The threshold current evoking self-stimulation or multiples of this current was used to investigate the effect of ACTH4–10 on response performance for brain stimulation reward in the medial septum (MS) and the medial forebrain bundle (MFB). ACTH4–10 in a dose of 50 μg administered SC enhanced bar-pressing for low intensity stimulation but failed to change self-stimulation rates when the base-line rate exceeded 100 responses per 6 min. The peptide lowered the threshold for producing self-stimulation but only in the MS. When an ascending sequence of threshold multiples was used within a session, ACTH4–10 treatment resulted in an increase in response rate at 1.2 and 1.5 threshold multiples and a decrease at 3.0 times the threshold but only in the MS. The results indicate that ACTH4–10 facilitated response performance at a low response rate by enhancing the rewarding effect of brain stimulation. Furthermore, the peptide changed the response pattern which normally followed reinforcement shift in the MS
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