Spontaneous and evoked activities of anterior thalamic neurons during waking and sleep states

Spectral organization of focal seizures within the thalamotemporal network

Objective

To investigate dynamic changes in neural activity between the anterior nucleus of the thalamus (ANT) and the seizure onset zone (SOZ) in patients with drug‐resistant temporal lobe epilepsy (TLE) based on anatomic location, seizure subtype, and state of vigilance (SOV).

Methods

Eleven patients undergoing stereoelectroencephalography for seizure localization were recruited prospectively for local field potential (LFP) recording directly from the ANT. The SOZ was identified using line length and epileptogenicity index. Changes in power spectral density (PSD) were compared between the two anatomic sites as seizures (N = 53) transitioned from interictal baseline to the posttermination stage.

Results

At baseline, the thalamic LFPs were significantly lower and distinct from the SOZ with the presence of higher power in the fast ripple band (P < 0.001). Temporal changes in ictal power of neural activity within ANT mimic those of the SOZ, are increased significantly at seizure onset (P < 0.05), and are distinct for seizures that impaired awareness or that secondarily generalized (P < 0.05). The onset of seizure was preceded by a decrease in the mean power spectral density (PSD) in ANT and SOZ (P < 0.05). Neural activity correlated with different states of vigilance at seizure onset within the ANT but not in the SOZ (P = 0.005).

Interpretation

The ANT can be recruited at the onset of mesial temporal lobe seizures, and the recruitment pattern differs with seizure subtypes. Furthermore, changes in neural dynamics precede seizure onset and are widespread to involve temporo‐thalamic regions, thereby providing an opportunity to intervene early with closed‐loop DBS.

Single-cell persistent activity in anterodorsal thalamus

The anterodorsal nucleus of the thalamus contains a high percentage of head-direction cells whose activities are correlated with an animal's directional heading in the horizontal plane. The firing of head-direction cells could involve self-sustaining reverberating activity in a recurrent network, but the thalamus by itself lacks strong excitatory recurrent synaptic connections to sustain tonic reverberating activity. Here we examined whether a single thalamic neuron could sustain its own activity without synaptic input by recording from individual neurons from anterodorsal thalamus in brain slices with synaptic blockers. We found that the rebound firing induced by hyperpolarizing pulses often decayed slowly so that a thalamic neuron could keep on firing for many minutes after stimulation. The hyperpolarization-induced persistent firing rate was graded under repeated current injections, and could be enhanced by serotonin. The effect of depolarizing pulses was much weaker and only slightly accelerated the decay of the hyperpolarization-induced persistent firing. Our finding provides the first direct evidence for single-cell persistent activity in the thalamus, supporting the notion that cellular mechanisms at the slow time scale of minutes might potentially contribute to the operations of the head-direction system.

Fos-like immunoreactivity in the mamillary body and thalamus following injections of muscimol into the ventral tegmental nucleus of Gudden in the rat

In many neurons, increased rates of firing are accompanied by expression of the proto-oncoprotein Fos. The current study examined Fos-like immunoreactivity in the mamillary body and the anterior thalamus following unilateral injections of the inhibitory GABA-A agonist muscimol into the ventral tegmental nucleus of Gudden (VTN). These injections resulted in a marked increase in Fos-like immunoreactivity ipsilaterally in both the medial mamillary nucleus and in its principle thalamic projection targets, the anteroventral and anteromedial thalamic nuclei. Since the projection from the VTN to the mamillary body has been shown to contain a substantial GABAergic component, these results are likely to reflect a disinhibition of mamillothalamic circuitry resulting from suppression of tonic inhibitory inputs arising in the VTN.

