Petit mal epilepsy and parkinsonian tremor: hypothesis of a common pacemaker

Annotation: Tourette syndrome: a relentless drumbeat--driven by misguided brain oscillations

OBJECTIVE

This annotation reviews recent evidence that points to the likely role of aberrant neural oscillations in the pathogenesis of Tourette syndrome (TS).

METHODS

The available anatomic and electrophysiological findings in TS are reviewed in the context of an emerging picture of the crucial role that neural oscillations play in maintaining normal central nervous system (CNS) function.

RESULTS

Neurons form behavior-dependent oscillating networks of various sizes and frequencies that bias input selection and facilitate synaptic plasticity, mechanisms that cooperatively support temporal representation as well as the transfer and long-term consolidation of information. Coherent network activity is likely to modulate sensorimotor gating as well as focused motor actions. When these networks are dysrhythmic, there may be a loss of control of sensory information and motor action. The known electrophysiological effects of medications and surgical interventions used to treat TS likely have an ameliorative effect on these aberrant oscillations. Similarly, a strong case can be made that successful behavioral treatments involve the willful training regions of the prefrontal cortex to engage in tic suppression and the performance of competing motor responses to unwanted sensory urges such that these prefrontal regions become effective modulators of aberrant thalamocortical rhythms.

CONCLUSIONS

A deeper understanding of neural oscillations may illuminate the complex, challenging, enigmatic, internal world that is TS.

Difference in surgical strategies between thalamotomy and thalamic deep brain stimulation for tremor control

Stereotactic targeting strategies differ between thalamotomy and thalamic deep brain stimulation (DBS) for tremor control. In thalamotomy, a minimal radiofrequency lesion created within the lateral portion of the nucleus ventralis intermedius (Vim) often affords the best control of parkinsonian tremor, supporting the assumption that there is a concentrated cluster of cells within this area which is responsible for tremor. However, this assumption may not always be true; such neural elements sometimes appear to spread out across wide areas. Cells with tremor-frequency activity are widely distributed over the areas extending from the Vim to the nuclei ventralis oralis posterior and anterior (Vop and Voa). All of these cells appear to be more or less involved in tremor generation, especially in patients with essential tremor and post-stroke tremor. In contrast to radiofrequency lesions for thalamotomy, electrodes for DBS can be arranged in such a way that wide areas can be stimulated, if necessary. For this purpose, it is critically important to determine optimal placement and orientation of DBS leads for arranging the electrodes to yield maximal benefits in patients with tremor.

Facilitation of spike-wave discharge activity by lipopolysaccharides in Wistar Albino Glaxo/Rijswijk rats

In normal rats the proinflammatory cytokines like interleukin-1beta, interleukin-6, which are induced by bacterial lipopolysaccharides, are able to control thalamo-cortical excitability by exerting strong effects on physiological synchronization such as sleep and on pathological synchronization like that in epileptic discharges. To investigate whether proinflammatory cytokines or lipopolysaccharides could modulate absence seizures resulting from a very different generator mechanism than the already investigated bicuculline-, kindling- and kainate-induced seizures, we used a genetically epileptic Wistar Albino Glaxo/Rijswijk rat strain, which is spontaneously generating high voltage spike-wave discharges. Wistar Albino Glaxo/Rijswijk rats responded with an increase of the number of spike-wave discharges to lipopolysaccharide injection (from 10 microg/kg to 350 microg/kg). Repetitive administration of 350 microg/kg lipopolysaccharides daily for 5 days increased the number of spike-wave discharges on the first, second and third days but the number of spike-wave discharges returned to the control value on day 5, at the 5th injection of lipopolysaccharides, showing a tolerance to lipopolysaccharides. The lipopolysaccharide-induced increase in spike-wave discharges was not directly correlated with the elevation of the core body temperature, as it is in febrile seizures, although lipopolysaccharide induced prostaglandin and is clearly pyrogenic at the doses used. Indomethacin, the prostaglandin synthesis inhibitor, efficiently blocked lipopolysaccharide-induced enhancement of spike-wave discharge genesis suggesting that the spike-wave discharge facilitating effect of lipopolysaccharides involves induction of cyclooxygenase 2 and subsequent synthesis and actions of prostaglandin E2. Low dose (40 mg/kg, i.p.) of competitive N-methyl-d-aspartate receptor antagonist 2-amino-5-phosphonopentanoic acid, and low dose of lipopolysaccharide (20 microg/kg) showed a synergistic interaction to increase the number of spike-wave discharges, whereas at supramaximal doses of lipopolysaccharide and the N-methyl-D-aspartate antagonist no synergy was present. The data reveal a functional connection between absence epileptic activity and lipopolysaccharide induction of prostaglandin synthesis and prostaglandin action and suggest some common cellular targets in epilepsy and lipopolysaccharide-induced inflammation.

Vers une théorie unifiée des symptômes positifs

Defined more than one century ago, the concept of positive symptoms has become obsolete, except in the psychiatric domain. However, its relevance remains intact today when considering such pathophysiologies as neuropathic (phantom) pain, movement disorders, tinnitus, epilepsy, and psychiatric disorders. Beside their very different clinical characteristics, all these symptoms arise from a lesion in the nervous system. Furthermore, they are paradoxical in the sense that they correspond to a spontaneous hyperactivity of the injured functional system, concomitant to the usual deficits resulting from the lesion. Could these similarities reflect the existence of some common pathophysiological process? A peculiar electrophysiological property of thalamic cells is likely to be compatible with this hypothesis. A thalamic cell produces action potentials when depolarised by excitatory inputs. Conversely, its ability to produce action potentials is decreased or even completely suppressed when the same cell is hyperpolarized by inhibitory influences. However, depending on its level of hyperpolarization, this cell can also produce rhythmic paradoxical bursts of activity at low frequency (3-4 Hz). In this context, a lesion involving, for example, the somatosensory excitatory fibres gives rise to hyperpolarization of the corresponding thalamic cells, which may produce such rhythmic bursting activity. This causes an increase of low frequency thalamo-cortical activity, which, through reduction of collateral cortico-cortical inhibition, induces high frequency activity in neighbouring thalamo-cortical loops ("edge effect"). This leads to the appearance of the clinical symptoms, in this case, pain. Electrophysiological recordings performed in patients suffering from sensory or motor positive symptoms have shown the presence of such deleterious sequence of events. Furthermore, the efficiency of neurosurgical treatments that are used against some positive symptoms can be explained on the basis of such a dynamic process. Both considerations support the validity of the proposed hypothesis and open avenues for the control of other positive symptoms.

Thalamic mechanisms of EEG alpha rhythms and their pathological implications

During relaxed wakefulness, the human brain exhibits pronounced rhythmic electrical activity in the alpha frequency band (8-13 Hz). This activity consists of 3 main components: the classic occipital alpha rhythm, the Rolandic mu rhythm, and the so-called third rhythm. In recent years, the long-held belief that alpha rhythms are strongly influenced by the thalamus has been confirmed in several animal models and, in humans, is well supported by numerous noninvasive imaging studies. Of specific importance is the emergence of 2 key cellular thalamic mechanisms, which come together to generate locally synchronized alpha activity. First, a novel form of rhythmic burst firing, termed high-threshold (HT) bursting, which occurs in a specialized subset of thalamocortical (TC) neurons, and second, the interconnection of this subset via gap junctions (GJs). Because repetitive HT bursting in TC neurons occurs in the range of 2 to 13 Hz, with the precise frequency increasing with increasing depolarization, the same cellular components that underlie thalamic alpha rhythms can also lead to theta (2-7 Hz) rhythms when the TC neuron population is less depolarized. As such, this scenario can explain both the deceleration of alpha rhythms that takes place during early sleep and the chronic slowing that characterizes a host of neurological and psychiatric disorders.

