Reverse tolerance to ambulation-increasing effects of methamphetamine and morphine in 6 mouse strains

Effects of single administration of methamphetamine (1, 2 and 4 mg/kg, s.c.) and morphine (5, 10 and 20 mg/kg, s.c.) and repeated administration of methamphetamine (2 mg/kg, s.c.) and morphine (10 mg/kg, s.c.) on ambulatory activity were investigated in 6 mouse strains: dd, ICR, BALB/c, C57BL/6, C3H/He and DBA/2. Although there were differences in the drug sensitivities among mouse strains, methamphetamine and morphine increased the ambulatory activity in all the strains except for the DBA/2 strain that showed an increase only after morphine. Repeated 5 times administration of methamphetamine at intervals of 3-4 days induced a reverse tolerance (an enhancement in the sensitivity) to the ambulation-increasing effect in all the strains with a marked degree in dd, ICR, C3H/He and DBA/2 strains and a slight degree in BALB/c and C57BL/6 strains. The same treatment with morphine induced reverse tolerance to the effect of morphine markedly in C57BL/6 and C3H/He strains and moderately in dd, ICR and BALB/c strains, but the DBA/2 strain showed no significant change in the ambulatory activity throughout the repeated 5 times administration of morphine. There was positive correlation between the initial drug sensitivities of animals and the degrees of the reverse tolerance in either methamphetamine or morphine. Furthermore, the reverse tolerance to methamphetamine and morphine was sometimes transferable, although such cross interaction varied among mouse strains.