Effect of posttraining and pretest beta-endorphin and ACTH administration in normal and protein malnourished rats

Rats were submitted to a normal (25% casein) or a low protein diet (8% casein) from the day of birth until the age of 110 to 120 days. Hypothalamic beta-endorphin-like immunoreactivity was lower in the animals raised and maintained with the low protein diet, and, in addition, it did not respond to training in a step-down inhibitory avoidance task with or without footshock with a depletion, as was the case with the normal diet animals. In the animals submitted to the normal protein diet posttraining ACTH (0.2 micrograms/kg) and beta-endorphin (1.0 micrograms/kg) caused retrograde amnesia of a step-down inhibitory avoidance task, and pretest administration of these substances had no effect of its own, but was able to reverse the amnesia induced by their previous posttraining administration. In the animals submitted to the low protein diet, results were similar except that pretest beta-endorphin caused amnesia on its own. On the basis of previous findings which suggest that pretest actions of ACTH and beta-endorphin depend on their endogenous release at the time of training, the present results are compatible with a malfunction of the brain beta-endorphin system in the undernourished animals.