A quantitative study of electroencephalography, eye movements and neck electromyography characterizing the sleep-wake cycle of the guinea-pig

Reductions in cortical alpha activity, enhancements in neural responses and impaired gap detection caused by sodium salicylate in awake guinea pigs

Tinnitus chronically affects between 10-15% of the population but, despite its prevalence, the underlying mechanisms are still not properly understood. One experimental model involves administration of high doses of sodium salicylate, as this is known to reliably induce tinnitus in both humans and animals. Guinea pigs were implanted with chronic electrocorticography (ECoG) electrode arrays, with silver-ball electrodes placed on the dura over left and right auditory cortex. Two more electrodes were positioned over the cerebellum to monitor auditory brainstem responses (ABRs). We recorded resting-state and auditory evoked neural activity from awake animals before and 2 h following salicylate administration (350 mg/kg; i.p.). Large increases in click-evoked responses (> 100%) were evident across the whole auditory cortex, despite significant reductions in wave I ABR amplitudes (in response to 20 kHz tones), which are indicative of auditory nerve activity. In the same animals, significant decreases in 6-10 Hz spontaneous oscillations (alpha waves) were evident over dorsocaudal auditory cortex. We were also able to demonstrate for the first time that cortical evoked potentials can be inhibited by a preceding gap in background noise [gap-induced pre-pulse inhibition (PPI)], in a similar fashion to the gap-induced inhibition of the acoustic startle reflex that is used as a behavioural test for tinnitus. Furthermore, 2 h following salicylate administration, we observed significant deficits in PPI of cortical responses that were closely aligned with significant deficits in behavioural responses to the same stimuli. Together, these data are suggestive of neural correlates of tinnitus and oversensitivity to sound (hyperacusis).

Phasic bursts of the antagonistic jaw muscles during REM sleep mimic a coordinated motor pattern during mastication

Sleep-related movement disorders are characterized by the specific phenotypes of muscle activities and movements during sleep. However, the state-specific characteristics of muscle bursts and movement during sleep are poorly understood. In this study, jaw-closing and -opening muscle electromyographic (EMG) activities and jaw movements were quantified to characterize phenotypes of motor patterns during sleep in freely moving and head-restrained guinea pigs. During non-rapid eye movement (NREM) sleep, both muscles were irregularly activated in terms of duration, activity, and intervals. During rapid eye movement (REM) sleep, clusters of phasic bursts occurred in the two muscles. Compared with NREM sleep, burst duration, activity, and intervals were less variable during REM sleep for both muscles. Although burst activity was lower during the two sleep states than during chewing, burst duration and intervals during REM sleep were distributed within a similar range to those during chewing. A trigger-averaged analysis of muscle bursts revealed that the temporal association between the bursts of the jaw-closing and -opening muscles during REM sleep was analogous to the temporal association during natural chewing. The burst characteristics of the two muscles reflected irregular patterns of jaw movements during NREM sleep and repetitive alternating bilateral movements during REM sleep. The distinct patterns of jaw muscle bursts and movements reflect state-specific regulations of the jaw motor system during sleep states. Phasic activations in the antagonistic jaw muscles during REM sleep are regulated, at least in part, by the neural networks involving masticatory pattern generation, demonstrating that waking jaw motor patterns are replayed during sleep periods.

Heterogeneous activity level of jaw-closing and -opening muscles and its association with arousal levels during sleep in the guinea pig

Exaggerated jaw motor activities during sleep are associated with muscle symptoms in the jaw-closing rather than the jaw-opening muscles. The intrinsic activity of antagonistic jaw muscles during sleep remains unknown. This study aims to assess the balance of muscle activity between masseter (MA) and digastric (DG) muscles during sleep in guinea pigs. Electroencephalogram (EEG), electroocculogram, and electromyograms (EMGs) of dorsal neck, MA, and DG muscles were recorded with video during sleep-wake cycles. These variables were quantified for each 10-s epoch. The magnitude of muscle activity during sleep in relation to mean EMG activity of total wakefulness was up to three times higher for MA muscle than for DG muscle for nonrapid eye movement (NREM) and rapid-eye-movement (REM) sleep. Although the activity level of the two jaw muscles fluctuated during sleep, the ratio of activity level for each epoch was not proportional. Epochs with a high activity level for each muscle were associated with a decrease in δEEG power and/or an increase in heart rate in NREM sleep. However, this association with heart rate and activity levels was not observed in REM sleep. These results suggest that in guinea pigs, the magnitude of muscle activity for antagonistic jaw muscles is heterogeneously modulated during sleep, characterized by a high activity level in the jaw-closing muscle. Fluctuations in the activity are influenced by transient arousal levels in NREM sleep but, in REM sleep, the distinct controls may contribute to the fluctuation. The above intrinsic characteristics could underlie the exaggeration of jaw motor activities during sleep (e.g., sleep bruxism).

