The thalamic clock: emergent network properties

Environmental factors during postnatal period modify activity and non-selective attention in the Naples High-Excitability rat

The involvement of epigenetic factors in the phenotypic expression of the neural systems underlying activity and attentive processes has been investigated in an animal model of hyperactivity and attention-deficit, the Naples High-Excitability rat (NHE). To this aim, male NHE pups have been reared in small (four) or normal litter size (nine) during the first 4 weeks of postnatal life. Both groups underwent a differential handling procedure occurring once, twice or four times a week. After weaning (28 days), rats were housed in groups of two and tested as young adults for activity and non-selective attention in a spatial novelty situation for three consecutive tests at 24-h intervals. The behaviour was videotaped and analysed off line for the frequency of corner crossings and rearings and duration of rearings. The results indicate that the increased maternal care and high fat diet induced by the small litter size produced long lasting effects on activity and duration of rearing episodes that indexes non-selective attention. These effects were complex as differential handling was beneficial only at low stimulation level. Thus, these findings suggest that epigenetic factors acting during critical periods of post-natal development may interact with genetic determinants that in turn influence the maturation of the neural systems controlling activity, orienting and scanning time.

Cortical focus drives widespread corticothalamic networks during spontaneous absence seizures in rats

Absence seizures are the most pure form of generalized epilepsy. They are characterized in the electroencephalogram by widespread bilaterally synchronous spike-wave discharges (SWDs), which are the reflections of highly synchronized oscillations in thalamocortical networks. To reveal network mechanisms responsible for the initiation and generalization of the discharges, we studied the interrelationships between multisite cortical and thalamic field potentials recorded during spontaneous SWDs in the freely moving WAG/Rij rat, a genetic model of absence epilepsy. Nonlinear association analysis revealed a consistent cortical "focus" within the peri-oral region of the somatosensory cortex. The SWDs recorded at other cortical sites consistently lagged this focal site, with time delays that increased with electrode distance (corresponding to a mean propagation velocity of 1.4 m/sec). Intra-thalamic relationships were more complex and could not account for the observed cortical propagation pattern. Cortical and thalamic sites interacted bi-directionally, whereas the direction of this coupling could vary throughout one seizure. However, during the first 500 msec, the cortical focus was consistently found to lead the thalamus. These findings argue against the existence of one common subcortical pacemaker for the generation of generalized spike-wave discharges characteristic for absence seizures in the rat. Instead, the results suggest that a cortical focus is the dominant factor in initiating the paroxysmal oscillation within the corticothalamic loops, and that the large-scale synchronization is mediated by ways of an extremely fast intracortical spread of seizure activity. Analogous mechanisms may underlie the pathophysiology of human absence epilepsy.

Eeg alpha rhythm and glucose metabolic rate in the thalamus in schizophrenia

Positron emission tomography with uptake of [(18)F]fluorodeoxyglucose (FDG) and quantitative EEG were simultaneously performed in 18 medication-free patients with schizophrenia and in 13 normal volunteers. Subjects performed the Continuous Performance Task (CPT) during FDG uptake. Correlations were calculated between alpha power during the CPT and glucose metabolic rate (GMR) in thalamic regions and between alpha power during the CPT and GMR in occipital cortices. Regression analyses were used to describe the prediction of GMR in the occipital cortices and in the thalamic regions of occipital alpha power. In normal controls, we found (1) significant negative correlations between absolute alpha power and GMR in the left occipital cortex, (2) significant positive correlations between normalized alpha power and GMR in the right and left lateral thalamus and (3) combined effects of GMR in the thalamic regions and the occipital cortices on alpha power, which accounted for 98% of the variance of alpha power. In patients with schizophrenia, we found no significant correlations between alpha power and GMR in the occipital cortices or between alpha power and GMR in the thalamic regions. Correlation coefficients between absolute alpha power and GMR in the left occipital cortex and between normalized alpha power and GMR in the left lateral thalamus were significantly different in normal subjects compared to schizophrenic patients. The present findings provide evidence for involvement of the thalamus in the generation of alpha rhythm in humans. Furthermore, the present results suggest differences in thalamocortical circuits between normal controls and schizophrenic subjects.

Modal analysis of corticothalamic dynamics, electroencephalographic spectra, and evoked potentials

The effects of cortical boundary conditions and resulting modal aspects of continuum corticothalamic electrodynamics are explored, including feedbacks. Dispersion relations, electroencephalographic spectra, and stimulus response functions are calculated from the underlying physiology, and the effects of discrete mode structure are determined. Conditions under which modal effects are important are obtained, along with estimates of the point at which modal series can be truncated, and the limit in which only a single globally uniform mode need be retained. It is found that for physiologically plausible parameters only the lowest cortical spatial eigenmode together with the set of next-lowest modes can produce distinct modal structure in spectra and response functions, and then only at frequencies where corticothalamic resonances reduce dissipation to the point where the spatial eigenmodes are weakly damped. The continuum limit is found to be a good approximation, except at very low frequencies and, under some circumstances, near the alpha resonance. It is argued that the major electroencephalographic rhythms result from corticothalamic feedback resonances, but that cortical modal effects can contribute to weak substructure in the alpha resonance. This mechanism is compared and contrasted with purely cortical and pacemaker-based alternatives and testable predictions are formulated to enable experimental discrimination between these possibilities.

Model for olfactory discrimination and learning in Limax procerebrum incorporating oscillatory dynamics and wave propagation

We extend our model of the procerebral (PC) lobe of Limax, which is comprised of a layer of coupled oscillators and a layer of memory neurons, each layer 4 rows by 20 columns, corresponding to the cell body layer (burster cells) and neuropil layer (nonburster cells) of the PC lobe. A gradient of connections in the layer of model burster cells induces periodic wave propagation, as measured in the PC lobe. We study odor representations in the biological PC lobe using the technique of Kimura and coworkers. Lucifer yellow injection into intact Limax after appetitive or aversive odor learning results in a band or patch of labeled cells in the PC lobe with the band long axis normal to the axis of wave propagation. Learning two odors yields two parallel bands of labeled PC cells. We introduce olfactory input to our model PC lobe such that each odor maximally activates a unique row of four cells which produces a short-term memory trace of odor stimulation. A winner-take-all synaptic competition enabled by collapse of the phase gradient during odor presentation produces a single short-term memory band for each odor. The short-term memory is converted to long-term memory if odor stimulation is followed by activation of an input pathway for the unconditioned stimulus (US) which presumably results in release of one or more neuromodulatory amines or peptides in the PC lobe.

