Hippocampal seizures and striatal regulation: a possible functional pathway

Do the basal ganglia inhibit seizure activity in temporal lobe epilepsy?

There is substantial evidence in the literature that the basal ganglia (BG), namely the striatum and pallidum, are involved in temporal lobe epilepsy (TLE). The BG are probably not involved in elaborating clinical seizures, as they do not produce specific epileptiform activity and there is no evident change in the electrical activity in the BG immediately after seizure onset. The data we obtained by direct ictal recording in the BG [1,2], as well as a large body of experimental and clinical evidence reported by other groups, suggest an inhibitory role of the BG during temporal lobe seizures. The BG may have a remote influence on cortical oscillatory processes related to control of epileptic seizures via their feedback pathways to the cortex.

Effects of diazepam on extracellular brain neurotransmitters in pilocarpine-induced seizures in rats

The present study was undertaken to gain insights into the mechanism of action of diazepam in focally-evoked pilocarpine-induced seizures by concomitantly assessing the changes produced in the extracellular levels of glutamate, GABA (gamma-aminobutyric acid) and dopamine. In vivo microdialysis, coupled to continuous monitoring of electrocorticographic (ECoG) recordings, was performed in freely moving rats. Intrahippocampal perfusion with 10 mM pilocarpine (40 min, 2 microl/min) produced limbic seizures. A single dose of intraperitoneal diazepam (5 mg/kg) was administered 2 h after pilocarpine perfusion was started. Dialysates were sampled both from hippocampus and cerebellum and analysed by microbore liquid chromatography. Diazepam produced instant inhibition of behavioural and ECoG seizure activity. Pilocarpine-induced increases in the extracellular levels of glutamate and dopamine in hippocampus were promptly reduced by diazepam. No concurrent alterations in pilocarpine-induced increases in the extracellular levels of GABA in either hippocampus or cerebellum were seen. Pilocarpine enhanced cerebellar glutamate levels only transiently and levels returned to baseline before diazepam administration. No further changes in cerebellar glutamate levels were observed with diazepam. Our findings suggest that the anti-convulsant action of diazepam against pilocarpine-induced seizures is associated with a prompt attenuation of extracellular hippocampal glutamate overflow without concurrent alteration of pilocarpine-induced increases in endogenous GABA levels. Diazepam also significantly decreased pilocarpine-induced increases in extracellular dopamine levels within the hippocampus. No immediate alterations of the basal levels of the neurotransmitters monitored were observed with diazepam.

Hippocampal cell density and subcortical metabolism in temporal lobe epilepsy

PURPOSE

Correlations between hippocampal cell density and subcortical metabolism in patients with temporal lobe epilepsy (TLE) were studied to explore possible links between subcortical function and the regulation of hippocampal excitability.

METHODS

Resected hippocampal cell densities were correlated with cortical and subcortical regional cerebral metabolic rate for glucose (CMRglu), as measured by [18F]-fluorodeoxyglucose positron emission tomography (18-FDG-PET), in 39 patients with intractable TLE who underwent anterior temporal lobectomy (ATL). CMRglu was measured ipsilateral and contralateral to the resected temporal lobe. Linear regression techniques were used for statistical analysis.

RESULTS

Hilar cell densities correlated positively and significantly with CMRglu in the bilateral thalamus, putamen and globus pallidus, and the ipsilateral caudate. Dentate granule cell densities correlated positively and significantly with CMRglu in the bilateral thalamus and putamen. There was no significant correlation between cell densities and CMRglu in any cortical region, including the hippocampus.

CONCLUSIONS

We postulate that hippocampal cell loss results in decreased efferent synaptic activity to the thalamus and basal ganglia, causing decreased neuronal activity in these structures with consequent hypometabolism. This synaptic activity has a significant bilateral component. Subcortical hypometabolism in patients with TLE may reinforce the epileptogenic potential of mesial temporal lobe discharges.

Presurgical contribution of quantitative stereotactic positron emission tomography in temporo limbic epilepsy

BACKGROUND

We quantified the interictal metabolic changes associated with temporal lobe epilepsy by using an accurate stereotactic method.

