Increased seizure susceptibility in adult rats with neuronal migration disorders.

Recent data show that neuronal migration disorders (NMD) lower the seizure threshold in the immature brain. To assess if this is an age-related phenomenon, kainic acid (KA) was administered to induce status epilepticus in adult rats with NMD. Results of the present study demonstrate that adult rats with NMD had a shorter latency to seizures and longer duration of status epilepticus compared to age-related controls. Furthermore, in rats with NMD seizures were more severe and status epilepticus-induced mortality was worse than in age-matched controls. These data confirm that NMD lower the seizure threshold in the adult rat. The results of the present study combined with our previous studies in the immature rat, suggest that the facilitating effects of NMD on seizures are not age dependent.

Age-related differences in seizure susceptibility to flurothyl.

The purpose of the present study was to determine whether the susceptibility to seizures induced by inhalation of flurothyl ether (FE) varies with age. Adult rats and 16-day-old rat pups were tested in different sized chambers to also determine whether the size of the FE-test chamber influences seizure thresholds. Results indicate that pups developed age-specific seizure patterns; their seizure latency thresholds were shorter than those of adult animals. For both age-groups, seizure thresholds varied as a function of chamber size; the smaller the chamber the faster seizures occurred.

Age-related changes of muscimol binding in the substantia nigra.

Receptor binding studies of the substantia nigra (SN) and cerebellum revealed two affinity sites for muscimol binding in the SN and cerebellum of adult and 16-day-old rats. Scatchard analysis revealed a paucity of high-affinity muscimol receptors in the SN of 16-day-old rat pups. These results suggest that the lack of anticonvulsant action of muscimol in the SN of 16-day-old rat pups may be due to the paucity of high-affinity muscimol receptors as compared to adult rats.

Maturation and segregation of brain networks that modify seizures.

The mature brain is less susceptible to seizures than the immature brain. We demonstrate that in the mature substantia nigra (SN) there are two topographically discrete GABAA-sensitive regions which differ in the amount of mRNA expression of the GABAA receptor alpha 1 subunit. These two regions mediate separate anticonvulsant and proconvulsant effects and use divergent projection networks. By contrast, in the immature SN there is no special topography of mRNA expression of the alpha 1 subunit and only the proconvulsant network is present. The decreased seizure susceptibility of the mature brain may be related to postnatal segregation of GABAA-sensitive networks.

Effects of substantia nigra gamma-vinyl-GABA infusions on flurothyl seizures in adult rats.

There is evidence implicating the nigral gamma-aminobutyric acid (GABA) system in the control of seizures. Our previous studies have demonstrated that, in rat pups, intranigrally infused gamma-vinyl-GABA (GVG, 5-20 micrograms) strongly suppresses flurothyl-induced tonic but not clonic seizures. Furthermore, nigral infusions of bicuculline or muscimol abolish the anticonvulsant effect of GVG. In this study, we report that in adult rats bilateral infusions of GVG (20 micrograms) into the substantia nigra pars reticulata (SNR) significantly elevated the thresholds for both clonic and tonic seizures induced by flurothyl. Lower doses (5 and 10 micrograms) did not significantly protect adult rats against seizures, but there was a significant effect of GVG dose. Unilateral infusion of GVG (20 micrograms) in the SNR did not alter the thresholds for flurothyl-induced seizures. Intranigral infusions of bicuculline following pretreatment with GVG abolished the protective effect of GVG on flurothyl-induced seizures, indicating that the anticonvulsant effect of GVG is most likely mediated by the nigral GABAA receptor. Intranigral administration of muscimol after GVG pretreatment significantly suppressed flurothyl-induced seizures, but the combined effect of the two drugs was not as strong as that of GVG alone. The data suggest that GVG protects adult rats against flurothyl-induced seizures. In adults, however, the dose of GVG required to protect against both clonic and tonic seizures is higher than that needed in rat pup SNR.

Age-related changes in striatal dopamine activity following nigral muscimol infusions.

Microinfusions of muscimol into the substantia nigra pars reticulata produced marked increases in striatal dopamine (DA) utilization without affecting striatal DA concentration in adult rats. In contrast, muscimol increased striatal DA concentration and decreased DA utilization in 16-day-old rat pups. The striatal norepinephrine concentration was not altered in either group. Since previous studies have shown that similar infusions of muscimol are anticonvulsant in adults and proconvulsant in rat pups, our results suggest that the nigrostriatal pathway may play an important role in mediating the nigral effects on seizures.

