Substantia nigra and pentylenetetrazol threshold in rats: correlation with striatal dopamine metabolism

Transient reduction of cerebral blood flow leads to longlasting increase in GABA content in vulnerable structures and decreased susceptibility to bicuculline induced seizures

Rats were exposed for 24 min to bilateral clamping of the common carotid arteries (BCCA) in pentobarbital anaesthesia. The GABA content was measured 24 hours, 48 hours, 4 days, 14 days and 3 months after BCCA. In other groups of rats seizures were elicited by i.p. injection of (+)-bicuculline (3 mg/kg) 24 hours, 48 hours, 4 days, 14 days and 3 months after BCCA. Analysis of the GABA content revealed significant increase compared with controls in the hippocampus, frontal cortex and substantia nigra from 24 hours up to 3 months. Bicuculline treatment induced tonic/clonic seizures and status epilepticus in sham operated animals; these effects were drastically diminished at various time points after BCCA. The present results suggest that BCCA produces a longlasting increase in GABA content and as a consequence protection from bicuculline-induced seizures.

Selective stimulation of kainate but not quisqualate or NMDA receptors in substantia nigra evokes limbic motor seizures

Bilateral microinjection of kainic acid (30-117 pmol) into the substantia nigra induced convulsive seizures resembling those elicited from limbic system structures. The convulsive seizures, which consisted of facial and forelimb clonus with rearing and falling, developed after a latency of more than 30 min and were preceded by wet dog shakes and non-convulsive seizure activity registered electroencephalographically. The convulsant effect of intranigral kainic acid was strictly dose-dependent (ED50 = 60 pmol) and anatomically site-specific. Stimulation of nigral neurons by focal application of agonists for NMDA or quisqualate receptors, or by focal application of the GABA antagonist, bicuculline, was without convulsant effects. The convulsant action of intranigral kainic acid was prevented by the focal application of kynurenic acid (100 nmol) but not by 2-amino-7-phosphonoheptanoic acid (AP-7) (25 nmol) or 7-chlorokynurenic acid (20 nmol), suggesting that the convulsant effect of kainic acid in the substantia nigra does not depend upon activation of NMDA receptors in this region.

Dopamine D1 receptor modulation of pilocarpine-induced convulsions

The contribution of dopaminergic mechanisms to the generalization of epileptic activity was studied in rats given pilocarpine after pretreatment with selective dopamine agonists. At the dose of 200 mg/kg, pilocarpine produced limbic stereotypes but not convulsions or seizure-related brain damage. Pilocarpine, 200 mg/kg, following pretreatment with the D1 agonist (RS)-2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3 benzazepine, but not its (S)-enantiomer, induced convulsive activity as revealed by behavioral, electroencephalographic alterations and widespread brain damage. These features were identical to those produced by a higher, convulsant dose of pilocarpine (400 mg/kg). On the other hand, pretreatment with the D2 agonist 4,4a,5,6,7,8,8a,9-octahydro-5-n-propyl-2H-pyrazolo-3,4-g-quinoline failed to induce convulsions. Furthermore, the D1 receptor antagonist (R)-(+)-8-chloro-2,3,4,5-n-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine -7-ol prevented the convulsive activity induced by both 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3 benzazepine plus pilocarpine (200 mg/kg) and pilocarpine (400 mg/kg), given alone. However, neither dopamine agonists nor antagonists altered the limbic stereotypes induced by pilocarpine, suggesting a dopamine system involvement primarily in the mechanisms of epilepsy generalization. The results suggest that pharmacological manipulation of dopaminergic transmission may provide an alternative approach to therapy of secondarily generalized epilepsy.

Progression and generalization of seizure discharge: anatomical and neurochemical substrates

Seizure activity is generated and propagated by specific subcortical circuits. The substantia nigra (SN) and the area tempestas (AT) have been identified as two exemplary substrates for the control of experimental seizures. In animal models, GABAergic transmission has been shown to protect against seizures of different origins and methods of induction. Neuroactive peptides and excitatory amino acids may work with GABA in the SN to control the propagation of a wide variety of seizure types. In contrast, inhibition of AT pons selectively protects against seizures associated with limbic circuits. The AT is also a site from which bilaterally synchronous convulsions can be triggered in response to manipulations of cholinergic, GABAergic, and excitatory amino acid receptors. Definition of other pathways of seizure development and the effects of pharmacologic treatments on discrete brain regions await further research efforts.

MPTP and convulsive responses in rodents

Administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (30 mg/kg s.c. for 5 days) to mice resulted in complete abolishment of strychnine seizure and of the tonic phase of the maximal electroshock response. Bicuculline and picrotoxin convulsions were not significantly affected by MPTP treatment. The severity of the pentylenetetrazole seizures was mildly, but significantly affected in the protective way. MPTP depleted neostriatal dopamine and its metabolites, together with hippocampal norepinephrine. No nigral neuronal loss was detected histologically. Strychnine seizures and the tonic phase of the maximal electroshock response are thought to depend mostly on hindbrain (bulbo-spinal) structures. Thus, these experiments suggest that a caudally projecting system originates from the substantia nigra, pars compacta, and/or locus coeruleus, controlling seizures that involve bulbo-spinal centers. While neostriatal dopamine depletion offers a good index of seizure resistance, its role in the protection from seizures remains to be established.