Distribution of D1-like and D2-like dopamine receptors in the brain of genetic epileptic WAG/Rij rats

The densities of the dopamine (DA) D1-like and D2-like receptors were studied by autoradiography in brain regions of rats with (WAG/Rij strain) and without (ACI strain) genetic absence epilepsy. The core of the nucleus accumbens in WAG/Rij rats had a lower density of D1-like receptors than in ACI rats, a reduction of both D1-like and D2-like DA receptors was also found for the dorsal striatum (dorsal caudate-putamen). On the other hand, the density of D2-like receptors was higher in cortical (frontal and parietal) regions and lower in the CA3 region of the hippocampus of WAG/Rij, as compared to ACI rats. These results give new information about possible malfunction of the brain dopaminergic system in the WAG/Rij rat model of absence epilepsy. It seems that there are differences between WAG/Rij and other models of absence epilepsy, especially concerning the role of striatum.

Relation Between Activities of the Cortex and Vibrissae Muscles During High-Voltage Rhythmic Spike Discharges in Rats

Paroxysmal 5- to 12-Hz high-voltage rhythmic spike (HVRS) activities, which are accompanied by whisker twitching (WT), are found in Long Evans rats, but the function of these HVRS activities is still debated. In four major functional hypotheses of HVRS discharges, i.e., alpha tremor, attention/mu rhythm, idling/mu rhythm, and absence seizure, the first two hypotheses emphasize WT behavior in HVRS bouts. Whisker movement is primarily determined by activation of intrinsic and extrinsic muscles. To clarify the role of WT in HVRS activities, simultaneous recording of the activities from the cortex and intrinsic/extrinsic and neck muscles were performed. Most HVRS bouts (68.8%) revealed no time-locked WT behavior in a 2-h recording session. In addition, WT primarily arose from active protraction due to activation of intrinsic muscles followed by passive retraction. A small portion of WT resulted from activation of both vibrissae muscles with dynamic frequency-dependent phase shifts. Onset of the rhythmic vibrissae EMG significantly lagged behind HVRS onset, and the mean duration of vibrissae muscle activity was one-third to a one-half of a HVRS bout. Moreover, a greater number of HVRS bouts were associated with a longer HVRS duration and higher oscillation frequency. Oscillation frequencies of HVRS activities without WT behavior were significantly lower than those with WT. Under peripheral sensory/motor blockade by xylocaine injection, oscillation frequencies of HVRS bouts significantly decreased, but no remarkable changes in the number or duration of HVRS bouts were observed. Compared with vibrissa muscle activity during WT and exploratory whisking, the duration of muscular activity in each cycle was apparently longer during whisking bouts. Based on these results, overemphasis of the role of WT on HVRS activities might not be appropriate. Instead, HVRS discharges may be associated with absence seizure or idling state. In addition, peripheral inputs, including WT, may elevate the oscillation frequency of HVRS bouts. Moreover, different muscular controls may exist between WT and whisking.

Degenerative neuronal changes in the rat thalamus induced by status epilepticus at different developmental stages

SE was induced in Wistar rats at post-natal (P) days 12, 15, 18, 21, and 25 to determine distribution and severity of thalamic damage in relation to time after SE. Six different intervals from 4 h up to 1 week were studied using Fluoro-Jade B (FJB) staining. Severity of damage was semi-quantified for every age-and-interval group. Distribution of neuronal damage within various thalamic nuclei was mapped by a computer-aided digitizing system. A consistent neuronal damage occurred in functionally heterogenous thalamic nuclei. Damage was found in all age groups although its extension and time course as well as the number of involved thalamic nuclei varied. Number of injured thalamic nuclei rapidly increased with age on SE-onset. In P12 group, degenerating neurons were consistently seen in the mediodorsal and lateral dorsal thalamic nuclei. Since P15, neurodegeneration was observed additionally in midline, ventral and caudal thalamic nuclei (visual and auditory thalamic nuclei), in the lateral posterior and in the reticular nucleus. In P21 and P25 animals, the majority of thalamic nuclei exhibited marked neuronal damage. Nuclei with a small number (anterior and intralaminar) or no FJB-positive neurons (the ventral nucleus of the lateral geniculate body) were exceptional. The pattern of thalamic damage is age-specific; its extent and severity increases with age.

7–12 Hz cortical oscillations: Behavioral context and dynamics of prefrontal neuronal ensembles

7-12 Hz Oscillations, characterized by spindle-like high-voltage rhythmic spike components, appear in quiet immobile states of rats. However, it remains unclear what their relationships with preceding behavioral activities are and how prefrontal neuronal dynamics during these oscillations is. In the present study, we first determined the relationship of 7-12 Hz oscillations with the wake-sleep cycle and preceding behavioral activities in several normal rat strains by recording electroencephalograms from the multiple cortical regions. Prolonged awake period transiently enhanced the following appearance of 7-12 Hz oscillations, which were frequently followed by slow-wave sleep. The degree of transient enhancement under the task condition was similar to that by prolonged wakefulness under the no-task condition. In addition, by recording local-field potential and multi-unit activities in the medial prefrontal cortex, we determined the temporal dynamics of prefrontal neuronal activities in relation to 7-12 Hz oscillations. Collective neuronal activities in medial prefrontal cortex were gradually organized into phase-locked patterns and showed highly synchronization during these oscillations. These dynamics were in temporal proximity to those of slow-wave activities (<4 Hz). Since slow-wave activities are thought to synchronize large spatial domains, these results suggest that 7-12 Hz oscillations are involved in the transition from the awake to sleep states by oscillatory entrainment of global cortical networks including the prefrontal neurons.

7 to 12 Hz activity in rat gustatory cortex reflects disengagement from a fluid self-administration task

The 7 to 12 Hz rhythm is a high-voltage oscillatory phenomenon recorded in many rat neocortical regions, largely analogous to the rodent and human somatosensory mu rhythm. Central to any interpretation of the functional significance of this pattern is the analysis of the behavioral context associated with it. Much of the debate on the function of mu, variously believed to represent either an environment-oriented or -isolated state, has relied primarily on its association with quiet immobility. In this report, we describe the relationship between the 7 to 12 Hz rhythm and a more complex behavioral setting, in which we were able to dissociated task orientation from disengagement. We trained head-restrained, water-restricted rats to perform a simple variant of a timed fluid self-administration task, while recording local field potentials from gustatory cortex (GC). Rats progressed through two behavioral states that were clearly distinguishable on the basis of lever-pressing regimes: a task-oriented state and a second state that reflected disengagement from the task. Concurrent GC neural recordings revealed bilaterally coherent oscillations in the 7 to 12 Hz range associated solely with the latter state. Consistent with published recordings of mu rhythm from somatosensory cortex, these rhythmic episodes were endogenously quenched when the rats prepared to lever-press; this inhibition of rhythmic episodes lasted through fluid delivery and consumption, making it clear that GC rhythms are not related to gustatory processing itself. By showing a direct relationship between the 7 to 12 Hz rhythm and disengagement from a task, these data provide strong and novel evidence that this gustatory rhythm in rats is associated with withdrawal from experimental contingencies.

