Excitatory Interneurons Research Articles

Functional subdivision of feline spinal interneurons in reflex pathways from group Ib and II muscle afferents; an update

A first step towards understanding the operation of a neural network is identification of the populations of neurons that contribute to it. Our aim here is to reassess the basis for subdivision of adult mammalian spinal interneurons that mediate reflex actions from tendon organs (group Ib afferents) and muscle spindle secondary endings (group II afferents) into separate populations. Re-examining the existing experimental data, we find no compelling reasons to consider intermediate zone interneurons with input from group Ib afferents to be distinct from those co-excited by group II afferents. Similar patterns of distributed input have been found in subpopulations that project ipsilaterally, contralaterally or bilaterally, and in both excitatory and inhibitory interneurons; differences in input from group I and II afferents to individual interneurons showed intra- rather than inter-population variation. Patterns of reflex actions evoked from group Ib and II afferents and task-dependent changes in these actions, e.g. during locomotion, may likewise be compatible with mediation by premotor interneurons integrating information from both group I and II afferents. Pathological changes after injuries of the central nervous system in humans and the lineage of different subclasses of embryonic interneurons may therefore be analyzed without need to consider subdivision of adult intermediate zone interneurons into subpopulations with group Ib or group II input. We propose renaming these neurons 'group I/II interneurons'.

Distinctive properties of CXC chemokine receptor 4‐expressing Cajal–Retzius cells versus GABAergic interneurons of the postnatal hippocampus

The CXC chemokine receptor 4 (CXCR4) for the chemokine (C-X-C motif) ligand 12/stromal cell-derived factor-1 alpha (CXCL12/SDF-1 alpha) is highly expressed in the postnatal CA1 stratum lacunosum-moleculare. However, both the network events triggered by SDF-1 alpha in this microcircuit and the cellular targets of this chemokine remain virtually unexplored. Here, we have studied SDF-1 alpha-mediated neuromodulation of the stratum lacunosum-moleculare by directly comparing the properties of CXCR4-expressing Cajal-Retzius cells vs. CXCR4-non-expressing interneurons, and by recording the electrophysiological effects caused by application of SDF-1 alpha on either cell type. We demonstrate that SDF-1 alpha dramatically reduces spontaneous firing in Cajal-Retzius cells via hyerpolarization, and that cessation of firing is prevented by the CXCR4-specific antagonist AMD3100. In contrast, no effects on the excitability of interneurons of the same layer were observed following exposure to the chemokine. We also provide evidence that, despite the expression of functional glutamate receptors, Cajal-Retzius cells are integrated in the synaptic network of the stratum lacunosum-moleculare via excitatory GABAergic input. Furthermore, we show that the axons of Cajal-Retzius cells target specifically the stratum lacunosum-moleculare and the dentate gyrus, but lack postsynaptic specializations opposite to their axonal varicosities. These results, taken together with our observation that SDF-1 alpha reduces evoked field responses at the entorhinal cortex-CA1 synapse, suggest that Cajal-Retzius cells produce a diffuse output that may impact information processing of stratum lacunosum-moleculare. We propose that pathological alterations of local levels of SDF-1 alpha or CXCR4 expression may affect the functions of an important hippocampal microcircuit.

Endogenous SNAP-25 Regulates Native Voltage-gated Calcium Channels in Glutamatergic Neurons

In addition to its primary role as a fundamental component of the SNARE complex, SNAP-25 also modulates voltage-gated calcium channels (VGCCs) in various overexpression systems. Although these studies suggest a potential negative regulatory role of SNAP-25 on VGCC activity, the effects of endogenous SNAP-25 on native VGCC function in neurons are unclear. In the present study, we investigated the VGCC properties of cultured glutamatergic and GABAergic rat hippocampal neurons. Glutamatergic currents were dominated by P/Q-type channels, whereas GABAergic cells had a dominant L-type component. Also, glutamatergic VGCC current densities were significantly lower with enhanced inactivation rates and shifts in the voltage dependence of activation and inactivation curves compared with GABAergic cells. Silencing endogenous SNAP-25 in glutamatergic neurons did not alter P/Q-type channel expression or localization but led to increased VGCC current density without changes in the VGCC subtype proportions. Isolation of the P/Q-type component indicated that increased current in the absence of SNAP-25 was correlated with a large depolarizing shift in the voltage dependence of inactivation. Overexpressing SNAP-25 in GABAergic neurons reduced current density without affecting the VGCC subtype proportion. Accordingly, VGCC current densities in glutamatergic neurons from Snap-25(+/-) mice were significantly elevated compared with wild type glutamatergic neurons. Overall, this study demonstrates that endogenous SNAP-25 negatively regulates native VGCCs in glutamatergic neurons which could have important implications for neurological diseases associated with altered SNAP-25 expression.

