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

Patterns of bursting occurring in thalamic cells during parkinsonian tremor

It has been proposed that parkinsonian tremor is produced either by the activity of an intrinsic thalamic pacemaker or by the oscillation of an unstable long loop reflex arc. The former (central) hypothesis proposes that overactivity of neurons in the internal segment of the globus pallidus inhibits or hyperpolarizes thalamic neurons. When hyperpolarized, thalamic cells oscillate with bursting of the type associated with low threshold calcium spikes (low threshold spike-bursts). Low threshold spike-bursts can be identified by particular patterns of interspike intervals within the burst. The alternative (peripheral) hypothesis proposes that tremor results from oscillation of a reflex arc transmitting activity from muscle stretch receptors to thalamus, motor cortex, and back to the stretched muscle. When the gain of this reflex is increased, the arc may become unstable and oscillate. Oscillations produced by peripheral inputs may produce an acceleration-deceleration pattern within the burst which results in sinusoidal modulation of a spike train if bursting is periodic. We have assessed these two hypotheses by studying the pattern of interspike intervals occurring within bursts recorded in patients with parkinsonian tremor. The spike trains were analysed for 118 cells located in the ventral nuclear group including ventralis intermedius (thalamic cerebellar relay nucleus, n=48) and ventralis oralis posterior (thalamic pallidal relay nucleus, n=39) of patients with parkinsonian tremor. Two cells recorded in ventralis intermedius of a sleeping patient with chronic pain showed bursting activity similar to the low threshold spike-bursts recorded in sleeping animals, suggesting a common mechanism for low threshold spike-bursts across species. Forty-two cells recorded in patients with parkinsonian tremor (ventralis intermedius, n=19; ventralis oralis posterior, n=12) were classified as tremor-related cells because their activity was characterized by both a concentration of power at tremor frequency and significant correlation with tremor. Eleven tremor-related cells, 10 located in ventralis intermedius or ventralis oralis posterior and most responding to sensory inputs, had an acceleration-deceleration pattern of intraburst firing. Only one cell, a tremor-related cell in ventralis intermedius, showed the pattern expected of presumed low threshold spike-bursts. Therefore, intraburst interspike interval patterns consistent with either the central or the peripheral hypothesis were recorded in the thalamus of patients with parkinsonian tremor. Twenty-one tremor-related cells (15 cells in ventralis intermedius or ventralis oralis posterior) had bursts with intraburst interspike intervals which were independent of position of the interspike interval within the burst. Therefore, the activity of the majority of cells was not consistent with either hypothesis, suggesting that another oscillatory process may contribute to parkinsonian tremor.

Functional properties of perigeniculate inhibition of dorsal lateral geniculate nucleus thalamocortical neurons in vitro

The properties of the inhibitory influence of neurons in the perigeniculate (PGN) nucleus on thalamocortical cells were examined with intracellular recordings in the ferret geniculate slice maintained in vitro. Activation of PGN neurons with the local application of glutamate caused IPSPs in thalamocortical neurons that were mediated by both GABAA and GABAB receptors, as well as the activation of spindle waves. With low intensity stimulation of the PGN, local application of bicuculline to the dorsal lateral geniculate nucleus (LGNd) strongly inhibited evoked and spindle-associated IPSPs, indicating that these are largely mediated by GABAA receptors. The generation of GABAB receptor-mediated IPSPs in thalamocortical cells that were large enough to generate rebound low threshold Ca2+ spikes required substantially increased activation of the PGN with glutamate. The activation of synchronous bicuculline-induced slowed oscillations in thalamocortical neurons required the block of GABAA receptors in the LGNd as well as in the PGN. These results indicate that bursts of action potentials in PGN neurons can result in the activation of both GABAA and GABAB receptors in thalamocortical neurons, with the strong activation of GABAB receptors requiring an intense, simultaneous discharge of a number of PGN neurons. Functionally, these results suggest that PGN neurons inhibit thalamocortical cells preferentially through the activation of GABAA receptors, although the strong activation of GABAB receptors may occur under pathological conditions and contribute to the generation of abnormal, synchronous slow oscillations.

Current controversies and future directions in basal ganglia research. Integrating basic neuroscience and clinical investigation

This article discusses current controversies and future directions in basal ganglia research, detailing behavioral aspects, anatomic models, neurochemistry, pharmacology, and diagnostic methods as well as surgical techniques. A neuroethologic perspective is highlighted. Furthermore, the relevant literature pertaining to contemporary molecular approaches such as brain microinjections of embryonic or genetically modified cells, for therapeutic purposes and the use of transgenic and knockout animals.

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.

Inhibition of voluntary activity by thalamic stimulation in humans: relevance for the control of tremor

The motor effects of stimuli delivered through four-channel, quadripolar macroelectrodes chronically implanted in the ventrolateral thalamus were studied in 20 awake cooperating human subjects. Single stimuli could inhibit voluntary contraction of the contralateral first dorsal interosseous muscle (FDI) for up to 200 ms. The inhibition was often followed by a rebound facilitation or by oscillatory activity. This inhibition appeared to arise from the ventrolateral thalamus and could not be obtained in other patients by stimulation of the periventricular grey matter (PVG), the globus pallidus internus (GPI), or the subthalamic nucleus (STN). The neural elements activated by the stimulus had a short chronaxie and a short refractory period, implying that they were large-diameter axons. Similar effects were obtained from each of the four electrodes in the row, suggesting that this fiber system lay parallel rather than perpendicular to the implanted macroelectrode. The inhibition resulting from a single stimulus was diminished by a prior stimulus or train of stimuli. A continuous train of stimuli produced inhibition for only the first 200 ms. We propose that the thalamic stimulus activates a neural network which includes thalamic relay cells and neurons of the thalamic reticular nucleus and that the inhibition of thalamic relay cells habituates with repeated stimuli. It has been suggested that parkinsonian rest tremor results from synchronization of the oscillatory activity of this network. If this is the case, continuous thalamic stimulation might disrupt this oscillation by diminishing the inhibitory phase.

