The effect of 5-HT and opioid receptor antagonists on morphine-induced Stra
ub tail was studied in mice. Straub tail behavior was induced by subcutaneo
us administration of different doses (20, 30, and 40 mg/kg) of morphine hyd
rochloride to mice. The effect of morphine was dose-dependent. Maximum resp
onse was obtained with 40 mg/kg of the drug. The response induced by morphi
ne (20 and 40 mg/kg) was decreased by different doses of intraperitoneal in
jection of naloxone (1 and 2 mg/kg) or methysergide, mianserin, and ritanse
rin (1 and 2 mg/ kg). The effect of morphine (40 mg/kg) was also reduced by
intracerebroventricular injection of naloxone (0.4-0.8 mug/animal) or mian
serin (2 and 4 mug/animal). Different groups of mice received one daily dos
e (50 mg/kg se) of morphine sulfate for 3 days to develop tolerance to morp
hine. The Straub tail reaction induced by morphine hydrochloride (40 mg/kg)
was tested on the fourth day. Naloxone injection (1 and 2 mg/kg ip) on Day
3 (1 h after morphine sulfate injection) or on Day 4 (1 h before test dose
of morphine hydrochloride), decreased tolerance induced to morphine. Methy
sergide, mianserin, or ritanserin (intraperitoneal) on Days 2 and 3 (1 h af
ter morphine sulfate injection) or on Day 4 (1 h before test dose of morphi
ne hydrochloride), also decreased tolerance induced to morphine. Intracereb
roventricular injection of either naloxone or mianserin also reduced tolera
nce to morphine. It is concluded that 5-HT2 and opioid receptor mechanisms
are involved in morphine-induced Straub tail reaction and tolerance induced
to morphine also may be mediated through these receptors. (C) 2001 Elsevie
r Science Inc. All rights reserved.