Neural substrates of two different rhythmical vibrissal movements in the rat

Anatomical pathways involved in generating and sensing rhythmic whisker movements

The rodent whisker system is widely used as a model system for investigating sensorimotor integration, neural mechanisms of complex cognitive tasks, neural development, and robotics. The whisker pathways to the barrel cortex have received considerable attention. However, many subcortical structures are paramount to the whisker system. They contribute to important processes, like filtering out salient features, integration with other senses, and adaptation of the whisker system to the general behavioral state of the animal. We present here an overview of the brain regions and their connections involved in the whisker system. We do not only describe the anatomy and functional roles of the cerebral cortex, but also those of subcortical structures like the striatum, superior colliculus, cerebellum, pontomedullary reticular formation, zona incerta, and anterior pretectal nucleus as well as those of level setting systems like the cholinergic, histaminergic, serotonergic, and noradrenergic pathways. We conclude by discussing how these brain regions may affect each other and how they together may control the precise timing of whisker movements and coordinate whisker perception.

The WAG/Rij strain: a genetic animal model of absence epilepsy with comorbidity of depression [corrected]

A great number of clinical observations show a relationship between epilepsy and depression. Idiopathic generalized epilepsy, including absence epilepsy, has a genetic basis. The review provides evidence that WAG/Rij rats can be regarded as a valid genetic animal model of absence epilepsy with comorbidity of depression. WAG/Rij rats, originally developed as an animal model of human absence epilepsy, share many EEG and behavioral characteristics resembling absence epilepsy in humans, including the similarity of action of various antiepileptic drugs. Behavioral studies indicate that WAG/Rij rats exhibit depression-like symptoms: decreased investigative activity in the open field test, increased immobility in the forced swimming test, and decreased sucrose consumption and preference (anhedonia). In addition, WAG/Rij rats adopt passive strategies in stressful situations, express some cognitive disturbances (reduced long-term memory), helplessness, and submissiveness, inability to make choice and overcome obstacles, which are typical for depressed patients. Elevated anxiety is not a characteristic (specific) feature of WAG/Rij rats; it is a characteristic for only a sub-strain of WAG/Rij rats susceptible to audiogenic seizures. Interestingly, WAG/Rij rats display a hyper-response to amphetamine similar to anhedonic depressed patients. WAG/Rij rats are sensitive only to chronic, but not acute, antidepressant treatments, suggesting that WAG/Rij rats fulfill a criterion of predictive validity for a putative animal model of depression. However, more and different antidepressant drugs still await evaluation. Depression-like behavioral symptoms in WAG/Rij rats are evident at baseline conditions, not exclusively after stress. Experiments with foot-shock stress do not point towards higher stress sensitivity at both behavioral and hormonal levels. However, freezing behavior (coping deficits) and blunted response of 5HT in the frontal cortex to uncontrollable sound stress, increased c-fos expression in the terminal regions of the meso-cortico-limbic brain systems and greater DA response of the mesolimbic system to forced swim stress suggest that WAG/Rij rats are vulnerable to some, but not to all types of stressors. We propose that genetic absence epileptic WAG/Rij rats have behavioral depression-like symptoms, are vulnerable to stress and might represent a model of chronic low-grade depression (dysthymia). Both 5HT and DAergic abnormalities detected in the brain of WAG/Rij rats are involved in modulation of vulnerability to stress and provocation of behavioral depression-like symptoms. The same neurotransmitter systems modulate SWDs as well. Recent studies suggest that the occurrence and repetition of absence seizures are a precipitant of depression-like behavior. Whether the neurochemical changes are primary to depression-like behavioral alterations remains to be determined. In conclusion, the WAG/Rij rats can be considered as a genetic animal model for absence epilepsy with comorbidity of dysthymia. This model can be used to investigate etiology, pathogenic mechanisms and treatment of a psychiatric comorbidity, such as depression in absence epilepsy, to reveal putative genes contributing to comorbid depressive disorder, and to screen novel psychotropic drugs with a selective and/or complex (dual) action on both pathologies.

Non-invasive stimulation of the vibrissal pad improves recovery of whisking function after simultaneous lesion of the facial and infraorbital nerves in rats

We have recently shown that manual stimulation of target muscles promotes functional recovery after transection and surgical repair to pure motor nerves (facial: whisking and blink reflex; hypoglossal: tongue position). However, following facial nerve repair, manual stimulation is detrimental if sensory afferent input is eliminated by, e.g., infraorbital nerve extirpation. To further understand the interplay between sensory input and motor recovery, we performed simultaneous cut-and-suture lesions on both the facial and the infraorbital nerves and examined whether stimulation of the sensory afferents from the vibrissae by a forced use would improve motor recovery. The efficacy of 3 treatment paradigms was assessed: removal of the contralateral vibrissae to ensure a maximal use of the ipsilateral ones (vibrissal stimulation; Group 2), manual stimulation of the ipsilateral vibrissal muscles (Group 3), and vibrissal stimulation followed by manual stimulation (Group 4). Data were compared to controls which underwent surgery but did not receive any treatment (Group 1). Four months after surgery, all three treatments significantly improved the amplitude of vibrissal whisking to 30° versus 11° in the controls of Group 1. The three treatments also reduced the degree of polyneuronal innervation of target muscle fibers to 37% versus 58% in Group 1. These findings indicate that forced vibrissal use and manual stimulation, either alone or sequentially, reduce target muscle polyinnervation and improve recovery of whisking function when both the sensory and the motor components of the trigemino-facial system regenerate.

Effect of intracranial administration of ethosuximide in rats with spontaneous or pentylenetetrazol-induced spike-wave discharges

PURPOSE

Generalized absence seizures are characterized by bilateral spike-wave discharges (SWDs), particularly in the frontoparietal cortical region. In WAG/Rij and GAERS rats with absence epilepsy, recent evidence indicates that SWDs arise first from the lateral somatosensory cortex (LSC), that is, the cortical focus theory. To further understand the cortical role in SWD generation, two epileptic rat models were assessed.

METHODS

Two models, Long-Evans rats with spontaneous SWDs and Wistar rats with low-dose pentylenetetrazol-induced SWDs (20 mg/kg, i.p.), were administered intracortical or intrathalamic ethosuximide (ESM) or saline. Electroencephalographic recordings were analyzed before and after intracranial microinfusion to evaluate onset, frequency, and duration of SWDs.

KEY FINDINGS

In both epileptic rat models, ESM in the LSC significantly reduced SWD number, shortened SWD duration, and delayed SWD onset compared to saline. By contrast, ESM in the medial somatosensory cortex had little effect compared to saline. Intrathalamic infusion of ESM only delayed SWD onset.

SIGNIFICANCE

These findings suggest that the LSC may be essential for the occurrence of SWDs. Our data support the cortical focus theory for the generation of absence seizures.

Cross-approximate entropy of cortical local field potentials quantifies effects of anesthesia--a pilot study in rats

Background

Anesthetics dose-dependently shift electroencephalographic (EEG) activity towards high-amplitude, slow rhythms, indicative of a synchronization of neuronal activity in thalamocortical networks. Additionally, they uncouple brain areas in higher (gamma) frequency ranges possibly underlying conscious perception. It is currently thought that both effects may impair brain function by impeding proper information exchange between cortical areas. But what happens at the local network level? Local networks with strong excitatory interconnections may be more resilient towards global changes in brain rhythms, but depend heavily on locally projecting, inhibitory interneurons. As anesthetics bias cortical networks towards inhibition, we hypothesized that they may cause excessive synchrony and compromise information processing already on a small spatial scale. Using a recently introduced measure of signal independence, cross-approximate entropy (XApEn), we investigated to what degree anesthetics synchronized local cortical network activity. We recorded local field potentials (LFP) from the somatosensory cortex of three rats chronically implanted with multielectrode arrays and compared activity patterns under control (awake state) with those at increasing concentrations of isoflurane, enflurane and halothane.

