Seizure onset times for rats receiving systemic lithium and pilocarpine: sources of variability

The impact of early-life environment on absence epilepsy and neuropsychiatric comorbidities

This review discusses the long-term effects of early-life environment on epileptogenesis, epilepsy, and neuropsychiatric comorbidities with an emphasis on the absence epilepsy. The WAG/Rij rat strain is a well-validated genetic model of absence epilepsy with mild depression-like (dysthymia) comorbidity. Although pathologic phenotype in WAG/Rij rats is genetically determined, convincing evidence presented in this review suggests that the absence epilepsy and depression-like comorbidity in WAG/Rij rats may be governed by early-life events, such as prenatal drug exposure, early-life stress, neonatal maternal separation, neonatal handling, maternal care, environmental enrichment, neonatal sensory impairments, neonatal tactile stimulation, and maternal diet. The data, as presented here, indicate that some early environmental events can promote and accelerate the development of absence seizures and their neuropsychiatric comorbidities, while others may exert anti-epileptogenic and disease-modifying effects. The early environment can lead to phenotypic alterations in offspring due to epigenetic modifications of gene expression, which may have maladaptive consequences or represent a therapeutic value. Targeting DNA methylation with a maternal methyl-enriched diet during the perinatal period appears to be a new preventive epigenetic anti-absence therapy. A number of caveats related to the maternal methyl-enriched diet and prospects for future research are discussed.

In Up to My Ears and Temporal Lobes: Effects of Early Life Stress on Epilepsy Development

Epilepsy and stress are each significant concerns in today's society, bearing heavy impacts on mental and physical health and overall quality of life. Unfortunately, the intersection between these is potentially even more concerning, as stress is a frequent trigger of seizures and may contribute to neural hyperexcitability. A growing body of research suggests a connection between early life stress (occurring in the prenatal or postnatal stage) and later development of epilepsy. While the larger part of this literature suggests that early life stress increases vulnerability for epilepsy development, there are a number of interacting factors influencing this relationship. These factors include developmental stage at which both stressor and seizure assessment occur, type of stressor, sex effects, and type of seizure (convulsive or non-convulsive). Additionally, a number of potential mechanisms have been identified, including activation of the hypothalamic-pituitary-adrenal axis, neuroinflammation, altered inhibitory/excitatory balance, and temporal lobe structures. Developing a clearer understanding of this relationship between early life stress and epilepsy, the factors that influence it, and underlying mechanisms that may serve as targets for intervention is crucial to improving quality of life for persons with epilepsy.

Functional neuroimaging of post-mortem tissue: lithium-pilocarpine seized rats express reduced brain mass and proportional reductions of left ventral cerebral theta spectral power

Structural imaging tools can be used to identify neuropathology in post-mortem tissue whereas functional imaging tools including quantitative electroencephalography (QEEG) are thought to be restricted for use in living subjects. We are not aware of any study which has used electrophysiological methods decades after death to infer pathology. We therefore attempted to discriminate between chemically preserved brains which had incurred electrical seizures and those that did not using functional imaging. Our data indicate that modified QEEG technology involving needle electrodes embedded within chemically fixed neural tissue can be used to discriminate pathology. Forty (n = 40) rat brains preserved in ethanol-formalin-acetic acid (EFA) were probed by needle electrodes inserted into the dorsal and ventral components of the left and right cerebral hemispheres. Raw microvolt potentials were converted to spectral power densities within classical electroencephalographic frequency bands (1.5 Hz to 40 Hz). Brain mass differences were shown to scale with left hemispheric ventral theta-band spectral power densities in lithium-pilocarpine seized rats. This relationship was not observed in non-seized rats. A conspicuous absence of pathological indicators within dorsal regions as inferred by microvolt fluctuations was expected given the known localization of post-ictal damage in lithium-pilocarpine seized rats. Together, the data demonstrate that post-mortem neuroimaging is both possible and potentially useful as a means to identify neuropathology without structural imaging techniques or dissection.