Involvement of pallidal and nigral GABA mechanisms in the generation of tremulous jaw movements in rats

Four experiments were conducted to investigate the role of pallidal and nigral GABA in the generation of tremulous jaw movements in rats. In these experiments, tremulous jaw movements were induced by i.p. injections of the anticholinesterase tacrine (5.0 mg/kg). Previous work has shown that the tremulous jaw movements induced by cholinomimetics and dopamine depletion are dependent upon striatal mechanisms. Thus, the present study investigated potential striatal output pathways that could be involved in the generation of these movements. Because there are GABAergic projections from neostriatum to entopeduncular nucleus (medial globus pallidus) and substantia nigra pars reticulata, the GABA agonist muscimol was injected directly into these structures to study the effects of GABA stimulation on tacrine-induced jaw movements. Injections of muscimol into the entopeduncular nucleus (25-100 ng) failed to have any significant effects on tacrine-induced vacuous jaw movements. However, injections of muscimol (12.5-50 ng) into the substantia nigra pars reticulata blocked the jaw movements induced by tacrine. In the third experiment, it was again demonstrated that 25.0 ng of muscimol injected directly into the substantia nigra pars reticulata blocked the jaw movements induced by tacrine; in addition, it was shown that injections of this dose 2.0 mm dorsal to the substantia nigra pars reticulata failed to affect tacrine-induced tremulous jaw movements. It was shown in the fourth experiment that injections of muscimol into a more medial portion of the substantia nigra pars reticulata also reduced tacrine-induced tremulous jaw movements. These results indicate that stimulation of GABA(A) receptors in substantia nigra pars reticulata can block tacrine-induced tremulous jaw movements. This finding is consistent with the notion that striatonigral GABA projections are involved in the generation of tremulous jaw movements. It is also possible that striatonigral GABA mechanisms are involved in human clinical phenomena such as parkinsonian tremor.

Sleep and arousal: thalamocortical mechanisms

Thalamocortical activity exhibits two distinct states: (a) synchronized rhythmic activity in the form of delta, spindle, and other slow waves during EEG-synchronized sleep and (b) tonic activity during waking and rapid-eye-movement sleep. Spindle waves are generated largely through a cyclical interaction between thalamocortical and thalamic reticular neurons involving both the intrinsic membrane properties of these cells and their anatomical interconnections. Specific alterations in the interactions between these cells can result in the generation of paroxysmal events resembling absence seizures in children. The release of several different neurotransmitters from the brain stem, hypothalamus, basal forebrain, and cerebral cortex results in a depolarization of thalamocortical and thalamic reticular neurons and an enhanced excitability in many cortical pyramidal cells, thereby suppressing the generation of sleep rhythms and promoting a state that is conducive to sensory processing and cognition.

No loss in total neuron number in the thalamic reticular nucleus and neocortex in the genetic absence epilepsy rats from Strasbourg

The thalamic reticular nucleus (nRt) as well as the neocortex are involved in the bilateral spike- and wave-discharge loop in genetic absence epilepsy rats from Strasbourg (GAERS). Neuron loss in different brain areas has been described in relation to epilepsy with convulsive seizures. We have previously investigated the ventrolateral/posterior nucleus of thalamus in GAERS and found no neuron loss. We applied the same efficient and unbiased stereological methods to nRt and to neocortex and again found no loss of neurons. The oscillatory properties of nRt are not related to neurons loss.

Generalized absence epilepsy and catalepsy in rats

Adult WAG/Rij rats are considered adequate genetic models for human generalized absence epilepsy. Rats of this strain of 8, 12, and 18 weeks old and age-matched control Wistar rats were exposed to sound stimulation. After offset of stimulation, all WAG/Rij rats showed cataleptic or even cataplexic reactions, which could persist for up to 20 min. Age effects could be demonstrated. None of the Wistar rats showed cataleptic reactions. Electroencephalographic studies in WAG/Rij rats of 21 weeks showed that spike-wave discharges were abundantly present in the background electroencephalogram prior to sound stimulation. Age-matched Wistar rats had almost no spike-wave discharges. Spike-wave discharges in WAG/Rij rats disappeared during sound stimulation and were then increased compared to the prestimulation and stimulation periods. The electroencephalogram during the cataleptic state was also characterized by the presence of large amplitude 2 Hz waves, interspersed with spike-wave discharges. The data suggest that the cataleptic state can be elicited in genetically epilepsy-prone rats. The youngest WAG/Rij rats showed no spike-wave discharges during the cataleptic state. In all, the data suggest that epilepsy-prone animals are sensitive for catalepsy at an age at which the EEG signs of generalized absence epilepsy are not yet manifest.

Simultaneous quantitative evaluation of visual-evoked responses and background EEG activity in rat: normative data

An integrated quantitative electroencephalography system (Phegra) for pharmacological and toxicological research in rat is described. Peak latencies and amplitudes of visual-evoked potentials, occurrence, duration, and linear excursions of photically evoked afterdischarges, "activity," "mobility," "complexity" of Hjorth, and absolute spectral powers of delta, theta, alpha, and beta frequency bands of background activity of visual cortex and frontal-visual leads were measured in freely moving rats. Counts of small and large movements were also registered. Data of baseline measurements performed in large amount of animals are presented. None of the parameters except the occurrence of photically evoked afterdischarge and the linear excursion of its averaged waveshape changed significantly in five measurements performed within six hours following the intraperitoneal and oral administration of two commonly used drug vehicles.

Unilateral nigrostriatal lesions induce a bilateral increase in glutamate decarboxylase messenger RNA in the reticular thalamic nucleus

The reticular thalamic nucleus consists of densely packed neurons containing the neurotransmitter GABA. It surrounds the lateral border of the thalamus, has extensive reciprocal connections with thalamocortical neurons, and is thought to be involved in attentional processes. The reticular thalamic nucleus also receives direct and indirect inputs from the basal ganglia, suggesting that it may be involved in relaying motor information to the thalamus and cortex. We examined the possibility that decreased dopaminergic transmission in the basal ganglia indirectly affects the reticular thalamic nucleus. Rats received unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta and were killed two or three weeks after the lesion. Sections of the reticular thalamic nucleus were processed for in situ hybridization histochemistry at the single cell level with RNA probes for both isoforms of glutamate decarboxylase (M(r) 65,000: glutamate decarboxylase 65 and M(r) 67,000: glutamate decarboxylase 67), the rate limiting enzyme of GABA synthesis. Unilateral nigrostriatal dopaminergic lesions induced a topographically specific, bilateral increase in glutamate decarboxylase 67 messenger RNA in neurons of the lateral and ventral reticular thalamic nucleus. A much smaller increase in glutamate decarboxylase 65 messenger RNA was observed which was significant only ipsilateral to the lesion. Short- (seven day) and long-term (eight month) treatments with the antipsychotic drug haloperidol, in regimens that preferentially block D2 dopamine receptors, induced catalepsy and orofacial dyskinesia, respectively, but did not alter glutamate decarboxylase 67 messenger RNA levels in the reticular thalamic nucleus. Thus, loss of dopaminergic terminals, but not blockade of D2 dopamine receptors, induced the effects observed in the reticular thalamic nucleus. The results reveal a novel bilateral effect of unilateral dopamine depletion. In view of the role of the reticular thalamic nucleus in tremor and attentional processes, which are altered in Parkinson's disease, this effect may contribute to the clinical manifestations of nigrostriatal dopamine depletion.

