The lemniscal-cuneate recurrent excitation is suppressed by strychnine and enhanced by GABAA antagonists in the anaesthetized cat

Plastic changes in primate motor cortex following paired peripheral nerve stimulation

Repeated paired stimulation of two peripheral nerves can produce lasting changes in motor cortical excitability, but little is known of the underlying neuronal basis. Here, we trained two macaque monkeys to perform selective thumb and index finger abduction movements. Neural activity was recorded from the contralateral primary motor cortex during task performance, and following stimulation of the ulnar and median nerves, and the nerve supplying the extensor digitorum communis (EDC) muscle. Responses were compared before and after 1 h of synchronous or asynchronous paired ulnar/median nerve stimulation. Task performance was significantly enhanced after asynchronous and impaired after synchronous stimulation. The amplitude of short latency neural responses to median and ulnar nerve stimulation was increased after asynchronous stimulation; later components were reduced after synchronous stimulation. Synchronous stimulation increased neural activity during thumb movement and decreased it during index finger movement; asynchronous stimulation decreased activity during both movements. To assess how well neural activity could separate behavioral or sensory conditions, linear discriminant analysis was used to decode which nerve was stimulated, or which digit moved. Decoding accuracy for nerve stimulation was decreased after synchronous and increased after asynchronous paired stimulation. Decoding accuracy for task performance was decreased after synchronous but was unchanged after asynchronous paired stimulation. Paired stimulation produces changes in motor cortical circuits that outlast the stimulation. Some of these changes depend on precise stimulus timing.

NEW & NOTEWORTHY Paired stimulation of peripheral nerves for 1 h induced lasting changes in neural responses within the motor cortex to nerve stimulation and to performance of a behavioral task. These changes were sufficient to alter the efficiency with which activity could encode stimulus type. Stimuli that can be easily applied noninvasively in human subjects can alter central motor circuits.

AMPA and GABA(A/B) receptor subunit expression in the cuneate nucleus of adult squirrel monkeys during peripheral nerve regeneration

The primate somatosensory neuroaxis provides an excellent model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for AMPA and GABAA/B receptor subunits in the cuneate nucleus of adult squirrel monkeys 1 and 5 months after median nerve compression. This method of nerve injury allowed the investigation of the way in which patterns of receptor correlates change during peripheral nerve regeneration. These results are compared to cortical data collected within the same animals. As observed in the cortex, the pattern of subunit staining in the brainstem 1 month after nerve compression suggests that the sensory deprived nucleus enters a state of reorganization. That is, the expression of GluR2/3 AMPA receptor subunits is significantly increased, while GABA α1 and GABABR1b receptor subunits are significantly decreased. Five months after nerve injury, the pattern of subunit expression is again very similar to that observed in the infragranular layers of cortex. At this later time we observe a significant increase in GluR2/3 and GABABR1a, with no change in GABAAα1, and a significant decrease in GABABR1b. Together these results suggest that during reorganization and recovery from injury the brainstem and cortex are governed by homogeneous mechanisms of plasticity.

Processing afferent proprioceptive information at the main cuneate nucleus of anesthetized cats

Medial lemniscal activity decreases before and during movement, suggesting prethalamic modulation, but the underlying mechanisms are largely unknown. Here we studied the mechanisms underlying proprioceptive transmission at the midventral cuneate nucleus (mvCN) of anesthetized cats using standard extracellular recordings combined with electrical stimulation and microiontophoresis. Dual simultaneous recordings from mvCN and rostroventral cuneate (rvCN) proprioceptive neurons demonstrated that microstimulation through the rvCN recording electrode induced dual effects on mvCN projection cells: potentiation when both neurons had excitatory receptive fields in muscles acting at the same joint, and inhibition when rvCN and mvCN cells had receptive fields located in different joints. GABA and/or glycine consistently abolished mvCN spontaneous and sensory-evoked activity, an effect reversed by bicuculline and strychnine, respectively; and immunohistochemistry data revealed that cells possessing strychnine-sensitive glycine receptors were uniformly distributed throughout the cuneate nucleus. It was also found that proprioceptive mvCN projection cells sent ipsilateral collaterals to the nucleus reticularis gigantocellularis and the mesencephalic locomotor region, and had slower antidromic conduction speeds than cutaneous fibers from the more dorsally located cluster region. The data suggest that (1) the rvCN-mvCM network is functionally related to joints rather than to single muscles producing an overall potentiation of proprioceptive feedback from a moving forelimb joint while inhibiting, through GABAergic and glycinergic interneurons, deep muscular feedback from other forelimb joints; and (2) mvCN projection cells collateralizing to or through the ipsilateral reticular formation allow for bilateral spreading of ascending proprioceptive feedback information.

