Avian photogenic epilepsy and embryonic brain chimeras: neuronal activity of the adult prosencephalon and mesencephalon

Photogenic genetic epilepsy was studied in an avian model, using either the Fayoumi epileptic chicken (Fepi) or neural chimeras obtained by replacement of em bryonic brain vesicles in normal chickens with those of Fepi embryos. In these two kinds of animals motor seizures accompanied by electroencephalographic (EEG) desynchronization and flattening (DF) were evoked by intermittent light stimulation (ILS). In chimeras with on ly the prosencephalon grafted, motor seizures were less severe but DF remained. ILS-induced DF persisted un der paralysis by gallamine triethiodide (Flaxedil). Extra cellular recordings were made in the prosencephalon (wulst) and in the mesencephalon (optic tectum) of paralysed animals. Units recorded in the prosencephalon of Fepi and chimeras showed abnormal interictal burst ing activity, distinctly different from the non-epileptic Fayoumi heterozygotes (Fhtz) and normal chickens. The mesencephalic units of Fepi and chimeras having both prosencephalon and mesencephalon grafted showed two types of abnormal activities during ILS-induced DF, which were distinct from the non-epileptic chickens: type I neurons displaying early, high sensitivity to ILS fol lowed by a prolonged suppression of activity; type II neurons displaying an early and prolonged suppression of activity. The results are discussed with respect to the brain structures generating ictal and interictal EEG ac tivities and motor seizures.