Effects of bilateral partial diencephalic lesions on cortical epileptic activity in generalized penicillin epilepsy in the cat

The dynamics of cortico-thalamo-cortical interactions at the transition from pre-ictal to ictal LFPs in absence epilepsy

PURPOSE

Generalized spike and wave discharges (SWD) are generated within the cortico-thalamo-cortical system. However the exact interactions between cortex and different thalamic nuclei needed for the generation and maintenance of SWD are still to be elucidated. This study aims to shed more light on these interactions via multisite cortical and thalamic local-field-potential recordings.

METHODS

WAG/Rij rats were equipped with multiple electrodes targeting layers 4 to 6 of the somatosensory-cortex, rostral and caudal RTN, VPM, anterior (ATN)- and posterior (Po) thalamic nucleus. The maximal-association-strength between signals was calculated for pre-ictal→ictal transition periods and in control periods using non-linear-association-analysis. Dynamics of changes in coupling-direction and time-delays between channels were analyzed.

RESULTS

Earliest and strongest increases in coupling-strength were seen between cortical layers 5/6 and Po. Other thalamic nuclei became later involved in SWD activity. During the first 500ms of SWDs the cortex guided most thalamic nuclei while cortex and Po kept a bidirectional crosstalk. Most thalamic nuclei started to guide the Po until the end of the SWD. While the rostral RTN showed increased coupling with Po, the caudal RTN decoupled. Instead, it directed its activity to the rostral RTN.

CONCLUSIONS

Next to the focal cortical instigator zone of SWDs, the Po seems crucial for their occurrence. This nucleus shows early increases in coupling and is the only nucleus which keeps a bidirectional crosstalk to the cortex within the first 500ms of SWDs. Other thalamic nuclei seem to have only a function in SWD maintenance. Rostral and caudal-RTN have opposite roles in SWD occurrence.

Secondary epileptogenesis in humans

Secondary epileptogenesis as it applies to humans remains a controversial topic despite 40 years of investigation. Part of the controversy stems from disagreement about the definition of secondary epileptogenesis, and part of the controversy stems from the imperfect fit of animal models to the human epileptic syndromes. It may be that models of secondary epileptogenesis can be useful to describe specific epileptic syndromes such as bitemporal epilepsy and secondary bilateral synchrony, but other models may be required for remitting syndromes such as the Landau-Kleffner syndrome. The concept of secondary epileptogenesis may also provide a useful construct for evaluating patients with partial epileptic syndromes, especially those under consideration for epilepsy surgery, and for the evaluation of preventive strategies in epilepsy.

The role of GABAB mechanisms in animal models of absence seizures

Generalized absence seizures in humans are a unique type of epilepsy characterized by a synchronous, bilateral 3-Hz spike and wave discharge emanating from a cortical and thalamic network within the brain. The availability of a number of pharmacological and genetic animal models has provided us with the means with which to investigate the cellular and molecular mechanisms underlying these seizures. Over the last few years a significant amount of research in these models has focused on the role of the inhibitory GABAB receptors, which have been previously described in a number of brain areas as being responsible for a long-lasting hyperpolarization and depression in neurotransmitter release. Initial studies provided evidence that the GABAB receptor was capable of generating the low threshold calcium spike required for initiation of the burst firing, leading researchers to hypothesize that the GABAB receptors played a significant role in these seizures. Subsequent research took advantage of the new generation of GABAB antagonists that became available in the early 1990s and demonstrated that in a number of models the seizures could be abolished by the administration of one of these compounds. Further biochemical, molecular, and electrophysiological experiments have been carried out to determine the exact involvement of GABAB receptors and their mechanism of action. The current evidence and interpretations of this work are presented here.

