Maturational changes in postictal refractoriness and seizure susceptibility in developing rats

The ability of the central nervous system to suppress recurrent seizures as a function of age was determined in rats. Eight electrical stimulations were delivered to the amygdala at 2-minute intervals in adult and suckling rats that were previously kindled from the left amygdala. During this 16-minute period, prolonged and severe convulsions were repeatedly elicited in the 16-day-old rat pups, whereas convulsions were not always elicited in adul rats. The results suggest that immature rats are more prone to develop status epilepticus than adult rats. Subsequently, the rat pus and implanted but not stimulated littermates were allowed to grow. At maturity, all the surviving animals were kindled from the contralateral (right) amygdala. The previously kindled rats developed seizures significantly faster than did their littermates. However, their respective abilities to suppress recurrent seizures did not differ at 150 and 210 days of age, being similar to the seizure suppression abilities of naive controls and significantly greater than those evidenced in infancy. We conclude that the propensity to develop single seizures in adulthood is directly related to infant seizure history, whereas the ability to suppress multiple seizures is a dynamic process that is modified by age, being minimal early in life and enhanced with maturation independently of history of infantile convulsions.     

Estrogen modulation of NMDA-induced seizures in ovariectomized and non-ovariectomized rats

The objective of this study was to examine the neuroprotective effects of estrogen in response to N-methyl-D-aspartate (NMDA)-induced seizures in both male and female rats. Thirty-eight Long-Evans rats were divided into five groups: ovariectomized females, non-ovariectomized females, ovariectomized females with estrogen replacement (10 microg 17beta-estradiol in 100 microl sesame oil), males given exogenous estrogen and males receiving no estrogen. Using stereotaxic surgery, a cannula was placed in the lateral ventricle for convulsant agent administration (20 microg of NMDA), while an electrode was placed into the hippocampus for seizure recording. Seizure activity was monitored for 20 min. Onset to first seizure, first seizure duration, seizure frequency and total duration of seizures were determined. Rats were pretreated with either sesame oil (vehicle) or estrogen given subcutaneously for 4 days prior to seizure induction on the fourth day. Rats were euthanized 72 h later and the brains removed for histological processing. Electrode and cannula placement were verified microscopically and neuronal integrity was assessed via hematoxylin and eosin staining. Total seizure number was significantly higher in the ovariectomized females compared to the non-ovariectomized females and the ovariectomized females receiving estrogen (P<0.05). Moreover, hippocampal neuronal damage following seizure induction was significant in the ovariectomized rats compared to the non-ovariectomized rats (P<0.05). Pretreatment with estrogen did not affect any of the seizure parameters measured in the male rats. We conclude that estrogen appears to be neuroprotective against NMDA-induced seizures in female ovariectomized rats.     

Prenatal morphine exposure differentially alters seizure susceptibility in developing female rats.

We examined the effect of prenatal morphine exposure (5-10 mg/kg on days 11-18 of gestation) on seizure susceptibility in female rats during development. The effect of morphine exposure on flurothyl-induced seizures was age-dependent. At postnatal day (PN) 15, morphine exposure decreased both clonic and tonic-clonic seizure thresholds compare to saline controls. At PN 25, morphine exposure did not alter the clonic seizure threshold but increased the threshold to tonic-clonic seizure. At PN 38, morphine exposure did not influence either threshold. The data suggest that the effects of prenatal exposure to opioids on seizures are age-related and transient.     

Gender difference in acquired seizure susceptibility in adult rats after early complex febrile seizures

Gender differences are involved in many neurological disorders including epilepsy. However, little is known about the effect of gender difference on the risk of epilepsy in adults with a specific early pathological state such as complex febrile seizures（FSs） in infancy. Here we used a well-established complex FS model in rats and showed that：（1） the susceptibility to seizures induced by hyperthermia, pentylenetetrazol（PTZ）, and maximal electroshock（MES） was similar in male and female rat pups, while males were more susceptible to PTZ- and MES-induced seizures than age-matched females in normal adult rats;（2） adult rats with complex FSs in infancy acquired higher seizure susceptibility than normal rats; importantly, female FS rats were more susceptible to PTZ and MES than male FS rats; and（3） the protein expression of interleukin-1β, an infl ammatory factor associated with seizure susceptibility, was higher in adult FS females than in males, which may reflect a gender-difference phenomenon of seizure susceptibility. Our results provide direct evidence that the acquired seizure susceptibility after complex FSs is gender-dependent.     

