Maturation of kainic acid seizure-brain damage syndrome in the rat. III. Postnatal development of kainic acid binding sites in the limbic system

Prenatal betamethasone exposure increases corticotropin-releasing hormone expression along with increased hippocampal slice excitability in the developing hippocampus

BACKGROUND

The objective of this study was to determine whether prenatal exposure to betamethasone alters hippocampal expression of corticotropin-releasing hormone (CRH) and resultant hippocampal circuit excitability.

METHODS

Real time (RT)-PCR and western blots were used to determine CRH mRNA and protein expression levels, respectively, in hippocampal extracts of two-week old rat pups prenatally primed with betamethasone or saline on gestational day 15. The data were compared to changes in epileptiform activity induced by kainic acid (KA) or depletion of [Mg²⁺]₀ in combined hippocampus-entorhinal cortex slices.

RESULTS

RT-PCR analysis showed 3-fold increased levels of CRH mRNA in hippocampal extracts from prenatally betamethasone-primed pups compared to saline controls (p < 0.05), but no changes in mRNA expression of CRH receptors (1 and 2). Changes in CRH protein isoform ratio in hippocampal extracts suggest 30 % increase in mature CRH levels in betamethasone-primed hippocampi (p < 0.05). No changes in mRNA expression in CRH feedback loop associated genes, GR and FKBP51, were found. Compared to saline-exposed pups, slices from betamethasone-primed pups had faster onset of epileptiform-like activity (inter-ictal discharges and seizure-like-events) after bath application of 4 μM KA (p < 0.05) suggesting a "more hyperexcitable" state. The epileptiform-like activity after KA application was significantly reduced following bath application of a CRH R2 antagonist (p < 0.05) but CRH R1 antagonist had no effect (p > 0.05). Also in the low-Mg²⁺-induced epileptiform activity, there was increased excitability, in the form of enhanced inter-ictal discharges, in slices from betamethasone primed compared to saline exposed rat pups (p < 0.05).

CONCLUSIONS

Our study suggests a possible mechanistic link to prenatal betamethasone priming-induced increase in postnatal hippocampal excitability that involves enhanced expression of CRH acting at CRH R2. This is important in regards to the links between prenatal stress/corticosteroid-exposure and syndromes, such as epilepsy, autism spectrum disorders and other psychiatric disorders associated with neuronal hyperexcitability.

Chronic subconvulsive activity during early postnatal life produces autistic behavior in the absence of neurotoxicity in the juvenile weanling period

The diagnosis of autism spectrum disorder (ASD) varies from very mild to severe social and cognitive impairments. We hypothesized that epigenetic subconvulsive activity in early postnatal life may contribute to the development of autistic behavior in a sex-related manner. Low doses of kainic acid (KA) (25-100 μg) were administered to rat pups for 15 days beginning on postnatal (P) day 6 to chronically elevate neuronal activity. A battery of classical and novel behavioral tests was used, and sex differences were observed. Our novel open handling test revealed that ASD males nose poked more often and ASD females climbed and escaped more frequently with age. In the social interaction test, ASD males were less social than ASD females who were more anxious in handling and elevated plus maze (EPM) tasks. To evaluate group dynamics, sibling and non-sibling control and experimental animals explored 3 different shaped novel social environments. Control pups huddled quickly and more frequently in all environments whether they socialized with littermates or non-siblings compared to ASD groups. Non-sibling ASD pups were erratic and huddled in smaller groups. In the object recognition test, only ASD males spent less time with the novel object compared to control pups. Data suggest that chronic subconvulsive activity in early postnatal life leads to an ASD phenotype in the absence of cell death. Males were more susceptible to developing asocial behaviors and cognitive pathologies, whereas females were prone to higher levels of hyperactivity and anxiety, validating our postnatal ASD model apparent in the pre-juvenile period.

Lessons from the laboratory: the pathophysiology, and consequences of status epilepticus

Status epilepticus (SE) is the most common neurologic emergency of childhood. Experimental models parallel several clinical features of SE including (1) treatment is complicated by an increasing probability that benzodiazepines will fail with increasing seizure duration and (2) outcome varies with age and etiology. Studies using these models showed that the activity-dependent trafficking of GABA(A) receptors contributes in part to the progressive decline in GABA-mediated inhibition and the failure of the benzodiazepines. Furthermore, laboratory studies have provided evidence that age and inciting stimulus interact to determine the neuronal circuits activated during SE (ie, functional anatomy) and that differences in functional anatomy can partially account for variations in SE outcome. Future laboratory studies are likely to provide an additional understanding of the cellular and molecular mechanisms that underlie SE and its consequences. Such studies are necessary in the development of rational emergent therapy for SE and its long-term outcomes.

The long-term effects of neonatal seizures

The highest incidence of seizures occurs during the first hours to days after birth. The immature brain is prone to seizures because of reduced inhibition. GABA, which is the primary inhibitory neurotransmitter in the mature brain, is depolarizing and excitatory in the immature brain. Seizures are an ominous sign, indicating either an acquired brain insult or a genetic abnormality. While the primary outcome determinant of neonatal seizures is etiology, whether seizures can result in long-term adverse consequences independently is not clear. While the clinical data is uncertain, there is now a considerable body of evidence indicating that in animals, neonatal seizures can adversely alter the developing brain. Animal data indicates that the sequelae of seizures are strongly age dependent; seizures will affect the developing and plastic neuronal circuitry much differently than the fixed circuitry of the mature brain. Seizures at an early developmental stage can dramatically affect the construction of networks, resulting in severe and permanent handicaps in some patients. In the young brain, the long-lasting detrimental consequences of seizures are caused by an alteration of developmental programs rather than by neuronal cell loss, as occurs in adults. In animal models, neonatal seizures result in decreases in neurogenesis, sprouting of mossy fibers, and long-standing changes in signaling properties. Seizures in rat pups are also associated with abnormalities in firing patterns of single cells in the hippocampus. Furthermore, these anatomic and physiologic changes correlate well with behavioral dysfunction.

The 2008 Judith Hoyer lecture: epilepsy in children: listening to mothers

The incidence of epilepsy is significantly higher in children than adults. When faced with the diagnosis of epilepsy, parents have many questions regarding cause, treatment, and prognosis. Although the majority of children with epilepsy have an excellent prognosis and respond well to therapy, some children are refractory to therapy and suffer from cognitive decline. Animal models are now providing insights into the mechanisms responsible for the high incidence of seizures during development and age-dependent seizure-induced damage. One of the causes of the increased susceptibility of the young brain to seizures is the depolarizing effects of GABA secondary to high intracellular concentrations of chloride in young neurons. Although cell loss is not a feature of seizures in the young brain, recurrent seizures do result in aberrant sprouting of mossy fibers, reduce neurogenesis, and alter excitatory and inhibitory neurotransmitter receptor structure and function. Behavioral consequences of early-life seizures include impaired spatial cognition, which now can be assessed using single-cell recordings from the hippocampus. Antiepileptic drugs have had a tremendous positive influence in epilepsy management, although there are now a number of studies demonstrating that antiepileptic drugs at therapeutic concentrations can impair cognition and result in increased apoptosis. While clinical judgment and experience are paramount when discussing the consequences of seizures and their treatment, awareness of studies from animals can provide the clinician with guidance in addressing these important issues with parents.