The thalamic reticular nucleus: structure, function and concept

On the basis of theoretical, anatomical, psychological and physiological considerations, Francis Crick (1984) proposed that, during selective attention, the thalamic reticular nucleus (TRN) controls the internal attentional searchlight that simultaneously highlights all the neural circuits called on by the object of attention. In other words, he submitted that during either perception, or the preparation and execution of any cognitive and/or motor task, the TRN sets all the corresponding thalamocortical (TC) circuits in motion. Over the last two decades, behavioural, electrophysiological, anatomical and neurochemical findings have been accumulating, supporting the complex nature of the TRN and raising questions about the validity of this speculative hypothesis. Indeed, our knowledge of the actual functioning of the TRN is still sprinkled with unresolved questions. Therefore, the time has come to join forces and discuss some recent cellular and network findings concerning this diencephalic GABAergic structure, which plays important roles during various states of consciousness. On the whole, the present critical survey emphasizes the TRN's complexity, and provides arguments combining anatomy, physiology and cognitive psychology.

Effect of ethosuximide on rest tremor in the MPTP monkey model

Based on the hypothesis that low-threshold calcium conductance in the thalamus might be involved in the pathophysiology of parkinsonian tremor, ethosuximide was given chronically to a monkey previously treated with MPTP and displaying exceptionally a typical rest tremor. After 5 days of daily treatment, the tremor was reduced by 60%. Diltiazem and verapamil which act on different calcium channels had no such effect. Ethosuximide also potentiated the anti-tremor effect of the dopamine D2 agonist LY-171555.

Effect of Vim thalamic DBS in Parkinson’s disease on F wave duration

F waves were recorded from abductor hallucis muscle in eight Parkinsonian patients with deep brain stimulation (DBS) electrodes surgically implanted in their Vim thalamic nucleus in two conditions of DBS ON and OFF. Patients with relatively anteriorly located electrodes exhibited a significant reduction in F wave duration and also in the UPDRS rigidity score of the corresponding foot when the DBS was ON. In contrast, patients with relatively posteriorly located electrodes exhibited no significant difference in F wave duration in the two DBS ON and OFF conditions. The rigidity UPDRS score in the corresponding foot diminished very little in the latter group. Both groups had great improvement in their tremor at rest UPDRS score in that foot when the DBS was ON. Vim surgery is generally accepted to affect tremor mechanisms. However, surgical intervention in anterior parts of Vim has been reported to affect rigidity mechanisms. This correspondence of these two symptoms of rigidity and tremor with the two locations of anterior and relatively posterior Vim may indicate the contribution of mechanisms of rigidity, but not tremor, in enhancement of F wave duration and hyper excitability of spinal motoneuron.

Oscillatory Entrainment of Striatal Neurons in Freely Moving Rats

Oscillations and synchrony in basal ganglia circuits may play a key role in the organization of voluntary actions and habits. We recorded single units and local field potentials from multiple striatal and cortical locations simultaneously, over a range of behavioral states. We observed opposite gradients of oscillatory entrainment, with dorsal/lateral striatal neurons entrained to high-voltage spindle oscillations ("spike wave discharges") and ventral/medial striatal neurons entrained to the hippocampal theta rhythm. While the majority of units were likely medium-spiny projection neurons, a second neuronal population showed characteristic features of fast-spiking GABAergic interneurons, including tonic activity, brief waveforms, and high-frequency bursts. These fired at an earlier spindle phase than the main neuronal population, and their density within striatum corresponded closely to the intensity of spindle oscillations. The orchestration of oscillatory activity by networks of striatal interneurons may be an important mechanism in the pathophysiology of neurological disorders such as Parkinson's disease.

Heritable differences in the dopaminergic regulation of sensorimotor gating. II. Temporal, pharmacologic and generational analyses of apomorphine effects on prepulse inhibition

RATIONALE AND OBJECTIVES

The disruption of prepulse inhibition (PPI) of startle in rats by dopamine agonists has been used in a predictive model for antipsychotics, and more recently, to study the neural basis of strain differences in dopaminergic function. We have previously reported that Sprague-Dawley (SDH) and Long Evans (LEH) rats differed in their sensitivity to the PPI-disruptive effects of the D(1)/D(2) agonist apomorphine (APO) in two distinct ways: 1) compared to LEH rats, SDH rats were more sensitive to the ability of APO to disrupt PPI with relatively long prepulse intervals (60-120 ms), and 2) APO enhanced PPI in LEH rats with 10-30 ms prepulse intervals, but this effect was limited to 10 ms prepulse intervals in SDH rats.

METHODS

In the present study, we replicated this temporal profile in SDH versus LEH rats, assessed the role of D(1) versus D(2) substrates in the two components of this strain difference, and assessed the heritability of these temporally distinct processes.

RESULTS

Pharmacologic studies revealed that: 1) D(2) blockade prevented the long interval PPI-disruptive effects of APO in both strains, and extended the temporal range of the PPI-enhancing effects of APO from 10 to 30 ms in SDH rats, and 2) D(1) blockade increased PPI and blocked the PPI-enhancing effects of APO at short intervals in both strains. Generational studies in adult F0 (SDH and LEH), F1 (SDHxLEH) and N2 (SDHxF1) rats demonstrated that sensitivity to APO of both short and long interval PPI were inherited in a manner suggestive of relatively simple additive effects of multiple genes.

CONCLUSIONS

The present findings demonstrate that inherited differences in the dopaminergic regulation of sensorimotor gating are manifested not only in quantitative shifts (more versus less), but also in qualitative shifts in the temporal properties of sensorimotor gating that appear to be under separate control of D(1) and D(2) substrates.

Pathological theta oscillations in idiopathic generalised epilepsy

OBJECTIVE

To investigate spectral power, inter- and intra-hemispheric coherence in the interictal scalp electroencephalography (EEG) of 41 patients with idiopathic generalised epilepsy.

METHODS

Two minutes of eyes-closed waking interictal EEG activity was analysed. Fast Fourier transformation was performed. Raw and age-regressed, Z-transformed values were computed for 19 electrodes and 4 frequency bands: absolute power (AP, ZAP), percent power (RP, ZRP), band mean frequency (MF, ZMF), inter-hemispheric (CO, ZCO) and intra-hemispheric (IC, ZIC) coherence. Compressed values (scalp averages) were computed for each parameter and 4 frequency bands. Compressed data of the patients (GE group) and the healthy controls (C group) were compared.

RESULTS

ZAP across the entire 1.5-25.0 Hz range was greater in the GE than in the C group. Delta and theta ZRP was greater, alpha ZRP was less in GE than in C. ZMF and ZIC was about the same in the GE and C groups. The crucial, band-specific finding was significantly greater theta-ZCO in the GE group, in contrast to normal or decreased ZCO in the other bands. In addition, within-group correlation between ZAP of the frequency bands, correlation of ZAP and ZCO, correlation of ZIC and ZCO were different for the two groups.

CONCLUSIONS

The pattern of enhanced ZAP, ZRP, ZCO, together with normal ZIC and ZMF in the theta range was a peculiar, novel finding in GE. It was incompatible with any of the known patterns of altered power and coherence due to lesional or metabolic aetiology. This pattern presumably indicates the presence of a powerful, diffuse, hypersynchronous, hypercoherent theta oscillation at the thalamo-cortical level of the brain. The role of inter-hemispheric connections in maintaining this oscillation was suggested. The other findings suggest disturbed central regulation of EEG power and coherence in the interictal state.