Central neuron-glia interactions and neuropathic pain

Neuropathic pain results from injury or disease causing dysfunction at any level of the somatosensory (primarily spinothalamic) system, including peripheral nociceptive axons, dorsal root ganglion (DRG), dorsal horn, spinothalamic pathway, and thalamus. The manifestations of neuropathic pain, including spontaneous pain, hyperalgesia, and thermal and mechanical allodynia, reflect excessive excitability of peripheral nociceptors (peripheral sensitization), central nociceptive neurons (central sensitization), or both. Sensitization reflects maladaptive plastic changes in the nociceptive system that result directly from axonal injury and from the effects of products of inflammation. There is abundant evidence that activation of microglia and astrocytes in the dorsal horn is common and constitutes an important amplification mechanism leading to neuropathic pain in the setting of peripheral nerve or spinal cord injury. Activated glial cells are also involved in sensitization of brainstem and thalamic neurons at a distance from the site of injury. The complex roles of the central glia and its mediators in the mechanisms of neuropathic pain have been extensively reviewed.1–9 The components of transmission of nociceptive information (“pain system”) include the nociceptive neurons of the DRG with unmyelinated C- and small myelinated Aδ axons that activate different types of neurons in the dorsal horn, particularly in lamina I and lamina V, which project via parallel pathways to the thalamus and to the brainstem. The primary DRG afferents also activate excitatory and inhibitory interneurons in lamina II that exert local control on nociceptive transmission.10–12 Similar organization occurs in the nociceptive trigeminal system. The nociceptive DRG neurons express different types of voltage-gated sodium (Na+) channels (particularly Nav1.7, Nav1.8, and Nav1.9), transient receptor potential (TRP) channels (including TRPV1 and TRPA), acid-sensitive channels, serotonin 5-HT3 receptors, and purinergic P2X receptors that are activated by noxious mechanical or chemical stimuli.11 In normal conditions, activation of Aδ and C-fiber nociceptors leads …

Periodically forced neural mass model: entrainment and complex behavior

Neural mass models (NMM) provide a parsimonious, yet biophysically plausible, way to model mass action in the nervous system [1]. It has been demonstrated that, even in the absence of any time-variant input, they are capable of producing a number of biologically relevant behavior [2]. Moreover, by slowly traversing the parameter space, they can serve as generative models for ordered sequences of oscillatory regimes observed in EEG or MEG data [2]. In the brain, neural circuits are embedded in global networks and may experience high amplitude input from other areas. Such input is very often periodic. Here, we systematically investigate the behavior of a NMM of a cortical area, comprising three neural masses for pyramidal cells and inhibitory and excitatory interneurons, in response to periodic input, as a function of input amplitude and frequency. In our previous work we studied the system with constant input [2]. Two mechanisms for oscillations could be identified: homoclinic and Hopf cycles. Here, we extend our study by considering periodic pulse-like input and systematically vary amplitude and frequency of the input for the widely used parameter set proposed by Jansen and Rit [1]. The input frequency ranges between 4 Hz and 19 Hz including the intrinsic frequency (correponding to the Hopf cycle), and the amplitude values are 1/3, 2/3, 1, 4/3 and 5/3 of amplitude of the Hopf cycle in the unperturbed system. We study the phase space orbits and the time evolution of all variables. The largest Lyapunov exponents and the spectra of the time series are calculated. For studying the relationship of system input and response we compute the frequency-detuning curves, i.e., the frequency difference of response and input plotted against the input frequency. For slight perturbation we found frequency entrainment meaning that the cortical area responds with the stimulus frequency instead of the intrinsic frequency. Away from the entrainment chaotic behavior could be observed, indicated by broadband frequencies in the spectra and positive Lyapunov exponents, whose value in our example is 0.153 (see Figure ​Figure1).1). We also found quasi-periodic and multi-periodic behavior (high-dimensional tori), where the corresponding spectra are discrete with multiple frequency components. Figure 1 Chaotic behavior in neural mass models represented in A time series, B power spectrum, C phase space.