Prediction of electroencephalographic spectra from neurophysiology

A recent neurophysical model of propagation of electrical waves in the cortex is extended to include a physiologically motivated subcortical feedback loop via the thalamus. The electroencephalographic spectrum when the system is driven by white noise is then calculated analytically in terms of physiological parameters, including the effects of filtering of signals by the cerebrospinal fluid, skull, and scalp. The spectral power at low frequencies is found to vary as f(-1) when awake and f(-3) when asleep, with a breakpoint to a steeper power-law tail at frequencies above about 20 Hz in both cases; the f(-1) range concurs with recent magnetoencephalographic observations of such a regime. Parameter sensitivities are explored, enabling a model with fewer free parameters to be proposed, and showing that spectra predicted for physiologically reasonable parameter values strongly resemble those observed in the laboratory. Alpha and beta peaks seen near 10 Hz and twice that frequency, respectively, in the relaxed wakeful state are generated via subcortical feedback in this model, thereby leading to predictions of their frequencies in terms of physiological parameters, and of correlations in their occurrence. Subcortical feedback is also predicted to be responsible for production of anticorrelated peaks in deep sleep states that correspond to the occurrence of theta rhythm at around half the alpha frequency and sleep spindles at 3/2 times the alpha frequency. An additional positively correlated waking peak near three times the alpha frequency is also predicted and tentatively observed, as are two new types of sleep spindle near 5/2 and 7/2 times the alpha frequency, and anticorrelated with alpha. These results provide a theoretical basis for the conventional division of EEG spectra into frequency bands, but imply that the exact bounds of these bands depend on the individual. Three types of potential instability are found: one at zero frequency, another in the theta band at around half the alpha frequency, and a third at the alpha frequency itself.

Stimulus-based state control in the thalamocortical system

Neural systems operate in various dynamic states that determine how they process information (Livingstone and Hubel, 1981; Funke and Eysel, 1992; Morrow and Casey, 1992; Abeles et al., 1995; Guido et al., 1995; Mukherjee and Kaplan, 1995; Kenmochi and Eggermont, 1997; Wörgötter et al., 1998; Kisley and Gerstein, 1999). To investigate the function of a brain area, it is therefore crucial to determine the state of that system. One grave difficulty is that even under well controlled conditions, the thalamocortical network may undergo random dynamic state fluctuations which alter the most basic spatial and temporal response properties of the neurons. These uncontrolled state changes hinder the evaluation of state-specific properties of neural processing and, consequently, the interpretation of thalamocortical function. Simultaneous extracellular recordings were made in the auditory thalamus and cortex of the ketamine-anesthetized cat under several stimulus conditions. By considering the cellular and network mechanisms that govern state changes, we develop a complex stimulus that controls the dynamic state of the thalamocortical network. Traditional auditory stimuli have ambivalent effects on thalamocortical state, sometimes eliciting an oscillatory state prevalent in sleeping animals and other times suppressing it. By contrast, our complex stimulus clamps the network in a dynamic state resembling that observed in the alert animal. It thus allows evaluation of neural information processing not confounded by uncontrolled variations. Stimulus-based state control illustrates a general and direct mechanism whereby the functional modes of the brain are influenced by structural features of the external world.

Increased synchrony with increase of a low-threshold calcium conductance in a model thalamic network: a phase-shift mechanism

A computer model of a thalamic network was used in order to examine the effects of an isolated augmentation in a low-threshold calcium current. Such an isolated augmentation has been observed in the reticular thalamic (RE) nucleus of the genetic absence epilepsy rat from the Strasbourg (GAERS) model of absence epilepsy. An augmentation of the low-threshold calcium conductance in the RE neurons (gTs) of the model thalamic network was found to lead to an increase in the synchronized firing of the network. This supports the hypothesis that the isolated increase in gTs may be responsible for epileptic activity in the GAERS rat. The increase of gTs in the RE neurons led to a slight increase in the period of the isolated RE neuron firing. In contrast, the low-threshold spike of the RE neuron remained relatively unchanged by the increase of gTs. This suggests that the enhanced synchrony in the network was primarily due to a phase shift in the firing of the RE neurons with respect to the thalamocortical neurons. The ability of this phase-shift mechanism to lead to changes in synchrony was further examined using the model thalamic network. A similar increase in the period of RE neuron oscillations was obtained through an increase in the conductance of the calcium-mediated potassium channel. This change was once again found to increase synchronous firing in the network.

Relationship of activity in the subthalamic nucleus-globus pallidus network to cortical electroencephalogram

One of the functions of the excitatory subthalamic nucleus (STN) is to relay cortical activity to other basal ganglia structures. The response of the STN to cortical input is shaped by inhibition from the reciprocally connected globus pallidus (GP). To examine the activity in the STN-GP network in relation to cortical activity, we recorded single and multiple unit activity in STN and/or GP together with cortical electroencephalogram in anesthetized rats during various states of cortical activation. During cortical slow-wave activity (SWA), STN and GP neurons fired bursts of action potentials at frequencies that were similar to those of coincident slow ( approximately 1 Hz) and spindle (7-14 Hz) cortical oscillations. Spontaneous or sensory-driven global activation was associated with a reduction of SWA and a shift in STN-GP activity from burst- to tonic- or irregular-firing. Rhythmic activity in STN and GP neurons was lost when the cortex was inactivated by spreading depression and did not resume until SWA had recovered. Although rhythmic STN-GP activity was correlated with SWA, the phase relationships of activities of neurons within the STN and GP and between the nuclei were variable. Even when neurons displayed synchronous bursting activity, correlations on the millisecond time scale, which might indicate shared synaptic input, were not observed. These data indicate that (1) STN and GP activity is intimately related to cortical activity and hence the sleep-wake cycle; (2) rhythmic oscillatory activity in the STN-GP network in disease states may be driven by the cortex; and (3) activity of the STN-GP network is regulated in space in a complex manner.

Non-selective attention in a rat model of hyperactivity and attention deficit: subchronic methylphenydate and nitric oxide synthesis inhibitor treatment

The involvement of dopamine (DA) and nitric oxide (NO) in the process of non-selective attention (NSA) to environmental stimuli has been investigated in the juvenile Spontaneously Hypertensive rat (SHR). To this aim the frequency and duration of rearing episodes in a novelty situation, which is thought to monitor NSA, have been measured in male SHR and Wistar-Kyoto (WKY) control rats following subchronic treatment with methylphenidate (MP; 3 mg/kg) or the nitric oxide synthase (NOS) inhibitor L-Nitro-arginine-methylester (L-NAME; 1 mg/kg) or vehicle daily for two weeks. Different groups were tested at 0.5 h or 24 h after the last injection in a Làt-maze. Tests were repeated twice at a 24 h interval and lasted 10 min each. Upon first exposure, there was a differential drug effect only in the SHR. In fact, MP and L-NAME yielded a shift to the left and to the right, i.e. towards episodes of lower or higher duration, respectively. This shift was more pronounced in the group tested 0.5 h after the last injection. In contrast, both drugs produced a significant lengthening of the rearing episodes in the SHR only in comparison with the vehicle-treated rats over days of testing. Therefore both MP and L-NAME appear to shear a similar effect on non-selective attention, although the effect of L-NAME is somewhat paradoxical. The latter is likely to be due to increased arginine selective uptake due to negative feedback with the NO production. The consequent increased arginine availability displaces the NOS inhibitor, thus leading to increased NO production. In conclusion, dopamine and nitric oxide play a role in non-selective attention by synaptic and extrasynaptic mechanisms, respectively, in a rat model of hyperactivity and attention-deficits.