Flurothyl seizures susceptibility is increased in prenatally methamphetamine-exposed adult male and female rats

The purpose of the present study was to examine the effect of prenatal methamphetamine (MA) exposure on flurothyl seizures. Adult prenatally MA-exposed male rats had decreased threshold of the first fasciculation and clonic seizure. In adult female rats, prenatal MA exposure decreased the threshold of the first fascicualtion in diestrous females, while there were no changes in proestrous or estrous females. The tonic-clonic seizure threshold was not altered by prenatal MA exposure.     

Selective and long-term learning impairment following neonatal hypoxic-ischemic brain insult in rats

We examined four different learning and memory tasks in rats which had been subjected to left carotid artery ligation followed by 2 h hypoxia (8% oxygen) when they were 7 days old. The examination began on the 4th week after insult and continued to 18 weeks post-insult. Compared with the control group, the hypoxic-ischemic group showed significant learning impairments in choice reaction time tasks relating to the attention process, and in plus-maze tasks and water maze tasks which examine long-term reference memory. In eight-arm radial maze tasks representing both short-term working memory and long-term reference memory, inferiority of the hypoxic-ischemic group was transient. Results of the sensorimotor test were normal in the hypoxic-ischemic group although slight flexion and twisting in the right forelimb was observed in 30% of the hypoxic-ischemic group when suspended by the tail. These abnormalities did not affect the results of learning tests. Findings of the study indicate that left-side brain damage produced by hypoxia-ischemia at 7 days of age resulted in selective and long-lasting learning and memory impairment.     

Seizures in the developing brain cause adverse long-term effects on spatial learning and anxiety

PURPOSE: Seizures in the developing brain cause less macroscopic structural damage than do seizures in adulthood, but accumulating evidence shows that seizures early in life can be associated with persistent behavioral and cognitive impairments. We previously showed that long-term spatial memory in the eight-arm radial-arm maze was impaired in rats that experienced a single episode of kainic acid (KA)-induced status epilepticus during early development (postnatal days (P) 1-14). Here we extend those findings by using a set of behavioral paradigms that are sensitive to additional aspects of learning and behavior. METHODS: On P1, P7, P14, or P24, rats underwent status epilepticus induced by intraperitoneal injections of age-specific doses of KA. In adulthood (P90-P100), the behavioral performance of these rats was compared with that of control rats that did not receive KA. A modified version of the radial-arm maze was used to assess short-term spatial memory; the Morris water maze was used to evaluate long-term spatial memory and retrieval; and the elevated plus maze was used to determine anxiety. RESULTS: Compared with controls, rats with KA seizures at each tested age had impaired short-term spatial memory in the radial-arm maze (longer latency to criterion and more reference errors), deficient long-term spatial learning and retrieval in the water maze (longer escape latencies and memory for platform location), and a greater degree of anxiety in the elevated plus maze (greater time spent in open arms). CONCLUSIONS: These findings provide additional support for the concept that seizures early in life may be followed by life-long impairment of certain cognitive and behavioral functions. These results may have clinical implications, favoring early and aggressive control of seizures during development.     

Decreased levels of brain cyclo-oxygenase products as a possible cause of increased seizure susceptibility in convulsion-prone gerbils

Basal levels of 5 cerebral prostanoids (PGD₂, PGF_2α, PGE₂, 6-keto-PGF_1α and throm☐ane/TX/B₂) were measured radioimmunologically in normal and convulsion-prone gerbils. Significantly less PGD₂, PGE₂ and 6-keto-PGF_1α was found in the brain of seizure-sensitive animals. After treatment with indomethacin, which reduced the amount of brain cyclo-oxygenase products, also normal gerbils exhibited convulsions following environmental stress. The results are in accordance with the hypothesis that endogenous prostanoids play a role in the regulation of seizure susceptibility.     