Thalamic mediodorsal and intralaminar nuclear lesions disrupt the generation of experimentally induced generalized absence-like seizures in rats

The effect of bilateral electrolytic lesions of various thalamic sites on the generation of bilaterally synchronous spike and wave discharges (SWD) was studied in two experimental rat models of absence-like seizures. SWD induced by both pentylenetetrazole (20 mg/kg, i.p.) and gamma-hydroxybutyric acid (gamma-butyrolactone, 100 mg/kg, i.p.) were recorded simultaneously from the thalamus and cortex. In both models generation of SWD from the mediodorsal, intralaminar (central lateral and paracentral), ventroposterolateral (VPL) and the reticular thalamic (RT) nucleus was synchronous with that of frontoparietal cortex. Bilateral lesions in mediodorsal and intralaminar thalamic nuclei abolished SWD from both cortex and thalamus in both models. Similar lesions in VPL did not abolish, but attenuated the duration of pentylenetetrazole- and gamma-hydroxybutyric acid-induced SWD, more significantly from the thalamus than from the cortex. RT lesions were associated with more pronounced suppression of pentylenetetrazole-, but not gamma-hydroxybutyric acid-induced SWD in the thalamus. These findings suggest a potential role for mediodorsal and intralaminar thalamic nuclei in the generation of experimental absence-like seizures in rats.

Kainic acid-induced thalamic seizure in cats--a possible model of petit mal seizure

Bipolar depth electrodes were implanted stereotaxically in the thalamus, hippocampus and midbrain reticular formation of cats. Cortical screw electrodes were placed over the bilateral sensorimotor cortex. A guide cannula with an inner injection cannula was inserted unilaterally into the posterolateral ventral nuclei (VPL) of the thalamus. Eight days after the procedures, kainic acid (2.0 micrograms) was injected unilaterally into the VPL via the injection cannula in freely moving animals and electro-clinical observations were made. About 1 h after the kainic acid injection, multiple spikes were observed in the VPL (injection site), which propagated to the subcortical structures. These seizures finally propagated bilaterally to the cortex about 2 h after the injection. About 3-4 h after the injection, small spike and wave complexes repeatedly appeared for a short period of time in cortical leads and cats exhibited behavioral arrest with unresponsiveness during the seizures. About 24 h after the injection, generalized small spike and wave complexes were observed intermittently in cortical and subcortical structures. They persisted for 4-5 s and were associated with behavioral arrest and staring. The results demonstrate that a unilateral microinjection of kainic acid into VPL induced petit mal-like seizure, and suggest that VPL plays an important role in the generation or transfer of spike and wave complexes.

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.

Cortical and thalamic lesions in rats with genetic absence epilepsy

In generalized, non-convulsive, absence epilepsy, spike-and-wave discharges (SWD) are recorded in both the cortex and the thalamus. The effect of various cortical and thalamic lesions on the occurrence of spontaneous SWD was examined in rats from a strain with genetic absence epilepsy. Cortical ablations suppressed SWD recorded in the thalamus. KCl induced unilateral cortical spreading depression and transiently suppressed SWD in the ipsilateral cortex and thalamus; SWD recovered simultaneously in both structures. Bilateral thalamic lesions of the anterior nuclei, the ventromedial nuclei, the posterior area, or lesion of the midline nuclei did not suppress cortical SWD. However, large lesions of the lateral thalamus, including the specific relay and reticular nuclei, definitely suppressed ipsilateral SWD, and pentylenetetrazol, THIP or gammabutyrolactone failed to restore the cortical SWD. These results demonstrate that the neocortex and the specific thalamic nuclei are both necessarily involved in the generation of SWD in absence epilepsy.

