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 beta-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), beta-endorphin (2.0 microgram/Kg), or naloxone (0.8 mg/kg), was administered ip immediately after training, and saline or beta-endorphin was administered 6 min before testing. The retrieval enhancing effects of post-training naloxone and pretest beta-endorphin, and the retrieval impairing effect of post-training beta-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 shuttlebox) 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 beta-endorphin, in the doses used, and the novelty-induced antinociception response, which are possibly dependent on the activity of hypothalamic beta-endorphin system, become established between 45 and 60 days postnatal in the rat.
Netto
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CA
Departamento de Bioquímica, Universidade Federal do Rio Grande do Sul (Campus Central), Porto Alegre, Brazil.
Maltchik
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D000818:Animals; D000831:Animals, Newborn; D001143:Arousal / Q000187:drug effects*; D001288:Attention / Q000187:drug effects; D001921:Brain / Q000187:drug effects* / Q000254:growth & development; D004305:Dose-Response Relationship, Drug; D011939:Mental Recall / Q000187:drug effects*; D009270:Naloxone / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects*; D051381:Rats; D011919:Rats, Inbred Strains; D011930:Reaction Time / Q000187:drug effects; D012153:Retention, Psychology / Q000187:drug effects; D001615:beta-Endorphin / Q000494:pharmacology*
D009270:Naloxone; D001615:beta-Endorphin