Spike timing, spike count, and temporal information for the discrimination of tactile stimuli in the rat ventrobasal complex

The aim of this work was to investigate the role of spike timing for the discrimination of tactile stimuli in the thalamic ventrobasal complex of the rat. We applied information-theoretic measures and computational experiments on neurophysiological data to test the ability of single-neuron responses to discriminate stimulus location and stimulus dynamics using either spike count (40 ms bin size) or spike timing (1 ms bin size). Our main finding is not only that spike timing provides additional information over spike count alone, but specifically that the temporal aspects of the code can be more informative than spike count in the rat ventrobasal complex. Virtually all temporal information--i.e., information exclusively related to when the spikes occur--is conveyed by first spikes, arising mostly from latency differences between the responses to different stimuli. Although the imprecision of first spikes (i.e., the jitter) is highly detrimental for the information conveyed by latency differences, jitter differences can contribute to temporal information, but only if latency differences are close to zero. We conclude that temporal information conveyed by spike timing can be higher than spike count information for the discrimination of somatosensory stimuli in the rat ventrobasal complex.

Elimination of muscle afferent boutons from the cuneate nucleus of the rat medulla during development

There is developmental refinement of the proprioreceptive muscle afferent input to the rat ventral horn. This study explored the extent to which this occurs in the medulla. Muscle afferents were transganglionically labeled from the extensor digitorum communis forelimb muscle with cholera toxin B subunit. Tracer amounts and transport times were adjusted for animal size. Immunohistochemistry revealed tracer localization in the medulla and dorsal root ganglia. Labeled muscle afferent boutons were counted in the cuneate nucleus between postnatal days 7 and 42, during which time a large decrease in the density of labeled boutons was observed qualitatively. Localization of input to dorsolateral parts of the nucleus remained broadly the same at different ages, although disappearance of a marked innervation of ventromedial regions in more caudal sections was observed. Bouton counts were corrected for growth of the medulla with age, and any spread of tracer to adjacent muscles indicated by counts of labeled dorsal root ganglion neurons. There was a statistically significant, approximately 40% reduction in the number of muscle afferent boutons in the cuneate nucleus during this developmental period. Previous studies suggest that perturbations to the corticospinal input during a developmental critical period influence the eventual size of the muscle afferent input to the ventral horn. Corticocuneate fibers invade the nucleus during the same period and may influence reorganization of its muscle afferent input, making it another potential site for aberrant reflex development in cerebral palsy.

Giant subcortical high-frequency SEPs in idiopathic generalized epilepsy: a protective mechanism against seizures?

OBJECTIVE

Recently, we found that high-frequency somatosensory evoked potentials (HF-SEPs), which are modulated by arousal-related structures, were abnormally enhanced during N-REM sleep in two seizure-free IGE patients [Restuccia D, Rubino M, Valeriani M, Della Marca G. Increase of brainstem high-frequency SEP subcomponents during light sleep in seizure-free epileptic patients. Clin Neurophysiol 2005; 116: 1774-1778]. Here, we aimed at verifying whether similar HF-SEP abnormalities were significantly correlated to the clinical outcome in a larger population of untreated IGE patients.

METHODS

Patients were classified as Juvenile Myoclonic epilepsy (JME; six patients) and Childhood or Juvenile Absence epilepsy (CAE and JAE, six patients). They were untreated because newly diagnosed, or because seizure-free. HF-SEPs from patients were compared with those obtained from 21 healthy volunteers.

RESULTS

HF-SEPs were abnormally enhanced in all seizure-free CAE-JAE patients, whereas they were normal in all JME patients and in CAE-JAE patients with frequent seizures. Not only scalp distribution, but also dipolar source analysis suggested a subcortical origin for these enhanced subcomponents, possibly in the brainstem.