Differential expression of activating transcription factor-2 and c-Jun in the immature and adult rat hippocampus following lithium-pilocarpine induced status epilepticus

PURPOSE

Lithium-pilocarpine induced status epilepticus (LPSE) causes selective and age-dependent neuronal death, although the mechanism of maturation-related injury has not yet been clarified. The activating transcription factor-2 (ATF-2) protein is essential for the normal development of mammalian brain and is activated by c-Jun N-terminal kinase (JNK). It induces the expression of the c-jun gene and modulates the function of the c-Jun protein, a mediator of neuronal death and survival. Therefore, we investigated the expression of c-Jun and ATF-2 protein in the immature and adult rat hippocampus to understand their roles in LPSE-induced neuronal death.

MATERIALS AND METHODS

Lithium chloride was administrated to P10 and adult rats followed by pilocarpine. Neuronal injury was assessed by silver and cresyl violet staining, performed 72 hours after status epilepticus. For evaluation of the expression of ATF-2 and c-Jun by immunohistochemical method and Western blot, animals were sacrificed at 0, 4, 24, and 72 hours after the initiation of seizure.

RESULTS

Neuronal injury and expression of c-Jun were maturation-dependently increased by LPSE, whereas ATF-2 immunoreactivity decreased in the mature brain. Since both c-Jun and ATF-2 are activated by JNK, and targets and competitors in the same signal transduction cascade, we could speculate that ATF-2 may compete with c-Jun for JNK phosphorylation.

CONCLUSION

The results suggested a neuroprotective role of ATF-2 in this maturation-related evolution of neuronal cell death from status epilepticus.

In utero domoic acid toxicity: a fetal basis to adult disease in the California sea lion (Zalophus californianus)

California sea lions have been a repeated subject of investigation for early life toxicity, which has been documented to occur with increasing frequency from late February through mid-May in association with organochlorine (PCB and DDT) poisoning and infectious disease in the 1970’s and domoic acid poisoning in the last decade. The mass early life mortality events result from the concentrated breeding grounds and synchronization of reproduction over a 28 day post partum estrus cycle and 11 month in utero phase. This physiological synchronization is triggered by a decreasing photoperiod of 11.48 h/day that occurs approximately 90 days after conception at the major California breeding grounds. The photoperiod trigger activates implantation of embryos to proceed with development for the next 242 days until birth. Embryonic diapause is a selectable trait thought to optimize timing for food utilization and male migratory patterns; yet from the toxicological perspective presented here also serves to synchronize developmental toxicity of pulsed environmental events such as domoic acid poisoning. Research studies in laboratory animals have defined age-dependent neurotoxic effects during development and windows of susceptibility to domoic acid exposure. This review will evaluate experimental domoic acid neurotoxicity in developing rodents and, aided by comparative allometric projections, will analyze potential prenatal toxicity and exposure susceptibility in the California sea lion. This analysis should provide a useful tool to forecast fetal toxicity and understand the impact of fetal toxicity on adult disease of the California sea lion.

Effects of seizures on developmental processes in the immature brain

Infants and children are at a high risk for seizures compared with adults. Although most seizures in children are benign and result in no long-term consequences, increasing experimental animal data strongly suggest that frequent or prolonged seizures in the developing brain result in long-lasting sequelae. Such seizures may intervene with developmental programmes and lead to inadequate construction of cortical networks rather than induction of neuronal cell loss. As a consequence, the deleterious actions of seizures are strongly age dependent: seizures have different effects on immature or migrating neurons endowed with few synapses and more developed neurons that express hundreds of functional synapses. This differential effect is even more important in human beings and subhuman primates who have an extended brain development period. Seizures also beget seizures during maturation and result in a replay of development programmes, which suggests that epileptogenesis recapitulates ontogenesis. Therefore, to understand seizures and their consequences in the developing brain, it is essential to determine how neuronal activity modulates the main steps of cortical formation. In this Review, we present basic developmental principles obtained from animal studies and examine the long-lasting consequences of epilepsy.

Animal models for the development of new neuropharmacological therapeutics in the status epilepticus

Status epilepticus (SE) is a major medical emergency associated with significant morbidity and mortality. SE is best defined as a continuous, generalized, convulsive seizure lasting > 5 min, or two or more seizures during which the patient does not return to baseline consciousness. The relative efficacy and safety of different drugs in the treatment of human SE should be determined in a prospective, randomized, blinded study. However, complementary animal models of SE are required to answer important questions concerning the treatment of SE because of the obvious difficulties of setting up such studies in clinical emergency conditions. This review offers an overview of the implementation and characteristics of some of the most prevalent animal models of SE currently in use. A description is also provide about how animal models of SE may facilitate the use of neurobiological techniques to successfully address critical questions in the drug treatment of SE. In particular, the experience with recently introduced drugs such as intravenous valproate will be addressed. Finally, the importance of some animal models and pharmacological approaches is explained and we discuss their impact in the development of therapeutic strategies to improve pharmacological treatment for SE is discussed.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Effects of early seizures on later behavior and epileptogenicity

Both clinical and laboratory studies demonstrate that seizures early in life can result in permanent behavioral abnormalities and enhance epileptogenicity. Understanding the critical periods of vulnerability of the developing nervous system to seizure-induced changes may provide insights into parallel or divergent processes in the development of autism. In experimental rodent models, the consequences of seizures are dependent on age, etiology, seizure duration, and frequency. Recurring seizures in immature rats result in long-term adverse effects on learning and memory. These behavioral changes are paralleled by changes in brain connectivity, changes in excitatory neurotransmitter receptor distribution, and decreased neurogenesis. These changes occur in the absence of cell loss. Although impaired cognitive function and brain changes have been well-documented following early-onset seizures, the mechanisms of seizure-induced dysfunction remain unclear.

Long-term effects of status epilepticus in the immature brain are specific for age and model

PURPOSE

Status epilepticus (SE) is more common in children than adults and has a high mortality and morbidity rate. SE in adult rats results in long-term disturbances in learning and memory, as well as an enhanced seizure susceptibility to further seizures. In contrast, a number of studies suggest that the immature brain is less vulnerable to the morphologic and physiologic alterations after SE. The goal of this study was to determine whether the long-term consequences of SE during development on hippocampal plasticity and cognitive function are age and model specific.

METHODS

We used lithium-pilocarpine (Li-PC) to induce SE at different age points during development (P12, P16, P20) and evaluated the effects of this abnormal neural activity on spatial memory performance and seizure susceptibility in the animals beginning at P55, corresponding to young adulthood.

RESULTS

We demonstrated that SE at P12 did not result in any structural or functional changes detectable in adulthood, whereas SE at both P16 and P20 induced cell loss and mossy fiber sprouting within the hippocampus and cognitive impairment when the animals were tested as adults.

CONCLUSIONS

Whereas the seizure threshold to generalized seizures was not altered, animals with SE at P20 showed an increased susceptibility to kindling in adulthood.