Emergent properties of CNS neuronal networks as targets for pharmacology: application to anticonvulsant drug action

CNS drugs may act by modifying the emergent properties of complex CNS neuronal networks. Emergent properties are network characteristics that are not predictably based on properties of individual member neurons. Neuronal membership within networks is controlled by several mechanisms, including burst firing, gap junctions, endogenous and exogenous neuroactive substances, extracellular ions, temperature, interneuron activity, astrocytic integration and external stimuli. The effects of many CNS drugs in vivo may critically involve actions on specific brain loci, but this selectivity may be absent when the same neurons are isolated from the network in vitro where emergent properties are lost. Audiogenic seizures (AGS) qualify as an emergent CNS property, since in AGS the acoustic stimulus evokes a non-linear output (motor convulsion), but the identical stimulus evokes minimal behavioral changes normally. The hierarchical neuronal network, subserving AGS in rodents is initiated in inferior colliculus (IC) and progresses to deep layers of superior colliculus (DLSC), pontine reticular formation (PRF) and periaqueductal gray (PAG) in genetic and ethanol withdrawal-induced AGS. In blocking AGS, certain anticonvulsants reduce IC neuronal firing, while other agents act primarily on neurons in other AGS network sites. However, the NMDA receptor channel blocker, MK-801, does not depress neuronal firing in any network site despite potently blocking AGS. Recent findings indicate that MK-801 actually enhances firing in substantia nigra reticulata (SNR) neurons in vivo but not in vitro. Thus, the MK-801-induced firing increases in SNR neurons observed in vivo may involve an indirect effect via disinhibition, involving an action on the emergent properties of this seizure network.

Dynamics of non-convulsive epileptic phenomena modeled by a bistable neuronal network

It is currently believed that the mechanisms underlying spindle oscillations are related to those that generate spike and wave (SW) discharges. The mechanisms of transition between these two types of activity, however, are not well understood. In order to provide more insight into the dynamics of the neuronal networks leading to seizure generation in a rat experimental model of absence epilepsy we developed a computational model of thalamo-cortical circuits based on relevant (patho)physiological data. The model is constructed at the macroscopic level since this approach allows to investigate dynamical properties of the system and the role played by different mechanisms in the process of seizure generation, both at short and long time scales. The main results are the following: (i) SW discharges represent dynamical bifurcations that occur in a bistable neuronal network; (ii) the durations of paroxysmal and normal epochs have exponential distributions, indicating that transitions between these two stable states occur randomly over time with constant probabilities; (iii) the probabilistic nature of the onset of paroxysmal activity implies that it is not possible to predict its occurrence; (iv) the bistable nature of the dynamical system allows that an ictal state may be aborted by a single counter-stimulus.

Brain implantations of engineered GABA-releasing cells suppress tremor in an animal model of Parkinsonism

Traditional approaches in the treatment of Parkinson's disease have typically been directed at restoring dopaminergic tone in the neostriatum of the basal ganglia. Nevertheless, the vast majority of neostriatal efferent projections use GABA as their neurotransmitter. Substantia nigra pars reticulata (SNr) is a major basal ganglia output area that is a target of these GABAergic projections, and research from animal models has indicated that stimulation of GABA receptors in SNr can produce motor effects consistent with an antiparkinsonian action. In the present study, implantation of engineered GABA-releasing cells into SNr reduced tremulous movements in an animal model of parkinsonian tremor. These results suggest that implantation of GABA cells into SNr, or possibly into other basal ganglia structures, could provide an alternative transplantation strategy for the treatment of Parkinsonism.

Suppression of limbic motor seizures by electrical stimulation in thalamic reticular nucleus

Kindling is a model of temporal lobe epilepsy in which repeated electrical stimulations in limbic areas lead to progressive increase of seizure susceptibility, culminating in generalized convulsions and the establishment of a permanent epileptic syndrome. We studied here the effect of stimulations in the thalamic reticular nucleus (TRN) on the development of seizures and hippocampal hyperexcitability in kindling elicited from the ventral hippocampus in rats. Animals given 12 kindling stimulations per day with 30-min intervals for 4 consecutive days developed generalized convulsions on day 4. Stimulations in TRN delivered simultaneously with those in the hippocampus induced marked suppression of seizure generalization. Similarly, the number of generalized seizures and the duration of behavioral convulsions were reduced when rats subjected to 40 kindling stimulations with 5-min intervals during about 3 h were costimulated in the TRN. The anticonvulsant effect of TRN costimulation was detected also when rats were test-stimulated in the hippocampus at 24 h and 2 and 4 weeks after the initial 40 hippocampal stimulations. Our data provide the first evidence that TRN stimulations can act to suppress limbic motor seizures in hippocampal kindling and suggest a new approach for seizure control in temporal lobe epilepsy.

Somatosensory input to the ventrolateral thalamic region in the macaque monkey: potential substrate for parkinsonian tremor

In the present study, we determined the anatomic relationships between somatosensory and motor pathways within ventrolateral (VL) thalamic nuclei of the motor thalamus of macaque monkeys. In labeling experiments, four macaque monkeys (Macaca mulatta) received injections of biotinylated dextran amine and wheat germ agglutinin conjugated to horseradish peroxidase into the cerebellar nuclei or internal segment of the globus pallidus and cervical segments of the spinal cord, respectively. Each tracer was visualized in brain sections by sequentially using a different chromogen. Labeled terminals were plotted and superimposed on adjacent brain sections processed for Nissl substance, acetylcholinesterase, and the antigens for calbindin and Cat-301 to reveal thalamic nuclei. The labeled cerebellar terminals were distributed throughout the posterior VL (VLp), whereas the labeled pallidothalamic terminals were concentrated in the anterior VL and the ventral anterior nucleus. The spinothalamic input was directed mostly to the ventral posterior complex and cells just caudal to it. In addition, the patches of spinothalamic terminations intermingled and partly overlapped with the cerebellothalamic, but not with the pallidothalamic terminations within VLp. The regions of overlap of somatosensory and cerebellar inputs within the VLp of the present study appear to correspond to the reported locations of the tremor-related cells in parkinsonian patients. Thus, the overlapping spinothalamic and cerebellar inputs may provide a substrate for the altered activity of motor thalamic neurons in such patients.

The physiological basis of clinical deficits in Parkinson's disease

Despite the fact that Parkinson's disease (PD) is a relatively common neurological condition, the physiological derangements that result in its clinical features remain unclear. On combining findings from psychophysical, clinical and electrophysiological studies, an overriding theme is proposed that PD deficits are essentially quantitative rather than qualitative in nature. This may arise because the normal function of the basal ganglia is to activate neural processes selectively, providing appropriate diversion of "attentional" resources for decision-making aspects of motor tasks and appropriate "energising" of the executive aspects of such tasks. It is suggested that these concepts of attention, an idea stemming from psychophysical studies, and of energisation, which has derived from kinematic studies, may in fact reflect the same universal process of selective facilitation of particular processes and inhibition of others. In PD, without efficient facilitation, tasks may still be performed but less well than in normal individuals. Possible underlying mechanisms of basal ganglial function are discussed in the context of new findings on direct and indirect pathway actions and the role that oscillatory modulations may play in achieving selective facilitation is explored. Further investigation of disturbances of such mechanisms in PD may prove important in understanding the underlying pathophysiology of the condition.

