Elsevier

Experimental Neurology

Volume 207, Issue 2, October 2007, Pages 329-349
Experimental Neurology

Behavioral alterations in the pilocarpine model of temporal lobe epilepsy in mice

https://doi.org/10.1016/j.expneurol.2007.06.021Get rights and content

Abstract

Psychiatric disorders frequently occur in patients with epilepsy, but the relationship between epilepsy and psychopathology is poorly understood. Frequent comorbidities in epilepsy patients comprise major depression, anxiety disorders, psychosis and cognitive dysfunction. Animal models of epilepsy, such as the pilocarpine model of acquired epilepsy, are useful to study the relationship between epilepsy and behavioral dysfunctions. However, despite the advantages of mice in studying the genetic underpinning of behavioral alterations in epilepsy, mice have only rarely been used to characterize behavioral correlates of epilepsy. This prompted us to study the behavioral and cognitive alterations developing in NMRI mice in the pilocarpine model of epilepsy, using an anxiety test battery as well as tests for depression, drug-induced psychosis, spatial memory, and motor functions. In order to ensure the occurrence of status epilepticus (SE) and decrease mortality, individual dosing of pilocarpine was performed by ramping up the dose until onset of SE. This protocol was used for studying the consequences of SE, i.e. hippocampal damage, incidence of epilepsy with spontaneous recurrent seizures, and behavioral alterations. SE was terminated by diazepam after either 60, 90 or 120 min. All mice that survived SE developed epilepsy, but the severity of hippocampal damage varied depending on SE length. In all anxiety tests, except the elevated plus maze test, epileptic mice exhibited significant increases of anxiety-related behavior. Surprisingly, a decrease in depression-like behavior was observed in the forced swimming and tail suspension tests. Furthermore, epileptic mice were less sensitive than controls to most of the behavioral effects induced by MK-801 (dizocilpine). Learning and memory were impaired in epileptic mice irrespective of SE duration. Thus, the pilocarpine-treated mice seem to reflect several of the behavioral and cognitive disturbances that are associated with epilepsy in humans. This makes these animals an ideal model to study the neurobiological mechanisms underlying the association between epilepsy and psychopathology.

Introduction

Most patients with epilepsies have to cope not only with the disease itself but also with many additional medical problems that may be associated with epilepsies (Boro and Haut, 2003). Of all medical comorbidities associated with epilepsy, psychiatric disorders are the most common (Boro and Haut, 2003, Swinkels et al., 2005). Mood disorders, predominantly major depression, and anxiety disorders have the highest frequency, followed by psychosis (Boro and Haut, 2003, Devinsky, 2003). Psychopathology may coexist with epilepsy, may be directly related to seizure activity, or may be associated with the postictal state (Boro and Haut, 2003). The neurobiological mechanisms of the relationship between epilepsy and psychopathology are poorly understood and systematic research in this area is still lacking (Devinsky, 2003, Swinkels et al., 2005). Apart from psychiatric disorders, many patients with epilepsy suffer from impaired cognitive performance (Motamedi and Meador, 2003). The type of neuropathology associated with epilepsy may affect the type of cognitive dysfunction. For instance, temporal lobe epilepsy (TLE) with hippocampal sclerosis is often associated with memory impairment (Motamedi and Meador, 2003).

Animal models of epilepsy are useful to enhance our understanding of causal mechanisms underlying the association between epilepsy and behavioral abnormalities (Post, 2004, Majak and Pitkanen, 2004, Heinrichs and Seyfried, 2006). Most studies in this respect have used the kindling model and post-status epilepticus (SE) models of TLE in rats, whereas mouse models of TLE have received much less attention. Mice offer gene targeted technologies for in vivo gene function analysis that may be invaluable in unraveling the complex relationship between epileptogenesis, neurodegeneration and behavioral and cognitive abnormalities.