Results

Cortical LFP signals were more synchronous, as expressed by XApEn, in the presence of anesthetics. Specifically, XApEn was a monotonously declining function of anesthetic concentration. Isoflurane and enflurane were indistinguishable; at a concentration of 1 MAC (the minimum alveolar concentration required to suppress movement in response to noxious stimuli in 50% of subjects) both volatile agents reduced XApEn by about 70%, whereas halothane was less potent (50% reduction).

Conclusions

The results suggest that anesthetics strongly diminish the independence of operation of local cortical neuronal populations, and that the quantification of these effects in terms of XApEn has a similar discriminatory power as changes of spontaneous action potential rates. Thus, XApEn of field potentials recorded from local cortical networks provides valuable information on the anesthetic state of the brain.

Cannabinoids reveal separate controls for whisking amplitude and timing in rats

Whisking is controlled by multiple, possibly functionally segregated, motor sensory-motor loops. While testing for effects of endocannabinoids on whisking, we uncovered the first known functional segregation of channels controlling whisking amplitude and timing. Channels controlling amplitude, but not timing, were modulated by cannabinoid receptor type 1 (CB1R). Systemic administration of CB1R agonist Δ(9)-tetrahydrocannabinol (Δ(9)-THC) reduced whisking spectral power across all tested doses (1.25-5 mg/kg), whereas whisking frequency was affected at only very high doses (5 mg/kg). Concomitantly, whisking amplitude and velocity were significantly reduced in a dose-dependent manner (25-43 and 26-50%, respectively), whereas cycle duration and bilateral synchrony were hardly affected (3-16 and 3-9%, respectively). Preadministration of CB1R antagonist SR141716A blocked Δ(9)-THC-induced kinematic alterations of whisking, and when administered alone, increased whisking amplitude and velocity but affected neither cycle duration nor synchrony. These findings indicate that whisking amplitude and timing are controlled by separate channels and that endocannabinoids modulate amplitude control channels.

Control of the firing patterns of vibrissa motoneurons by modulatory and phasic synaptic inputs: a modeling study

Vibrissa motoneurons (vMNs) generate rhythmic firing that controls whisker movements, even without cortical, cerebellar, or sensory inputs. vMNs receive serotonergic modulation from brain stem areas, which mainly increases their persistent sodium conductance (g(NaP)) and, possibly, phasic input from a putative central pattern generator (CPG). In response to serotonergic modulation or just-suprathreshold current steps, vMNs fire at low rates, below the firing frequency of exploratory whisking. In response to periodic inputs, vMNs exhibit nonlinear suprathreshold resonance in frequency ranges of exploratory whisking. To determine how firing patterns of vMNs are determined by their 1) intrinsic ionic conductances and 2) responses to periodic input from a putative CPG and to serotonergic modulation, we construct and analyze a single-compartment, conductance-based model of vMNs. Low firing rates are supported in extended regimes by adaptation currents and the minimal firing rate decreases with g(NaP) and increases with M-potassium and h-cation conductances. Suprathreshold resonance results from the locking properties of vMN firing to stimuli and from reduction of firing rates at low frequencies by slow M and afterhyperpolarization potassium conductances. h conductance only slightly affects the suprathreshold resonance. When a vMN is subjected to a small periodic CPG input, serotonergically induced g(NaP) elevation may transfer the system from quiescence to a firing state that is highly locked to the CPG input. Thus we conclude that for vMNs, the CPG controls firing frequency and phase and enables bursting, whereas serotonergic modulation controls transitions from quiescence to firing unless the CPG input is sufficiently strong.

Taste-guided decisions differentially engage neuronal ensembles across gustatory cortices

Much remains to be understood about the differential contributions from primary and secondary sensory cortices to sensory-guided decision making. To address this issue we simultaneously recorded activity from neuronal ensembles in primary [gustatory cortex GC)] and secondary gustatory [orbitofrontal cortex (OFC)] cortices while rats made a taste-guided decision between two response alternatives. We found that before animals commenced a response guided by a tastant cue, GC ensembles contained more information than OFC about the response alternative about to be selected. Thereafter, while the animal's response was underway, the response-selective information in ensembles from both regions increased, albeit to a greater degree in OFC. In GC, this increase depends on a representation of the taste cue guiding the animal's response. The increase in the OFC also depends on the taste cue guiding and other features of the response such as its spatiomotor properties and the behavioral context under which it is executed. Each of these latter features is encoded by different ensembles of OFC neurons that are recruited at specific times throughout the response selection process. These results indicate that during a taste-guided decision task both primary and secondary gustatory cortices dynamically encode different types of information.

Design and construction of a photoresistive sensor for monitoring the rat vibrissal displacement

Rats sweep their vibrissae in a rhythmic and coordinated fashion in order to acquire tactile information from their environment. Measuring vibrissae movement has become a matter of increased attention, from several labs, over the last few years. We describe the design and construction of an inexpensive photoresistive sensor that registers horizontal vibrissae movement. The device consists of an LED array and a light-dependent resistor (LDR) covered by a coating with varying transparency along its axis. When a vibrissa is located in the sensor, it generates a shadowy line over the photosensitive material, thus changing the LDR resistance. These changes are transduced into voltage changes. Our measurements on vibrissa show that this simple and inexpensive sensor effectively monitors the movement of a single vibrissa.

The synergistic inhibitory actions of oxcarbazepine on voltage-gated sodium and potassium currents in differentiated NG108-15 neuronal cells and model neurons

Oxcarbazepine (OXC), one of the newer anti-epileptic drugs, has been demonstrating its efficacy on wide-spectrum neuropsychiatric disorders. However, the ionic mechanism of OXC actions in neurons remains incompletely understood. With the aid of patch-clamp technology, we first investigated the effects of OXC on ion currents in NG108-15 neuronal cells differentiated with cyclic AMP. We found OXC (0.3-30 microm) caused a reversible reduction in the amplitude of voltage-gated Na+ current (INa). The IC50 value required for the inhibition of INa by OXC was 3.1 microm. OXC (3 microm) could shift the steady-state inactivation of INa to a more negative membrane potential by approximately -9 mV with no effect on the slope of the inactivation curve, and produce a significant prolongation in the recovery of INa inactivation. Additionally, OXC was effective in suppressing persistent INa (INa(P)) elicited by long ramp pulses. The blockade of INa by OXC does not simply reduce current magnitude, but alters current kinetics. Moreover, OXC could suppress the amplitude of delayed rectifier K+ current (IK(DR)), with no effect on M-type K+ current (IK(M)). In current-clamp configuration, OXC could reduce the amplitude of action potentials and prolong action-potential duration. Furthermore, the simulations, based on hippocampal pyramidal neurons (Pinsky-Rinzel model) and a network of the Hodgkin-Huxley model, were analysed to investigate the effect of OXC on action potentials. Taken together, our results suggest that the synergistic blocking effects on INa and IK(DR) may contribute to the underlying mechanisms through which OXC affects neuronal function in vivo.