The impact of age on the hypnotic effects of eszopiclone and zolpidem in the guinea pig

RATIONALE

Eszopiclone and zolpidem are hypnotics that differentially affect sleep and waking states in adult animals. Therefore, it was of interest to compare their effects on the states of sleep and wakefulness in aged animals.

OBJECTIVES

Our objective was to determine the responses to eszopiclone and zolpidem vis-à-vis sleep and waking states in aged guinea pigs and to compare them with the effects of these hypnotics in adult animals.

METHODS

Aged guinea pigs were prepared to monitor sleep and waking states and to perform a frequency analysis of the EEG. Eszopiclone and zolpidem were administered intraperitoneally (1, 3, and 10 mg/kg).

RESULTS

Eszopiclone produced a more rapid and greater increase in NREM sleep as well as longer duration episodes of NREM sleep compared with zolpidem. There was also a significant increase in the latency to REM sleep with eszopiclone, but not with zolpidem. EEG power during NREM sleep increased in the delta band and decreased in the theta band following eszopiclone administration, whereas zolpidem had no effect on any of the frequency bands analyzed.

CONCLUSIONS

In aged as well as adult guinea pigs, eszopiclone is a more effective hypnotic insofar as it produces a shorter latency to NREM sleep, a greater amount of NREM sleep and EEG delta waves. Differences in the effects produced by eszopiclone and zolpidem as a function of the aging process likely reflect the fact that they bind to different subunits of the GABA(A) receptors, which are differentially reactive to the aging process.

Tonic and phasic phenomena underlying eye movements during sleep in the cat

Mammalian sleep is not a homogenous state, and different variables have traditionally been used to distinguish different periods during sleep. Of these variables, eye movement is one of the most paradigmatic, and has been used to differentiate between the so-called rapid eye movement (REM) and non-REM (NREM) sleep periods. Despite this, eye movements during sleep are poorly understood, and the behaviour of the oculomotor system remains almost unknown. In the present work, we recorded binocular eye movements during the sleep-wake cycle of adult cats by the scleral search-coil technique. During alertness, eye movements consisted of conjugated saccades and eye fixations. During NREM sleep, eye movements were slow and mostly unconjugated. The two eyes moved upwardly and in the abducting direction, producing a tonic divergence and elevation of the visual axis. During the transition period between NREM and REM sleep, rapid monocular eye movements of low amplitude in the abducting direction occurred in coincidence with ponto-geniculo-occipital waves. Along REM sleep, the eyes tended to maintain a tonic convergence and depression, broken by high-frequency bursts of complex rapid eye movements. In the horizontal plane, each eye movement in the burst comprised two consecutive movements in opposite directions, which were more evident in the eye that performed the abducting movements. In the vertical plane, rapid eye movements were always upward. Comparisons of the characteristics of eye movements during the sleep-wake cycle reveal the uniqueness of eye movements during sleep, and the noteworthy existence of tonic and phasic phenomena in the oculomotor system, not observed until now.

Tonic inhibition and ponto-geniculo-occipital-related activities shape abducens motoneuron discharge during REM sleep

Eye movements, ponto-geniculo-occipital (PGO) waves, muscular atonia and desynchronized cortical activity are the main characteristics of rapid eye movement (REM) sleep. Although eye movements designate this phase, little is known about the activity of the oculomotor system during REM sleep. In this work, we recorded binocular eye movements by the scleral search-coil technique and the activity of identified abducens (ABD) motoneurons along the sleep-wake cycle in behaving cats. The activity of ABD motoneurons during REM sleep was characterized by a tonic decrease of their mean firing rate throughout this period, and short bursts and pauses coinciding with the occurrence of PGO waves. We demonstrate that the decrease in the mean firing discharge was due to an active inhibition of ABD motoneurons, and that the occurrence of primary and secondary PGO waves induced a pattern of simultaneous but opposed phasic activation and inhibition on each ABD nucleus. With regard to eye movements, during REM sleep ABD motoneurons failed to codify eye position as during alertness, but continued to codify eye velocity. The pattern of tonic inhibition and the phasic activations and inhibitions shown by ABD motoneurons coincide with those reported in other non-oculomotor motoneurons, indicating that the oculomotor system - contrary to what has been accepted until now - is not different from other motor systems during REM sleep, and that all motor systems are receiving similar command signals during this period.