Ethosuximide alters intrathalamic and thalamocortical synchronizing mechanisms: a possible explanation of its antiabsence effect

Effects of systemic administration of a single dose (50 mg/kg) of ethosuximide (ESM) on extracellularly recorded thalamic (nucleus centralis lateralis, CL; nucleus reticularis, RE) and cortical neurons and on cortical EEG activity of acute cats, have been studied. In intact animals ESM led to: (a) desynchronization of cortical EEG activity; (b) reduction of cortical recruiting responses to 6 Hz stimulation of nucleus centralis medialis (CeM); (c) increased firing rate of CL units; and (d) reduction of incremental responses (IRs) of CL neurons to CeM stimulation. In midbrain reticular formation (MRF)-lesioned animals, ESM induced: (a) reduction of cortical spindle waves; (b) increment of their intraburst frequency; (c) reduction of the IR of CL neurons to 3 and 6 Hz CeM stimulation; (d) shortening of the inhibitory period following each response; and (e) no increment of spontaneous firing rate of CL units. Moreover, ESM led to important changes in the spontaneous activity of RE neurons: spike barrages, typical of these neurons in MRF-lesioned animals, became less frequent and of longer duration, being also constituted by longer interspike intervals. However, responses of RE neurons to low frequency CeM stimulation, when present, did not show any incremental phenomenon and appeared unchanged after ESM. Responses of cortical neurons to paired stimuli, applied with different interstimulus intervals, to nucleus ventralis posterolateralis or in animals with isolated cortex, to subcortical white matter, disclosed a reduction of the cortical inhibitory period following the response to the conditioning stimulus. These data suggest that ESM exerts a moderate diffuse anti-inhibitory action at both cortical and thalamic levels and an activating effect on MRF, which could also be accomplished through disinhibition. The reduction of the inhibitory phases in thalamic nuclei would alter spontaneous intrathalamic synchronizing mechanisms, leading to a decreased effectiveness of thalamocortical volleys, which are believed to be fundamental for the appearance of cortical spike and wave discharges. This hypothesis would therefore explain the specific efficacy of ESM against absence seizures.

Anticonvulsant drugs selectively affect kindled and penicillin epilepsy, especially during seizure-prone sleep or awakening states in cats

Carbamazepine (CBZ) selectively suppressed kindled convulsions, whereas ethosuximide (ESM) suppressed spike-wave activity accompanying systemic penicillin epilepsy in cats. Evoked potential data indicated that CBZ acted at the thalamic level, whereas ESM acted at cortex. Reduction of seizures and thalamic or cortical excitability occurred throughout the sleep-wake cycle, but effects were most pronounced in seizure-prone sleep or awakening states. These findings extend previous work showing differential antiepileptic drug (AED) effects on temporal lobe and absence seizures. The results are also consistent with recent work suggesting that thalamocortical pathways provide a final common pathway for the manifestation of sleep and awakening epilepsy and also reflect a chronic, latent pathophysiology.

Temporal lobe and petit mal antiepileptics differentially affect ventral lateral thalamic and motor cortex excitability patterns

Evoked potential (EP) analysis of the somatomotor pathway in cats revealed that the temporal lobe anticonvulsant carbamazepine suppresses the thalamic relay, whereas the petit mal antiepileptic ethosuximide acts on the cortex. Moreover, a history of temporal lobe epilepsy (amygdala kindling) maximized thalamic response to carbamazepine, especially during sleep states vulnerable to generalized kindled seizures. Ethosuximide accentuated cortical response during sleep and awakening states vulnerable to generalized spike-wave complexes, regardless of a history of petit mal seizures (systemic penicillin epilepsy). The findings provide a neural basis for differential drug effects on generalized temporal lobe and petit mal epilepsy and further suggest that a chronic seizure disorder can create a predisposition for antiepileptic drug sensitivity.