Nucleus reticularis thalami and neocortical paroxysms in the rat

The role of the nucleus reticularis thalami in spike-wave discharges in rats with genetic absence epilepsy has already been demonstrated. This study further investigated the role of the nucleus reticularis thalami in paroxysmal synchronizations in Sprague-Dawley rats; this strain shows no propensity to epileptic activity. Electroencephalographic patterns were followed in chronically implanted, unrestrained rats. After both electrolytic and chemical unilateral lesions, stereotaxically placed in the anterolateral sectors of this nucleus (verified post mortem), abnormal electroencephalographic rhythms (high-voltage polyspikes and spike-wave complexes) were recorded from the frontoparietal cortex, primarily in the contralateral hemisphere. Stereotyped discharges at 3 Hz developed progressively from multiple spikes within the alpha frequency range through the lengthening of the wave component. The excessive synchronized activity recorded from the intact hemisphere was of greater amplitude and occurred slightly earlier than from the lesioned hemisphere. These EEG patterns were associated with behavioural manifestations closely resembling those seen during absence seizures in humans. Bilateral lesions did not induce paroxysmal activity, both hemispheres being characterized by dominant delta/theta activity without signs of EEG-synchronized sleep. The seizures may thus have been due to disinhibition of the contralateral reticularis nucleus, recently shown to project to the reticularis nucleus of the other side in rats. This working hypothesis is supported by callosal cuts. The results indicate that the reticular neurons exert a control over neocortical paroxysmal activity even in animals which do not present genetic absence epilepsy.

Modulation of rat neocortical high-voltage spindle activity by 5-HT1/5-HT2 receptor subtype specific drugs

To investigate the role of serotonin (5-hydroxytryptamine; 5-HT) receptors in the modulation of rat thalamocortical oscillations, we studied the effects of 5-HT1/5-HT2 receptor subtype specific drugs on neocortical high-voltage spindle activity in adult male rats. A 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.03, 0.1, 0.3 and 1.0 mg/kg s.c.), had no effect on neocortical high-voltage spindle activity. Furthermore, a mixed 5-HT1/5-HT2 receptor antagonist, methysergide (1.0, 5.0 and 15.0 mg/kg i.p.), had no effect, whereas a non-specific mixed 5-HT1/5-HT2 receptor antagonist, methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.), significantly increased neocortical high-voltage spindles. Of the 5-HT2 receptor antagonists, ritanserin (0.1, 1.0 and 5.0 mg/kg s.c.) had no effect, whereas ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) increased neocortical high-voltage spindles, but only at the highest dose used. A 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.5, 1.0 and 2.0 mg/kg s.c.), at the two highest doses significantly decreased neocortical high-voltage spindle activity, and this effect was blocked by the 5-HT2 receptor antagonists, ketanserin (1.0, 5.0 and 20.0 mg/kg s.c.) and ritanserin (1.0 and 5.0 mg/kg s.c.), as well as by methiothepin (0.2, 1.0 and 5.0 mg/kg i.p.) and methysergide (1.0, 5.0 and 15.0 mg/kg i.p.). Furthermore, unilateral intrathalamic infusions, but not intrahippocampal control infusions, of DOI (10 and 50 micrograms/1.0 microliters/rat) decreased neocortical high-voltage spindle activity and systemic administration of ketanserin (20.0 mg/kg s.c.) completely blocked this effect. The present results suggest that (1) the serotonergic system modulates rat thalamocortical oscillations as measured by neocortical high-voltage spindle activity, (2) activation of 5-HT2 receptors, possibly located in the thalamus, with a specific 5-HT2 receptor agonist, DOI, causes a reduction in rat neocortical high-voltage spindle activity.

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.

Role of the ferret perigeniculate nucleus in the generation of synchronized oscillations in vitro

1. The cellular mechanisms by which neurons of the ferret perigeniculate nucleus (PGN) participate in the generation of spindle waves and slowed absence seizure-like oscillations were investigated with intracellular and extracellular recording techniques in geniculate slices maintained in vitro. 2. During spindle wave generation, PGN neurons generated repetitive (2-9 Hz) high frequency (up to 500 Hz) burst discharges mediated by the activation of a low threshold Ca2+ spike by the arrival of barrages of excitatory postsynaptic potentials (EPSPs). In most PGN cells at resting membrane potentials (-60 to -70 mV) spindle waves were associated with a progressive hyperpolarization that persisted as a prolonged after-hyperpolarization. 3. The EPSPs occurring in PGN cells were highly synchronized with burst firing in the neighbouring portion of the dorsal lateral geniculate nucleus (LGNd) and were intermixed with short duration inhibitory postsynaptic potentials (IPSPs). After block of GABAergic receptors, the EPSPs occurring during the generation of spindle waves reversed polarity at around 0 mV. In addition, these EPSPs were completely blocked with the bath application of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), as was spindle wave generation in both the PGN and LGNd. 4. Slowing the intraspindle frequency to 2-4 Hz with pharmacological block of GABAA receptors resulted in a marked increase in the intensity of burst firing by PGN cells such that the number of action potentials per burst increased from a maximum of thirteen to a maximum of sixty. Block of GABAA receptors also resulted in a marked increase in the amplitude and duration of the EPSP barrages arriving from the relay laminae during generation of the slowed oscillation. 5. These findings indicate that spindle waves are generated in the ferret LGNd in vitro through an interaction between the GABAergic neurons of the PGN and relay neurons, such that burst firing in relay neurons activates a barrage of EPSPs and a subsequent low threshold Ca2+ spike in PGN cells. This activation of PGN neurons inhibits a substantial number of relay cells, a few of which rebound burst after this IPSP, thus starting the cycle again. Block of GABAA receptors results in a marked enhancement of activity in PGN cells through increased excitation from relay cells and disinhibition from neighbouring PGN cells. This increased activity in PGN neurons results in a markedly enhanced activation of GABAB receptors in relay neurons and the subsequent generation of paroxysmal activity that is similar to that associated with absence seizures.

Parkinsonism and rest tremor secondary to supratentorial tumours sparing the basal ganglia

Intracranial neoplasms are an uncommon cause of symptomatic Parkinsonism and rest tremor. We found an incidence of 0.3% in a prospective evaluation of 907 patients with supratentorial tumours. Eight patients with Parkinsonism and rest tremor secondary to supratentorial tumours sparing the basal ganglia are reported. Neuro-imaging revealed compression and distortion of the basal ganglia by large tumours which were identified histopathologically as meningiomas in four patients and as an epidermoid, a fibrillary astrocytoma, an anaplastic oligodendroglioma and a glioblastoma. Six patients underwent tumour removal by craniotomy, in two the histopathology was obtained by stereotactic biopsy. Four patients were free of Parkinsonian symptoms and signs on long-term follow-up. The possible pathophysiological mechanisms involved are discussed. Since some of these patients closely resemble cases of idiopathic Parkinson's disease, and the movement disorder can precede other symptoms and signs or will remain isolated in the further course, the diagnosis of an intracranial neoplasm was generally delayed in these patients. Increased awareness of this rare entity may lead to an earlier diagnosis. Early computed tomography in patients with Parkinsonism might help to detect these patients with a potentially curable cause of their condition.