Kainic acid modifies mu-receptor binding in young, adult, and elderly rat brain

Mu-receptor binding changes were evaluated following the kainic acid (KA)-induced status epilepticus (SE) in young, adult, and elderly animals. Male Wistar rats were used as follows: young rats (15 days old) were treated with KA (7 mg/kg) and sacrificed 72 h (YKA3d) or 35 days (YKA35d) after SE; adult (90 days old) (AKA1d and AKA40d) and elderly rats (1-year-old) (EKA1d and EKA40d) were injected with KA (10 mg/kg) and then sacrificed 24 h or 40 days following SE. Their brains were processed for an autoradiography assay for mu-receptors. The YKA3d group showed increased values in dentate gyrus (39%) and a decrease in substantia nigra (26%); YKA35d animals had a reduction in caudate putamen (29%) and in substantia nigra (20%). The AKA1d group exhibited increased mu-receptors in caudate putamen (49%), cingulate (415%), frontal (52%), and temporal (53%) cortices: substantia nigra (56%), dentate gyrus (48%). and CA2 field of hippocampus (53%). The AKA40d group showed increased values in sensorimotor cortex (45%), anterior (39%), medial (65%), basolateral (202%), and central (32%) amygdaloid nuclei; dentate gyrus (80%) as well as CA2 (80%) and CA3 (49%) fields of hippocampus. The EKA1d group presented decreased mu-receptor binding in piriform (16%) and enthorinal (22%) cortices as well as in anterior amygdala nucleus (17%). The EKA40d group showed reduced values in sensorimotor cortex (14%) and substantia nigra (27%). The present results indicate that the mu-binding changes following SE depend on the rate of brain maturation.

Seizure-induced damage in the developing human: relevance of experimental models

A considerable amount of money and effort is spent every year investigating the effects of seizure on the developing rodent brain. A critical question is the relevance of these studies to children. The goal of this chapter is to review the relationship between seizures during early development and cognitive impairment in children and rodents. While the majority of children with epilepsy have normal cognitive development, a small group of children with frequent, recurrent seizures show progressive cognitive impairment. Likewise, in rodent models recurrent seizures during early development are associated with cognitive impairment and histological changes including mossy fiber sprouting and reduced neurogenesis. Status epilepticus is associated with a lower morbidity and mortality rate in children than in adults. Status epilepticus in rodent models is associated with less cell loss and cognitive impairment than in adults. While rodent studies can offer a great deal of insight into mechanisms of seizure-induced brain damage, they also have significant limitations. No animal models have yet been developed that mimic human epileptic syndromes, such as infantile spasms, Lennox-Gastaut syndrome, or the severe myoclonic epilepsies. In addition, rodent studies supply only crude measures of learning and memory. Disturbances of language or higher cortical functions such as visual or auditory processing cannot be tested in animal models.

Immunohistochemistry of c-fos in Mouse Brain During Postnatal Development: Basal Levels and Changing Response to Metrazol and Kainate Injection

Levels and cellular distribution of FOS, the product of c-fos (onco)gene, were studied by immunohistochemistry during the development of mouse brain at rest and after the administration of convulsants. Basal FOS immunoreactivity became detectable only after postnatal day 20 (P20). Metrazol and kainate at the appropriate doses induced convulsions at all ages but, in both cases, FOS accumulated in limbic areas (particularly in the dentate gyrus) only after a certain age: P20 for kainate and P30 for Metrazol. Surprisingly, considering the different molecular targets of Metrazol and kainate, respectively, and the different type of convulsions elicited, the cell groups in the limbic areas in which FOS increased were the same in the two cases. These results suggest that both drugs produced FOS increase by finally activating the same circuit. During ontogeny, the ability to accumulate FOS, which appears after P20, could be the sign of the attained maturity of signal transduction mechanisms in the cells of the hippocampal formation; endogenous signals originating from the activity of the nervous system increase the basal FOS levels and exogenous signals (i.e. like those given, probably locally, by kainate) further increase these levels. Metrazol manifests its capability to induce FOS accumulation only at later ages. We suggest that this occurs because the Metrazol target is probably distant from the hippocampal region and thus the maturity of a nerve pathway(s) is also required for c-fos induction.

Modulation of GABA-mediated Synaptic Potentials by Glutamatergic Agonists in Neonatal CA3 Rat Hippocampal Neurons

Intracellular recordings were made from slices of adult and neonatal hippocampal neurons. During the first 2 weeks of life the majority of pyramidal cells exhibited spontaneous gamma-aminobutyric acid (GABA)-mediated synaptic potentials, which were depolarizing at birth and became hyperpolarizing by the end of the first postnatal week. These synaptic potentials were reduced in frequency or blocked by the N-methyl-d-aspartate (NMDA) receptor antagonist d(-)2-amino-5-phosphonovalerate (AP-5, 50 microM) (13/15 cells). The non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 - 10 microM) abolished the GABA-mediated synaptic potentials in all the cells tested (n=12), Superfusion of l-glutamate (up to 100 microM) increased the frequency of both depolarizing and hyperpolarizing GABA-mediated synaptic potentials. This effect was reduced by AP-5 or dl-2-amino-7-phosphonoheptanoate (AP-7, 50 microM) and fully blocked by concomitant application of AP-5 (50 microM) and CNQX (5 - 10 microM). NMDA (0.5 - 2 microM) increased the frequency of the GABA-mediated synaptic potentials. These effects were blocked by AP-5 (50 microM) and by bicuculline (10 microM). Quisqualate (100 - 300 nM), (RS)-alpha-amino-3-hydroxy-5-methyl-4-izopropionate (AMPA, 100 - 300 nM) and kainate (100 nM) also increased the frequency of the GABA-mediated synaptic potentials. These effects were blocked by CNQX (5 - 10 microM) and by bicuculline (10 microM) but not by AP-5 (50 microM). In the presence of tetrodotoxin (TTX, 1 microM), quisqualate (up to 300 nM), AMPA (up to 500 nM) and kainate (100 nM) had no effect on membrane potential or input resistance. In conclusion, our experiments suggest that, in early postnatal life, NMDA and non-NMDA receptors located on GABAergic interneurons modulate GABAergic synaptic potentials.

Resistance of immature hippocampus to morphologic and physiologic alterations following status epilepticus or kindling

Seizures in adult rats result in long-term deficits in learning and memory, as well as an enhanced susceptibility to further seizures. In contrast, fewer lasting changes have been found following seizures in rats younger than 20 days old. This age-dependency could be due to differing amounts of hippocampal neuronal damage produced by seizures at different ages. To determine if there is an early developmental resistance to seizure-induced hippocampal damage, we compared the effects of kainic acid (KA)-induced status epilepticus and amygdala kindling on hippocampal dentate gyrus anatomy and electrophysiology, in immature (16 day old) and adult rats. In adult rats, KA status epilepticus resulted in numerous silver-stained degenerating dentate hilar neurons, pyramidal cells in fields CA1 and CA3, and marked numerical reductions in CA3c pyramidal neuron counts (-57%) in separate rats. Two weeks following the last kindled seizure, some, but significantly less, CA3c pyramidal cell loss was observed (-26%). Both KA status epilepticus and kindling in duced mossy-fiber sprouting, as evidenced by ectopic Timm staining in supragranular layers of the dentate gyrus. In hippocampal slices from adult rats, paired-pulse stimulation of perforant path axons revealed a persistent enhancement of dentate granule-cell inhibition following KA status epilepticus or kindling. While seizures induced by KA or kindling in 16-day-old rats were typically more severe than in adults, the immature hippocampus exhibited markedly less KA-induced cell loss (-22%), no kindling-induced loss, no detectable synaptic rearrangement, and no change in dentate inhibition. These results demonstrate that, in immature rats, neither severe KA-induced seizures nor repeated kindled seizures produce the kind of hippocampal damage and changes associated with even less severe seizures in adults. The lesser magnitude of seizure-induced hippocampal alterations in immature rats may explain their greater resistance to long-term effects of seizures on neuronal function, as well as future seizure susceptibility. Conversely, hippocampal neuron loss and altered synaptic physiology in adults may contribute to increased sensitivity to epileptogenic stimuli, spontaneous seizures, and behavioral deficits.

