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Minimal clonic seizures

Tonic phase of generalized seizures

FIGURE 1 Motor seizures elicited by electroshock stimulation. A-F: Minimal seizures induced by corneal suprathreshold stimulation in an 18-day-old rat. Righting ability is preserved, and the animal exhibits clonic movements of forelimbs. A, F: Classified as stage 2 (only head movements, both forelimbs on the floor). C-E: Stage 3 (forelimb clonus). B: Stage 4 (forelimb clonus with rearing; not yet perfect at this age). G: Tonic extension of forelimbs and hindlimbs in an adult rat. H: Tonic flexion of the forelimbs in 7-day-old rat pup (hindlimbs are not involved).

FIGURE 1 Motor seizures elicited by electroshock stimulation. A-F: Minimal seizures induced by corneal suprathreshold stimulation in an 18-day-old rat. Righting ability is preserved, and the animal exhibits clonic movements of forelimbs. A, F: Classified as stage 2 (only head movements, both forelimbs on the floor). C-E: Stage 3 (forelimb clonus). B: Stage 4 (forelimb clonus with rearing; not yet perfect at this age). G: Tonic extension of forelimbs and hindlimbs in an adult rat. H: Tonic flexion of the forelimbs in 7-day-old rat pup (hindlimbs are not involved).

corneal electrodes. The animal is restrained in hand during stimulation. It is necessary to release the animal immediately after the end of stimulation to see all phases of the seizures.

Monitoring these tests is visual; the presence or absence of individual components of seizures and their duration may be measured. In the most common MES test, only the presence of tonic hindlimb extension is taken as a criterion.

Threshold estimation may be performed using the staircase method (Finney, 1952). Statistical evaluation, that is, estimation of a current eliciting seizures in 50% of animals (CD50) as well as 50% effective doses (ED50) of anticonvulsants can be performed using the Lichtfield and Wilcoxon graphic method (1953) or the Finney's probit analysis (1952).

These tests are very easy to perform. It is necessary to have a good stimulator with a constant current output allowing application of high current intensities (up to 150mA). Reliability of data is high. It can be affected by the experience of the observer, and video recording could be used if more than tonic hindlimb extension is measured.

TABLE 1 Stages of seizures

1 Mouth and facial movements

2 Head nodding

3 Forelimb clonus

4 Rearing

5 Rearing and falling

From Racine, 1972.

Characteristics

Behavioral Features

Minimal threshold electroshock seizures consist of clonic seizures of the head and the forelimb muscles (Figure 1A-F). To quantify these seizures, the 5-point scale of Racine (1972) can be used (Table 1).

Maximal electroshock seizures consist of tonic flexion followed by tonic extension and clonus accompanied by a loss of posture. The endpoint is tonic extension of hindlimb with the limbs extended 180 degrees at the plane of the body (Figure 1G).

The duration of hindlimb tonic extension is in the range of seconds (tens of seconds at maximum). It is possible but not common to include a 5-second duration as the limit for a positive result. MESs are followed by a period of postic-tal depression when the same stimulation cannot induce MES (Velisek and Mares, 1992). This depression lasts longer in rats than in mice, and its duration depends on the intensity of stimulation (Swinyard, 1972).

There are no consistent data on EEG during MES because of the high intensity of stimulation current and violent movements during the clonic phase.

Morphologic studies have not found any significant neu-ropathological changes after single MES (Meldrum, 1986). To induce neuronal death, repeated electroshocks must be applied, but single electroshock seizures are sufficient to induce immediate early gene expression (French et al., 2001).

Inducing and evaluating MESs are easy tasks that are not time consuming. They thus represent an ideal screening tool and are used as a routine testing method for potential antiepileptic drugs (AEDs). The AEDs that block voltage-gated sodium channels (phenytoin, carbamazepine, val-proate, lamotrigine, topiramate, zonisamide, and others) are effective (Swinyard and Woodhead, 1982). There are data not only for all clinically used and potential AEDs but also for many other drugs. The 6-Hz stimulation inducing minimal seizures was also extensively studied pharmacologically (Barton et al., 2001).

There are two limitations of the MES test: (1) animals are to be used only once; and (2) evaluation of hindlimb tonic extension only is a restriction of possibilities of this model. Low-frequency electroshock seizures may be repeated, especially if high-stimulation currents are avoided. Electroshock seizures (usually repeated) may serve as a tool in studies of brain chemistry (e.g., Wasterlain and Plum, 1973) or molecular biology (e.g., expression of genes; see French et al., 2001; Altar et al., 2004).

Ontogeny

Developmental data were published by Millichap (1958) and Vernadakis and Woodbury (1969). The maturation patterns are not the same using the two basic stimulation frequencies. A mature pattern of MES induced by 60-Hz stimulation appears at the age of 16 days in rats; younger animals exhibited only forelimb flexion (10-12 days or earlier) (Figure 1H) or forelimb flexion, followed by fore-limb extension and hindlimb flexion (13-15 days). The lowest threshold for elicitation of seizures with the 60-Hz stimulation is around postnatal day 16 in both rats and mice. Low-frequency electroshock threshold with minimal seizures as an endpoint decreases steeply up to the end of the third postnatal week, and then a moderate decrease continues up to postnatal day 40 (Vernadakis and Woodbury, 1969).

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