One of the most prominent models of TLE in rats and mice is the pilocarpine model (Turski et al., 1989, Cavalheiro et al., 1996, Coulter et al., 2002, Cavalheiro et al., 2006). The induction of SE by systemic administration of pilocarpine, a cholinergic muscarinic agonist, in rodents leads to epilepsy with spontaneous recurrent seizures (SRS) and hippocampal alterations that are reminiscent of hippocampal sclerosis in patients with TLE. Furthermore, alterations in learning and memory as well as behavioral dysfunction have been described in the pilocarpine model in rats. Surprisingly, although the acute convulsive and neurodegenerative effects of pilocarpine in mice were first reported in 1984 (Turski et al., 1984) and SRS following a pilocarpine-induced SE in mice in 1996 (Cavalheiro et al., 1996), to our knowledge the long-term behavioral alterations occurring in this model have not been characterized in detail in this species since then. This prompted us to perform a study with behavioral phenotyping of pilocarpine-treated mice, using behavioral test batteries to investigate locomotor activity and function, learning and memory, emotionality, depression-related behavior and other types of psychopathology. The study was undertaken in three subsequent parts as follows. (1) Because the systemic administration of convulsant doses of pilocarpine in mice is often associated with high mortality and/or a low percentage of mice developing or surviving SE (Shibley and Smith, 2002, Borges et al., 2003, Winokur et al., 2004, Borges et al., 2006), we compared different application and dosing protocols for pilocarpine to obtain a protocol yielding a high percentage of mice with SE but low mortality (“study #1”). (2) Because mortality, hippocampal damage and incidence of animals with SRS is affected by the duration of SE (Lemos and Cavalheiro, 1995, Goodman, 1998, Glien et al., 2001), we terminated SE after 1, 1.5 or 2 h and investigated the long-term consequences (“study #2”). (3) The pilocarpine protocol thus developed was used to study the behavioral and cognitive alterations developing in mice in this model of TLE (“study #3”). Some of the results have been presented in abstract form (Groeticke et al., 2006).

Section snippets

Animals

As in previous studies of our group (Löscher and Lehmann, 1996, Löscher and Lehmann, 1998, Wlaz et al., 1998b, Potschka and Löscher, 1999, Löscher et al., 2003), we used adult female outbred NMRI mice, which were obtained from a commercial breeder (Harlan-Winkelmann, Borchen) at an age of about 5–6 weeks. Female mice were used for the present study to ease housing in groups for the long period needed to perform all experiments (see below). After arriving at our Department, animals were housed

Comparison of different application and dosing protocols for pilocarpine

In a first preliminary experiment, single i.p. doses of pilocarpine were tested in mice (Table 1). The range of doses (200–360 mg/kg) covered the dose range previously reported for different mouse strains (Turski et al., 1984, Cavalheiro et al., 1996, Shibley and Smith, 2002, Borges et al., 2003, Winokur et al., 2004). Within a few minutes after pilocarpine injection, immobility, staring, Straub tail, head nodding and occasional clonic seizures occurred, but a SE with continuous seizure

Discussion

Pilocarpine is frequently used to induce SE in mice, but systemic (i.p.) injection of high doses is often associated with high mortality and/or a low percentage of mice developing or surviving SE (Shibley and Smith, 2002, Borges et al., 2003, Winokur et al., 2004, Borges et al., 2006). In a first series of preliminary experiments with administration of high i.p. doses of pilocarpine in NMRI mice, only few of the mice developed SE, but mortality was high. This prompted us to perform experiments

Acknowledgments

We thank Prof. H. Emrich (Department of Clinical Psychiatry and Psychotherapy, Medical School Hannover, Germany), Prof. S. Steinlecher (Department of Zoology, University of Veterinary Medicine Hannover, Germany), and Prof. J.P. Huston and his colleagues (Institute of Physiological Psychology, Center for Biological and Medical Research, University of Düsseldorf, Germany) for helpful discussions and advice during establishment of the behavioral models and interpretation of data obtained in these

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