Pavlos Rigas

Thalamocortical Up States: Differential Effects of Intrinsic and Extrinsic Cortical Inputs on Persistent Activity

During behavioral quiescence, the neocortex generates spontaneous slow oscillations that consist of Up and Down states. Up states are short epochs of persistent activity that resemble the activated neocortex during arousal and cognition. Although Up states are generated within the cortex, the impact of extrinsic (thalamocortical) and intrinsic (intracortical) inputs on the persistent activity is not known. Using thalamocortical slices, we found that the persistent cortical activity during spontaneous Up states effectively drives thalamocortical relay cells through corticothalamic connections. However, thalamic activity can also precede the onset of cortical Up states, which suggests a role of thalamic activity in triggering cortical Up states through thalamocortical connections. In support of this hypothesis, we found that cutting the connections between thalamus and cortex reduced the incidence of spontaneous Up states in the cortex. Consistent with a facilitating role of thalamic activity on Up states, electrical or chemical stimulation of the thalamus triggered cortical Up states very effectively and enhanced those occurring spontaneously. In contrast, stimulation of the cortex triggered Up states only at very low intensities but otherwise had a suppressive effect on Up states. Moreover, cortical stimulation suppressed the facilitating effect of thalamic stimulation on Up states. In conclusion, thalamocortical inputs facilitate and intracortical inputs suppress cortical Up states. Thus, extrinsic and intrinsic cortical inputs differentially regulate persistent activity, which may serve to adjust the processing state of thalamocortical networks during behavior.

Impact of Persistent Cortical Activity (Up States) on Intracortical and Thalamocortical Synaptic Inputs

The neocortex generates short epochs of persistent activity called up states, which are associated with changes in cellular and network excitability. Using somatosensory thalamocortical slices, we studied the impact of persistent cortical activity during spontaneous up states on intrinsic cellular excitability (input resistance) and on excitatory synaptic inputs of cortical cells. At the intrinsic excitability level, we found that the expected decrease in input resistance (high conductance) resulting from synaptic barrages during up states is counteracted by an increase in input resistance due to depolarization per se. The result is a variable but on average relatively small reduction in input resistance during up states. At the synaptic level, up states enhanced a late synaptic component of short-latency thalamocortical field potential responses but suppressed intracortical field potential responses. The thalamocortical enhancement did not reflect an increase in synaptic strength, as determined by measuring the evoked postsynaptic current, but instead an increase in evoked action potential (spike) probability due to depolarization during up states. In contrast, the intracortical suppression was associated with a reduction in synaptic strength, apparently driven by increased presynaptic intracortical activity during up states. In addition, intracortical suppression also reflected a reduction in evoked spike latency caused by depolarization and the abolishment of longer-latency spikes caused by stronger inhibitory drive during up states. In conclusion, depolarization during up states increases the success of excitatory synaptic inputs to reach firing. However, activity-dependent synaptic depression caused by increased presynaptic firing during up states and the enhancement of evoked inhibitory drive caused by depolarization suppress excitatory intracortical synaptic inputs.

Resonance (approximately 10 Hz) of excitatory networks in motor cortex: effects of voltage-dependent ion channel blockers.

The motor cortex generates synchronous network oscillations at frequencies between 7 and 14 Hz during disinhibition or low [Mg2+]o buffers, but the underlying mechanisms are poorly understood. These oscillations, termed here ∼10 Hz oscillations, are generated by a purely excitatory network of interconnected pyramidal cells because they are robust in the absence of GABAergic transmission. It is likely that specific voltage‐dependent currents expressed in those cells contribute to the generation of ∼10 Hz oscillations. We tested the effects of different drugs known to suppress certain voltage‐dependent currents. The results revealed that drugs that suppress the low‐threshold calcium current and the hyperpolarization‐activated cation current are not critically involved in the generation of ∼10 Hz oscillations. Interestingly, drugs known to suppress the persistent sodium current abolished ∼10 Hz oscillations. Furthermore, blockers of K+ channels had significant effects on the oscillations. In particular, blockers of the M‐current abolished the oscillations. Also, blockers of both non‐inactivating and slowly inactivating voltage‐dependent K+ currents abolished ∼10 Hz oscillations. The results indicate that specific voltage‐dependent non‐inactivating K+ currents, such as the M‐current, and persistent sodium currents are critically involved in generating ∼10 Hz oscillations of excitatory motor cortex networks.

High-Affinity Nicotinic Receptors Modulate Spontaneous Cortical Up States In Vitro.

Nicotinic acetylcholine receptors (nAChRs) play an important role in the modulation of many cognitive functions but their role in integrated network activity remains unclear. This is at least partly because of the complexity of the cholinergic circuitry and the difficulty in comparing results from in vivo studies obtained under diverse experimental conditions and types of anesthetics. Hence the role of nAChRs in the synchronization of cortical activity during slow-wave sleep is still controversial, with some studies showing they are involved in ACh-dependent EEG desynchronization, and others suggesting that this effect is mediated exclusively by muscarinic receptors. Here we use an in vitro model of endogenous network activity, in the form of recurring self-maintained depolarized states (Up states), which allows us to examine the role of high-affinity nAChRs on network dynamics in a simpler form of the cortical microcircuit. We find that mice lacking nAChRs containing the β2-subunit (β2-nAChRs) have longer and more frequent Up states, and that this difference is eliminated when β2-nAChRs in wild-type mice are blocked. We further show that endogenously released ACh can modulate Up/Down states through the activation of both β2- and α7-containing nAChRs, but through distinct mechanisms: α7-nAChRs affect only the termination of spontaneous Up states, while β2-nAChRs also regulate their generation. Finally we provide evidence that the effects of β2-subunit-containing, but not α7-subunit-containing nAChRs, are mediated through GABAB receptors. To our knowledge this is the first study documenting direct nicotinic modulation of Up/Down state activity. SIGNIFICANCE STATEMENT Through our experiments we were able to uncover a clear and previously disputed effect of nicotinic signaling in synchronized activity of neuronal networks of the cortex. We show that both high-affinity receptors (containing the β2-subunit, β2-nAChRs) and low-affinity receptors (containing the α7-subunit, α7-nAChRs) can regulate cortical network function exhibited in the form of Up/Down states. We further show that the effects of β2-nAChRs, but not α7-nAChRs, are mediated through the activation of GABAB receptors. These results suggest a possible synthesis of seemingly contradictory results in the literature and could be valuable for informing computational models of cortical function and for guiding the search for therapeutic interventions.