Nigral infusions of muscimol or bicuculline facilitate seizures in developing rats.

The substantia nigra (SN) appears to be a crucial site involved in the modification of seizures. The aim of this study was to elucidate the role of the GABA nigral system in the expression of seizures by comparing the effects of multiple doses of a GABA agonist (muscimol) and a GABA antagonist (bicuculline methobromide) on the development of flurothyl seizures in 16-day-old rat pups. The drugs were infused bilaterally either in the SN or dorsal to the SN. An additional group of pups were infused with bicuculline in the corpus striatum. Results indicate that both drugs facilitated the development of seizures in a dose-related manner when infused into the SN. Infusions of muscimol dorsal to the SN had no effect on seizure latencies while infusions of bicuculline dorsal to SN or corpus striatum still increased the susceptibility of rat pups to seizures. The data suggest that only the effects of muscimol on seizures are specific for the SN and that early in life muscimol may exert its proconvulsant effects via a different receptor site or mechanism than bicuculline.

Evidence for the involvement of nigral GABAA receptors in seizures of adult rats.

Several studies have implicated the substantia nigra GABAergic system in the mediation of seizures in adult rats. The present study examines whether the different GABA receptors (GABAA and GABAB), are preferentially involved in this GABAergic seizure suppression mechanism. Adult rats were intranigrally infused with muscimol (GABAA receptor agonist), bicuculline (GABAA receptor antagonist) or baclofen (GABAB receptor agonist) and were exposed to flurothyl seizures. Results indicated that while infusions of muscimol had an anticonvulsant effect, infusions of bicuculline had a proconvulsant effect. Baclofen infusions were found to have no effect on seizures. These findings suggest an involvement of the nigral GABAA receptors in the mediation of seizures in adult rats.

Is the anticonvulsant effect of substantia nigra infusion of gamma-vinyl-GABA (GVG) mediated by the GABAA receptor in rat pups?

The substantia nigra GABAergic system is considered important for the modification of seizures. Our previous studies have shown that, in rat pups, nigral infusions of baclofen suppressed flurothyl-induced seizures. In the present study, we determined, in rat pups, the effect of nigral infusions of gamma-vinyl-GABA (GVG) on clonic-tonic seizures induced by flurothyl, generated a dose-response curve of the GVG effect and investigated the possible role of the nigral GABAA receptor in mediating the GVG effect. Bilateral nigral infusions of GVG profoundly suppressed flurothyl-induced tonic seizures in a dose-dependent fashion. Flurothyl-induced clonic seizures were not modified. The lowest effective dose of nigral GVG administration was 5 micrograms/0.25 microliter per site. Nigral infusions of GVG at doses greater than 10 micrograms/0.25 microliter induced sedation as well. Infusions of GVG, 2 mm dorsal to the substantia nigra, did not alter seizure latencies. Bilateral nigral infusions of bicuculline, a specific GABAA receptor antagonist, reduced the protective potency of GVG against flurothyl-induced seizures. Nigrally administered muscimol, a GABAA receptor agonist, also attenuated the anticonvulsant effect of GVG. These findings suggest that the optimal dose of nigrally infused GVG against flurothyl-induced seizures is in the range of 5-10 micrograms/0.25 microliter and that GVG may be more efficient as an anticonvulsant for the treatment of tonic seizures in developing animals. The anticonvulsant effect of GVG may, in part, involve the nigral GABAA receptor. The data, together with the previous experiments, indicate that both nigral GABAA and GABAB receptors may play a role in the regulation of seizures in rat pups.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the involvement of nigral GABAB receptors in seizures of rat pups.

