Progressive changes in hippocampal cytoarchitecture in a neurodevelopmental rat model of epilepsy: implications for understanding presymptomatic epileptogenesis, predictive diagnosis, and targeted treatments

Epilepsies affect about 4% of the population and are frequently characterized by a prolonged "silent" period before the onset of spontaneous seizures. Most current animal models of epilepsy either involve acute seizure induction or kindling protocols that induce repetitive seizures. We have developed a rat model of epilepsy that is characterized by a slowly progressing series of behavioral abnormalities prior to the onset of behavioral seizures. In the current study, we further describe an accompanying progression of cytoarchitectural changes in the hippocampal formation. Groups of male and female SD rats received serial injections of a low dose of domoic acid (0.020 mg/kg) (or vehicle) throughout the second week of life. Postmortem hippocampal tissue was obtained on postnatal days 29, 64, and 90 and processed for glial fibrillary acidic protein (GFAP), NeuN, and calbindin expression. The data revealed no significant changes on postnatal day (PND) 29 but a significant increase in hilar NeuN-positive cells in some regions on PND 64 and 90 that were identified as ectopic granule cells. Further, an increase in GFAP positive cell counts and evidence of reactive astrogliosis was found on PND 90 but not at earlier time points. We conclude that changes in cellular expression, possibly due to on-going non-convulsive seizures, develop slowly in this model and may contribute to progressive brain dysfunction that culminates in a seizure-prone phenotype.

Neuroteratology and Animal Modeling of Brain Disorders

Over the past 60 years, a large number of selective neurotoxins were discovered and developed, making it possible to animal-model a broad range of human neuropsychiatric and neurodevelopmental disorders. In this paper, we highlight those neurotoxins that are most commonly used as neuroteratologic agents, to either produce lifelong destruction of neurons of a particular phenotype, or a group of neurons linked by a specific class of transporter proteins (i.e., dopamine transporter) or body of receptors for a specific neurotransmitter (i.e., NMDA class of glutamate receptors). Actions of a range of neurotoxins are described: 6-hydroxydopamine (6-OHDA), 6-hydroxydopa, DSP-4, MPTP, methamphetamine, IgG-saporin, domoate, NMDA receptor antagonists, and valproate. Their neuroteratologic features are outlined, as well as those of nerve growth factor, epidermal growth factor, and that of stress. The value of each of these neurotoxins in animal modeling of human neurologic, neurodegenerative, and neuropsychiatric disorders is discussed in terms of the respective value as well as limitations of the derived animal model. Neuroteratologic agents have proven to be of immense importance for understanding how associated neural systems in human neural disorders may be better targeted by new therapeutic agents.

Perinatal Domoic Acid as a Neuroteratogen

In mammals, the period shortly before and shortly after birth is a time of massive brain growth, plasticity and maturation. It is also a time when the developing brain is exquisitely sensitive to insult, often with long-lasting consequences. Many of society's most debilitating neurological diseases arise, at least in part, from trauma around the time of birth but go undetected until later in life. For the past 15 years, we have been studying the consequences of exposure to the AMPA/kainate agonist domoic acid (DOM) on brain development in the rat. Domoic acid is a naturally occurring excitotoxin that enters the food chain and is known to produce severe neurotoxicity in humans and other adult wildlife. Our work, and that of others, however, has demonstrated that DOM is also toxic to the perinatal brain and that toxicity occurs at doses much lower than those required in adults. This raises concern about the current regulatory limit for DOM contamination that is based on data in adult animals, but has also allowed creation of a novel model of neurological disease progression. Herein, we review briefly the toxicity of DOM in adults, including humans, and describe features of the developing nervous system relevant to enhanced risk. We then review the data on DOM as a prenatal neuroteratogen and describe in detail the work of our respective laboratories to characterize the long-term behavioural and neuropathological consequences of exposure to low-dose DOM in the newborn rat.

Low-dose neonatal domoic acid causes persistent changes in behavioural and molecular indicators of stress response in rats