Trial-to-trial variability and state-dependent modulation of auditory-evoked responses in cortex

Recent experimental work has provided evidence that trial-to-trial variability of sensory-evoked responses in cortex can be explained as a linear superposition of random ongoing background activity and a stationary response. While studying single trial variability and state-dependent modulation of evoked responses in auditory cortex of ketamine/xylazine-anesthetized rats, we have observed an apparent violation of this model. Local field potential and unit spike trains were recorded and analyzed during different anesthesia depths-deep, medium, and light-which were defined by the pattern of ongoing cortical activity. Estimation of single trial evoked response was achieved by considering whole waveforms, rather than just one or two peak values from each wave. Principal components analysis was used to quantitatively classify waveforms on the basis of their time courses (i.e., shapes). We found that not only average response but also response variability is modulated by depth of anesthesia. Trial-to-trial variability is highest under medium levels of anesthesia, during which ongoing cortical activity exhibits rhythmic population bursting activity. By triggering the occurrence of stimuli from the spontaneously occurring burst events, we show that the observed variability can be accounted for by the background activity. In particular, the ongoing activity was found to modulate both amplitude and shape (including latency) of evoked local field potentials and evoked unit activity in a manner not predicted by linear superposition of background activity and a stereotyped evoked response. This breakdown of the linear model is likely attributable to rapid transitions between different levels of thalamocortical excitability (e.g., spike-wave discharges), although brain "state" is relatively fixed.

Oscillatory activity in the cerebellar hemispheres of unrestrained rats

We recorded multiunit neural activity in the granule cell layer of cerebellar folium Crus IIa in unrestrained rats. Seven- to 8-Hz oscillatory activity was seen during behavioral states in which the animal was immobile; any movement the animal made coincided with termination of the oscillations. However, nearly one-third of oscillatory episodes appeared to cease spontaneously, in the absence of any observable sensory input or movement. Oscillations were synchronized both within and between cerebellar hemispheres, demonstrating precise temporal coordination among multiple, bilateral levels of the somatosensory system. We interpret these data in the context of similar oscillations observed in other brain structures and suggest that the oscillations are an underlying dynamic property of the entire somatosensory network.

Spike-wave complexes and fast components of cortically generated seizures. I. Role of neocortex and thalamus

We explored the relative contributions of cortical and thalamic neuronal networks in the generation of electrical seizures that include spike-wave (SW) and polyspike-wave (PSW) complexes. Seizures were induced by systemic or local cortical injections of bicuculline, a gamma-aminobutyric acid-A (GABAA) antagonist, in cats under barbiturate anesthesia. Field potentials and extracellular neuronal discharges were recorded through arrays of eight tungsten electrodes (0.4 or 1 mm apart) placed over the cortical suprasylvian gyrus and within the thalamus. 1) Systemic injections of bicuculline induced SW/PSW seizures in cortex, whereas spindle sequences continued to be present in the thalamus. 2) Cortical suprasylvian injection of bicuculline induced focal paroxysmal single spikes that developed into full-blown seizures throughout the suprasylvian cortex. The seizures were characterized by highly synchronized SW or PSW complexes at 2-4 Hz, interspersed with runs of fast (10-15 Hz) activity. The intracellular aspects of this complex pattern in different types of neocortical neurons are described in the following paper. Complete decortication abolished the seizure, leaving intact thalamic spindles. Injections of bicuculline in the cortex of athalamic cats resulted in similar components as those occurring with an intact thalamus. 3) Injection of bicuculline in the thalamus decreased the frequency of barbiturate spindles and increased the synchrony of spike bursts fired by thalamocortical and thalamic reticular cells but did not induce seizures. Decortication did not modify the effects of bicuculline injection in the thalamus. Our results indicate that the minimal substrate that is necessary for the production of seizures consisting of SW/PSW complexes and runs of fast activity is the neocortex.

Periodicity of thalamic spindle waves is abolished by ZD7288,a blocker of Ih

The actions of the novel bradycardiac agent ZD7288 [4-(N-ethyl-N-phenylamino)-1, 2-dimethyl-6-(methylamino)pyrimidinium chloride] were investigated on the hyperpolarization-activated cation current Ih and on network activity in spontaneously spindling ferret lateral geniculate (LGNd) slices in vitro using intracellular recording techniques. In voltage-clamp recordings, local application of ZD7288 (1 mM in micropipette) resulted in a complete block of Ih, whereas in current-clamp recordings, application of this agent resulted in an abolition of the depolarizing sag activated by hyperpolarization and decreased the frequency of intrinsic delta-oscillations for which Ih acts as a pacemaker current. In addition, block of Ih with ZD7288 resulted in an abolition of the afterdepolarization (ADP) that follows repetitive hyperpolarization and rebound burst firing as well as that occurring in between spindle waves. The block of the ADP was associated with a block of the spindle wave refractory period such that continuous 6- to 10-Hz oscillations were generated throughout the network. These findings give further support to the hypothesis that Ih is critically involved in the generation of slow rhythmicity in synchronized thalamic activity.

A possible role for cholinergic neurons of the basal forebrain and pontomesencephalon in consciousness

Excitation at widely dispersed loci in the cerebral cortex may represent a neural correlate of consciousness. Accordingly, each unique combination of excited neurons would determine the content of a conscious moment. This conceptualization would be strengthened if we could identify what orchestrates the various combinations of excited neurons. In the present paper, cholinergic afferents to the cerebral cortex are hypothesized to enhance activity at specific cortical circuits and determine the content of a conscious moment by activating certain combinations of postsynaptic sites in select cortical modules. It is proposed that these selections are enabled by learning-related restructuring that simultaneously adjusts the cytoskeletal matrix at specific constellations of postsynaptic sites giving all a similar geometry. The underlying mechanism of conscious awareness hypothetically involves cholinergic mediation of linkages between microtubules and microtubule-associated protein-2 (MAP-2). The first reason for proposing this mechanism is that previous studies indicate cognitive-related changes in MAP-2 occur in cholinoceptive cells within discrete cortical modules. These cortical modules are found throughout the cerebral cortex, measure 1-2 mm2, and contain approximately 10(3)-10(4) cholinoceptive cells that are enriched with MAP-2. The subsectors of the hippocampus may function similarly to cortical modules. The second reason for proposing the current mechanism is that the MAP-2 rich cells throughout the cerebral cortex correspond almost exactly with the cortical cells containing muscarinic receptors. Many of these cholinoceptive, MAP-2 rich cells are large pyramidal cell types, but some are also small pyramidal cells and nonpyramidal types. The third reason for proposing the current mechanism is that cholinergic afferents are module-specific; cholinergic axons terminate wholly within individual cortical modules. The cholinergic afferents may be unique in this regard. Finally, the tapering apical dendrites of pyramidal cells are proposed as primary sites for cholinergic mediation of linkages between MAP-2 and microtubules because especially high amounts of MAP-2 are found here. Also, the possibility is raised that muscarinic actions on MAP-2 could modulate microtubular coherence and self-collapse, phenomena that have been suggested to underlie consciousness.

Functional dynamics of GABAergic inhibition in the thalamus

The inhibitory gamma-aminobutyric acid-containing (GABAergic) neurons of the thalamic reticular and perigeniculate nuclei are involved in the generation of normal and abnormal synchronized activity in thalamocortical networks. An important factor controlling the generation of activity in this system is the amplitude and duration of inhibitory postsynaptic potentials (IPSPs) in thalamocortical cells, which depend on the pattern of activity generated in thalamic reticular and perigeniculate cells. Activation of single ferret perigeniculate neurons generated three distinct patterns of GABAergic IPSPs in thalamocortical neurons of the dorsal lateral geniculate nucleus: Low-frequency tonic discharge resulted in small-amplitude IPSPs mediated by GABAA receptors, burst firing resulted in large-amplitude GABAA IPSPs, and prolonged burst firing activated IPSPs mediated by GABAA and GABAB receptors. These functional properties of GABAergic inhibition can reconfigure the operations of thalamocortical networks into patterns of activity associated with waking, slow-wave sleep, and generalized seizures.

