ACTH, but not corticosterone impairs habituation and reduces exploration

The habituation of an orienting response to auditory stimuli was impaired in rats by the administration of ACTH1-24. This impairment is unlikely to be due to the action of corticosterone because of the time course of the effect, because injected corticosterone had no effect on hibituation and because the peptide fragment ACTH4-10, which does not release corticosterone, also impaired habituation. Both ACTH1-24 and ACTH4-10 reduced the level of exploration measured in a holeboard, but corticosterone had no significant effect. However ACTH did not impair habituation and exploration, thus providing further evidence that there are different mechanisms underlying habituation of orienting and habituation of exploration.