New concepts in neonatal seizures

The immature brain is more prone to seizures than the older brain as a result of an imbalance between excitatory and inhibitory input. The depolarizing, rather than hyperpolarizing effect of GABA(A) during the first week of life in the rodent, and the delay in postsynaptic GABA(B) inhibition coupled with the over-expression of glutamatergic synapses contribute to this increased propensity toward seizures. It is now clear that seizures can be injurious to the immature brain, although the pattern of seizure-induced injury is age-related. While the immature brain is resistant to acute seizure-induced cell loss, there are functional abnormalities following seizures with impairment of visual-spatial memory and reduced seizure threshold. Neonatal seizures are also associated with a number of activity-dependent changes in brain development including altered synaptogenesis and reduction in neurogenesis. These results argue that neonatal seizures should no longer be considered as benign events.

The expression of Fos following kainic acid-induced seizures is age-dependent

The expression of limbic seizures following kainic acid (KA) administration starts at approximately postnatal day (P) 19 in rats. In this study we investigated whether the expression of Fos-like immunoreactivity (Fos-IR) in limbic regions occurs concomitantly with the behavioural expression of limbic seizures. Immunohistochemistry for c-Fos protein was examined 1, 2, 4, 12 and 24 h following seizure onset (KA-treated rats) or saline injections (controls) in immature and adult rats at P7, P13, P20 and P60. The expression of Fos-IR in limbic structures following KA-induced seizures is age-dependent. There is a strong and selective induction of Fos-IR in the CA3 region of the hippocampus following KA-induced seizures in rats at P7. However, the expression of Fos-IR in KA-treated rats at P13, P20 and P60 involved other hippocampal structures in addition to CA3. Abundant induction of Fos-IR was found in the CA1, CA3 and dentate gyrus (DG) in KA-treated rats at P13, P20 and P60. While immature rats at P7 and P13 showed very few or no Fos-IR neurons in most amygdala nuclei, rat pups at P20 showed strong induction of Fos-IR in the amygdala. Our results demonstrated that the induction of Fos-IR in most amygdala nuclei and the full expression of behavioural limbic seizures occur at the same developmental age, which is consistent with the idea that the amygdala may play a role in the modulation of limbic seizures.

Prolonged seizures exacerbate perinatal hypoxic-ischemic brain damage

This study was undertaken to clarify whether seizures in the newborn cause damage to the healthy brain and, more specifically, to determine the extent to which seizures may contribute to the brain-damaging effects of hypoxia-ischemia (HI). Seizures were induced in 10-d-old rat pups with kainic acid (KA). Seizure duration was determined electrographically. HI was induced by common carotid artery ligation followed by exposure to 8% oxygen for either 15 or 30 min. Six groups of animals were assessed: 1) controls [neither KA nor HI (group I)]; 2) group II, KA alone; 3) group III, 15 min HI alone; 4) group IV,15 min HI plus KA; 5) group V, 30 min HI alone; and 6) group VI, 30 min HI plus KA. Animals were assessed neuropathologically at 3 (early) and 20 (late) d of recovery. KA injection without hypoxia resulted in continuous clinical and electrographic seizures lasting a mean of 282 min. No neuropathologic injury was seen in groups I (no HI or KA), II (KA alone), III (15 min HI alone), or IV (15 min HI and KA). Animals in group V (30 min HI alone) displayed brain damage with a mean score of 2.3 and 0.60 at 3 and 20 d of recovery, respectively. Animals in group VI (30 min HI and KA) had a mean score of 12.1 and 3.65 at 3 and 20 d of recovery, respectively. Compared with group V, the increased damage as a result of the seizure activity in group VI occurred exclusively in the hippocampus. Status epilepticus in the otherwise "healthy" neonatal brain does not cause neuropathologic injury. However, seizures superimposed on HI significantly exacerbate brain injury in a topographically specific manner.

The neurobiology and consequences of epilepsy in the developing brain

Epilepsy is a disorder in which the balance between cerebral excitability and inhibition is tipped toward uncontrolled excitability. There is now clear evidence that there are distinct differences between the immature and mature brain in the pathophysiology and consequences of seizures. Both the enhanced excitability of the immature brain compared with the mature brain and the unique pathologic consequences of seizures are related to the sequential development and expression of essential signaling pathways. Although the immature brain is less vulnerable than the mature brain to seizure-induced cell death, seizures in the developing brain can result in irreversible alterations in neuronal connectivity. Developing novel strategies to treat and avert the consequences of seizures in children will require further understanding of the unique mechanisms of seizure initiation and propagation in the immature brain.

Patterns of status epilepticus-induced substance P expression during development

Substance P, which modulates synaptic excitability, can be induced by a variety of stimuli. We studied the expression of hippocampal substance P in rats in using lithium-pilocarpine model of status epilepticus during development. Status epilepticus resulted in an age-specific manner of substance P expression that was anatomically distinctive in hippocampal subfields. Maximal induction of substance P immunoreactivity was seen in the CA1 region of the two-week-old rats, and progressively decreased in the three-, four-week-old rats and adults. Meanwhile, the number of substance P-immunoreactive neurons in the CA3 region and dentate granule cell layer was minimal in the two-week-old animals, but approximated the adult level in the three- and four-week-old rats. No substance P-immunoreactive axon terminals were seen in the strata pyramidale and lucidum in the CA3 region of the two-week-old rats, but they were found to progressively increase in the three-, four-week-old rats and adults. To confirm substance P expression after status epilepticus, we studied the expression of preprotachykinin-A mRNA in the hippocampus of the three-week-old rats by in situ hybridization. Two hours following injection of lithium-pilocarpine, preprotachykinin-A mRNA dramatically increased in the granule cells, as well as in the CA3 and CA1 pyramidal cell layers of the hippocampus. To evaluate the relationship between behavioral seizures and substance P induction, we used the NMDA receptor antagonist MK-801. Injection of MK-801 completely blocked lithium-pilocarpine-induced behavioral seizures and SP induction in the two-week-old rats. These results indicate that seizure activity selectively evokes age-dependent and region-selective expression of substance P.

Anticonvulsant action and long-term effects of gabapentin in the immature brain

The anticonvulsant action and the long-term effects on learning, memory and behavior of the new generation antiepileptic drug gabapentin (GBP) were investigated in immature animals. Kainic acid (KA) was administered to rats on postnatal day (P) 35. Animals were treated with GBP or saline from P36 to P75 and spontaneous seizure frequency was monitored. After tapering the drug, the rats were tested in the water maze and open field test. Brains were then analyzed for histological lesions. Animals treated with GBP following KA-induced status epilepticus had a reduced incidence of spontaneous recurrent seizures, a better pathology score, and less aggressiveness compared to saline-treated controls. Effectiveness of GBP on seizure threshold was tested using flurothyl inhalation in 10 separate age groups of animals ranging from the newborn period to adulthood. Furthermore, GBP plasma concentration peaks were determined in all age groups. At all ages, GBP pre-treated animals demonstrated a higher seizure threshold. Plasma GBP concentrations did not significantly change with age. These data suggest that acute administration of a single therapeutic dose of GBP increases the seizure threshold at all ages studied, while chronic treatment following the status reduces spontaneous seizure frequency and cell damage and has no long-term adverse consequences on cognitive processes during development.