Visual and computer-based detection of slow eye movements in overnight and 24-h EOG recordings

OBJECTIVE

The present work aimed to evaluate the performance of an automatic slow eye movement (SEM) detector in overnight and 24-h electro-oculograms (EOG) including all sleep stages (1, 2, 3, 4, REM) and wakefulness.

METHODS

Ten overnight and five 24-h EOG recordings acquired in healthy subjects were inspected by three experts to score SEMs. Computerized EOG analysis to detect SEMs was performed on 30-s epochs using an algorithm based on EOG wavelet transform, recently developed by our group and initially validated by considering only pre-sleep wakefulness, stages 1 and 2.

RESULTS

The validation procedure showed the algorithm could identify epochs containing SEM activity (concordance index k=0.62, 80.7% sensitivity, 63% selectivity). In particular, the experts and the algorithm identified SEM epochs mainly in pre-sleep wakefulness, stage 1, stage 2 and REM sleep. In addition, the algorithm yielded consistent indications as to the duration and position of SEM events within the epoch.

CONCLUSIONS

The study confirmed SEM activity at physiological sleep onset (pre-sleep wakefulness, stage 1 and stage 2), and also identified SEMs in REM sleep. The algorithm proved reliable even in the stages not used for its training.

SIGNIFICANCE

The study may enhance our understanding of SEM meaning and function. The algorithm is a reliable tool for automatic SEM detection, overcoming the inconsistency of manual scoring and reducing the time taken by experts.

Muscle activities are differently modulated between masseter and neck muscle during sleep-wake cycles in guinea pigs

Sleep bruxism is a sleep-related movement disorder characterized by an exaggerated jaw motor activity during sleep. Currently, the magnitude of jaw motor activation in normal sleep remains poorly understood. In this study, we aim to assess the state-dependent changes in the magnitude of electromyographic activities of the jaw-closing masseter muscle in comparison with those of a neck muscle (specifically, the obliquus capitis) during sleep-wake cycles in guinea pigs. These electromyographic activities were integrated for 10-s epochs during wakefulness, non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The masseter activity per epoch was found to be five times lower in both sleep stages while the neck muscle activity also decreased to 30% in NREM sleep and was lowest (16%) in REM sleep. In the periods without motor activity, masseter tone did not differ between the three states, whereas neck muscle tone decreased from wakefulness to NREM sleep and further to REM sleep. Moreover, in the epochs with masseter activation, the neck muscle activity did not increase during sleep. These results suggest that masseter activity decreases but is occasionally activated during sleep, and that state-dependent changes in electromyographic activity can be differently modulated in time and intensity between the masseter and the obliquus capitis.

GABAergic processes in the mesencephalic tegmentum modulate the occurrence of active (rapid eye movement) sleep in guinea pigs

The ventrolateral subdivision of the periaqueductal gray (vlPAG) and the adjacent dorsal mesencephalic reticular formation (dMRF) are involved in the modulation of active (rapid eye movement) sleep (AS). In order to determine the effects on AS of the suppression of neuronal activity in these regions, muscimol, a GABA receptor A (GABA(A)) receptor agonist, and bicuculline, a GABA(A) receptor antagonist, were microinjected bilaterally in guinea pigs and the states of sleep and wakefulness were examined. The main effect of muscimol was an increase in AS; this increase occurred in conjunction with a reduction in the time spent in wakefulness. The powerful effect of muscimol was striking especially when considering the small amount of naturally-occurring AS that is present in this species. Additional observable effects that were induced by muscimol were: 1) long lasting episodes of hypotonia/atonia during wakefulness and quiet sleep that included a lack of extensor tone in the hind limbs, and 2) frequently occurring cortical spindles, similar to those observed during naturally-occurring quiet sleep (sleep spindles), that were present during wakefulness. Conversely, bilateral microinjections of bicuculline induced a prolonged state of wakefulness and blocked the effect of subsequent injections of muscimol. These data suggest that endogenous GABA acts on GABA(A) receptors within the vlPAG and dMRF to promote AS in the guinea pig.

