Layer Interneurons Research Articles

Gap Junctions Link Regular-Spiking and Fast-Spiking Interneurons in Layer 5 Somatosensory Cortex

Gap junctions form electrical synapses that modulate neuronal activity by synchronizing action potential (AP) firing of cortical interneurons (INs). Gap junctions are thought to form predominantly within cortical INs of the same functional class and are therefore considered to act within discrete neuronal populations. Here, we challenge that view and show that the probability of electrical coupling is the same within and between regular-spiking (RS) and fast-spiking (FS) cortical INs in 16–21 days old mice. Firing properties of these two populations were distinct from other INs types including neurogliaform and low-threshold spiking (LTS) cells. We also demonstrate that pre-junctional APs can depolarize post-junctional neurons and increase the probability of firing. Our findings of frequent gap junction coupling between functionally distinct IN subtypes suggest that cortical IN networks are much more extensive and heterogeneous than previously thought. This may have implications on mechanisms ranging from cognitive functions to modulation of pathological states in epilepsy and other neurological disorders.

Fast voltage-sensitive dye imaging of excitatory and inhibitory synaptic transmission in the rat granular retrosplenial cortex.

Rodent granular retrosplenial cortex (GRS) has dense connections between the anterior thalamic nuclei (ATN) and hippocampal formation. GRS superficial pyramidal neurons exhibit distinctive late spiking (LS) firing property and form patchy clusters with prominent apical dendritic bundles. The aim of this study was to investigate spatiotemporal dynamics of signal transduction in the GRS induced by ATN afferent stimulation by using fast voltage-sensitive dye imaging in rat brain slices. In coronal slices, layer 1a stimulation, which presumably activated thalamic fibers, evoked propagation of excitatory synaptic signals from layers 2-4 to layers 5-6 in a direction perpendicular to the layer axis, followed by transverse signal propagation within each layer. In the presence of ionotropic glutamate receptor antagonists, inhibitory responses were observed in superficial layers, induced by direct activation of inhibitory interneurons in layer 1. In horizontal slices, excitatory signals in deep layers propagated transversely mainly from posterior to anterior via superficial layers. Cortical inhibitory responses upon layer 1a stimulation in horizontal slices were weaker than those in the coronal slices. Observed differences between coronal and horizontal planes suggest anisotropy of the intracortical circuitry. In conclusion, ATN inputs are processed differently in coronal and horizontal planes of the GRS and then conveyed to other cortical areas. In both planes, GRS superficial layers play an important role in signal propagation, which suggests that superficial neuronal cascade is crucial in the integration of multiple information sources.NEW & NOTEWORTHY Superficial neurons in the rat granular retrosplenial cortex (GRS) show distinctive late-spiking (LS) firing property. However, little is known about spatiotemporal dynamics of signal transduction in the GRS. We demonstrated LS neuron network relaying thalamic inputs to deep layers and anisotropic distribution of inhibition between coronal and horizontal planes. Since deep layers of the GRS receive inputs from the subiculum, GRS circuits may work as an integrator of multiple sources such as sensory and memory information.

Characterization of focal cortical dysplasia with balloon cells by layer‐specific markers: Evidence for differential vulnerability of interneurons