Bone marrow-derived mesenchymal stem cell transplantation does not improve quality of muscle reinnervation or recovery of motor function after facial nerve transection in rats

Recently, we devised and validated a novel strategy in rats to improve the outcome of facial nerve reconstruction by daily manual stimulation of the target muscles. The treatment resulted in full recovery of facial movements (whisking), which was achieved by reducing the proportion of pathologically polyinnervated motor endplates. Here, we posed whether manual stimulation could also be beneficial after a surgical procedure potentially useful for treatment of large peripheral nerve defects, i.e., entubulation of the transected facial nerve in a conduit filled with suspension of isogeneic bone marrow-derived mesenchymal stem cells (BM-MSCs) in collagen. Compared to control treatment with collagen only, entubulation with BM-MSCs failed to decrease the extent of collateral axonal branching at the lesion site and did not improve functional recovery. Post-operative manual stimulation of vibrissal muscles also failed to promote a better recovery following entubulation with BM-MSCs. We suggest that BM-MSCs promote excessive trophic support for regenerating axons which, in turn, results in excessive collateral branching at the lesion site and extensive polyinnervation of the motor endplates. Furthermore, such deleterious effects cannot be overridden by manual stimulation. We conclude that entubulation with BM-MSCs is not beneficial for facial nerve repair.

Biomechanics of the vibrissa motor plant in rat: rhythmic whisking consists of triphasic neuromuscular activity

The biomechanics of a motor plant constrain the behavioral strategies that an animal has available to extract information from its environment. We used the rat vibrissa system as a model for active sensing and determined the pattern of muscle activity that drives rhythmic exploratory whisking. Our approach made use of electromyography to measure the activation of all relevant muscles in both head-fixed and unrestrained rats and two-dimensional imaging to monitor the position of the vibrissae in head-fixed rats. Our essential finding is that the periodic motion of the vibrissae and mystacial pad during whisking results from three phases of muscle activity. First, the vibrissae are thrust forward as the rostral extrinsic muscle, musculus (m.) nasalis, contracts to pull the pad and initiate protraction. Second, late in protraction, the intrinsic muscles pivot the vibrissae farther forward. Third, retraction involves the cessation of m. nasalis and intrinsic muscle activity and the contraction of the caudal extrinsic muscles m. nasolabialis and m. maxillolabialis to pull the pad and the vibrissae backward. We developed a biomechanical model of the whisking motor plant that incorporates the measured muscular mechanics along with movement vectors observed from direct muscle stimulation in anesthetized rats. The results of simulations of the model quantify how the combination of extrinsic and intrinsic muscle activity leads to an enhanced range of vibrissa motion than would be available from the intrinsic muscles alone.

Manually-stimulated recovery of motor function after facial nerve injury requires intact sensory input

We have recently shown in rat that daily manual stimulation (MS) of vibrissal muscles promotes recovery of whisking and reduces polyinnervation of muscle fibers following repair of the facial nerve (facial-facial anastomosis, FFA). Here, we examined whether these positive effects were: (1) correlated with alterations of the afferent connections of regenerated facial motoneurons, and (2) whether they were achieved by enhanced sensory input through the intact trigeminal nerve. First, we quantified the extent of total synaptic input to motoneurons in the facial nucleus using synaptophysin immunocytochemistry following FFA with and without subsequent MS. We found that, without MS, this input was reduced compared to intact animals. The number of synaptophysin-positive terminals returned to normal values following MS. Thus, MS appears to counteract the deafferentation of regenerated facial motoneurons. Second, we performed FFA and, in addition, eliminated the trigeminal sensory input to facial motoneurons by extirpation of the ipsilateral infraorbital nerve (IONex). In this paradigm, without MS, vibrissal motor performance and pattern of end-plate reinnervation were as aberrant as after FFA without MS. MS did not influence the reinnervation pattern after IONex and functional recovery was even worse than after IONex without MS. Thus, when the sensory system is intact, MS restores normal vibrissal function and reduces the degree of polyinnervation. When afferent inputs are abolished, these effects are eliminated or even reversed. We conclude that rehabilitation strategies must be carefully designed to take into account the extent of motor and/or sensory damage.

Using hardware models to quantify sensory data acquisition across the rat vibrissal array

Our laboratory investigates how animals acquire sensory data to understand the neural computations that permit complex sensorimotor behaviors. We use the rat whisker system as a model to study active tactile sensing; our aim is to quantitatively describe the spatiotemporal structure of incoming sensory information to place constraints on subsequent neural encoding and processing. In the first part of this paper we describe the steps in the development of a hardware model (a 'sensobot') of the rat whisker array that can perform object feature extraction. We show how this model provides insights into the neurophysiology and behavior of the real animal. In the second part of this paper, we suggest that sensory data acquisition across the whisker array can be quantified using the complete derivative. We use the example of wall-following behavior to illustrate that computing the appropriate spatial gradients across a sensor array would enable an animal or mobile robot to predict the sensory data that will be acquired at the next time step.

Manual stimulation of facial muscles improves functional recovery after hypoglossal–facial anastomosis and interpositional nerve grafting of the facial nerve in adult rats

The facial nerve in humans is often prone to injuries requiring surgical intervention. In the best case, nerve reconstruction is achieved by a facial-facial anastomosis (FFA), i.e. suture of the proximal and distal stumps of the severed facial nerve. Although a method of choice, FFA rarely leads to a satisfactory functional recovery. We have recently devised and validated, in an established experimental paradigm in rats, a novel strategy to improve the outcome of FFA by daily manual stimulation (MS) of facial muscles. This treatment results in full recovery of facial movements (whisking) and is achieved by reducing the proportion of functionally detrimental poly-innervated motor end-plates. Here we asked whether MS could also be beneficial after two other commonly used surgical methods of clinical facial nerve reconstruction namely hypoglossal-facial anastomosis (HFA) and interpositional nerve grafting (IPNG) which, however, seem to have a poorer outcome compared to FFA. Compared to FFA, daily MS for 2 months after HFA and IPGN did not completely restore function but, nevertheless, significantly improved the amplitude of whisker movements by 50% compared with untreated animals. Functional improvement was associated with a reduction in the proportion of polyinnervated end-plates. MS did not reduce the extent of axonal branching at the lesion site nor the subsequent misdirected axonal regrowth to inappropriate targets. Our data show that a simple approach leading to improved quality of muscle fiber reinnervation is functionally beneficial after different types of clinically relevant surgical interventions.

The whisking rhythm generator: a novel mammalian network for the generation of movement

Using the rat vibrissa system, we provide evidence for a novel mechanism for the generation of movement. Like other central pattern generators (CPGs) that underlie many movements, the rhythm generator for whisking can operate without cortical inputs or sensory feedback. However, unlike conventional mammalian CPGs, vibrissa motoneurons (vMNs) actively participate in the rhythmogenesis by converting tonic serotonergic inputs into the patterned motor output responsible for movement of the vibrissae. We find that, in vitro, a serotonin receptor agonist, alpha-Me-5HT, facilitates a persistent inward current (PIC) and evokes rhythmic firing in vMNs. Within each motoneuron, increasing the concentration of alpha-Me-5HT significantly increases the both the magnitude of the PIC and the motoneuron's firing rate. Riluzole, which selectively suppresses the Na(+) component of PICs at low concentrations, causes a reduction in both of these phenomena. The magnitude of this reduction is directly correlated with the concentration of riluzole. The joint effects of riluzole on PIC magnitude and firing rate in vMNs suggest that the two are causally related. In vivo we find that the tonic activity of putative serotonergic premotoneurons is positively correlated with the frequency of whisking evoked by cortical stimulation. Taken together, these results support the hypothesized novel mammalian mechanism for movement generation in the vibrissa motor system where vMNs actively participate in the rhythmogenesis in response to tonic drive from serotonergic premotoneurons.