The substantia nigra GABA-sensitive system is important for the modification of seizures. In adult rats, nigral infusions of baclofen (GABAB agonist) had no effect on flurothyl seizures. This suggests that the nigral GABAB receptor system may not be involved in the mediation of flurothyl seizures in this age group. The present study examines whether the nigral GABAB receptor is involved in the modification of flurothyl seizures in rat pups and whether systemic infusions of baclofen could alter their seizure susceptibility. Baclofen (50-200 ng/0.25 microliters) was intranigrally administered to 16-day-old rat pups which were then exposed to flurothyl seizures. Results indicated that intranigral infusions of baclofen (100 and 200 ng/0.25 microliters) protected the rat pups against seizures. Systemic injections of baclofen also protected pups from flurothyl seizures suggesting that it may be worthwhile to examine baclofen as a potential antiepileptic drug in children.

Kindling in developing animals: expression of severe seizures and enhanced development of bilateral foci.

In adult rats, alternating stimulations between two limbic sites can result in one site kindling normally, while the other is retarded in an early non-generalized kindling stage. This phenomenon has been named kindling antagonism. In this report, we present data indicating that kindling antagonism does not occur in 16-day-old rats. Instead, 16-day-old rats receiving alternating stimulations in the amygdala and hippocampus develop progressively more severe seizures. Kindling with alternate stimulations is elicited at a much faster rate at the two foci compared to kindling from a single site, either the hippocampus or amygdala. All groups develop generalized seizures including seizure stages 6 and 7, consisting of wild jumping, running with vocalizations and tonus. These seizures appear after relatively few stimulations in the pup, in comparison to the adult. The results indicate that the immature brain is less able to suppress the generalization of seizures than the adult. The age-specific enhanced development of bilateral foci may be due to underdeveloped inhibitory systems and may underlie the propensity of the immature CNS to develop multifocal seizures.

Kindling in developing animals: interactions between ipsilateral foci.

In adult rats, concurrent kindling of two limbic sites, alternating stimulation to each site, often results in the retarding of kindling at one or both sites. This inhibitory interaction between two limbic kindling foci is termed kindling antagonism and occurs irrespective of whether the sites are contralateral or ipsilateral. We have previously shown that kindling antagonism does not occur when 16- to 17-day-old rat pups are concurrently kindled in the hippocampus and contralateral amygdala or between the two amygdalae. In this study, adult and 16- to 17-day-old rats were concurrently kindled in the hippocampus and ipsilateral amygdala to determine if the local intrahemispheric mechanisms suppressing multiple kindled foci are age-dependent. Kindling antagonism occurred in 7 out of 10 adult rats. In contrast, in rat pups, kindling development was not suppressed. Concurrent kindling of the two limbic sites enhanced the development of severe seizures. Two 16- to 17-day-old rats receiving alternating stimulations exhibited spontaneous seizures. The age-specific failure of both inter- and intrahemispheric mechanisms to suppress the development of multiple kindling foci may explain the high incidence of multifocal seizures in the immature CNS.

Resistance of the immature hippocampus to seizure-induced synaptic reorganization.

Temporal lobe epilepsy is a common form of epilepsy in human adults and is associated with a unique pattern of damage in the hippocampus. The damage includes cell loss of the CA3 and CA4 areas and synaptic growth (sprouting) of mossy fibers in the supragranular layer of the dentate gyrus. Experimental evidence indicates that in adult rats the excitatory amino acid, kainic acid, induces a similar pattern of changes in hippocampal circuitry associated with alterations in perforant path excitation and inhibition. It has been suggested that, in humans, this type of damage may be a result of seizures early in life. In this study we examined the effects of kainic acid-induced status epilepticus on synaptic reorganization and paired-pulse electrophysiology in developing rats and adults. Kainic acid induced more severe seizures in 15-day-old rat pups than in adults. In contrast to adult rats, these seizures did not produce CA3/CA4 neuronal loss, mossy fiber sprouting or changes in paired-pulse excitation or inhibition in the hippocampus of rat pups tested 2-4 weeks after status epilepticus. Our results provide evidence that the immature hippocampus may be more resistant to seizure-induced changes than the mature hippocampus.

Flurothyl status epilepticus in developing rats: behavioral, electrographic histological and electrophysiological studies.