Brain malformations, epilepsy, and infantile spasms

The models of cortical dysplasia discussed earlier--the Lis1 knockout, the MAM-induced cobblestone LIS, the spontaneous tish mutant, and focal freeze injury-induced PMG--illustrate several important insights into epileptogenesis in malformed brain. First, the appearance of epilepsy varies according to the pathogenesis of the dysplasia and may well depend more on the intrinsic properties of the neurons in these models rather than on the disturbed position of the cells. This is supported by models such as the reeler mouse, in which the dysfunctional extracellular matrix molecule leads to a form of lissencephaly in mouse and human, but there is a far less impressive association with seizures than for LIS1 mutations. However, Lis1 and Dex mutations that appear to affect the cytoskeleton and perhaps intracellular protein trafficking are frequently associated with infantile spasms and epilepsy. Second, the possible mechanisms of epileptogenesis in these models include (a) a loss of subsets of neurons, (b) altered neurotransmitter release, (c) differences in neurotransmitter receptor levels and changes in receptor subunit composition, (d) altered neurite density and/or synaptogenesis, (e) changed membrane properties (e.g., altered voltage-gated channels), (f) altered cell morphology (neuronal differentiation), and (g) effects on cytoskeletal function. Finally, it is important to note that the "generator" of excitability in affected brain may be within the heterotopia or in the normotopic cortex. As additional genetic models come to light and the ability to distinguish their clinical counterparts improves, more individually tailored therapies, including standards for surgical interventions, will surely evolve.

Assessment of disturbances in internal time counting and emotional behavior in rats with lesions to the striatum and hippocampus

Bilateral electrolytic lesioning of the striatum led to a delay in the acquisition of a conditioned reflex to time, along with an anxiogenic effect and disturbances to the normal relationship between the level of anxiety and chronometric ability. These changes progressed with increases in the size of the destruction zone in the striatum. Combined surgery consisting of lesions to the striatum and hippocampus, like hippocampectomy alone, completely eliminated the ability to count time regardless of the size of brain lesions. The anxiogenic effect was greater in animaLs with combined procedures. Thus, the striatum and hippocampus are involved to different extents in the mechanism of internal time counting. The two structures function synergistically.

Noradrenergic Control of Thalamic Oscillation: the Role of alpha-2 Receptors

The effects of alpha-adrenergic drugs on neocortical high voltage spike and wave spindles (HVS), reflecting thalamic oscillation, was investigated in freely moving rats. HVS occurred spontaneously in the awake but immobile animal. Peripheral administration of the alpha-1 antagonist, prazosin and alpha-2 agonists, xylazine and clonidine increased the incidence and duration of HVS in a dose-dependent manner. The alpha-2 antagonist, yohimbine and the tricyclic antidepressants, desipramine and amitriptyline, significantly decreased the incidence of the neocortical HVS. Bilateral microinjections of the alpha-2 agonists into the nucleus ventralis lateralis area of the thalamus, but not into the hippocampus or corpus callosum, was as effective as peripheral injection of these drugs. Xylazine was most effective in Fischer 344 rats that display high spontaneous rate of HVS and less effective in the Sprague - Dawley and Buffalo strains. The HVS-promoting effect of clonidine was antagonized by prior intrathalamic injection of the alpha-2 antagonist, yohimbine. The amplitude of the HVS was increased by picomole amounts of unilaterally-injected clonidine. Neurotoxic destruction of the thalamopetal noradrenergic afferents by intracisternal or intrathalamic injection of 6-hydroxydopamine, but not by peripheral administration of DSP-4, increased the incidence of HVS. Importantly, intrathalamic administration of xylazine continued to induce HVS after destroying the thalamic noradrenergic terminals. Following downregulation of the alpha-2 adrenoceptors by chronic administration (3 weeks) of amitriptylene the incidence of HVS decreased and the effectiveness of intrathalamic xylazine on the induction of HVS was significantly reduced. Based on these findings, we suggest that a major action of alpha-2 adrenergic drugs on thalamic oscillation may be mediated by postsynaptic alpha-2 adrenoceptors located on the thalamocortical neurons. We hypothesize that noradrenaline in the thalamus has a dual effect on the relay cells: blocking and promoting thalamic oscillation via alpha-1 and alpha-2 receptors, respectively. The final physiological effect is assumed to be a function of the relative density and affinity of these adrenergic receptor subtypes.

Pathophysiology of Parkinson's disease: from clinical neurology to basic neuroscience and back

Parkinson's disease (PD) is characterized by motor and nonmotor (cognitive and limbic) deficits. The motor signs of PD include hypokinetic signs such as akinesia/bradykinesia, rigidity and loss of normal postural reflexes, and hyperkinetic signs such as tremor. Dopamine depletion in the striatum is the hallmark of PD and of its animal models, still the pathophysiology of the parkinsonian symptoms and especially of parkinsonian tremor are under debate. The most extreme hypotheses argue about peripheral versus central nervous system origin, intrinsic cellular oscillator versus network oscillators, and basal ganglia-based pathophysiology versus cerebellar-thalamic based pathophysiology. Recent studies support the view that parkinsonian symptoms are most likely due to abnormal synchronous oscillating neuronal activity within the basal ganglia. Peripheral factors do only play a minor role for the generation, maintenance, and modulation of PD tremor and other signs. The most likely candidates producing these neuronal oscillations are the weakly coupled neural networks of the basal ganglia-thalamo-cortical loops. However, the present evidence supports the view that the basal ganglia loops are influenced by other neuronal structures and systems and that the tuning of these loops by cerebello-thalamic mechanisms and by other modulator neurotransmitter systems entrain the abnormal synchronized oscillations. Neurosurgical procedures, such as lesions or high-frequency stimulation of different parts of the loop, might resume the normal unsynchronized activity of the basal ganglia circuitry, and, therefore, ameliorate the clinical symptoms of Parkinson's disease.

Intraoperative microelectrode and semi-microelectrode recording during the physiological localization of the thalamic nucleus ventral intermediate

We review the techniques of physiological localization of the site for ventralis intermedius (Vim) thalamotomy or implantation of Vim-deep brain stimulation (DBS) for treatment of parkinsonian, essential, and intention tremor. Both microelectrode and semi-microelectrode techniques are reviewed. We believe the use of microelectrode and semi-microelectrode recordings in combination with Radiological landmarks provide the most accurate localization of the target. In addition to recording, microstimulation of subcortical structures such as Vim and thalamic nucleus ventralis caudal through the microelectrode may improve physiological identification by altering the tremor and evoking somatic sensations, respectively. Microelectrode recording provides the highest resolution picture of the target site at a cost of increased time to locate the target. We also review the relationship between thalamic neuronal firing and electromyographic activity during tremor. Implications of these results for the mechanisms for parkinsonian, essential, and intention tremors are discussed.

Tremor in Parkinson's disease

Rest tremor is a common feature of Parkinson's disease, but its underlying pathophysiology remains unknown. This review hypothesizes that tremor is related to selective loss of components of the substantia nigra. The relative scarcity of tremor in related Parkinsonian conditions may indicate a dissociation associated with different pathological involvement of the substantia nigra and its connections. Connections of the subthalamic nucleus with the pallidum, modified by cortical and nigral inputs, allow for the transfer of tremorogenic activity to the thalamus. Thalamo-cortical interactions, tempered by cerebellar input, generate the final common pathway for tremor production.

Deactivation of thalamocortical activity is responsible for suppression of parkinsonian tremor by thalamic stimulation: a 99mTc-ECD SPECT study

Four patients with Parkinson's disease (PD) achieved excellent improvement of their unilateral tremor by chronic deep brain stimulation (DBS) of the contralateral ventral intermediate (Vim) nucleus of the thalamus. Repeated measurements of cerebral blood flow were obtained 14 days apart off and on stimulation using 99mTc-ECD SPECT. Subjects were scanned at rest and the data were compared with those of normal healthy volunteers. During stimulation, there were highly significant deactivations in the motor area and supplementary motor area on the electrode side and in the prefrontal area and the anterior cingulum bilaterally. No cerebellar deactivation was detected. We conclude that the mechanism responsible for suppression of parkinsonian tremor by thalamic stimulation is deactivation of thalamocortical activity.