Appropriate stress responses rely on a finely-tuned neuronal balance that must continually adapt to a frequently changing external environment. Alterations in this balance can result in susceptibility to a variety of stress-related disorders, as well as exacerbate already existing conditions. We have previously reported that rat pups injected with a very low dose (20 μg/kg) of domoic acid during the second postnatal week of life display low-grade seizure behaviours when challenged with stressful tasks, and also exhibit a variety of structural and functional changes similar to those seen in temporal lobe epilepsy. The current study was designed to investigate markers of altered stress-response in this model. Following neonatal treatment, adult rats were tested in the elevated plus maze, as well as two water maze tasks, both of which involved a platform reversal challenge. Results indicated a modified behavioural stress/anxiety response, increased perseveration, and alterations in search strategy for all domoate-treated rats, as well as male-specific deficits in cognitive flexibility. In addition, 80% of treated males and 20% of treated females exhibited seizure behaviour. Western blot analysis revealed male-only increases in adrenergic receptor (α2a and α2c) and mineralocorticoid receptor expression, and subtle sex-specific changes in glucocorticoid receptor expression, but no differences in corticotropin-releasing factor receptors I/II, or dopamine D2 receptor expression. A significant decrease in glucocorticoid:mineralocorticoid ratio was also noted. We conclude that early exposure to DOM alters central mechanisms underlying stress response, and that this model may be valuable for investigating the connection between stress and neurological disorders.

Modeling epileptogenesis and temporal lobe epilepsy in a non-human primate

Here we describe a new non-human primate model of temporal lobe epilepsy (TLE) to better investigate the cause/effect relationships of human TLE. Status epilepticus (SE) was induced in adult marmosets by pilocarpine injection (250mg/kg; i.p.). The animals were divided in 2 groups: acute (8h post-SE) and chronic (3 and 5 months post-SE). To manage the severity of SE, animals received diazepam 5min after the SE onset (acute group: 2.5 or 1.25mg/kg; i.p.; chronic group/; 1.25mg/kg; i.p). All animals were monitored by video and electrocorticography to assess SE and subsequent spontaneous recurrent seizures (SRS). To evaluate brain injury produced by SE or SRS we used argyrophil III, Nissl and neo-Timm staining techniques. Magnetic resonance image was also performed in the chronic group. We observed that pilocarpine was able to induce SE followed by SRS after a variable period of time. Prolonged SE episodes were associated with brain damage, mostly confined to the hippocampus and limbic structures. Similar to human TLE, anatomical disruption of dentate gyrus was observed after SRS. Our data suggest that pilocarpine marmoset model of epilepsy has great resemblance to human TLE, and could provide new tools to further evaluate the subtle changes associated with human epilepsy.

Ischemic cardiomyopathy following seizure induction by domoic Acid

Exposure to the excitotoxin domoic acid (DOM) has been shown to produce cardiac lesions in both clinical and animal studies. We have previously shown that DOM failed to directly affect cardiomyocyte viability and energetics, but the development of this cardiomyopathy has remained unexplained. The present study compared effects of high-level seizure induction obtained by intraperitoneal (2 mg/kg) or intrahippocampal (100 pmol) bolus administration of DOM on development of cardiac pathologies in a rat model. Assessment of cardiac pressure derivatives and coronary flow rates revealed a significant time-dependent decrease in combined left ventricular (LV) systolic and diastolic function at 1, 3, 7, and 14 days after intraperitoneal administration and at 7 and 14 days after intrahippocampal DOM administration. LV dysfunction was matched by a similar time-dependent decrease in mitochondrial respiratory control, associated with increased proton leakage, and in mitochondrial enzyme activities. Microscopic examination of the LV midplane revealed evidence of progressive multifocal ischemic damage within the subendocardial, septal, and papillary regions. Lesions ranged from reversible early damage (vacuolization) to hypercontracture and inflammatory necrosis progressing to fibrotic scarring. Plasma proinflammatory IL-1α, IL-1β, and TNF-α cytokine levels were also increased from 3 days after seizure induction. The observed cardiomyopathies did not differ between intraperitoneal and intrahippocampal groups, providing strong evidence that cardiac damage after DOM exposure is a consequence of a seizure-evoked autonomic response.

New animal models of progressive neurodegeneration: tools for identifying targets in predictive diagnostics and presymptomatic treatment

Mental and neurological disorders are increasingly prevalent and constitute a major societal and economic burden worldwide. Many of these diseases and disorders are characterized by progressive deterioration over time, that ultimately results in identifiable symptoms that in turn dictate therapy. Disease-specific symptoms, however, often occur late in the degenerative process. A better understanding of presymptomatic events could allow for the development of new diagnostics and earlier interventions that could slow or stop the disease process. Such studies of progressive neurodegeneration require the use of animal models that are characterized by delayed or slowly developing disease phenotype(s). This brief review describes several examples of such animal models that have recently been developed with relevance to various neurological diseases and disorders, and delineates the potential of such models to aid in predictive diagnosis, early intervention and disease prevention.