Nonconvulsive Kainic Acid-Induced Seizures Elicit Age-Dependent Impairment of Memory for the Elevated Plus-Maze

The goal of this study was to evaluate changes in spatial learning in adult and immature rats during and after nonconvulsive seizures. An elevated plus-maze was used in 18- and 25-day-old and adult rats. Kainic acid (KA 6 mg/kg) was administered 60 minutes before the first exposure (Experiment 1) or after a 3-day pretraining (Experiment 2, only adult rats). Animals were retested three times with 24-hour intervals. EEG activity was monitored in 18-day-old rats. KA prolonged the transfer latency (TL) in all age groups. In the youngest group the TL was prolonged 24 hours after KA when epileptic EEG graphoelements were still registered. In both older groups, prolonged TL was measured only 60 minutes after KA. In the pretrained adults, significantly prolonged TLs persisted for 24 hours after KA. KA changed the performance of adult and immature rats in the elevated plus maze not only during nonconvulsive seizures but also 24 hours later.

Is epilepsy a progressive disease? The neurobiological consequences of epilepsy

While primary, or idiopathic, epilepsies may exist, in the vast majority of cases epilepsy is a symptom of an underlying brain disease or injury. In these cases, it is difficult if not impossible to dissociate the consequences of epilepsy from the consequences of the underlying disease, the treatment of either the disease or the epilepsy, or the actual seizures themselves. Several cases of apparent complications of epilepsy are presented to illustrate the range of consequences encountered in clinical practice and the difficulty in assigning blame for progressive symptomatology in individual cases. Because of the difficulty in interpreting clinical material, many investigators have turned to epilepsy models in order to address the potential progressive consequences of recurrent seizures. The authors review experimental data, mainly from animal models, that illustrate short-, medium-, and long-term morphological and biochemical changes in the brain occurring after seizures, and attempt to relate these observations to the human condition.

Nitrone spin trapping compound N-tert-butyl-alpha-phenylnitrone prevents seizures induced by anticholinesterases

The neuroprotection afforded by spin trapping agents such as N-tert-butyl-alpha-phenylnitrone (PBN) has lent support to the hypothesis that increased production of reactive oxygen species (ROS) is a major contributing factor to excitotoxicity, aging and cognitive decline. Little is known, however, about the pharmacological properties of PBN. We have compared the acute effects of PBN on the development of seizures induced by the irreversible acetylcholinesterase (AChE) inhibitor diisopropylphosphorofluoridate (DFP), the reversible AChE inhibitor physostigmine (PHY), the muscarinic cholinergic receptor agonist pilocarpine (PIL) and the glutamatergic receptor agonist kainic acid (KA). Rats were sacrificed 90 min after the injection of seizure-inducing agents. In situ hybridization was used to detect the induction of immediate early gene (IEG) c-fos and c-jun mRNA's and the levels of AChE mRNA. The activity of AChE was visualized by AChE staining and quantified using an in vitro AChE assay. The seizures correlated with the induction of IEG mRNA's with all agents used. The pre-treatment with 150 mg/kg of PBN prevented DFP- and PHY-induced seizures and the related expression of IEG mRNA's, but had no effect on PIL- or KA-induced seizures and associated IEG mRNA's changes. PBN prevented seizures and significantly protected AChE activity against DFP inhibition when given before, but not when given after DFP. This study shows that PBN specifically protects against anticholinesterase-induced seizures by reversible protection of AChE activity and not by the blockade of muscarinic or glutamate receptors, reactivation of AChE or scavenging of ROS. The anticholinesterase properties should be considered when using PBN in studies of cholinergic dysfunction.

Impact of neonatal kainate treatment on hippocampal insulin-like growth factor receptors

The insulin-like growth factors-I and -II have neurotrophic properties and act through specific membrane receptors. High levels of binding sites for these growth factors are distributed discretely throughout the brain, being concentrated in the hippocampal formation. Functionally, the insulin-like growth factors, in addition to their growth-promoting actions, are considered to play important roles in normal cell functions, as well as in response to pharmacological or surgical manipulations. In adult rats, we have previously shown that systemic injection of kainate produces an overall decrease, in a time-dependent manner, in insulin-like growth factor-I and -II receptor binding sites in the hippocampus [Kar S. et al. (1997) Neuroscience 80, 1041-1055]. Given the evidence that insulin-like growth factors play a critical role during the early stages of brain development, the present study is a logical extension of this earlier report and established the effect of neonatal kainate injection on the developmental profile of insulin-like growth factor receptors. We have evaluated the time-course alteration of these receptors following systemic injection of kainate to newborn rats. After injection of a sublethal dose of kainate (5 mg/kg, i.p.) to postnatal one-day-old pups, [125I]insulin-like growth factor-I, [125I]insulin-like growth factor-II and [125I]insulin binding sites were studied at different postnatal days (7, 14, 21, 28 and 35) using receptor autoradiography. In the developing hippocampus, insulin-like growth factor-I and insulin binding sites are concentrated primarily in the dentate gyrus and the CA2/CA3 subfields, whereas insulin-like growth factor-II binding is discretely localized to the pyramidal layer and the granular layer of the dentate gyrus. Following kainate injection, we observed a slight increase in insulin-like growth factor-I binding sites in given hippocampal subfields starting at postnatal day 14, being significant at day 21. At later days, a progressive decrease was noted. This transient increase may represent an attempt for neuronal plasticity by up-regulating receptor levels. In contrast, insulin-like growth factor-II and insulin receptor binding sites are found to be decreased in various regions of the hippocampus in kainate-treated pups. Taken together, these results provide further evidence for the existence and differential alterations of insulin-like growth factor-I, insulin-like growth factor-II and insulin receptors in the developing rat hippocampus following kainate-induced lesion, suggesting possible involvement of these growth factors in brain plasticity.

Effect of seizures on cerebral hypoxic-ischemic lesions in immature rats

The present investigation was designed to study the effect of chemically induced seizures on cerebral hypoxic-ischemic (HI) damage in immature animals. Accordingly, cerebral HI was produced in 7-day postnatal (p7) rats and p13 rats by combined unilateral common carotid artery ligation and hypoxia with 8% oxygen. Seizures were induced chemically by the subcutaneous injection of kainic acid (KA) or inhalation of flurothyl vapor. Three types of experiments were conducted in each age group and for each convulsant. In some animals (group 1), seizures were produced at 24 h and again at 6 h prior to HI. In groups 2 and 3, seizures were induced 2 h or 24 h post HI, respectively. The results indicate that in group 1 animals, the first seizure significantly reduced duration of the second seizure challenge 18 h later at both p7 and p13 (p=0.001). Histologic examination of brains of animals in group 1 subjected to seizures prior to HI and their HI-only controls showed that seizures prior to HI conferred protection against cerebral damage. This effect was significant for flurothyl seizures in p13 rats for all cerebral regions, especially hippocampal CA1 (p=0.0004), and in p7 rats for hippocampus (p=0.04) and particularly cerebral cortex (p=0.007). For KA seizures, the protective effect was only significant in p13 rats and was limited to hippocampal CA regions and subiculum (p=0.0009). Histologic assessment of cerebral lesions of p7 and p13 rats in the other two groups showed no significant difference between the animals subjected to seizures 2 h or 24 h post HI and their HI-only controls (p>0.05). In conclusion, the results of the present study provide no evidence that seizures in early postnatal development aggravate pre-existing cerebral HI damage. They do suggest that seizures prior to HI or prior to a second seizure confer tolerance to both conditions.