Cellular-synaptic generation of sleep spindles, spike-and-wave discharges, and evoked thalamocortical responses in the neocortex of the rat

Thalamocortical neuronal oscillations underlie various field potentials that are expressed in the neocortex, including sleep spindles and high voltage spike-and-wave patterns (HVSs). The mechanism of extracellular current generation in the neocortex was studied in the anesthetized and awake rat. Field potentials and unit activity were recorded simultaneously along trajectories perpendicular to the cortical layers at spatial intervals of 100 microm by multiple-site recording silicon probes. Current source density (CSD) analysis revealed that the spatial positions of sinks in layers IV, V-VI, and II-III and of the accompanying sources were similar during sleep spindles, HVSs, and thalamic-evoked responses, although their relative strengths and timings differed. The magnitude and relative timing of the multiple pairs of sinks and sources determined the amplitude variability of HVSs and sleep spindles. The presence of temporally shifted dipoles was also supported by the time distribution of unit discharges in different layers. Putative interneurons discharged with repetitive bursts of 300-500 Hz. The spike component of HVSs was associated with fast field oscillations (400-600 Hz "ripples"). Discharges of pyramidal cells were phase-locked to the ripples. These findings indicate that the major extracellular currents underlying sleep spindles, HVSs, and evoked responses result from activation of intracortical circuitries. We hypothesize that the fast field ripples reflect summed IPSPs in pyramidal cells resulting from the high frequency barrage of interneurons.

Central versus peripheral determinants of patterned spike activity in rat vibrissa cortex during whisking

We report on the relationship between single-unit activity in primary somatosensory vibrissa cortex of rat and the rhythmic movement of vibrissae. Animals were trained to whisk freely in air in search of food. Electromyographic (EMG) recordings from the mystatial pads served as a reference for the position of the vibrissae. A fast, oscillatory component in single-unit spike trains is correlated with vibrissa position within the whisk cycle. The phase of the correlation for different units is broadly distributed. A second, slowly varying component of spike activity correlates with the amplitude of the whisk cycle. For some units, the phase and amplitude correlations were of sufficient strength to allow the position of the whiskers to be accurately predicted from a single spike train. To determine whether the observed patterned spike activity was driven by motion of the vibrissae, as opposed to central pathways, we reversibly blocked the contralateral facial motor nerve during the behavioral task so that the rat whisked only on the ipsilateral side. The ipsilateral EMG served as a reliable reference signal. The fast, oscillatory component of the spike-EMG correlation disappears when the facial motor nerve is blocked. This implies that the position of vibrissae within a cycle is encoded through direct sensory activation. The slowly varying component of the spike-EMG correlation is unaffected by the block. This implies that the amplitude of whisking is likely to be mediated by corollary discharge. Our results suggest that motor cortex does not relay a reference signal to sensory cortex for positional information of the vibrissae during whisking.

Nonconvulsive status epilepticus with generalized 'fast activity'

We report the case of a 55-year-old patient with infrequent generalized tonic-clonic seizures since the age of 40 years and additional frequent episodes of disturbed behaviour and impaired cognition. The latter last from several hours up to one day with sudden onset and end of electroencephalographic (EEG) changes and clinical manifestations. They occurred about once a week before they were successfully treated with valproate and lamotrigine. From clinical, therapeutic and EEG findings we conclude that the patient suffers from nonconvulsive status epileptici, although the ictal EEG showed an untypical pattern of monomorphic generalized alpha rhythm. Many EEG characteristics point towards a primary generalized seizure disorder but a focal origin with secondary bilateral synchrony has to be considered as well. However, this is obviously a rare type which has not been described previously.

Global and serial neurons form A hierarchically arranged interface proposed to underlie memory and cognition

It is hypothesized that the cholinergic and monoaminergic neurons of the brain from a global network. What is meant by a global network is that these neurons operate as a unified whole, generating widespread patterns of activity in concert with particular electroencephalographic states, moods and cognitive gestalts. Apart from cholinergic and monoaminergic global systems, most other mammalian neurons relay sensory information about the external and internal milieu to serially ordered loci. These "serial" neurons are neurochemically distinct from global neurons and commonly use small molecule amino acid neurotransmitters such as glutamate or aspartate. Viewing the circuitry of the mammalian brain within the global-serial dichotomy leads to a number of novel interpretations and predictions. Global systems seem to be capable of transforming incoming sensory data into cognitive-related activity patterns. A comparative examination of global and serial systems anatomy, development and physiology reveals how global systems might turn sensation into mentation. An important step in this process is the permanent encoding of memory. Global neurons are particularly plastic, as are the neurons receiving global inputs. Global afferents appear to be capable of reorganizing synapses on recipient serial cells, thus leading to enhanced responding to a signal, in a particular context and state of arousal.

Thalamic NMDA transmission in a genetic model of absence epilepsy in rats

In the selected strain of GAERS Wistar rats (Genétic Absence Epilepsy Rats from Strasbourg), all animals present spontaneously recurrent absence seizures characterized by bilateral and synchronous generalized spike-and-wave discharges (SWD) accompanied by behavioural arrest. SWD depend on a thalamo-cortical network connecting the reticular and relay nuclei of the thalamus and their cortical projection areas. This loop involves both GABAergic and glutamatergic synapses. In the present study, we investigated the implication of NMDA transmission in the genesis of absence seizures in GAERS. Intra-peritoneal or intra-cerebroventricular injections of NMDA, the competitive NMDA antagonist CGP 40116, the non-competitive NMDA antagonist (+)-MK 801 and the antagonist of the glycine modulatory site 5,7-dichlorokynurenic acid dose-dependently suppressed SWD. Bilateral infusions of the same drugs in the lateral relay nuclei of the thalamus had similar suppressive effects. Intra-cerebroventricular or intrathalamic administration of D-serine, an agonist of the glycine modulatory site, had no effect on SWD. These data show that NMDA neurotransmission, especially within the thalamus, plays a major role in the control of absence seizures in GAERs. Disregulation of NMDA-mediated transmission by NMDA or antagonists, interacting with various sites of the receptor complex, may suppress the thalamo-cortical oscillatory activity which underlies SWD.