Status epilepticus and repeated seizures have age-dependent morphological and neurophysiological alterations in the hippocampus. In the present study, effects of flurothyl-induced status epilepticus were examined in awake and free moving immature (2 weeks old) and adult rats. Without exception, adult rats died of respiratory arrest before the onset of status epilepticus. We were unable to find a concentration of flurothyl that produced status epilepticus and a low mortality in adult rats. In contrast, immature rats survived flurothyl status epilepticus for up to 60 min with a very low mortality. In rat pups, behavioral manifestations correlated with electrographic seizures in both the cortex and hippocampus. Neuropathological damage (cell loss, pyknotic cells or gliosis) was not observed in the immature hippocampus, thalamus, amygdala, substantia nigra or cortex at 24 h, 2 days or 2 weeks after status epilepticus. In addition, no aberrant mossy fiber reorganization or decrease in cells counts were observed in the hippocampus. Young rats did not show alterations in paired-pulse perforant path inhibition following flurothyl status epilepticus. The present findings are consistent with studies in other seizure models, indicating that immature rats are highly resistant to seizure-induced changes.

The proconvulsant effect of nigral infusions of THIP on flurothyl-induced seizures in rat pups.

The substantia nigra gamma-aminobutyric acid (GABA) system is crucial for seizure control. Our previous work indicates that in 16-day-old rat pups, nigral administration of the GABAA receptor agonist muscimol facilitates flurothyl-induced seizures, whereas it suppresses seizures in adult rats. To determine whether the proconvulsant effect of muscimol in rat pups may be mediated by nigral GABAA receptors, in the present study we applied a selective GABAA receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). Bilateral nigral infusions of THIP (500 or 700 ng) significantly decreased the thresholds for flurothyl seizures in a dose-dependent fashion. Doses of 350 ng or less did not significantly modify the susceptibility to seizures. An anticonvulsant action of THIP could not be detected at any dose. Administration of an effective THIP dose (500 ng) 2 mm dorsal to the SNR had no influence on seizures. These findings suggest that in rat pups the proconvulsant effect of nigral GABAA receptor agonists may be attributed to unique pharmacologic characteristics of GABAA receptors during development.

Baclofen inhibits amygdala kindling in immature rats.

The possible effectiveness of systemic infusions of baclofen (a GABAB receptor agonist) in the modification of seizures in developing animals has not yet been established. In this report, we studied the effects of systemic baclofen infusions on amygdala kindling in 16-day-old rat pups. Baclofen suppressed the development of kindling without altering the local afterdischarge threshold. Moreover, baclofen suppressed the severity and duration of established kindled seizures and increased the intensity of postictal refractoriness. The data suggest that baclofen may be a useful antiepileptic agent in this age group.

The influence of thalamic GABA transmission on the susceptibility of adult rats to flurothyl induced seizures.

There is considerable evidence that thalamic nuclei are involved in the propagation and regulation of seizures. In the present study, we investigated the possible role in seizure mechanisms of GABAergic transmission in two thalamic nuclei, the posterior nucleus (PO) and the ventromedial nucleus (VM). Several GABAergic drugs were bilaterally microinfused into PO or VM of adult rats via chronically implanted cannulae, before testing the rats' susceptibility to seizures induced by flurothyl. In PO, infusions of the GABA elevating agent gamma-vinyl-GABA (20 micrograms) or of the GABAA receptor agonist muscimol (100 ng) suppressed both clonic and tonic seizures. Infusions into PO of the GABAA receptor antagonist bicuculline (100 ng) facilitated both these seizure types. Administration of the GABAB receptor agonist baclofen (200 ng) also suppressed clonic seizure susceptibility. Drug infusions into VM, however, did not significantly modify the susceptibility to seizures. These findings lead us to conclude that GABAergic transmission in the vicinity of the PO, but perhaps not in VM, affects flurothyl seizure susceptibility. We hypothesize that GABA synapses in PO may be part of a seizure propagation or control circuit including striatum, substantia nigra, and superior colliculus.

Age-dependent vulnerability to seizures.

