity, and β4 knock-out animals exhibit spontaneous seizures. Here we investigate β4's effect on BK channel trafficking to the plasma membrane. Using a novel genetic tag to track the cellular location of the pore-forming BKα subunit in living cells, we find that β4 expression profoundly reduces surface localization of BK channels via a C-terminal ER retention sequence. In hippocampal CA3 neurons from C57BL/6 mice with endogenously high β4 expression, whole-cell BK channel currents display none of the characteristic properties of BKα+β4 channels observed in heterologous cells. Finally, β4 knock-out animals exhibit a 2.5-fold increase in whole-cell BK channel current, indicating that β4 also regulates current magnitude in vivo. Thus, we propose that a major function of the brain-specific β4 subunit in CA3 neurons is control of surface trafficking.

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.

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.

Effects of sensory learning on intracortical synaptic transmission in the barrel cortex of mice

Pairing tactile stimulation of a row of whiskers with a tail shock results in an expansion of the functional representation of the stimulated whiskers within the primary somatosensory cortex of mice. Using the same paradigm, the present study examined field potentials evoked in ex vivo slices of the barrel cortex. The amplitude of responses, evoked by single and repetitive stimuli in layer IV-layer II/III pathway contained within the barrel column corresponding to the whisker stimulated during training, was unchanged. In contrast, in a transcolumnar pathway from the "trained" barrel to layer II/III of the neighboring, "untrained" column, the amplitude of responses was reduced and responses to trains of stimuli applied at 40 Hz, but not at lower frequencies, depressed faster. These data are suggestive of a selective weakening of excitatory transmission and/or enhancement of inhibitory transmission in transcolumnar pathways, which accompany associative learning-induced cortical plasticity.