Conscious and pre-conscious processes as seen from the standpoint of sleep-waking cycle neurophysiology

The literature on state-dependent fluctuations in thalamocortical activities indicates that in electrophysiological terms, waking and paradoxical sleep are fundamentally identical states, with the provision that the handling of sensory information is altered in REM sleep. The central paradox of REM sleep, namely the apparent lack of cognitive responsiveness to sensory stimulation in spite of increased thalamocortical responsiveness to sensory stimuli, will lead us to hypothesize that the processing of sensory inputs in REM sleep is similar to that underlying preconscious processing of sensory inputs in the waking state. This will lead to a general discussion of the role of fast (approximately equal to 40 Hz) thalamocortical oscillations and temporal binding in sensory processing and conscious experience.

Projections from the anterodorsal and anteroventral nucleus of the thalamus to the limbic cortex in the rat

The present study characterized the projections of the anterodorsal (AD) and the anteroventral (AV) thalamic nuclei to the limbic cortex. Both AD and AV project to the full extent of the retrosplenial granular cortex in a topographic pattern. Neurons in caudal parts of both nuclei project to rostral retrosplenial cortex, and neurons in rostral parts of both nuclei project to caudal retrosplenial cortex. Within AV, the magnocellular neurons project primarily to the retrosplenial granular a cortex, whereas the parvicellular neurons project mainly to the retrosplenial granular b cortex. AD projections to retrosplenial cortex terminate in very different patterns than do AV projections: The AD projection terminates with equal density in layers I, III, and IV of the retrosplenial granular cortex, whereas, in contrast, the AV projections terminate very densely in layer Ia and less densely in layer IV. Further, both AD and AV project densely to the postsubicular, presubicular, and parasubicular cortices and lightly to the entorhinal (only the most caudal part) cortex and to the subiculum proper (only the most septal part). Rostral parts of AD project equally to all three subicular cortices, whereas neurons in caudal AD project primarily to the postsubicular cortex. Compared to AD, neurons in AV have a less extensive projection to the subicular cortex, and this projection terminates primarily in the postsubicular and presubicular cortices. Further, the AD projection terminates in layers I, II/III, and V of postsubiculum, whereas the AV projection terminates only in layers I and V.

Sleep-wake cycle abnormalities in fatal familial insomnia. Evidence of the role of the thalamus in sleep regulation

Alterations in sleep organization were longitudinally studied in 6 new cases of fatal familial insomnia (FFI) by 24 h polygraphic recording. All patients showed an early reduction in sleep spindles and K complexes, and a drastic reduction in total sleep time and disruption of the cyclic sleep organization. Complete abolition of NREM sleep and persistence of only brief residual periods of REM sleep without atonia were features characteristic of the 3 patients with a short (less than 1 year) clinical course, and lacking in the 3 cases with a longer (more than 2 years) disease course. In the latter, sudden transitions from waking to NREM or REM sleep occurred, sometimes recurring periodically. Our findings confirm that impairment of sleep-wake regulation is a consistent distinctive feature of FFI.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Of dreaming and wakefulness

Following a set of studies concerning the intrinsic electrophysiology of mammalian central neurons in relation to global brain function, we reach the following conclusions: (i) the main difference between wakefulness and paradoxical sleep lies in the weight given to sensory afferents in cognitive images; (ii) otherwise, wakefulness and paradoxical sleep are fundamentally equivalent brain states probably subserved by an intrinsic thalamo-cortical loop. From this assumption, we conclude that wakefulness is an intrinsic functional realm, modulated by sensory parameters. In support of this hypothesis, we review morphological studies of the thalamocortical system, which indicate that only a minor part of its connectivity is devoted to the transfer of direct sensory input. Rather, most of the connectivity is geared to the generation of internal functional modes, which may, in principle, operate in the presence or absence of sensory activation. These considerations lead us to challenge the traditional Jamesian view of brain function according to which consciousness is generated as an exclusive by-product of sensory input. Instead, we argue that consciousness is fundamentally a closed-loop property, in which the ability of cells to be intrinsically active plays a central role. We further discuss the importance of spatial and temporal mapping in the elaboration of cognitive and perceptual constructs.