METHODS

We selected 16 patients who had proven unilateral focal or regional temporal onset defined by SEEG criteria. Each patient underwent stereotactic MRI and stereotactic [18 fluoro] fluorodeoxyglucose positron emission tomography (PET).

RESULTS

Asymmetries (mean, +/- SD) were found in mesio-temporal structures: amygdala (-0.033+/-0.027, p = 0.0002), hippocampus (-0.035+/-0.032, p = 0.0006), and superior temporal gyrus (-0.036+/-0.032, p = 0.0004). Four of the sixteen patients had previously had unlocalized qualitative nonstereotactic PET analysis.

CONCLUSIONS

The quantitative stereotactical PET method allows a higher resolution study of mesio-temporal structures.

Characteristics of dorsal and ventral striatal kindling in rats

We examined the characteristics of kindling of seizures with stimulation of the dorsal or ventral striatum in rats. Different groups of rats carried electrodes directed towards the nucleus accumbens or the head, middle, or tail of the caudate. Thresholds for afterdischarge (AD) were high at all sites, and stimulation often produced forced motor responses (motor responses that occurred during the stimulation and were not a consequence of kindling). Kindling at sites in the accumbens proceeded more slowly than at the sites in the caudate, which did not differ among themselves. The duration of accumbens seizures increased dramatically over the course of kindling, whereas the duration of caudate seizures remained relatively short and invariant. Although the kindled seizures resembled seizures kindled from limbic sites, they also contained aspects of seizures triggered from stimulation of the anterior neocortex. We conclude that striatal kindling comprises elements of both limbic and neocortical kindling.

Accumbens-caudate-septal circuit as a system for hippocampal regulation: involvement of a GABAergic neurotransmission

Hippocampal-based epileptiform activity may reach the basal ganglia via the nucleus accumbens. Previous data suggested that caudate nucleus is able to influence hippocampal epilepsy, probably sending a projection to the septum. In order to test the hypothesis of a retrograde activation of accumbens-caudate pathway in hippocampal regulation, we electrically stimulated both caudate nucleus and nucleus accumbens and studied modifications of hippocampal EEG in the feline focal epilepsy model. We also performed bilateral electrolytic lesion of nucleus accumbens and repeated caudate stimulation. Results showed that nucleus accumbens stimulation was ineffective in modifying hippocampal epilepsy; on the contrary, caudate stimulation caused a statistically significant decrease of hippocampal spike frequency and amplitude. On the other hand, in accumbens-lesioned animals caudate activation consistently reduced hippocampal epilepsy to a significant degree. As the caudate nucleus influences hippocampal activity and the septum may constitute a relay station of this functional relation, a possibility was tested concerning a GABAergic mediation. To this end, after a stable caudate-induced effect was reached, an intraseptal microinjection of picrotoxin (GABA receptor antagonist) was made and caudate stimulation repeated at the same parameters. Such a study showed that after intraseptal picrotoxin, caudate stimulation failed to elicit any type of modification of hippocampal activity. Experimental findings support the notion that the striatal modulation on hippocampus is mediated by an anterograde rather than a retrograde pathway, and underline the possibility of a GABAergic caudate-septal influence.

Relay stations and neurotransmitters between the pallidal region and the hippocampus

The effects of internal pallidum and lateral habenula stimulation on epileptiform activity of cat's hippocampus were studied. A steady interictal activity was induced by locally applied sodium penicillin (PCN) solution. Both pallidal and habenular electrical stimulation caused an increase in spike frequency and amplitude. Intraperitoneally injected atropine sulphate failed to modify pallidal and habenular influences. Intraperitoneal methysergide bimaleate (5-HT antagonist) suppressed the effects of habenular stimulation. In contrast to the effects of pallidal and habenular stimulation, raphe electrical stimulation inhibited hippocampal spiking and intra-raphal muscimol (a GABA receptor agonist) enhanced hippocampal-based epilepsy. After muscimol, raphe stimulation at the same threshold parameters failed to affect hippocampal activity. In cats with habenular lesions hippocampal spike frequency and amplitude were reduced and intra-raphal muscimol did not affect the hippocampus. The results are discussed in the light of a complex interrelationship between basal ganglia and hippocampus. The role of the lateral habenula and of the medial raphe as relay stations between the two regions is emphasized.