Seizure disorders frequently occur early in life. Seizures are classified as reactive, symptomatic, or idiopathic depending on whether their cause can be identified. Reactive seizures are the result of acute environmental perturbations. Early in life, many stressors can produce seizures and the ultimate outcome may depend on the particular precipitating factor and its intensity. Febrile convulsions are the most common reactive seizures, although they must be differentiated from symptomatic seizures precipitated by fever. Symptomatic seizures are often associated with varying degrees of central nervous system (CNS) insults, including congenital malformations and metabolic storage diseases of the gray matter. These seizures may have age-specific characteristics and may at times be difficult to treat with conventional antiepileptic treatments. To develop a better understanding of the pathophysiology of seizures early in life, we have extensively used animal models of epilepsy. In this chapter, we report our findings with a rat model of developmental cortical dysplasias produced by intrauterine injections of methylazoxymethanol acetate. These rats are more susceptible to kainic acid, flurothyl, and hyperthermic seizures than normal rats. Rats with severe cortical dysplasia are most susceptible to seizures. We have also studied the mechanisms involved in the control of seizures during development because status epilepticus is more prevalent in infants than in adults. Our data suggest that the substantia nigra may play a crucial role in status epilepticus as a function of age. In the adult substantia nigra two regions mediate opposing effects on seizures following infusions of gamma-aminobutyric acid type A (GABAA) agents. One region is located in the anterior substantia nigra, and muscimol infusions in this region mediate anticonvulsant effects. The second region is in the posterior substantia nigra, and here muscimol infusions produce proconvulsant effects. In situ hybridization data demonstrate that, at the cellular level, neurons in the two substantia nigra regions differ in the amount of hybridization grains for GABAA receptor alpha 1 and gamma 2L subunit mRNAs. In developing male rats, only the "proconvulsant" region is present up to the age of 21 days. The transition from the immature to mature substantia nigra mediated seizure control occurs between age 25 and 30 days. The identification of age-dependent functional networks involved in the containment of seizures may lead to possible new pharmacologic strategies to control seizures, thus aiding the development of age-appropriate treatments of seizure disorders.

Intranigral GABAergic drug effects on striatal dopamine activity.

Concentrations of striatal dopamine (DA), serotonin (5-HT) and their metabolites were measured following infusions of the GABAA receptor agonist, muscimol, or GABAA receptor antagonist, bicuculline, into the substantia nigra (SN) or areas dorsal to the SN in adult rats and 16-day-old rat pups. Results indicated that intranigral infusions of muscimol produced site-specific increases in the concentrations of striatal DA metabolites in adults, while in pups, intranigral muscimol infusions produced site-specific increases in the concentrations of striatal DA. Intranigral infusions of bicuculline had no effect on striatal DA or its metabolites in either age group. Neither GABAergic drug had any effect on striatal 5-HT or its metabolite. The data suggest that the effect of nigral GABAA agonist infusions on the activity of the nigrostriatal pathway is age-specific. The lack of opposing effects following the nigral infusion of a GABAA receptor antagonist indicates that the influences of GABAA agonists may be mediated by different mechanisms as a function of age.

Hippocampal sclerosis revisited.

Studies dating back more than 150 years reported a relationship between hippocampal sclerosis and epilepsy. Retrospective studies of patients who underwent temporal lobectomy for intractable partial epilepsy found a relationship between a history of early childhood convulsions, hippocampal sclerosis, and the development of temporal lobe epilepsy. Many believe that febrile seizures lead to hippocampal damage and this in turn predisposes the patient to the development of temporal lobe epilepsy. Studies in adult rats have shown that seizures can lead to hippocampal damage and unprovoked recurrent seizures. However, many questions remain as to the relevance of early childhood seizures to hippocampal sclerosis and temporal lobe epilepsy. Human prospective epidemiologic studies have not shown a relationship between early childhood seizures and temporal lobe epilepsy. Recent MRI studies in humans suggest that a preexisting hippocampal lesion may predispose infants to experience febrile seizures, later on hippocampal sclerosis, and possibly temporal lobe epilepsy may occur. Unlike the studies in adult rats, normal immature rats with seizures have not been shown to develop hippocampal damage or unprovoked seizures in adulthood. Furthermore, animal studies reveal that preexisting brain abnormalities can predispose to hippocampal damage following seizures early in life. This paper reviews evidence for and against the view that early childhood convulsions, hippocampal sclerosis, and temporal lobe epilepsy are related, while also exploring clinical and animal studies on how seizures can lead to hippocampal damage, and how this can result in temporal lobe epilepsy. By better understanding the cause and effect relationship between early childhood seizures and hippocampal injury in normal and abnormal brains specific treatments can be developed that target the pathogenesis of epilepsy.