Effects of acute cerebellectomy on maximal electroshock seizures and anticonvulsant efficacy of diazepam in the rat

Cortical and subcortical networks in human secondarily generalized tonic-clonic seizures

Generalized tonic-clonic seizures are among the most dramatic physiological events in the nervous system. The brain regions involved during partial seizures with secondary generalization have not been thoroughly investigated in humans. We used single photon emission computed tomography (SPECT) to image cerebral blood flow (CBF) changes in 59 secondarily generalized seizures from 53 patients. Images were analysed using statistical parametric mapping to detect cortical and subcortical regions most commonly affected in three different time periods: (i) during the partial seizure phase prior to generalization; (ii) during the generalization period; and (iii) post-ictally. We found that in the pre-generalization period, there were focal CBF increases in the temporal lobe on group analysis, reflecting the most common region of partial seizure onset. During generalization, individual patients had focal CBF increases in variable regions of the cerebral cortex. Group analysis during generalization revealed that the most consistent increase occurred in the superior medial cerebellum, thalamus and basal ganglia. Post-ictally, there was a marked progressive CBF increase in the cerebellum which spread to involve the bilateral lateral cerebellar hemispheres, as well as CBF increases in the midbrain and basal ganglia. CBF decreases were seen in the fronto-parietal association cortex, precuneus and cingulate gyrus during and following seizures, similar to the 'default mode' regions reported previously to show decreased activity in seizures and in normal behavioural tasks. Analysis of patient behaviour during and following seizures showed impaired consciousness at the time of SPECT tracer injections. Correlation analysis across patients demonstrated that cerebellar CBF increases were related to increases in the upper brainstem and thalamus, and to decreases in the fronto-parietal association cortex. These results reveal a network of cortical and subcortical structures that are most consistently involved in secondarily generalized tonic-clonic seizures. Abnormal increased activity in subcortical structures (cerebellum, basal ganglia, brainstem and thalamus), along with decreased activity in the association cortex may be crucial for motor manifestations and for impaired consciousness in tonic-clonic seizures. Understanding the networks involved in generalized tonic-clonic seizures can provide insights into mechanisms of behavioural changes, and may elucidate targets for improved therapies.

Cerebellar and thalamic stimulation treatment for epilepsy

The present chapter describes the most important available experimental and clinical evidence on the role of electrical stimulation of the cerebellum or the thalamus in the control of epilepsy. Cerebellum serves as an integrator of sensory information and regulator of motor coordinating and training. The sole output of the cerebellum is inhibitory Purkinje cell projections to deep cerebellar nuclei in the brainstem. Cerebellar stimulation in animal models of epilepsy has given mixed results. Nevertheless, more than 130 epileptic patients have been subjected to cerebellar stimulation and the results from uncontrolled studies have been encouraging. The anterior thalamic nucleus (ATN) is part of the Papez circuit, a group of limbic structures with demonstrated role in epilepsy. The centromedian thalamic nucleus (CMN) is considered part of the thalamic reticular system. Stimulation of either of these nuclei in experimental animals has been associated with considerable antiepileptic effects. On the basis of the research evidence, numerous studies have been done on humans, which gave promising results. Currently, a multicenter trial on stimulation of the ATN, the SANTE trial is in progress in the USA. On the basis of the reported studies, the authors aim to provide insights into how the electrical stimulation of the above structures exerts an antiepileptic effect and also provide suggestions regarding the future progress in this field.

Causes and consequences of pathogenic processes in evolution: implications from experimental epilepsy in animals

Examples from experimental epilepsy in animals are used to illustrate the view that a crucial role of the transfer of mechanisms from compensatory into pathogenic (e.g. lethal ones in the course of a disease), is played by the power of pathologic stimuli. In the genesis of epilepsy it is suggested that a critical increase of endogenous factors may underlie the conversion of the absence form of epilepsy into a generalized self-supporting form. The ability to precipitate endogenous self-augmenting mechanisms of diseases may have increased in the course of evolution. The lethal result of a serious pathogenic process leads to the suggestion that organisms cope with the disease by dying. This prevents spreading of the putative infectious disease within the population. This mechanism of disease aggravation could play a role in the survival of the species and in further evolutionary progress. This may explain why certain species may have survived in evolution and supports the theory of synthetic evolution.