Effects of 7-nitroindazole, NG-nitro-L-arginine, and D-CPPene on harmaline-induced postural tremor, N-methyl-D-aspartate-induced seizures, and lisuride-induced rotations in rats with nigral 6-hydroxydopamine lesions

The present behavioral study was undertaken to investigate whether neuronal nitric oxide (NO) synthase mediates the abnormal consequences of increased NMDA receptor-mediated synaptic transmission in models of postural tremor, Parkinson's disease and epilepsy. We used 7-nitroindazole, a selective inhibitor of neuronal NO synthase, and NG-nitro-L-arginine (L-NAME), an unspecific NO synthase inhibitor, and compared their action with that of the competitive NMDA receptor antagonist 3-[(R)-2-carboxypiperazin-4-yl]-prop-2-enyl-1-phosphonic acid (D-CPPene). In both mice and rats, 7-nitroindazole, L-NAME and D-CPPene dose dependently reversed the harmaline-induced increase of cerebellar cyclic guanosine-5'-monophosphate (cGMP) levels. For subsequent behavioral experiments we used doses of 7-nitroindazole, L-NAME and D-CPPene which were equipotent in preventing harmaline-induced cGMP increase. Harmaline-induced tremor in mice and rats was suppressed by D-CPPene, but not by 7-nitroindazole or by L-NAME. This effect of D-CPPene was not due to unspecific suppression of motor activity, since D-CPPene did not affect locomotor activity at doses which reduced tremor. D-CPPene, but not 7-nitroindazole and L-NAME potentiated the antiparkinsonian action of the dopamine agonist lisuride in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra. D-CPPene antagonized seizures induced by intracerebroventricular injection of NMDA in mice. In contrast, 7-nitroindazole and L-NAME had only a tendency to prevent seizures and to delay the latency to onset of seizures. We conclude from these results that neuronal NO synthase does not serve as a major mediator of increased NMDA receptor-mediated synaptic transmission in animal models of Parkinson's disease, postural tremor and epilepsy. The novel observation that D-CPPene suppresses harmaline-induced tremor leads us to suggest that NMDA receptor antagonists should be considered as novel therapeutics for postural tremor.

Central mechanisms of tremor

Physiologic and pathologic tremors are mechanistically classified into two broad groups: (1) those produced by oscillation in sensorimotor loops, so-called mechanical-reflex tremors, and (2) those produced by the oscillatory properties of central neuronal networks. This review provides a contemporary perspective of tremor pathophysiology while acknowledging that no form of tremor is understood completely. Indeed, the origin of oscillation in most forms of tremor is undefined, and in many instances the underlying pathology is unknown.

Lack of hippocampal involvement in a rat model of petit mal epilepsy

Although petit mal epilepsy is believed to arise from the thalamocortical system, it has been suggested recently that high-voltage spike-and-wave activity (HVS) in mutant mice can cause structural reorganization in the hippocampus, similar to that seen in temporal lobe epilepsy. We tested this assumption in rat strains with low and high incidence of HVS. No relationship was found between mossy fiber sprouting and HVS incidence. Hippocampal neurons were not driven by HVS. These findings suggest that petit mal seizures do not invade the hippocampus and are not causally related to sprouting of the mossy fibers.

The brain as a hermeneutic device

An attempt has been made to reconcile the 'device approach' and the 'philosophical approach' to the brain. Systems exhibiting high structural and dynamic complexity may be candidates of being hermeneutic devices. The human brain, which is a structurally and dynamically complex device, not only perceives but also creates new reality: it is a hermeneutic device.

Effects of acute and chronic cocaine administration on EEG and behaviour in intact and castrated male and intact and ovariectomized female rats

Intact and gonadectomized male and female WAG/ Rij rats were used to study the effects of gender and gonadal hormones on the development of sensitization and tolerance to cocaine-induced changes in EEG and behaviour. The four groups of WAG/Rij rats differed in the number of spontaneously occurring spike-wave discharges: ovariectomy decreased and castration increased the number of spike-wave discharges. This confirms that testosterone has antiabsence effects and that female gonadal hormones may promote the occurrence of spike-wave discharges. Cocaine [10 and 20 mg/kg, intraperitoneally (IP)] was administered before and after chronic cocaine administration (9 days, one daily injection with 10 mg/kg) and EEG and behaviour were monitored. Cocaine strongly suppressed the occurrence of spike-wave discharges before and after chronic administration in all four groups, although the decrease was less in the intact males. Sensitization or tolerance induced by cocaine on EEG could not be established. Acute cocaine administration eliminated explorative, automatic, and passive behaviour, whereas various stereotypical activities such as uncoordinated head and body movements and head swaying emerged. Differences between groups were observed as intact males were less likely than subjects in the three other groups to engage in intense stereotyped behaviour. These data suggest that testosterone inhibits EEG and behavioural effects of acute cocaine administration. All four groups displayed less head swaying and more uncoordinated head and body movements after chronic cocaine administration, suggesting that behavioural sensitization had occurred. Differences between the four groups had faded away. Although pharmacokinetic differences in levels of cocaine and benzoylecgonine between the four groups were found, they could not easily be related to the behavioural differences between groups.

Impairment of intracortical GABAergic inhibition in a rat model of absence epilepsy

The WAG/Rij rat strain is characterized in its EEG by the manifestation of spike-wave discharges which resemble in their spontaneous appearance and pharmacological sensitivity the absence epilepsy observed in humans. In order to test the hypothesis whether cellular intrinsic membrane and/or synaptic network properties in the neocortex are modified in this form of epilepsy, we analyzed with extra- and intracellular recording techniques the functional status of neocortical slices obtained from adult epileptic WAG/Rij rats and compared them with the data acquired from non-epileptic control Wistar rats. Intrinsic membrane properties, like resting membrane potential, neuronal input resistance and basic cellular firing characteristics, did not differ between these two strains. However, the analysis of extra- and intracellularly recorded synaptic responses revealed an intracortical hyperexcitability which was accompanied by a significant reduction in the efficiency of GABAergic inhibition. Our data indicate that the imbalance between intracortical excitatory and inhibitory mechanisms may at least contribute to the expression and augmentation of spike-wave discharges in epileptic WAG/Rij rats.

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.

Emergent spindle oscillations and intermittent burst firing in a thalamic model: specific neuronal mechanisms

The rhythmogenesis of 10-Hz sleep spindles is studied in a large-scale thalamic network model with two cell populations: the excitatory thalamocortical (TC) relay neurons and the inhibitory nucleus reticularis thalami (RE) neurons. Spindle-like bursting oscillations emerge naturally from reciprocal interactions between TC and RE neurons. We find that the network oscillations can be synchronized coherently, even though the RE-TC connections are random and sparse, and even though individual neurons fire rebound bursts intermittently in time. When the fast gamma-aminobutyrate type A synaptic inhibition is blocked, synchronous slow oscillations resembling absence seizures are observed. Near-maximal network synchrony is established with even modest convergence in the RE-to-TC projection (as few as 5-10 RE inputs per TC cell suffice). The hyperpolarization-activated cation current (Ih) is found to provide a cellular basis for the intermittency of rebound bursting that is commonly observed in TC neurons during spindles. Such synchronous oscillations with intermittency can be maintained only with a significant degree of convergence for the TC-to-RE projection.

Sensorimotor encoding by synchronous neural ensemble activity at multiple levels of the somatosensory system

Neural ensemble processing of sensorimotor information during behavior was investigated by simultaneously recording up to 48 single neurons at multiple relays of the rat trigeminal somatosensory system. Cortical, thalamic, and brainstem neurons exhibited widespread 7- to 12-hertz synchronous oscillations, which began during attentive immobility and reliably predicted the imminent onset of rhythmic whisker twitching. Each oscillatory cycle began as a traveling wave of neural activity in the cortex that then spread to the thalamus. Just before the onset of rhythmic whisker twitching, the oscillations spread to the spinal trigeminal brainstem complex. Thereafter, the oscillations at all levels were synchronous with whisker protraction. Neural structures manifesting these rhythms also exhibited distributed spatiotemporal patterns of neuronal ensemble activity in response to tactile stimulation. Thus, multilevel synchronous activity in this system may encode not only sensory information but also the onset and temporal domain of tactile exploratory movements.

