DIFFERENTIAL SENSITIVITY TO MORPHINE-INDUCED ANALGESIA AND MOTOR-ACTIVITY IN 2 INBRED STRAINS OF MICE - BEHAVIORAL AND BIOCHEMICAL CORRELATIONS

The effects of morphine have been evaluated in two strains of mice showing different behavioral responses. Morphine elicits an increase of locomotor activity only in the C57BL/6J(C57) strain, whereas no effect is found when the opiate is injected to the DBA/2J mice (DBA). On the other hand, the DBA strain, unlike the C57 mice, is very sensitive to the analgesic effects of morphine. The following biochemical parameters have been studied: dopamine (DA) release in the striatum as shown by the measurements of 3-methoxytyramine and cyclic adenosine 3':5'-monophosphate (cyclic AMP) levels in the striatum, cyclic AMP and cyclic guanosine 3':5'-monophosphate (cylic GMP) levels in the cerebellum. The levels of 3-methoxytyramine and cyclic AMP in the striatum are increased in the C57 strain. Haloperidol, a DA receptor blocker, and naltrexone, a pure morphine antagonist, reverse this effect. Neither DA release nor cyclic AMP is increased by morphine in the DBA strain. The results demonstrate that the increased motor activity after morphine in the C57 strain, but not analgesia, is related to an activation of DA neurons in the striatum. Cerebellar cyclic GMP concentrations increase in the C57 strain and decrease in the DBA after morphine, again suggesting that the levels of this nucleotide may also be affected by input to the cerebellum which regulates motor coordination.