Focal cortical dysplasia (FCD) is a major cause of pharmacoresistant focal epilepsy. Little is known about the pathomechanisms underlying the characteristic cytoarchitectural abnormalities associated with FCD. In the present study, a broad panel of markers identifying layer-specific neuron subpopulations was applied to characterize dyslamination and structural alterations in FCD with balloon cells (FCD 2b). Pan-neuronal neuronal nuclei (NeuN) and layer-specific protein expression (Reelin, Calbindin, Calretinin, SMI32 (nonphosphorylated neurofilament H), Parvalbumin, transducin-like enhancer protein 4 (TLE4), and Vimentin) was studied by immunohistochemistry on paraffin sections of FCD2b cases (n = 22) and was compared to two control groups with (n = 7) or without epilepsy (n = 4 postmortem cases). Total and layer-specific neuron densities were systematically quantified by cell counting considering age at surgery and brain region. We show that in FCD2b total neuron densities across all six cortical layers were not significantly different from controls. In addition, we present evidence that a basic laminar arrangement of layer-specific neuron subtypes was preserved despite the severe disturbance of cortical structure. SMI32-positive pyramidal neurons showed no significant difference in total numbers, but a reduction in layers III and V. The densities of supragranular Calbindin- and Calretinin-positive interneurons in layers II and III were not different from controls, whereas Parvalbumin-expressing interneurons, primarily located in layer IV, were significantly reduced in numbers when compared to control cases without epilepsy. In layer VI, the density of TLE4-positive projection neurons was significantly increased. Altogether, these data show that changes in cellular composition mainly affect deep cortical layers in FCD2b. The application of a broad panel of markers defining layer-specific neuronal subpopulations revealed that in FCD2b neuronal diversity and a basic laminar arrangement are maintained despite the severe disturbance of cytoarchitecture. Moreover, it showed that Parvalbumin-positive, inhibitory interneurons are highly vulnerable in contrast to other interneuron subtypes, possibly related to the epileptic condition.

Thalamo-cortical axons regulate the radial dispersion of neocortical GABAergic interneurons

Neocortical GABAergic interneuron migration and thalamo-cortical axon (TCA) pathfinding follow similar trajectories and timing, suggesting they may be interdependent. The mechanisms that regulate the radial dispersion of neocortical interneurons are incompletely understood. Here we report that disruption of TCA innervation, or TCA-derived glutamate, affected the laminar distribution of GABAergic interneurons in mouse neocortex, resulting in abnormal accumulation in deep layers of interneurons that failed to switch from tangential to radial orientation. Expression of the KCC2 cotransporter was elevated in interneurons of denervated cortex, and KCC2 deletion restored normal interneuron lamination in the absence of TCAs. Disruption of interneuron NMDA receptors or pharmacological inhibition of calpain also led to increased KCC2 expression and defective radial dispersion of interneurons. Thus, although TCAs are not required to guide the tangential migration of GABAergic interneurons, they provide crucial signals that restrict interneuron KCC2 levels, allowing coordinated neocortical invasion of TCAs and interneurons.

Structural organization of the dendritic reticulum linked by gap junctions in layer 4 of the visual cortex

Neuronal gap junctions are ubiquitous in the brain, but their precise positions in actual neuronal circuits have been uncertain, and their functional roles remain unclear. In this study, I visualized connexin36-immunoreactive gap junctions and examined the structural features of the interconnected dendrites arising from parvalbumin (PV)-positive interneurons in layer 4 of the feline visual cortex. I observed evidence for net-like dense linkages of dendrites among virtually all PV neurons (56/58 cells, 96.6%) in the tissue. This dendritic reticulum established connections among clustered cells and further among remote cells. The latter connectivity exhibited a preference for vertical direction, including translaminar linkages, but was also characterized by lateral continuity. Measurement of the distances from each dendritic gap junction back to the two connected somata revealed that at least one of two somata was within 50μm from the junction in 77.5% of the cases and within 75μm in 91.2% of the cases. Thus, distal gap junctions mediated morphologically asymmetrical connection where one soma was close to, but the other soma was far from the connecting junction. This connectivity was typically observed between neurons located apart in the same columnar space, where a long vertical dendrite bridged two neurons through an asymmetrically positioned gap junction. In contrast, gap junctions formed between nearby cells were close to both somata. Thalamocortical afferents established synapses densely on somata of layer 4 PV neurons, indicating the possible involvement of proximal gap junctions in visual stimulus-driven feedforward regulation. These findings provide a new structural basis for cortical investigations.

Identifying Anatomical Origins of Coexisting Oscillations in the Cortical Microcircuit.

