"Subclinical" dosages of lithium and pilocarpine that do not evoke overt seizures affect long-term spatial memory but not learning in rats

Brain pathology in focal status epilepticus: evidence from experimental models

Status Epilepticus (SE) is often a neurological emergency characterized by abnormally sustained, longer than habitual seizures. The new ILAE classification reports that SE "…can have long-term consequences including neuronal death, neuronal injury…depending on the type and duration of seizures". While it is accepted that generalized convulsive SE exerts detrimental effects on the brain, it is not clear if other forms of SE, such as focal non-convulsive SE, leads to brain pathology and contributes to long-term deficits in patients. With the available clinical and experimental data, it is hard to discriminate the specific action of the underlying SE etiologies from that exerted by epileptiform activity. This information is highly relevant in the clinic for better treatment stratification, which may include both medical and surgical intervention for seizure control. Here we review experimental studies of focal SE, with an emphasis on focal non-convulsive SE. We present a repertoire of brain pathologies observed in the most commonly used animal models and attempt to establish a link between experimental findings and human condition(s). The extensive literature on focal SE animal models suggest that the current approaches have significant limitations in terms of translatability of the findings to the clinic. We highlight the need for a more stringent description of SE features and brain pathology in experimental studies in animal models, to improve the accuracy in predicting clinical translation.

A Low Mortality, High Morbidity Reduced Intensity Status Epilepticus (RISE) Model of Epilepsy and Epileptogenesis in the Rat

Animal models of acquired epilepsies aim to provide researchers with tools for use in understanding the processes underlying the acquisition, development and establishment of the disorder. Typically, following a systemic or local insult, vulnerable brain regions undergo a process leading to the development, over time, of spontaneous recurrent seizures. Many such models make use of a period of intense seizure activity or status epilepticus, and this may be associated with high mortality and/or global damage to large areas of the brain. These undesirable elements have driven improvements in the design of chronic epilepsy models, for example the lithium-pilocarpine epileptogenesis model. Here, we present an optimised model of chronic epilepsy that reduces mortality to 1% whilst retaining features of high epileptogenicity and development of spontaneous seizures. Using local field potential recordings from hippocampus in vitro as a probe, we show that the model does not result in significant loss of neuronal network function in area CA3 and, instead, subtle alterations in network dynamics appear during a process of epileptogenesis, which eventually leads to a chronic seizure state. The model’s features of very low mortality and high morbidity in the absence of global neuronal damage offer the chance to explore the processes underlying epileptogenesis in detail, in a population of animals not defined by their resistance to seizures, whilst acknowledging and being driven by the 3Rs (Replacement, Refinement and Reduction of animal use in scientific procedures) principles.

The frequency of spontaneous seizures in rats correlates with alterations in sensorimotor gating, spatial working memory, and parvalbumin expression throughout limbic regions

Cognitive deficits and psychotic symptoms are highly prevalent in patients with temporal lobe epilepsy (TLE). Imaging studies in humans have suggested that these comorbidities are associated with atrophy in temporal lobe structures and other limbic regions. It remains to be clarified whether TLE comorbidities are due to the frequency of spontaneous seizures or to limbic structural damage per se. Here, we used the pilocarpine model of chronic spontaneous seizures to evaluate the possible association of seizure frequency with sensorimotor gating, spatial working memory, and neuropathology throughout limbic regions. For TLE modeling, we induced a 2-h status epilepticus by the systemic administration of lithium-pilocarpine. Once spontaneous seizures were established, we tested the locomotor activity (open field), spatial working memory (eight-arm radial maze), and sensorimotor gating (prepulse inhibition of acoustic startle). After behavioral testing, the brains were sectioned for hematoxylin-eosin staining (cell density) and parvalbumin immunohistochemistry (GABAergic neuropil) in the prefrontal cortex, nucleus accumbens, thalamus, amygdala, hippocampus, and entorhinal cortex. The animal groups analyzed included chronic epileptic rats, their controls, and rats that received lithium-pilocarpine but eventually failed to express status epilepticus or spontaneous seizures. Epileptic rats showed deficits in sensorimotor gating that negatively correlated with the radial maze performance, and impairments in both behavioral tests correlated with seizure frequency. In addition to neuronal loss at several sites, we found increased parvalbumin immunostaining in the prefrontal cortex (infralimbic area), thalamus (midline and reticular nuclei), amygdala, Ammon's horn, dentate gyrus, and entorhinal cortex. These tissue changes correlated with seizure frequency and impairments in sensorimotor gating. Our work indicates that chronic seizures might impact the inhibitory-excitatory balance in the temporal lobe and its interconnected limbic regions, which could increase the likelihood of cognitive deficits and interictal psychiatric disorders.