Isodomoic acids A and C exhibit low KA receptor affinity and reduced in vitro potency relative to domoic acid in region CA1 of rat hippocampus

Several natural isomers of the seizurogenic neurotoxin domoic acid (DA) have been found to occur at up to mg/kg levels in shellfish. The aim of the current study was to assess the neurotoxic potency of isodomoic acids A and C (Iso-A and Iso-C), recently isolated from commercial shellfish. Hippocampal slices were obtained from young adult rats and maintained in a tissue recording chamber. Synaptically evoked population spikes were recorded in region CA1 before and after exposure to DA or its isomers. Both Iso-A and Iso-C produced transient neuronal hyperexcitability followed by a dose-dependent suppression of population spikes, but were, respectively, 4- and 20-fold less potent than DA (spike area: EC50 DA=237 nM; Iso-A=939 nM; Iso-C=4.6 microM). In the hippocampus, DA preconditioning induces tolerance to subsequent DA toxicity. However, in the present study neither Iso-A nor Iso-C were effective as preconditioning agents. Competitive binding studies using homomeric GluR6 kainate (kainic acid, KA) receptors showed the affinity of Iso-A to be 40-fold lower than DA (Ki DA=3.35 nM; Iso-A=130 nM). Together with earlier work showing Iso-C affinity at GluR6 receptors to be 240-fold lower than DA, our results suggest that neuroexcitatory effects of Iso-A in CA1 may involve both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and KA receptors, while Iso-C likely involves the activation of AMPA receptors alone.

Hippocampal mossy fiber sprouting and elevated trkB receptor expression following systemic administration of low dose domoic acid during neonatal development

We have previously reported that serial systemic injections of low-dose (subconvulsive) domoic acid (DOM) during early postnatal development produces changes in both behavior and hippocampal cytoarchitecture in aged rats (17 months) that are similar to those seen in existing animal models of temporal lobe epilepsy. Herein we report further hippocampal changes, consisting of mossy fiber sprouting and associated changes in the trkB receptor population in young adult (3 months) rats, and further, report that these changes show regional variation throughout the septo-temporal axis of the hippocampus. Groups of Sprague Dawley rat pups were injected daily from postnatal day 8-14 with either saline (n = 23) or 20 microg/kg DOM (n = 25), tested for key indicators of neonatal neurobehavioral development, and then left undisturbed until approximately 90 days of age, at which time brain tissue was removed, hippocampi were dissected, fixed and processed using either Timm's stain to visualize hippocampal mossy fiber sprouting (MFS) or trkB immunohistochemistry to visualize full length trkB receptors. Multiple sections from dorsal, mid, and ventral hippocampus were analyzed separately and all measures were conducted using image analysis software. The results indicate significant increases in MFS in the inner molecular layer in treated animals with corresponding changes in trkB receptor density. Further we identified significant increases in trkB receptor density in the hilus of the dentate gyrus and area CA3 and report increased mossy fiber terminal density in the stratum lucidum in treated rats. The magnitude of these changes differed between sections from dorsal, mid, and ventral hippocampus. We conclude that low dose neonatal DOM produces cytoarchitectural changes indicative of abnormal development and/or synaptic plasticity that are progressive with age and show regional variation within the hippocampal formation.

Gender-based changes in cognition and emotionality in a new rat model of epilepsy

Epilepsy research relies heavily on animal models that mimic some, or all, of the clinical symptoms observed. We have previously described a new developmental rat model of epilepsy that demonstrates both behavioural seizures and changes in hippocampal morphology. In the current study we investigated whether these rats also show changes in cognitive performance as measured using the Morris water maze task, and emotionality as measured using the Elevated plus maze task. In the water maze, significant differences between male and female rats were found in several performance variables regardless of treatment. In addition, female but not male rats, treated neonatally with domoic acid had significant impairments in learning new platform locations in the water maze. In the elevated plus maze, a significant proportion of female rats spent more time in the open arm of the maze following prior exposure to the maze whereas this effect was not seen in male rats. We conclude that perinatal treatment with low doses of domoic acid results in significant gender-based changes in cognition and emotionality in adult rats.

Histopathological and molecular changes produced by hippocampal microinjection of domoic acid

