Electroencephalogram and somatosensory evoked potential changes after administration of six convulsant drugs

Physiological and pathological spindling phenomena have similar regional EEG power distributions

Sleep spindles in human and in rat are known to have a thalamocortical substrate. It has also been suggested that absence epilepsy spike and wave discharges may be generated by a similar mechanism. In addition, we have previously reported a possible thalamocortical origin of the EEG spindling rhythmic discharges associated with myoclonic jerks in the picrotoxin rat model of primary generalised epilepsy. To investigate whether pathological and physiological brain rhythms have common mechanisms of generation, we analysed four electroencephalographic (EEG) spindling activities in the rat. These were the non-convulsive spindle discharges induced by picrotoxin (picrotoxin spindles), naturally occurring absence epilepsy spike and wave discharges (absence spindles), spindle discharges during natural sleep (sleep spindles) and spindling activity that occurs under barbiturate anaesthesia (barbiturate spindles). We used power spectral analysis to define and compare the strength and brain distribution of EEG power during the spindling activities in 12 forebrain and 7 brainstem regions. There were brain-wide differences in power for each of the different spindle types with the pathological rhythms of the epilepsies containing more power than the physiological rhythms. There were also similar differences in the expression of spindles related to the region examined and no thalamic emphasis. These results provide evidence for a similar regional EEG power distribution for all four types of spindling activity and thus for the different spindles being expressions of a single phenomenon.

Frontal cortex leads other brain structures in generalised spike-and-wave spindles and seizure spikes induced by picrotoxin

Generaliszed spike-and-wave (SW) spindles (5-7 Hz) associated with myoclonic jerks precede the occurrence of regular spikes (2-3 Hz) associated with convulsive seizure induced by picrotoxin. SW spindles occur spontaneously in rodent and cat under some experimental conditions and are considered to be models of human generalised epilepsy. These spindles have been proposed as being led by a thalamic pacemaker. To examine this possibility in picrotoxin-induced SW spindles and seizure spikes, we recorded EEG using chronically implant unipolar electrodes during intravenous picrotoxin infusion in freely behaving rat. The 6 EEG signals were digitally sampled at 1000 Hz. Linear correlation, spectral, coherence and phase analyses were undertaken to determine time differences (TDs) between EEG channels and the brain structure leading seizure activity. One frontal cortex led all other structures during SW spindles. TD between SW spindles in the leading frontal cortex (Fr1) and the contralateral Fr1 was 3.6 + / - 0.5 msec. All ipsilateral structures (hippocampus, thalamus, amygdala, caudate nucleus and occipital cortex) were delayed by more than 3 msec from Fr1 (intralaminar thalamic nuclei - by 6.3 + / - 0.9 msec). TDs of SW spindles between subcortical regions were less than 1.5 msec. Similar relationships with slightly smaller TDs were found with spikes during convulsive seizure except TDs between frontal cortices did not significantly differ from zero. We suggest that seizure activity induced by picrotoxin is led by one Fr1 during SW spindles and by both frontal cortices working as one system during convulsive seizure.

Kainic acid microinjected into the cat raphe dorsal nucleus modulates the somatosensory evoked potentials and their cycles of excitability

Studies were made on the effect of the neuroexcitatory agent kainic acid, microinjected into raphe dorsal nucleus by glass micropipette and an air pressure system in doses ranging from 0.2 to 24.0 nmol (in volumes from 0.05 microliter to 0.47 microliter), on the somatosensory evoked potentials and their cycles of recovery (excitability) obtained from cortex (primary somatosensory and parietal associative), thalamus (ventral posterolateral nucleus and centre median nucleus), mesencephalic reticular formation and raphe dorsal nucleus. Kainic acid in doses higher than 3 nmol exerted an activating effect on the evoked potentials and their recovery cycles especially in thalamus and mesencephalic reticular formation. The analysis of these electrophysiological parameters revealed that the non-specific structures were involved to a larger extent in the activating effect of kainic acid than the specific ones. The morphological changes were not severe and were limited to a part of the raphe dorsal nucleus neurons. Our data indicate that kainic acid injected into raphe dorsal nucleus modulates (in direction of facilitation) the somatosensory evoked potentials and their cycles of excitability obtained in some brain structures. The results suggest that this nucleus is involved in the somatosensory information processing in a non-specific manner.

