The course of the decrease of hypothalamic beta-endorphin induced by training, and the development of the effect of beta-endorphin on the retrieval of inhibitory avoidance in rats.

Step-down inhibitory avoidance training or the simple exposure of rats to the training apparatus is followed by a decrease of hypothalamic beta-endorphin immunoreactivity at 0.1, 1.0 or 2.0 h after training. Immunoreactivity returns to normal at 6.0 h. The ip administration of 1.0 microgram/kg of human beta-endorphin 6 min prior to training produces an inhibition of the retrieval of the step-down task at 6.0 h, but not at 0, 1.0 or 2.0 h after training. This effect is reversed by a second injection of the substance immediately before testing. The possible physiological significance of this parallel development of the effect of beta-endorphin on retrieval and the depletion of the substance caused by training is discussed. The data indicate that retrieval is insensitive to the peptide when its hypothalamic stores are depleted.