The role of substantia nigra in the development of kindling: pharmacologic and lesion studies

The role of substantia nigra (SN) in the development of kindling was investigated. Microinjection of gamma-vinyl gamma-aminobutyric acid (GVG), a gamma-aminobutyric acid (GABA) transaminase inhibitor, into the SN bilaterally retarded kindling development by 77%. GVG injected dorsal to the SN did not alter the kindling rate. By contrast, lesions of the SN, whether by thermocoagulation or by microinjected neurotoxin, N-methyl-D,L-aspartate, facilitated kindling development by 27-44%. Thermocoagulative lesions dorsal to the SN did not affect the rate of kindling development. Thus these two manipulations, each presumed to suppress the activity of the SN, resulted in opposite effects on kindling development. We interpret the pharmacologic findings to indicate that the intact SN can powerfully facilitate kindling development. However, the SN is not vital for kindling development, since kindling can be established after destruction of a considerable portion of SN. Whether the increased rate of kindling development following SN lesions is due solely to the absence of SN remains unclear.

Intranigral muscimol attenuates electrographic signs of seizure activity induced by intravenous bicuculline in rats

Rats were implanted with infusion cannulas in the substantia nigra (SN), and intracranial electrodes for recording the electroencephalogram (EEG). These rats received intravenous injections of bicuculline, producing repetitive high voltage spike-and-wave complexes. Pretreatment with bilateral intranigral infusions of the GABA agonist muscimol reduced the duration of EEG epileptic activity by over 80%, as compared to the effect of pretreatment with intranigral saline in the same rats. This antiepileptic effect of intranigral muscimol may reflect a primary influence of SN outputs on brain excitability.

Susceptibility to seizures produced by pilocarpine in rats after microinjection of isoniazid or gamma-vinyl-GABA into the substantia nigra

Pilocarpine, given intraperitoneally to rats, reproduces the neuropathological sequelae of temporal lobe epilepsy and provides a relevant animal model for studying mechanisms of buildup of convulsive activity and pathways operative in the generalization and propagation of seizures within the forebrain. In the present study, the effects of manipulating the activity of the gamma-aminobutyric acid (GABA)-mediated synaptic inhibition within the substantia nigra on seizures produced by pilocarpine in rats, were investigated. In animals pretreated with microinjections of isoniazid, 150 micrograms, an inhibitor of activity of the GABA-synthesizing enzyme, L-glutamic acid decarboxylase, into the substantia nigra pars reticulata (SNR), bilaterally, non-convulsant doses of pilocarpine, 100 and 200 mg/kg, resulted in severe motor limbic seizures and status epilepticus. Electroencephalographic and behavioral monitoring revealed a profound reduction of the threshold for pilocarpine-induced convulsions. Morphological analysis of frontal forebrain sections with light microscopy revealed seizure-related damage to the hippocampal formation, thalamus, amygdala, olfactory cortex, substantia nigra and neocortex, which is typically observed with pilocarpine in doses exceeding 350 mg/kg. Bilateral intrastriatal injections of isoniazid did not augment seizures produced by pilocarpine, 200 mg/kg. Application of an irreversible inhibitor of GABA transaminase, gamma-vinyl-GABA (D,L-4-amino-hex-5-enoic acid), 5 micrograms, into the SNR, bilaterally, suppressed the appearance of electrographic and behavioral seizures produced by pilocarpine, 380 mg/kg. This treatment was also sufficient to protect animals from the occurrence of brain damage. Microinjections of gamma-vinyl-GABA, 5 micrograms, into the dorsal striatum, bilaterally, failed to prevent the development of convulsions produced by pilocarpine, 380 mg/kg. The results demonstrate that the threshold for pilocarpine-induced seizures in rats is subjected to the regulation of the GABA-mediated synaptic inhibition within the substantia nigra.

Genetically determined predisposition to convulsions as the result of a generalized defect in the metabolism of catecholamines in the central nervous system

The author compared the functional properties of the striatal system in KM rats sensitive to the convulsive effects of sound with those in Wistar rats, which are insensitive to these effects. It was shown that bulbocapnine (an antagonist of dopamine) administered to the Wistar rats at a dose of 40 mg/kg body weight caused catalepsy, depressed the motor cortex excitability, and raised the threshold of the generalized Jacksonian-type convulsions. The KM rats showed neither catalepsy nor a rise in the generalized convulsion threshold, and the depression of the motor cortex excitability in them was only slight. Examinations of the apomorphine-induced stereotypy (dose 1.0-10 mg/kg) showed that in the KM rats the sensitivity of the receptors to dopamine was changed. The hyperproduction of catecholamines in the striatum, the hypothalamus, and adrenals in the KM rats suggests that the predisposition to epileptiform states correlates with the generalized defect in the metabolism of catecholamines. It is suggested that the hypersensitivity of KM rats to epileptogenic effects is due to a deficiency (caused by an excess of dopamine) in the depressing function of the striatum.