Spontaneous Up states in vitro: a single-metric index of the functional maturation and regional differentiation of the cerebral cortex

Understanding the development and differentiation of the neocortex remains a central focus of neuroscience. While previous studies have examined isolated aspects of cellular and synaptic organization, an integrated functional index of the cortical microcircuit is still lacking. Here we aimed to provide such an index, in the form of spontaneously recurring periods of persistent network activity -or Up states- recorded in mouse cortical slices. These coordinated network dynamics emerge through the orchestrated regulation of multiple cellular and synaptic elements and represent the default activity of the cortical microcircuit. To explore whether spontaneous Up states can capture developmental changes in intracortical networks we obtained local field potential recordings throughout the mouse lifespan. Two independent and complementary methodologies revealed that Up state activity is systematically modified by age, with the largest changes occurring during early development and adolescence. To explore possible regional heterogeneities we also compared the development of Up states in two distinct cortical areas and show that primary somatosensory cortex develops at a faster pace than primary motor cortex. Our findings suggest that in vitro Up states can serve as a functional index of cortical development and differentiation and can provide a baseline for comparing experimental and/or genetic mouse models.

High-Throughput Analysis of in-vitro LFP Electrophysiological Signals: A validated workflow/software package

Synchronized brain activity in the form of alternating epochs of massive persistent network activity and periods of generalized neural silence, has been extensively studied as a fundamental form of circuit dynamics, important for many cognitive functions including short-term memory, memory consolidation, or attentional modulation. A key element in such studies is the accurate determination of the timing and duration of those network events. The local field potential (LFP) is a particularly attractive method for recording network activity, because it allows for long and stable recordings from multiple sites, allowing researchers to estimate the functional connectivity of local networks. Here, we present a computational method for the automatic detection and quantification of in-vitro LFP events, aiming to overcome the limitations of current approaches (e.g. slow analysis speed, arbitrary threshold-based detection and lack of reproducibility across and within experiments). The developed method is based on the implementation of established signal processing and machine learning approaches, is fully automated and depends solely on the data. In addition, it is fast, highly efficient and reproducible. The performance of the software is compared against semi-manual analysis and validated by verification of prior biological knowledge.

Spontaneous Neuronal Network Persistent Activity in the Neocortex: A(n) (Endo)phenotype of Brain (Patho)physiology

Abnormal synaptic homeostasis in the cerebral cortex represents a risk factor for both psychiatric and neurodegenerative disorders, from autism and schizophrenia to Alzheimers disease. Neurons via synapses form recurrent networks that are intrinsically active in the form of oscillating activity, visible at increasingly macroscopic neurophysiological levels: from single cell recordings to the local field potentials (LFPs) to the clinically relevant electroencephalography (EEG). Understanding in animal models the defects at the level of neural circuits is important in order to link molecular and cellular phenotypes with behavioral phenotypes of neurodevelopmental and/or neurodegenerative brain disorders. In this study we introduce the novel idea that recurring persistent network activity (Up states) in the neocortex at the reduced level of the brain slice may be used as an endophenotype of brain disorders that will help us understand not only how local microcircuits of the cortex may be affected in brain diseases, but also when, since an important issue for the design of successful treatment strategies concerns the time window available for intervention.

Resonance (∼10 Hz) of excitatory networks in motor cortex: effects of voltage-dependent ion channel blockers: Mechanisms of cortical oscillations

The motor cortex generates synchronous network oscillations at frequencies between 7 and 14 Hz during disinhibition or low [Mg2+]o buffers, but the underlying mechanisms are poorly understood. These oscillations, termed here ∼10 Hz oscillations, are generated by a purely excitatory network of interconnected pyramidal cells because they are robust in the absence of GABAergic transmission. It is likely that specific voltage‐dependent currents expressed in those cells contribute to the generation of ∼10 Hz oscillations. We tested the effects of different drugs known to suppress certain voltage‐dependent currents. The results revealed that drugs that suppress the low‐threshold calcium current and the hyperpolarization‐activated cation current are not critically involved in the generation of ∼10 Hz oscillations. Interestingly, drugs known to suppress the persistent sodium current abolished ∼10 Hz oscillations. Furthermore, blockers of K+ channels had significant effects on the oscillations. In particular, blockers of the M‐current abolished the oscillations. Also, blockers of both non‐inactivating and slowly inactivating voltage‐dependent K+ currents abolished ∼10 Hz oscillations. The results indicate that specific voltage‐dependent non‐inactivating K+ currents, such as the M‐current, and persistent sodium currents are critically involved in generating ∼10 Hz oscillations of excitatory motor cortex networks.