Neural field model of rat's cortex based on realistic connectivity from diffusion weighted MRI and neural morphology

Generative models of neural circuits may help to create a link between neural mechanisms and observable data. We propose a model of rat's cortex using a neural field model containing biologically plausible anatomical connections from tractography based on dwMRI data and from the neural morphological database NeuroMorpho [1]. There are three principal types of anatomical connections in the cortex: Local, long-range and distal connections [2]. For specifying local connections we use neural morphologies from [1]. We consider each voxel in the model as a neural mass and distribute randomly drawn neurons from the database therein. After that we use bootstrap methods to determine the total number and variability of synaptic contacts. For the distal connectivity we estimated the degree of anatomical connectedness using white matter tractography on the basis of diffusion weighted MRI [3]. Our neural field consists of 5 layers. For each layer we assume three different neural masses: pyramidal cells, excitatory and inhibitory interneurons [4]. The mutual interactions between neural masses will be described by a system of integral differential equations: where V is the state vector (mean membrane potentials), T the time delays in the dendritic arbors, S the sigmoidal output function, W the connection coefficients between the neural elements, I the input, and t(d) is the distance dependent time delays. Figure ​Figure1A1A shows the estimated spatial dependency of local connectivity, which is in accordance with anatomical observations [2]. The distal connectivity map of 28 regions is displayed in Fig. ​Fig.1B.1B. An example map of simulated activity on the cortex in response to a stimulus to somatosensory cortex is illustrated in Fig. ​Fig.1C.1C. Figure 1 (A) Local connectivity profile between a pyramidal cell in layer V and an interneuron in layer III (grid size 250 µm). (B) Distal connectivity map estimated from dwMRI between 28 regions. (C) Spatio-temporal activity pattern 100 ms after the onset ... To summarize, we developed a method for estimating the local connectivity and constructed a neural field model of the entire cortex enriched by estimated local and distal connectivities. With this model we are able to simulate spatio-temporal activity patterns. This is a first step for a comprehensive dynamic brain model and thereby for understanding complex brain processes.

Circuit breaking the synaptic connections of the superficial dorsal horn of the spinal cord

Circuit breaking the synaptic connections of the superficial dorsal horn of the spinal cord

Bidirectional Neuron–Glia Interactions Triggered by Deficiency of Glutamate Uptake at Spinal Sensory Synapses

Bidirectional interactions between neurons and glial cells are crucial to the genesis of pathological pain. The mechanisms regulating these interactions and the role of this process in relaying synaptic input in the spinal dorsal horn remain to be established. We studied the role of glutamate transporters in the regulation of such interactions. On pharmacological blockade of glutamate transporters, slow inward currents (SICs) appeared spontaneously and/or were evoked by peripheral synaptic input in the spinal superficial dorsal horn neurons, including the spinothalamic tract neurons. We showed that the SICs were induced by the release of glutamate from glial cells. On inhibition of glutamate uptake, the stimulation-induced, synaptically released glutamate activated glial cells and caused glial cells to release glutamate. Glial-derived glutamate acted on extrasynaptic N-methyl-d-aspartate (NMDA) receptors mainly composed of NR2B receptors and generated SICs, which led to depolarization and action potential generation in superficial spinal dorsal horn neurons. Thus glutamate transporters regulate glutamatergic neuron-glia interactions at spinal sensory synapses. When glutamate uptake is impaired, glial cells function like excitatory interneurons-they are activated by peripheral synaptic input and release glutamate to activate postsynaptic neurons in spinal pain pathways.