CONCLUSIONS

The enhancement of HF-SEPs might reflect the hyperactivity of arousal-related brainstem structures; such an enhancement was found in all seizure-free CAE-JAE patients, while it was never observed in JME patients.

SIGNIFICANCE

We speculate that the hyperactivity of arousal-related brainstem structures might account for the different clinical outcome among IGE subsyndromes.

Spontaneous bursting and rhythmic activity in the cuneate nucleus of anaesthetized rats

Spontaneous and rhythmic neuronal activity in dorsal column nuclei has long been identified in anesthetized cats. Here, we have studied the spontaneous behavior of cuneate cells in anesthetized rats through extracellular recording, showing that most cuneate neurones recorded (155 of 185) fired spontaneously. Overall, 74% of these spontaneously firing neurones were single-spiking and 26% were bursting. Cells were considered "bursting" when more than 50% of the spontaneous spikes belonged to bursts. Nevertheless, occasional bursts were seen in 33% of spontaneous cuneate cells which were classified as single-spiking. Rhythmic firing was observed in about 14% of both spontaneously bursting and single-spiking cells, and these cells were located close to the obex (+/-0.5 mm). Although the spike-frequency was mostly in the range 0-15 spikes/s, spontaneous rhythmic activity was circumscribed mainly to the alpha/beta-like range, both in single-spiking (26.1+/-3.6 Hz, n=16) and bursting cells (19.5+/-4.1 Hz, n=6). Lemniscal stimulation often activated several antidromic units with the same latency. About 65% of cuneolemniscal cells were spontaneously active and of these, 83% were single-spiking and 11% rhythmic (all single-spiking). In cells that were not antidromically activated from the medial lemniscus, short latency orthodromic responses consistent with excitation by recurrent lemniscal collaterals were often observed following lemniscal activation. Interestingly, only cells completely unresponsive to lemniscal stimulation showed rhythmic bursting. Most spontaneous cells responded with a burst to natural receptive field stimulation, while rhythmic cells became temporally arrhythmic. These results demonstrate, for the first time, that rat cuneate neurones can fire bursts spontaneously. Besides, this bursting activity can be rhythmic. These two properties, and the fact that groups of cuneolemniscal cells share the same conduction velocity, probably imply the reinforcement of temporal and spatial summation at their targets when they are synchronously recruited by the stimulation of overlapping receptive fields.

Cortical modulation of dorsal column nuclei: A computational study

We present a computational study aimed at exploring the sensorimotor cortex modulation of the behaviour of dorsal column nuclei, specifically the impact of synaptic parameters, during both sleep and waking conditions. On the basis of the circuit proposed by Canedo et al. (2000), we have developed realistic computational models that have been tested with simultaneous electrocorticographic as well as intracellular cuneate recordings performed in anaesthetized cats. The results show that, (1) under sleep conditions, the model can block the transmission of afferent sensory information and, (2) operations expected during wakefulness, such as filtering and facilitation, can be performed if synaptic parameters are appropriately tuned.

GABA(B) receptor-mediated modulation of cutaneous input at the cuneate nucleus in anesthetized cats

This study examined the modulatory influence exerted by GABA(B) receptors on the transmission of cutaneous afferent input to cuneate nucleus neurons in anesthetized cats. Electrical stimulation at the center of a receptive field activated cuneate nucleus cells at latencies of < or = 7 ms whereas stimulation at neighboring sites (receptive field edge) increased the response latency. Extracellular recording combined with microiontophoresis demonstrated that GABA(B) receptors are tonically active. Blockade of GABA(B) receptors prolonged sensory-evoked response durations and decreased times of occurrence of successive bursts whereas the agonist baclofen suppressed both these effects. Ejection of baclofen delayed the evoked response from the receptive field edge with respect to the receptive field center response and inhibited responses from the receptive field edge more effectively than responses from the receptive field center. From these results it is concluded that activation of GABA(B) receptors precludes cuneate cells from reaching firing threshold when afferent inputs are weak, spatially modulate cuneate nucleus excitability, play a major role in temporal pattern of discharges, and shape cutaneous receptive fields.

