α-Terpineol attenuates morphine-induced physical dependence and tolerance in mice: role of nitric oxide.

OBJECTIVES Dependence and tolerance to opioid analgesics are major problems limiting their clinical application. α-Terpineol is a monoterpenoid alcohol with neuroprotective effects which is found in several medicinal plants such as Myrtus communis, Laurus nobilis, and Stachys byzantina. It has been shown that some of these medicinal plants such as S. byzantina attenuate dependence and tolerance to morphine. Since α-terpineol is one of the bioactive phytochemical constituent of these medicinal plants, the present study was conducted to investigate the effects of α-terpineol on morphine-induced dependence and tolerance in mice. MATERIALS AND METHODS The mice were rendered dependent or tolerant to morphine by a 3-day administration schedule. The hot-plate test and naloxone-induced withdrawal syndrome were used to evaluate tolerance and dependence on morphine, respectively. To investigate a possible role for nitric oxide (NO) in the protective effect of α-terpineol, the NO synthase inhibitor, L-N(G)-nitroarginine methyl ester (L-NAME) and NO precursor, L-arginine, were used. RESULTS Administration of α-terpineol (5, 10, and 20 mg/kg, IP) significantly decreased the number of jumps in morphine dependent animals. Moreover, α-terpineol (20 and 40 mg/kg, IP) attenuated tolerance to the analgesic effect of morphine. The inhibitory effects of α-terpineol on morphine-induced dependence and tolerance were enhanced by pretreatment with L-NAME (10 mg/kg, IP). However, L-arginine (300 mg/kg, IP) antagonized the protective effects of α-terpineol on dependence and tolerance to morphine. CONCLUSION These findings indicate that α-terpineol prevents the development of dependence and tolerance to morphine probably through the influence on NO production.