Modifications of local cerebral glucose utilization in thalamic structures following injection of a dopaminergic agonist in the nucleus accumbens--involvement in antiepileptic effects?

Dopaminergic transmission in the nucleus accumbens (NAcc) is implicated in different aspects of reward and motivational mechanisms. More recently, it has been suggested that this nucleus could also be involved in the modulation of generalized epileptic seizures. In particular, microinjection of dopaminergic agonists in the NAcc suppresses the occurrence of epileptic seizures in a model of absence seizures, the GAERS (generalized absence epileptic rats from Strasbourg). The aim of this study was to identify the structures involved in this effect. Local cerebral metabolic rates for glucose utilization (LCMRglc) were measured in different parts of the basal ganglia and output structures after apomorphine injection in the NAcc in GAERS and in the inbred non-epileptic rats (NE), concomitantly with seizure suppression. Apomorphine injection in the NAcc induced a significant increase of glucose intake in the anteromedial, mediodorsal and ventrolateral nuclei of the thalamus in NE rats, while no significant changes were observed in the basal ganglia structures (globus pallidus, subthalamic nucleus, substantia nigra). Furthermore, microinjections of muscimol (100 and 200 pmol/side) in the mediodorsal nucleus of the thalamus in GAERS rats suppressed seizures. These results suggest that the mediodorsal nucleus of the thalamus could be involved in absence seizures modulation. Along with data from the literature, our data suggest that this nucleus could participate in the control of the basal ganglia over generalized epileptic seizures.

Suppression of absence seizures by electrical and pharmacological activation of the caudal superior colliculus in a genetic model of absence epilepsy in the rat.

Activation of the superior colliculus has been shown to reproduce the antiepileptic effect of the inhibition of the substantia nigra reticulata. A circuit involving neurons of the caudal deep layers of the superior colliculus has been suggested to control brain stem convulsive seizures. The present study was designed to examine whether a similar circuit is also involved in the control of absence seizures. For this, activation of either the rostral or caudal parts of the deep and intermediate layers of the superior colliculus was applied in a genetic model of absence seizures in the rat (GAERS). Single-shock (5 s) electrical stimulation of the rostral and caudal superior colliculus interrupted ongoing spike-and-wave discharges at an intensity (antiepileptic threshold) significantly lower than the intensity inducing behavioral effects. At this intensity, no interruption of licking behavior was observed in water-deprived rats. Repeated stimulations (5 s on/5 s off) at the antiepileptic threshold reduced absence seizures only during the first 10 min. Bilateral microinjection of a GABA antagonist (picrotoxin, 33 pmol/side) significantly suppressed spike-and-wave discharges when applied in the caudal aspect of the superior colliculus. This antiepileptic effect appears dissociated from an anxiogenic effect, as tested in an elevated plus maze test. Finally, bilateral injection of picrotoxin (33 pmol/side) appeared more effective in the superficial and intermediate layers of the caudal superior colliculus, whereas such injections had only weak effects on absence seizures when applied in the deep layers. These results suggest that a specific population of neurons located in the intermediate and superficial layers of the caudal superior colliculus is involved in the inhibitory control of absence seizures. It may constitute an important relay for the control of absence seizures by the basal ganglia via the substantia nigra reticulata.

Inhibition of the substantia nigra suppresses absences and clonic seizures in audiogenic rats, but not tonic seizures: evidence for seizure specificity of the nigral control.

GABAergic inhibition of the substantia nigra pars reticulata has been shown to suppress seizures in most models of epilepsy involving forebrain networks, such as absences or clonic seizures. No such antiepileptic effects were observed, however, in genetically audiogenic rats exhibiting tonic seizures generated in the brainstem. This suggests a constitutive dysfunction of the nigral GABAergic neurotransmission in this strain of rat or a selective action of the nigral control on specific networks. In the present study, we first confirmed that bilateral injection of muscimol (700 pmol/side) in the substantia nigra had no effect in Wistar rats with audiogenic seizures (Wistar AS). [3H]Muscimol autoradiography suggested a 40% reduced density of GABA(A) receptors in the substantia nigra of Wistar AS, whereas no change was observed in the cortex and the superior colliculus (superficial and intermediate layers), as compared to control animals. In Wistar AS where 40 repetitions of audiogenic stimulations progressively induced generalised convulsive seizures with both tonic and clonic components, bilateral injection of muscimol (350 pmol/side) in the substantia nigra suppressed the clonic component but had no effect on tonic seizures. In hybrid rats issued from cross-breeding between Wistar AS and rats with spontaneous absence seizures, bilateral injection of muscimol (18 pmol/side) in the substantia nigra abolished cortical spike-and-wave discharges, but had no effect on tonic audiogenic seizures at doses up to 700 pmol/side. These results show that despite a decreased number of GABA(A) receptors in the substantia nigra, inhibition of this structure in Wistar AS still leads to inhibition of seizures involving forebrain structures. These results confirm that GABAergic inhibition of the substantia nigra has antiepileptic effects through the control of forebrain circuits. They suggest that this control mechanism has no inhibitory effect on circuits underlying audiogenic tonic seizures.

