Can ACTH analogs support discriminative learning in rats?

Ten rats were trained to discriminate between the stimulus properties of subcutaneously (SC) administered MSH/ACTH4-10 and saline in a two-lever, food-motivated operant task. After 12 weeks of discriminative training with 100 micrograms/kg MSH/ACTH4-10, half the rats received 200 micrograms/kg MSH/ATCH4-10, whereas the other half were administered 400 micrograms/kg, for 6 additional weeks. Subsequently, all rats continued training on 50 micrograms/kg ORG 2766 (SC) and, after 12 weeks of training, were randomly assigned to receive either 100 or 200 micrograms/kg ORG 2766. The results of this extensive 36 week training schedule indicate that only 1 of the 10 rats learned to discriminate the interoceptive cues produced by the ACTH analogs. However, this rat's performance was so sustained and errorless that the possibility exists that it was relatively more sensitive to the effects of MSH/ACTH4-10 and its analogs and that these substances may support discriminative learning in the rat.