Fig. 1.
Left, Male Sprague Dawley rats received an intrathecal infusion of saline or of DAMGO (1 nmol · μl−1 · hr−1) for 7 d. The spinal cords were removed and assayed for dynorphin content with an enzyme immunoassay. The rats with DAMGO infusions showed a significant (*p ≤ 0.05; Student'st test) increase in spinal dynorphin content when compared with saline-infused rats. Right, Antinociceptive tolerance on day 8 to intrathecal DAMGO (1 μg/5 μl;n = 8; top) or cross-tolerance to intrathecal morphine (10 μg/5 μl; n = 8;bottom) in the 52°C water tail-flick test is demonstrated.