Oscillatory and Intrinsic Membrane Properties of Guinea Pig Nucleus Prepositus Hypoglossi Neurons In Vitro

Numerous models of the oculomotor neuronal integrator located in the prepositus hypoglossi nucleus (PHN) involve both highly tuned recurrent networks and intrinsic neuronal properties; however, there is little experimental evidence for the relative role of these two mechanisms. The experiments reported here show that all PHN neurons (PHNn) show marked phasic behavior, which is highly oscillatory in ∼25% of the population. The behavior of this subset of PHNn, referred to as type D PHNn, is clearly different from that of the medial vestibular nucleus neurons, which transmit the bulk of head velocity-related sensory vestibular inputs without integrating them. We have investigated the firing and biophysical properties of PHNn and developed data-based realistic neuronal models to quantitatively illustrate that their active conductances can produce the oscillatory behavior. Although some individual type D PHNn are able to show some features of mathematical integration, the lack of robustness of this behavior strongly suggests that additional network interactions, likely involving all types of PHNn, are essential for the neuronal integrator. Furthermore, the relationship between the impulse activity and membrane potential of type D PHNn is highly nonlinear and frequency-dependent, even for relatively small-amplitude responses. These results suggest that some of the synaptic input to type D PHNn is likely to evoke oscillatory responses that will be nonlinearly amplified as the spike discharge rate increases. It would appear that the PHNn have specific intrinsic properties that, in conjunction with network interconnections, enhance the persistent neural activity needed for their function.

Radiotelemetry recording of electroencephalogram in piglets during rest

A wireless recording system was developed to study the electroencephalogram (EEG) in unrestrained, male Landrace piglets. Under general anesthesia, ball-tipped silver/silver chloride electrodes for EEG recording were implanted onto the dura matter of the parietal and frontal cortex of the piglets. A pair of miniature preamplifiers and transmitters was then mounted on the surface of the skull. To examine whether other bioelectrical activities interfere with the EEG measurements, an electrocardiogram (ECG) or electromyogram (EMG) of the neck was simultaneously recorded with the EEG. Next, wire electrodes for recording movement of the eyelid were implanted with EEG electrodes, and EEG and eyelid movements were simultaneously measured. Power spectral analysis using a Fast Fourier Transformation (FFT) algorithm indicates that EEG was successfully recorded in unrestrained piglets, at rest, during the daytime in the absence of interference from ECG, EMG or eyelid movements. These data indicate the feasibility of using our radiotelemetry system for measurement of EEG under these conditions.

Vestibulo-oculomotor behaviour in rats following a transient unilateral vestibular loss induced by lidocaine

The effects of a transient vestibular nerve blockade, achieved by intra-tympanic instillation of lidocaine, were studied in rats by recording horizontal eye movements in darkness. Evaluation of the dose-response relationship showed that a maximal effect was attained with a concentration of 4% lidocaine. Within 15 min of lidocaine instillation, a vigorous spontaneous nystagmus was observed which reached maximal frequency and velocity of the slow phase after about 20 min. Subsequently, the nystagmus failed for approximately half an hour before it reappeared. This could be avoided by providing visual feedback in between the recordings in darkness or by a contralateral instillation of 2.5% lidocaine. It is suggested that the failure reflects an overload of the vestibulo-oculomotor circuits. After recovery from the nerve blockade, when the gaze was stable, dynamic vestibular tests were performed. They revealed that a decrease of the slow phase velocity gain and the dominant time constant during, respectively, sinusoidal- and step stimulation toward the unanaesthetised side, had developed with the nerve blockade. These modulations were impaired by a nodulo-uvulectomy but not by bilateral flocculectomy, which is consistent with the concept of vestibular habituation. A GABA(B) receptor antagonist, CGP 56433A, given systemically during the nerve blockade, aggravated the vestibular asymmetry. The same effect has previously been demonstrated in both short- (days) and long-term (months) compensated rats, by antagonising the GABA(B) receptor. In summary, this study provides the first observations of vestibulo-oculomotor disturbances during the first hour after a rapid and transient unilateral vestibular loss in the rat. By using this method, it is possible to study immediate behavioural consequences and possible neural changes that might outlast the nerve blockade.

Gene expression in rat vestibular and reticular structures during and after space flight

