Synchronous activity of inhibitory networks in neocortex requires electrical synapses containing connexin36

Inhibitory interneurons often generate synchronous activity as an emergent property of their interconnections. To determine the role of electrical synapses in such activity, we constructed mice expressing histochemical reporters in place of the gap junction protein Cx36. Localization of the reporter with somatostatin and parvalbumin suggested that Cx36 was expressed largely by interneurons. Electrical synapses were common among cortical interneurons in controls but were nearly absent in knockouts. A metabotropic glutamate receptor agonist excited LTS interneurons, generating rhythmic inhibitory potentials in surrounding neurons of both wild-type and knockout animals. However, the synchrony of these rhythms was weaker and more spatially restricted in the knockout. We conclude that electrical synapses containing Cx36 are critical for the generation of widespread, synchronous inhibitory activity.     

Gap junctions promote synchronous activities in a network of inhibitory interneurons

By using a single compartment biophysical model of a fast spiking interneuron the synchronization properties of a pair of cells, coupled by electrical and inhibitory synapses, are investigated. The inhibitory and excitatory synaptic couplings are modeled in order to reproduce the experimental time course of the corresponding currents. It is shown that increasing the conductance value of the electrical synapses enhances the synchronization between the spike trains of the two cells. Moreover, increasing either the decay time constant of the inhibitory current or the firing frequency of the cells favours the emergence of synchronous discharges.     

A novel network of multipolar bursting interneurons generates theta frequency oscillations in neocortex

GABAergic interneurons can phase the output of principal cells, giving rise to oscillatory activity in different frequency bands. Here we describe a new subtype of GABAergic interneuron, the multipolar bursting (MB) cell in the mouse neocortex. MB cells are parvalbumin positive but differ from fast-spiking multipolar (FS) cells in their morphological, neurochemical, and physiological properties. MB cells are reciprocally connected with layer 2/3 pyramidal cells and are coupled with each other by chemical and electrical synapses. MB cells innervate FS cells but not vice versa. MB to MB cell as well as MB to pyramidal cell synapses exhibit paired-pulse facilitation. Carbachol selectively induced synchronized theta frequency oscillations in MB cells. Synchrony required both gap junction coupling and GABAergic chemical transmission, but not excitatory glutamatergic input. Hence, MB cells form a distinct inhibitory network, which upon cholinergic drive can generate rhythmic and synchronous theta frequency activity, providing temporal coordination of pyramidal cell output.     

Synchronization of Firing in Cortical Fast-Spiking Interneurons at Gamma Frequencies: A Phase-Resetting Analysis

Fast-spiking (FS) cells in the neocortex are interconnected both by inhibitory chemical synapses and by electrical synapses, or gap-junctions. Synchronized firing of FS neurons is important in the generation of gamma oscillations, at frequencies between 30 and 80 Hz. To understand how these synaptic interactions control synchronization, artificial synaptic conductances were injected in FS cells, and the synaptic phase-resetting function (SPRF), describing how the compound synaptic input perturbs the phase of gamma-frequency spiking as a function of the phase at which it is applied, was measured. GABAergic and gap junctional conductances made distinct contributions to the SPRF, which had a surprisingly simple piecewise linear form, with a sharp midcycle break between phase delay and advance. Analysis of the SPRF showed how the intrinsic biophysical properties of FS neurons and their interconnections allow entrainment of firing over a wide gamma frequency band, whose upper and lower frequency limits are controlled by electrical synapses and GABAergic inhibition respectively.     

Noise-tolerant stimulus discrimination by synchronization with depressing synapses

 Some synapses between cortical pyramidal neurons exhibit a rapid depression of excitatory postsynaptic potentials for successive presynaptic spikes. Since depressing synapses do not transmit information on sustained presynaptic firing rates, it has been speculated that they are favorable for temporal coding. In this paper, we study the dynamical effects of depressing synapses on stimulus-induced transient synchronization in a simple network of inhibitory interneurons and excitatory neurons, assuming that the recurrent excitation is mediated by depressing synapses. This synchronization occurs in a temporal pattern which depends on a given stimulus. Since the presence of noise is always a potential hazard in temporal coding, we investigate the extent to which noise in stimuli influences the synchronization phenomena. It is demonstrated that depressing synapses greatly contribute to suppressing the influences of noise on the stimulus-specific temporal patterns of synchronous firing. The timing-based Hebbian learning revealed by physiological experiments is shown to stabilize the temporal patterns in cooperation with synaptic depression. Thus, the times at which synchronous firing occurs provides a reliable information representation in the presence of synaptic depression. Received: 5 July 2000 / Accepted in revised form: 12 January 2001     