Genetic threshold hypothesis of neocortical spike-and-wave discharges in the rat: an animal model of petit mal epilepsy

Neocortical high-voltage spike-and-wave discharges (HVS) in the rat are an animal model of petit mal epilepsy. Genetic analysis of total duration of HVS (s/12 hr) in reciprocal F1 and F2 hybrids of F344 and BN rats indicated that the phenotypic variability of HVS cannot be explained by a simple, monogenic Mendelian model. Biometrical analysis suggested the presence of additive, dominance, and sex-linked-epistatic effects, buffering maternal influence, and heterosis. High correlation was observed between average duration (s/episode) and frequency of occurrence of spike-and-wave episodes (n/12 hr) in parental and segregating generations, indicating that common genes affect both duration and frequency of the spike-and-wave pattern. We propose that both genetic and developmental-environmental factors control an underlying quantitative variable, which, above a certain threshold level, precipitates HVS discharges. These findings, together with the recent availability of rat DNA markers for total genome mapping, pave the way to the identification of genes that control the susceptibility of the brain to spike-and-wave discharges.

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.

Some unusual effects of alpha 2-adrenergic drugs on cortical high voltage spindles in rats

The dose-response curves for a number of alpha-adrenergic drugs were investigated to estimate a possible role of the alpha 2/alpha 1 selectivity of these drugs on the incidence of cortical high voltage spindles (HVS), reflecting level of vigilance. The alpha 2 antagonists yohimbine (0.25-4 mg/kg) and idazoxan (0.5-4 mg/kg), but not atipamezole induced a biphasic effect on the incidence of HVS in rats. This effect of relatively small doses of yohimbine and idazoxan should be taken into consideration when using these drugs as alpha 2 antagonists in behavioral and neurophysiological tests. On the other hand the linearity of the dose-response curve for atipamezole (0.01-4 mg/kg) indicates that this drug is a good candidate for use in such tests.

Effects of VA-045, a novel apovincaminic acid derivative, on age-related impairment evidence in electroencephalograph, caudate spindle, a passive avoidance task and cerebral blood flow in rats

1. The ability of VA-045 to improve aged-related impairment on electroencephalograph (EEG), caudate spindle, performance on a passive avoidance task and cerebral blood flow (CBF) were evaluated in rats. 2. The cortical EEG of the aged rats showed a higher incidence of spontaneous spindle burst (SSB) than seen in young rats. VA-045 decreased the incidence of SSB in aged rats. In contrast, vinpocetine increased the incidence of SSB in aged rats. 3. Electrical stimulation of the striatum in aged rats lead to a higher incidence of neocortical high voltage spindle (CS) than seen in young rats. In young rats, VA-045 had no effect on the CS, whereas an age-related increase in CS was blocked by VA-045, but was enhanced by vinpocetine. 4. There were no differences in the cortical EEG arousal response elicited by stimulation of the reticular formation of the brain stem in rats of all ages. VA-045 and vinpocetine had no effect on the cortical EEG arousal response in both young and aged rats. 5. VA-045, but not vinpocetine, attenuated the age-related decreased step through latency (STL) on a passive avoidance task. VA-045 and vinpocetine did not enhance the acquisition of learning behavior in a passive avoidance task in young rats. 6. VA-045 increased the cerebral blood flow (CBF) in both young and aged rats and the potency in aged rats was greater than that in young rats. Vinpocetine had no effect on CBF in either young or aged rats. 7. The pharmacological effects of VA-045 on age-related neuronal dysfunction are discussed.

A model of intrinsic and driven spindling in thalamocortical neurons

We add a slow hyperpolarization-activated inward current IH = gHmH (v-vH) to our previous model of rebound bursting (Hindmarsh & Rose Phil. Trans. R. Soc. Lond. B 346, 129-150 (1994a)) to give a four-dimensional physiological model, and a corresponding four-dimensional model of the model. The physiological model generates periodic 'bursts of bursts' or 'spindles' resembling those recorded experimentally in thalamocortical (TC) neurons. The model of the model is simplified to a two-dimensional system having a limit cycle which corresponds to the slow spindle oscillation of the physiological model. Analysis of the stability of this two-dimensional model allows us to divide the parameter space of the slope (gamma mH) and shift (theta mH) parameters of mH infinity (v) into regions in which the model generates spindles or continuous bursting. This enables us to determine the parameter values required for spindling in the physiological model and to explain the experimentally observed effects of noradrenaline. Next we examine whether a cell at a stable equilibrium point can be driven into spindling by applying a sinusoidal input at the resonant frequency. This is done by averaging the equations for the driven model of the model. Analysis of the stability of these averaged equations shows how the regions of the (theta mH, gamma mH) parameter space change when the system is driven by a sinusoidal input. This enables us to choose parameter values for a physiological model of a driven spindle. We show that if the physiological model is modified to include a voltage-dependent time constant for mH, spindles, similar to those of TC cells, can be obtained with a small Ca(2+)-activated K+ current. Finally our knowledge of the form of the bifurcation diagram and the conditions for resonance leads to a new suggestion for the roles of GABAA and GABAB inhibitory postsynaptic potentials when TC cells are driven into spindling by neurons of the nucleus reticularis thalami.

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.

Seizures associated with propofol anesthesia

Propofol is a new, fast-acting intravenous (i.v.) anesthetic. Involuntary movements or epileptic seizures have occurred during or after propofol-induced anesthesia in approximately 50 reported cases; a third of the patients have had epilepsy. We report 5 patients with seizures in association with propofol anesthesia. A female epileptic patient developed severe status epilepticus; the other patients with short-lasting seizures had no previous epilepsy. Although propofol has been used in treatment of patients of status epilepticus, the risk of precipitation of epileptic seizures warrants consideration especially when planning anesthesia for epileptic patients.

Effects of muscarinic receptor agonists and anticholinesterase drugs on high voltage spindles and slow waves

The effects of muscarinic agonists (AF102B, pilocarpine, oxotremorine) and anticholinesterases (physostigmine, tetrahydroaminoacridine) were investigated on the incidence of thalamically generated rhythmic high voltage spindles and on scopolamine (0.2 mg/kg)-induced neocortical slow wave activity (i.e. increased sum amplitude value of the 1-20 Hz band in a quantitative electroencephalography (qEEG) analysis). AF102B and pilocarpine decreased high voltage spindles and scopolamine increased sum amplitude values at 3 and 9 mg/kg, but not at 1 mg/kg. Oxotremorine was less potent than AF102B or pilocarpine in suppressing high voltage spindles. Oxotremorine had no effect on the scopolamine-induced qEEG changes. Tetrahydroaminoacridine decreased high voltage spindles at 1, 3 and 9 mg/kg and slow waves at 9 mg/kg. Physostigmine decreased high voltage spindles and slow waves at 0.12 and 0.36 mg/kg. Based on the present results we propose that agonists possessing muscarinic M1 receptor activity are effective in decreasing high voltage spindles and scopolamine-induced slow wave activity, but agonists showing predominant muscarinic M2 receptor activity may be less effective in decreasing high voltage spindles and slow waves. Furthermore, tetrahydroaminoacridine decreased high voltage spindles at doses lower than those required to decrease scopolamine-induced slow waves. Physostigmine decreased high voltage spindles and slow waves over the same dose range. This result may indicate that non-cholinergic mechanisms are involved in the tetrahydroaminoacridine-induced decrease in high voltage spindles.