Midline thalamic lesion and feline amygdaloid kindling. I. Effect of lesion placement prior to kindling

Nine cats with electrolytic lesion in the massa intermedia (MI, N = 8) and ipsilateral nucleus centrum medianum (CM, N = 1) underwent primary and secondary site amygdaloid (AM) kindling and primary site retest. During primary site kindling, MI- but not CM-lesioned animals showed a pattern of seizure development strikingly similar to that of forebrain-bisected cats and the development of contralateral hemiconvulsive seizure was coincident with the onset of afterdischarge (AD) in the ipsilateral motor cortex. Unlike the forebrain-bisected ones, the MI-lesioned animals eventually developed bisymmetrical clonic convulsion with initial contralateral and then ipsilateral clonic involvement. At the secondary site, no positive transfer effect was noted in 1 CM-lesioned and 6/7 MI-lesioned cats. When the primary site was retested, a marked interference effect was observed. The lesion site responsible for these unique kindling features was in the rostral half of the MI with varying involvement of the nucleus centralis medialis (NCM). It is concluded that the MI plays an important role both for transhemispheric ictal propagation and for positive transfer effect in feline AM kindling. However, the mechanisms underlying these two phenomenon are unlikely to be the same since these effects can be totally divorced.

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.

Anterior thalamic mediation of generalized pentylenetetrazol seizures

The effects of microinjection of various neuroactive compounds into the anterior thalamic nucleus (AN) and other selected subcortical regions of guinea pig brain on the expression of pentylenetetrazol (PTZ)-induced behavioral and electrical seizure activity were examined. Excitatory agents, kainic acid (KA), bicuculline (BIC) or PTZ, injected into the AN or other thalamic nuclei, striatum, but not the mammillary bodies (MB), facilitated the EEG convulsant action of systemically administered PTZ. Injection of muscimol into the AN protected against the expression of PTZ-induced repetitive high-voltage EEG seizure discharges and inhibited the facilitatory effects of subcortically applied KA or BIC. Injection of muscimol into the AN was also able to terminate established ongoing seizure discharges. Unilateral application of muscimol to the AN did not prevent the repetitive hypersynchronous EEG discharges following systemic PTZ but did result in the delay in the onset of cortical hypersynchrony in the ipsilateral hemisphere. Muscimol injections into other thalamic nuclei, MB, cortex, striatum or directly into the CSF space had no anticonvulsant effect, however. Microinjection of gamma-vinyl-gamma-aminobutyric acid, a selective GABA transaminase inhibitor, resulted in protection from the behavioral convulsant action and lethal effects of PTZ when administered into the thalamus, especially the AN, but not when injected into the striatum or CSF. These data demonstrate that the AN is an important subcortical nucleus for the mediation of both cortical EEG synchrony and behavioral seizure expression induced by PTZ. In light of previous results establishing a role for the brainstem and diencephalon in PTZ seizure expression, the AN may serve, in part, as a gating mechanism for the propagation of paroxysmal activity between subcortical areas and the cerebral cortex.

Multiunitary activity analysis of cortical and subcortical structures in paroxysmal discharges and grand mal seizures in photosensitive baboons

Cortical and subcortical multiunitary activities (MUA) and EEG were simultaneously recorded in baboons made photosensitive by a subconvulsant dose of DL-allylglycine. Intermittent light stimulation (ILS) trains induced in these animals fronto-rolandic (FR) paroxysmal discharges (PDs, constituted as spikes and waves) and grand mal seizures. During the induction of FR PDs by ILS trains, the visual structures (occipital cortex, colliculi superioris, pulvinar) showed a significant MUA increase which was not related to the PD spike or wave but correlated with the flashes. The first structure showing bursts of MUA that frequently preceded the PD appearance was the FR cortex. When PDs appeared, the bursts were related to the spikes of PDs and were followed by an inhibition during the slow wave. The pontine and mesencephalic reticular formations and the facial nuclei were activated in bursts after the FR PDs had reached a certain amplitude. The thalamic nuclei ventralis lateralis, centrum medianum and lateralis posterior were activated only later, when the FR PDs had reached an even greater amplitude. It is suggested that the activation of visual structures is necessary for FR PD appearance. The secondary pontine and mesencephalic activation could reinforce that of the FR cortex and then the thalamus, and could determine the myoclonus observed in unparalysed animals. When the ILS is continued, grand mal seizures appear. The onset of the seizures could be linked to the loss of FR cortical control of the subcortical structures. The resulting reticular activation would be responsible for the vasomotor modifications which constitute the first clinical signs of a seizure.

