RETRIEVAL EFFECTS OF BETA-ENDORPHIN AND NALOXONE, AND THE NOVELTY-INDUCED ANTINOCICEPTION IN THE DEVELOPING RAT

Three experiments were conducted to assess the retrieval effects of a single dose of β-endorphin and of naloxone, and of the novelty-induced antinociception response in the developing rat. Wistar rats 30, 45, 60, and 90 days old from our breeding stock were used. Animals were trained and tested, with a 24-h interval between sessions, in a two-way active avoidance task (using 20 presentations of a 5-s, 1-kHz tone and a 0.4-mA footshock) or in a step-down inhibitory avoidance task (using a 60-Hz, 0.2-mA footshock). Saline (1.0 ml/kg), β-endorphin (2.0 μg/Kg), or naloxone (0.8 mg/kg), was administered ip immediately after training, and saline or β-endorphin was administered 6 min before testing. The retrieval enhancing effects of post-training naloxone and pretest β-endorphin, and the retrieval impairing effect of post-training β-endorphin, were consistently observed only in 60- and 90-day-old rats, on both tasks. In a third experiment, another group of naive rats was placed for 2 min in a novel environment (the shuttle-box) and nociception was assessed by the tail-flick method. Novelty-induced antinociception was observed only for 60- and 90-day-old rats, and this response was cancelled by naloxone given 6 min before exposure to novelty. These results suggest that both the retrieval effects of naloxone and β-endorphin, in the doses used, and the novelty-induced antinociception response, which are possibly dependent on the activity of hypothalamic β-endorphin system, become established between 45 and 60 days postnatal in the rat. © 1991 Academic Press, Inc.