Noradrenaline-induced spontaneous inhibitory postsynaptic currents in mouse basolateral nucleus of amygdala pyramidal neurons: Comparison with dopamine-induced currents

The basolateral nucleus of the amygdala (BLA) receives both noradrenergic and dopaminergic projections. These projections are thought to be important for modulation of amygdala neural circuits. In BLA pyramidal neurons, noradrenaline (NA) is known to facilitate gamma-aminobutyric acid (GABA)ergic spontaneous inhibitory postsynaptic currents (sIPSCs) through excitation of interneurons. Dopamine (DA) also is known to facilitate GABAergic sIPSCs in pyramidal neurons of the amygdala region including the BLA. It is unclear which neurotransmitter, NA or DA, is predominant in facilitating sIPSC in the BLA. Whether NA and DA facilitate sIPSC in different or the same pyramidal neurons also remains unknown. Herein, we employed the patch clamp recording technique on BLA pyramidal neurons in mouse brain slices, and compared the facilitating actions of NA and DA on sIPSCs. First NA and then DA, or first DA and then NA, were applied to a slice. NA enhanced sIPSC frequency in the majority (80–90%) of pyramidal neurons tested, whereas DA enhanced sIPSC frequency in relatively few neurons (approximately 30%). Neurons responding to NA alone and DA alone accounted, respectively, for 54.3% and 2.9% of the pyramidal neurons tested (11.4% of neurons responded to neither NA nor DA). Pyramidal neurons in which NA and DA both facilitated sIPSCs accounted for 31.4% of neurons tested. These results suggest that NA facilitates GABAergic sIPSCs in a larger proportion of mouse BLA pyramidal neurons than DA.

Functional organization of V2a‐related locomotor circuits in the rodent spinal cord

Studies of mammalian locomotion have been greatly facilitated by the use of the isolated rodent spinal cord preparation that retains the locomotor circuits needed to execute the movement. Physiological and molecular genetic experiments in this preparation have started to unravel the basic circuit organization responsible for walking in mammals. Here, we review these experiments with a focus on the functional role of excitatory V2a interneurons in the mammalian locomotor network. With regard to these neurons and other network structures we also discuss similarities and differences between the mammalian walking central pattern generator (CPG) and the fish swimming CPG.

Synaptic integration of rhythmogenic neurons in the locomotor circuitry: the case of Hb9 interneurons

Innovative molecular and genetic techniques have recently led to the identification of genetically defined populations of ipsilaterally projecting excitatory interneurons with probable functions in the rhythm-generating kernel of the central pattern generators (CPGs). The role of interneuronal populations in specific motor function is determined by their synaptic inputs, intrinsic properties, and target neurons. In this review we examine whether Hb9-expressing interneurons (Hb9 INs) fulfill a set of criteria that are the hallmarks of rhythm generators in the locomotor circuitry. Induced locomotor-like activity in this distinct population of ventral interneurons is in phase with bursts of motor activity, raising the possibility that they are part of the locomotor generator. To increase our understanding of the integrative function of Hb9 INs in the locomotor CPG, we investigated the cellular mechanisms underlying their rhythmic activity and examined the properties of synaptic inputs from low-threshold afferents and possible synaptic contacts with segmental motoneurons. Our findings suggest that the rhythmogenic Hb9 INs are integral components of the sensorimotor circuitry that regulate locomotor-like activity in the spinal cord.

Mechanisms of excitation of spinal networks by stimulation of the ventral roots

It has recently been demonstrated that motoneurons in neonatal rodents release an excitatory amino acid, in addition to acetylcholine, from their central terminals onto Renshaw cells. Although the function of this amino acid release is not understood, it may mediate the excitatory actions of motor axon stimulation on spinal motor networks. Stimulation of motor axons in the ventral roots or muscle nerves can activate the locomotor central pattern generator or entrain bursting in the disinhibited cord. Both of these effects persist in the presence of cholinergic antagonists and are abolished or diminished by ionotropic and metabotropic glutamate antagonists. Calcium imaging in the disinhibited cord shows that a ventral root stimulus evokes ventrolateral activity initially, which subsequently propagates to the rest of the cord. This finding suggests that excitatory interneurons excited by motoneuron recurrent collaterals are located in this region. However, motoneurons do not exhibit short latency excitatory potentials in response to ventral root stimulation indicating that the excitatory effects are mediated polysynaptically. We discuss the significance of these findings.