Benzodiazepine-GABAA receptor binding during absence seizures

The role of benzodiazepine (BZD)-gamma-aminobutyric acidA (GABAA) receptors in the pathogenesis of absence seizures is uncertain. In this study, we examined the effect of absence seizures on the binding of flumazenil to the BZD binding site of the GABAA receptor. Five patients with idiopathic generalized epilepsy (IGE) were studied at rest and during absence seizures with [11C]flumazenil and positron emission tomography (PET). Normalized regional cerebral time-activity curves from the resting and ictal scans were compared with each other and with computed simulations showing the effects of changes in cerebral blood flow (CBF) and [11C]flumazenil binding. No evidence was found for a change in [11C]flumazenil binding with absence seizures. This result, together with those of a recent study showing no abnormality of [11C]flumazenil binding interictally in patients with childhood and juvenile absence epilepsy (JAE) does not support a primary role for the BZD binding site of the GABAA receptor in the pathogenesis of absence seizures.

Late onset absence seizures in multiple sclerosis: a case report

A 38-year-old woman was admitted to hospital because of generalized tonic-clonic attacks and late onset absence seizures. EEG and Video-EEG showed 3-4 hz generalized spike and wave discharges lasting 1-8 seconds, which were associated with impairment of consciousness and unresponsiveness. MR scan revealed multiple demyelinating lesions, including the most prominent one in the mesial frontal region that we suppose might be responsible for electroclinical absence seizures. After investigation the diagnosis of multiple sclerosis was made. Possible pathophysiological mechanisms and differential diagnosis of tonic-clonic and absence seizures in MS are discussed.

Synaptic and membrane mechanisms underlying synchronized oscillations in the ferret lateral geniculate nucleus in vitro

1. The cellular basis for generation of spindle waves and a slower synchronized oscillation resembling absence seizures was investigated with extracellular and intracellular recording techniques in slices of ferret dorsal lateral geniculate nucleus (LGNd) maintained in vitro. 2. Intracellular recording from LGNd relay cells in vitro revealed that spindle waves occurred once every 3-30 s and were associated with barrages of inhibitory postsynaptic potentials (IPSPs) occurring at a frequency of 6-10 Hz. These IPSPs resulted in the generation of rebound low threshold Ca2+ spikes at 2-4 Hz, owing to the intrinsic propensity of LGNd relay cells to generate oscillatory burst firing in this frequency range. These rebound bursts of action potentials were highly synchronized with local multiunit and single unit activity. 3. The spindle wave-associated IPSPs in LGNd relay cells exhibited a mean reversal potential of -86 mV. This reversal potential was shifted to more depolarized membrane potentials with the intracellular injection of Cl- through the use of KCl-filled microelectrodes. Simultaneous recording from the perigeniculate nucleus (PGN) and LGNd revealed the IPSPs to be synchronous with the occurrence of burst firing in the PGN. Excitation of PGN neurons with local electrical stimulation after pharmacological block of excitatory amino acid transmission resulted in bicuculline-sensitive IPSPs in relay neurons similar in amplitude and time course to those occurring during spindle waves. 4. Application of (-)-bicuculline methiodide resulted in the abolition of spindle wave-associated IPSPs or in the slowing of the rate of rise, an increase in amplitude and a prolongation of these IPSPs; this resulted in a synchronized 2-4 Hz oscillation, in which each relay cell strongly burst on nearly every cycle, thus forming a paroxysmal event. Bath application of the GABAB receptor antagonist 2-OH-saclofen blocked these slowed oscillations, indicating that they are mediated by the activation of GABAB receptors. In contrast, pharmacological antagonism of GABAB receptors did not block the generation of normal spindle waves. 5. These and other results indicate that spindle waves are generated in the ferret LGNd in vitro as a network phenomenon occurring through an interaction between the relay cells of the LGNd and the GABAergic neurons of the PGN. We propose that burst firing in PGN cells hyperpolarizes relay neurons through activation of GABAA receptors. These IPSPs result in rebound burst firing in LGNd cells, which then excite PGN neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Benzodiazepine-GABAA receptors in idiopathic generalized epilepsy measured with [11C]flumazenil and positron emission tomography

The neurochemical basis of absence seizures and the mechanism of their suppression by valproate (VPA) are uncertain. We used positron emission tomography (PET) to determine whether an abnormality of [11C]flumazenil binding to benzodiazepine (BZD)-GABAA receptors exists in patients with childhood and juvenile absence epilepsy and to examine the effects of VPA on [11C]flumazenil binding. The regional cerebral volume of distribution (Vd) of [11C]flumazenil in patients not treated with VPA was not different from that in normal controls; Vd was lower in patients treated with VPA, and the number of receptors available for binding was significantly reduced in such patients as compared with normal controls. There was no evidence of a primary abnormality of the BZD-GABAA receptor in childhood and juvenile absence epilepsy (CAE/JAE), but the data suggest that treatment with VPA is associated with a reduction in [11C]flumazenil binding that may be relevant to its mode of action in CAE/JAE.

Spike-and-wave epilepsy in rats: sex differences and inheritance of physiological traits

Spontaneously occurring spike-and-wave patterns were examined in seven to eight-month-old rats of the inbred Fischer 344 and Brown Norway strains and their F1 and F2 hybrids. Neocortical activity and movement were monitored for 12 night h. Spike-and-wave episodes were identified by a three-layer back-propagation neural network. The incidence, average duration and total duration of spike-and-wave episodes were significantly higher in F1 males and F2 hybrids than in the parental strains. Male rats of the Brown Norway strain had significantly more and longer episodes than females, whereas no sex differences were present in Fischer rats. The average intraepisodic frequency of spike-and-wave patterns was significantly lower in Fischer rats than in the other groups and significantly higher in males than females. Tremor (myoclonic movements) associated with spike-and-wave episodes was absent or of very small amplitude in Fischer rats but frequent and of large amplitude in Brown Norway rats and their F1 and F2 descendants. Most of the interstrain differences were limited to male rats. Spike-and-wave episodes recurred at predictable short-term (10-30 s) and long-term (15-30 min) periods. The long-term oscillation corresponded to a similar fluctuation of motor activity. The maximum probability of spike-and-wave patterns occurred at a relatively narrow range of delta power (0-3.1 Hz) of the background EEG activity. Systemic administration of the adrenergic alpha-2 agonist, clonidine, increased the incidence of spike-and-wave episodes several-fold. The total duration of spike-and-wave episodes in the clonidine sessions (15 min) and night sessions (12 h test) correlated significantly. We suggest that several genes interact with maturational, environmental and endocrine factors, resulting in sex differences, and produce the variety of EEG and behavioral findings encountered. In addition, we submit that the clonidine test may be useful in genetic investigations of human absence epilepsies. The findings of this work demonstrate that genetic manipulation of rodents is a promising method for producing analogous models for the various forms of human absence epilepsies.