Increase of brain-stem high-frequency SEP subcomponents during light sleep in seizure-free epileptic patients

OBJECTIVE

Three hertz spike-and-wave (SW) occurrence is caused by the abnormal functioning of the same thalamo-cortical loop generating sleep spindles. In fact, SW preferably occurs during light sleep and transitional phases of the vigilance status. Since high-frequency somatosensory evoked potentials (HF-SEPs) are powerfully modulated by sleep and arousal, we verified whether they can reveal abnormalities of arousal-related structures in two patients having showed sporadic SW discharges during light sleep.

METHODS

We recorded right median nerve SEPs in two adult patients who suffered since the infancy from childhood absence epilepsy (CAE). Sleep stage-related changes of HF-SEPs were compared to those observed in five healthy volunteers.

RESULTS

HF-SEPs decreased during sleep in controls. By contrast, the amplitude of the subcortical component dramatically increased in CAE patients during phase II NREM sleep. Simultaneous EEG showed normally represented sleep spindles, but not SW discharges.

CONCLUSIONS

HF-SEP increase probably reflects the hyperactivation of brain-stem arousal-related structures. During such a hyperactivation no EEG abnormalities were observed.

SIGNIFICANCE

We hypothesize that HF-SEP increase might reflect a protective mechanism against seizure occurrence during light sleep.

Intracuneate mechanisms underlying primary afferent cutaneous processing in anaesthetized cats

The cutaneous primary afferents from the upper trunk and forelimbs reach the medial cuneate nucleus in their way towards the cerebral cortex. The aim of this work was twofold: (i) to study the mechanisms used by the primary afferents to relay cutaneous information to cuneate cuneolemniscal (CL) and noncuneolemniscal (nCL) cells, and (ii) to determine the intracuneate mechanisms leading to the elaboration of the output signal by CL cells. Extracellular recordings combined with microiontophoresis demonstrated that the primary afferent cutaneous information is communicated to CL and nCL cells through AMPA, NMDA and kainate receptors. These receptors were sequentially activated: AMPA receptors participated mainly during the initial phase of the response, whereas kainate- and NMDA-mediated activity predominated during a later phase. The involvement of NMDA receptors was confirmed by in vivo intracellular recordings. The cutaneous-evoked activation of CL cells was decreased by GABA and increased by glycine acting at a strychnine-sensitive site, indicating that glycine indirectly affects CL cells. Two subgroups of nCL cells were distinguished based on their sensitivity to iontophoretic ejection of glycine and strychnine. Overall, the results support a model whereby the primary afferent cutaneous input induces a centre-surround antagonism in the cuneate nucleus by activating (via AMPA, NMDA and kainate receptors) and disinhibiting (via serial glycinergic-GABAergic interactions) a population of CL cells with overlapped receptive fields that at the same time inhibit (via GABAergic cells) other neighbouring CL cells with different receptive fields.

Brain-stem components of high-frequency somatosensory evoked potentials are modulated by arousal changes: nasopharyngeal recordings in healthy humans

OBJECTIVE

Until now, the demonstration that early components of high-frequency oscillations (HFOs) evoked by electrical upper limb stimulation are generated in the brain-stem has been based on the results of scalp recordings. To better define the contribution of brain-stem components to HFOs building, we recorded high-frequency somatosensory evoked potentials (SEPs) in 6 healthy volunteers by means of a nasopharyngeal (NP) electrode. Moreover, since HFOs are highly susceptible to arousal fluctuations, we investigated whether eyes opening can influence HFOs at this level.

METHODS

We recorded right median nerve SEPs from the ventral surface of the medulla by means of a NP electrode as well as from the scalp, in 6 healthy volunteers under two different arousal states (eyes opened versus eyes closed). SEPs have been further analyzed after digital narrow bandpass filtering (400-800 Hz).

RESULTS

NP recordings demonstrated in all subjects a well-defined burst, occurring in the same latency window of the low-frequency P13-P14 complex. Eyes opening induced a significant amplitude increase of the NP-recorded HFOs, whereas scalp-recorded HFOs as well as low-frequency SEPs remained unchanged.