M 1-like muscarinic acetylcholine receptors regulate fast-spiking interneuron excitability in rat dentate gyrus

Cholinergic transmission through muscarinic acetylcholine receptors (mAChRs) plays a key role in cortical oscillations. Although fast-spiking (FS), parvalbumin-expressing basket cells (BCs) are proposed to be the cellular substrates of gamma oscillations, previous studies reported that FS nonpyramidal cells in neocortical areas are unresponsive to cholinergic modulation. Dentate gyrus (DG) is an independent gamma oscillator in the hippocampal formation. However, in contrast to other cortical regions, the direct impact of mAChR activation on FS BC excitability in this area has not been investigated. Here, we show that bath-applied muscarine or carbachol, two mAChR agonists, depolarize DG BCs in the acute brain slices, leading to action potential firing in the theta-gamma bands in the presence of blockers of ionotropic glutamate and γ-aminobutyric acid type A receptors at physiological temperatures. The depolarizing action persists in the presence of tetrodotoxin, a voltage-gated Na + channel blocker. In voltage-clamp recordings, muscarine markedly reduces background K + currents. These effects are mimicked by oxotremorine methiodide, an mAChR-specific agonist, and largely reversed by atropine, a non-selective mAChR antagonist, or pirenzepine, an M 1 receptor antagonist, but not by gallamine, an M 2/4 receptor antagonist. Interestingly, in contrast to M 1-receptor-mediated depolarization, M 2 receptor activation by the specific agonist arecaidine but-2-ynyl ester tosylate down-regulates GABA release at BC axons—the effect is occluded by gallamine, an M 2 receptor antagonist. Overall, muscarinic activation results in a net increase in phasic inhibitory output to the target cells. Thus, cholinergic activation through M 1-like receptor enhances BC activity and promotes the generation of nested theta and gamma rhythms, thereby enhancing hippocampal function and associated performance.

Lesioning alters functional properties in isolated spinal cord hemisegmental networks

Hemisegmental networks produced by longitudinal lesions of the spinal cord midline are able to generate rhythmic bursting activity. This has led to the suggestion that hemisegmental networks can independently burst in the intact spinal cord. Previous analyses in the lamprey spinal cord failed to show hemisegmental bursting in NMDA. This was subsequently attributed to the failure to wait sufficient time for NMDA-evoked hemisegmental activity to recover after being abolished by the lesion, which can take tens of minutes to hours. The reason for this delay in the onset of NMDA-evoked activity was not previously addressed. We have investigated it here by examining two hypotheses: that hemisegmental networks intrinsically burst under normal conditions but that NMDA-evoked bursting was temporarily silenced by lesion-induced transmitter release; or that lesioning altered functional properties in the hemisegment that subsequently led to the development of bursting. We found no evidence to support transmitter-induced silencing of ongoing NMDA-evoked hemisegmental activity, but did find evidence for significant changes in the cellular and synaptic properties of motor neurons and premotor excitatory interneurons in lesioned hemisegmental networks. These results thus suggest that there are lesion-induced changes in functional properties in hemisegmental networks. As the interpretation of lesion studies rests on the assumption that the functional properties of hemisegmental components are not altered, further work is needed before conclusions can be made about the function of the intact system.

ClC-2 Voltage-Gated Channels Constitute Part of the Background Conductance and Assist Chloride Extrusion

The function of voltage-gated chloride channels in neurons is essentially unknown. The voltage-gated chloride channel ClC-2 mediates a chloride current in pyramidal cells of the hippocampus. We directly show that ClC-2 assists chloride extrusion after high chloride load. Furthermore, the loss of this chloride channel leads to a dramatic increase of the input resistance of CA1 pyramidal cells, making these cells more excitable. Surprisingly, basal synaptic transmission, as judged from recordings of field EPSPs, was decreased. This difference was eliminated when GABAergic inhibition was blocked. Recordings from hippocampal interneurons revealed ClC-2-mediated currents in a subset of these cells. An observed increase in GABAergic inhibition could thus be explained by an increase in the excitability of interneurons, caused by the loss of ClC-2. Together, we suggest a dual role for ClC-2 in neurons, providing an additional efflux pathway for chloride and constituting a substantial part of the background conductance, which regulates excitability. In ClC-2 knock-out mice, an increased inhibition seemingly balances the hyperexcitability of the network and thereby prevents epilepsy.