Mechanical stimulation of paralyzed vibrissal muscles following facial nerve injury in adult rat promotes full recovery of whisking

Many patients suffer lifelong disabilities after peripheral nerve injury. Insufficient recovery has been attributed to excessive axonal branching, axonal regrowth to improper targets and polyneuronal reinnervation of motor endplates. We used the rat facial nerve transection/suture model to quantify the effects of mechanical stimulation on the paralyzed whisker musculature. "Manual" stimulation involved briskly stroking the whiskers by hand in a manner that specifically mimicked normal whisker movement. "Environmental" stimulation involved enhanced whisker use as rats encountered objects in an enriched environment. Manual and environmental stimulation were also combined. Video-based motion analysis of vibrissal motor performance showed that daily manual, but not environmental, stimulation for 2 months resulted in full recovery of whisking. Polyneuronal reinnervation of motor endplates was reduced but not misdirected axonal regrowth. Our findings indicate the potential of use-specific training to enhance appropriate functional outcome after peripheral nerve injury and may be useful in a clinical rehabilitation setting.

Development of rodent whisking: trigeminal input and central pattern generation

To examine the contribution of whisker inputs to the initial emergence and subsequent refinement of the rodent whisking pattern we combined surgical treatments producing varying degrees of postnatal whisker deafferentation with observations and video analysis of whisking across the first month of life. Whisking emerges during the second postnatal week, preceding eye opening by a few days. In contrast to the absence of deafferentation effects in adults, whisker deafferentation in pups, if carried out between the second and third postnatal week, delays (but does not prevent) the emergence of whisker movements and disrupts the development of normal whisking kinematics and coordination. The extent of the delay varies directly with the reduction in whisker input. When regeneration of the nerve is prevented by a cyanoacrylate block emergence of the normal pattern may be delayed indefinitely. Moreover, section of the whisker motor nerve contralateral to the deafferented side, substantially potentiates the effects of the initial deafferentation. These results confirm and extend an earlier description of the development of whisking in normal rat pups (Welker, Behaviour 12:223-244, 1964), fix the time of its initial emergence more precisely at P (postnatal day) 11-13, and suggest a critical role for trigeminal afference in the development of the normal whisking pattern. They are discussed in relation to the development of pattern generating mechanisms in the rodent whisker system.

Improved outcome of facial nerve repair in rats is associated with enhanced regenerative response of motoneurons and augmented neocortical plasticity

Within a recent study on the vibrissae motor performance after facial nerve repair in strains of blind (SD/RCS) and sighted (SD) rats we found that, despite persisting myotopic disorganization in the facial nucleus, the blind animals fully restored vibrissal whisking. Here we searched for morphological substrates of better recovery in the regenerating motoneurons and in the cerebral motor cortex. Expression analyses of the neurite growth-related proteins f-actin, neuronal class III beta-tubulin and plasticity-related gene-1, and stereological estimates of growth cone densities revealed a more vigorous regenerative response in the proximal nerve stump of blind SD/RCS rats compared with SD animals at 5-7 days after buccal nerve transection. Using c-Fos immunoreactivity as a marker for neuronal activation, we found that the volume of the cortex acutely responding to nerve transection (facial muscles reactive volume, FMRV) in both hemispheres of intact sighted rats was twofold smaller than that measured in blind animals. One month after transection and suture of the right facial nerve (FFA) we found a twofold increase in the FMRV in both rat strains compared with intact animals. The FMRV in SD/RCS animals, but not in SD rats, returned to the values in intact rats 2 months after FFA. Our findings suggest that enhanced plasticity in the CNS and an augmented regenerative response of the injured motoneurons contribute to better functional recovery in blind rats.

Right-left asymmetries in the whisking behavior of rats anticipate head movements

Rats use rhythmic movements of their vibrissae (whiskers) to tactually explore their environment. This "whisking" behavior has generally been reported to be strictly synchronous and symmetric about the snout, and it is thought to be controlled by a brainstem central pattern generator. Because the vibrissae can move independently of the head, however, maintaining a stable perception of the world would seem to require that rats adjust the bilateral symmetry of whisker movements in response to head movements. The present study used high-speed videography to reveal dramatic bilateral asymmetries and asynchronies in free-air whisking during head rotations. Kinematic analysis suggested that these asymmetric movements did not serve to maintain any fixed temporal relationship between right and left arrays, but rather to redirect the whiskers to a different region of space. More specifically, spatial asymmetry was found to be strongly correlated with rotational head velocity, ensuring a "look-ahead" distance of almost exactly one whisk. In contrast, bilateral asynchrony and velocity asymmetry were only weakly dependent on head velocity. Bilateral phase difference was found to be independent of the whisking frequency, suggesting the presence of two distinct left and right central pattern generators, connected as coupled oscillators. We suggest that the spatial asymmetries are analogous to the saccade that occurs during the initial portion of a combined head-eye gaze shift, and we begin to develop the rat vibrissal system as a new model for studying vestibular and proprioceptive contributions to the acquisition of sensory data.

Haptic object localization in the vibrissal system: behavior and performance

Using their large mystacial vibrissas, rats perform a variety of tasks, including localization and identification of objects. We report on the discriminatory thresholds and behavior of rats trained in a horizontal object localization task. Using an adaptive training procedure, rats learned to discriminate offsets in horizontal (anteroposterior) location with all, one row, or one arc of whiskers intact, but not when only a single whisker (C2) was intact on each cheek. However, rats initially trained with multiple whiskers typically improved when retested later with a single whisker intact. Individual rats reached localization thresholds as low as 0.24 mm (approximately 1 degree). Among the tested groups, localization acuity was finest (<1.5 mm) with rats that were initially trained with all whiskers and then trimmed to one arc of whiskers intact. Horizontal acuity was finer than the typical inter-vibrissal spacing (approximately 4.8 mm at contact points). Performance correlated with the net whisking spectral power in the range of 5-25 Hz but not in nonwhisking range of 30-50 Hz. Lesioning the facial motor nerves reduced performance to chance level. We conclude that horizontal object localization in the rat vibrissal system can reach hyperacuity level and is an active sensing process: whisker movements are both required and beneficiary, in a graded manner, for making accurate positional judgments.