Role of L-type calcium channel modulation in nonconvulsive epilepsy in rats

Old male Wistar rats spontaneously showing hundreds of spike-wave discharges daily were used to investigate the role of calcium ions in nonconvulsive epilepsy. The effects of the L-type calcium channel blocker nimodipine and the L-type channel opener BAY K 8644 on number and duration of these spike-wave discharges were investigated. In rats aged 84-94 weeks standard EEG electrodes were chronically implanted; animals were allowed to recover for 10 days. After a baseline registration, nimodipine 2.2, 8.8, and 35.2 mg/kg or BAY K 8644 in dosages of 0.12, 0.47, and 1.88 mg/kg was administered. A control group received the solvent. EEG recordings were made to evaluate drug effects. The highest dose of nimodipine increased the number of spike-wave discharges, whereas BAY K 8644 reduced the number of spike-wave discharges dose dependently. The highest dose of BAY K 8644 also induced fatal convulsions in 3 animals. Our results demonstrate that the L-type calcium antagonist nimodipine facilitates spike-wave discharges and that the L-type calcium agonist BAY K 8644 protects against these discharges, in contrast to previous results suggesting that calcium channel blockers act as antiepileptic drugs (AEDs) and that calcium channel openers act as convulsants. Our results are a further example of the different pharmacologic profile of convulsive and nonconvulsive epilepsy and are also in contrast to what has been described for T-type calcium channel modulation. We therefore propose that modulation of L-type and T-type calcium channels have opposite effects in nonconvulsive epilepsy.

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: in vivo electrophysiology

In vivo intracellular recordings were obtained from identified thalamocortical neurons in the ventroanterior-ventrolateral complex in urethane-anesthetized rats. This thalamic nucleus has few interneurons. Neurons that responded to cerebellar stimulation were injected intracellularly with horseradish peroxidase or biocytin and examined with light and electron microscopy (see companion paper). Intrinsic membrane properties and voltage-dependent rhythmic activity of cerebellar-responsive ventroanterior-ventrolateral neurons were similar to those described previously for thalamic neurons. Thus, in addition to conventional "fast" Na(+)-dependent spikes, rat ventroanterior-ventrolateral neurons had "slow" Ca(2+)-mediated low-threshold spikes and membrane conductances that supported rhythmic oscillations. Two modes of spontaneous activity were observed: (i) a tonic firing pattern that consisted of irregularly occurring fast spikes that predominated when the membrane potential was more positive than about -60 mV, and (ii) a rhythmic firing pattern, observed when the membrane potential was more negative than about -65 mV, composed of periodic (4-8 Hz) membrane hyperpolarizations and ramp depolarizations that often produced a low-threshold spike and a burst of fast spikes. In some neurons, spontaneous fast prepotentials were also observed, often with a relatively constant rate (up to 70 Hz). Cerebellar stimulation elicited excitatory postsynaptic potentials that in some cases appeared to be all-or-none and were similar in form to fast prepotentials. Stimulation of ipsilateral motor cortex elicited a short-latency antidromic response followed by a monosynaptic excitatory postsynaptic potential, which had a slower rise time than excitatory postsynaptic potentials evoked from cerebellum, suggesting that cortical inputs were electrotonically distal to cerebellar inputs. In the presence of moderate membrane hyperpolarization, the cortically evoked excitatory postsynaptic potential was followed by a long-lasting hyperpolarization (100-400 ms duration), a rebound depolarization and one or two cycles resembling spontaneous rhythmic activity. Membrane conductance was increased during the initial component of the long hyperpolarization, much of which was probably due to an inhibitory postsynaptic potential. In contrast, membrane conductance was unchanged or slightly decreased during the latter three-quarters of the long hyperpolarization. The amplitude of this component of the long hyperpolarization usually decreased when the membrane was hyperpolarized with intracellular current injection. Thus, both disfacilitation and an inhibitory postsynaptic potential may have contributed to the latter portion of the cortically-evoked long hyperpolarization. The cortically-evoked inhibitory postsynaptic potentials likely originated predominantly from feedforward activation of GABAergic neurons in the thalamic reticular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

A neurosynaptic model of state-dependent EEG wave generation in the subcortico-cortical system

A neurosynaptic model of the subcortico-cortical system is presented in order to analyze the mechanism for the generation of EEG rhythms with specific state-dependent spectral patterns. The model is based on the interaction among the infraslow, as well as basic, rhythms of the PSP's (postsynaptic potentials) trains from which CSD's (current source densities) or cortical surface potentials emerge. The model system was simulated by two trains of positive and negative cortical surface potentials within the same period, according to the thalamic clock as modulated by the infraslow rhythms of the midbrain reticular system. The simulated EEG's showed rhythmic waxing and waning sawtooth-like waves with no frequency fluctuation, but with some spectral broadband peaks at the basic repetitive frequency, as well as its harmonics.

Opiate receptors in idiopathic generalised epilepsy measured with [11C]diprenorphine and positron emission tomography

The neurochemical basis of absence seizures is uncertain. A previous PET study has provided evidence for release of endogenous opioids from cerebral cortex at the time of absence seizures, but it is has not yet been established whether there is an abnormality of opiate receptor numbers interictally. In the present study, the non-specific opiate receptor ligand, [11C]diprenorphine, was used to measure cerebral opiate receptors interictally in patients with childhood and juvenile absence epilepsy. Eight patients and eight normal controls had a single scan after a high specific activity injection of [11C]diprenorphine. The cerebral volume of distribution (Vd) of [11C]diprenorphine relative to plasma was calculated on a pixel-by-pixel basis. There were no significant differences in [11C]diprenorphine Vd between patients and control subjects in either cortex or thalamus, structures thought to be involved in the pathogenesis of absence seizures. The results suggest that there is no overall abnormality of opioid receptors in patients with childhood and juvenile absence epilepsy. Studies with specific ligands may provide information about the different receptor subtypes.

The effects of chronic treatment with a calcium channel antagonist on two types of generalized epilepsies in rats

Although calcium antagonists possess antiepileptic properties in various models of epilepsy, their role after chronic administration and in models for generalized absence epilepsy has not been studied. Twenty-four male Wistar rats, aged 84-94 weeks, were chronically provided with EEG electrodes. Two groups received dietary nimodipine (860 ppm) for 14 and 21 weeks, respectively, while a control group received the same rat chow without nimodipine. The EEG was recorded for 3 h to establish the effects of nimodipine on spike-wave discharges. Next, 50 mg/kg pentylenetetrazol (PTZ) was injected to establish the effects on convulsive epilepsy, and the EEG was recorded for 30 min. All animals had spontaneous spike-wave discharges (SWD), but there were no differences between the three groups. However, chronic nimodipine treatment had a significant effect on PTZ-induced seizures: the group that had been treated with nimodipine for 21 weeks showed significantly more and longer-lasting seizures than the control group. The facilitating effects of chronically administered of nimodipine on PTZ-induced seizures are striking and opposite to those reported in the literature. In a second study, nimodipine was administered acutely, but no effects of nimodipine on PTZ-induced epilepsy could be detected. It can be concluded that chronic dietary administration of a calcium antagonist induces different effects on PTZ-induced seizures than acute administration in aged Wistar rats with spontaneous occurring SWD.