Development of a rat pilocarpine model of seizure/status epilepticus that mimics chemical warfare nerve agent exposure

We developed a rat pilocarpine seizure/status epilepticus (SE) model, which closely resembles 1.6-2.0 x LD50 soman exposure, to analyse the molecular mechanism of neuronal damage and to screen effective neuroprotectants against cholinergic agonist and chemical warfare nerve agent (CWNA) exposure. Rats implanted with radiotelemetry probes capable of recording electroencephalogram (EEG), electrocardiogram (ECG), temperature, and physical activity were treated with lithium chloride (5 mEq/kg, im), followed 24 h later by (ip) doses of pilocarpine hydrochloride. Based on radiotelemetry analysis, a dose of 240 mg/kg (ip) pilocarpine generated seizure/SE analogous to 1.6-2.0 x LD50 of soman. The model was refined by reducing the peripheral convulsions without affecting the central nervous system (CNS) by administering methylscopolamine bromide (1 mg/kg, ip), an anti-cholinergic that does not cross the blood-brain barrier. However, when methylscopolamine bromide was administered, a higher dose of pilocarpine (320 mg/kg, ip) was required to generate the equivalent seizure/SE. Histopathology data indicated that pilocarpine induces significant damage to the hippocampal region of the brain, with similar neuropathology to that of 1.6-2.0 x LD50 soman exposure. There was a reduction in body temperature after the administration of pilocarpine, as observed in organophosphate (OP) nerve agents exposure. The heart-rate of pilocarpine-treated animals increased compared to the normal range. The pilocarpine seizure/SE model was also reproducible in the absence of lithium chloride. These results support that pilocarpine seizure/SE model is useful in studying the molecular mechanisms of neuropathology and screening neuroprotectants following cholinergic agonist and CWNA exposure.

Behavioral changes resulting from the administration of cycloheximide in the pilocarpine model of epilepsy

Cycloheximide influences synaptic reorganization resulting from pilocarpine-induced status epilepticus (SE). To investigate the possible behavioral consequences of this effect, we subjected animals to pilocarpine-induced SE either in the absence (Pilo group) or presence of cycloheximide (Chx group). Animals were further divided regarding the occurrence of spontaneous recurrent seizures (SRS). Two months after SE induction animals were exposed to different behavioral tests. Age-matched naïve animals were used as controls. All epileptic groups showed a significantly diminished freezing time in contextual and tone fear conditioning, performed poorly in the Morris water maze and present less seconds in immobility position as compared to controls. Only Pilo animals explored more extensively the open arms of the elevated plus maze and showed increased in horizontal exploratory activity in the open field as compared to controls. With the exception of Pilo animals without recorded SRS, all other groups had extensive tissue shrinkage in central nucleus of the amygdala as compared to controls. Cycloheximide-treated animals differed from Pilo animals in the extent of hilar loss and supragranular mossy fiber sprouting as well as tissue shrinkage in the dorsal hippocampus. Despite the histological differences seen in the dorsal hippocampus between experimental groups, no differences were encountered in the cognitive tests used to evaluate dorsal hippocampal function. The encountered histological differences between Chx and Pilo animals, however, might underlie the different emotional responses between the two groups.

Long-term behavioral and morphological consequences of nonconvulsive status epilepticus in rats

The aims of the present study were to ascertain whether nonconvulsive status epilepticus (NCSE) could give rise to long-term behavioral deficits and permanent brain damage. Two months after NCSE was elicited with pilocarpine (15 mg/kg i.p.) in LiCl-pretreated adult male rats, animals were assigned to either behavioral (spontaneous behavior, social interaction, elevated plus-maze, rotorod, and bar-holding tests) or EEG studies. Another group of animals was sacrificed and their brains were processed for Nissl and Timm staining as well as for parvalbumin and calbindin immunohistochemistry. Behavioral analysis revealed motor deficits (shorter latencies to fall from rotorod as well as from bar) and disturbances in the social behavior of experimental animals (decreased interest in juvenile conspecific). EEGs showed no apparent abnormalities. Quantification of immunohistochemically stained sections revealed decreased amounts of parvalbumin- and calbindin-immunoreactive neurons in the motor cortex and of parvalbumin-positive neurons in the dentate gyrus. Despite relatively inconspicuous manifestations, NCSE may represent a risk for long-term deficits.

An animal model of nonconvulsive status epilepticus: a contribution to clinical controversies

PURPOSE

To characterize electroencephalographic and behavioral effects as well as electrophysiologic and morphologic consequences of a subconvulsive dose of pilocarpine in lithium chloride-pretreated rats.

METHODS

Pilocarpine (15 mg/kg) was administered intraperitoneally to adult rats pretreated with lithium chloride (3 mEq/kg, i.p.). Behavior was observed for 2 h and videotaped in three consecutive sessions. At the same time, EEG was recorded from the sensorimotor cortex and the dorsal hippocampus. Threshold intensities of currents necessary to elicit hippocampal afterdischarges were determined 24 h and 1 week after the pilocarpine administration. The brains were histologically examined 1 week after pilocarpine administration using Nissl stain.

