Lamotrigine-Resistant Amygdala Kindling Model (rat)

The Lamotrigine-resistant amygdala kindled rat model is a model in which drugs are challenged against lamotrigine-resistant seizures induced via repeated electrical stimulation of the amygdala. This model is useful for not only identifying compounds effective against secondarily generalized partial seizures, but also allows for the differentiation of compounds that may be effective in therapy-resistant patients. Daily administration of lamotrigine (LTG; 5 mg/kg) during the kindling acquisition phase does not prevent the development of kindling in the test animals but leads to an LTG-resistant state in the fully kindled rat. Other sodium channel blockers, such as phenytoin and carbamazepine, also do not block kindling acquisition despite being highly effective against fully kindled seizures in drug-naïve rats. However, in the LTG-resistant fully kindled rats, both carbamazepine and phenytoin lose their ability to block fully kindled seizures (Metcalf et al., 2019). These findings suggest that the presence of lamotrigine during the epileptogenic process leads to a subsequent resistance to other sodium channel blockers, thereby making this a useful model of pharmacoresistance. This model may serve as a means to identify compounds which may be effective against therapy-resistant seizures.

For kindling, adult male Sprague-Dawley CD rats (275-300 g) are implanted with a bipolar stimulating electrode under stereotaxic guidance and anesthesia in the right amygdala (AP = - 2.2 mm, ML = 4.7 mm, DV = -8.7 mm). Beginning one week after the surgery, animals are administered 5 mg/kg LTG intraperitoneally (IP) and after one hour stimulated with a 200 µAmp, 50 Hz stimulus for 2 seconds once daily, five days a week, until all animals achieve fully kindled state defined as occurrence of 4 or 5 consecutive Racine scale stage 4-5 seizures. Any rat not achieving the fully kindled state is excluded from further experiments. Two days after all animals are kindled, the animals receive a challenge dose of LTG (30 mg/kg, IP) before being stimulated to confirm lamotrigine resistance. The animals are then allowed a washout of 3 days. On day 3 of the washout, the animals are pre-stimulated to ensure recovery of the fully kindled seizure. On day 4 or afterwards, kindled rats are challenged with a dose of an investigational agent and then challenged with the kindling stimulus at the predetermined time of peak effect (TPE) of the investigational drug. The average seizure scores and afterdischarge duration are noted, as are the number of animals protected from seizure (defined as a < stage 3 seizure on Racine scale) over the number of animals tested. The dose selection is based upon results of acute rat screening models (6Hz and MES model). When a drug treatment is observed to significantly lower seizure score and decrease afterdischarge, a dose-response study can be conducted. For this study, the ability of a candidate substance to reduce afterdischarge duration and behavioral seizure scores is quantitated by varying the dose between 0 and 100% effect. To maximize the efficiency of the screening test, animals are reused multiple times for dose/drug tests. Animals are allowed at least four days of washout period between tests.