Sexual dimorphism and developmental regulation of substantia nigra function

The substantia nigra is an important brain nucleus involved in the expression of movement disorders and seizures. The two most common movement disorders affecting the substantia nigra, Parkinson's disease and Tourette syndrome, show gender differences and age-related onset. To assess the substrates for the gender and age specificity of substantia nigra-related disorders, we determined the functional properties of the substantia nigra gamma-aminobutyric acid (GABAA) system along its anterior-posterior axis, using localized microinfusions of muscimol (a GABAA agonist) and susceptibility to motor seizures in rats. In the substantia nigra, there are sex-specific differences in the topographic segregation and functionality of GABAA systems. In mature male rats there are two distinct regions mediating opposite effects on seizures; in female rats there is only one region that can affect seizures. In the neonatal period, the presence of circulating testosterone is essential for the development of a substantia nigra region that exerts proconvulsant effects throughout the rat's life, a unique feature of the male substantia nigra. The final maturation of the substantia nigra occurs in the peripubertal period, and is in part regulated by testosterone as well. The recognition of the existence of distinct sex- and age-specific substantia nigra features can be translated into new cures of disorders affecting the substantia nigra.

Dopamine characteristics in rat genotypes with distinct susceptibility to epileptic activity: apomorphine-induced stereotyped gnawing and novelty/amphetamine-induced locomotor stimulation

Rat genotypes differ in their susceptibility to spontaneously occurring spike-wave discharges and in their dopaminergic properties. In a previous study, it was found that spike-wave discharge incidence decreased in the following order in four rat genotypes during baseline and following injection with the dopamine antagonist haloperidol: apomorphine-susceptible (APO-SUS) > WAG/Rij > apomorphine-unsusceptible (APO-UNSUS) and ACI rats. The question in the present study was to what extent certain dopaminergic properties are pathognomonic for epileptic rats. Therefore, behavioral responses were assessed in order to investigate the dopaminergic properties in the four rat genotypes. Apomorphine-induced gnawing data imply that the dopamine activity of the nigrostriatal system in the WAG/Rij rats is higher than in APO-SUS but lower than in the ACI and APO-UNSUS rats. Furthermore, in previous studies APO-SUS have been shown to have a higher novelty/amphetamine-induced locomotion, indicative of a higher dopamine reactivity of the mesolimbic system as compared to APO-UNSUS rats. Results from the present study showed that WAG/Rij rats have a higher locomotor responsiveness to novelty/amphetamine, indicating a higher dopamine reactivity of the mesolimbic system in comparison to the ACI rats. It is suggested that the functional dopaminergic mesolimbic dominance is an important factor in the susceptibility to show spontaneously occurring spike-wave discharges.

Up-regulation of D3 dopaminergic receptor mRNA in the core of the nucleus accumbens accompanies the development of seizures in a genetic model of absence-epilepsy in the rat

The basal ganglia system is thought to play a key role in the control of absence-seizures and there is ample evidence that epileptic seizures modify brain dopamine function. We recently reported that local injections of dopamine D1 or D2 agonists in the core of the nucleus accumbens suppressed absence-seizures in a spontaneous, genetic rodent model of absence-epilepsy whereas injections of D1 or D2 antagonists had aggravating effects. These findings raised the possibility that the dopaminergic system may be altered in absence-epilepsy prone rats. Therefore, we studied by in situ hybridization histochemistry the expression of pre- and postsynaptic components of the dopaminergic system in this strain of rats. When compared to non-epileptic control rats, epileptic rats displayed no change in the expression of mRNAs coding for the neuronal dopaminergic markers (tyrosine hydroxylase, membraneous and vesicular dopamine transporters). In addition, there was no difference between the two strains concerning the expression of the dopamine receptor transcripts D1, D2 and D5. In adult absence-epilepsy prone rat with an overt epileptic phenotype, however, an elevated level of D3 mRNA expression was observed in neurons of the core of the nucleus accumbens (+23% increase in silver grain density compared to non-epileptic control rats). D3 transcripts were not increased in juvenile epileptic rats without seizures. These findings suggests that up-regulation of D3 receptor mRNA is part of the epileptic phenotype in absence-epilepsy prone rats. Its localization in the core of the nucleus accumbens bears close resemblance to the dopamine-sensitive antiepileptic sites in ventral striatum and further support the involvement of ventral structures of the basal ganglia system in the control of absence-seizures.

Electrophysiological and pharmacological characteristics of two types of spike-wave discharges in WAG/Rij rats

Rats of the WAG/Rij strain are commonly seen as a genetic model for generalised absence epilepsy in man. Interestingly, generalised absence epilepsy shows, in addition to the fully generalised spike-wave discharges, a second type of spike-wave discharge, which lasts for a shorter time, has a lower frequency, and a lower incidence. The originally described distinction between the two types of spike-wave discharges was mainly based on the shape, polarity and duration of the discharges. In the present study other characteristics such as the spatial and temporal distribution of the spike and wave components of the two discharges and frequency spectra were found to differ between the two types. In addition, a reciprocal regulation of the two types of spike-wave discharges by drugs affecting the dopaminergic system (haloperidol and apomorphine) was observed. The results convincingly demonstrate the difference between the two phenomena and warrant the search for neurobiological mechanisms underlying both types of spike-wave discharges.

The pathophysiology of tremor

Tremor is defined as rhythmic oscillatory activity of body parts. Four physiological basic mechanisms for such oscillatory activity have been described: mechanical oscillations; oscillations based on reflexes; oscillations due to central neuronal pacemakers; and oscillations because of disturbed feedforward or feedback loops. New methodological approaches with animal models, positron emission tomography, and mathematical analysis of electromyographic and electroencephalographic signals have provided new insights into the mechanisms underlying specific forms of tremor. Physiological tremor is due to mechanical and central components. Psychogenic tremor is considered to depend on a clonus mechanism and is thus believed to be mediated by reflex mechanisms. Symptomatic palatal tremor is most likely due to rhythmic activity of the inferior olive, and there is much evidence that essential tremor is also generated within the olivocerebellar circuits. Orthostatic tremor is likely to originate in hitherto unidentified brainstem nuclei. Rest tremor of Parkinson's disease is probably generated in the basal ganglia loop, and dystonic tremor may also originate within the basal ganglia. Cerebellar tremor is at least in part caused by a disturbance of the cerebellar feedforward control of voluntary movements, and Holmes' tremor is due to the combination of the mechanisms producing parkinsonian and cerebellar tremor. Neuropathic tremor is believed to be caused by abnormally functioning reflex pathways and a wide variety of causes underlies toxic and drug-induced tremors. The understanding of the pathophysiology of tremor has made significant progress but many hypotheses are not yet based on sufficient data. Modern neurology needs to develop and test such hypotheses, because this is the only way to develop rational medical and surgical therapies.

