Stressor predictability influences open field behavior, pain sensitivity and brain MAO inhibitory activity (tribulin) in the rat.

The effects of predictable or unpredictable shocks on ambulation, pain sensitivity, plasma catecholamines and heart and brain monoamine oxidase (MAO) inhibitory activity were investigated in rats. Animals showed plasma catecholamines and heart MAO inhibitory activity sensitization irrespective of type of treatment, while differences between groups were observed when open field and hot plate tests and brain MAO inhibitory activity were considered. These effects parallel those observed using the classic triadic design leading to the helpless state. Our results suggest that predictability per se is able to generate this phenomenon.