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

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

Hippocampal CA1 spiking during encoding and retrieval: relation to theta phase

The hippocampal theta rhythm is a prominent oscillation in the field potential observed throughout the hippocampus as a rat investigates stimuli in the environment. A recent computational model [Hasselmo, M. E., Bodelon, C., & Wyble, B. P. (2002a). A proposed function for hippocampal theta rhythm: separate phases of encoding and retrieval enhance reversal of prior learning. Neural Computation, 14, 793-817. Neuromodulation, theta rhythm and rat spatial navigation. Neural Networks, 15, 689-707] suggested that the theta rhythm allows the hippocampal formation to alternate rapidly between conditions that promote memory encoding (strong synaptic input from entorhinal cortex to areas CA3 and CA1) and conditions that promote memory retrieval (strong synaptic input from CA3 to CA1). That model predicted that the preferred theta phase of CA1 spiking should differ for information being encoded versus information being retrieved. In the present study, the spiking activity of CA1 pyramidal cells was recorded while rats performed either an odor-cued delayed nonmatch-to-sample recognition memory test or an object recognition memory task based on the animal's spontaneous preference for novelty. In the test period of both tasks, the preferred theta phase exhibited by CA1 pyramidal cells differed between moments when the rat inspected repeated (match) and non-repeated (nonmatch) items. Also in the present study, additional modeling work extended the previous model to address the mean phase of CA1 spiking associated with stimuli inducing varying levels of retrieval relative to encoding, ranging from novel nonmatch stimuli with no retrieval to highly familiar repeated stimuli with extensive retrieval. The modeling results obtained here demonstrated that the experimentally observed phase differences are consistent with different levels of CA3 synaptic input to CA1 during recognition of repeated items.

Exploratory whisking by rat is not phase locked to the hippocampal theta rhythm

The rat has a strong 6-9 Hz rhythm of electrical activity in the hippocampus, known as the theta rhythm. Exploratory whisking, i.e., the rhythmic movement of the rat's vibrissas to acquire tactile information, occurs within the same frequency range as the theta rhythm and provides a model system to examine the relationship between theta rhythm and active sensory movements. In particular, it has been postulated that these two rhythms are phase locked as a means to synchronize sensory and hippocampal processing. We tested this hypothesis in rats trained to whisk in air. Theta activity was measured via field electrodes in the hippocampus, and whisking was measured via the mystacial electromyogram. We calculated the spectral coherence between these two signals as a means to quantify the extent of phase locking. First, we found that the fraction of epochs with high coherence is not significantly greater than that expected by chance (seven of eight animals and as a population average). Second, we found that the trial-averaged coherence is low (coherence, < 0.1) and, as an average across all animals, statistically insignificant. We further asked whether the strength of the theta rhythm correlated with that of whisking, independent of the lack of cycle-by-cycle coherence. We observe that the correlation is weak and insignificant (six of eight animals and as a population average). We conclude that there is no relationship between the whisking and theta rhythms, at least when animals whisk in air.

Annotation: Tourette syndrome: a relentless drumbeat--driven by misguided brain oscillations

OBJECTIVE

This annotation reviews recent evidence that points to the likely role of aberrant neural oscillations in the pathogenesis of Tourette syndrome (TS).

METHODS

The available anatomic and electrophysiological findings in TS are reviewed in the context of an emerging picture of the crucial role that neural oscillations play in maintaining normal central nervous system (CNS) function.

RESULTS

Neurons form behavior-dependent oscillating networks of various sizes and frequencies that bias input selection and facilitate synaptic plasticity, mechanisms that cooperatively support temporal representation as well as the transfer and long-term consolidation of information. Coherent network activity is likely to modulate sensorimotor gating as well as focused motor actions. When these networks are dysrhythmic, there may be a loss of control of sensory information and motor action. The known electrophysiological effects of medications and surgical interventions used to treat TS likely have an ameliorative effect on these aberrant oscillations. Similarly, a strong case can be made that successful behavioral treatments involve the willful training regions of the prefrontal cortex to engage in tic suppression and the performance of competing motor responses to unwanted sensory urges such that these prefrontal regions become effective modulators of aberrant thalamocortical rhythms.

CONCLUSIONS

A deeper understanding of neural oscillations may illuminate the complex, challenging, enigmatic, internal world that is TS.

Cellular mechanisms of motor control in the vibrissal system

In this article we discuss the experimental advantages that the vibrissal motor system offers for analysis of motor control and the specializations of this system related to the unique characteristics of whisker movements. Whisker movements are often rhythmic, fast, and bilateral. Movements of individual whiskers have simple characteristics, whereas, movements of the entire vibrissae array are complex and sophisticated. In the last few years, powerful methods for high precision tracking of whisker movements have become available. The whisker musculature is arranged to permit forward movements of individual whiskers and consists-depending on the species-mainly or exclusively of fast contracting, fast fatigable muscle fibers. Whisker motor neurons are located in the lateral facial nucleus and their cellular properties might contribute to the rhythmicity of whisking. Numerous structures provide input to the lateral facial nucleus, the most mysterious and important one being the putative central pattern generator (CPG). Although recent studies identified candidate structures for the CPG, the precise identity and the functional organization of this structure remains uncertain. The vibrissa motor cortex (VMC) is the largest motor representation in the rodent brain, and recent work has clarified its localization, subdivisions, cytoarchitectonics, and connectivity. Single-cell stimulation experiments in VMC allow determining the cellular basis of cortical motor control with unprecedented precision. The functional significance of whisker movements remains to be determined.

Cortical control of a whisking central pattern generator

Whether the motor cortex regulates voluntary movements by generating the motor pattern directly or by acting through subcortical central pattern generators (CPGs) remains a central question in motor control. Using the rat whisker system, an important model system of mammalian motor control, we develop an anesthetized preparation to investigate the interaction between the motor cortex and a whisking CPG. Using this model we investigate the involvement of a serotonergic component of the whisking CPG in determining whisking kinematics and the mechanisms through which drive from the CPG is converted into movements by vibrissa motor units. Consistent with an action of the vibrissa motor cortex (vMCx) on a subcortical CPG, the frequency of whisking evoked by intracortical microstimulation (ICMS) of vMCx differed significantly from the stimulation frequency, whereas whisking onset latencies correlated negatively with stimulation intensity. Further, ICMS-evoked whisking was suppressed by a serotonin receptor antagonist, supporting previous findings that the whisking CPG contains a significant serotonergic component. The amplitude of ICMS-evoked whisking was correlated with the number of active motor units-isolated from vibrissal EMGs or recorded directly from vibrissa motoneurons-and their activity level. In addition, whisking frequency was correlated with the firing rate of these motoneurons. These findings support the hypothesis that vMCx regulates whisking through its actions on a subcortical CPG.

Intracellular activity of cortical and thalamic neurones during high-voltage rhythmic spike discharge in Long-Evans rats in vivo

Spontaneous high-voltage rhythmic spike (HVRS) discharges at 6-12 Hz have been widely described in the electrocorticogram (EcoG) of Long-Evans rats. These ECoG oscillations have been proposed to reflect a state of attentive immobility allowing the optimization of sensory integration within the corticothalamic pathway. This hypothesis has been challenged by recent studies emphasizing similarities between HVRS discharges and spike-and-wave discharges (SWDs) in well-established rat genetic models of absence epilepsy. Here, we made in vivo intracellular recordings to determine, for the first time, the cellular mechanisms responsible for the synchronized oscillations in the corticothalamic loop during HVRS discharges in the Long-Evans rats. We show that HVRS discharges are associated in corticothalamic neurones with rhythmic suprathreshold synaptic depolarizations superimposed on a tonic hyperpolarization, likely due to a process of synaptic disfacilitation. Simultaneously, thalamocortical neurones exhibit a large-amplitude 'croissant'-shaped membrane hyperpolarization with a voltage sensitivity suggesting a potassium-dependent mechanism. This thalamic hyperpolarizing envelope was associated with a membrane oscillation resulting from interactions between excitatory synaptic inputs, a chloride-dependent inhibitory conductance and voltage-gated intrinsic currents. These cortical and thalamic cellular mechanisms underlying HVRS activity in Long-Evans rats are remarkably similar to those previously described in the thalamocortical networks during SWDs. Thus, the present study provides an additional support to the hypothesis that HVRS activity in Long-Evans rats is an absence-like seizure activity.

