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Strain-dependent effects of morphine injected into the periaqueductal gray area of mice.

1. Microinjection of morphine (0.31, 1.25 and 5.0 micrograms) into the periaqueductal grey area (PAG) of C57BL/6 (C57) and DBA/2 (DBA) mice increased the pain threshold in the tail-flick test. The highest dose also caused a behavioral reaction in both strains characterized by periods of immobility alternating with explosive motor behavior. 2. In the DBA strain, the analgesic effect was demonstrated with all doses of morphine, while in the C57 strain only the highest dose induced analgesia. 3. DBA mice presented a decrease in activity with the lowest dose of morphine, whereas in the C57 strain, this effect was obtained only with the highest dose of morphine. 4. These data corroborate at the PAG level the results of other studies which have shown that central and peripheral injections of morphine produce analgesia and alter motor activity in C57 and DBA strains. They also confirm that these two strains of mice present genotype-dependent differences in sensitivity to opioids as determined after injections of morphine into the PAG.