Dopamine characteristics in different rat genotypes: the relation to absence epilepsy

Dopaminergic neurotransmission has been shown to participate in the control of absence epilepsy. This type of epilepsy, a generalized non-convulsive form, is associated with bursts of bilateral synchronous spike wave discharges (SWDs) recorded in the EEG. In a previous study, it was suggested that two features of the apomorphine-susceptible (APO-SUS) rat genotype, a relatively low dopaminergic reactivity of the nigrostriatal system and relatively high dopaminergic reactivity of the mesolimbic system, contribute to the high incidence of SWDs. Indeed, apomorphine-unsusceptible (APO-UNSUS) rats, characterized by opposite dopaminergic features, show considerably less SWDs than APO-SUS rats. The first goal of the present study was to assess the baseline SWD incidence in four rat genotypes (WAG/Rij, ACI, APO-SUS and APO-UNSUS) in order to replicate previous findings. It was expected that both the APO-SUS and WAG/Rij rats would show a considerably higher SWD incidence in comparison to the APO-UNSUS and ACI rats. For this purpose, rats were registered for a 19 hour period. Assuming that haloperidol decreases dopaminergic transmission in the nigrostriatal system via inhibition of the dopamine receptors and enhances dopaminergic transmission in the mesolimbic system via inhibition of the noradrenergic receptors, it was postulated that haloperidol would enhance the difference in dopaminergic reactivity between both systems in favor of the accumbens. Therefore, the second purpose in the present study was to investigate whether haloperidol (2 mg/kg, IP) could further potentiate SWD incidence when injected in the APO-SUS rats, already characterized by a relatively low dopaminergic reactivity of the nigrostriatal system and relatively high dopaminergic reactivity of the mesolimbic system, in comparison to the APO-UNSUS rat genotype. Finally, the third aim was to study if another epileptic rat genotype, the WAG/Rij, would show similar increases in SWD incidence following an injection with haloperidol as expected for the APO-SUS. First, previous findings were replicated: the value of the hourly number of SWDs decreased in the following order: APO-SUS > WAG/Rij > APO-UNSUS and ACI. Secondly, earlier data were extended by the fact that the APO-SUS responded to a systemic injection of haloperidol with an increase in SWD number and duration, in contrast to the APO-UNSUS rats. The hypothesis that the SWD incidence would be mostly affected by haloperidol in the APO-SUS rats, was confirmed by these findings. It is suggested that haloperidol increases the SWD incidence in APO-SUS rats by enhancing the difference between the dopaminergic reactivity in the nigrostriatal and mesolimbic system. Finally, further research is required to provide evidence in favor of the hypothesis that the relative dominance of the dopaminergic mesolimbic system is smaller in WAG/Rij than in APO-SUS.

Spike-and-wave discharges of absence seizures as a transformation of sleep spindles: the continuing development of a hypothesis

OBJECTIVES

This review aims to offer a critical account of recent scientific developments relevant to the hypothesis which Pierre Gloor proposed in the 1970s for the generation of spike and wave discharges (SWDs) of primary generalized absence seizures.

RESULTS

According to this hypothesis SWDs develop in the same circuits, which normally generate sleep spindles, by an initially cortical transformation of one every two or more spindle waves to a 'spike' component of SWDs, while the next one or more spindle waves are eliminated and replaced by a slow negative wave. This hypothesis was based on experiments in feline generalized penicillin epilepsy showing the possibility of transition from spindles to SWDs, when cortical neurons become hyper-responsive to thalamocortical volleys, which normally induce spindles, and thus engage feedback cortical inhibition, rebound excitation, recurrent intracortical dissemination of excitation during the 'spike' and strong excitation of thalamus for further augmentation of a brain wide synchronous oscillation. In the 1980s, electrophysiological studies in vitro and in vivo revealed the basic features of spindle rhythm generation by neurons in nucleus reticularis thalami and thalamocortical-corticothalamic oscillatory reverberations.

CONCLUSIONS

In the light of this knowledge, experimental studies in several genetic and pharmacological animal models of absence seizures, clinical observations and theoretical studies in computer models have considered, tested, modified and challenged this hypothesis. It may still be found useful in the era of dynamic digital EEG analysis of SWDs and its current sources.

Backpropagation of the delta oscillation and the retinal excitatory postsynaptic potential in a multi-compartment model of thalamocortical neurons

Uniform and non-uniform somato-dendritic distributions of the ion channels carrying the low-threshold Ca(2+) current (I(T)), the hyperpolarization-activated inward current (I(h)), the fast Na(+) current (I(Na)) and the delayed rectifier current (I(K)) were investigated in a multi-compartment model of a thalamocortical neuron for their suitability to reproduce the delta oscillation and the retinal excitatory post-synaptic potential recorded in vitro from the soma of thalamocortical neurons. The backpropagation of these simulated activities along the dendritic tree was also studied. A uniform somato-dendritic distribution of the maximal conductance of I(T) and I(K) (g(T) and g(K), respectively) was sufficient to simulate with acceptable accuracy: (i) the delta oscillation, and its phase resetting by somatically injected current pulses; as well as (ii) the retinal excitatory postsynaptic potential, and its alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate and/or N-methyl-D-aspartate components. In addition, simulations where the dendritic g(T) and g(K) were either reduced (both by up to 34%) or increased (both by up to 15%) of their respective value on the soma still admitted a successful reproduction of the experimental activity. When the dendritic distributions were non-uniform, models where the proximal and distal dendritic g(T) was up to 1.8- and 1. 2-fold larger, respectively, than g(T(s)) produced accurate simulations of the delta oscillation (and its phase resetting curves) as well as the synaptic potentials without need of a concomitant increase in proximal or distal dendritic g(K). Furthermore, an increase in proximal dendritic g(T) and g(K) of up to fourfold their respective value on the soma resulted in acceptable simulation results. Addition of dendritic Na(+) channels to the uniformly or non-uniformly distributed somato-dendritic T-type Ca(2+) and K(+) channels did not further improve the overall qualitative and quantitative accuracy of the simulations, except for increasing the number of action potentials in bursts elicited by low-threshold Ca(2+) potentials. Dendritic I(h) failed to produce a marked effect on the simulated delta oscillation and the excitatory postsynaptic potential. In the presence of uniform and non-uniform dendritic g(T) and g(K), the delta oscillation propagated from the soma to the distal dendrites with no change in frequency and voltage-dependence, though the dendritic action potential amplitude was gradually reduced towards the distal dendrites. The amplitude and rising time of the simulated retinal excitatory postsynaptic potential were only slightly decreased during their propagation from their proximal dendritic site of origin to the soma or the distal dendrites. These results indicate that a multi-compartment model with passive dendrites cannot fully reproduce the experimental activity of thalamocortical neurons, while both uniform and non-uniform somato-dendritic g(T) and g(K) distributions are compatible with the properties of the delta oscillation and the retinal excitatory postsynaptic potential recorded in vitro from the soma of these neurons. Furthermore, by predicting the existence of backpropagation of low-threshold Ca(2+) potentials and retinal postsynaptic potentials up to the distal dendrites, our findings suggest a putative role for the delta oscillation in the dendritic processing of neuronal activity, and support previous hypotheses on the interaction between retinal and cortical excitatory postsynaptic potentials on thalamocortical neuron dendrites.

