Effects of ACTH, epinephrine and met-enkephalin on brain B-endorphin-like immunoreactivity, and of ACTH, epinephrine, met-enkephalin and naloxone on retention, in normal and in protein- malnourished rats

ABSTRACT

Rats raised and maintained on a normal-protein diet (25% protein) responded to the ip adminsitration of ACTH-(1-24), epinephrine or Met-enkephalin with a decrease in hypothalamic Beta-endorphin-like immunoreactivity, which is attributable to a release of this substance. This effect was not seen in rats raised an maintained on a low-protein diet (8% protein). In the normal animals, the pre-test administration of ACTH, epinephrine or Met-enkephalin and the post-training adminsitration of naloxone enhanced retention-test performance of a step-down inhibitory avoidance task. These behavioral effects were absent in the protein-malnourished rats. Previous studies have shown that the behavioral effect of post-training naloxone is secondary to the release of brain Beta-endorphin during training, and that the pre-test it is not likely that the differences were caused by hyperreactivity to the aversive stimuli employed, the suggested interpretation is that protein-malnourished rats present a dysfunction in the brain Beta-endorphin system which renders it unresponsive not only to novel training experiences, but also to the pre-test retrieval enchancing effects of ACTH, epinephrine and Met-enkephalin