Brain chimeras for the study of an avian model of genetic epilepsy: structures involved in sound and light-induced seizures.

The epileptic homozygotes of the Fayoumi strain of chickens (Fepi) are affected by photogenic reflex epilepsy with complete penetrance. Here we demonstrate that they are equally affected by audiogenic reflex epilepsy induced by intense sound stimulation. All the Fepi display sound-induced seizures from hatching to adulthood consisting of initial 'ictal arousal' and running fits usually followed by generalized clonico-tonic convulsions. A running fit is the preconvulsive motor symptom specifically induced by auditory stimulation while neck myoclonus is the preconvulsive motor symptom specifically induced by photic stimulation. The EEG interictal spikes and spike and waves are suppressed and replaced by a desynchronized trace during the seizures of both kinds. Viable neural chimeras were obtained by graft of embryonic brain vesicles from Fepi donors into normal chick embryos. Transfer of the complete audiogenic and photogenic phenotypes was obtained in chimeras resulting from embryonic substitution of both the prosencephalon and mesencephalon. The substitution of the prosencephalon alone resulted in transfer of interictal paroxysmal EEG activity accompanied by the sound and light-induced desynchronization and 'ictal arousal' with no motor seizures. Chimeras with embryonic substitution of the mesencephalon alone displayed running fits and convulsions induced by sound stimulation but only neck myoclonus following light stimulation. The conclusions are reached that: (i) the Fepi is a model of audiogenic and photogenic reflex epilepsy; (ii) in both types, the seizure initiator and the convulsion generator are localized in the brainstem, although reinforcement from telencephalic visual structures is needed to trigger photogenic generalized convulsions.

Mapping of the early neural primordium in quail-chick chimeras. I. Developmental relationships between placodes, facial ectoderm, and prosencephalon.

Defined fragments of the anterolateral neural ridge and of the associated region of the neural plate of presomitic to three-somite stage quail embryos were grafted isotopically and isochronically into chick hosts. This resulted in the development of apparently normal brain and facial structures to which the contribution of the grafted tissue could be observed by means of the quail nuclear marker. It was shown that the anterolateral neural ridge contains the progenitor cells of the adenohypophyseal and olfactory placodes and also of the superficial ectoderm lining the nasal cavity and conchae and the superficial ectoderm of the beak. When the appropriate region of the neural ridge was involved in the quail-chick substitution, the egg tooth was made up of graft-derived cells. Grafting of the neural plate area adjacent to the "ridge" territory containing the placodal ectoderm revealed that the presumptive region of the hypothalamus is in contiguity with that of the adenohypophyseal placode. The same observation was made for the olfactory placode and the floor of the telencephalon from which the olfactive bulb later develops.

Differentiation of autonomic neuron precursors in vitro: cholinergic and adrenergic traits in cultured neural crest cells.

The development of autonomic neuronal precursors was studied in cultures of microsurgically excised quail neural crest grown alone and associated with other young embryonic tissues. Biochemical differentiation in the cultures was followed by measuring their ability to synthesize acetylcholine (ACh) and catecholamines (CA) from radioactive precursors; cytochemical aspects of their differentiation were examined by techniques including electron microscopy, cholinesterase histochemistry, and CA cytofluorescence. Mesencephalic crest, which can make ACh before explantation, always synthesized ACh after 7 d in culture and often, but not invariably, elaborated small quantities of CA as well. Association with 2-d somite and notochord, 2-d heart, or 4-d hindgut, in medium supplemented with horse serum, resulted in the synthesis of increased amounts of both transmitters. ACh-synthesizing activity was lower and the cholinergic-stimulating effects of somite and heart were abolished in the presence of fetal calf serum. Cervicothoracic (trunk) crest, taken from the level where the dorsal mesoderm is still unsegmented, always produced ACh after culture, but CA was detectable only when the cultures were obtained by initially explanting the entire neural primordium. Co-culture of trunk crest with young embryonic tissue increased ACh-synthesizing ability and initiated CA production. Despite their capacity to elaborate neurotransmitter, cultures of either type of neural crest, alone or in association with the above-mentioned tissues, contained very few cells resembling neurons in their phase contrast appearance and none that reacted positively to any of the cytological tests applied. On the other hand, when the sclerotomic moiety of 3-d somite was cultured, trunk neural crest cells that had already migrated into the rudiment in vivo but which had not yet begun to produce detectable amounts of CA underwent rapid differentiation into neurons that synthesized and accumulated large quantities of CA. Stores of CA were detectable cytochemically as early as 24 hr after explantation and the presence of many small, dense core vesicles in neurons and processes was revealed by electron microscopy. ACh-synthesizing activity, demonstrable in freshly dissected sclerotomes, was also present in all of the cultures examined. These results show that (1) during ontogeny, cholinergic traits appear earlier than adrenergic ones in the neuronal precursors contained in the neural crest; (2) some decisive step in the differentiation of the precursor cells of the sympathetic ganglia takes place in vivo within a few hours of the onset of trunk neural crest migration. This coincides with a maturation of the somitic mesenchyme. A similar developmental process does not occur in vitro when 2-d somites and neural crest are associated in histiotypic cultures.