Oscillations are omnipresent in neural population signals, like multi-unit recordings, EEG/MEG, and the local field potential. They have been linked to the population firing rate of neurons, with individual neurons firing in a close-to-irregular fashion at low rates. Using a combination of mean-field and linear response theory we predict the spectra generated in a layered microcircuit model of V1, composed of leaky integrate-and-fire neurons and based on connectivity compiled from anatomical and electrophysiological studies. The model exhibits low- and high-γ oscillations visible in all populations. Since locally generated frequencies are imposed onto other populations, the origin of the oscillations cannot be deduced from the spectra. We develop an universally applicable systematic approach that identifies the anatomical circuits underlying the generation of oscillations in a given network. Based on a theoretical reduction of the dynamics, we derive a sensitivity measure resulting in a frequency-dependent connectivity map that reveals connections crucial for the peak amplitude and frequency of the observed oscillations and identifies the minimal circuit generating a given frequency. The low-γ peak turns out to be generated in a sub-circuit located in layer 2/3 and 4, while the high-γ peak emerges from the inter-neurons in layer 4. Connections within and onto layer 5 are found to regulate slow rate fluctuations. We further demonstrate how small perturbations of the crucial connections have significant impact on the population spectra, while the impairment of other connections leaves the dynamics on the population level unaltered. The study uncovers connections where mechanisms controlling the spectra of the cortical microcircuit are most effective.

Diverse Ensembles of Inhibitory Interneurons

In this issue of Neuron, Karnani etal. (2016) show that ensembles of specific types of inhibitory interneurons generate coordinated activity in mouse visual cortex. They also describe chemical and electrical synaptic mechanisms that may enable diverse interneuron ensembles to function as distinct operational units.

Layer Specific Development of Neocortical Pyramidal to Fast Spiking Cell Synapses.

All cortical neurons are engaged in inhibitory feedback loops which ensure excitation-inhibition balance and are key elements for the development of coherent network activity. The resulting network patterns are strongly dependent on the strength and dynamic properties of these excitatory-inhibitory loops which show pronounced regional and developmental diversity. Therefore we compared the properties and postnatal maturation of two different synapses between rat neocortical pyramidal cells (layer 2/3 and layer 5, respectively) and fast spiking (FS) interneurons in the corresponding layer. At P14, both synapses showed synaptic depression upon repetitive activation. Synaptic release properties between layer 2/3 pyramidal cells and FS cells were stable from P14 to P28. In contrast, layer 5 pyramidal to FS cell connections showed a significant increase in paired pulse ratio by P28. Presynaptic calcium dynamics also changed at these synapses, including sensitivity to exogenously loaded calcium buffers and expression of presynaptic calcium channel subtypes. These results underline the large variety of properties at different, yet similar, synapses in the neocortex. They also suggest that postnatal maturation of the brain goes along with increasing differences between synaptically driven network activity in layer 5 and layer 2/3.

Immunocytochemical heterogeneity of somatostatin-expressing GABAergic interneurons in layers II and III of the mouse cingulate cortex: A combined immunofluorescence/design-based stereologic study.

Many neurological diseases including major depression and schizophrenia manifest as dysfunction of the GABAergic system within the cingulate cortex. However, relatively little is known about the properties of GABAergic interneurons in the cingulate cortex. Therefore, we investigated the neurochemical properties of GABAergic interneurons in the cingulate cortex of FVB-Tg(GadGFP)45704Swn/J mice expressing green fluorescent protein (GFP) in a subset of GABAergic interneurons (GFP-expressing inhibitory interneurons [GINs]) by means of immunocytochemical and design-based stereologic techniques. We found that GINs represent around 12% of all GABAergic interneurons in the cingulate cortex. In contrast to other neocortical areas, GINs were only found in cortical layers II and III. More than 98% of GINs coexpressed the neuropeptide somatostatin (SOM), but only 50% of all SOM + neurons were GINs. By analyzing the expression of calretinin (CR), calbindin (CB), parvalbumin, and various neuropeptides, we identified several distinct GIN subgroups. In particular, we observed coexpression of SOM with CR and CB. In addition, we found neuropeptide Y expression almost exclusively in those GINs that coexpressed SOM and CR. Thus, with respect to the expression of calcium-binding proteins and neuropeptides, GINs are surprisingly heterogeneous in the mouse cingulate cortex, and the minority of GINs express only one marker protein or peptide. Furthermore, our observation of overlap between the SOM + and CR + interneuron population was in contrast to earlier findings of non-overlapping SOM + and CR + interneuron populations in the human cortex. This might indicate that findings in mouse models of neuropsychiatric diseases may not be directly transferred to human patients. J. Comp. Neurol. 524:2281-2299, 2016. © 2015 Wiley Periodicals, Inc.