Suppression of magnetic mu rhythm during parkinsonian tremor

We recorded spontaneous magnetoencephalographic (MEG) activity and somatosensory-evoked fields (SEFs) with a 24-channel planar SQUIDgradiometer in five patients with hemiparkinsonism. The SEFs of the patients were within normal limits. During tremorless periods, the spontaneous activity over the somatomotor cortex had a frequency peak at approximately 10 Hz in all five patients and another at approximately 20 Hz in three. Tremor dampened the 10-Hz activity in all patients; in three the effect was bilateral. Tremor did not increase MEG activity at the tremor frequency. The suppression of the mu rhythm by the parkinsonian tremor resembled that occurring during voluntary movements in healthy subjects.

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.

Cellular mechanisms of a synchronized oscillation in the thalamus

Spindle waves are a prototypical example of synchronized oscillations, a common feature of neuronal activity in thalamic and cortical systems in sleeping and waking animals. Spontaneous spindle waves recorded from slices of the ferret lateral geniculate nucleus were generated by rebound burst firing in relay cells. This rebound burst firing resulted from inhibitory postsynaptic potentials arriving from the perigeniculate nucleus, the cells of which were activated by burst firing in relay neurons. Reduction of gamma-aminobutyric acidA (GABAA) receptor-mediated inhibition markedly enhanced GABAB inhibitory postsynaptic potentials in relay cells and subsequently generated a slowed and rhythmic population activity resembling that which occurs during an absence seizure. Pharmacological block of GABAB receptors abolished this seizure-like activity but not normal spindle waves, suggesting that GABAB antagonists may be useful in the treatment of absence seizures.

Epileptogenic spikes and seizures but not high voltage spindles are induced by local frontal cortical application of gamma-hydroxybutyrate

Combining the methods of microdialysis and EEG recording, we have examined the effect of unilaterally, intracortically applied gamma-hydroxybutyrate (GHB) on frontal cortical EEG activity in freely moving rats. GHB, a natural endogenous GABA metabolite, is known to induce rhythmic spike and wave activity, resembling generalized petit mal epilepsy. Without GHB, spontaneous high voltage spindles (HVS, 6-9 Hz) were observed during awake and immobile behavior in most of the animals (HVS rats), while others never had any HVS. In those both groups of animals intracortical application of GHB induced epileptogenic spikes (< 0.5 Hz) behaviorally accompanied by occasional myoclonic jerks and epileptic discharges (< 2 Hz) with behavioral convulsions and contraversive movements towards the left hindlimb (seizures) but did not induce HVS or spike and waves, as reported after systemic application. In the group of rats with spontaneous occurring HVS the amplitude of the HVS on the side of the microdialysis probe was suppressed by GHB and GHB-induced spikes invading the contralateral cortex frequently triggered and terminated local HVS. The results point to different neural mechanisms for the generation of HVS and spikes and epileptic discharges (seizures) induced after local intracortical application of GHB.

Alpha 1-adrenoceptor antagonist decreases alpha 2-adrenoceptor antagonist-induced high voltage spindle suppression in adult and aged rats

The present experiments were undertaken to study whether blockade of alpha 1-adrenoceptors would prevent the high-voltage spindle suppressing effect of atipamezole, an alpha 2-adrenoceptor antagonist. In adult rats, an alpha 1-adrenoceptor antagonist, prazosin, increased dose dependently the high voltage spindles in saline and atipamezole-pretreated rats. In addition, in aged rats prazosin blocked the high-voltage spindle suppressing action of atipamezole. Prazosin produced a smaller increase in high-voltage spindle values in aged than in adult rats. According to the present results, alpha 2-adrenoceptor antagonists may suppress high-voltage spindles indirectly by activating alpha 1-adrenoceptors in young and aged rats.

Spindle rhythmicity in the reticularis thalami nucleus: synchronization among mutually inhibitory neurons

The sleep spindle rhythm of thalamic origin (7-14 Hz) displays widespread synchronization among thalamic nuclei and over most of the neocortex. The mechanisms which mediate such global synchrony are not yet well understood. Here, we theoretically address the hypothesis of Steriade and colleagues that the reticularis thalami nucleus may be considered as a genuine pacemaker for thalamocortical spindles. Interestingly, the reticularis consists of a population of neurons which are GABAergic and synaptically coupled. These cells, as do thalamic relay cells, exhibit a transient depolarization following release from sustained hyperpolarization. This postinhibitory rebound property is due to a T-type calcium ionic current which is inactivated at rest but de-inactivated by hyperpolarization. Theoretically, rebound-capable cells coupled by inhibition can generate rhythmic activity, although such oscillations are usually alternating (out-of-phase), rather than synchronous (in-phase). Here, we develop and apply to Steriade's pacemaker hypothesis our earlier finding that mutual inhibition can in fact synchronize cells, provided that the postsynaptic conductance decays sufficiently slowly. Indeed, postsynaptic receptors of the GABAB subtype mediate inhibition with a large decay time-constant (approximately 200 ms). In contrast, chloride-dependent, GABAA-mediated inhibitory postsynaptic potentials are fast and brief. Both GABAA and GABAB receptor binding sites are present in most thalamic regions, including the reticularis. We suggest that if GABAB receptors exist postsynaptically in the reticularis, they may play a critical role in the rhythmic synchronization among reticular neurons, hence in the thalamocortical system.

Pattern recognition of the electroencephalogram by artificial neural networks

A back-propagation network was trained to recognize high voltage spike-and-wave spindle (HVS) patterns in the rat, a rodent model of human petit mal epilepsy. The spontaneously occurring HVSs were examined in 137 rats of the Fisher 344 and Brown Norway strains and their F1, F2 and backcross hybrids. Neocortical EEG and movement of the rat were recorded for 12 night hours in each animal and analog data were filtered (low cut: 1 Hz; high cut: 50 Hz) and sampled at 100 Hz with 12 bit precision. A training data set was generated by manually marking durations of HVS epochs in 16 representative animals selected from each group. Training data were presented to back-propagation networks with variable numbers of input, hidden and output cells. The performance of different types of networks was first examined with the training samples and then the best configuration was tested on novel sets of the EEG data. FFT transformation of EEG significantly improved the pattern recognition ability of the network. With the most effective configuration (16 input; 19 hidden; 1 output cells) the summed squared error dropped by 80% as compared with that of the initial random weights. When testing the network with new patterns the manual and automatic evaluations were compared quantitatively. HVSs which were detected properly by the network reached 93-99% of the manually marked HVS patterns, while falsely detected events (non-HVS, artifacts) varied between 18% and 40%. These findings demonstrate the utility of back-propagation networks in automatic recognition of EEG patterns.