Electrical synapses between GABA-releasing interneurons

Although gap junctions were first demonstrated in the mammalian brain about 30 years ago, the distribution and role of electrical synapses have remained elusive. A series of recent reports has demonstrated that inhibitory interneurons in the cerebral cortex, thalamus, striatum and cerebellum are extensively interconnected by electrical synapses. Investigators have used paired recordings to reveal directly the presence of electrical synapses among identified cell types. These studies indicate that electrical coupling is a fundamental feature of local inhibitory circuits and suggest that electrical synapses define functionally diverse networks of GABA-releasing interneurons. Here, we discuss these results, their possible functional significance and the insights into neuronal circuit organization that have emerged from them.     

Dynamics of spiking neurons connected by both inhibitory and electrical coupling

We study the dynamics of a pair of intrinsically oscillating leaky integrate-and-fire neurons (identical and noise-free) connected by combinations of electrical and inhibitory coupling. We use the theory of weakly coupled oscillators to examine how synchronization patterns are influenced by cellular properties (intrinsic frequency and the strength of spikes) and coupling parameters (speed of synapses and coupling strengths). We find that, when inhibitory synapses are fast and the electrotonic effect of the suprathreshold portion of the spike is large, increasing the strength of weak electrical coupling promotes synchrony. Conversely, when inhibitory synapses are slow and the electrotonic effect of the suprathreshold portion of the spike is small, increasing the strength of weak electrical coupling promotes antisynchrony (see Fig. 10). Furthermore, our results indicate that, given a fixed total coupling strength, either electrical coupling alone or inhibition alone is better at enhancing neural synchrony than a combination of electrical and inhibitory coupling. We also show that these results extend to moderate coupling strengths.     

Desynchronization of Neocortical Networks by Asynchronous Release of GABA at Autaptic and Synaptic Contacts from Fast-Spiking Interneurons

Networks of specific inhibitory interneurons regulate principal cell firing in several forms of neocortical activity. Fast-spiking (FS) interneurons are potently self-inhibited by GABAergic autaptic transmission, allowing them to precisely control their own firing dynamics and timing. Here we show that in FS interneurons, high-frequency trains of action potentials can generate a delayed and prolonged GABAergic self-inhibition due to sustained asynchronous release at FS-cell autapses. Asynchronous release of GABA is simultaneously recorded in connected pyramidal (P) neurons. Asynchronous and synchronous autaptic release show differential presynaptic Ca²⁺ sensitivity, suggesting that they rely on different Ca²⁺ sensors and/or involve distinct pools of vesicles. In addition, asynchronous release is modulated by the endogenous Ca²⁺ buffer parvalbumin. Functionally, asynchronous release decreases FS-cell spike reliability and reduces the ability of P neurons to integrate incoming stimuli into precise firing. Since each FS cell contacts many P neurons, asynchronous release from a single interneuron may desynchronize a large portion of the local network and disrupt cortical information processing.     

A cortical attractor network with Martinotti cells driven by facilitating synapses

The population of pyramidal cells significantly outnumbers the inhibitory interneurons in the neocortex, while at the same time the diversity of interneuron types is much more pronounced. One acknowledged key role of inhibition is to control the rate and patterning of pyramidal cell firing via negative feedback, but most likely the diversity of inhibitory pathways is matched by a corresponding diversity of functional roles. An important distinguishing feature of cortical interneurons is the variability of the short-term plasticity properties of synapses received from pyramidal cells. The Martinotti cell type has recently come under scrutiny due to the distinctly facilitating nature of the synapses they receive from pyramidal cells. This distinguishes these neurons from basket cells and other inhibitory interneurons typically targeted by depressing synapses. A key aspect of the work reported here has been to pinpoint the role of this variability. We first set out to reproduce quantitatively based on in vitro data the di-synaptic inhibitory microcircuit connecting two pyramidal cells via one or a few Martinotti cells. In a second step, we embedded this microcircuit in a previously developed attractor memory network model of neocortical layers 2/3. This model network demonstrated that basket cells with their characteristic depressing synapses are the first to discharge when the network enters an attractor state and that Martinotti cells respond with a delay, thereby shifting the excitation-inhibition balance and acting to terminate the attractor state. A parameter sensitivity analysis suggested that Martinotti cells might, in fact, play a dominant role in setting the attractor dwell time and thus cortical speed of processing, with cellular adaptation and synaptic depression having a less prominent role than previously thought.     

