Further studies on the understanding of Octodon degus natural resistance to morphine: a comparative study with the Wistar rat.

1. Octodon degus shows higher levels of tolerance to morphine when compared with the Wistar rat. 2. In the formalin algesiometric test, this caviomorph is more resistant to pain (P less than 0.01) and to the analgesic effect of morphine (P less than 0.001). 3. CD50 and LD50 were significantly higher in Octodon degus as compared with Wistar rat. 4. Morphine caused in rat severe hypotension, while doses eight times higher in O. degus had a transient effect. 5. 3H-naloxone binding in adrenal glands of O. degus is higher than in other tissue samples assayed from the same animal or rats.