Somatostatin release in rat neocortex during gamma-hydroxybutyrate-provoked seizures: Microdialysis combined with EEG recording

Gamma-hydroxybutyrate (GHB) was intracortically applied in two doses (first 10 and then 20 mg/ml) to awake Wistar rats using microdialysis. Simultaneously, EEG and the release of somatostatin-like immunoreactivity (SLI) were measured from the frontal cortex. Intracerebrally administered GHB induced cortical epileptogenic spikes, but not high voltage spindles (HVS) as reported after systemic administration, and seizures with myoclonic jerks and contraversive head movements. Compared to the basal level, GHB (10 mg/ml) initially increased the release of SLI (p < 0.05). However, when the frequency of spikes and seizures rose rapidly (p < 0.001), SLI release decreased significantly (p < 0.001). Minimum release of SLI occurred when seizures were most frequent (during perfusion with 20 mg/ml GHB), while after removal of the drug it rose above the basal level (p < 0.05). According to these results, intracortically applied GHB increases the release of SLI in the surrounding tissue. However, further exposure of GHB leads to a manifestation of epileptic spikes and seizures, during which the release of SLI is significantly attenuated. This suggests that release of somatostatin is affected during epileptic phenomena induced also by intracortical GHB application.

Excitotoxic lesions of the pedunculopontine tegmental nucleus of the rat. I. Comparison of the effects of various excitotoxins, with particular reference to the loss of immunohistochemically identified cholinergic neurons

The pedunculopontine tegmental nucleus (PPTg) has been shown to have cholinergic connections with the thalamus and basal ganglia. The ability of various doses of the excitotoxins (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) (AMPA), folate, ibotenate, kainate, N-methyl-D-aspartate (NMDA), quinolinate and quisqualate to make lesions in the PPTg was examined, with particular reference to their ability to destroy cholinergic neurons identified using choline acetyltransferase (ChAT) immunohistochemistry. All of the toxins induced convulsive activity on recovery from surgical anesthesia and all except folate made lesions in the PPTg and surrounding structures. The size of the lesions was computed following examination of Cresyl violet stained sections. The largest lesions were made by kainate = AMPA greater than NMDA = ibotenate greater than quisqualate = quinolinate. All of the toxins destroyed cholinergic neurons, higher doses producing greater loss than lower. The ratio of cholinergic cell loss to general neuronal loss (assessed by Cresyl violet staining) was also computed, revealing marked differences between the toxins. Statistical analysis showed that there were significant differences between excitotoxins in terms of this ratio, but these were accounted for by the low dose of quinolinate (24 nmol) producing a significantly greater ratio of damage (12.18:1) than every other toxin. (Next highest ratio: quisqualate 60 nmol, 6.22:1.) Between the other toxins (kainate, AMPA, ibotenate, quisqualate, NMDA and the high dose of quinolinate) there were no statistically significant differences. Intense calcium deposits (stained by Alizarin red) were found frequently and often defined the borders of the lesion. Tyrosine hydroxylase immunohistochemistry revealed axons running below and into the area of lesioned tissue suggesting strongly that fibers were undamaged by the lesions. We conclude that in the PPTg, different excitotoxins make discriminably different lesions, both quantitatively and qualitatively. Unlike excitotoxic lesions in the basal forebrain quinolinate, not quisqualate, made the most selective lesions of cholinergic neurons and, unlike excitotoxic lesions in the septal nuclei, non-myelinated fibers were spared by ibotenate. The implications of these data for research into brainstem mechanisms of Parkinson's disease are discussed.

Excitotoxic lesions of the pedunculopontine tegmental nucleus of the rat. II. Examination of eating and drinking, rotation, and reaching and grasping following unilateral ibotenate or quinolinate lesions

The pedunculopontine tegmental nucleus (PPTg) contains a population of cholinergic neurons thought to be part of the ascending reticular activating system, and non-cholinergic neurons. In the previous study it was shown that various excitotoxins made effective lesions of cholinergic neurons in the PPTg but that quinolinate made smaller lesions in the non-cholinergic population, making it more selective than any other excitotoxin. The purpose of the present experiment was, first, to make lesions of cholinergic neurons throughout the length of the PPTg by infusing toxin at two different sites within it; and second, to examine simple motor activities in rats bearing either quinolinate or ibotenate lesions of the PPTg, and contrast these with the deficits seen after 6-hydroxydopamine (6-OHDA) induced lesions of mesostriatal dopamine (DA)-containing neurons. Post-mortem examination was carried out using choline acetyltransferase (ChAT) and tyrosine hydroxylase (TOH) immunohistochemistry, and routine Nissl staining. Both quinolinate and ibotenate destroyed approximately 75% of ChAT-positive neurons in the PPTg, but damage to non-cholinergic neurons (assessed by Nissl staining) was twice as great following ibotenate as quinolinate. 6-OHDA induced almost complete lesions of mesostriatal DA neurons, assessed by TOH immunohistochemistry. DA depleted rats showed deficits in drinking and spilled more food in the first 2 weeks after surgery, and were unable to reach or grasp food pellets in the staircase test. They also showed strong ipsilateral turning in response to amphetamine and contralateral turning to apomorphine. Quinolinate lesioned rats had no eating or drinking impairment in the home cage but showed a reaching (though not grasping) disability in the staircase test. They had a mild ipsilateral bias following amphetamine. Ibotenate lesioned rats, despite having larger lesions than the quinolinate, showed no deficits in eating or drinking in the home cage, or reaching or grasping disabilities in the staircase test. They did have a mild contralateral bias in response to amphetamine. This dissociation of the effects of quinolinate and ibotenate lesions of the PPTg is consistent with the suggestion that the PPTg has two functionally distinct components, and is attributed to the differential lesion of non-cholinergic neurons by the two excitotoxins.

Nitric oxide controls oscillatory activity in thalamocortical neurons

Nitric oxide (NO) is considered a diffusible messenger involved in neuronal communication, although the post-synaptic target cells of NO action and the associated biological function in the CNS are still a matter of controversy. Within the discrete pattern of NO-synthesizing neurons in the brain, NO synthase is specifically colocalized with the cholinergic brain stem-thalamic system, which is thought to regulate the state-dependent activity of the thalamocortical circuit. Here we report evidence indicating that the release of NO onto thalamocortical neurons results in an alteration in voltage dependence of the hyperpolarization-activated cation conductance, probably mediated via the cGMP system. NO selectively dampens oscillatory neuronal activity, indicating a rapidly diffusing signaling mechanism that controls the functional state of the thalamocortical network.

