Unusual dose-related effect of an endorphin analog in a complex maze.

KASTIN, A. J., M. D. MAUK, A. V. SCHALLY AND D. H. COY. Unusual dose-related effecr of an endorphin a n d o g in u complex maze. PHYSIOL. BEHAV. 25(6) 95S962, 1980.-Hungry adult rats were tested in a 12-choice maze for their ability to find a reward of food I5 minutes after being injected peripherally with (D-Ala‘)-D-endorphin. Injection of the endorphin analog in a dose of 80 &kg body weight resulted in rats running the maze significantly slower and with more errors than rats injected with diluent. Animals receiving a dose ten times larger (800 d k g ) were indistinguishable from controls in both running speed and c l~ors , thus making a toxic effect unlikely. Possible changes in appetite, thirst, olfaction, emotionality, and general motor activity did not seem to explain the results. The inverted U-shaped dose-related response seemed to represent a variant from previous observations with CNS-active peptides in that the smaller dose impaired rather than enhanced pelformance.