Multiple dynamical modes of thalamic relay neurons: rhythmic bursting and intermittent phase-locking

A model of thalamocortical relay neuron is studied to assess whether a 7-14 Hz (spindle) oscillation and a 0.5-4 Hz (delta) oscillation may result from the interplay between a T-type calcium current and a non-specific cation sag current. With moderate change of membrane parameter values, the model neuron can exhibit both the spindle and delta rhythms, at different levels of hyperpolarization; only the slower (delta) one or none. In the case when the model neuron is not intrinsically oscillatory, its response to rhythmic hyperpolarization is complex, and displays the "intermittent phase-locking" phenomenon where bursts of Na+ action potentials occur infrequently but their occurrence is phase-locked to the rhythmic input. The rhythmic bursting, whenever possible, is shown to emerge (bifurcate) from a subthreshold oscillation. Near the bifurcation chaotic discharge patterns are observed, where spikes occur intermittently at randomly chosen cycles of a mostly subthreshold slow oscillation. Furthermore, when both the spindle and delta modes can be realized, the transition between the two appears as a sudden drop of the rhythmic frequency with increased hyperpolarization. The T-type calcium current and the sag current may explain the "intermittent phase-locking" phenomenon that is characteristic to thalamic relay neurons during spindle oscillation and provide a cellular basis for the 7-14 Hz rhythm and the slower 0.5-4 Hz rhythm.

Effects of mu and delta opioid receptor agonists and antagonists on absence epilepsy in WAG/Rij rats

The effects of various types of opioid receptor agonists and antagonists were determined in a genetic rat model for generalized absence epilepsy. Rats of the WAG/Rij strain spontaneously showed several hundred spike-wave discharges per day. Intracerebroventricular (i.c.v.) injections of the selective mu agonist DAMGO (0.2, 0.7 microgram) resulted in a dose-related increase in the number of spike-wave discharges, while the selective delta agonist DPDPE (20, 60 micrograms) was without effect. DAMGO reduced the duration of automatic behavior, enhanced the immobile behavior (after the low dose) and had no effect on exploratory behavior. On the other hand, DPDPE significantly enhanced the total time spent on exploration, but did not influence other behavioral parameters. There was no correlation between the ability to the drug to modulate the epileptic activity and behavioral changes. The nonselective antagonist naloxone, administered either i.p. (0.4, 2.0, 10 mg/kg) or i.c.v. (10, 50 micrograms), increased the number of spike-wave discharges in a dose-dependent way. The specific delta receptor antagonist naltrindole (0.3, 1 mg/kg) was without effect, as was the irreversible mu receptor antagonist beta-funaltrexamine (beta-FNA). Pretreatment with beta-FNA diminished the action of DAMGO. These results clearly indicate that activation of the mu opioid receptor increases the number of spike-wave discharges, and that modulation of delta receptors is not effective. On the other hand, the naloxone-induced enhancement of spike-wave discharges, suggests a tonic control of the epileptic activity by another opioid system. These results point to an important role of the mu-, but not delta-, receptor in facilitation of the epileptic activity in WAG/Rij rats.

Thalamocortical rhythm generation in vitro: extra- and intracellular recordings in mouse thalamocortical slices perfused with low Mg2+ medium

Mouse thalamocortical slices maintain synaptic connections between thalamus and cortex. When perfused with low Mg2+ medium, thalamocortical slices exhibited spontaneous thalamocortical 2-6 s long bursts of rhythmic activity every 15-30 s, which was potentiated by pentylenetetrazol and abolished by severing thalamocortical connections. These oscillations were similar in frequency and duration to normal and pathological thalamocortical rhythms in vivo. In vitro studies of these thalamocortical rhythms may prove valuable in understanding cellular factors important in rhythm generation.

Aberrant transients in the EEG of epileptic rats: a spectral analytical approach

Aberrant transients in the cortical electroencephalogram of rats of the epileptic WAG/Rij strain were studied by means of spectral analysis. The EEG of rats of this strain contains, besides normal sleep spindles, high-voltage spiky phenomena, epileptic spike-wave discharges, and deviant intermediate stage. Spectral analysis of these transient phenomena shows that some features, like their peak frequency, are alike, but that they differ in other spectral characteristics, as in the first harmonic of the peak frequency and in the domain of the high frequencies. The results provide arguments for the view that spike-wave discharges might be considered as unique aberrant phenomena, presumably related but dissimilar to the high-voltage spiky phenomena and intermediate stage. Next to this, spectral analysis was used to study the intraphenomenal dynamics of spike-wave discharges. The peak frequency was found to decrease monotonously from about 10 Hz at the beginning of the spike-wave discharge to about 8 Hz at the end. Other spike-wave discharge frequency bands showed an intraphenomenal increase followed by a decrease. These time-variant EEG dynamics in spike-wave discharges might correlate with the cognitive disturbances during absence seizures in man.

Cerebellar neuronal activity correlates with spike and wave EEG patterns in the rat

In this study we investigated the involvement of the cerebellum in high voltage spike-and-wave spindles, a rodent model of petit mal epilepsy. High voltage spindles, recorded epidurally from the sensorimotor neocortex, were correlated with single or multiple unit activity in the cerebellar cortex and deep cerebellar nuclei. The majority of neurons or neuronal groups in the cerebellum (77.9%) fired rhythmically and phase-locked with the high voltage spindles, either during the spike (43.2%; n = 41) or during the wave (34.7%; n = 33) component of the high voltage spindle. Tremor of the head and neck musculature, recorded with an accelero-meter, occurred during the high voltage spindle in approximately half of the rats. When present, rhythmic movement occurred predominantly during the wave phase of the high voltage spindle. The remaining half of the rats did not show tremor during high voltage spindles but, nevertheless, had cerebellar units that burst during the spike or wave phase of the high voltage spindle. These latter results demonstrate that phase-locked bursting of cerebellar units during high voltage spindle is independent of rhythmic movement. The findings suggest that rhythmic output from the cerebellum may contribute to the maintenance of generalized petit mal seizures.

The effects of alpha 2-adrenoceptor stimulation on neocortical EEG activity in control and 6-hydroxydopamine dorsal noradrenergic bundle-lesioned rats

The present study investigated the effects of a alpha 2-adrenoceptor agonist, D-medetomidine (0.3, 3.0, 30.0 and 300.0 micrograms/kg, s.c.), on neocortical EEG activity in control and 6-hydroxydopamine dorsal noradrenergic bundle-lesioned rats. D-Medetomidine at 0.3, 3.0, and 30.0 micrograms/kg dose dependently increased waking-immobility-related high-voltage spike and wave spindles. Movement and waking-immobility-related slow wave activity was increased at doses of 3.0, 30.0 and 300.0 micrograms/kg. D-Medetomidine at 300.0 micrograms/kg produced continuous 1-2 Hz slow wave activity and the animals were markedly sedated. In rats injected with D-medetomidine at 0.3, 3.0 and 30.0, micrograms/kg EEG activity could be desynchronized (block of high-voltage spindles and slow waves) by pinching the tail. However, rats injected with D-medetomidine at 300.0 micrograms/kg showed no change in EEG activity or behavior following tail pinching. D-Medetomidine induced similar EEG activity (high-voltage spindles and slow waves) and behavioral changes (sedation) in 6-hydroxydopamine dorsal noradrenergic bundle-lesioned rats. Atipamezole, an alpha 2-adrenoceptor antagonist, blocked D-medetomidine-induced EEG and behavioral changes in control and 6-hydroxydopamine dorsal noradrenergic bundle-lesioned rats. Based on the present results we suggest that stimulation of presynaptic noradrenergic fibers is not a prerequisite for the increase of high-voltage spindle and slow wave activity induced by an alpha 2-adrenoceptor agonist and that the magnitude of EEG slowing induced by D-medetomidine correlates with the decreased behavioral response to sensory stimulation.