Topographical evolution of spike-wave complexes

Computer-generated 3-dimensional field potential maps of spike-wave complexes from two 4 X 4 electrode grids on the scalp were studied. A visual analysis of these field maps throughout the spike-wave evolution permitted quantification of the spike, trough and slow wave components in terms of distribution, origin and propagation. In addition, a more objective morphological analyzer also quantified the discriptive parameters of distribution, origin and propagation for the spike component of the patients' spike-wave complexes. We found that field distributions of spikes differed from that of waves. Succeeding positive troughs evolved more symmetrically than did spikes but less than ensuing negative waves. Negative waves were more diffuse, more symmetrical in evolution, and more posteriorly centred than either spikes or troughs. Unlike the troughs and slow waves whose fields tended to remain stationary during their evolution, spikes always moved from their points of origin. Spikes originated at the most lateral points of the grids and propagated laterally and anteriorly in one of two ways: a simultaneous origin at both lateral positions, then propagation toward the midline, and then usually anteriorly, a clearly unilateral origin with spread contralaterally to the homologous electrode position of the contralateral hemisphere followed again by anterior propagation. Interhemispheric lag times of spikes ranged from 0 to 25 ms with an average of 10.5 ms. Inter- and intrapatient variability was considerable. This type of analysis reveals properties of spike-wave complexes which may not be appreciated by standard paper writeout.

The influence of light stimulation on subcortical potentials evoked by single flashes in photosensitive Papio papio

Visual evoked potentials (VEPs) from the frontorolandic (FR) cortex and from subcortical nuclei (colliculi superioris, pulvinar, corpori geniculati lateralis, centrum medianum, ventralis lateralis) and from pontine reticular formation were analyzed in Papio papio monkeys rendered photosensitive by a subconvulsant dose of allylglycine. The VEPs induced by single flashes were compared statistically with those induced by flashes preceded by trains of intermittent light stimulation (ILS). This latter mode of stimulation provoked the appearance of paroxysmal VEPs (PVEPs) in the FR cortex with the same morphology as the spikes and waves induced in this area by the ILS. The aim of our research was to provide evidence for the possible implication of the subcortical structures which we have studied in the elaboration of PVEPs and thus of spikes and waves. The VEPs recorded at the thalamic and pontine levels were modified when PVEPs were present. These modifications varied according to the site, but subcortical VEPs were never paroxysmal. In structures with visual functions (colliculi superioris, corpori geniculati lateralis), the VEPs were modified by the ILS, but showed more marked changes when PVEPs were present. Thus, these structures may contribute to the genesis of PVEPs. In the other structures (centrum medianum, ventralis lateralis, and pontine reticular formation), modifications of the VEPs occurred only when PVEPs were present. Thus, these structures would be only secondarily involved. We also present preliminary results concerning the effects of lesioning the pulvinar and the ventralis lateralis on the susceptibility of the FR cortex to produce spikes and waves during ILS.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrographic and clinical correlates of secondary bilateral synchrony

Reverting to the more strict definition of secondary bilateral synchrony (SBS) of Tukel and Jasper (1952), we reexamined clinical and EEG correlates of this phenomenon. SBS occurred in 57 of 10,410 consecutive patients (0.5%) recorded in our laboratory. SBS originated in the frontal lobe in 51% of patients, significantly more often than the incidence of frontal spikes among controls. The best indicator of SBS origin was the most active spike focus which localized the onset in 52 of the 57 patients (91%) and falsely identified the origin in only one. Other focal or lateralizing clinical and EEG features appeared each in a minority of patients and lateralized and/or localized SBS origin with the following incidences when present: seizure manifestations (86%), neurological examination (95%), focal delta (100%), focal or unilateral theta (100%), and offset of bisynchronous paroxysms (96%). However, the most active spike focus triggered SBS in all but two instances when these other features appeared. Reliability of such indicators of SBS origin may obviate the need for depth electroencephalography in such patients. The presence of more than one spike focus in 96% of patients and three or more foci in 77% suggests that SBS results from a complex interaction of multiple potentially epileptogenic regions, instead of spread from a single focus as previously thought.