Post-seizure drug treatment in young rats determines clear incremental losses of frontal cortical and hippocampal neurons: the resultant damage is similar to very old brains

Loss of neurons occurs with aging and following lithium/pilocarpine-induced epileptic seizures. In the present study, the numbers of neurons within the layers from sample areas of the four lobes of the neocortices and the hippocampus were counted by light microscopy in brains of rats that had been administered lithium or pilocarpine and then injected immediately or shortly after seizure onset with either acepromazine, ketamine, or prazosin. The mean numbers of neocortical and hippocampal neurons were lowest in rats treated with acepromazine or prazosin 1h after seizure onset, while those of rats immediately treated with ketamine displayed the least decrements and were most similar to normal rats. The largest loss of neurons occurred within the CA1 field and layers 5 and 6 of the frontal cortices. The mean numbers of neurons within the cortices in rats whose treatments had been delayed for 1h were similar to those of normal rats over 700 days of age. These results support the hypothesis that neuronal loss from cumulative effects of seizure-induced brain damage simulates aging.

Prenatal stress potentiates pilocarpine-induced epileptic behaviors in infant rats both time and sex dependently

Stressful events during gestation have important effects on the later physical and mental health of the offspring. In the study described here, the pilocarpine-induced seizure model was used to test the hypothesis that prenatal stress affects seizure susceptibility in infant rats. Prenatal stress consisted of daily restraint of the dam under normal room conditions (for 120minutes, twice daily) during the first, second, and third weeks of gestation. The pups were then compared with pups born to unstressed dams. Both second- and third-week-gestation stress significantly reduced pilocarpine-induced seizures in 19-day-old rat offspring, as compared with nonstressed control offspring. Mid- and late-gestation stress increased the rate and time of tonic-clonic seizures. Mortality rate 2 and 24hours after pilocarpine administration increased significantly in all stressed rats. Stress induced a significant rise in circulating corticosterone levels (2- to 8-fold, P<0.001) in the offspring. Female offspring differed little from male offspring with respect to blood corticosterone levels and epileptic behaviors. These findings indicate that prenatal stress, particularly during the second and third weeks of pregnancy, may play an important role in increasing seizure vulnerability in the unborn offspring. Female rats are more resistant to stress than males probably because of the lower susceptibility of their hypothalamic-pituitary-adrenal axis.

Large differences in blood measures, tissue weights, and focal areas of damage 1 year after postseizure treatment with acepromazine or ketamine

Approximately 1 year after rats were seized as young adults with lithium (3 mEq/kg) and pilocarpine (30 mg/kg) and given acepromazine or ketamine, 18 blood measures, wet tissue weights, and detailed damage scores for 107 brain structures were completed. Compared with normal and ketamine-treated rats, acepromazine-treated seized rats (total n=54) had lighter pancreata and spleens and elevated aspartate aminotransferase and alanine aminotransferase blood levels. Even though average damage did not differ, the mosaic of brain damage completely discriminated the two seized groups. Differential effects of postseizure treatment on functions of the thyroid, pancreas, and spleen were indicated. Ketamine-treated seized rats were healthier than acepromazine-treated seized rats or normal rats. This experiment demonstrates the importance of whole-organism assessment and that the single administration of a specific drug after onset of status epilepticus can produce marked differences in the evolution of brain damage and its influence on specific organs for the rest of the animal's life.

High fat diet and food restriction differentially modify the behavioral effects of quinpirole and raclopride in rats