Space flight produces profound changes of neuronal activity in the mammalian vestibular and reticular systems, affecting postural and motor functions. These changes are compensated over time by plastic alterations in the brain. Immediate early genes (IEGs) are useful indicators of both activity changes and neuronal plasticity. We studied the expression of two IEG protein products [Fos and Fos-related antigens (FRAs)] with different cell persistence times (hours and days, respectively) to identify brainstem vestibular and reticular structures involved in adaptation to microgravity and readaptation to 1 G (gravity) during the NASA Neurolab Mission (STS-90). IEG protein expression in flight animals was compared to that of ground controls using Fisher 344 rats killed 1 and 12 days after launch and 1 and 14 days after landing. An increase in the number of Fos-protein-positive cells in vestibular (especially medial and spinal) regions was observed 1 day after launch and 1 day after landing. Fos-positive cell numbers were no different from controls 12 days after launch or 14 days after landing. No G-related changes in IEG expression were observed in the lateral vestibular nucleus. The pattern of FRA protein expression was generally similar to that of Fos, except at 1 day after landing, when FRA-expressing cells were observed throughout the whole spinal vestibular nucleus, but only in the caudal part of the medial vestibular nucleus. Fos expression was found throughout the entire medial vestibular nucleus at this time. While both Fos and FRA expression patterns may reflect the increased G force experienced during take-off and landing, the Fos pattern may additionally reflect recent rebound episodes of rapid eye movement (REM) sleep following forced wakefulness, especially after landing. Pontine activity sources producing rhythmic discharges of vestibulo-oculomotor neurons during REM sleep could substitute for labyrinthine signals after exposure to microgravity, contributing to activity-related plastic changes leading to G readaptation. Reticular structures exhibited a contrasting pattern of changes in the numbers of Fos- and FRA-positive cells suggestive of a major influence from proprioceptive inputs, and plastic re-weighting of inputs after landing. Asymmetric induction of Fos and FRAs observed in some vestibular nuclei 1 day after landing suggests that activity asymmetries between bilateral otolith organs, their primary labyrinthine afferents, and vestibular nuclei may become unmasked during flight.

Auditory thalamus neurons during sleep: changes in frequency selectivity, threshold, and receptive field size

The present study describes how the frequency receptive fields (RF) of auditory thalamus neurons are modified when the state of vigilance of an unanesthetized animal naturally fluctuates among wakefulness (W), slow-wave sleep (SWS), and paradoxical sleep (PS). Systematic quantification of several RF parameters-including strength of the evoked responses, response latency, acoustic threshold, shape of rate-level function, frequency selectivity, and RF size-was performed while undrugged, restrained guinea pigs presented spontaneous alternances of W, SWS, and PS. Data are from 102 cells recorded during W and SWS and from 53 cells recorded during W, SWS, and PS. During SWS, thalamic cells behaved as an homogeneous population: as compared with W, most of them (97/102 cells) exhibited decreased evoked spike rates. The frequency selectivity was enhanced and the RF size was reduced. In contrast during PS, two populations of cells were identified: one (32/53 cells) showed the same pattern of changes as during SWS, whereas the other (21/53 cells) expressed values of evoked spike rates and RF properties that did not significantly differ from those in W. These two populations were equally distributed in the different anatomical divisions of the auditory thalamus. Last, during both SWS and PS, the responses latency was longer and the acoustic threshold was higher than in W but the proportion of monotonic versus nonmonotonic rate-level functions was unchanged. During both SWS and PS, no relationship was found between the changes in burst percentage and the changes of the RF properties. These results point out the dual aspect of sensory processing during sleep. On the one hand, they show that the auditory messages sent by thalamic cells to cortical neurons are reduced both in terms of firing rate at a given frequency and in terms of frequency range. On the other hand, the fact that the frequency selectivity and the rate-level function are preserved suggests that the messages sent to cortical cells are not deprived of informative content, and that the analysis of complex acoustic sounds should remain possible. This can explain why, although attenuated, reactivity to biologically relevant stimuli is possible during sleep.

The vestibular system as a model of sensorimotor transformations. A combined in vivo and in vitro approach to study the cellular mechanisms of gaze and posture stabilization in mammals

To understand the cellular mechanisms underlying behaviours in mammals, the respective contributions of the individual properties characterizing each neuron, as opposed to the properties emerging from the organization of these neurons in functional networks, have to be evaluated. This requires the use, in the same species, of various in vivo and in vitro experimental preparations. The present review is meant to illustrate how such a combined in vivo in vitro approach can be used to investigate the vestibular-related neuronal networks involved in gaze and posture stabilization, together with their plasticity, in the adult guinea-pig. Following first a general introduction on the vestibular system, the second section describes various in vivo experiments aimed at characterizing gaze and posture stabilization in that species. The third and fourth parts of the review deal with the combined in vivo-in vitro investigations undertaken to unravel the physiological and pharmacological properties of vestibulo-ocular and vestibulo-spinal networks, together with their functional implications. In particular, we have tried to use the central vestibular neurons as examples to illustrate how the preparation of isolated whole brain can be used to bridge the gap between the results obtained through in vitro, intracellular recordings on slices and those collected in vivo, in the behaving animal.