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.

Reciprocal positive transfer between kindling of audiogenic seizures and electrical kindling of inferior colliculus.

The behavioral and EEG concomitants of kindling produced by daily electrical stimulation of the inferior colliculus have been recorded in three series of Wistar rats: (1) non epileptic controls (NE), (2) rats susceptible to audiogenic seizures (AS), (3) acoustically susceptible rats with prior kindling of audiogenic seizures by repeated sound exposure (KAS). Repeated collicular stimulation produced behavioral and EEG changes which were similar in the AS and the NE rats. The tonic seizure without cortical discharges elicited by the first stimulation progressively changed into tonic-clonic seizures with sustained cortical EEG discharges after more than 20 stimulations. In the KAS group, the electrical collicular kindling was clearly accelerated: kindled tonic-clonic seizures and their EEG discharges already occurred after one to five electrical stimulations. Similarly, after completion of electrical collicular kindling in AS, sound stimulations immediately induced characteristic kindled audiogenic seizures. The immediate reciprocal positive transfer observed between kindling of audiogenic seizures and kindling of seizures induced by electrical stimulation of the inferior colliculus suggests that kindling of these two brain-stem seizures involves similar structures and mechanisms.

Positive transfer of audiogenic kindling to electrical hippocampal kindling in rats.

Audiogenic seizures in genetically susceptible rodents are provoked by intense acoustic stimulations which result in a tonic seizure associated with a short flattening of the EEG. These seizures have been shown to involve primarily brainstem structures. Daily exposure to sound for 30-40 days produced a permanent change in the evoked seizure with development of facial myoclonias, rearing and falling, or of tonic-clonic seizures accompanied by high amplitude cortical spike-and-wave discharges. Kindled audiogenic seizures appear similar to seizures kindled from amygdala or hippocampus, suggesting that repeated auditory stimulations cause a progressive propagation of the epileptic discharge toward limbic structures. To verify this hypothesis, the behavioral and EEG development of electrical hippocampal kindling has been studied in 7 non epileptic controls (NE), 8 acoustic susceptible (AS), and 8 audiogenic kindled rats (KAS). The behavioral and EEG development of the electrical hippocampal kindling was similar in the AS and the NE rats. However, 2 animals in the AS group but no controls exhibited behavioral running and bouncing during the course of hippocampal kindling. In the KAS group, the hippocampal kindling was clearly facilitated as compared to NE and AS: behavioral stage greater than or equal to 5 was reached in a mean of 4 stimulations in KAS versus 30 and 22 stimulations respectively in NE and AS groups. This positive transfer phenomenon suggests that during kindling of audiogenic seizures, epileptic discharge spreads from the brainstem to the forebrain and progressively involves the hippocampus.

Involvement of intrathalamic GABAB neurotransmission in the control of absence seizures in the rat.

The role of intrathalamic GABAB neurotransmission in the control of absence seizures was investigated. In rats with genetic absence epilepsy, bilateral injections of R-baclofen (50, 100 and 200 ng/side), a selective GABAB receptor agonist, into the specific relay nuclei and the reticular nuclei of the thalamus increased spontaneous spike and wave discharges in a dose-dependent fashion, whereas injections of a GABAB antagonist CGP 35,348 (1, 2.5 and 5 micrograms/side) into the same sites decreased these seizures dose-dependently. The effect of R-baclofen (200 ng/side) on spike and wave discharges could be blocked by a subsequent injection of CGP 35,348 (1 microgram/side) at the same site. Injections of R-baclofen (200 ng) or CGP 35,348 (5 micrograms) into the midline thalamus had no effect on these seizures. In non-epileptic rats, bilateral injections of R-baclofen (1 microgram/side) into the specific relay nuclei induced synchronized rhythmic oscillations on the cortical electroencephalogram. The results suggest that GABAB receptors in the ventrolateral thalamus and in the reticular nuclei are involved in an oscillatory activity which underlies the rhythmic spike and wave discharges recorded during spontaneous generalized non-convulsive seizures.