Q/R editing of the rat GluR5 and GluR6 kainate receptors in vivo and in vitro: evidence for independent developmental, pathological and cellular regulation

Kainate (KA) is a potent neuroexcitatory agent in several areas of the adult brain, with convulsant and excitotoxic properties that increase as ontogeny proceeds. Besides its depolarizing actions, KA may enhance intracellular accumulation of Ca2+ to promote selective neuronal damage. The effects of KA are mediated by specific receptors recently considered to be involved in fast neurotransmission and that can be activated synaptically. KA receptors, e.g. GluR5 and GluR6 have been characterized by molecular cloning. Structure-function relationships indicate that in the MII domain of these KA receptors, a glutamine (Q) or arginine (R) residue determines ion selectivity. The arginine stems from post-transcriptional editing of the GluR5 and GluR6 pre-RNAs, and the unedited and edited versions of GluR6 elicit distinct Ca2+ permeability. Using a PCR-based approach, we show that in vivo, Q/R editing in the GluR5 and GluR6 mRNAs is modulated during ontogeny and differs substantially in a variety of nervous tissues. GluR5 editing is highest in peripheral nervous tissue, e.g. the dorsal root ganglia, where GluR6 expression is barely detectable. In contrast, GluR6 editing is maximal in forebrain and cerebellar structures where GluR5 editing is lower. Intra-amygdaloid injections of KA provide a model of temporal lobe epilepsy, and we show that following seizures, the extent of GluR5 and GluR6 editing is altered in the hippocampus. However, in vitro, high levels of glutamate and potassium-induced depolarizations have no effect on GluR5 and GluR6 Q/R editing. GluR6 editing is rapidly enhanced to maximal levels in primary cultures of cerebellar granule neurons but not in cultured hippocampal pyramidal neurons. Finally, we show that cultured glial cells express partially edited GluR6 mRNAs. Our results indicate that Q/R editing of GluR5 and GluR6 mRNAs is structure-, cell type- and time-dependent, and suggest that editing of these mRNAs is not co-regulated.

A gene expression approach to mapping the functional maturation of the hippocampus

Previous studies have shown an association among seizures, neuronal death and the expression of cellular immediate-early genes (cIEG). To understand further the relationship between these processes, we investigated the ability of kainic acid (KAI) to induce behavioral responses and gene expression in the hippocampus of developing fos-lacZ transgenic mice. Despite the fact that KAI elicited seizure-like activity from P2 onwards, Fos-lacZ was first detected at P5 in CA3 pyramidal neurons. Thus, intense behavioral responses were not invariably associated with fos-lacZ expression. Furthermore, while adult CA3 neurons are highly susceptible to KAI toxicity, they are resistant at P5. Therefore, the presence of Fos-lacZ in CA3 neurons is not necessarily predictive of their fate. By P10, Fos-lacZ was induced in CA3 neurons and in the most mature granule neurons of the dentate gyrus (DG). Between P15 and P20, KAI induced fos-lacZ in all CA1 and CA3 pyramidal neurons and most granule neurons of the DG. This stereotypical pattern of fos-lacZ expression mirrors the ontogeny of hippocampal circuitry and glutamate signalling. Thus the fos-lacZ mice can be used to map the functional maturation of the nervous system with single cell resolution. The scope of this approach was extended by administration of additional chemoconvulsants to fos-lacZ mice and by analysis of fos-lacZ transgenic mice with mutations in their FAP site. These additional studies revealed anatomical and mechanistic differences in glutamate receptor-mediated transcriptional responses in the nervous system.

Patterns of status epilepticus-induced neuronal injury during development and long-term consequences

The lithium-pilocarpine model of status epilepticus (SE) was used to study the type and distribution of seizure-induced neuronal injury in the rat and its consequences during development. Cell death was evaluated in hematoxylin- and eosin-stained sections and by electron microscopy. Damage to the CA1 neurons was maximal in the 2- and 3-week-old pups and decreased as a function of age. On the other hand, damage to the hilar and CA3 neurons was minimal in the 2-week-old rat pups but reached an adult-like pattern in the 3-week-old animals, and damage to amygdalar neurons increased progressively with age. The 3-week-old animals also demonstrated vulnerability of the dentate granule cells. To evaluate neuronal apoptosis, we used terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) stain, confocal fluorescence microscopy of ethidium bromide-stained sections, electron microscopy, and DNA electrophoresis. Neurons displaying all of those features of apoptotic death in response to SE were seen in the CA1 region of the 2-week-old pups and in the hilar border of the dentate granule cells of the 3-week-old animals. Some (3/11) of the animals that underwent SE at 2 weeks of age and most of the animals that underwent SE at 3 or 4 weeks of age (8/11 and 6/8, respectively) developed spontaneous seizures later in life; the latter showed SE-induced synaptic reorganization as demonstrated by Timm methodology. These results provide strong evidence for the vulnerability of the immature brain to seizure-induced damage, which bears features of both necrotic and apoptotic death and contributes to synaptic reorganization and the development of chronic epilepsy.

Nurr1 mRNA expression in neonatal and adult rat brain following kainic acid-induced seizure activity

Nurr1 is an immediate early gene encoding a member of the steroid-thyroid hormone receptor family. In PC12 cells, Nurr1 is readily induced by membrane depolarization, but not by growth factors. Nurr1 is predominantly expressed in the brain, and is essential to the differentiation of midbrain dopaminergic neurons. However, Nurr1 is also expressed in brain regions unrelated to dopaminergic neurons, e.g., hippocampus and cerebral cortex, and its immediate induction following seizure activity suggests a potential involvement of this transcription factor in modulating gene expression in the nervous system. To investigate the response of Nurr1 to neuronal activation, we analyzed Nurr1 mRNA expression in neonatal and adult rat brain following kainic acid (KA)-induced seizure. In P7 animals, systemic injection of KA increased Nurr1 mRNA levels in a few hilar cells of the dentate gyrus and some pyramidal cells of the CA3 region of the hippocampus. In older animals, Nurr1 induction progressively expanded to all hippocampal regions (P14, P21) and eventually to cortical regions (adult). The increase was rapid and transient in the dentate gyrus, a structure resistant to the neurotoxic effect of KA, and was more prolonged in other regions more susceptible to KA toxicity. Induction of Nurr1 at early postnatal stages and rapid increase in the dentate gyrus following KA-induced seizure, suggest that Nurr1 expression is modulated by neuronal activity. On the other hand, prolonged Nurr1 induction in regions sensitive to KA toxicity indicates a possible involvement of Nurr1 in selective neuronal vulnerability.

Developmental changes in calpain activity, GluR1 receptors and in the effect of kainic acid treatment in rat brain