Ultra-slow oscillation (0.025 Hz) triggers hippocampal afterdischarges in Wistar rats

Oscillations in neuronal networks are assumed to serve various physiological functions, from coordination of motor patterns to perceptual binding of sensory information. Here, we describe an ultra-slow oscillation (0.025 Hz) in the hippocampus. Extracellular and intracellular activity was recorded from the CA1 and subicular regions in rats of the Wistar and Sprague-Dawley strains, anesthetized with urethane. In a subgroup of Wistar rats (23%), spontaneous afterdischarges (4.7+/-1.6 s) occurred regularly at 40.8+/-15.7 s. The afterdischarge was initiated by a fast increase of population synchrony (100-250 Hz oscillation; "tonic" phase), followed by large-amplitude rhythmic waves and associated action potentials at gamma and beta frequency (15-50 Hz; "clonic" phase). The afterdischarges were bilaterally synchronous and terminated relatively abruptly without post-ictal depression. Single-pulse stimulation of the commissural input could trigger afterdischarges, but only at times when they were about to occur. Commissural stimulation evoked inhibitory postsynaptic potentials in pyramidal cells. However, when the stimulus triggered an afterdischarge, the inhibitory postsynaptic potential was absent and the cells remained depolarized during most of the afterdischarge. Afterdischarges were not observed in the Sprague-Dawley rats. Long-term analysis of interneuronal activity in intact, drug-free rats also revealed periodic excitability changes in the hippocampal network at 0.025 Hz. These findings indicate the presence of an ultra-slow oscillation in the hippocampal formation. The ultra-slow clock induced afterdischarges in susceptible animals. We hypothesize that a transient failure of GABAergic inhibition in a subset of Wistar rats is responsible for the emergence of epileptiform patterns.

Thalamic reticular nucleus parcellation delineated by VIP and TRH gene expression in the rat

The distribution of the mRNAs encoding VIP (vasoactive intestinal peptide) and TRH (thyrotropin releasing hormone) was examined in the thalamic reticular nucleus of the adult rat using hybridization histochemistry with S35-labeled oligoprobes. Low levels of TRH expression were found in a medial tier. High levels of VIP expression were found in neurons located in a lateral shell of the same portion. High levels of TRH expression were found in a tier located dorsally and in a tier located ventrally to the first one. In these regions no VIP expression could be detected. These data suggest a parcellation of this nucleus according to the differential expression patterns of TRH and VIP.

Glutamate inhibits thalamic reticular neurons

Activation of metabotropic glutamate receptors (mGluRs) can result in long-lasting modulation of neuronal excitability. Multiple mGluR subtypes are localized within the rat thalamic reticular nucleus (TRN), and we have examined the effects of activating these different receptor subtypes on the excitability of these neurons using an in vitro slice preparation. Typical of most mGluR-sensitive preparations, the general mGluR agonist, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) produced a robust, long-lasting excitatory response. Surprisingly, ACPD produced a membrane hyperpolarization in some neurons. Using selective mGluR agonists, we found that activation of group II mGluRs produces the hyperpolarization, whereas the depolarization is mediated by group I mGluRs. While the polarity of the postsynaptic response (hyperpolarization vs depolarization) was dependent on the mGluR subtype activated, both actions appear to result from modification of a linear K(+) conductance. The inhibitory action of Glutamate, via group II mGluRs, provides an avenue for a disinhibitory effect that could have interesting consequences upon a well-investigated, model neuronal circuit, turning its assumed functional role upside down.

Evidence for the involvement of the pallidum in the modulation of seizures in a genetic model of absence epilepsy in the rat

Inhibition of the subthalamic nucleus (STN) has been shown to suppress seizures in different animal models of epilepsy. The aim of this study was to examine the role of the pallidal inputs to the STN in the control of absence seizures in a genetic model in the rat. Disinhibition of the globus pallidus or the ventral pallidum, by local injections of a GABA(A) antagonist, suppressed absence seizures. Conversely, inhibition of the ventral pallidum by a GABA(A) agonist aggravated absence seizures. Furthermore, the antiepileptic effects of intrapallidal injections of a GABA(A) antagonist were correlated with a decrease of extracellular levels of glutamate in the substantia nigra. Our results show that both the globus pallidus and the ventral pallidum exert a modulatory influence on absence seizures and suggest that these effects are mediated through the STN.

Tremulous jaw movements in rats: a model of parkinsonian tremor

Several pharmacological and neurochemical conditions in rats induce 'vacuous' or 'tremulous' jaw movements. Although the clinical significance of these movements has been a subject of some debate, considerable evidence indicates that the non-directed, chewing-like movements induced by cholinomimetics, dopamine antagonists and dopamine depletions have many of the characteristics of parkinsonian tremor. These movements occur within the 3-7 Hz peak frequency range that is characteristic of parkinsonian tremor. Tremulous jaw movements are induced by many of the conditions that are associated with parkinsonism, and suppressed by several different antiparkinsonian drugs, including scopolamine, benztropine, L-DOPA, apomorphine, bromocriptine, amantadine and clozapine. Striatal cholinergic and dopaminergic mechanisms are involved in the generation of tremulous jaw movements, and substantia nigra pars reticulata appears to be a major basal ganglia output region through which the jaw movements are regulated. Future research on the neurochemical and anatomical characteristics of tremulous jaw movements could yield important insights into the brain mechanisms that generate tremulous movements.

Spike-and-wave oscillations based on the properties of GABAB receptors

Neocortical and thalamic neurons are involved in the genesis of generalized spike-and-wave (SW) epileptic seizures. The cellular mechanism of SW involves complex interactions between intrinsic neuronal firing properties and multiple types of synaptic receptors, but because of the complexity of these interactions the exact details of this mechanism are unclear. In this paper these types of interactions were investigated by using biophysical models of thalamic and cortical neurons. It is shown first that, because of the particular activation properties of GABAB receptor-mediated responses, simulated field potentials can display SW waveforms if cortical pyramidal cells and interneurons generate prolonged discharges in synchrony, without any other assumptions. Here the "spike" component coincided with the synchronous firing, whereas the "wave" component was generated mostly by slow GABAB-mediated K+ currents. Second, the model suggests that intact thalamic circuits can be forced into a approximately 3 Hz oscillatory mode by corticothalamic feedback. Here again, this property was attributable to the characteristics of GABAB-mediated inhibition. Third, in the thalamocortical system this property can lead to generalized approximately 3 Hz oscillations with SW field potentials. The oscillation consisted of a synchronous prolonged firing in all cell types, interleaved with a approximately 300 msec period of neuronal silence, similar to experimental observations during SW seizures. This model suggests that SW oscillations can arise from thalamocortical loops in which the corticothalamic feedback indirectly evokes GABAB-mediated inhibition in the thalamus. This mechanism is shown to be consistent with a number of different experimental models, and experiments are suggested to test its consistency.

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.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

High-voltage electroencephalogram spindles in rats, aging and 5-HT2 antagonism

We examined the effects of serotonin-2 (5-hydroxytryptamine-2, 5-HT2) receptor antagonists on the so-called high-voltage spindles (HVS, electroencephalographic patterns, characterized by large amplitude rhythmic waves mainly in the alpha band), recorded from the frontal cortex of young, middle-aged and old freely-moving rats during waking immobility. The study was based on the assumption that the effects of 5-HT2 receptor antagonists on the HVS activity depend on the age of rats, because there is evidence for an age-related decrease in the 5-HT2 binding sites density. Four parameters of the electroencephalogram (EEG) were used to characterize the HVS activity: the square root-transformed EEG peak power in the alpha band, the frequency corresponding to this peak (both measured from the EEG power spectra using the fast Fourier transform), the HVS mean duration, and the HVS incidence (both measured from the EEG records). The EEG parameters were analyzed after i.p. administration of three 5-HT2 receptor antagonists: ketanserin, ritanserin and cyproheptadine. In young rats, the three drugs increased the alpha power, but did not change the alpha peak-corresponding frequency. Ketanserin and ritanserin did not change the HVS mean duration and HVS incidence, while cyproheptadine increased both these parameters in young rats. In middle-aged and old untreated rats, the HVS activity was significantly increased. The three 5-HT2 antagonists did not change the HVS activity in aged rats, which could be due to age-related suppression of the 5-HT2 receptor functions.