Effects of ACTH on seizure susceptibility in the developing brain

Adrenocorticotropic hormone (ACTH) has been frequently used as an anticonvulsant drug in some childhood seizure disorders. Despite its widespread use, few studies have evaluated the effects of ACTH on seizure susceptibility in the developing animal or the long-term consequences of ACTH treatment on brain development. In this study, ACTH was given either for 2 days or 14 days prior to kindling in 15-, 22-, and 30-day-old rats. Morphological changes in the brain were studied using routine light microscopy and dendrite branch counting following Golgi staining. Both acute and chronic ACTH treatment inhibited the rate of kindling in all three age groups. There were no differences in brain morphology between the controls and the ACTH-treated rats killed shortly after kindling. Rats treated with ACTH and killed as adults, however, had significantly more dendrite branches than did controls. In the immature brain, ACTH treatment reduces seizure susceptibility and has no long-term deleterious effects on neuronal growth.     

Reduced density of NMDA receptors and increased sensitivity to dizocilpine-induced learning impairment in aged rats

About 20 min prior to training in a shock-motivated 14-unit T-maze, young (3-4 months) and aged (24-25 months) male Fischer-344 rats were given s.c. injections of either saline or dizocilpine (MK-801, 0.02 or 0.04 mg/kg), a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor. The aged rats showed a dose-dependent impairment in maze performance. Deficiencies were manifested as increases in errors, in runtime from start to goal, and in the number and duration of shocks received. In contrast, young rats exhibited no detrimental effects of dizocilpine on maze performance. Analysis of [3H]glutamate binding in these rats revealed a marked age-related decline in NMDA receptor binding in hippocampus. A significant correlation was observed between errors in the maze and hippocampal [3H]-glutamate binding, but the correlation was positive, i.e., rats that made the most errors had the highest level of NMDA receptor binding. Thus, compared to young rats, aged rats were more sensitive to the behavioral effects of NMDA receptor antagonism and they showed a hippocampal loss of [3H]glutamate in binding, which may be related to the increased sensitivity to dizocilpine. The positive correlation between poor maze performance and NMDA receptor binding suggests that the behaviors assessed involve complex interactions between NMDA receptors and other neuronal systems in the hippocampus.     

Medium- and long-term effects of repeated bicuculline-induced seizures in developing rats on local cerebral energy metabolism

To assess long-term metabolic consequences of recurrent ictal events arising during development, seizures were repeatedly generated in rats at different stages of cerebral maturation. Seizures were induced by i.p. injections of bicuculline for three consecutive days, starting from postnatal day 5 (P5), when the brain is very immature, or from P15, a period at which the brain is more structurally organized. Local cerebral metabolic rates for glucose were measured in 74 structures at P15, P25 and in adults (P60), by the autoradiographic method using 2-

Full-size image

-[

Full-size image

]deoxyglucose. Repeated seizures in P5 to P7 pups led to a reduction (16–34%) of glucose consumption at P15, mainly significant in sensory, motor and functionally non-specific areas as well as in cerebellar nuclei. Selective decreases in metabolic activity were still recorded in adults, mostly in auditory system (20%) and cerebellar nuclei (27%). Seizures generated from P15 to P17 led to an overall mortality rate of 62% (versus 22% at P5 to P7). Surviving animals exhibited reduced metabolic rates for glucose (by 7–27%) at P25, significant in 23 structures, and depicting pronounced changes in limbic, hypothalamic, sensory and white matter areas, whereas brain functional activity finally returned to basal values at P60. Therefore, while younger rats seemed to better tolerate repeated bicuculline-induced seizures than older animals, the reverse was true for long-term metabolic effects, and the more immature the brain when seizures arise, the more persistent the functional consequences.     