Recent advances in pain medicine: from bench to bed; August 17, 2009, Kobe, Japan

In late 2000, the US Congress passed into law a provision, which the President signed, that declared a 10-year period as the Decade of Pain Control and Research which began from January 1, 2001. The American Pain Society has actively supported the Decade of Pain Control and Research, and it has been a focal point for the development of numerous programs to heighten awareness and treatment of pain and research funding. Cutaneous sensation produced by noxious and innocuous stimuli is transmitted to the dorsal horn of the spinal cord via primary afferent fibers and recognized as pain and touch in the somatosensory cortex. While acute pain has a fundamental role as a protective system against the occurrence or threat of injuries and diseases, chronic pain associated with diseases such as postherpetic neuralgia, and rheumatoid arthritis often outlasts its biological usefulness and in itself causes functional disturbance. Neuropathic pain is a significant and largely unresolved medical problem characterized by spontaneous pain, hyperalgesia, and tactile pain (allodynia). Over several decades, non-steroidal antiinflammatory drugs (NSAIDs) and opioids have been used for clinical management of pain. Neuropathic pain is persistent and can be particularly problematic because they are often poorly managed by NSAIDs and conventional opioid analgesics. Kohno demonstrated that although excitatory transmission remained intact, presynaptic GABA release was reduced in lamina II neurons of an isolated adult rat spinal cord slice preparation in neuropathic pain models. He also reviewed the role of neuron–microglia interactions in the spinal cord in neuropathic pain [1]. Pain associated with cancer, particularly when tumors metastasize to bone, is often severe and debilitating. Better understanding of the neurobiological mechanisms underlying cancer pain are likely to lead to the development of more effective treatments. Kawamata et al. demonstrated that transient receptor potential vanilloid subfamily 1 (TRPV1) was up-regulated, and the l opioid receptor was down-regulated in peripheral neurons in bone cancer. Activation of spinal cannabinoid receptor type 1 (CB1) has been shown to inhibit glutamatergic excitatory postsynaptic currents in the spinal cord slices of naive rats and C-fiber-evoked neuronal response of the dorsal horn. He found that CB1 was located in the axon terminals of excitatory spinal interneuron, and its expression is preserved in bone cancer. Furthermore, he reported that spinal CB1 activation by an exogenously administered CB1 agonist reduced bone cancer-related pain. Therefore, spinal T. Minami (&) Department of Anesthesiology, Osaka Medical College, 2-7 Daigaku-machi, Takatsuki, Osaka 569-8686, Japan e-mail: ane022@poh.osaka-med.ac.jp

Embryonic MGE Precursor Cells Grafted into Adult Rat Striatum Integrate and Ameliorate Motor Symptoms in 6-OHDA-Lesioned Rats

We investigated a strategy to ameliorate the motor symptoms of rats that received 6-hydroxydopamine (6-OHDA) lesions, a rodent model of Parkinson's disease, through transplantation of embryonic medial ganglionic eminence (MGE) cells into the striatum. During brain development, embryonic MGE cells migrate into the striatum and neocortex where they mature into GABAergic interneurons and play a key role in establishing the balance between excitation and inhibition. Unlike most other embryonic neurons, MGE cells retain the capacity for migration and integration when transplanted into the postnatal and adult brain. We performed MGE cell transplantation into the basal ganglia of control and 6-OHDA-lesioned rats. Transplanted MGE cells survived, differentiated into GABA(+) neurons, integrated into host circuitry, and modified motor behavior in both lesioned and control rats. Our data suggest that MGE cell transplantation into the striatum is a promising approach to investigate the potential benefits of remodeling basal ganglia circuitry in neurodegenerative diseases.

Corticofugal control of vibrissa-sensitive neurons in the interpolaris nucleus of the trigeminal complex.

