Studies of behavioral and physiological bases of genetically controlled epileptiform seizures in domestic fowl
The present studies are an attempt to obtain further information on the behavioral and physiological bases of epileptiform seizures in chickens, caused by the autosomal recessive gene epi. The stock used in these studies was a mongrel population obtained from crosses of Fayoumi, in which the gene was first discovered, with several other breeds. Birds used in these studies were obtained from three successive generations produced from this mongrel population. A total of 2025 birds which consisted of 1603 epileptic, 120 carrier, 105 normal, and 197 non-epileptic birds (Epi-) were used. They were obtained from 48 hatches over a 2 1/2- year period, and were studied between one day and 104 weeks of age. Some individuals were used in more than one experiment. Seizures could be induced from epileptic chickens by heat, complex sound and photic stimulation. Intermittent light stimulation (ILS) was the most satisfactory seizure-inducing stimulus. The response of epileptic chickens was found to be affected by age and by ILS frequency. The seizure susceptibility and incidence of complete seizures were relatively high at one day, decreased sharply during three to seven days, and increased rapidly again after seven days of age. Incomplete seizures were not common and were found most often in birds tested between three days and 26 weeks of age. Seizures were mostly induced by less than 60 seconds of ILS and usually lasted less than 60 seconds, except in day-old chicks which tended to have prolonged seizures. Seizure latency decreased and the frequency of seizures of short duration increased gradually with age. The most effective ILS frequency for inducing seizures in epileptic chickens ranged from 10 to 20 flashes per second (fps). Birds tested with low ILS frequencies tended to have longer seizure latency than birds tested with high ILS frequencies. Seizure duration did not seem to be affected by ILS frequency. No differences were found between male and female chickens in seizure susceptibility, incidence of complete and incomplete seizures and seizure latency, but males tended to have longer seizures than females. No prominent effect of parental genotype was found on the response of epileptic chickens to ILS. Prolonged ILS during a seizure had no effect on seizure duration or on the duration of post-seizure depression which usually lasted less than ten minutes, but it significantly increased the incidence of post-seizure depression. Prior exposure of epileptic chickens to ineffective ILS and to heat stress did not affect susceptibility of birds immediately exposed to effective ILS. Seizure susceptibility diminished significantly in day-old chicks which had been previously subjected to cold stress, and in birds which had been previously subjected to emotional disturbances. The resting period needed for day-old chicks to produce a second seizure was much shorter than in birds at older ages. As many as eight successive seizures could be induced from birds if effective ILS frequencies were used and the resting period after a seizure was sufficient. Expressivity of the epi gene was complete when ILS was used for seizure induction. Sex distribution in epileptic and carrier chickens was normal. Presence of the epi gene had no effect on fertility, embryonic mortality and hatchability. The resting EEG of epileptic chickens was characterized by relatively slow waves with high amplitude as compared to the EEG of carriers and normals. Abnormal spiking waves with frequency identical to the stimulus frequency were obtained from epileptic chickens during ILS prior to the onset of severe clonic convulsions, but were not found in the EEG of carrier and normal chickens. Carrier and normal chickens did not differ much in wave frequency and amplitude in resting EEG and in EEG during ILS.
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