The inhibitory control of the substantia nigra over generalized non-convulsive seizures in the rat.

A system exerting inhibitory control over generalized epilepsies and involving neurons from the substantia nigra has been described by several authors in experimental models of convulsive seizures. In the present study, the existence of such a control system governing absence epilepsy was investigated using models of non-convulsive seizures in the rat. Activation of the GABAergic neurotransmission within the substantia nigra by local injection of GABA agonists (muscimol, THIP) or an inhibitor of GABA degradation (gamma-vinyl GABA) suppresses generalized non convulsive seizures, whether they are genetically determined or induced by systemic injections of gamma-butyrolactone (100 and 200 mg/kg), pentylenetetrazole (20 mg/kg) or THIP (7.5 mg/kg). The ascending dopaminergic nigral output or the GABAergic fibres to the ventromedial thalamus are not critically involved in this control system. By contrast, the GABAergic nigro-collicular pathway appears crucial: bilateral lesion of the superior colliculus abolishes the anti-epileptic effects of intranigral injection of muscimol and blockade of the GABAergic transmission within the superior colliculus results in a suppression of generalized non-convulsive seizures. Finally, activation of collicular cell bodies by low doses of kainic acid significantly suppresses absence seizures. These results suggest the existence of a control system inhibiting generalized non-convulsive seizures which is activated by the release of the tonic inhibition exerted by the nigral GABAergic fibres on collicular neurons. The similarities between this system and the control system described for convulsive seizures are discussed.

Intrathalamic injections of gamma-hydroxybutyric acid increase genetic absence seizures in rats.

The effects of intrathalamic injections of gamma-hydroxybutyric acid (GHB) and of NCS 382 85, a specific antagonist of GHB receptors, were evaluated in rats with spontaneous generalized absence epilepsy. Bilateral injections of GHB (25 and 50 micrograms/side) into the mediolateral thalamus increased spontaneous spike and wave discharges (SWD) in a dose-dependent fashion. This effect was suppressed by administration of NCS 382 85 (50 micrograms/side). Bilateral injection of NCS 382 85 alone (50 and 100 micrograms/side) into the same sites had no effect on SWD duration. Injection of GHB or NCS 382 85 into the midline thalamus and the area of reticular nuclei did not modify the SWD. These data suggest that GHB receptors in the mediolateral thalamus may be involved in the control of spontaneous SWD in this rat model of petit mal epilepsy.

Evidence for a critical role of GABAergic transmission within the thalamus in the genesis and control of absence seizures in the rat.

The involvement of GABAergic transmission within the thalamus in the generation and control of spike and wave discharges (SWD) in generalized non-convulsive or absence epilepsy was studied in rats with spontaneous SWD and in non-epileptic rats. In epileptic rats, bilateral injections of gamma-vinyl GABA (GVG, 10 micrograms/side) or muscimol (10 ng/side) into the medial part of the ventral lateral thalamus, i.e. the specific relay nuclei, significantly increased spontaneous cortical SWD whereas similar injections into the most lateral part of the thalamus, i.e. the area of the reticular nuclei, significantly suppressed these seizures. Injections of GVG (20 micrograms) or muscimol (20 ng) into the midline thalamus had no direct effect on the spontaneous SWD. In non-epileptic rats, injections of GVG (25 micrograms/side) or muscimol (100 ng/side) into the thalamic relay nuclei produced short SWD on the cortical EEG. These results suggest that GABAergic neurons in the reticular nuclei and their projections to the specific relay nuclei of the thalamus are involved in the elicitation and control of generalized non-convulsive seizures.

Mapping of spontaneous spike and wave discharges in Wistar rats with genetic generalized non-convulsive epilepsy.