Inhibitory network of spiking neurons may express a sharp peak of synchrony at low frequency band

Spike synchronization remains an important issue in neuroscience, and inhibitory networks are the best candidates to provide such synchrony. Increasing evidence indicates that in many brain area inhibitory interneurons of similar properties make reciprocal connections. We found that a hybrid, as well as model network, consisting of two reciprocally inhibitory spiking neurons may express a peak of synchronization in a narrow range of low spiking frequencies in addition to classically described plateau of synchrony at a wide range of high frequencies. Occurrence of the low frequency peak of synchrony requires a moderate-to-strong inhibitory coupling and relatively fast synapses. This novel possibility of synchronization in a narrow range of network parameters may have an important implication in discrimination and encoding of signals of precise intensity, as well as in altering network ability to process information.     

Dynamical responses to external stimuli for both cases of excitatory and inhibitory synchronization in a complex neuronal network

For studying how dynamical responses to external stimuli depend on the synaptic-coupling type, we consider two types of excitatory and inhibitory synchronization (i.e., synchronization via synaptic excitation and inhibition) in complex small-world networks of excitatory regular spiking (RS) pyramidal neurons and inhibitory fast spiking (FS) interneurons. For both cases of excitatory and inhibitory synchronization, effects of synaptic couplings on dynamical responses to external time-periodic stimuli S(t) (applied to a fraction of neurons) are investigated by varying the driving amplitude A of S(t). Stimulated neurons are phase-locked to external stimuli for both cases of excitatory and inhibitory couplings. On the other hand, the stimulation effect on non-stimulated neurons depends on the type of synaptic coupling. The external stimulus S(t) makes a constructive effect on excitatory non-stimulated RS neurons (i.e., it causes external phase lockings in the non-stimulated sub-population), while S(t) makes a destructive effect on inhibitory non-stimulated FS interneurons (i.e., it breaks up original inhibitory synchronization in the non-stimulated sub-population). As results of these different effects of S(t), the type and degree of dynamical response (e.g., synchronization enhancement or suppression), characterized by the dynamical response factor \(D_f\) (given by the ratio of synchronization degree in the presence and absence of stimulus), are found to vary in a distinctly different way, depending on the synaptic-coupling type. Furthermore, we also measure the matching degree between the dynamics of the two sub-populations of stimulated and non-stimulated neurons in terms of a “cross-correlation” measure \(M_c\). With increasing A, based on \(M_c\), we discuss the cross-correlations between the two sub-populations, affecting the dynamical responses to S(t).     

Changes in inhibitory CA1 network in dual pathology model of epilepsy

The combination of two precipitating factors appears to be more and more recognized in patients with temporal lobe epilepsy. Using a two-hit rat model, with a neonatal freeze lesion mimicking a focal cortical malformation combined with hyperthermia-induced seizures mimicking febrile seizures, we have previously reported an increase of inhibition in CA1 pyramidal cells at P20. Here, we investigated the changes affecting excitatory and inhibitory drive onto CA1 interneurons to better define the changes in CA1 inhibitory networks and their paradoxical role in epileptogenesis, using electrophysiological recordings in CA1 hippocampus from rat pups (16–20 d old). We investigated interneurons in CA1 hippocampal area located in stratum oriens (Or) and at the border of strata lacunosum and moleculare (L-M). Our results revealed an increase of the excitatory drive to both types of interneurons with no change in the inhibitory drive. The mechanisms underlying the increase of excitatory synaptic currents (EPSCs) in both types of interneurons are different. In Or interneurons, the amplitude of spontaneous and miniature EPSCs increased, while their frequency was not affected suggesting changes at the post-synaptic level. In L-M interneurons, the frequency of spontaneous EPSCs increases, but the amplitude is not affected. Analyses of miniature EPSCs showed no changes in both their frequency and amplitude. We concluded that L-M interneurons increase in excitatory drive is due to a change in Shaffer collateral axon excitability. The changes described here in CA1 inhibitory network may actually contribute to the epileptogenicity observed in this dual pathology model by increasing pyramidal cell synchronization.     

