[Influence of ACTH on Kamin effect in passive avoidance learning in rats].

The purpose of the present experiments was to compare the validity of the associative or "state dependent learning" hypothesis and the nonassociative ones, which had been proposed to explain the Kamin effect. In Experiment 1, rats were given one-trial passive avoidance trial with one of the three shock intensities (0.5-, 1.0- and 2.0-mA), and then the retention test was given after either one of the intervals of 0.05-, 2.5- and 24-hr. The clear Kamin effect (U-shaped retention function) was obtained for the animals treated with the 2.0-mA shock. In Experiment 2, the adrenocorticotropic hormone (ACTH) was injected two hours before the retention test to investigate the involvement of adreno-pituitary system in the retrieval of the memory. The retention performances at the intermediate (2.5-hr) interval were reinstated, whereas those at the 24-hr interval were impaired, as a function of dosage of ACTH. The results of these experiments favored the associative or state-dependent learning hypothesis on the Kamin effect.