Nonconvulsive Seizures Result in Behavioral but Not Electrophysiological Changes in Developing Rats

It is not known if nonconvulsive seizures lead to functional or morphological changes in immature rats. Therefore we studied consequences of such seizures induced by kainic acid (KA) on Postnatal Day (PD) 12 (2 mg/kg ip). The animals were examined electrophysiologically (cortical epileptic after-discharges (ADs) were elicited in rats with implanted electrodes on PD 14, 18 or 25) and behaviorally (open field was studied in another group of animals on PDs 18 and 25). Hippocampal and cortical morphology was checked by light microscopy (Nissl staining) on PDs 18 and/or 25. Another group of rats was injected with a 6 mg/kg dose of KA on PD 18 and examined on PD 25. The dose of KA used induced only nonconvulsive seizures characterized by automatisms (scratching on PD 12, wet dog shakes on PD 18). Cortical ADs in animals stimulated on PD 14, 18, or 25 did not differ from those in control rats. KA-Treated rats exposed to open field two times (on PDs 18 and 25) exhibited more exploratory activities during the second exposure than control animals. A similar difference was noted in PD 25 rats injected with KA on PD 18. Qualitative histology did not reveal any obvious neuronal damage in hippocampus and cortex. These results demonstrate that nonconvulsive seizures induced at early developmental stages that do not result in observable electrophysiological and morphological changes may have delayed behavioral consequences. (C) 2001 Academic Press