Opioid agonists such as morphine have been found to exert excitatory and inhibitory receptor-mediated effects at low and high doses, respectively. Ultra-low doses of opioid antagonists (naloxone and naltrexone), which selectively inhibit the excitatory effects, have been reported to augment systemic morphine analgesia and inhibit the development of tolerance/physical dependence. This study investigated the site of action of the paradoxical effects of naltrexone and the generality of this effect. The potential of ultra-low doses of naltrexone to influence morphine-induced analgesia was investigated in tests of nociception. Administration of intrathecal (0.05 and 0.1 ng) or systemic (10 ng/kg i.p.) naltrexone augmented the antinociception produced by an acute submaximal dose of intrathecal (5 g) or systemic (7.5 mg/kg i.p.) morphine in the tail-flick test. Chronic intrathecal (0.005 and 0.05 ng) or systemic (10 ng/kg) naltrexone combined with morphine (15 g i.t.; 15 mg/kg i.p.) over a 7-day period inhibited the decline in morphine antinociception and prevented the loss of morphine potency. In animals rendered tolerant to intrathecal (15 g) or systemic (15 mg/kg) morphine, administration of naltrexone (0.05 ng i.t.; 10 and 50 ng/kg i.p.) significantly restored the antinociceptive effect and potency of morphine. Thus, in ultra-low doses, naltrexone paradoxically enhances morphine analgesia and inhibits or reverses tolerance through a spinal action. The potential of naltrexone to influence morphine-induced reward was also investigated using a place preference paradigm. Systemic administration of ultra-low doses of naltrexone (16.7, 20.0, and 25.0 ng/kg) with morphine (1.0 mg/kg) extended the duration of the morphine-induced conditioned place preference. These effects of naltrexone on morphine-induced reward may have implications for chronic treatment with agonist-antagonist combinations.Opioid drugs such as morphine are widely used in the treatment of severe pain; however, their chronic administration results in the development of tolerance to their analgesic effects, limiting their clinical usefulness in pain management. Although the mechanisms underlying the development of opioid tolerance are poorly understood, recent studies have suggested that alterations in the coupling of opioid receptors to G protein-linked effectors may play a significant role (Crain and Shen, 2000). Morphine and related agonists are recognized to produce their characteristic acute and chronic effects by activating spinal and supraspinal -, ␦-, and -opioid receptors. Classically, morphine activates G i protein-coupled -opioid receptors to inhibit adenylyl cyclase activity and decrease neuronal cAMP levels (Uhl et al., 1994). At the presynaptic level, -opioid receptor activation inhibits voltage-sensitive Ca 2ϩ channels (Tallent et al., 1994) and reduces neurotransmitter release, whereas at the postsynaptic level, it opens potassium channels and hyperpolarizes neurons (North and Williams, 1983;Ikeda et al., 1995). The net result of th...