Electrical activity was recorded in different parts of the brain in Wistar rats from a strain with genetic generalized non-convulsive epilepsy (GNCE or absence epilepsy). Movable bipolar electrodes were lowered stereotaxically by 1 mm steps into the brain in immobilized animals. Spontaneous spike and wave discharges (SWD) of the largest amplitude were recorded in the cortex and in lateral nuclei of the thalamus where they appeared occasionally to precede. Smaller amplitude SWD were recorded in the striatum, hypothalamus, tegmentum and substantia nigra. No SWD were recorded in limbic structures. Partial limbic seizures induced by the introduction of the electrode did not interfere with occurrence of cortical SWD. These results confirm the primacy of thalamocortical involvement in SWD of GNCE. The absence of spread to limbic structures and the implication of a precisely limited substrate in GNCE accounts for the clinical and pharmacological specificity of this particular kind of epilepsy.

Potentiation of gamma-vinyl GABA (vigabatrin) effects by glycine.

Vigabatrin, because of its ability to increase brain GABA concentration, acts as an anticonvulsant on convulsive epileptic seizures and increases seizures in generalized non-convulsive epilepsy. Next to GABA, glycine is one of the most important inhibitory neurotransmitter amino acids. We studied the influence of glycine on the effects of treatment with vigabatrin in two rat models of generalized convulsive seizures and a rat model of spontaneous generalized non-convulsive seizures. Glycine (750 mg/kg i.p.) or vigabatrin (200 mg/kg i.p.), when given alone, provided partial protection against convulsive seizures, while combined treatment with the two drugs significantly suppressed the convulsive seizures in both the mercaptopropionic acid (MPA)-induced seizures and audiogenic seizures. In contrast to the response to treatment with each individual drug, the drug combination nearly abolished the appearance of isolated spikes on the EEG in MPA seizures. On the other hand, glycine also enhanced the aggravating effect of vigabatrin on spontaneous spike and wave discharges in a rat model of genetic absence epilepsy, whereas glycine or vigabatrin alone, at the above doses, produced only a slight, non-significant increase in spontaneous spike and wave discharges. The GABA-glycine interaction is the first example of a synergistic action of two inhibitory neurotransmitters on seizure-related pathological discharges.

Audiogenic seizures in Wistar rats before and after repeated auditory stimuli: clinical, pharmacological, and electroencephalographic studies.

A strain of Wistar rats was inbred for susceptibility to audiogenic seizures characterized by one or two wild running fits followed by tonic dorsiflexion with open mouth and then a catatonic state. During the tonic phase, the cortical EEG was flat for 1 to 2 sec, then changed to a slow, regular low-amplitude discharge, 9 to 12 c/s, for 25 to 60 sec. In these rats exposed to 40 daily 90-sec auditory stimuli, behavior and EEG changed. The wild running became disorganized by myoclonic jerks of the limbs and body. In some animals, the tonic extension disappeared and a myoclonic seizure developed progressively, with facial and forelimb clonus, and rearing and falling. In others, the tonic phase was followed by a generalized clonic phase. The EEG during the myoclonic and tonic-clonic seizures showed high-amplitude rhythmic spikes, polyspikes and spike-waves, 1 to 10 c/s, for 40 to 120 sec, often outlasting the sound stimulus. The effects of ethosuximide, carbamazepine and phenytoin were the same on primary and modified audiogenic seizures. The progressive behavioral and EEG modifications of audiogenic seizures following repeated auditory stimuli suggest that kindling had developed, the seizures being propagated from the brain stem to forebrain structures.

Kindling of audiogenic seizures in the rat.