Changes in local cerebral glucose utilization induced by convulsants

With the six convulsants studied (Soman, intrahippocampal penicillin, bicuculline, pentylenetetrazol, picrotoxin and strychnine), the anatomical distribution of changes in local cerebral glucose utilization was related to the type of seizure observed. Strychnine induced a few very intense motor convulsions during the 2-deoxyglucose experimental period without having a major effect on brain local cerebral glucose utilization, in support of the view that its actions are predominantly in the spinal cord. Pentylenetetrazol and picrotoxin induced intermittent intense seizures and marked increases in local cerebral glucose utilization in the globus pallidus and substantia nigra. Soman, intrahippocampal penicillin and bicuculline all induced persistent status epilepticus associated with increases in local cerebral glucose utilization in many brain areas; those with striking increases in glucose use include: cortical areas, the limbic system, basal ganglia and substantia nigra. The glucose use changes produced by Soman, penicillin and bicuculline greatly exceeded those induced by pentylenetetrazol and picrotoxin. Activation of the substantia nigra and basal ganglia occurred with all centrally mediated convulsions and with status epilepticus there was also marked activation of cortical and limbic structures.

Nigral muscimol infusions facilitate the development of seizures in immature rats

Since recent data utilizing GABAergic stimulation of the substantia nigra (SN) suggest that the SN is a crucial site in a circuitry involved in the modification of seizures in adult rats, the role of the SN was investigated in seizures of rat pups. Bilateral nigral infusions of the GABA agonist muscimol partially protected adult rats against flurothyl-induced seizures, while similar infusions actually facilitated the development of flurothyl seizures in 15-day-old rat pups. These results suggest that age-related differences in the nigral GABA sensitive system may account for the increased susceptibility to generalized seizures of the developing brain.

Kainic-acid-induced seizures: a developmental study

Developmental dose-response curves for kainic-acid-induced seizures were generated in rats. Rats at 15-18 days (pups), 33-37 days ( pubescents ) and over 90 days (adults) were administered kainic acid intraperitoneally. Seizures were elicited in all 3 age groups, but some of the behavioral manifestations differed in the pups. This group also had the lowest convulsive threshold, the most severe seizures and the highest mortality. Forelimb convulsions and status epilepticus were associated with the occurrence of necrotic lesions in the adults and pubescents but not in the pups. Deoxyglucose (DG) autoradiographic studies of the convulsing rats disclosed differences in the DG uptake pattern of the substantia nigra across the 3 age groups. Increases in the DG uptake were present in the two older age groups but not in the pups. Since recent data have implicated the substantia nigra as a crucial site in a seizure modifying circuitry in adult animals, our results suggest that the lack of substantia nigra involvement in the pups may account for the early generalization and the increased severity of seizures in this age group.

The perspective of GABA replenishment therapy in the epilepsies: a critical evaluation of hopes and concerns

Impaired GABA-mediated inhibition is probably one of the cellular abnormalities leading to Focal Epilepsy. The role of GABA in generalized seizures, particularly of Petit Mal type, is unknown. Various approaches are available to potentiate GABA function. Merits and flaws of each one of them are critically evaluated. In some forms of epilepsy, GABA agonists may replenish depleted pools, and in some others may nonspecifically raise the general excitability threshold of the brain, yet in other forms they may exert a glutamate/aspartate antagonistic effect. The available experimental evidence suggests that in bilaterally synchronous spike and wave epilepsies, GABA agonists are either ineffective or pejorative.

Lesions of substantia nigra protect against experimentally induced seizures

Bilateral lesions of substantia nigra (SN) were evaluated for anticonvulsant effects in rats. Electrolytic and kainic acid lesions of SN reduced bicuculline-elicited seizure activity; lesions in the ventral midbrain tegmentum adjacent to SN were without an anticonvulsant effect. Lesions of SN also reduced the incidence and duration of tonic hindlimb extension in the maximal electroshock seizure test. Bilateral SN lesions appeared to decrease seizure susceptibility to a given stimulus intensity rather than altering the motor pattern of the seizures. Our results support the view that SN efferents constitute a critical gating mechanism in the propagation of seizure activity.

Substantia nigra: site of anticonvulsant activity mediated by gamma-aminobutyric acid

Localization of the anatomic substrate for anticonvulsant activity mediated by gamma-aminobutyric acid (GABA) was examined using intracerebral injections of GABA agonists. Blockade of tonic hindlimb extension in the maximal electroshock test and blockade of tonic and clonic seizures produced by pentylenetetrazole and bicuculline were obtained by elevating GABA in the ventral midbrain tegmentum. Elevation of GABA in forebrain and hindbrain areas had no effect on convulsant activity. Blockade of tonic and clonic seizures was also obtained after microinjections of the direct GABA receptor agonist, muscimol, into the midbrain. The substantia nigra was identified as the critical midbrain site for GABA-mediated anticonvulsant activity. Local injection of GABA agonists into the midbrain provided seizure protection without a widespread augmentation of GABA-mediated activity throughout the brain and without impairing either alertness or motor function. Synapses in the substantia nigra appear to represent an important control mechanism for inhibiting the propagation of generalized convulsions.