What is the function of hippocampal theta rhythm?--Linking behavioral data to phasic properties of field potential and unit recording data

The extensive physiological data on hippocampal theta rhythm provide an opportunity to evaluate hypotheses about the role of theta rhythm for hippocampal network function. Computational models based on these hypotheses help to link behavioral data with physiological measurements of different variables during theta rhythm. This paper reviews work on network models in which theta rhythm contributes to the following functions: (1) separating the dynamics of encoding and retrieval, (2) enhancing the context-dependent retrieval of sequences, (3) buffering of novel information in entorhinal cortex (EC) for episodic encoding, and (4) timing interactions between prefrontal cortex and hippocampus for memory-guided action selection. Modeling shows how these functional mechanisms are related to physiological data from the hippocampal formation, including (1) the phase relationships of synaptic currents during theta rhythm measured by current source density analysis of electroencephalographic data from region CA1 and dentate gyrus, (2) the timing of action potentials, including the theta phase precession of single place cells during running on a linear track, the context-dependent changes in theta phase precession across trials on each day, and the context-dependent firing properties of hippocampal neurons in spatial alternation (e.g., "splitter cells"), (3) the cholinergic regulation of sustained activity in entorhinal cortical neurons, and (4) the phasic timing of prefrontal cortical neurons relative to hippocampal theta rhythm.

Olivocerebellar modulation of motor cortex ability to generate vibrissal movements in rat

The vibrissal movements known as whisking are generated in a pulsatile, or non-continuous, fashion and comprise sequences of brief regularly spaced movements. These rhythmic timing sequences imply the existence of periodically issued motor commands. As inferior olivary (IO) neurones generate periodic synchronous discharges that could provide the underlying timing signal, this possibility was tested by determining whether the olivocerebellar system modulates motor cortex (MCtx)-triggered whisker movements in rats. Trains of current pulses were applied to MCtx, and the resulting whisker movements were recorded using a high speed video camera. The evoked movement patterns demonstrated properties consistent with the existence of an oscillatory motor driving rhythm. In particular, movement amplitude showed a bell-shaped dependence on stimulus frequency, with a peak at 11.5+/-2.3 Hz. Moreover, movement trajectories showed harmonic and subharmonic entrainment patterns within specific stimulus frequency ranges. By contrast, movements evoked by facial nerve stimulation showed no such frequency-dependent properties. To test whether the IO was the oscillator in question, IO neuronal properties were modified in vivo by intra-IO picrotoxin injection, which enhances synchronous oscillatory IO activity and reduces its natural frequency. The ensuing changes in the evoked whisker patterns were consistent with these pharmacological effects. Furthermore, in cerebellectomized rats, oscillatory modulation of MCtx-evoked movements was greatly reduced, and intra-IO picrotoxin injections did not affect the evoked movement patterns. Additionally, multielectrode recording of Purkinje cell complex spikes showed a temporal correlation of olivocerebellar activity during MCtx stimulus trains to evoked movement patterns. In sum, the results indicate that MCtx's ability to generate movements is modulated by an oscillatory signal arising in the olivocerebellar system.

Temporal characteristics of tactile stimuli influence the response profile of cerebellar Golgi cells

An increasing number of studies have investigated the effect of stimulation parameters on neuronal response properties. Here, we describe the effect of temporal characteristics of tactile stimuli, more specifically the stimulation frequency and duration, on the response profile of simultaneously recorded cerebellar and cerebral cortical units in ketamine-xylazine anaesthetized rats. Long stimulus durations (>50 ms) elicited ON and OFF excitatory components in response to the stimulus onset and offset respectively, in both the cortex and the cerebellum. Golgi cells responded on average 7.5 ms later to the stimulus withdrawal than to the stimulus onset. Furthermore, the corticopontine OFF responses in the cerebellum and OFF responses in the cortex showed congruent latency decreases and amplitude increases for longer stimulus durations (50-200 ms). Decreasing the stimulus frequency similarly affected the latency and amplitude of the responses for inter-stimuli intervals shorter than 200 ms. In view of these results, we speculate that the stimulus offset is regarded as a novel input, because both paradigms resulted in similar response amplitude and latency modifications. Finally, the results suggest that a 100-200 ms time window can be of particular importance for cerebellar processing of information in the somatosensory system.

Spatial segregation of different modes of movement control in the whisker representation of rat primary motor cortex

What is mapped on the surface of the primary motor cortex (M1)? The classic somatotopic map holds true on the level of limb representations. However, on the small scale (at within-limb representations), neither somatotopy nor movement dynamics/kinematics seem to be organizational principles. We investigated the hypothesis that integrated into the body representation of M1 there may be separate representation of different modes of motor control, using different subcortical computations but sharing the same motor periphery. Using awake rats and long intracortical stimulation trains in M1 whisker representation (wM1) revealed that natural-like, rhythmic whisking (normally used for tactile exploration) can be evoked from a posteromedial subregion of wM1. Nonrhythmic whisker retraction, on the other hand, was evoked in an adjacent but more anterolaterally located region within wM1. Evoked whisker retraction was always accompanied by complex movements of the face, suggesting that the respective subregion is able to interact with other representations in specific behavioral contexts. Such associations were absent for evoked rhythmic whisking. The respective subregion rather seemed to activate a downstream central pattern generator, the oscillation frequency of which was dependent on the average evoked cortical activity. Nevertheless, joint stimulation of the two neighboring subregions demonstrated their potency to interact in a functionally useful way. Therefore, we suggest that the cause of cortical separation is the specific drive of subcortical structures needed to generate different types of movements rather than different behavioral contexts in which the movements are performed.

Computing with thalamocortical ensembles during different behavioural states

A series of recent studies have indicated that ensembles of neurones, distributed within the neural structures that form the primary thalamocortical loop (TCL) of the trigeminal component of the rat somatosensory system, change the way they respond to similar tactile stimuli, according to both the behavioural strategy employed by animals to gather information and the animal's internal brain states. These findings suggest that top-down influences, which are more likely to play a role during active discrimination than during passive whisker stimulation, may alter the pattern of neuronal firing within both the distinct layers of the primary somatosensory cortex (S1) and the ventral posterior medial nucleus (VPM). We propose that through this physiological process, which involves concurrent dynamic modulations at both cellular and circuit levels in the TCL, rats can either optimize the detection of novel or hard to sense stimuli or they can analyse complex patterns of multi-whisker stimulation, during natural exploration of their surrounding environment.