Involvement of cortical, thalamic and midbrain reticular formation neurons in spike and wave discharges: extracellular study in feline generalized penicillin epilepsy

Extracellular activity of single units, simultaneously recorded in cortex, thalamus, and midbrain reticular formation was investigated during feline generalized penicillin epilepsy. The firing activity of neurons recorded in the cortex was invariably and consistently enhanced in coincidence with the positive peak and the positive-negative transient of the "spike" of the spike and wave complex, and it was greatly decreased during the wave. In the nonspecific thalamic nuclei three classes of neurons were identified according to their patterns of activity during the spike and wave complex: (i) neurons behaving like cortical units, (ii) neurons with enhanced firing activity during the wave and a decreased activity during the "spike," and (iii) unmodified neurons. In the nucleus lateralis posterior neurons of the third class were not found. Most midbrain reticular neurons could be classified in the same three classes of the nonspecific thalamic nuclei; however, 11% of those units increased their activity 20 to 30 ms earlier than did the cortical units (class IV). Investigation of the activities of all these neuronal populations immediately prior to a spike and wave discharge showed that the rhythmic cycle of excitation-inhibition commenced earlier in the cortical neurons than in any other subcortical neuron. Moreover, there were some nonspecific thalamic neurons of class II with an inhibitory phase exactly coincident with the activation of class IV midbrain reticular neurons. These data suggest (i) a leading role of cortical neurons in initiating and maintaining a spike and wave burst; (ii) the involvement of a corticothalamocortical circuit in timing the bursts, and (iii) an accessory reticulothalamic loop also involved in regulating the intraburst frequency of the spike and wave complex.

Differential participation of some 'specific' and 'non-specific' thalamic nuclei in generalized spike and wave discharges of feline generalized penicillin epilepsy

Extracellular single unit and electroencephalographic (EEG) activity during generalized spike and wave discharges (SW) induced by i.m. penicillin was recorded simultaneously in the cortex, in a 'specific' thalamic nucleus (n. lateralis posterior, LP) and in some 'non-specific' thalamic nuclei (n. centralis medialis, NCM; n. centrum medianum, CM; n. centralis lateralis, CL) Computer-generated EEG averages and histograms of single unit activity were triggered by either peaks of EEG transients or action potentials. The time at which cortical neurons (66/66) were most likely to fire was during the 'spike' of the SW complex while absence of firing was the rule during the 'wave'. Most LP neurons (23/26) showed a similar pattern, 3 cells firing preferentially during the 'wave'. In NCM only 17 of 39 neurons fired during the 'spike', 8 of 39 neurons during the 'wave' while the others showed no change in their firing pattern during SWs. Twenty-six of 30 CM and 20 of 24 CL neurons fired during the 'spike' of SW; the other cells in these nuclei did not change their firing pattern during SWs. When present, rhythmic fluctuations in firing linked to SW discharge were less prominent in these 'non-specific' thalamic nuclei than in cortex and LP. Furthermore, participation of NCM, CM and CL neurons in the SW rhythm occurred only after neurons in cortex and LP had become involved in it. Thus, as is the case for cortical neurons, the main firing pattern of thalamic cells during SWs consists of an oscillation between 'excitation' during the 'spike' and 'inhibition' during the 'wave' of the SW complex. However, the coupling between cortical and thalamic neuronal firing is less intimate for cells of the 'non-specific' thalamic nuclei than for a 'specific' nucleus such as LP. Thus, at least some 'specific' thalamic nuclei are more intimately involved in the mechanism of SW discharge than the midline intralaminar nuclei.