Signal processing properties of fast spiking interneurons

Fast spiking interneurons receive excitatory synaptic inputs from pyramidal cells and a relevant problem is to understand how these cells readout this information. Here this topic is investigated theoretically by using a biophysical modeling of a pair of coupled fast spiking interneuron models. The model predicts, in agreement with the experimental findings, that these cells are capable of transmitting pre-synaptic signals with high temporal precision and transferring high frequency inputs while preserving their relative timing. Moreover, it is shown that a pair of fast spiking interneurons, coupled through both inhibitory and electrical synapses, behaves as a coincidence detector. Lastly, to understand the mechanisms underlying these phenomena, a theoretical analysis is carried out by using a simpler modeling approach.     

Developmental downregulation of GABAergic drive parallels formation of functional synapses in cultured mouse neocortical networks

Networks of cortical neurons in vitro spontaneously develop synchronous oscillatory electrical activity at around the second week in culture. However, the underlying mechanisms and in particular the role of GABAergic interneurons in initiation and synchronization of oscillatory activity in developing cortical networks remain elusive. Here, we examined the intrinsic properties and the development of GABAergic and glutamatergic input onto presumed projection neurons (PNs) and large interneurons (L-INs) in cortical cultures of GAD67-GFP mice. Cultures developed spontaneous synchronous activity already at 5-7 days in vitro (DIV), as revealed by imaging transient changes in Fluo-3 fluorescence. Concurrently, spontaneous glutamate-mediated and GABA(A)-mediated postsynaptic currents (sPSCs) occured at 5 DIV. For both types of neurons the frequency of glutamatergic and GABAergic sPSCs increased with DIV, whereas the charge transfer of glutamatergic sPSCs increased and the charge transfer of GABAergic sPSCs decreased with cultivation time. The ratio between GABAergic and the overall charge transfer was significantly reduced with DIV for L-INs and PNs, indicating an overall reduction in GABAergic synaptic drive with maturation of the network. In contrast, analysis of miniature PSCs (mPSCs) revealed no significant changes of charge transfer with DIV for both types of neurons, indicating that the reduction in GABAergic drive was not due to a decreased number of functional synapses. Our data suggest that the global reduction in GABAergic synaptic drive together with more synaptic input to PNs and L-INs during maturation may enhance rhythmogenesis of the network and increase the synchronization at the level of population bursts.     

The synchronization properties of a network of inhibitory interneurons depend on the biophysical model

The synchronization properties of a pair of coupled fast spiking interneurons are studied by using the theory of weakly coupled oscillators. Four different biophysical models of the single fast spiking interneuron are used and the corresponding results are compared. It is shown that for a pair of identical coupled cells, the synchronization properties are model-dependent. In particular, the firing coherence of the network is strongly affected by the reversal potential, the kinetics of the inhibitory postsynaptic current and the electrical coupling; the activation properties of the sodium and potassium currents play a significant role too.     

LTS and FS inhibitory interneurons, short-term synaptic plasticity, and cortical circuit dynamics

Somatostatin-expressing, low threshold-spiking (LTS) cells and fast-spiking (FS) cells are two common subtypes of inhibitory neocortical interneuron. Excitatory synapses from regular-spiking (RS) pyramidal neurons to LTS cells strongly facilitate when activated repetitively, whereas RS-to-FS synapses depress. This suggests that LTS neurons may be especially relevant at high rate regimes and protect cortical circuits against over-excitation and seizures. However, the inhibitory synapses from LTS cells usually depress, which may reduce their effectiveness at high rates. We ask: by which mechanisms and at what firing rates do LTS neurons control the activity of cortical circuits responding to thalamic input, and how is control by LTS neurons different from that of FS neurons? We study rate models of circuits that include RS cells and LTS and FS inhibitory cells with short-term synaptic plasticity. LTS neurons shift the RS firing-rate vs. current curve to the right at high rates and reduce its slope at low rates; the LTS effect is delayed and prolonged. FS neurons always shift the curve to the right and affect RS firing transiently. In an RS-LTS-FS network, FS neurons reach a quiescent state if they receive weak input, LTS neurons are quiescent if RS neurons receive weak input, and both FS and RS populations are active if they both receive large inputs. In general, FS neurons tend to follow the spiking of RS neurons much more closely than LTS neurons. A novel type of facilitation-induced slow oscillations is observed above the LTS firing threshold with a frequency determined by the time scale of recovery from facilitation. To conclude, contrary to earlier proposals, LTS neurons affect the transient and steady state responses of cortical circuits over a range of firing rates, not only during the high rate regime; LTS neurons protect against over-activation about as well as FS neurons.     

