Substantia nigra pars reticulata lesion facilitates kainic acid-induced seizures

Not Part of the Temporal Lobe, but Still of Importance? Substantia Nigra and Subthalamic Nucleus in Epilepsy

The most researched brain region in epilepsy research is the temporal lobe, and more specifically, the hippocampus. However, numerous other brain regions play a pivotal role in seizure circuitry and secondary generalization of epileptic activity: The substantia nigra pars reticulata (SNr) and its direct input structure, the subthalamic nucleus (STN), are considered seizure gating nuclei. There is ample evidence that direct inhibition of the SNr is capable of suppressing various seizure types in experimental models. Similarly, inhibition via its monosynaptic glutamatergic input, the STN, can decrease seizure susceptibility as well. This review will focus on therapeutic interventions such as electrical stimulation and targeted drug delivery to SNr and STN in human patients and experimental animal models of epilepsy, highlighting the opportunities for overcoming pharmacoresistance in epilepsy by investigating these promising target structures.

Intranigral transplants of immortalized GABAergic cells decrease the expression of kainic acid-induced seizures in the rat

Repeated systemic administration of low doses of kainic acid (KA) induces spontaneous convulsive seizures [Hellier JL, Patrylo PR, Buckmaster PS, Dudek FE. Recurrent spontaneous motor seizures after repeated low-dose systemic treatment with kainate: assessment of a rat model of temporal lobe epilepsy. Epilepsy Res 1998;31:73-84]. In this study, male Sprague-Dawley animals received intranigral transplants of a control cell line M213-2O, or a cell line transfected with human GAD67 cDNA (M213-2O CL4) [Conejero-Goldberg C, Tornatore C, Abi-Saab W, Monaco MC, Dillon-Carter O, Vawter M, et al. Transduction of human GAD67 cDNA into immortalized striatal cell lines using an Epstein-Barr virus-based plasmid vector increases GABA content. Exp Neurol 2000;161:453-61], or no transplant. Eight weeks after transplantation surgery, KA was administered (5 mg/kg/h) until animals reached stage V seizures as described by Racine [Racine RJ. Modification of seizure activity by electrical stimulation. II. Motor seizure. Electroencephalogr Clin Neurophysiol 1972;32:281-94]. The group transplanted with CL4 required a larger dose of KA and a longer latency to reach a stage V seizure. In addition, this group exhibited significantly fewer stage III and IV seizures. These results indicate that intranigral transplants of a GABA-producing cell line can decrease the number of kainic acid-induced seizures.

Congenital brain dysplasias of different genesis can differently affect susceptibility to pilocarpine- or kainic acid-induced seizures in the rat

Interruption of neurogenesis and/or neuronal migration produces brain dysplasia modifying susceptibility to epileptic seizures in adulthood. The course of neurogenesis has a strictly defined time-table. Consequently, the developmental stage at which the interruption occurs determines what functional subsystem potentially involved in epileptogenesis will suffer from irreversible neuronal deficit. The present study attempts to verify a hypothesis that brain dysplasias of different genesis should also lead to different susceptibility to seizures evoked by receptor agonists of different functional specificity, like kainic acid or pilocarpine, a cholinergic or glutaminergic agonist, respectively. Pregnant Wistar rats were exposed to gamma-rays on gestation days 13, 15, 17 or 19 (E13, E15, E17 or E19). Sixty-day-old offsprings of the females were injected with kainic acid or pilocarpine to evoke status epilepticus. During a 6-h period following the injection, motor manifestations of seizure activity were recorded. Generally, the intensity of pilocarpine-induced symptoms was relatively low in rats irradiated on E13 or E15 but high in rats irradiated on E17 or E19. In rats treated with kainic acid, the trend was opposite, viz. the later the prenatal irradiation was performed, the less intense epileptic symptoms were induced in adulthood. The data provide evidence that dysplasias acquired during prenatal development may significantly amplify or reduce the brain susceptibility to seizures. However, this relation depends not only on the developmental stage at which the dysplasias were produced but also on the functional specificity of epileptogenic stimuli used in the experimental model of epilepsy.

The nicotinic acetylcholine receptor subunit alpha 5 mediates short-term effects of nicotine in vivo

Nicotine, acting at pentameric neuronal nicotinic acetylcholine receptors (nAChRs), is the primary addictive component in tobacco. At low doses, it affects attention, learning, memory, anxiety, cardiovascular responses, thermoregulation, and nociception. At high doses, nicotine produces more drastic behaviors and eventually induces tonic-clonic seizures in rodents. In mammals, several subunits of the nAChRs have been cloned, including eight alpha and three beta subunits. To study the physiological role of the alpha 5 subunit, we have generated alpha 5-deficient mice. These mice have a generally healthy appearance and are normal in a standard battery of behavioral tests. However, the sensitivity of alpha 5 mutant mice to nicotine-induced behaviors and seizures is dramatically reduced compared with their wild-type littermates. These animals have a normal brain anatomy and normal levels of mRNA for other nAChR subunits, namely alpha 4, alpha 6, alpha 7, beta 2, and beta 4. In addition, (125)I-epibatidine and [(125)I]alpha-bungarotoxin binding in the brains of alpha 5-deficient mice is normal. Together, these results suggest a direct involvement of the alpha 5 subunit in the observed phenotypes.

Electrical stimulation of substantia nigra pars reticulata is anticonvulsant in adult and young male rats

Electrical stimulation of deep brain structures has been used for pain relief and treatment of refractory Parkinson's disease. Recently, stimulation of the subthalamic nucleus or anterior nuclei of the thalamus was introduced for the treatment of refractory epilepsy when other treatments failed. The substantia nigra pars reticulata (SNR) is another crucial site involved in the control of seizures. We studied the effects of continuous electrical stimulation of the SNR as a function of age in male rats. Adult [postnatal day (PN) 60] and young (PN 15) rats with electrodes symmetrically implanted in the SNR were used. The rats were stimulated with continuous constant current pulses (130 Hz) and simultaneously challenged with flurothyl to induce seizures. Control rats had the electrodes implanted but were not stimulated. High-frequency electrical stimulation of the SNR had anticonvulsant effects in both age groups. However, we identified age-specific features: In PN 60 rats, both unilateral and bilateral stimulation of the anterior region of the SNR produced anticonvulsant effects against clonic seizures, while stimulation of the posterior region of the SNR was ineffective. Stimulation of either SNR region had no effects on tonic-clonic seizures. In PN 15 rats, irrespective of the stimulation site within the SNR, bilateral stimulations of the SNR produced anticonvulsant effects against both clonic and tonic-clonic flurothyl-induced seizures, while unilateral stimulation was without effect. The data suggest that the SNR may be a candidate site for deep brain stimulation for the treatment of epilepsy.