RESULTS

Pilocarpine induced time-limited nonconvulsive status epilepticus (NCSE). Epileptic EEG activity concurrent with prominent behavioral features was observed both in the neocortex and, predominantly, in the hippocampus. No changes in afterdischarge thresholds were observed in the dorsal hippocampus 24 h and 1 week after NCSE. One week after NCSE, seizure-related brain damage was found mainly in the motor neocortical fields.

CONCLUSIONS

Pilocarpine-induced NCSE in rats strongly resembles a short-term human complex partial status epilepticus. Our animal model is suitable for studying the possible adverse effects of prolonged nonconvulsive seizures.

Nonconvulsive status epilepticus in rats: impaired responsiveness to exteroceptive stimuli

An animal model of human complex partial status epilepticus induced by lithium chloride and pilocarpine administration was developed in our laboratory. The objective of the study was to provide a detailed analysis of both ictal and postictal behavior and to quantify seizure-related morphological damage. In order to determine the animal's responsiveness to either visual or olfactory stimuli, adult male rats were submitted to the following behavioral paradigms: the object response test, the social interaction test, and the elevated plus-maze test. The rotorod test was used to evaluate motor performance. Two weeks after status epilepticus, brains were morphologically examined and quantification of the brain damage was performed. Profound impairment of behavior as well as responsiveness to exteroceptive stimuli correlated with the occurrence of epileptic EEG activity. When the epileptic EEG activity ceased, responsiveness of the pilocarpine-treated animals was renewed. However, remarkable morphological damage persisted in the cortical regions two weeks later. This experimental study provides support for the clinical evidence that even nonconvulsive epileptic activity may cause brain damage. We suggest that the model can be used for the study of both functional and morphological consequences of prolonged nonconvulsive seizures.

Lithium therapy and signal transduction

Lithium is the simplest therapeutic agent available for the treatment of depression and has been used for over 100 years, yet no definitive mechanism for its effect has been established. Among the proposed mechanisms, two lithium-sensitive signal transduction pathways are active in the brain; these are mediated by glycogen synthase kinase 3beta (GSK-3beta) and inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] signalling. This article describes recent experiments in cell and developmental biology that advance our understanding of how lithium works and it presents new directions for the study of both depression and Alzheimer's disease (AD).

Infiltration of lymphocytes in the limbic brain following stimulation of subclinical cellular immunity and low dosages of lithium and a cholinergic agent

This experiment was designed to investigate the hypothesis that single small dosages of lithium (1.5 mEq/kg), the muscarinic agent pilocarpine (15 mg/kg) and spinal cord emulsion encourage perivascular infiltration of lymphocytes into the brain even when overt symptoms of experimental allergic encephalomyelitis are not apparent. The brains of rats that had received this small dosage of lithium and pilocarpine exhibited discernable infiltrations of lymphocytes within limbic tracts but no discernable neuronal loss. Although the brains of the rats that displayed overt seizures following larger dosages of lithium (3 mEq/kg) and pilocarpine (30 mg/kg) exhibited the usual pattern of neuronal loss within multiple thalamic and limbic structures and conspicuous foci of lymphocytic infiltration (particularly within the hippocampal formation) the correlation between the numbers of foci and the proportions of neuronal damage in these structures was not significant statistically. These results indicate that infiltrations of lymphocytes into brain parenchyma are not simple artifacts of the amount of neuronal damage and may be sensitive toxicological markers for subclinical interactions between drugs and immune responses.

Loss of a prolyl oligopeptidase confers resistance to lithium by elevation of inositol (1,4,5) trisphosphate

The therapeutic properties of lithium ions (Li+) are well known; however, the mechanism of their action remains unclear. To investigate this problem, we have isolated Li+-resistant mutants from Dictyostelium. Here, we describe the analysis of one of these mutants. This mutant lacks the Dictyostelium prolyl oligopeptidase gene (dpoA). We have examined the relationship between dpoA and the two major biological targets of lithium: glycogen synthase kinase 3 (GSK-3) and signal transduction via inositol (1,4,5) trisphosphate (IP3). We find no evidence for an interaction with GSK-3, but instead find that loss of dpoA causes an increased concentration of IP3. The same increase in IP3 is induced in wild-type cells by a prolyl oligopeptidase (POase) inhibitor. IP3 concentrations increase via an unconventional mechanism that involves enhanced dephosphorylation of inositol (1,3,4,5,6) pentakisphosphate. Loss of DpoA activity therefore counteracts the reduction in IP3 concentration caused by Li+ treatment. Abnormal POase activity is associated with both unipolar and bipolar depression; however, the function of POase in these conditions is unclear. Our results offer a novel mechanism that links POase activity to IP3 signalling and provides further clues for the action of Li+ in the treatment of depression.