The phytoplankton-derived neurotoxin, domoic acid (DOM), frequently causes poisoning of marine animals and poses an increasing threat to public health through contamination of seafood. In this study, we used stereotactic microinjection technique to administer varying amounts of DOM into the hippocampal CA1 region in order to examine potential histopathological changes after injection of sub-lethal concentrations to CA1 pyramidal neurons. Gross anatomical abnormalities in CA1 were observed at above 10 microM DOM (3 pmol in 0.3 microl saline). At 1mM concentration, DOM produces both ipsilateral and contralateral neuronal cell death in CA1, CA3 as well as dentate gyrus subfields. Animal behavioral changes after microinjection were similar to those observed by previous studies through systemic DOM injection. Neuronal degeneration was paralleled by reduced glutamate receptor (NR1, GluR1 and GluR6/7) immunolabeling throughout the whole hippocampal formation. Pre-injection of the AMPA/KA receptor antagonist NBQX (10 microM, 0.3 microl) blocked 1mM DOM-induced neuronal degeneration as well as behavioral symptoms. At concentrations lower than 10 microM, no histopathological changes were observed microscopically, nor were the levels of immunostaining of NR1, GluR1, GluR6/7 different. However, increased immunolabeling of autophosphorylated calcium-calmodulin-dependent kinase II (CaMKII, p-Thr286) and phosphorylated cAMP response element binding protein (CREB, p-Ser133) were observed at 24 h post-injection, suggesting that altered intracellular signal transduction mediated by GluRs might be an adaptive cellular protective mechanism against DOM-induced neurotoxicity.

Low doses of domoic acid during postnatal development produce permanent changes in rat behaviour and hippocampal morphology

It is well established that the developing brain is a highly dynamic environment that is susceptible to toxicity produced by a number of pharmacological, chemical and environmental insults. We report herein on permanent behavioural and morphological changes produced by exposing newborn rats to very low (subconvulsive) doses of kainate receptor agonists during a critical window of brain development. Daily treatment of SD rat pups with either 5 or 20 microg/kg of domoic acid (DOM) from postnatal day 8-14 resulted in a permanent and reproducible seizure-like syndrome when animals were exposed to different tests of spatial cognition as adults. Similar results were obtained when animals were treated with equi-efficacious doses of kainic acid (KA; 25 or 100 microg/kg). Treated rats had significant increases in hippocampal mossy fiber staining and reductions in hippocampal cell counts consistent with effects seen in adult rats following acute injections of high doses of kainic acid. In situ hybridization also revealed an elevation in hippocampal brain derived neurotrophic factor (BDNF) mRNA in region CA1 without a corresponding increase in neuropeptide Y (NPY) mRNA. These results provide evidence of long-lasting behavioural and histochemical consequences arising from relatively subtle changes in glutamatergic activity during development, that may be relevant to understanding the aetiology of seizure disorders and other forms of neurological disease.

Low doses of non-NMDA glutamate receptor agonists alter neurobehavioural development in the rat

While it is known that glutamate is critical to CNS development and function, less is known about the role of kainate receptors, a subclass of ionotropic glutamate receptors, during ontogeny. This is especially true with respect to the emergence and expression of behaviour. It is also known that the neonatal CNS differs from that of adults with respect to excitatory amino acid (EAA) toxicity. Our aim was to determine the effects of early low-dose stimulation of kainate receptors on physical and behavioural development in the rat. Saline, one of two subtoxic doses of domoic acid (DOM) (5 and 20 microg/kg), or in a parallel study, saline, or one of two pharmacologically equivalent doses of kainic acid (KA) (25 and 100 microg/kg), were systemically administered once daily from postnatal days (PNDs) 8-14. While DOM or KA had no effect on typical measures of toxicity such as weight gain, acoustic startle, ultrasonic vocalizations (UVs), or maternal retrieval, these doses were shown to be physiologically relevant, producing particular group differences in eye opening, conditioned place preference, and activity levels. We conclude that administration of very low doses of selective kainate receptor agonists during the second postnatal week produces changes in neurobehavioural development in the rat.

Domoic acid neurotoxicity in cultured cerebellar granule neurons is controlled preferentially by the NMDA receptor Ca(2+) influx pathway

We have monitored real-time alterations in [Ca(2+)](i) in fluo-3-loaded cerebellar granule neurons exposed to domoate, and ascertained the influence of pharmacological blockers of various Ca(2+) entry pathways on intracellular Ca(2+) accumulation, excitatory amino acid (EAA) release and neuronal death. Domoate produced a rapid and concentration-dependent increase in [Ca(2+)](i), the magnitude of which correlated closely with the severity of neuron loss. The increase in [Ca(2+)](i) was derived from activation of NMDA receptors, L-type voltage-sensitive calcium channels (VSCC) and the reversed mode of operation of the Na(+)/Ca(2+) exchanger. When the level of neuroprotection conferred by pharmacological manipulation of these calcium entry pathways was regressed with the corresponding reductions in [Ca(2+)](i) load, it was observed that neuronal vulnerability is controlled preferentially by NMDA receptors. This observation is consistent with our previous study of brevetoxin-induced autocrine excitotoxicity and with the source specificity hypothesis of others [J. Neurochem. 71 (1998) 2349], which suggests that elevation of [Ca(2+)](i) in the vicinity of the NMDA receptor ion channel activates processes leading to neuronal death.