Relation Between Activities of the Cortex and Vibrissae Muscles During High-Voltage Rhythmic Spike Discharges in Rats

Paroxysmal 5- to 12-Hz high-voltage rhythmic spike (HVRS) activities, which are accompanied by whisker twitching (WT), are found in Long Evans rats, but the function of these HVRS activities is still debated. In four major functional hypotheses of HVRS discharges, i.e., alpha tremor, attention/mu rhythm, idling/mu rhythm, and absence seizure, the first two hypotheses emphasize WT behavior in HVRS bouts. Whisker movement is primarily determined by activation of intrinsic and extrinsic muscles. To clarify the role of WT in HVRS activities, simultaneous recording of the activities from the cortex and intrinsic/extrinsic and neck muscles were performed. Most HVRS bouts (68.8%) revealed no time-locked WT behavior in a 2-h recording session. In addition, WT primarily arose from active protraction due to activation of intrinsic muscles followed by passive retraction. A small portion of WT resulted from activation of both vibrissae muscles with dynamic frequency-dependent phase shifts. Onset of the rhythmic vibrissae EMG significantly lagged behind HVRS onset, and the mean duration of vibrissae muscle activity was one-third to a one-half of a HVRS bout. Moreover, a greater number of HVRS bouts were associated with a longer HVRS duration and higher oscillation frequency. Oscillation frequencies of HVRS activities without WT behavior were significantly lower than those with WT. Under peripheral sensory/motor blockade by xylocaine injection, oscillation frequencies of HVRS bouts significantly decreased, but no remarkable changes in the number or duration of HVRS bouts were observed. Compared with vibrissa muscle activity during WT and exploratory whisking, the duration of muscular activity in each cycle was apparently longer during whisking bouts. Based on these results, overemphasis of the role of WT on HVRS activities might not be appropriate. Instead, HVRS discharges may be associated with absence seizure or idling state. In addition, peripheral inputs, including WT, may elevate the oscillation frequency of HVRS bouts. Moreover, different muscular controls may exist between WT and whisking.

7 to 12 Hz activity in rat gustatory cortex reflects disengagement from a fluid self-administration task

The 7 to 12 Hz rhythm is a high-voltage oscillatory phenomenon recorded in many rat neocortical regions, largely analogous to the rodent and human somatosensory mu rhythm. Central to any interpretation of the functional significance of this pattern is the analysis of the behavioral context associated with it. Much of the debate on the function of mu, variously believed to represent either an environment-oriented or -isolated state, has relied primarily on its association with quiet immobility. In this report, we describe the relationship between the 7 to 12 Hz rhythm and a more complex behavioral setting, in which we were able to dissociated task orientation from disengagement. We trained head-restrained, water-restricted rats to perform a simple variant of a timed fluid self-administration task, while recording local field potentials from gustatory cortex (GC). Rats progressed through two behavioral states that were clearly distinguishable on the basis of lever-pressing regimes: a task-oriented state and a second state that reflected disengagement from the task. Concurrent GC neural recordings revealed bilaterally coherent oscillations in the 7 to 12 Hz range associated solely with the latter state. Consistent with published recordings of mu rhythm from somatosensory cortex, these rhythmic episodes were endogenously quenched when the rats prepared to lever-press; this inhibition of rhythmic episodes lasted through fluid delivery and consumption, making it clear that GC rhythms are not related to gustatory processing itself. By showing a direct relationship between the 7 to 12 Hz rhythm and disengagement from a task, these data provide strong and novel evidence that this gustatory rhythm in rats is associated with withdrawal from experimental contingencies.

Estimation of neurophysiological parameters from the waking EEG using a biophysical model of brain dynamics

This paper presents the results from using electroencephalographic (EEG) data to estimate the values of key neurophysiological parameters using a detailed biophysical model of brain activity. The model incorporates spatial and temporal aspects of cortical function including axonal transmission delays, synapto-dendritic rates, range-dependent connectivities, excitatory and inhibitory neural populations, and intrathalamic, intracortical, corticocortical and corticothalamic pathways. Parameter estimates were obtained by fitting the model's theoretical spectrum to EEG spectra from each of 100 healthy human subjects. Statistical analysis was used to infer significant parameter variations occurring between eyes-closed and eyes-open states, and a correlation matrix was used to investigate links between the parameter variations and traditional measures of quantitative EEG (qEEG). Accurate fits to all experimental spectra were observed, and both inter-subject and between-state variability were accounted for by the variance in the fitted biophysical parameters, which were in turn consistent with known independent experimental and theoretical estimates. These values thus provide physiological information regarding the state. transitions (eyes-closed vs. eyes-open) and phenomena including cortical idling and alpha desynchronization. The parameters are also consistent with traditional qEEG, but are more informative, since they provide links to underlying physiological processes. To our knowledge, this is the first study where a detailed biophysical model of the brain is used to estimate neurophysiological parameters underlying the transitions in a broad range (0.25-50 Hz) of EEG spectra obtained from a large set of human data.

Oscillatory Entrainment of Striatal Neurons in Freely Moving Rats

Oscillations and synchrony in basal ganglia circuits may play a key role in the organization of voluntary actions and habits. We recorded single units and local field potentials from multiple striatal and cortical locations simultaneously, over a range of behavioral states. We observed opposite gradients of oscillatory entrainment, with dorsal/lateral striatal neurons entrained to high-voltage spindle oscillations ("spike wave discharges") and ventral/medial striatal neurons entrained to the hippocampal theta rhythm. While the majority of units were likely medium-spiny projection neurons, a second neuronal population showed characteristic features of fast-spiking GABAergic interneurons, including tonic activity, brief waveforms, and high-frequency bursts. These fired at an earlier spindle phase than the main neuronal population, and their density within striatum corresponded closely to the intensity of spindle oscillations. The orchestration of oscillatory activity by networks of striatal interneurons may be an important mechanism in the pathophysiology of neurological disorders such as Parkinson's disease.

Cortical control of generalized absence seizures: effect of lidocaine applied to the somatosensory cortex in WAG/Rij rats

The role of the somatosensory cortex (SmI) in the incidence of spike-wave discharges (SWDs) was studied in a genetic model of absence epilepsy, WAG/Rij rats. SWDs were recently shown to initiate at the perioral area of the SmI and spread over the cortex and thalamus within a few milliseconds [J. Neurosci. 22 (2002) 1480]. It was hypothesized that functional deactivation of the SmI might reduce the appearance of SWDs. This was tested using unilateral microinjections (1 microl) of 2% lidocaine into the SmI in 13 WAG/Rij rats. Electrocorticogram (ECoG) was recorded in free moving animals from four cortical sites after lidocaine and control (saline) injections. Lidocaine effectively diminished the power of the ECoG spectra mostly in the area surrounding the injection site. Deactivation of the perioral region of the SmI reduced the incidence of SWDs at the entire cortex in both hemispheres. The number of SWDs gradually reached control level at the end of the second hour after injections of lidocaine. These data show that proper functioning of SmI is important for the occurrence of SWDs, supporting the idea that absence seizures might have a focal cortical origin.

Vagus nerve stimulation inhibits harmaline-induced tremor

Excessive olivo-cerebellar burst-firing occurs during harmaline-induced tremor. This system receives rich sensory inputs, including visceral. We hypothesized that electrical vagus nerve stimulation (VNS) would suppress harmaline tremor, as measured with digitized motion power in the rat. Cervical vagus nerve stimulation suppressed power in the 8-12-Hz tremor range by 40%, whereas sham stimulation was ineffective. This study raises the possibility that activation of various sensory modalities, as well as visceral, may reduce tremor.