Granule cell excitability mediates gamma and beta oscillations in a model of the dendrodendritic microcircuit

Odors evoke gamma (60 - 100 Hz) and beta (20 - 30 Hz) oscillations in the local field potential (LFP) of the rat olfactory bulb (OB). These oscillations arise from activity in the dendrodendritic microcircuit between excitatory mitral cells (MCs) and inhibitory granule cells (GCs) [1]. When cortical feedback inputs to the OB are blocked, beta oscillations are extinguished while gamma oscillations persist [2]. Much of this cortical feedback targets inhibitory interneurons in the GC layer and regulates the excitability of GCs [3], which suggests a causal link between the emergence of beta oscillations and the GC excitability. We investigate the effect that GC excitability has on network oscillations in a biophysical model of the MC-GC dendrodendritic network with graded inhibition. When GC excitability is low, there is transient activation of NMDAR channels by AMPARs, which produces fast inhibitory pulses in the gamma frequency range. When GC excitability is increased, the activation of NMDARs and other VDCCs is prolonged, allowing the slow decay time constants of these channels to drive beta frequency oscillations. The power of the beta oscillation peaks when inhibitory and excitatory currents onto MCs are balanced, which could explain the relationship between beta power and odor volatility measured experimentally [4].

Optogenetic Mapping of Intracortical Circuits Originating from Semilunar Cells in the Piriform Cortex.

Despite its comparatively simple trilaminar architecture, the primary olfactory (piriform) cortex of mammals is capable of performing sophisticated sensory processing, an ability that is thought to depend critically on its extensive associational (intracortical) excitatory circuits. Here, we used a novel transgenic mouse model and optogenetics to measure the connectivity of associational circuits that originate in semilunar (SL) cells in layer 2a of the anterior piriform cortex (aPC). We generated a mouse line (48L) in which channelrhodopsin-2 (ChR) could be selectively expressed in a subset of SL cells. Light-evoked excitatory postsynaptic currents (EPSCs) could be evoked in superficial pyramidal cells (17.4% of n = 86 neurons) and deep pyramidal cells (33.3%, n = 9) in the aPC, but never in ChR- SL cells (0%, n = 34). Thus, SL cells monosynaptically excite pyramidal cells, but not other SL cells. Light-evoked EPSCs were also selectively elicited in 3 classes of GABAergic interneurons in layer 3 of the aPC. Our results show that SL cells are specialized for providing feedforward excitation of specific classes of neurons in the aPC, confirming that SL cells comprise a functionally distinctive input layer.

Specific Early and Late Oddball-Evoked Responses in Excitatory and Inhibitory Neurons of Mouse Auditory Cortex.

A major challenge for sensory processing in the brain is considering stimulus context, such as stimulus probability, which may be relevant for survival. Excitatory neurons in auditory cortex, for example, adapt to repetitive tones in a stimulus-specific manner without fully generalizing to a low-probability deviant tone ("oddball") that breaks the preceding regularity. Whether such stimulus-specific adaptation (SSA) also prevails in inhibitory neurons and how it might relate to deviance detection remains elusive. We obtained whole-cell recordings from excitatory neurons and somatostatin- and parvalbumin-positive GABAergic interneurons in layer 2/3 of mouse auditory cortex and measured tone-evoked membrane potential responses. All cell types displayed SSA of fast ("early") subthreshold and suprathreshold responses with oddball tones of a deviant frequency eliciting enlarged responses compared with adapted standards. SSA was especially strong when oddball frequency matched neuronal preference. In addition, we identified a slower "late" response component (200-400 ms after tone onset), most clearly in excitatory and parvalbumin-positive neurons, which also displayed SSA. For excitatory neurons, this late component reflected genuine deviance detection. Moreover, intracellular blockade of NMDA receptors reduced early and late responses in excitatory but not parvalbumin-positive neurons. The late component in excitatory neurons thus shares time course, deviance detection, and pharmacological features with the deviant-evoked event-related potential known as mismatch negativity (MMN) and provides a potential link between neuronal SSA and MMN. In summary, our results suggest a two-phase cortical activation upon oddball stimulation, with oddball tones first reactivating the adapted auditory cortex circuitry and subsequently triggering delayed reverberating network activity. Significance statement: Understanding how the brain encodes sensory context in addition to stimulus feature has been a main focus in neuroscience. Using in vivo targeted whole-cell recordings from excitatory and inhibitory neurons of mouse primary auditory cortex, we report two temporally distinct components of membrane potential responses encoding oddball tones that break stimulus regularity. Both components display stimulus-specific adaptation upon oddball paradigm stimulation in the three recorded cell types. The late response component, in particular, carries signatures of genuine deviance detection. In excitatory but not parvalbumin-positive inhibitory neurons, both early and late components depend on NMDA receptor-signaling. Our work proposes a potential neuronal substrate of a known deviant-evoked event-related potential, which is of fundamental significance in basic and clinical neuroscience.