Differential spatial patterns of Fos induction following generalized clonic and generalized tonic seizures

The expression of generalized clonic and generalized tonic seizures has been suggested to result from the activation of different and independent neuronal circuits. Using the induction of the c-fos protein (Fos) as a marker of neuronal activity, we identified brain structures that are differentially associated with the expression of electroconvulsive shock-induced generalized clonic and generalized tonic seizures. Expression of either seizure phenotype resulted in a similar bilaterally symmetrical increase in Fos immunoreactivity in many forebrain structures, including the bed nucleus of the stria terminalis, hippocampal dentate gyrus, amygdala, and piriform cortex, compared to controls. However, following tonic hindlimb extension (THE), the degree of labeling in specific thalamic, hypothalamic, and brain stem areas was significantly greater than that of either controls or animals exhibiting clonic seizures. While a greater number of neurons in the hypothalamus (e.g., ventromedial nucleus), subparafascicular thalamic nucleus, peripeduncular area, deep medial superior colliculus, dorsal and lateral central gray, and paralemniscal nuclei were robustly labeled following THE, noticeably fewer cells were immunoreactive following face and forelimb clonic seizure behaviors. These differences were also found to be independent of the stimulus magnitude. In animals stimulated with the same current intensity but expressing either of the two seizure phenotypes, the pattern of Fos induction was consistent with the seizure phenotype expressed. These results demonstrate that specific subsets of neurons are differentially activated following the expression of different generalized seizure behaviors and that activity in discrete mesencephalic and diencephalic structures is more frequently associated with the expression of generalized tonic seizures than with the expression of generalized clonic seizures.

Interictal cerebral metabolic levels in Wistar rats sensitive to audiogenic seizures

In the present study, we compared interictal local cerebral metabolic rates for glucose (LCMRglcs) in a strain of audiogenic rats (Wistar AS) selected in our laboratory to interictal LCMRglcs in a strain of control non-epileptic (NE) rats. Two groups of Wistar AS were studied, one group exposed to a single audiogenic seizure and one group of kindled rats exposed to 40 daily repetitive seizures. Control NE animals were exposed to a single sound exposure which did not induce any behavioral disturbance. Interictal LCMRglcs were measured by the quantitative autoradiographic [14C]2-deoxyglucose technique 5 days after the last sound exposure. LCMRglcs were similar in the three groups of rats in 80% of the structures. Compared to the control NE strain, interictal metabolic levels were mainly decreased in auditory structures of Wistar AS, either naive or kindled, thus confirming auditory impairment in audiogenic animals. LCMRglcs were increased over control levels in both groups of Wistar AS in cerebellar regions. This increase of cerebellar functional activity in Wistar AS compared to control NE rats might reflect an increased cerebellar input which, together with auditory impairment, may facilitate the induction of seizure activity in Wistar AS. Finally, there was no difference between the interictal cerebral metabolic level of naive and kindled Wistar AS, except in the cerebellar dentate nucleus where LCMRglc was significantly higher in kindled than in naive animals.

Regional distribution of the anticonvulsant and behavioural effects of muscimol injected into the substantia nigra of rats

Previous anatomical investigations have reported a direct projection from substantia nigra pars lateralis to the dorsal midbrain anticonvulsant zone. The present study tested the hypothesis that the anticonvulsant properties of nigral inhibition previously attributed to substantia nigra pars reticulata were, in fact, due to the suppression of neural activity in the adjacent pars lateralis. Using the electroshock model of epilepsy, a systematic map of the anticonvulsant effects of bilateral injections of muscimol (60 ng/0.5 mu l per side) into different parts of substantia nigra was constructed. Electroshock (1 s of 40 mA 50 Hz AC) was administered via ear-clip electrodes 5 or 60 min following injections of muscimol, or 60 min after control injections of saline. To provide insight into the functional mechanisms whereby nigral inhibition might suppress tonic seizures the behavioural effects elicited by muscimol were also noted. No evidence supporting the experimental prediction was found. The most sensitive region of substantia nigra for suppressing tonic hindlimb extension was caudal pars reticulata. These data indicate a serious mismatch between the results of microinjection mapping studies and underlying patterns of anatomical connectivity. The behavioural reaction most closely associated with tonic seizure suppression was stereotyped locomotion; both were obtained maximally from caudal pars reticulata. Rostral substantia nigra was associated more with oral stereotypy, while a raised head position was observed at lateral injection sites and a lowered positioning of the head at medial locations. These data suggest that the rat substantia nigra may contain a functional organization based on a form of somatomotor topography. This organization may influence which part of the substantia nigra is most effective in suppressing seizures expressed by different muscle groups of the body.