Comparative behavioural toxicity of domoic acid and kainic acid in neonatal rats

Cumulative behavioural toxicity was measured in groups of male and female rat pups (n=6/sex) at different stages of postnatal development. Dose-response curves (DRCs) for toxicity produced by domoic acid (DOM) were generated using animals on postnatal days (PND) 0, 5, 14, and 22, using a behavioural rating scale. In a subsequent experiment, DRCs for toxicity generated by either DOM or kainic acid were produced in rats at PND 8 and 14 for comparison between the two toxins. DOM was found to be a very potent neurotoxin in newborn rats and the potency of DOM progressively decreased with increasing age (interpolated ED(50)=0.12, 0.15, 0.30, and 1.06 mg/kg at PND 0, 5, 14, and 22, respectively). In addition, the patterns of behavioural expression were found to differ with age. Comparisons between DOM and kainic acid revealed that DOM was approximately six-fold more potent than kainate at both PND 8 and PND 14 and that both toxins were approximately two-fold less potent in PND 14 rats, compared to PND 8. This implies that the mechanism(s) responsible for reduced potency is/are similar between the two compounds. Consistent with previous reports, however, there were both similarities and differences in the observed patterns of behavioural toxicity produced by the two toxins at both ages.

Mediation of ionotropic glutamate receptors in domoic acid-induced striatal dopamine release in rats

Our objective was to characterize the mechanism of action of intrastriatal infusion of domoic acid on extracellular dopamine levels, using in vivo dialysis in conscious and freely moving rats. The local infusion of domoic acid (500 microM) caused an increase (567.9+/-142.5%, versus basal) in dopamine extracellular levels associated with a decrease in its metabolites: dihydroxyphenylacetate (DOPAC) and homovanillate (HVA) (47.3+/-4.4% and 33.8+/-4.2%, respectively, compared to basal). Infusion of the amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/kainate) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX; 200 microM) reversed the effect of domoic acid infusion on striatal dopamine levels. However, the infusion of the selective non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (MK-801; 50 microM), did not change significantly the effect of domoic acid on dopamine extracellular levels. In conclusion, based on results with a microdialysis technique, we suggest that domoic acid may act through AMPA/kainate receptors in striatum.

The activation of glutamate receptors by kainic acid and domoic acid

The neurotoxins kainic acid and domoic acid are potent agonists at the kainate and alphaamino-5-methyl-3-hydroxyisoxazolone-4-propionate (AMPA) subclasses of ionotropic glutamate receptors. Although it is well established that AMPA receptors mediate fast excitatory synaptic transmission at most excitatory synapses in the central nervous system, the role of the high affinity kainate receptors in synaptic transmission and neurotoxicity is not entirely clear. Kainate and domoate differ from the natural transmitter, L-glutamate, in their mode of activation of glutamate receptors; glutamate elicits rapidly desensitizing responses while the two neurotoxins elicit non-desensitizing or slowly desensitizing responses at AMPA receptors and some kainate receptors. The inability to produce desensitizing currents and the high affinity for AMPA and kainate receptors are undoubtedly important factors in kainate and domoate-mediated neurotoxicity. Mutagenesis studies on cloned glutamate receptors have provided insight into the molecular mechanisms responsible for these unique properties of kainate and domoate.

High-affinity [3H] kainic acid binding to brain membranes: a re-evaluation of ligand potency and selectivity

[3H]Kainic acid ([3H]KA) is a widely used tool for studying the KA class of excitatory amino acid receptors. [3H]KA of significantly higher specific activity has become available permitting use of radioligand concentrations below the dissociation constant (K(D)) of the high-affinity binding site. We employed low radioligand (0.05-0.2 nM) and receptor concentrations (0.01 nM) to gain new insights into the binding characteristics of the high-affinity KA binding site in a standard preparation of lyzed synaptosomal membranes from the cerebral cortex of male Sprague-Dawley rats. Under these conditions, KA binds to a single class of high-affinity sites with a K(D) of 1.0+/- 0.3 nM. The potencies of competing agents are considerably higher than published reports. Specifically, domoic acid, glutamate, and glutamine exhibit IC(50) values for displacing [3H]KA of 0.37+/-0.02, 94+/-13, and 1500+/-500 nM, respectively. Domoate (1 microM) was tested against a panel of 32 central nervous system binding sites and found to be inactive at each, indicating this toxin displays considerable selectivity. This study illustrates the remarkable potency of domoic acid and underlines the importance of performing radioligand binding studies at concentrations of constituents that permit characterization of high-affinity interactions.