On the action of intravenous picrotoxin in the sensorimotor cortex of the cat

The action of topical convulsants, such as strychnine, is not exerted equally on all sets of neurons in the sensorimotor cortex of cats. Small-field neurons near the surface are influenced whereas deeper, wide-field neurons are affected only indirectly. The agent, picrotoxin, was used to test whether or not the deeper neurons are spared the effects of topical convulsants simply because of their location. Picrotoxin, injected intravenously, produced enhancement of the corticofugal reflex discharge evoked by stimulation of any of the four paws and by auditory stimulation. The surface-recorded primary evoked response was also enhanced but only slightly, far less than with topical application. The evoked response in the medial lemniscus was not altered by picrotoxin, but that in the thalamic radiations was enhanced. The drug diazepam, injected intravenously, diminished the enhancements of both the corticofugal reflex discharge and the evoked potential, in some cases reducing the responses to amplitudes smaller than control values. Single-cell recordings in other laboratories have shown that intravenous picrotoxin converts small-field cells to wide-field cells, and that diazepam reverses this effect. The increase in the surface-recorded primary response and the size of the increase in the corticofugal reflex suggest that there is also a direct effect on wide-field neurons or on the pathway leading to them.

Neural mechanisms distinguishing the neocortical EEG of C57BL/6 mice from that of DBA/2 mice

C57BL/6 inbred mice lack the 1-5 sec bursts of 6-7 cps spindles characteristic of the neocortical EEG of DBA/2 mice during waking. C57BL/6 mice (1) may be unable to generate any synchronized cortical EEG activity, (2) may lack the thalamocortical circuitry required to generate these brief spindle episodes (BSEs), (3) may lack mechanisms that can activate this circuitry or (4) may possess a potent mechanism to suppress BSE initiation and generation. Possibilities 1 and 2 have been eliminated because C57BL/6 mice generate pentobarbital, rostropontine-induced and sleep spindles, and because certain C57BL/6 sleep spindles resembled the BSEs seen in DBA/2 mice. Possibilities 3 and 4 were examined in the experiments reported here. In DBA/2 mice, pentylenetetrazol activates BSEs at subconvulsant doses. In contrast, neither 20 nor 50 mg/kg, IP, pentylenetetrazol activated BSEs in C57BL/6 mice, although the higher dose provoked 4-5 cps slow waves and myoclonic jerks. In DBA/2 mice, the beta-noradrenergic antagonist propranolol has been reported to powerfully release BSEs. In C57BL/6 mice, 10 and 15 mg/kg propanolol weakly released BSEs; fewer than 3 per hour occurred. Hence neither possibilities 3 and 4 are sufficient in themselves to explain the lack of BSEs during waking in C57BL/6 mice. However, simultaneous administration of 10 mg/kg propranolol and 20 mg/kg pentylenetetrazol provoked numerous BSEs in C57BL/6 mice. This suggests that perhaps C57BL/6 mice, as compared to DBA/2 mice, possess both a more powerful noradrenergic mechanism to suppress spindles and a more weakly functioning mechanism to activate BSEs. Hence possibilities 3 and 4 may both be correct.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of iontophoretic application of convulsants on the sensory responses of neurons in the brain-stem reticular formation

Iontophoretic or pneumatic application of convulsants produced enhancement of the sensory responses of brain-stem reticular formation (RF) neurons in cat and rat. Enhanced sensory responsiveness is observed with visual, auditory, somatosensory and vibrissa manipulation following application of strychnine, bicuculline or pentylenetetrazol. The effect was not transmitter-specific, since it could be produced by sequential application of bicuculline and strychnine which are thought to affect different neurotransmitters. Intravenous administration of convulsant also enhanced the responsiveness of RF neurons which had previously been affected by application of a different convulsant. Both systemic and iontophoretic effects could be antagonized by application of GABA and glycine. These data, coupled with previous findings in primary sensory pathways, indicate that sensory response enhancement in the RF induced by intravenous administration of these convulsants is in large part due to actions on synaptic elements of reticular formation neurons.

The influence of intermittent light stimulation on potentials evoked by single flashes in photosensitive and non-photosensitive Papio papio

The effects of intermittent light stimulation (ILS) on visual potentials (VEPs), evoked in different cortical areas, were statistically studied in baboons either naturally photosensitive or made photosensitive by allylglycine at a subconvulsant dose, as well as in non-photosensitive animals. VEPs were induced by single flashes (paradigm a) or by flashes preceded by trains of ILS (paradigm b). In every baboon, photosensitive or not, the VEPs induced by paradigm b in the striate area show a decrease of amplitude compared to VEPs induced by paradigm a. The ERG evolves in the same way. Therefore, these effects do not depend on photosensitivity; they depend on the intensity of stimulation. In photosensitive animals the single flash in paradigm b can induce a paroxysmal VEP in the fronto-rolandic (FR) area. In parietal and peristriate areas the VEPs induced by paradigm b show new late components when compared to those induced by paradigm a. These changes are observed even if no FR paroxysmal VEP is induced; they depend on the presence of a train of ILS preceding the single flash and on the predisposition to epilepsy (both natural and due to allylglycine); in the non-photosensitive animals the VEPs recorded in the same areas do not show such differences. We consider that among afferents which could act in inducing FR paroxysmal activities some cortico-cortical visual afferents can come from non-specific cortical areas (parietal or peristriate), but would not directly originate in the striate cortex; anatomical data in this species may support such a hypothesis.

