Lack of overt behavioral abnormalities
Lack of major compensatory changes within the opioid system
Abolition of the analgesic effect of morphine
Abolition of place-preference activity
Abolition of physical dependence
Lack of behavioral responses related to delta- or kappa-receptor activation with morphine

Matthes, R. Maldonado, F. Simonin, O. Valverde, S. Slowe, I. Kitchen, K. Befort, A. Dierich, M. Le Meur, P. Dolle, E. Tzavara, J. Hanoune, B. P. Roques & B. L. Kieffer:
Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the mu-opioid-receptor gene.
Nature 383, (1996)