Delayed mutual information infers patterns of synaptic connectivity in a proprioceptive neural network.

Understanding the patterns of interconnections between neurons in complex networks is an enormous challenge using traditional physiological approaches. Here we combine the use of an information theoretic approach with intracellular recording to establish patterns of connections between layers of interneurons in a neural network responsible for mediating reflex movements of the hind limb of an insect. By analysing delayed mutual information of the synaptic and spiking responses of sensory neurons, spiking and nonspiking interneurons in response to movement of a joint receptor that monitors the position of the tibia relative to the femur, we are able to predict the patterns of interconnections between the layers of sensory neurons and interneurons in the network, with results matching closely those known from the literature. In addition, we use cross-correlation methods to establish the sign of those interconnections and show that they also show a high degree of similarity with those established for these networks over the last 30 years. The method proposed in this paper has great potential to elucidate functional connectivity at the neuronal level in many different neuronal networks.

Abstract W P107: HDAC4 Shuttling in Interneurons and Pyramidal Neurons is Associated with Improved Axonal Density and Myelination after Stroke

Background: We have previously demonstrated that stroke induces nuclear shuttling of histone deacetylase 4 (HDAC4) in neurons during stroke recovery. Further examination of the distribution of HDAC4 shuttling in interneurons and pyramidal neurons may provide new insights into mechanisms of HDAC4 mediated neuronal circuitry remodeling after stroke. In the current study, we measured nuclear shuttling of HDAC4 in these two populations of neurons in the peri-infarct cortical layers after stroke. Methods: Adult male Wistar rats (n=6/group) were subjected to permanent middle cerebral artery occlusion (MCAO) and sacrificed at 2 or 14 days after MCAO. Sham operated rats were used as a control. Double immunohistochemistry was performed with antibodies against HDAC4 along with microtubule-associated protein 2 (MAP2, neurons), parvalbumin (PV, interneurons), Ctip2 (pyramidal neurons), phosphorylated neurofilament heavy chain (pNFH, axons) and myelin basic protein (MBP, myelination). Results: HDAC4 nuclear immunoreactivity in MAP2+ neurons of the normal cortex was evenly distributed throughout the upper (L1-4) and lower cortical layers (L5-6). Stroke significantly (p<0.05) increased the percentage of neurons with nuclear HDAC4 in L5-6 at 2 (37%) and 14 days (73%) compared to sham (19%), whereas in L1-4, a significant increase in neurons with nuclear HDAC4 was detected only at 14 days after MCAO (49% vs 17%). Among neurons, stroke significantly increased the number of Ctip2+ and PV+ neurons with nuclear HDAC4 in L5-6 at 2 (Ctip2 57%, PV 66%) and 14 days after stroke (Ctip2 73%, PV 74%) compared to sham (Ctip2 33%, PV 12%). Furthermore, in L5-6, but not in L1-4, nuclear HDAC4 shuttling was positively and significantly correlated with increased axons and myelination as determined by pNFH (r = 0.84, p<0.05) and MBP (r = 0.76, p<0.05) densities, respectively. Conclusion: Stroke induces nuclear shuttling of HDAC4 in pyramidal neurons and interneurons of layers 5-6 of the peri-infarct cortex which is strongly correlated with enhanced axonal density and myelination, suggesting a role for HDAC4 in promoting pyramidal neuron and interneuron axon remodeling during stroke recovery.