The role of GABAB receptor activation in absence seizures of lethargic (lh/lh) mice

Lethargic (lh/lh) mice, which function as an animal model of absence seizures, have spontaneous seizures that have behavioral and electrographic features and anticonvulsant sensitivity similar to those of human absence seizures. Antagonists of the gamma-aminobutyric acidB (GABAB) receptor suppressed these seizures in lethargic mice, whereas agonists of GABAB receptors exacerbated them. Furthermore, GABAB receptor binding and synaptically evoked GABAB receptor-mediated inhibition of N-methyl-D-aspartate responses were selectively increased in lh/lh mice. Therefore, enhanced GABAB receptor-mediated synaptic responses may underlie absence seizures in lh/lh mice, and GABAB receptor antagonists hold promise as anticonvulsants for absence seizures.

Effects of chronic dietary choline on temporal discrimination of BN and WAG rats

Using rats of the inbred BN and WAG strain, we tested the hypothesis that chronic dietary choline supplementation would especially affect the timing behavior of BN rats because of their lower cholinergic activity and their poor performance in aversively motivated learning and memory tasks. An apparent effect of chronic choline supplementation (2.5 mg choline chloride per ml water) on DRL-8" responding was not confirmed in a second experiment when the choline concentration was doubled. WAG rats treated chronically with choline showed a poorer temporal discrimination performance on a DRL-16" schedule than untreated WAG rats. In contrast, choline supplementation never had an effect on the performance of BN rats. The results of the DRL-16" experiment provide partial support for a hypothesis proposed by Church and Meck that the remembered time of reinforcement is inversely related to the functional activity of brain cholinergic activity: acetylcholine precursor treatment increases memory storage speed, which results in an overestimation of the time elapsed. An alternative explanation, which takes into account the aberrant EEG activities of WAG rats, is also discussed.

Neurophysiological consequences of combined cholinergic and noradrenergic lesions

The present study investigates the effects of an anti-cholinesterase, tetrahydroaminoacridine (THA), on combined nucleus basalis (NB, quisqualic acid) and dorsal noradrenergic bundle (DNB, 60HDA) lesion-induced high-voltage spindle (HVS) activity. THA at 3 mg/kg, but not at 1 mg/kg, decreased HVS activity in NB- and DNB-lesioned rats. HVS activity in NB- and DNB-lesioned rats treated with THA at 6 mg/kg was lower than in saline-treated controls. In NB-lesioned rats subjected to an additional DNB lesioning, the HVS suppressing effect of THA at 3 mg/kg was decreased. The present results suggest that NB cholinergic and DNB noradrenergic systems interact in the regulation of HVS activity and that the efficacy of an anticholinesterase drug (THA) in reversing NB cholinergic lesion-induced thalamocortical activation deficit is decreased by combined DNB noradrenergic lesion.

Endogenous opioid peptides in brain and pituitary of rats with absence epilepsy

The level of opioid peptides in several brain areas and in the pituitary was estimated in WAG/Rij rats, which are considered to be a genetic animal model for human absence epilepsy. In comparison with three groups of non-epileptic controls, these epileptic rats had an elevated level of the proenkephalin-derived peptide Met-enkephalin-Arg6-Gly7-Leu8 in the mesencephalon and striatum, while the level of the prodynorphin-derived peptide alpha-neoendorphin was increased in the striatum and hippocampus. In addition various age- and/or strain-related changes in these peptide levels were found in the hippocampus, thalamus, striatum, frontal cortex and neurointermediate lobe of the pituitary. No difference in the hypothalamic beta-endorphin level were found between epileptic and non-epileptic rats, though strain- and/or age-related changes in the peptide content were detected in both lobes of the pituitary. The increased level of proenkephalin and prodynorphin opioid peptides in brain structures, essential for the appearance of spike-wave discharges, suggests that these opioid systems, but not proopiomelanocortin one, may play a role in absence epilepsy.

DSP-4, a noradrenergic neurotoxin, produces more severe biochemical and functional deficits in aged than young rats

The present study examines the effects of noradrenergic lesions (either DSP-4 i.p. or 6-hydroxydopamine (6-OHDA) into the dorsal noradrenergic bundle on biochemical (noradrenaline (NA), dopamine (DA), serotonin (5-HT) and choline acetyltransferase (ChAT) activity) and cortical EEG (quantitative EEG (qEEG) and high-voltage spindle (HVS)) activity in young and aged rats. Near complete 6-OHDA NA lesions, but not partial DSP-4 NA lesions, increased HVS activity in young rats. DSP-4 and 6-OHDA lesions produced no significant changes in the 5-HT or DA levels or in the ChAT activity in young rats. In some of the aged rats, DSP-4 produced similar biochemical and HVS effects, as it induced in young rats. In the remainder of the aged rats, NA levels were greatly and 5-HT levels slightly decreased. DA levels and ChAT activity were unaltered in either set of aged rats. HVS activity was increased only in that group of aged rats with the greatly lowered NA content. These results suggest that: (1) some of the aged rats are more sensitive to DSP-4 treatment than young adult rats; and (2) NA depletions have to be complete to produce an increase in HVS activity in young and aged rats.

Differences in spike-wave discharges in two rat selection lines characterized by opposite dopaminergic activities

In the present study, 48 h electroencephalographic recordings were made in order to examine the incidence and duration of spike-wave discharges in apomorphine-susceptible (APO-SUS) and apomorphine-unsusceptible (APO-UNSUS) lines of an outbred strain of Wistar rats. In comparison with APO-UNSUS rats APO-SUS rats showed significantly more spike-wave discharges, especially during the dark period: both the mesor and the amplitude of the optimal cosine fitted to the data were significantly increased, whereas neither the acrophase nor the period length (24 h) differed. It is suggested that both the relatively low dopaminergic activity of the nigrostriatal fibres and the relatively high dopaminergic activity of the mesolimbic fibres, i.e. two well characterized features of the APO-SUS rats, significantly contribute to the high incidence of spike-wave discharges in these APO-SUS rats.