Dopaminergic control of feline hippocampal epilepsy: a nigro hippocampal pathway

Substantia nigra is a mesencephalic structure inserted along several circuits which appear to play a key role in epilepsy. In previous researches we postulated that substantia nigra pars compacta (SNpc) may be the site of a precise control of hippocampal epilepsy while substantia nigra pars reticulata (SNpr) may exert a modulation of both neocortical epilepsy and spreading of hyperactivity toward a motor target. In order to better understand mechanisms subserving nigral action in feline hippocampal epilepsy we electrically stimulated SNpc (dopaminergic), before and after sulpiride (dopamine receptor-antagonist) intravenous injection. Furthermore we compared hippocampal epileptiform activity prior to and after apomorphine (dopamine receptor-agonist) intrahippocampal injection as well as prior to and after SNpc electrolytic destruction. Results showed that SNpc is able to regulate hippocampal epilepsy. This effect is selectively antagonized by sulpiride while apomorphine exerts, synergically with SNpc stimulation, inhibitory effects. On the contrary SNpc lesions induces a significant enhancement of hippocampal epileptiform spikes. Experimental findings suggest that SNpc represents a strategic region for the control of hippocampal excitability and that this regulation appears to be dopaminergic in nature.

Nigral influence on focal epilepsy

The substantia nigra (SN) has been proposed as a structure involved in epileptiform phenomena. Previous investigations demonstrated that SN is able to elicit hippocampal rhythmic slow activity (RSA) as well as to inhibit hippocampal interictal spikes induced by parenteral administration of penicillin. The present series of experiments was carried out in order to characterize the action of SN on a focal model of hippocampal epilepsy. Experiments were performed on encéphale isolé cats in which steady epileptiform activity was induced by locally applied penicillin. Electrical stimulation of SN pars reticulata (pr) caused a statistically significant decrease of hippocampal spike frequency and amplitude in 30% of the total number of stimulation sessions. Stimulation of SN pars compacta (pc) was even more effective. It induced inhibitory effects on hippocampal spikes in 91% of the cases. In 30% of the cats, RSA was noted on hippocampal recordings in correspondence to nigral activation. Experimental data support the hypothesis that the SNpc influences hippocampal excitability: a differential role may be played by SNpc and SNpr in the control of seizure processes.

Substantia nigra-mediated anticonvulsant action: a possible role of a dopaminergic component

A number of neural pathways may mediate nigral control of epilepsy. According to the literature, a GABAergic nigrotectal pathway may be responsible for the control exerted by the substantia nigra on the diffusion of discharges toward spinal targets, while the nigrothalamic projection may transfer nigral influence on premotor neocortical epilepsy. Since there is probably an anatomical nigrohippocampal pathway arising from dopaminergic cells in the substantia nigra, we tested the effects of stimulating the substantia nigra pars compacta (SNpc) on focal hippocampal epilepsy induced by penicillin injection in the cat. The possibility of dopamine involvement was further tested by studying the effects of intraperitoneal injection of haloperidol, a dopamine receptor blocking agent on nigrohippocampal influences, while to verify the precise site of action, in other groups of cats, sulpiride and apomorphine (D-receptor antagonist and agonist, respectively) were locally administered in the dorsal hippocampus. Furthermore, modifications of hippocampal epileptiform EEG were studied in control conditions and following SNpc electrolytic lesions. Results showed a strong nigral suppressive effect on focal hippocampal epilepsy. Nigral stimulation induced a significant decrease in both frequency and amplitude of hippocampal spikes, which disappeared either about 10 min after i.p. injection of haloperidol 1 mg/kg or about 5 min after intrahippocampal administration of sulpiride, and did not return during a further hour or more of experimental observation. It should be emphasized that in the absence of nigral stimulation, both haloperidol and sulpiride did not modify hippocampal spike frequency. Apomorphine application to dorsal hippocampus induced a marked reduction of hippocampal epileptiform activity parallel to the effect observed during SNpc stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Subcortical metabolic alterations in partial epilepsy

