Mu Opioid Receptor Knockout Mice in the Morris Water Maze: a Learning or Motivation Deficit?

An earlier study done by Kas et al. [Kas MJ, van den Bos R, Baars AM, Lubbers M, Lesscher HM, Hillebrand JJ, et al. Mu-opioid receptor knockout mice show diminished food-anticipatory activity. Eur J Neurosci 2004;20(6):1624-32] suggested that mu opioid receptor (MOR) knockout mice have deficits in the motivational component rather than in the information processing component of learning. To substantiate this difference further, MOR knockout mice and wildtype littermates were tested in the Morris Water Maze (MWM), which allows for testing both components of learning. On traditional parameters for performance, no significant differences between genotypes were found. However, swimming velocity, indicative of motivation, decreased for MOR knockout mice during the course of training but not for wildtype mice. In contrast, probe trial performance was comparable between genotypes. Again, these results suggest normal information processing abilities but a decreased motivation in this MOR knockout mouse. Our conclusion is discussed in the light of other studies using MOR knockout mice that do find differences on information processing between genotypes in MWM performance.