Whisker motor cortex ablation and whisker movement patterns

Previous studies, based on qualitative observations, reported that lesions of the whisker motor cortex produce no deficits in whisking behavior. We used high-resolution optoelectronic recording methods to compare the temporal organization and kinematics of whisker movements before and after unilateral lesions of whisker motor cortex in rats. We now report that while the lesion did not abolish whisking, it significantly disrupted whisking kinematics, coordination, and temporal organization. Lesioned animals showed significant increases in the velocity and amplitude of whisker protractions contralateral to the lesions, as well as a reduction in the synchrony of whisker movements on the two sides of the face. There was a marked shift in the distribution of whisking frequencies, with reduction of activity in the 5-7 Hz bandwidth and increased activity at < 2 Hz. Disruptions of the normal whisking pattern were evident on both sides of the face, and the magnitude of these effects was proportional to the extent of the cortical ablation. We suggest that the observed deficits reflect an imbalance in cortical inputs to a brainstem central pattern generator.

Developmental regulation of active and passive membrane properties in rat vibrissa motoneurones

We characterized the electrophysiological properties of vibrissa motoneurones (vMNs) in rat. Intracellular recordings of vMNs in brainstem slices from animals aged P4 to P5 and P9 to P11, i.e. newborn animals, showed that the subthreshold membrane impedance has the form of passive decay. In particular, the impedance follows the 1/ radical f signature for long dendrites beyond a cut-off frequency of f(c)= 8 Hz. In contrast, the impedance has the form of a resonant filter in vMNs from slices prepared from animals aged P17 to P23, i.e. young animals. The resonance has a peak near 4 Hz and an amplitude of 1.2 times that at low frequencies (f approximately 0.1Hz). The low frequency onset of the resonance is shown to depend on a hyperpolarization-activated depolarizing current, I(h). This current functions as a high-pass filter. The high frequency cut-off of the resonance results from passive decay in long dendrites, similar to the case with newborn animals but with f(c)= 20Hz. In addition to a resonance in subthreshold properties, an enhanced resonance in spiking is observed in young as opposed to newborn animals. The transition from solely passive decay in vMNs from newborn animals to resonance in young animals coincides with the onset of whisking. Further, the width of the resonance encompasses the 4-15Hz range of exploratory whisking. Nonetheless, it remains to be shown if there is a causal relation between the regulation of currents in vMNs and the onset of whisking. In particular, we further observed that the membrane impedance of hypoglossal motoneurones from both newborn and young animals exhibits a subthreshold resonance that also peaks near 4Hz. The amplitude of this resonance increases from 1.1 to 1.4 times that at low frequencies in newborn versus young animals. We conjecture that resonance properties in vibrissa, hypoglossal, and potentially other motoneurones, may serve to transiently and purposely synchronize different orofacial behaviours.

Is spontaneous high-voltage rhythmic spike discharge in Long Evans rats an absence-like seizure activity?

A distinct high-voltage rhythmic spike (HVRS) discharge characterized by a barrage of negative spikes oscillating at 5-12 Hz was observed in chronically implanted Long Evans rats. Spontaneous HVRS discharges were exhibited in 90% of 40 Long Evans rats and occurred during sudden arrest of ongoing behavior (immobility) with occasional facial/whisker twitching. However, the function of HVRS discharges in Long Evans rats remains inconclusive to date and has been associated with alpha tremor/mu rhythm, attentive mu wave, and absence seizure. To elucidate the function of HVRS discharges in Long Evans rats, several experiments were performed. In a 6-h recording session (12:00-18:00), HVRS activities primarily occurred in several specific vigilance states, being particularly abundant in a short-lasting period before vigilance changes. Several characteristics, such as durations, oscillatory frequencies, and interspike intervals (ISIs) of HVRS discharges, were altered during wake-sleep states. Oscillatory frequencies were negatively correlated with durations of HVRS segments. In addition, ISIs of a HVRS episode exhibited a crescendo-decrescendo pattern. These variable ISIs could explain why a negative correlation was found between oscillatory frequencies and durations of HVRS episodes. Moreover, HVRS discharges were demonstrated to have widespread and near-synchronous distribution to bilateral cortical areas. In addition, innocuous electrical stimuli were unable to stop ongoing HVRS discharges. By contrast, noxious stimuli elicited behavioral arousal and immediately terminated most HVRS discharges. Cortical-evoked potentials in response to mild electrical stimulation under HVRS discharges were different from those under waking state but resemble those under slow-wave sleep with a smaller magnitude. Moreover, the temporal and spectral characteristics of spontaneous HVRS activities were analogous to those of seizure activities induced by penicillin and pentylenetetrazol. The incidence of spontaneous HVRS discharges was significantly decreased by ethosuximide administration. Based on these results, HVRS discharge might not be associated with a voluntary mu-rhythm behavior, instead it behaves as an absence-like seizure activity. These results were also collaborated using other genetic absence-seizure rats, such as WAG/Rij and GAERS rats. Possible mechanisms for the generation and termination of paroxysmal HVRS discharges are also discussed.

Unilateral vibrissa contact: changes in amplitude but not timing of rhythmic whisking

Electromyographic recordings from the mystacial pad of rats were used to assess the effect of unilateral vibrissa contact on the bilateral movement of the vibrissae. A first group of animals was trained to whisk freely in air and served to establish the baseline variability in bilateral symmetry. We observed that the electromyogram (EMG) activity across the two mystacial pads was rhythmic and synchronous to within 2 ms on a whisk-by-whisk basis; this value is small in comparison with the approximately 50 ms required for protraction during the whisk cycle. A second group of animals was trained to use their vibrissae to contact a sensor that was located on one side of the head. The average EMG activity across the two pads was synchronous at the time of vibrissa contact, albeit with higher variability than for the case of free whisking. In contrast, the average amplitude of the activity on the contact vs noncontact side of the face was transiently greater, by 25% or approximately 10 degrees, at the time of contact. These data show that the amplitude of the vibrissae on the two sides of the face can be controlled independently, while the timing of vibrissa movement is largely synchronous.

Cellular interactions in the rat somatosensory thalamocortical system during normal and epileptic 5-9 Hz oscillations

In Genetic Absence Epilepsy Rats from Strasbourg (GAERS), generalized spike-and-wave (SW) discharges (5-9 SW s(-1)) develop during quiet immobile wakefulness from a natural, medium-voltage, 5-9 Hz rhythm. This study examines the spatio-temporal dynamics of cellular interactions in the somatosensory thalamocortical system underlying the generation of normal and epileptic 5-9 Hz oscillations. Paired single-unit and multi-unit recordings between the principal elements of this circuit and intracellular recordings of thalamic, relay and reticular, neurones were conducted in neuroleptanalgesied GAERS and control, non-epileptic, rats. The identity of the recorded neurones was established following juxtacellular or intracellular marking. At least six major findings have emerged from this study. (1) In GAERS, generalized spike-and-wave discharges were correlated with synchronous rhythmic firings in related thalamic relay and reticular neurones. (2) Usually, corticothalamic discharges phase-led related relay and reticular firings. (3) A depolarizing wave emerging from a barrage of EPSPs was the cause of both relay and reticular discharges. (4) In some relay cells, which had a relatively high membrane input resistance, the depolarizing wave had the shape of a ramp, which could trigger a low-threshold Ca2+ spike. (5) In reticular cells, the EPSP barrage could further trigger voltage-dependent depolarizations. (6) The epilepsy-related thalamic, relay and reticular, intracellular activities were similar to the normal-related thalamic activities. Overall, these findings strongly suggest that, during absence seizures, corticothalamic neurones play a primary role in the synchronized excitation of thalamic relay and reticular neurones. The present study further suggests that absence-related spike-and-wave discharges correspond to hypersynchronous wake-related physiological oscillations.