The dorsal midbrain anticonvulsant zone--III. Effects of efferent pathway transections on suppression of electroshock seizures and defence-like reactions produced by local injections of bicuculline

Having provided an anatomical description of the efferent projections of the dorsal midbrain anticonvulsant zone [Shehab S. et al. (1995) Neuroscience 65, 681-695], our purpose in the present study was to establish which outputs from this region are responsible for mediating the anticonvulsant and behavioural properties of dorsal midbrain activation. The ability of unilateral injections of bicuculline into the dorsal midbrain anticonvulsant zone to suppress tonic hindlimb extension in the electroshock model of epilepsy was tested before and after three different knife cuts: (i) a transection of ipsilateral descending projections on the same side as the injection of bicuculline; (ii) an identical cut except it was placed contralateral to the injection; (iii) a cut which transected rostral projecting fibres from the dorsal midbrain anticonvulsant zone including most ipsilateral ascending and crossed descending projections. A fourth group of operated control animals was included to establish a baseline for the schedule of repeated testing. Qualitative observations of behaviour were taken immediately prior to administration of the electroshocks. Unilateral transection of ipsilateral descending efferents prevented the suppression of electroshock-induced hindlimb extension by injections of bicuculline into the dorsal midbrain anticonvulsant zone on the same side of the brain. Both the control cuts on the opposite side of the brain and the rostral cuts were ineffective. Transection of the ipsilateral descending projection on the same side as the injection of bicuculline also reduced the incidence of defensive reactions induced by the GABA antagonist, including explosive motor behaviour, oral attack and vocalization. Damage to this projection on the opposite side had little effect on the expression of behavioural reactions, neither did transection of the ascending efferents. These data suggest that ipsilateral descending efferents are critical for the suppression of electroshock-induced extension of the hindlimbs and the expression of defensive reactions elicited by activation of the dorsal midbrain.

Role of the fastigial nucleus in generalized seizures as demonstrated by GABA agonist microinjections

The cerebellum is electrically and metabolically active during seizures. Numerous studies have also shown that cerebellar electrical stimulation and lesions of the cerebellar cortex or nuclei influence seizure threshold, but there are significant contradictions, with different effects observed even in investigations using the same species and similar seizure types and experimental manipulations. Discrete intracerebral microinjection of neuroactive agents has been used to characterize the way in which other brain regions control seizures, but has not been applied to the cerebellar systems. This approach has advantages because effects are restricted to specific receptors and spare passing axons; experimental variables also can be simply specified and reproduced. We used this method to characterize the role of the cerebellar nuclei in seizures and to determine if observed effects could be reproduced with different agents at different doses. Effects of bilateral control microinjections in the fastigial (medial) cerebellar nucleus were compared with different doses of the GABAA agonist piperidine-4-sulfonic acid and the GABAB agonist (-)baclofen (Bf). Soon after injection, the animals were ataxic. After 4 min, seizures were induced by timed continuous intravenous (i.v.) bicuculline (BIC) infusion. Both GABA agonists produced significant reductions in myoclonic, clonic, and tonic seizure thresholds. Injections just dorsal or anterior to this nucleus and bilateral dentate (lateral) nucleus injections had little effect on seizures. These results demonstrate that the cerebellar system does control seizures, but does not provide support for the early concept that cerebellar stimulation and systemic phenytoin block seizures through inhibition of cerebellar nuclei secondary to Purkinje cell activation.

The dendritic dopamine projection of the substantia nigra: phenotypic denominator of weaver gene action in hetero- and homozygosity

While cerebellar granule cell migration and survival are affected by the weaver (wv) mutation both in the heterozygous and homozygous states, the dopamine (DA) deficit of the nigrostriatal projection has been shown to involve only midbrain DA cell bodies and nigrostriatal DA axons of homozygous mutants. We have identified a cellular site which is defective in the mesencephalic DA system of mice both heterozygous and homozygous for the wv gene. That deficit involves the dendritic DA projection which extends from the substantia nigra pars compacta (SNc) into the pars reticulata (SNr). In the midbrain of heterozygotes, dopaminergic dendrites are reduced by 60% at 20 days of age, when DA neurone number in the midbrain, DA content in the neostriatum and pattern of synaptic connectivity of nigrostriatal axon terminals are normal. At the same age, the deficit of dopaminergic dendrites in the SNr of homozygotes (76%) is disproportionate to the loss of DA cell bodies (42%). These findings: (a) may provide clues to the aetiopathogenetic mechanisms of wv gene operation; and (b) may explain the generalised convulsions intermittently manifested by weaver heterozygotes, as the SN has been implicated in the pathophysiology of experimental seizures.