Topographical organization of the brainstem afferents to the lateral posterior-pulvinar thalamic complex in the cat

Following stereotaxic injections of horseradish peroxidase in the dorsal thalamus of the cat which were restricted to the lateralis posterior-pulvinar complex, labelled neurons were found in the superficial layers of the superior colliculus and in the brainstem. The retrogradely-filled cells of the brainstem were situated principally in the nucleus tegmenti pedunculopontinus, the locus coeruleus complex, the parabrachial nuclei and the dorsal tegmental nucleus of Gudden; in each case, labelled cells were more numerous on the ipsilateral side. In addition, some scattered neurons were observed in the central grey matter, the mesencephalic reticular formation, the central superior and dorsal raphe nuclei, the cuneiform nucleus reticularis gigantocellularis, the nucleus praepositus hypoglossi and the oculomotor nuclei. The differential organization of these projections were observed. It is concluded that the rostrointermediate subdivision of the lateralis posterior-pulvinar complex receives most of its connections from the nucleus tegmenti pedunculopontinus, from the deep layers of the superior colliculus and from the other brainstem nuclei, while the caudal subdivision (extrageniculate visual subdivision) receives its main projection from the superficial layers of the superior colliculus. The findings may have functional implications for the role of the complex in oculomotor control.

The effects of transient functional depression of the thalamus on spindles and on bilateral synchronous epileptic discharges of feline generalized penicillin epilepsy

A transient functional depression of thalamic activity (TFDTA) was induced in acute experiments in cats by the microinjection of 25% KCl into the thalamus. Spontaneous and evoked thalamic electrical activity was markedly depressed at the site of KCl microinjection. Spread of this depression to other thalamic areas often occurred, mainly when KCl was injected into the midline thalamus. In normal cats both spontaneous and evoked cortical spindle bursts as well as other evoked thalamocortical responses were reduced or abolished during the KCl-induced TFDTA. The generalized spike-and-wave discharges of feline generalized epilepsy were also suppressed for the duration of TFDTA, while incidental focal cortical interictal and ictal epileptic discharges, as well as generalized tonic-clonic seizure discharge, remained unaffected. The same effects were observed in animals with lesions of the mesencephalic reticular formation, indicating that the suppression of spindles and spike-and-wave discharges cannot be attributed to a release of the activity of the reticular formation by the TFDTA. An unexplained occurrence of generalized tonic-clonic EEG seizure was observed in most cases late after thalamic KCl microinjection, usually after the spike-and-wave discharges had recovered. These data are consistent with the hypothesis that the spontaneous bilaterally synchronous epileptic bursts of feline generalized penicillin epilepsy are not only closely related to spindles but are crucially dependent on thalamic inputs to the cerebral cortex.

The role of the corpus callosum in bilateral interhemispheric synchrony of spike and wave discharge in feline generalized penicillin epilepsy

In this study on interhemispheric synchrony of spike and wave discharges in feline generalized penicillin epilepsy, four groups of cats were treated in the following manner: Group A underwent complete section of the corpus callosum and anterior commissure; group B underwent division of the massa intermedia alone; group C underwent partial section of the corpus callosum; and in group D, a slab of the cortex on one side, comprising the middle parts of the lateral and suprasylvian gyri, was severed from all its subcortical inputs, without disturbing its connections with the opposite hemisphere through the corpus callosum. Two weeks after surgery or later the cats received an i.m. injection of penicillin. Bilateral synchrony of the epileptic discharges was abolished in group A, but not in group B. In group C, bilateral synchrony of the epileptic bursts was impaired, but not abolished. In group D, epileptic bursts synchronous with those occurring in the intact hemisphere continued to occur in the slab, but at lower amplitude. It is concluded that the corpus callosum is the main, if not the exclusive, pathway ensuring bilateral synchrony of the epileptic discharges of feline generalized penicillin epilepsy.