Noise filtering in the auditory system of Locusta migratoria L

Many acoustically communicating grasshoppers live in crowded populations where sound of many individuals may cause permanent noise. Tympanic receptors and first-order auditory interneurons of Locusta migratoria code such noise tonically, whereas many higher order interneurons react only weakly. In response to simultaneously presented sound they exhibit a better signal-to-noise ratio than their presynaptic elements. Two possible filter mechanisms are suggested for noise reduction in higher-order interneurons: (i) high-pass filtering of receptor spike frequencies and (ii) filtering due to synchronization of receptor spikes. Different receptor spike frequencies were elicited by series of short noise pulses with variable repetition rates. Receptor activities differing in their degree of synchronization were elicited by sound stimuli with variable rising times. In contrast to the first order interneurons some higher order interneurons responded best to receptor spike frequencies above 150–200 Hz, thus showing the postulated filtering. Only one higher order interneuron (AN4) distinguished between synchronous and asynchronous receptor activities. It is suggested that high-pass filtering of receptor spike frequencies is responsible for the noise filtering observed in these interneurons. The synchronization selectivity of AN4 is proposed to be responsible for temporal pattern detection of conspecific sounds.     

Modulation of Oscillatory Synchronization in an Interneuronal Network under the Influence of Tonic GABA-ergic Inhibition: a Model Study

The influence of a tonic GABA-ergic current on the processes of network synchronization was examined using a computer model of the neural network with shunting GABA-ergic synapses and tonic excitation that initiated spiking. The tonic inhibitory current was characterized by two parameters, the reversal potential and the conductance introduced. We found that tonic current with a reversal potential more negative than the threshold for spike generation reduces the network spiking frequency and synchronization. A monotonic decrease in the network synchronization with augmentation of the tonic current conductance was shown. We also found that a particular range of tonic current conductance leads to a bistable character of the network dynamics. Depending on the initial conditions of the network examined, spontaneous synchronous oscillations similar to epileptiform activity could appear.     

Disynaptic inhibition between neocortical pyramidal cells mediated by Martinotti cells

Reliable activation of inhibitory pathways is essential for maintaining the balance between excitation and inhibition during cortical activity. Little is known, however, about the activation of these pathways at the level of the local neocortical microcircuit. We report a disynaptic inhibitory pathway among neocortical pyramidal cells (PCs). Inhibitory responses were evoked in layer 5 PCs following stimulation of individual neighboring PCs with trains of action potentials. The probability for inhibition between PCs was more than twice that of direct excitation, and inhibitory responses increased as a function of rate and duration of presynaptic discharge. Simultaneous somatic and dendritic recordings indicated that inhibition originated from PC apical and tuft dendrites. Multineuron whole-cell recordings from PCs and interneurons combined with morphological reconstructions revealed the mediating interneurons as Martinotti cells. Martinotti cells received facilitating synapses from PCs and formed reliable inhibitory synapses onto dendrites of neighboring PCs. We describe this feedback pathway and propose it as a central mechanism for regulation of cortical activity.     

Synchronization of an embryonic network of identified spinal interneurons solely by electrical coupling

There is a need to understand the mechanisms of neural synchronization during development because correlated rhythmic activity is thought to be critical for the establishment of proper connectivity. The relative importance of chemical and electrical synapses for synchronization of electrical activity during development is unclear. We examined the activity patterns of identified spinal neurons at the onset of motor activity in zebrafish embryos. Rhythmic activity appeared early and persisted upon blocking chemical neurotransmission but was abolished by inhibitors of gap junctions. Paired recordings revealed that active spinal neurons were electrically coupled and formed a simple network of motoneurons and a subset of interneurons. Thus, the earliest spinal central pattern generator consists of synchronously active, electrically coupled neurons.