Trigeminal sensory nuclei that give rise to ascending pathways of vibrissal information are heavily linked by intersubnuclear connections. This is the case, for instance, of the principal trigeminal nucleus, which receives strong inhibitory input from the caudal sector of the interpolaris subnucleus. Because this inhibitory input can gate the relay of sensory messages through the lemniscal pathway, a central issue in vibrissal physiology is how brain regions that project to the interpolaris control the activity of inhibitory cells. In the present study, we examined how corticotrigeminal neurons of the primary and second somatosensory cortical areas control the excitability of interpolaris cells. Results show that these two cortical areas exert a differential control over the excitability of projection cells and intersubnuclear interneurons, and that this control also involves the recruitment of inhibitory cells in the caudalis subnucleus. These results provide a basic circuitry for a mechanism of disinhibition through which the cerebral cortex can control the relay of sensory messages in the lemniscal pathway. It is proposed that top-down control of brainstem circuits is prompted by motor strategies, expectations, and motivational states of the animal.

Mechanisms and Dynamics of Cortical Motor Inhibition in the Stop-signal Paradigm: A TMS Study

The ability to stop ongoing motor responses in a split-second is a vital element of human cognitive control and flexibility that relies in large part on prefrontal cortex. We used the stop-signal paradigm to elucidate the engagement of primary motor cortex (M1) in inhibiting an ongoing voluntary motor response. The stop-signal paradigm taps the ability to flexibly countermand ongoing voluntary behavior upon presentation of a stop signal. We applied single-pulse TMS to M1 at several intervals following the stop signal to track the time course of excitability of the motor system related to generating and stopping a manual response. Electromyography recorded from the flexor pollicis brevis allowed quantification of the excitability of the corticospinal tract and the involvement of intracortical GABA(B)ergic circuits within M1, indexed respectively by the amplitude of the motor-evoked potential and the duration of the late part of the cortical silent period (SP). The results extend our knowledge of the neural basis of inhibitory control in three ways. First, the results revealed a dynamic interplay between response activation and stopping processes at M1 level during stop-signal inhibition of an ongoing response. Second, increased excitability of inhibitory interneurons that drives SP prolongation was evident as early as 134 msec following the instruction to stop. Third, this pattern was followed by a stop-related reduction of corticospinal excitability implemented around 180 after the stop signal. These findings point to the recruitment of GABA(B)ergic intracortical inhibitory circuits within M1 in stop-signal inhibition and support the notion of stopping as an active act of control.

Electrophysiological Characterization of V2a Interneurons and Their Locomotor-Related Activity in the Neonatal Mouse Spinal Cord

The V2a class of Chx10-expressing interneurons has been implicated in frequency-dependent control of left-right phase during locomotion in the mouse. We have used the Chx10::CFP mouse line to further investigate the properties and locomotion-related activity of V2a interneurons in the isolated neonatal spinal cord. V2a interneurons can be divided into three classes, based on their tonic, phasic, or delayed-onset responses to step depolarization. Electrical coupling is found only between neurons of same class and helps to synchronize neuronal activity within the class. Serotonin (5-HT) excites isolated tonic V2a interneurons by depolarizing the neurons and increasing their membrane input resistance, with no significant effects on action potential properties, a mechanism distinct from 5-HT excitation of commissural interneurons. During NMDA-/5-HT-induced locomotor-like activity, patch-clamp recordings and two-photon calcium imaging experiments show that approximately half of V2a interneurons fire rhythmically with ventral root-recorded motor activity; the rhythmic V2a interneurons fired during one half of the cycle, in phase with either the ipsilateral or the contralateral L2 ventral root bursts. The percentage of rhythmically firing V2a interneurons increases during higher-frequency fictive locomotion, and they become significantly more rhythmic in their firing during the locomotor cycle; this may help to explain the frequency-dependent shift in left-right coupling in Chx10::DTA mice, which lack these neurons. Our results together with data from the accompanying paper (Dougherty and Kiehn, 2009) reinforce earlier proposals that the V2a interneurons are components of the hindlimb central pattern generator, helping to organize left-right locomotor coordination in the neonatal mouse spinal cord.