Functional properties of GABA synaptic inputs onto GABA neurons in monkey prefrontal cortex.

In rodent cortex GABAA receptor (GABAAR)-mediated synapses are a significant source of input onto GABA neurons, and the properties of these inputs vary among GABA neuron subtypes that differ in molecular markers and firing patterns. Some features of cortical interneurons are different between rodents and primates, but it is not known whether inhibition of GABA neurons is prominent in the primate cortex and, if so, whether these inputs show heterogeneity across GABA neuron subtypes. We thus studied GABAAR-mediated miniature synaptic events in GABAergic interneurons in layer 3 of monkey dorsolateral prefrontal cortex (DLPFC). Interneurons were identified on the basis of their firing pattern as fast spiking (FS), regular spiking (RS), burst spiking (BS), or irregular spiking (IS). Miniature synaptic events were common in all of the recorded interneurons, and the frequency of these events was highest in FS neurons. The amplitude and kinetics of miniature inhibitory postsynaptic potentials (mIPSPs) also differed between DLPFC interneuron subtypes in a manner correlated with their input resistance and membrane time constant. FS neurons had the fastest mIPSP decay times and the strongest effects of the GABAAR modulator zolpidem, suggesting that the distinctive properties of inhibitory synaptic inputs onto FS cells are in part conferred by GABAARs containing α1 subunits. Moreover, mIPSCs differed between FS and RS interneurons in a manner consistent with the mIPSP findings. These results show that in the monkey DLPFC GABAAR-mediated synaptic inputs are prominent in layer 3 interneurons and may differentially regulate the activity of different interneuron subtypes.

Neocortical calretinin neurons in primates: increase in proportion and microcircuitry structure.

In this article we first point at the expansion of associative cortical areas in primates, as well as at the intrinsic changes in the structure of the cortical column. There is a huge increase in proportion of glutamatergic cortical projecting neurons located in the upper cortical layers (II/III). Inside this group, a novel class of associative neurons becomes recognized for its growing necessity in both inter-areal and intra-areal columnar integration. Equally important to the changes in glutamatergic population, we found that literature data suggest a 50% increase in the proportion of neocortical GABAergic neurons between primates and rodents. This seems to be a result of increase in proportion of calretinin interneurons in layers II/III, population which in associative areas represents 15% of all neurons forming those layers. Evaluating data about functional properties of their connectivity we hypothesize that such an increase in proportion of calretinin interneurons might lead to supra-linear growth in memory capacity of the associative neocortical network. An open question is whether there are some new calretinin interneuron subtypes, which might substantially change micro-circuitry structure of the primate cerebral cortex.

RELN-expressing neuron density in layer I of the superior temporal lobe is similar in human brains with autism and in age-matched controls

Reelin protein (RELN) level is reduced in the cerebral cortex and cerebellum of subjects with autism. RELN is synthesized and secreted by a subpopulation of neurons in the developing cerebral cortex termed Cajal–Retzius (CR) cells. These cells are abundant in the marginal zone during cortical development, many die after development is complete, but a small population persists into adulthood. In adult brains, RELN is secreted by the surviving CR cells, by a subset of GABAergic interneurons in layer I, and by pyramidal cells and GABAergic interneurons in deeper cortical layers. It is widely believed that decreased RELN in layer I of the cerebral cortex of subjects with autism may result from a decrease in the density of RELN expressing neurons in layer I; however, this hypothesis has not been tested. We examined RELN expression in layer I of the adult human cortex and found that 70% of cells express RELN in both control and autistic subjects. We quantified the density of neurons in layer I of the superior temporal cortex of subjects with autism and age-matched control subjects. Our data show that there is no change in the density of neurons in layer I of the cortex of subjects with autism, and therefore suggest that reduced RELN expression in the cerebral cortex of subjects with autism is not a consequence of decreased numbers of RELN-expressing neurons in layer I. Instead reduced RELN may result from abnormal RELN processing, or a decrease in the number of other RELN-expressing neuronal cell types.

Excitatory and inhibitory synaptic connectivity to layer V fast-spiking interneurons in the freeze lesion model of cortical microgyria.

