Contributions of taste factors and gender to opioid preference in C57BL and DBA mice

C57BL/6J and DBA/2J mouse strains have been characterized as morphine preferrers and avoiders, respectively (Horowitz et al. 1977). Previously, sweetened morphine solutions were presented with a water alternative, primarily with male subjects. Because sweetness may affect the endogenous opioid system and rodents have shown strain and sex differences in taste preferences, this study looked for strain- and gender-related taste preferences that might have affected opiate consumption. Preference for sweetened and unsweetened morphine and etonitazene was compared across gender and strain. In all choice tests, the control was a similar tasting quinine sulphate solution. Under these conditions, C57BL/6J mice continued to show strong preference for morphine. However, DBA/2J mice drank approximately equal amounts of morphine and quinine solutions, rather than avoiding morphine as when water was the alternative. Both strains appeared surprisingly indifferent to the synthetic opioid etonitazene, compared because it is potent at concentrations having barely perceptible bitterness. This raises the possibility of unexpected differences in post-ingestional effects between morphine and etonitazene. Contrary to reports of gender differences in sweet preference in rats, none were found in either strain of mouse. Neither were there any significant sex differences in opiate preference in either strain. C57 mice preferred sweetness more than did DBA mice.