Pharmacological characterization of the 6 Hz psychomotor seizure model of partial epilepsy
Introduction
All of the currently available AEDs were discovered in part through a comprehensive screening process that utilizes rodent models. Traditionally, most screening programs employ normal mice and rats to assess efficacy against either electrically (e.g. maximal electroshock, MES) or chemically (e.g. pentylenetetrazol (PTZ), bicuculline, or picrotoxin) induced seizures. The number of new AEDs currently available, or in development, for the management of epilepsy certainly attests to the success of this approach. However, this approach may overlook novel compounds that would be uniquely effective in the therapy-resistant population. One example supporting this hypothesis is provided by levetiracetam (Keppra®), which has demonstrated efficacy in refractory human partial epilepsies (Abou-Khalil et al., 1996, Cameron et al., 1996, Shorvon et al., 1996, van Rijckevorsel et al., 1996). Levetiracetam is novel in that it was not discovered by the traditional screening approach. For example, it was found to be inactive against MES and PTZ seizures even at high doses (i.e. 500 mg/kg) as described by several groups (Loscher and Honack, 1993, Klitgaard et al., 1998). However, in the kindled rat model of partial seizures, levetiracetam was found to be highly effective in blocking the fully expressed kindled seizure and delaying the acquisition of kindled seizures at doses less than 54 mg/kg (Klitgaard et al., 1996, Loscher et al., 1998, Matagne and Klitgaard, 1998). The lack of efficacy of levetiracetam in traditional screening tests demonstrates the importance of utilizing models that more closely resemble the human condition.
While the high frequency, short-duration stimulation employed in the MES test has become a standard for screening many AEDs; it was only one of several electroshock paradigms initially developed for animal models in the 1940s and 1950s (Swinyard, 1972). An alternative paradigm, described in 1953, was the low frequency (6 Hz), long-duration (3 s) corneal stimulation model, which produced ‘psychic’ or ‘psychomotor’ seizures. The early description of the 6 Hz model suggested that the seizure did not spread as thoroughly or intensely as a seizure evoked by high-frequency pulses, such as employed in the 50 Hz MES model (Toman, 1951, Toman et al., 1952, Brown et al., 1953). Instead of the tonic extension seizure characteristic of the MES test, the 6 Hz seizure was reported to involve a minimal, clonic phase followed by stereotyped, automatistic behaviors that were reminiscent of aura of human patients with partial or limbic epilepsy (Toman, 1951, Toman et al., 1952, Brown et al., 1953). At the time the 6 Hz model was evaluated, the authors were attempting to validate it as a screening model for human partial seizures; however, the pharmacological studies conducted were not wholly consistent with clinical practice (Brown, 1953, Brown et al., 1953). For example, phenobarbital, phenurone, mebaral, and mesantoin were effective, as were the anti-absence drugs trimethadione and paradione (Brown et al., 1953, Tedeschi et al., 1956). In contrast, the clinically effective hydantoins phenytoin and thiantoin were inactive (Brown et al., 1953). The lack of phenytoin sensitivity led the authors to suggest that the 6 Hz model was no more predictive of clinical utility than the other models available at the time. The 6 Hz model was subsequently abandoned except for some limited anticonvulsant screening of cannabinoids (Turkanis et al., 1974, Karler and Turkanis, 1980, Karler et al., 1982).
Given the relative resistance of some patients to phenytoin and carbamazepine in today's clinical setting and the lack of sensitivity of the MES and subcutaneous (sc) PTZ tests to levetiracetam, the utility of the 6 Hz seizure as a potential screen for therapy-resistant epilepsy was re-evaluated.
Section snippets
Animals
Adult, male CF#1 mice (Charles River Laboratories, Wilmington, MA) weighing between 20 and 30 g were used in all experiments. All animals were allowed free access to food and water and were housed in a temperature- and light-controlled environment (12-h on/12-h off).
‘Psychomotor’ seizures were induced via corneal stimulation (6 Hz, 0.2 ms rectangular pulse width, 3 s duration) using a Grass S48 stimulator. At the time of drug administration, a drop of 0.5% tetracaine was applied to the eyes of
Results
Increasing the current intensity of the stimulation between 13 and 23 mA resulted in a progressive increase in the number of mice displaying a ‘psychomotor’ seizure characterized by stun, forelimb clonus, twitching of the vibrissae, and Straub-tail. The calculated median convulsive current (CC50) was found to be 19.4 mA (95% confidence interval: 18.4–20.0 mA).
In initial studies utilizing a 32 mA stimulus, CZP, PB, TMO, ESM, VPA, FBM, TGB, and LEV all displayed dose-dependent protection (Fig. 1;
Discussion
In their earlier report, Brown and co-workers reported that an 8 mA stimulus corresponded to the median convulsive current required to produce a ‘psychomotor’ seizure in 50% (CC50) of the mice tested. Pharmacological testing was conducted at a current intensity that was 4-fold greater than the CC50 (i.e. 32 mA). In the present study, the calculated CC50 was found to reproducibly lie between the 95% confidence intervals of 18.4 and 20.0 mA. In keeping with the original study, the pharmacological
Acknowledgements
The authors would like to thank Laura Webb for her assistance in the pharmacological studies. This work was supported by NINDS contract N01-NS-9-2313.
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