Coping and seizure susceptibility: Control over shock protects against bicuculline-induced seizures

Rats were either given 80 escapable shocks, yoked inescapable shocks, restraint or given no treatment. Two hours later all subjects received i.p. injection of bicuculline (4, 6 or 8 mg/kg) and were immediately tested for latency to initial myoclonic jerk and clonus. The latency to clonic convulsion was dramatically affected by prior shock treatment, and the direction of this change depended upon the escapability/inescapability of the shock. Subjects that were given escapable shock a delay of onset to seizure, while subjects inescapably shocked demonstrated a decreased latency to clonus in comparison to restrained and naive controls. It was also demonstrated that if the subjects were tested immediately following a stress experience, both the 80 escapable and inescapable shock condition protected against bicuculline-induced seizures in comparison to the control condition. Finally Experiment 2 confirmed a previous finding that less stress, i.e., 20 inescapable shocks, protects against seizures when the animals are challenged with bicuculline either immediately or 2 h later. Our suggestion is that control over stress may facilitate GABAergic transmission, and this may be the mechanism whereby coping protects against the behavioral and physiological disruption produced by exposure to a stressor.     

Correlation between extracellular glucose and seizure susceptibility in adult rats

In adult diabetic patients, periods of hyperglycemia may be associated with exacerbation of focal seizures. Our objective was to determine in the adult rats the correlation between seizure susceptibility and extracellular glucose concentration in two models of seizures. Male rats were injected with two doses of streptozocin (40 mg/kg IP) on 2 consecutive days to induce diabetic hyperglycemia. Controls either received vehicle or were not injected. After 2 weeks, blood glucose concentration was measured, and the rats were subjected to flurothyl seizure test. Another group of rats received glucose solution (20%, 5 ml IP) 30 minutes before testing to induce nondiabetic hyperglycemia. Thresholds for flurothyl-induced clonic and tonic-clonic seizures were determined. Finally, in vitro epileptiform activity was induced in the entorhinal cortex-hippocampal slices from naive rats by perfusing with magnesium-free medium with various glucose concentrations. In additional slices, paired-pulse paradigm was determined in the perforant path. Susceptibility to clonic and tonic-clonic flurothyl-induced seizures positively correlated with blood glucose concentrations as the increased glucose concentration was associated with proconvulsant effects. Similarly, in the in vitro experiments, epileptiform activity was promoted by increased and suppressed by decreased glucose concentrations. Data indicate that, in the adult rats, high glucose concentrations are associated with proconvulsant effects.     

Noise exposure-induced audiogenic seizure susceptibility in Sprague-Dawley rats.

Parameters were evaluated for the optimum induction of audiogenic seizure susceptibility in Sprague-Dawley (SD) rats by noise exposure. The effect of maturation on this susceptibility was also examined. It was found that SD rats are most inducible between neonatal days 13 and 15 and that susceptibility requires a minimum of 2 days to develop. Noise exposure on day 14 results in universal susceptibility by day 20, but seizure severity is not maximal until days 32-36. Although susceptibility persists at high levels into adulthood, seizures in older rats revert to the wild-running-only type. Seizure latency (from stimulus onset to onset of wild running) becomes increasingly shorter during the prepubescent period (days 16-24) but is stable at older ages. The mean shortness of latency in adult seizures depends somewhat on the age when initial noise exposure occurred; day-14 noise exposures result in seizures with shortest latencies. Ontogenetic comparisons were made of susceptibility in these noise exposure-induced rats, genetically epilepsy prone rats (GEPRs, which are SD substrains)29 and noise exposure-induced Wistar (WI) rats28. It appears that epileptogenesis begins at virtually the same age in all four groups of rats but that considerable differences characterize the absolute severity of seizures and the age dependence of maximum seizure severity among the strains.     

Kainic acid seizure syndrome and binding sites in developing rats

In rats up to 16 days after birth, parenteral kainic acid (KA) produced tonico-clonic convulsions, metabolic activation limited to the hippocampus, and no brain damage. Starting with the 19th day after birth, KA produced limbic seizures associated with metabolic activation and subsequent damage in the hippocampus, the amygdala, and other limbic structures. Membranes prepared from hippocampi 10 days after birth bound [3H]KA with a high affinity component, which was localized in the mossy fiber region by slice autoradiography. In contrast, on amygdaloid membranes this component appeared only 17-19 days after birth. Our results further stress the crucial role of the amygdala in the KA seizure syndrome.     

