Mu-opioid receptor knockout mice show diminished food-anticipatory activity
Kas, Martien J H; van den Bos, Ruud; Baars, Annemarie M; Lubbers, Marianne; Lesscher, Heidi M B; Hillebrand, Jacquelien J G; Schuller, Alwin G; Pintar, John E; Spruijt, Berry M
(2004) European Journal of Neuroscience, volume 20, issue 6, pp. 1624 - 1632
(Article)
Abstract
We have previously suggested that during or prior to activation of anticipatory behaviour to a coming reward, mu-opioid receptors are activated. To test this hypothesis schedule induced food-anticipatory activity in mu-opioid receptor knockout mice was measured using running wheels. We hypothesized that mu-knockout mice show little food-anticipatory activity. In wildtype
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mice we observed that food-anticipatory activity increased proportional to reduced food intake levels during daily scheduled food access, and thus reflects the animal's physiological need for food. mu-Knockout mice do not adjust their schedule induced running wheel behaviour prior to and during feeding time in the same way as wildtype mice; rather than showing more running wheel activity before than during feeding, they showed an equal amount of activity before and during feeding. As food-anticipatory activity is dependent on the mesolimbic dopamine system and mu-opioid receptors regulate dopaminergic activity, these data suggest a change in the dopamine system's activity in mu-knockout mice. As we observed that mu-knockout mice tended to show a stronger locomotor activity response than wildtype mice to the indirect dopamine agonist d-amphetamine, it appears that the dopaminergic system per se is intact and sensitive to activation. We found no differences in the expression of pro-opiomelanocortin, a precursor of endogenous endorphin, in the arcuate nucleus between mu-knockout mice and wildtype mice during restricted feeding, showing that the mu-opioid receptor does not regulate endogenous endorphin levels. These data overall suggest a role for mu-opioid receptors in adapting reward related behaviour to the requirements of the environment.
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Keywords: Amphetamine, Analysis of Variance, Animals, Behavior, Animal, Central Nervous System Stimulants, Conditioning, Operant, Eating, Feeding Behavior, Female, Food, Gene Expression Regulation, Mice, Mice, Inbred C57BL, Mice, Knockout, Motor Activity, Pro-Opiomelanocortin, Receptors, Opioid, mu, Reinforcement Schedule, Time Factors
ISSN: 0953-816X
Publisher: Blackwell Publishing Ltd
(Peer reviewed)