Functional anatomy of pentylenetetrazol and electroshock seizures in the rat brainstem

The ability of discrete brainstem injections of gamma-vinyl-gamma-aminobutyric acid (GVG), an irreversible inhibitor of gamma-aminobutyric acid transaminase, to prevent pentylenetetrazol (PTZ) seizures and maximal electroshock seizures (MES) was studied and compared in rats. PTZ seizures were prevented by GVG injections in the anterior thalamus, the caudal hypothalamus, the superior colliculus, cerebellar nuclei, and in a large area of the medial medullary, pontine, and mesencephalic tegmentum encompassing the vestibular nuclei, the reticular formation, and portions of the central gray. GVG injections in the substantia nigra did not protect against PTZ seizures. In contrast, tonic hindlimb extension in MES was prevented consistently by injections in the substantia nigra. A minority of injections in the vestibular nuclei, cerebellar nuclei, and parts of the reticular formation also protected against tonic hindlimb extension of MES. These results indicate a striking difference in the functional anatomy of PTZ-induced seizures and MES. PTZ seizures appear to be mediated by an extensive system involving the reticular formation, diencephalic regions in the vicinity of the anterior medial thalamus and caudal hypothalamus, and bulbar regions which give rise to descending motor pathways to the spinal cord. In contrast to PTZ seizures, MES appears to be mediated by a different neuroanatomical substrate with the present data implicating only the substantia nigra definitely in that process.

Effect of inferior olive lesion on seizure threshold in the rat

Cerebellar stimulation has been associated with anticonvulsant activity in several experimental seizure models. We examined the effect of destruction of cerebellar climbing fibers, by systemic administration of 3-acetylpyridine (3AP) or electrothermic lesion of the inferior olive, on seizures produced by various chemical convulsants in rats. We found that inferior olive lesioned rats had lower threshold to seizures induced by strychnine and brucine, both glycine antagonists. The dose response curve for strychnine seizure was shifted 2.5 times to the left in 3AP lesioned rats. No difference in seizure threshold was seen when picrotoxin, bicuculline or pentylenetetrazole PTZ) were used to produce seizures. Abnormal motor behavior (AMB) including myoclonus, backward movement and hyperextension, produced by all of the convulsants tested, was significantly aggravated in 3AP pretreated rats. The inferior olive-climbing fiber projection to the cerebellum appears to modulate seizures induced by inhibition of glycinergic neurotransmission.

Neuroanatomical localization of structures responsible for seizures in the GEPR: lesion studies

Identification of the neural substrates subserving audiogenic convulsions in the GEPR is an important task and while it is not yet complete, many laboratories employing various techniques have contributed importantly to our current understanding. The present review focuses on the use of lesions to identify the neural substrates of audiogenic convulsions. Lesions in brain stem nuclei appear to have a much greater ability to attenuate audiogenic convulsions than do forebrain lesions. In fact, some forebrain lesions (dorsal hippocampus, caudate, intralaminar thalamic nuclei) appear to enhance the severity of audiogenic seizures. On the other hand, bilateral lesions in the inferior colliculus (IC) have been shown to completely abolish audiogenic convulsions, while lesions in the pontine reticular formation (PRF nucleus) abolish all aspects except the running episode suggesting that these two brain stem structures are important neural substrates involved in the expression of audiogenic convulsions. Large bilateral lesions of the substantia nigra also appear to attenuate audiogenic convulsions. The effect of lesions on audiogenic convulsions is basically similar to their effect on other generalized seizure models and the data appear to support the hypothesis that there are two anatomical systems involved in the expression of all generalized convulsions: a forebrain system responsible for the expression of face and forelimb clonus; and a brain stem system responsible in the expression of running-bouncing clonus and tonus.