Potentiation and modification of recruiting responses precedes the appearance of spike and wave discharges in feline generalized penicillin epilepsy

Recruiting responses (RR) were evoked by stimulation of nucleus centralis medialis in awake and painlessly immobilized cats. Following the administration of sodium penicillin G (350,000 IU/kg i.m.) and at a time preceding the development of generalized spike-and-wave discharge we observed a strong potentiation of RR in 10 out of 15 experiments (50-200% increase in amplitude). The major feature of wave form modification consisted mainly of a development or increase of positive phases of individual recruiting waves. In between such large amplitude negative-positive recruiting waves a slow negative wave developed. One of every two recruiting waves was often diminished when the preceding recruiting wave had reached considerable amplitude. The changes in the RR were antagonized by barbiturates and by caffeine. In conjunction with previous evidence these results support the hypothesis that spikes of spike-and-wave discharges in FGPE are generated by similar thalamocortical volleys as those creating RR and spindles. They further suggest that the crucial neuronal mechanism underlying this effect of penicillin is a shift in the emphasis from distal apical dendritic thalamocortical synapses on cortical pyramidal neurons to more proximal ones.

Effects of bemegride on the sensory responses of neurons in the hippocampus and brain stem reticular formation

Administration of subconvulsant doses of bemegride results in extensive enhancement of sensory responsiveness to auditory, visual, somatosensory and vibrissa stimulation of neurons in the medullary and mesencephalic reticular formation (RF). This effect is not altered by cord transection. Most neurons in the dorsal hippocampus do not show enhanced sensory responsiveness. In the minority of hippocampal neurons which do show enhancement the effect is much less extensive than that seen in the RF despite larger doses of bemegride. The enhancement of RF neuronal response may involve the reversal of repetition induced response attenuation ('habituation') and reductions in response threshold. During EEG seizures the firing of neurons in RF and hippocampus are temporally correlated with the spikes in the cortical EEG. The effects of bemegride on RF neurons are similar to those previously reported for strychnine and pentylenetetrazol and preliminary studies with physostigmine and bicuculline. These findings further extend the concept that a relatively selective enhancement of the sensory responses of brain stem reticular formation neurons may be indicative of a general neuronal effect of convulsants which may play an important role in the initiation of sensory-induced seizures.

Excitatory and depressant effects of delta 9-tetrahydrocannabinol and cannabidiol on cortical evoked responses in the conscious rat

The influences of delta 9-tetrahydrocannabinol (THC) and cannabidiol on electrically evoked cortical potentials of conscious rats with chronically implanted electrodes were investigated. Specifically, the cannabinoids' effects on a transcallosal evoked response were compared with those of ethosuximide, phenytoin, and pentylenetetrazol. THC produced dose-related opposite effects: Low doses increased the amplitude of the response, whereas higher doses reduced the response. Other drugs that can cause or exacerbate seizures, i. e., phenytoin and pentylenetetrazol, also increased the amplitude of the cortical response. In contrast, cannabidiol, over a wide dosage range, caused only depression. Ethosuximide, like cannabidiol, elicited a depressant effect. The data indicate that under the conditions of the present investigation, cannabidiol shares electrophysiological properties with ethosuximide but not with phenytoin, and that cannabidiol is a relatively selective, centrally acting drug. In addition, our findings support the suggestion that augmentation of neurotransmission in central pathways may contribute to the convulsant actions of THC, and the cannabinoids' depressant effects may, at least partially, account for their anticonvulsant actions.

Strychnine effects on the sensory response patterns of reticular formation neurons

Strychnine (ST) administration results in enhanced responses of reticular formation neurons of the cat to visual, auditory, or somatosensory stimuli. Neurons in the bulbar reticular formation (BRF) also exhibited ST-induced spike-bursting which coincides with the onset of 10--20 Hz high amplitude rhythmic discharge in the lower brain stem often obscuring sensory response enhancement. Mesencephalic reticular formation (MRF) neurons showed enhanced sensory responsiveness at higher doses of ST but rarely showed spike-bursting. The rhythmic discharge in the EEG of BRF and spike-bursting in BRF neurons was not observed in animals with spinal cord transection suggesting that this bursting activity is due to the action of ST on the spinal cord. The enhancement of sensory responses may be due in part to reversal of repetition-induced response attenuation (habituation) mechanisms in RF neurons. The ST-induced enhancement of RF neuronal responses to sensory stimuli is similar to that reported previously with pentylenetetrazol. These findings extend the concept that enhancement of RF responses to sensory stimuli may be indicative of a general neuronal action of convulsant agents which may be important in sensory-induced seizures.