A variety of major developmental cortical malformations are closely associated with clinically intractable epilepsy. Pathophysiological aspects of one such disorder, human polymicrogyria, can be modeled by making neocortical freeze lesions (FL) in neonatal rodents, resulting in the formation of microgyri. Previous studies showed enhanced excitatory and inhibitory synaptic transmission and connectivity in cortical layer V pyramidal neurons in the paramicrogyral cortex. In young adult transgenic mice that express green fluorescent protein (GFP) specifically in parvalbumin positive fast-spiking (FS) interneurons, we used laser scanning photostimulation (LSPS) of caged glutamate to map excitatory and inhibitory synaptic connectivity onto FS interneurons in layer V of paramicrogyral cortex in control and FL groups. The proportion of uncaging sites from which excitatory postsynaptic currents (EPSCs) could be evoked (hotspot ratio) increased slightly but significantly in FS cells of the FL vs. control cortex, while the mean amplitude of LSPS-evoked EPSCs at hotspots did not change. In contrast, the hotspot ratio of inhibitory postsynaptic currents (IPSCs) was significantly decreased in FS neurons of the FL cortex. These alterations in synaptic inputs onto FS interneurons may result in an enhanced inhibitory output. We conclude that alterations in synaptic connectivity to cortical layer V FS interneurons do not contribute to hyperexcitability of the FL model. Instead, the enhanced inhibitory output from these neurons may partially offset an earlier demonstrated increase in synaptic excitation of pyramidal cells and thereby maintain a relative balance between excitation and inhibition in the affected cortical circuitry.

Multiple rhythms from one network: phase plane and stochastic analyses of rhythmic activity in turtle motor circuits

A central pattern generator (CPG) is a population of neurons producing rhythmic or repetitive behavior (i.e. scratching, walking) without requiring rhythmic input to the population. Turtles are observed to produce several rhythmic motor patterns in response to stimuli, in particular rostral scratch and pocket scratch (see Figure ​Figure1B1B inset) [1,2]. The rostral scratch and pocket scratch rhythms are created through the activity of three motoneurons: hip extensor, knee extensor, and hip flexor. A CPG (see Figure ​Figure1A)1A) to produce both rhythms has been proposed, featuring a layer of populations of excitatory and inhibitory interneurons projecting to a layer of motoneurons [1]. We implement the CPG as a system of relaxation oscillators. The system successfully reproduces the desired rhythms by changing inputs to the layer of interneurons (see Figure ​Figure1B).1B). We consider a dynamical systems approach to determine the mechanisms underlying rhythm generation. We analyze a proposed central pattern generator’s ability to produce differing motor patterns from a single pool of neurons under different tonic drives. A key issue is the knee extensor motoneuron’s response to different phasic synaptic inputs. We study the impact of these phasic inputs on motoneuron phase space and on properties of associated trajectories and show how these yield sufficient conditions for reproduction of observed rhythms. A contraction argument leads to existence of a stable solution. We also present preliminary results from stochastic analysis to examine the role of strong synaptic conductances. Figure 1 A. Implemented network architecture, adapted from [1] featuring a layer of populations of excitatory (EP, ER) and inhibitory (IP, IR) receiving external stimulation and driving a layer of populations of motoneurons (HE, KE, HF). Dashed arrows indicate ...

Neuronal connectome of a sensory-motor circuit for visual navigation.

Animals use spatial differences in environmental light levels for visual navigation; however, how light inputs are translated into coordinated motor outputs remains poorly understood. Here we reconstruct the neuronal connectome of a four-eye visual circuit in the larva of the annelid Platynereis using serial-section transmission electron microscopy. In this 71-neuron circuit, photoreceptors connect via three layers of interneurons to motorneurons, which innervate trunk muscles. By combining eye ablations with behavioral experiments, we show that the circuit compares light on either side of the body and stimulates body bending upon left-right light imbalance during visual phototaxis. We also identified an interneuron motif that enhances sensitivity to different light intensity contrasts. The Platynereis eye circuit has the hallmarks of a visual system, including spatial light detection and contrast modulation, illustrating how image-forming eyes may have evolved via intermediate stages contrasting only a light and a dark field during a simple visual task.