CONCLUSIONS

Our findings demonstrate that slight arousal variations induce significant changes in brain-stem components of HFOs. According to the hypothesis that HFOs reflect the activation of central mechanisms, which modulate sensory inputs depending on variations of arousal state, our data suggest that this modulation is already effective at brain-stem level.

Influence of modafinil on somatosensory input processing in the human brain-stem

OBJECTIVE

Since high frequency oscillations (HFOs) evoked by upper limb stimulation are susceptible to arousal fluctuation, we verified whether administration of modafinil, a vigilance promoting drug, modifies such responses at different levels of the somatosensory system.

METHODS

HFOs were obtained in 6 healthy volunteers by 500-700 Hz filtering of right median nerve somatosensory evoked potentials, before and 2 hours after the administration of 100 mg modafinil. Raw data were further submitted to brain electrical source analysis.

RESULTS

Modafinil significantly increased subcortical HFOs, as well as the strength of a dipolar source at the base of the skull.

CONCLUSIONS

Our data suggest that modafinil exerts its action also at the level of the brain-stem, where it interferes with the processing of somatosensory ascending inputs.

Quantitative stereological evaluation of the gracile and cuneate nuclei and their projection neurons in the rat

Stereological methods were employed to estimate the volume and neuron numbers of the rat dorsal column nuclei (DCN). These methods were applied to Nissl-stained sections from control animals and cases that received injections of horseradish peroxidase in the thalamus, the cerebellum, or the spinal cord. Additional cases received combinations of fluorescent tracers in the same structures, to examine whether some of the retrogradely labeled neurons sent collaterals to different targets. The mean volume of the DCN is 0.81 mm(3) (range 0.65-1.10 mm(3)), of which 3%, 39%, and 59% correspond, respectively, to the nucleus of Bischoff (Bi), the gracile (Gr), and the cuneate (Cu) nuclei. Within Cu, the middle division (CuM) is the largest (42%), followed by the rostral (CuR; 36%) and caudal (CuC; 22%) divisions. The mean total number of neurons in the DCN is 16,000 (range 12,400-19,500), of which 2.4%, 34.0% and 63.6% correspond, respectively, to Bi, Gr, and Cu. Within Cu, CuM contains 48% of all neurons, and 27% correspond to CuR and 25% to CuC. Interanimal variability is moderate for the whole DCN and Cu but increases when individual nuclei are considered. About 80% of DCN neurons project to the thalamus, 3% to the spinal cord, and 7% to the cerebellum. Thalamic-projecting cells are more numerous in CuM and Gr (83%), and relatively less common in Bi and CuC (72-74%). Most of the DCN neurons projecting to the spinal cord appear in CuC and CuM. Two-thirds of the neurons projecting to the cerebellum are located in CuR, 20% in CuM, and 15% in Gr. A small fraction of neurons projects simultaneously to spinal cord and thalamus.

New corticocuneate cellular mechanisms underlying the modulation of cutaneous ascending transmission in anesthetized cats

The ascending cutaneous transmission through the middle cuneate nucleus is subject to cortico-feedback modulation. This work studied the intracuneate cellular mechanisms underlying the corticocuneate influence. Single unit extracellular records combined with iontophoresis showed that the corticocuneate input activates cuneo-lemniscal (CL) and noncuneo-lemniscal (nCL) cells via N-methyl-D-aspartate (NMDA) and non-NMDA receptors as shown by the decrease of the cortical-induced activation on ejection of CNQX and APV, either alone or in combination. These results were confirmed by in vivo intracellular recordings. Two subgroups of nCL cells were distinguished according to their sensitivity to iontophoretic ejection of glycine and its antagonist, strychnine. Finally, the corticalevoked activation of CL cells was decreased by GABA and increased by glycine acting at a strychnine-sensitive site, indicating that glycine indirectly affects the cuneo-lemniscal transmission. A model is proposed whereby the cortex influences CL cells through three different mechanisms, producing 1) activation via non-NMDA and NMDA receptors, 2) inhibition through GABAergic nCLs, and 3) disinhibition via serial glycinergic-GABAergic nCL cells. These corticocuneate feedback effects serve to potentiate the activity of CL cells topographically aligned through direct activation and disinhibition, while inhibiting, via GABAergic cells, other CL neurons not topographically aligned.