Nutritional status can impact dopamine systems in a manner that might be important to understanding possible common neurobiological mechanisms that mediate abnormal compulsive food (e.g., obesity) and drug taking. Limiting food intake, for example, can increase sensitivity to the behavioral effects of indirect-acting dopamine receptor agonists. Much less is known regarding possible diet-induced changes in sensitivity to direct-acting dopamine receptor drugs. The present study investigated the effects of a high fat diet and of food restriction on sensitivity of rats to the behavioral effects of a direct-acting dopamine receptor agonist and a dopamine receptor antagonist. Free access to high fat chow increased sensitivity to quinpirole-induced yawning without changing sensitivity to raclopride-induced catalepsy or quinpirole-induced hypothermia. Food restriction (10 g/day) decreased sensitivity to quinpirole-induced yawning and raclopride-induced catalepsy without affecting sensitivity to quinpirole-induced hypothermia. Free access to a standard chow restored sensitivity to the behavioral effects of both drugs in rats that were previously food-restricted but not in rats that previously ate a high fat diet. These data confirm that food restriction can decrease sensitivity to behavioral effects of direct-acting dopamine receptor drugs, they provide evidence (i.e., no change in hypothermic effects) indicating that these changes are not due to pharmacokinetic mechanisms, and they provide initial evidence showing enhanced sensitivity to behavioral effects of dopamine receptor drugs in rats eating a high fat diet. These changes in sensitivity of dopamine systems could be relevant to understanding the impact of nutrition on therapeutic and recreational drug use.

Food restriction alters N'-propyl-4,5,6,7-tetrahydrobenzothiazole-2,6-diamine dihydrochloride (pramipexole)-induced yawning, hypothermia, and locomotor activity in rats: evidence for sensitization of dopamine D2 receptor-mediated effects

Food restriction enhances sensitivity to the reinforcing effects of a variety of drugs of abuse including opiates, nicotine, and psychostimulants. Food restriction has also been shown to alter a variety of behavioral and pharmacological responses to dopaminergic agonists, including an increased sensitivity to the locomotor stimulatory effects of direct- and indirect-dopamine agonists, elevated extracellular dopamine levels in responses to psychostimulants, as well as suppression of agonist-induced yawning. Behavioral and molecular studies suggest that augmented dopaminergic responses observed in food-restricted animals result from a sensitization of the dopamine D2 receptor; however, little is known about how food restriction affects dopamine D3 receptor function. The current studies were aimed at better defining the effects of food restriction on D2 and D3 receptor function by assessing the capacity of N'-propyl-4,5,6,7-tetrahydrobenzothiazole-2,6-diamine dihydrochloride (pramipexole) to induce yawning, penile erection (PE), hypothermia, and locomotor activity in free-fed and food-restricted rats. Food restriction resulted in a suppression of pramipexole-induced yawning, a sensitized hypothermic response, and an enhanced locomotor response to pramipexole, effects that are suggestive of an enhanced D2 receptor activity; no effect on pramipexole-induced PE was observed. Antagonist studies further supported a food restriction-induced enhancement of the D2 receptor activity because the D2 antagonist 3-[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole (L741,626) recovered pramipexole-induced yawning to free-fed levels, whereas yawning and PE were suppressed following pretreatment with the D3 antagonist N-{4-[4-(2,3-dichlorophenyl)-piperazin-1-yl]-trans-but-2-enyl}-4-pyridine-2-yl-benzamide hydrochloride (PG01037). The results of the current studies suggest that food restriction sensitized rats to the D2-mediated effects of pramipexole while having no effect on the D3-mediated effects of pramipexole.

Sucrose ingestion decreases seizure onset time in female rats treated with lithium and pilocarpine

To extend previous work concerning diet and overt seizures in rats, we tested the hypothesis that ingestion of 10% sucrose-water could reduce seizure onset time (SOT) in rats given lithium and pilocarpine. We found that female but not male rats given free access to a 10% sucrose-water solution for 3 weeks exhibited shorter SOTs than age- and sex-matched control subjects. A separate experiment determined that SOT was significantly reduced whether female rats were provided 1, 2, 3, or 4 weeks of free access to sucrose. Moreover, the daily volume of sucrose ingested was significantly correlated (r=-0.42) with SOT regardless of the duration of sucrose treatment (in weeks). These findings suggest that a diet supplemented with sugar can facilitate the emergence of behavioral seizures in female rats given lithium and pilocarpine. We discuss the potential role of dopamine in mediating the sucrose-induced changes in SOT.