A strain of Wistar rats was inbred in our laboratory for its susceptibility to sound. The seizures are characterized by one or two wild running fits which terminate in a tonic dorsiflexion with open mouth, followed by a catatonic state. During the tonic phase of the seizure, the cortical EEG is flattened for 2 to 3 s. Then, a slow and regular low-voltage (9-12 c/s) activity is observed during 40 to 60 s. When these animals are submitted to daily sound-stimulations, the behavioral as well as the EEG manifestations of the audiogenic seizures change progressively. After 5 to 30 exposures, the wild running becomes disorganized by occurrence of myoclonic jerks of the limbs and the body. In some animals, the tonic extension disappears and a myoclonic seizure develops progressively with facial and forelimb clonus, rearing and falling. In other animals, the tonic phase still occurs and is followed by a generalized clonic phase. During both the myoclonic and the tonicoclonic seizures, rhythmic spikes, polyspikes and spike and waves of high amplitude (1-10 c/s) during 40 to 120 s are observed on EEG recordings. These EEG modifications often outlast the sound stimulation. The pharmacological reactivity in rats exposed to single or repeated audiogenic seizures is similar: phenytoin and carbamazepine suppress both kinds of seizures at low doses whereas ethosuximide is efficacious only at high doses. In order to know whether the repeated exposure to sound or the repetition of seizures are responsible of the observed changes in audiogenic seizures, animals susceptible to sound were exposed daily to the seizure-inducing sound after previous injection of Diazepam, which prevented them from convulsing. On the other hand, sound susceptible animals were injected daily with a dose of PTZ inducing one or several convulsions without exposure to sound. None of these treatments ever facilitated the development of kindled audiogenic seizures. The progressive modification of behavioral and EEG modifications occurring when audiogenic seizures are repeated suggests that kindling has developed, the seizure extending from the brainstem to forebrain structures.

Kindling of audiogenic seizures in Wistar rats: an EEG study.

The EEG of 20 Wistar rats inbred for audiogenic seizures was recorded during 40 daily auditory stimuli 90 s long. The first stimuli provoked wild running, with no cortical EEG abnormality, and then a tonic phase with a characteristic EEG of a brief flat trace 2 to 3 s long followed by low-amplitude regular activity, 10 to 12 c/s, lasting 40 to 60 s. The lack of paroxysmal EEG patterns suggests that the cortex plays only a minor role in audiogenic seizure development. After 5 to 15 daily stimuli, the EEG during the running period exhibited brief spike and spike-wave discharges preceding the EEG pattern of the tonic phase. After a few more daily stimuli these paroxysmal discharges progressively increased in amplitude and duration, overlapping with the regular activity of the tonic phase. After 20 to 30 stimuli, only high-amplitude spikes and spike-waves, 1 to 10 c/s, were seen for 40 to 120 s. The modified EEG persisted 2 to 4 months after daily stimulation was discontinued. Thus, with stimulus repetition, a paroxysmal discharge progressively involved cortical structures. These data suggest that repetition of audiogenic seizures induced a phenomenon related to kindling in Wistar rats susceptible to sound-induced epilepsy.

Spontaneous spike and wave discharges in thalamus and cortex in a rat model of genetic petit mal-like seizures.

In an inbred strain of Wistar rats, spontaneous spike and wave discharges (8 to 10 c/s) appeared regularly on the EEG during quiet wakefulness and were accompanied by an arrest of behavioral activity associated with vibrissal and facial myoclonia. These seizures were recorded over the entire neocortex, but predominantly in the frontoparietal cortex. Subcortical bipolar recordings in chronic preparations showed that the lateral thalamic nuclei were greatly involved in these discharges: high-voltage spike and waves always appeared either simultaneously with, or slightly before the cortical discharges. In some cases, thalamic discharges were not accompanied by cortical discharges. No discharges were recorded in medial thalamic nuclei, in the cingulate cortex, or in the hippocampus. These results confirm the thalamocortical prevalence in the development of these rats' petit mal-like seizures, with a possible driving from thalamic nuclei.

Antiepileptic drug evaluation in a new animal model: spontaneous petit mal epilepsy in the rat.

One-third of Wistar rats bred in our laboratory present recurrent seizures whose EEG and clinical symptomatology resemble those of human petit mal. Bilateral cortical synchronous spike- and wave discharges (7-11 c/s; 200-600 microV, lasting 0.5 to 40 s) accompany behavioral arrest and are associated frequently with facial myoclonia. These seizures, observed as long as the animals survive, appear spontaneously and seem to be unrelated to surgical procedures. Antiepileptics in common clinical use were tested. Ethosuximide (greater than 12.5 mg/kg), diazepam (greater than 0.5 mg/kg), trimethadione and sodium valproate (greater than 50 mg/kg) suppressed these discharges in a dose related manner. Carbamazepine and phenytoin were ineffective or aggravated the seizures. Phenobarbital, effective at 2.5 to 10 mg/kg, was ineffective at 20 mg/kg. The similar effects of these antiepileptics on both the rats' seizures and human petit mal confirm the hypothesis that this phenomenon constitutes a valid pharmacological model of petit mal epilepsy. Its predictive value appears to be superior to that of other currently used models.