Invasiveness of 2 selected strains of Toxoplasma gondii with respect to mice, rats and rabbits

In Experiment I, groups of 22-and 140+-day rats were trained in acquisition and extinction of 1-way avoidance with a CS that consisted of the opening of a guilotine door 5 sec before US onset or a combination of door-opening plus a tonal signal that remained on until the occurrence of the motor response. Under both CS conditions, avoidance acquistion was similar at each age level. The extinction date indicated comparable performance for the young subjects but differential performance and greater resistance to extinction for the adult subjects. Adults trained with the door-opening CS persisted in responding for an entire series of 100 extinction trials, whereas the adults trained with the compound CS extinguished well within the 100-trial, whereas the adults trained with the compound CS extinguished well within the 100-trial limit. A 2nd experiment included 10 pre-exposures of the simple or compound CS's prior to avoidance training. Although the pups were insensitive to pre-exposure effects, the adults that were pre-exposed and trained with identical CS's showed evidence of pre-exposure effects. Results of both experiments were interpreted as indicative of differential cue saliency between ages.