The function of subcortical nuclei in partial epilepsy was investigated using positron emission tomography (PET) to measure metabolism in the basal ganglia and thalamus. Sixteen patients undergoing surgical evaluation were studied with 18F-fluorodeoxyglucose (FDG) interictally and had intensive extracranial and intracranial electrophysiologic evaluations. Eight patients had left temporal lobe seizure foci, six had right temporal lobe foci, and two had right posterotemporal or parietal foci. The PET data were analyzed visually and quantitatively, using a multivariate analysis of variance on the quantitative data. Hypometabolism of subcortical nuclei was present ipsilateral to the cortical seizure focus. Cortical hypometabolism was noted focally in the temporal lobe in patients with left temporal lobe seizure foci, whereas patients with right temporal lobe seizure foci had diffuse hemispheric hypometabolism. We postulate that the subcortical hypometabolism is secondary to decreased efferent activity from temporal lobe structures, in particular amygdala and hippocampus, to subcortical nuclei. Diminished subcortical activity may then lead to defective regulation of cortical excitability in the temporal lobe, increasing the likelihood of seizure development and spread.

Premotor cortical kindling interferes with subsequent hippocampal kindling

Interaction between the neocortical motor system and the limbic system was investigated in terms of the transfer of kindling. Premotor cortical kindling induced limbic seizures accompanied by dissociated hippocampal afterdischarges (AD) which outlasted the premotor AD. Subsequent kindling of the hippocampus was markedly retarded despite prolonged hippocampal AD. These results suggest that premotor cortical kindling induces a neuronal mechanism which prevents limbic access to the motor system.

Inhibitory control by substantia nigra of generalized epilepsy in the cat

Previous investigations have shown that the basal ganglia may exert a regulating influence on cortical epilepsy. Stimulation of the caudate nucleus enhances cortical penicillin (PCN) spikes. Stimulation of globus pallidus internus reduces cortical spike frequency. Since the substantia nigra pars reticulata (SNpr) seems to have an inhibitory action on the ventro-anterior (VA) and ventro-lateral (VL) thalamic nuclei and thalamic neurones send an excitatory influence to the cortex, we undertook an investigation to study nigral influence on cortical epilepsy induced by PCN. Experiments were conducted on encéphale isolé cats in which steady interictal activity was induced by means of parenteral PCN administration (feline generalized PCN epilepsy). Variations occurring in cortical PCN spikes following activation of either pars compacta (SNpc) or SNpr were analyzed. Electrical stimulation of SNpc reduced spike frequency and amplitude in 19% of the total number of stimulations; SNpr stimulation significantly inhibited cortical spikes, especially in the precruciate gyrus, in 80% of cases. The experimental findings constitute an electrophysiological feature of the control exerted by SNpr on the thalamo-cortical re-exciting loop. A putative preferential role of SNpr in the regulation of abnormal phenomena involving the neocortex is emphasized.

A study of caudate inhibition on an epileptic focus in the cat hippocampus

The mechanisms whereby the caudate nucleus modifies hippocampal spiking activity have been studied. Epileptiform activity was induced in the cat hippocampus by topical application of sodium penicillin in different concentrations. The frequency of induced spikes appeared to be directly correlated to the two doses of epileptogenic agent. The inhibitory effect of 10 Hz caudate stimulation on spike frequency was present even when stimulation lasted for 180 s. Likewise 25 Hz caudate stimulation brought about an inhibition which was maintained by stimulus trains lasting up to 90 s, while the degree of inhibition was reduced by trains of longer duration (120, 150 and 180 s); similar results were also noted in some atropine-treated cats. The time course of spikes in cats with electrolytic lesions of the caudate exhibited an increase in both frequency and duration. The results indicate that there is an optimal parameter for caudate stimulation causing inhibition of penicillin-induced hippocampal spiking activity, and suggest the possibility of tonic control of hippocampal excitability exerted by the caudate nucleus.