C57 BL/6J (C57) mice display a particular pattern of responses following morphine administration, such as a rapid development of tolerance to the pharmacological action of the opiate and an increase in locomotor activity after a single injection of the drug. We have measured met-enkephalin content and the responsiveness of different opiate receptors after repeated administration of morphine and naloxone. Prolonged morphine administration changed neither met-enkephalin levels, nor the density of opiate receptors in mice brain. In contrast repeated administration of the opiate antagonist naloxone, produced a marked increase in the number of 3H-DHM and 3H-DADLE binding sites in striatum and brainstem without modifying met-enkephalin concentrations. Behavioral studies have indicated that the morphine-induced increase in locomotor activity is enhanced in naloxone pretreated mice, thus suggesting a possible correlation between the behavioral response to morphine in C57 mice and the higher number of opiate receptors in the striatum. © 1984.
Modulation of opioid system in C57 mice after repeated treatment with morphine and naloxone: Biochemical and behavioral correlates / Brunello, N.; Volterra, A.; Di Giulio, A. M.; Cuomo, V.; Racagni, G.. - In: LIFE SCIENCES. - ISSN 0024-3205. - 34:17(1984), pp. 1669-1678. [10.1016/0024-3205(84)90638-6]