Acute effect of carbamazepine on corticothalamic 5–9‐Hz and thalamocortical spindle (10–16‐Hz) oscillations in the rat

A major side effect of carbamazepine (CBZ), a drug used to treat neurological and neuropsychiatric disorders, is drowsiness, a state characterized by increased slow‐wave oscillations with the emergence of sleep spindles in the electroencephalogram (EEG). We conducted cortical EEG and thalamic cellular recordings in freely moving or lightly anesthetized rats to explore the impact of CBZ within the intact corticothalamic (CT)–thalamocortical (TC) network, more specifically on CT 5–9‐Hz and TC spindle (10–16‐Hz) oscillations. Two to three successive 5–9‐Hz waves were followed by a spindle in the cortical EEG. A single systemic injection of CBZ (20 mg/kg) induced a significant increase in the power of EEG 5–9‐Hz oscillations and spindles. Intracellular recordings of glutamatergic TC neurons revealed 5–9‐Hz depolarizing wave–hyperpolarizing wave sequences prolonged by robust, rhythmic spindle‐frequency hyperpolarizing waves. This hybrid sequence occurred during a slow hyperpolarizing trough, and was at least 10 times more frequent under the CBZ condition than under the control condition. The hyperpolarizing waves reversed at approximately ?70 mV, and became depolarizing when recorded with KCl‐filled intracellular micropipettes, indicating that they were GABA_A receptor‐mediated potentials. In neurons of the GABAergic thalamic reticular nucleus, the principal source of TC GABAergic inputs, CBZ augmented both the number and the duration of sequences of rhythmic spindle‐frequency bursts of action potentials. This indicates that these GABAergic neurons are responsible for the generation of at least the spindle‐frequency hyperpolarizing waves in TC neurons. In conclusion, CBZ potentiates GABA_A receptor‐mediated TC spindle oscillations. Furthermore, we propose that CT 5–9‐Hz waves can trigger TC spindles.     

14 & 6 Hz positive spikes coinciding with PLEDs

ObjectiveWe describe the coincidence of 14 &; 6 Hz positive spikes with PLEDs in a patient with clonic status epilepticus of the left upper extremity and the persistence of 14 &; 6 Hz positive spikes after cessation of status.MethodsDigital video-EEG recordings were performed using 32-channel EEG equipment (XLTEK, Canada) with all electrodes of the international 10–20 system and additional anterior temporal electrodes in a patient during clonic status epilepticus and 2 months later after cessation of status.ResultsThe initial EEG during clonic status epilepticus showed right hemispheric PLEDs and right lateral temporal 14 &; 6 Hz positive spikes in between the PLEDs. Follow up EEG recording 2 months later after cessation of status revealed an absence of PLEDs, a continuous slowing over the right hemisphere and the occipital background of 7 Hz. Right lateral temporal 14 &; 6 Hz positive spikes were recorded in the same frequency and the same localization as in the previous status EEG.ConclusionsThis case demonstrates that a hemisphere which is in a status epilepticus as clinically reflected by clonic status of the left hand and PLEDs in the EEG is still capable to produce a benign variant pattern like 14 &; 6 Hz positive spikes.SignificanceThe generator of 14 &; 6 Hz positive spikes may still persist despite the presence of severe structural and epileptogenic lesions in the same hemisphere.     

3‐Hz subthreshold oscillations of CA2 neurons In vivo

The CA2 region is unique in the hippocampus; it receives direct synaptic innervations from several hypothalamic nuclei and expresses various receptors of neuromodulators, including adenosine, vasopressin, and oxytocin. Furthermore, the CA2 region may have distinct brain functions, such as the control of instinctive and social behaviors; however, little is known about the dynamics of the subthreshold membrane potentials of CA2 neurons in vivo. We conducted whole‐cell current‐clamp recordings from CA2 pyramidal cells in urethane‐anesthetized mice and monitored the intrinsic fluctuations in their membrane potentials. The CA2 pyramidal cells emitted spontaneous action potentials at mean firing rates of ～0.8 Hz. In approximately half of the neurons, the subthreshold membrane potential oscillated at ～3 Hz. In two neurons, we obtained simultaneous recordings of local field potentials from the CA1 stratum radiatum and demonstrated that the 3‐Hz oscillations of CA2 neurons were not correlated with CA1 field potentials. In tetrodotoxin‐perfused acute hippocampal slices, the membrane potentials of CA2 pyramidal cells were not preferentially entrained to 3‐Hz sinusoidal current inputs, which suggest that intracellular 3‐Hz oscillations reflect the neuronal dynamics of the surrounding networks. © 2016 Wiley Periodicals, Inc.     

