Abstract
Behavioral effects of ACTH on habituation of the acoustic startle responses were examined. A tone stimulus (3, 400 Hz, 107 dB) was presented under three different conditions of inter-stimulus interval (variable 4, 8, and 16 sec). Animals underwent 100 habituation trials. Ten min prior to habituation, the baseline amplitude of the startle response was measured using 107 and 85 dB tones (Prehabituation test), and 5 min after the habituation period, the after-effect was examined by the same procedure (Post-habituation test).
Administration of ACTH (4 i. u./animal, s. c.) facilitated habituation when the inter-stimulus-interval (ISI) was short, but retarded habituation when the ISI was long. No such phenomena was observed in the saline treated animals. The effects of ACTH became more marked in the post-habituation test.
The results mentioned above suggest that the effects of ACTH on habituation are dependent upon the ISI condition.
