Phenotyping the untouchables: environmental enhancement of behavioral and physiological activation in seizure-prone El mice.

The onset and frequency of spontaneous and tail suspension-induced seizures in El mice appear to be influenced strongly by developmental and experiential factors over the first 3 months of life. To assess the impact of social factors on behavioral characteristics of El mice prior to the age of seizure susceptibility, locomotor activity and exploratory measures of arousal were recorded in 40-day-old El and control DDY mice exposed to group and isolation housing conditions. Once mice reached maturity, physiological reactivity to a tail suspension stressor was evaluated. The locomotor activity measure revealed circadian entrainment in both strains, nocturnal hyperactivity in El mice, and a locomotor activity-attenuating effect of group housing in El mice. In the two-compartment model of exploration, latency to enter, transitions to and from, and rearing in a brightly lit compartment were 50% higher in El relative to DDY mice, again suggesting a hyperactive phenotype. Finally, an acute 2-minute tail suspension stressor applied to 75-day-old mice implanted with radiotelemetry transmitters revealed a reactive tachycardia in El, but not DDY, mice. No seizures were observed during any of the experimental manipulations. Taken together, these results suggest that spontaneously occurring deviations in behavioral and cardiovascular measures of arousal characterize preseizure El mice and that motor features of hyperarousal can be exaggerated by the environmental manipulation of isolation housing.