Areas V1 and V2 show microsaccade‐related 3–4‐Hz covariation in gamma power and frequency

Neuronal gamma‐band synchronization (25–80 Hz) in visual cortex appears sustained and stable during prolonged visual stimulation when investigated with conventional averages across trials. However, recent studies in macaque visual cortex have used single‐trial analyses to show that both power and frequency of gamma oscillations exhibit substantial moment‐by‐moment variation. This has raised the question of whether these apparently random variations might limit the functional role of gamma‐band synchronization for neural processing. Here, we studied the moment‐by‐moment variation in gamma oscillation power and frequency, as well as inter‐areal gamma synchronization, by simultaneously recording local field potentials in V1 and V2 of two macaque monkeys. We additionally analyzed electrocorticographic V1 data from a third monkey. Our analyses confirm that gamma‐band synchronization is not stationary and sustained but undergoes moment‐by‐moment variations in power and frequency. However, those variations are neither random and nor a possible obstacle to neural communication. Instead, the gamma power and frequency variations are highly structured, shared between areas and shaped by a microsaccade‐related 3–4‐Hz theta rhythm. Our findings provide experimental support for the suggestion that cross‐frequency coupling might structure and facilitate the information flow between brain regions.     

Coherent neocortical 40‐Hz oscillations are not present during REM sleep

During cognitive processes there are extensive interactions between various regions of the cerebral cortex. Oscillations in the gamma frequency band (≈40 Hz) of the electroencephalogram (EEG) are involved in the binding of spatially separated but temporally correlated neural events, which results in a unified perceptual experience. The extent of these interactions can be examined by means of a mathematical algorithm called ‘coherence’, which reflects the ‘strength’ of functional interactions between cortical areas. The present study was conducted to analyse EEG coherence in the gamma frequency band of the cat during alert wakefulness (AW), quiet wakefulness (QW), non‐rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Cats were implanted with electrodes in the frontal, parietal and occipital cortices to monitor EEG activity. Coherence values within the gamma frequency (30–100 Hz) from pairs of EEG recordings were analysed. A large increase in coherence occurred between all cortical regions in the 30–45 Hz frequency band during AW compared with the other behavioral states. As the animal transitioned from AW to QW and from QW to NREM sleep, coherence decreased to a moderate level. Remarkably, there was practically no EEG coherence in the entire gamma band spectrum (30–100 Hz) during REM sleep. We conclude that functional interactions between cortical areas are radically different during sleep compared with wakefulness. The virtual absence of gamma frequency coherence during REM sleep may underlie the unique cognitive processing that occurs during dreams, which is principally a REM sleep‐related phenomenon.     

Evidence for genetic regulation of the human parieto‐occipital 10‐Hz rhythmic activity

Several functional and morphological brain measures are partly under genetic control. The identification of direct links between neuroimaging signals and corresponding genetic factors can reveal cellular‐level mechanisms behind the measured macroscopic signals and contribute to the use of imaging signals as probes of genetic function. To uncover possible genetic determinants of the most prominent brain signal oscillation, the parieto‐occipital 10‐Hz alpha rhythm, we measured spontaneous brain activity with magnetoencephalography in 210 healthy siblings while the subjects were resting, with eyes closed and open. The reactivity of the alpha rhythm was quantified from the difference spectra between the two conditions. We focused on three measures: peak frequency, peak amplitude and the width of the main spectral peak. In accordance with earlier electroencephalography studies, spectral peak amplitude was highly heritable (h² > 0.75). Variance component‐based analysis of 28 000 single‐nucleotide polymorphism markers revealed linkage for both the width and the amplitude of the spectral peak. The strongest linkage was detected for the width of the spectral peak over the left parieto‐occipital cortex on chromosome 10 (LOD = 2.814, nominal P < 0.03). This genomic region contains several functionally plausible genes, including GRID1 and ATAD1 that regulate glutamate receptor channels mediating synaptic transmission, NRG3 with functions in brain development and HRT7 involved in the serotonergic system and circadian rhythm. Our data suggest that the alpha oscillation is in part genetically regulated, and that it may be possible to identify its regulators by genetic analyses on a realistically modest number of samples.     

Task‐sensitive reconfiguration of corticocortical 6–20 Hz oscillatory coherence in naturalistic human performance

Electrophysiological oscillatory coherence between brain regions has been proposed to facilitate functional long‐range connectivity within neurocognitive networks. This notion is supported by intracortical recordings of coherence in singled‐out corticocortical connections in the primate cortex. However, the manner in which this operational principle manifests in the task‐sensitive connectivity that supports human naturalistic performance remains undercharacterized. Here, we demonstrate task‐sensitive reconfiguration of global patterns of coherent connectivity in association with a set of easier and more demanding naturalistic tasks, ranging from picture comparison to speech comprehension and object manipulation. Based on whole‐cortex neuromagnetic recording in healthy behaving individuals, the task‐sensitive component of long‐range corticocortical coherence was mapped at spectrally narrow‐band oscillatory frequencies between 6 and 20 Hz (theta to alpha and low‐beta bands). This data‐driven cortical mapping unveiled markedly distinct and topologically task‐relevant spatiospectral connectivity patterns for the different tasks. The results demonstrate semistable oscillatory states relevant for neurocognitive processing. The present findings decisively link human behavior to corticocortical coherence at oscillatory frequencies that are widely thought to convey long‐range, feedback‐type neural interaction in cortical functional networks. Hum Brain Mapp 36:2455–2469, 2015. © 2015 The Authors Human Brain Mapping Published by Wiley Periodicals, Inc. .