Age related increased threshold for electroshock seizure in BDF1 mice

The thresholds for inducing the minimal and maximal electroshock seizures were examined in relation to age in BDF1 mice of both sexes. The 50 percent effective intensities for the maximal electroshock seizure (tonic hindlimb extensor component) were lowest in the youngest age groups (6-month-old) for both male (10.68 mA) and female (9.18 mA) animals. The threshold increased with age and became significantly higher at 24 months (14.00 mA, 12.70 mA for male and female mice respectively). There was also a further increase in threshold at 30 months for both sexes. Similarly, the threshold for inducing the minimal seizure also increased with age but the differences in mean threshold levels between the youngest and oldest groups were much smaller in comparison to the maximal seizure. It was concluded that the threshold for inducing electroshock seizures significantly increases with age in mice of both sexes.

Effect of midbrain and pontine tegmental lesions on the maximal electroshock seizure pattern in rats

A bilaterally induced mechanical lesion of the midbrain was highly effective in abolishing the hindlimb extensor (HLE) component of the maximal electroshock seizures (MES) in rats. Although these lesions produced damage to a variety of midbrain structures, correlations between different lesion placements and effects in the MES test provided evidence that damage to superior cerebellar peduncle (PCS) and/or reticular formation (RF) was responsible for inhibition of hindlimb extension. Moreover, discretely placed electrolytic lesions disrupting either the PCS or the RF were found to abolish the hindlimb extensor component of the MES test. These findings are consistent with the work of other investigators showing that total cerebellectomy abolishes the HLE component of MES and suggest that activity in the cerebellum and the midbrain reticular formation plays a major role in regulating the tonic phase of electroshock induced seizures.

Antagonism of experimentally induced tonic seizures following a lesion in the midbrain tegmentum

Midbrain tegmental lesions, which prevent the hindlimb extensor (HLE) component of maximal electroshock seizures (MES), were found to have no effect on the electroshock, flurothyl, or pentylenetetrazol (PTZ) seizures thresholds. However, these lesions were found to antagonize the HLE component of the maximal PTZ seizure, and to elevate the threshold for electroshock induced tonic flexion. These findings suggest that lesion of the midbrain tegmentum involving the superior cerebellar peduncle and/or the midbrain reticular formation antagonize the tonic component of generalized seizures, but have little or no effect on the clonic component.

Elevation of seizure thresholds: a comparison of cerebellar stimulation, phenobarbital, and diphenylhydantoin

Generalized EEG seizures were induced in acute, conscious New Zealand albino rabbits with intravenous pentylenetetrazol (PTZ) (10 or 15 mg/kg) or electrical stimulation of the frontal cerebral cortex (ELEC) (50 Hz, 1 msec pulse duration, 2 sec train duration, 4.0-7.6 V). Three anticonvulsant treatments were compared: (1) electrical transhemispheral stimulation of the ansiform or simplex cerebellar lobes (10 Hz, 1.5 msec, 3-4 V), (2) phenobarbital (PB) (25 mg/kg, i.v.), and (3) diphenylhydantoin (DPH) (30 mg/kg, i.v.). After treatment, increments in PTZ dose or stimulation voltage were applied until a seizure was evoked that approximated the original in severity and duration. PTZ seizure thresholds were not elevated by DPH, and electrically induced seizure thresholds were not elevated by cerebellar stimulation (CBL). The four remaining seizure threshold elevations (increase in PTZ dose or stimulation voltage) were significant at a level of 0.025 or greater. Comparison of the elevations of seizure thresholds showed no differences at the 0.01 level of significance. Thus, no differences were seen between elevation of PTZ seizure thresholds by CBL or PB, or elevation of electrically induced seizure thresholds by PB or DPH, when examined on an acute basis.

Blockade of the tonic hindlimb extensor component of maximal electroshock and pentylenetetrazol-induced seizures by drugs acting on muscle and muscle spindle systems: a perspective on method

Blockade of tonic hindlimb extension (THE) in the maximal electroshock seizure test was demonstrated for C-28'882-Ba, a muscle spindle suppressant, chlorpromazine, a depressant of fusimotor drive, and gallamine, a neuromuscular blocking drug. These agents also blocked THE produced by very large doses of pentylenetetrazol. Dantrolene, a muscle contraction antagonist, and MI-65-S, a muscle spindle suppressant, significantly delayed THE. The data indicate that blockade of THE may be effected at a multiplicity of loci and may not be an expression of an "anticonvulsant" effect.