The cellular distribution of calpain activation and glutamate receptor 1 (GluR1) subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and their alterations following kainic acid-induced seizure were evaluated during postnatal development using antibodies specific for spectrin breakdown product and the C-terminus of GluR1 subunits. In the first postnatal week, most brain regions exhibited high levels of calpain activity that progressively decreased during the following weeks. The highest levels of spectrin breakdown product immunoreactivity were observed in the somata and proximal dendrites of hippocampal pyramidal cells, non-pyramidal neurons in stratum oriens, and cortical neurons. In general, during the first two postnatal weeks, kainic acid treatment induced a decrease in spectrin breakdown product immunoreactivity in neuronal cell bodies and an increase in dendritic fields. Obvious elevation in spectrin breakdown product immunoreactivity in selective non-pyramidal cells in stratum oriens started at postnatal day 14, and was further evidenced by postnatal day 21. Likewise, massive calpain activation in subpopulations of neurons in some thalamic nuclei, amygdala, and pyriform cortex was observed after the third postnatal week. GluR1 subunits were highly expressed throughout the forebrain in the first postnatal week, further increased during the second postnatal week, decreased thereafter, and reached adult levels after postnatal day 21. In cortex, intense GluR1 immunostaining was found in the somata and proximal processes of pyramidal and non-pyramidal neurons, with the non-pyramidal neurons in layers IV through VI exhibiting the densest immunolabelling. In the first two postnatal weeks, the somata of hippocampal pyramidal neurons exhibited intense GluR1 immunostaining that became more dendritic in the subsequent developmental period. While hilar cells exhibited a similar developmental pattern as CA regions, the molecular layer of dentate gyrus exhibited weak immunoreactivity from postnatal day 7 to postnatal day 14. The early increase in GluR1 immunoreactivity in hippocampal pyramidal layer following kainic acid treatment occurred throughout the developmental period, while the later decrease in CA regions, amygdala, and pyriform cortex was observed only in postnatal day 21 animals. The combined immunocytochemical studies of spectrin breakdown product localization and GluR1 expression indicate that calpain activation might play an important role in synaptic formation, developmental regulation of synaptic plasticity, and neuronal vulnerability to excitotoxicity during postnatal development. Moreover, calpain-mediated modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors might underlie these processes.

Basic fibroblast growth factor is highly neuroprotective against seizure-induced long-term behavioural deficits

Basic fibroblast growth factor has been reported to protect neurons of various structures from excitotoxic damage. To study the effects of basic fibroblast growth factor on seizure-induced brain damage we infused the growth factor into the lateral ventricles of 35-day-old rats receiving convulsant dosages of kainic acid. Artificial cerebrospinal fluid or basic fibroblast growth factor at dosages of 0.5 ng/h or 2.5 ng/h was infused into the lateral ventricle continuously for seven days starting two days before and continuing for five days after the animals had kainic acid-induced status epilepticus. At age 80 days the animals underwent behavioural testing using the water maze, open field, and handling tests and at age 95 days were tested for seizure threshold using flurothyl inhalation. Neither artificial cerebrospinal fluid or basic fibroblast growth factor modified the latency or duration of the acute seizures following kainic acid. However, rats infused with 2.5 ng/h, but not 0.5 ng/h of basic fibroblast growth factor, had fewer spontaneous recurrent seizures, a higher seizure threshold, better performance in the handling, open field and water maze test, and less cell loss in the hippocampus when compared to rats receiving artificial cerebrospinal fluid or 0.5 ng/h of basic fibroblast growth factor. These results show that basic fibroblast growth factor has a dose-related neuroprotective effect against seizure-induced long-term behavioural deficits when administered by osmotic pump prior to seizure onset. This neuroprotective effect is not related to an anticonvulsant effect.

Anatomical studies of DNA fragmentation in rat brain after systemic kainate administration

Rats treated systemically with kainate develop stereotyped epileptic seizures involving mainly limbic structures that may last for hours. This model of limbic status epilepticus has been widely studied using classical neuropathological techniques. We used in situ nick translation histochemistry to examine patterns of DNA fragmentation in this model. We found a stereotyped and reproducible pattern of neuronal populations that demonstrate evidence of DNA fragmentation from 24 h to one week after kainate treatment. Neither blockade of new protein synthesis nor blockade of the N-methyl-D-aspartate-type glutamate receptors significantly altered this response. Moreover, we saw no evidence of the regular internucleosomal cleavage of DNA that produces a characteristic laddered appearance of 180-200 bp DNA fragments after gel electrophoresis in samples obtained from microdissected affected regions. These studies suggest that DNA fragmentation after systemic kainate-induced seizures is not the result of programmed cell death. This assay may be useful for quantitative testing of both neuroprotective agents and mechanistic hypotheses.

Two synaptotagmin genes, Syt1 and Syt4, are differentially regulated in adult brain and during postnatal development following kainic acid-induced seizures

The synaptotagmins together with other vesicle proteins are thought to be essential for the docking and/or fusion of synaptic vesicles with the plasma membrane that occurs following depolarization and calcium influx in presynatic terminals. Syt4, the fourth identified member of the synaptotagmin family, is inducible in PC12 cells by depolarization and secretagogues, and in limbic regions of the adult rat brain by kainic acid-induced seizures. In the present study, we examined the time course of the seizure-induced changes in the expression of Syt4 and Syt1, both in adult animals and during the postnatal period. Syt4 was transiently induced in several structures of the adult rat brain following seizure activity with peak inductions between 4 and 8 h and overal return to control values by 30 h. No induction was observed following seizure activity in 7-day-old animals. The brain regions most sensitive to increased induction were, in decreasing order of sensitivity, hippocampal pyramidal cells dentate granule cells and piriform cortex pyramidal cells. The brain areas showing the greatest Syt4 stimulation in adults were also the areas in which Syt4 was induced by seizures earlier in development. In contrast, Syt1 mRNA was depressed in adult brains following seizure activity, particularly in the dentate granule cells. Our results suggest that the differential regulation of different synaptotagmin genes following excessive neuronal activity might participate in rapid adaptation of subsequent transmitter release.

Glutamate receptor binding in the human hippocampus and adjacent cortex during development and aging

Distinct patterns of age-related alterations in NMDA (MK801 binding) and non-NMDA, AMPA (CNQX), and kainate binding have been identified in human hippocampus and parahippocampal gyrus in normal individuals with no evidence of degenerative brain disease ranging in age from 24 gestational weeks to 94 years. Whereas MK801 binding did not alter substantially over this age range, CNQX binding rose from low levels in the fetus to maximum levels between neonate and middle age, and kainate binding declined extensively from the perinatal to adult stage. Following maturity, there were no significant changes in kainate binding, although MK801 binding increased in CA1 and CA3 and CNQX binding declined in several regions, particularly CA2 and subiculum. For each receptor binding the timing of these fluctuations ocurring during development and aging varied within different regions of the dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex examined. The transient peaks of receptor binding are likely to reflect processes of synaptogenesis and pruning and may provide clues regarding the role of the different glutamate receptor subtypes in various pathologies of the hippocampus and adjacent cortex associated with developmental disorders (of genetic origin or due to perinatal trauma or insult). The absence of substantial changes in any subtype examined from middle to old age suggests alterations in transmitter binding to these glutamate receptors are not involved in senescent neurodegeneration.

Effect of kainic acid-induced status epilepticus on inositol-trisphosphate and seizure-induced brain damage in mature and immature animals

We investigated the role of excitatory amino acids in the activation of the phosphoinositide pathway during kainic acid-induced seizures in mature and immature animals. Kainic acid caused more severe seizures in the immature animals, but no hippocampal damage or induction of phosphoinositide hydrolysis. In mature animals, seizures were mild but severe hippocampal damage was seen and was associated with a marked and sustained release of inositol-trisphosphate, suggesting a role of this pathway and intracellular calcium stores in seizure-induced brain damage.

Assessing the extent of RNA editing in the TMII regions of GluR5 and GluR6 kainate receptors during rat brain development

Kainate (KA) is a potent neuroexcitatory agent that induces seizure and brain damage syndromes with increasing efficiency during maturation. It has been suggested that the selective neuronal damage induced by KA may result not only from its depolarizing actions, but also from intracellular accumulation of Ca2+. The effects of KA are mediated by specific high-affinity receptors, enriched in the hippocampus. Members of this class of receptors, GluR5 and GluR6, have been characterized by cDNA cloning. Ca2+ permeability of the GluR6 receptor is determined by editing in the corresponding RNA. We report here a rapid PCR-based approach to assess in all experimental conditions the levels of GluR5 and GluR6 editing in the transmembrane TMII region. We show that editing in both GluR5 and GluR6 RNA is developmentally regulated and that different regions of the adult rat hippocampus demonstrate distinct levels of GluR6 editing.