Interleukin-6 attenuates hyperthermia-induced seizures in developing rats

Previous studies indicated that several cytokines influenced the seizure propensity in convulsive disorders and were the cause of encephalopathies in childhood. We studied the role of one inflammatory cytokine, interleukin-6 (IL-6), in hyperthermia-induced seizures in developing rats. Twenty-four male Lewis rats (23–28 days old) were divided into three groups (n = 8/IL-6 (500 ng), IL-6 (50 ng), and saline control groups). We applied human recombinant IL-6 intra-nasally to developing rats 1 h before seizures induced by moist heated air (50 °C). The seizure latency was defined as the time from hyperthermia onset until the appearance of continuous seizure discharges on electroencephalography (EEG), and the seizure duration as the duration of continuous spike and wave discharges on EEG. Five of the eight rats in the IL-6 (500 ng) group, two in the IL-6 (50 ng) group, and one in the control group exhibited no seizure discharges during the 360 s heating period. In these cases, the seizure latency time was regarded as 360 s and the seizure duration time as 0 s. The median seizure latency for the IL-6 (500 ng) group, 360 s (range: 256–360), was significantly longer than that for the control one, 249 (121–360) (P < 0.05). The seizure duration for the IL-6 (500 ng) group, 0 s (0–20), was significantly shorter than that for the control one, 33 (0–76) (P < 0.025). Also, the adenosine receptor antagonist, aminophylline, prevented these effects of IL-6 on hyperthermia-induced seizures. These results indicate that IL-6 plays an anti-convulsive role through the adenosine system in hyperthermia-induced seizures, which might be relevant as to human febrile seizures.     

Early life seizures cause long-standing impairment of the hippocampal map

Children with seizures are at risk for long-term cognitive deficits. Similarly, recurrent seizures in developing rats are associated with deficits in spatial learning and memory. However, the pathophysiological bases for these deficits are not known. Hippocampal place cells, cells that are activated selectively when an animal moves through a particular location in space, provides the animal with a spatial map. We hypothesized that seizure-induced impairment in spatial learning is a consequence of the rat's inability to form accurate and stable hippocampal maps. To directly address the cellular concomitants of spatial memory impairment, we recorded the activity of place cells from hippocampal subfield CA1 in freely moving rats subjected to 100 brief flurothyl-induced seizures during the first weeks of life and then tested them in the Morris water maze and radial-arm water maze followed by place cell testing. Compared to controls, rats with recurrent seizures had marked impairment in Morris water maze and radial-arm water maze. In parallel, there were substantial deficits in action potential firing characteristics of place cells with two major defects: i) the coherence, information content, center firing rate, and field size were reduced compared to control cells; and ii) the fields were less stable than those in control place cells. These results show that recurrent seizures during early development are associated with significant impairment in spatial learning and that these deficits are paralleled by deficits in the hippocampal map. This study thus provides a cellular correlate for how recurrent seizures during early development lead to cognitive impairment.     

Effects of adrenal dysfunction and high-dose adrenocorticotropic hormone on NMDA-induced spasm seizures in young Wistar rats

Infantile spasms (IS) is a devastating epilepsy syndrome treated with adrenocorticotropic hormone (ACTH). To demonstrate the effects of adrenal dysfunction, adrenalectomy (ADX) and N-methyl-d-aspartate (NMDA)-induced rat model studies of IS were performed. The latency of the seizure in the ADX group decreased and the severity of seizures increased significantly. Hippocampal corticotropin-releasing hormone (CRH) mRNA was overexpressed in ADX rats. After ACTH administration, the latency increased and the severity of seizures decreased significantly. ADX increased seizure susceptibility of the rats to NMDA. Pretreatment with a single high dose of ACTH caused an obvious reduction in susceptibility to NMDA-induced seizures and suppressed CRH mRNA expression. These findings are especially useful for IS patients with adrenal diseases and worthy of further clinical study.