Hippocampal melatonin receptors modulate seizure threshold

PURPOSE

The pineal hormone melatonin has been shown to enhance hippocampal excitability. We therefore investigated whether inactivation of hippocampal melatonin receptors affects behavioral seizures.

METHODS

Intrahippocampal infusions were performed in rats to study the effect of different melatonin receptor antagonists on behavioral activity, EEG, and seizure susceptibility. Experiments were conducted at 2 times of the day that coincided with the peak and trough of the daily melatonin rhythm.

RESULTS

Local infusion of the Mel(1b) receptor antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT) into the hippocampus, but not the overlying neocortex, significantly increased seizure latency and in some cases provided complete protection against seizure development. In addition, 4-P-PDOT suppressed open field activity and hippocampal EEG amplitude. The mixed Mel(1a)/Mel(1b) receptor antagonist luzindole also increased seizure latency but to a lesser degree than 4-P-PDOT. The behavioral effects of Mel(1b) receptor inhibition were comparable to those of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol and were observed during the dark phase (2400-0200 h) but not the light phase (1200-1400 h) of the daily photocycle. The anticonvulsant effect of intrahippocampal infusion of 4P-P-DOT was blocked by coadministration of the GABA(A) antagonist bicuculline.

CONCLUSIONS

Our results suggest that nocturnal activation of hippocampal Mel(1b) receptors depresses GABA(A) receptor function in the hippocampus and enhances seizure susceptibility.

The role of the environment on the development of spike-wave discharges in two strains of rats

Recently, we demonstrated that Type 1 and 2 spike-wave discharges (SWD) in the EEG of juvenile WAG/Rij rats were affected differently by housing before the period at which SWD start to occur. Here we consider possible sensitive periods by analyzing strain and housing influences before and after age of SWD onset. The effects of environment in WAG/Rij and ACI rats were investigated by manipulating housing during the period in which SWD become fully manifested in WAG/Rij rats. Rats were first housed from weaning in either an impoverished or enriched environment. Housing changed for half of the rats at three months, while for the other half housing stayed the same. EEG recordings at six months showed that enriched housing led to a worsening of seizure activity. The occurrence, number and mean duration of both types of discharges were influenced differently by strain, housing and age. Our data strengthen the strong genetic dependence of Type 1 SWD, but the mean duration seems to remain sensitive to housing during development. Type 2 SWD are more sensitive to environmental influences, especially in WAG/Rij rats. Moreover, the period after three months seems a sensitive period for housing effects on Type 2 SWD in this strain. Finally, our data further support the idea that Type 1 and 2 SWD are different phenomena, with their number and mean duration controlled by distinct mechanisms.

α2-Adrenergic inhibition prevents the accompanied anticonvulsant effect of swim stress on behavioral convulsions induced by lithium and pilocarpine

There has been much debate regarding the potential influence of stress on epilepsy. Many studies have reported that stress can affect seizure susceptibility through eliciting either proconvulsant or anticonvulsant effects within the nervous system. In this study, we investigated the potential anticonvulsant effect of a 10-min swim stress on convulsions induced by a single systemic injection of lithium chloride followed 4 h later with pilocarpine. Rats pretreated with lithium chloride and exposed to a 10-min swim stressor prior to pilocarpine injection displayed a significant delay to seizure onset compared to unstressed rats or rats exposed to swim stress 10 min after lithium chloride, 2 h after lithium chloride, or immediately after pilocarpine injection. We then determined whether administration of a glucocorticoid antagonist (mifepristone; 10 or 50 mg/kg), an alpha(2)-adrenergic antagonist (yohimbine; 2 or 5 mg/kg), or a nonspecific opioid blocker (naloxone; 0.2 or 1 mg/kg) could prevent the anticonvulsant effect of swim stress. Only the high dose of yohimbine was capable of inhibiting the anticonvulsant effect of swim stress on lithium-pilocarpine seizures. Our findings highlight the importance of an endogenous noradrenergic-dependent anticonvulsant system in mediating the effects of swim stress on seizures. Further studies exploring the benefits of treatments with noradrenergic acting drugs in epilepsy is well warranted.