Seizure patterns of Mongolian gerbils subjected to a prolonged weekly test schedule: Evidence for a kindling-like phenomenon in the adult population

Abstract

Purpose: Mongolian gerbils (Meriones unguiculatus) seize in response to sensory stimulation and forced exploratory behavior, but the incidence and severity of their seizures are variable. We wished to characterize the seizure pattern of gerbils from our breeding colony. Methods: Ninety-three gerbils aged 1-16 months were tested for a mean of 24 consecutive weeks and assigned to a category according to their seizure pattern. Frequency distribution histograms of the mean scores assigned every 5 weeks were plotted for each category. Mean age, number of seizures, onset of the first facial and forelimb myoclonus, and of the first generalized tonic-clonic seizure (GTCS) were compared among categories. We performed correlation analysis between onset of seizures and animal age. Results: From the 93 tested, no seizure-resistant gerbils could be isolated. Four major categories were distinguished. Category 1, highly seizure-sensitive gerbils (39%), exhibited seizures from the first few weeks of test on. Category 2, consisting of -37%, were seizure-free for the first three to six consecutive tests, later developing facial and forelimb myoclonus and eventually GTCS. Because such progressive development of seizures was similar to that occurring upon electrical kindling, the gerbils were classified as kindled- like (KL). Among KL gerbils, older individuals were significantly more refractory to seizures. In category 3, gerbils (10%) exhibited inconsistent seizure behavior. Category 4 consisted of significantly younger animals (11%) with rapid progress to generalized seizures. Conclusions: Seizures of progressive severity can be induced in adult gerbils with a prolonged test regimen. As a consequence, the number of regularly seizing gerbils in a colony can be increased. Prolonged tests starting at a defined age may help characterize seizure development better in this genetic model of limbic epilepsy.