Genetic absence epilepsy in rats from Strasbourg--a review

We have selected a strain of rats and designated it the Genetic Absence Epilepsy Rat from Strasbourg (GAERS). In this strain, 100% of the animals present recurrent generalized non-convulsive seizures characterized by bilateral and synchronous spike-and-wave discharges accompanied with behavioural arrest, staring and sometimes twitching of the vibrissae. Spontaneous SWD (7-11 cps, 300-1,000 microV, 0.5-75 sec) start and end abruptly on a normal background EEG. They usually occur at a mean frequency of 1.5 per min when the animals are in a state of quiet wakefulness. Drugs effective against absence seizures in humans (ethosuccimide, trimethadione, valproate, benzodiazepines) suppress the SWD dose-dependently, whereas drugs specific for convulsive or focal seizures (carbamazepine, phenytoin) are ineffective. SWD are increased by epileptogenic drugs inducing petit mal-like seizures, such as pentylenetetrazol, gamma-hydroxybutyrate, THIP and penicillin. Depth EEG recordings and lesion experiments show that SWD in GAERs depend on cortical and thalamic structures with a possible rhythmic triggering by the lateral thalamus. Most neurotransmitters are involved in the control of SWD (dopamine, noradrenaline, NMDA, acetylcholine), but GABA and gamma-hydroxybutyrate (GHB) seem to play a critical role. SWD are genetically determined with an autosomal dominant inheritance. The variable expression of SWD in offsprings from GAERS x control reciprocal crosses may be due to the existence of multiple genes. Neurophysiological, behavioural, pharmacological and genetic studies demonstrate that spontaneous SWD in GAERS fulfill all the requirements for an experimental model of absence epilepsy. As the mechanisms underlying absence epilepsy in humans are still unknown, the analysis of the genetic thalamocortical dysfunction in GAERS may be fruitful in investigations of the pathogenesis of generalized non-convulsive seizures.

Gammavinylgaba increases high voltage spindle activity in control and nucleus basalis-lesioned rats after sub-chronic treatment

The present study investigates the effects of gammavinylgaba (GVG, 4 days treatment at 50 and 200 mg/kg), a gabatransaminase inhibitor, on the high-voltage spindle (HVS) activity in control and nucleus basalis-lesioned rats. GVG treatment at 200 mg/kg, but not at 50 mg/kg increased HVS activity in controls. GVG greatly aggravated nucleus basalis magnocellularis (NBM) lesion-induced increase in HVS activity at 200 mg/kg, but not at 50 mg/kg. The present results demonstrate that partial NBM lesions increase the HVS inducing potency of GVG.

Arousal, performance and absence seizures in rats

Rats of the WAG/Rij strain show bilateral symmetrical spontaneous spike-wave discharges in the EEG, with clinical concomitants. The present experiment investigated whether, during a learning task, the number of discharges would be diminished compared to a period of rest. Additionally, it was investigated whether behavioural differences would be noticed within the task in trials with and without spike-wave discharges. The length of the post reinforcement pause in a fixed interval task was used as a performance index. Eleven rats were extensively trained to press a lever for food in a fixed (60 sec) interval task until a stable response pattern emerged: a post-reinforcement pause of about half the interval. Next, EEG electrodes were implanted and baseline EEGs were made, before and after the first and fifth test sessions. In addition, the behavior of the animals in the task was monitored when an EEG was recorded. During the task, a significantly smaller number of spike-wave discharges was found, compared to the preceding and succeeding baseline hours. This reduction is probably related to a higher level of vigilance during the task compared to the rest hour. Furthermore, the post-reinforcement pause was significantly enhanced in trials with spike-wave discharges compared to trials without discharges, indicating a clear change in performance. Both results are in agreement with what could be expected in patients with absence epilepsy and provide further evidence for the validation of the spike-wave discharges as genuine epileptic phenomena.

Effects of alpha 2-drugs and pilocarpine on the high-voltage spindle activity of young and aged control and DSP4-lesioned rats

The present study investigates the effects of alpha 2-drugs and pilocarpine on the neocortical high-voltage spindle (HVS) activity in young and aged control and DSP4-lesioned rats. DSP4 partially decreased cortical and thalamic noradrenaline levels, but had no effect on HVS activity. The alpha 2-adrenoceptor agonist guanfacine (0.004, 0.02, 0.1 mg/kg) increased HVS activity in young and aged control and DSP4-lesioned rats. Guanfacine produced a significantly smaller increase in HVS activity in aged rats. A combination of pilocarpine (3 mg/kg), a muscarinic agonist, and atipamezole (1 mg/kg), an alpha 2-adrenoceptor antagonist, suppressed HVS activity more effectively than either of the drugs alone in young or aged control and DSP4-lesioned rats. The present results demonstrate that 1) the alpha 2-adrenoceptor antagonist and muscarinic agonist interact in suppressing HVSs in noradrenergically lesioned young and aged rats; 2) alpha 2-adrenoceptor agonists produce a greater increase in HVS activity in young than aged rats; and 3) partial noradrenergic lesions do not affect the HVS-modulating effects of alpha 2-adrenoceptor active drugs in young or aged rats.

Low-frequency oscillatory activities intrinsic to rat and cat thalamocortical cells

1. Low-frequency membrane potential oscillations recorded intracellularly from thalamocortical (TC) cells of the rat and cat dorsal lateral geniculate nucleus (dLGN) and of the rat ventrobasal nucleus (VB) maintained in vitro were investigated. On the basis of their electrophysiological and pharmacological properties, four types of activity were distinguished and named: the pacemaker oscillations, the spindle-like oscillations, the 'very slow' oscillations and the 'N-methyl-D-aspartate' (NMDA) oscillations. 2. The pacemaker oscillations (95 out of 173 cells) consisted of rhythmic, large-amplitude (10-30 mV) depolarizations which occurred at a frequency of 1.8 +/- 0.3 Hz (range, 0.5-2.9 Hz) and could often give rise to single or a burst of action potentials. Pacemaker oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV, but in a given cell the upper and lower limits of this voltage range were separated by only 13.1 +/- 0.5 mV. Above -45 mV tonic firing consisting of single action potentials was seen in the cells showing this or the other types of low-frequency oscillations. 3. The spindle-like oscillations were observed in thirty-nine (out of 173) TC cells and consisted of rhythmic (2.1 +/- 0.3 Hz), large-amplitude depolarizations (and often associated burst firing) similar to the pacemaker oscillations but occurring in discrete periods every 5-25 s and lasting for 1.5-28 s. The spindle-like oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV and in two cells they were transformed into continuous pacemaker oscillations by depolarization of the membrane potential to -60 mV. 4. Pacemaker and spindle-like oscillations were unaffected by tetrodotoxin (TTX) or by selective blockade of NMDA, non-NMDA, GABAA, GABAB, nicotinic, muscarinic, alpha- and beta-noradrenergic receptors. 5. The 'very slow' oscillations consisted of a TTX-insensitive, slow hyperpolarization-depolarization sequence (5-15 mV in amplitude) which lasted up to 90 s and was observed in nine dLGN cells and in two VB cells. The pacemaker and the spindle-like oscillations were recorded in one cell each which also showed the 'very slow' oscillations. 6. The 'NMDA' oscillations were observed only in a 'Mg(2+)-free' medium (0 mM-Mg2+, 2-4 mM-Ca2+; 64 out of 72 cells) and consisted of large-amplitude (10-25 mV) depolarizations that did not occur at regular intervals and were intermixed with smaller depolarizations present on the baseline and on the falling phase of the larger ones.(ABSTRACT TRUNCATED AT 400 WORDS)