Induction of c-fos mRNA expression in an in vitro hippocampal slice model of adult rats after kainate but not gamma-aminobutyric acid or bicuculline treatment

Levels of gene expression following in vitro treatment of rat hippocampal slices with kainate, gamma-aminobutyric acid (GABA), or bicuculline were measured by the reverse transcription-coupled polymerase chain reaction method. Following a short-term exposure to kainate, c-fos gene expression was induced by 12-fold in the adult, but not the newborn, hippocampus. Under the same experimental conditions, zifl268 and brain-derived neurotrophic factor (BDNF) gene expression were unchanged. Our results also demonstrate a lack of induction of c-fos, zifl268 and BDNF after short-time treatment of either adult or newborn hippocampal slices with GABA or bicuculline. The relevance of the differential induction of gene expression in the adult and newborn in an in vitro hippocampal slice model as compared to previously described in vivo models is discussed.

Transcriptional activation of ornithine decarboxylase in adult and neonatal hippocampal slices

Ornithine decarboxylase (ODC) is the rate-limiting enzyme in polyamine synthesis and is regulated by both transcription-dependent and transcription-independent mechanisms. We compared the effects of asparagine, an amino acid previously shown to increase ODC activity in adult hippocampal slices, on ODC mRNA and activity in adult and neonatal hippocampal slices. In addition, we evaluated the effects of asparagine on ODC activity following seizure activity elicited by systemic administration of kainic acid (KA) in both adult and neonatal rats. Asparagine produced an increase in ODC gene expression and activity in both adult and neonatal hippocampal slices. The increase in ODC activity elicited by asparagine in hippocampal slices was the same in control animals as in animals sacrificed 16 h after KA-induced seizure activity. The asparagine-elicited increase in ODC activity in neonatal and adult hippocampal slices was blocked by the RNA synthesis inhibitor, actinomycin D. Finally, polyamines produced an inhibition of ODC activity in neonatal hippocampal slices. The results indicate that the regulation of the expression and activity of ODC is similar in neonatal and adult hippocampus.

Excitatory and inhibitory pathways modulate kainate excitotoxicity in hippocampal slice cultures

In organotypic hippocampal slice cultures, kainate (KA) specifically induces cell loss in the CA3 region while N-methyl-D-aspartate induces cell loss in the CA1 region. The sensitivity of slice cultures to KA toxicity appears only after 2 weeks in vitro which parallels the appearance of mossy fibers. KA toxicity is potentiated by co-application with the GABA-A antagonist, picrotoxin. These data suggest that the excitotoxicity of KA in slice cultures is modulated by both excitatory and inhibitory synapses.

Functional effects of glucocorticoid exposure during fetal life

1. Pregnant rats were exposed to hydrocortisone in a dose of 10 mg/kg on days 9-11 or days 13-15 of gestation. The offsprings born to these mothers were observed for their behavioral development and the response to kainic acid during the infantile period. The response to kainic acid was assessed by the frequency of wet-dog shakes behavior and limbic seizures. 2. The growth rate in the infantile offspring of the 13-15dHc-F1 group showed a slight but significant decrease. 3. All the 13-15dHc-F1 rats exhibited the rearing activity in an open-field at 17 days of age, earlier than in the controls. 4. The ambulatory activity in the 9-11dHc-F1 rats showed a significant decrease at 15 and 17 days of age, whereas no change was shown in the 9-11dHc-F1 rats. 5. The frequency of the wet-dog shakes during the 60 minutes observation after the s.c. injection of 9 mg/kg kainic acid was significantly low in both the 9-11dHc-F1 and 13-15dHc-F1 groups as compared with that in the controls when tested at 25 days of age. The decrease in the response to kainic acid was slightly greater in the 9-11dHC-F1 rats. 6. The frequency of seizures with forelimb clonus and rearing during the 60 minutes observation in the 13-15dHc-F1 was less than that in the controls, whereas no significant difference in the frequency of seizures between the 9-11dHc-F1 and paired control groups was noted. 7. The second generation rats raised from the 9-11dHc-F1 rats by brother-sister mating showed a decrease in the frequency in the kainic acid-induced wet-dog shakes as shown in the F1 offspring. No change in the response to kainic acid was shown in the 13-15dHc-F2 rats.

Ornithine decarboxylase is differentially induced by kainic acid during brain development in the rat

The induction of brain ornithine decarboxylase (ODC) as a consequence of systemic kainic acid administration was studied in the hippocampus and the olfactory cortex-amygdala area of 10-day-old rat pups and 30-day-old young rats. In pups, ODC levels were moderately increased (plus 50-80%) 4 h after kainic acid administration, coming back quickly to control levels afterwards. In young rats, instead, ODC levels were dramatically increased by 17-25-fold, 16 h after kainic acid administration and decreased towards basal levels 48-72 h after injection. The present results suggest that the process of excitotoxic ODC induction can be split in two phases: a first phase characterized by moderate induction and essentially linked to the overstimulation of brain circuits and a second phase, during which a dramatic enzyme stimulation is accompanied by the appearance of neurodegenerative pathology.

Pathophysiological mechanisms of brain damage from status epilepticus

Human status epilepticus (SE) is consistently associated with cognitive problems, and with widespread neuronal necrosis in hippocampus and other brain regions. In animal models, convulsive SE causes extensive neuronal necrosis. Nonconvulsive SE in adult animals also leads to widespread neuronal necrosis in vulnerable regions, although lesions develop more slowly than they would in the presence of convulsions or anoxia. In very young rats, nonconvulsive normoxic SE spares hippocampal pyramidal cells, but other types of neurons may not show the same resistance, and inhibition of brain growth, DNA and protein synthesis, and of myelin formation and of synaptogenesis may lead to altered brain development. Lesions induced by SE may be epileptogenic by leading to misdirected regeneration. In SE, glutamate, aspartate, and acetylcholine play major roles as excitatory neurotransmitters, and GABA is the dominant inhibitory neurotransmitter. GABA metabolism in substantia nigra (SN) plays a key role in seizure arrest. When seizures stop, a major increase in GABA synthesis is seen in SN postictally. GABA synthesis in SN may fail in SE. Extrasynaptic factors may also play an important role in seizure spread and in maintaining SE. Glial immaturity, increased electronic coupling, and SN immaturity facilitate SE development in the immature brain. Major increases in cerebral blood flow (CBF) protect the brain in early SE, but CBF falls in late SE as blood pressure falters. At the same time, large increases in cerebral metabolic rate for glucose and oxygen continue throughout SE. Adenosine triphosphate (ATP) depletion and lactate accumulation are associated with hypermetabolic neuronal necrosis. Excitotoxic mechanisms mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors open ionic channels permeable to calcium and play a major role in neuronal injury from SE. Hypoxia, systemic lactic acidosis, CO2 narcosis, hyperkalemia, hypoglycemia, shock, cardiac arrhythmias, pulmonary edema, acute renal tubular necrosis, high output failure, aspiration pneumonia, hyperpyrexia, blood leukocytosis and CSF pleocytosis are common and potentially serious complications of SE. Our improved understanding of the pathophysiology of brain damage in SE should lead to further improvement in treatment and outcome.