Foraging activity is reduced in a mouse model of depression.

Depression interferes with the human ability to make decisions. Multiple criteria have been adopted for the diagnosis of depression in humans, but no clear indicators are available in animal models to reflect the depressive mood, involving higher cognitive functions. The act of foraging is a species-specific behaviour which is believed to involve the decision-making and higher cognitive functions. We previously established a method to detect the foraging behaviour of rodents, in which our results demonstrated that NMDA and dopamine receptors were involved. Conversely, increased NMDA receptors and reduced dopamine have been reported in depression model rodents. However, we hypothesise that foraging activities may also be impaired in depression. To test the theory, we successfully established a mouse model of depression using the chronic unpredictable mild stress (CUMS) paradigm. Most interestingly, the food foraging activity of mice after CUMS was significantly reduced. In addition, the treatment of anti-depressant fluoxetine reversed most depressive symptoms and reduced glial fibrillary associated protein (GFAP) expression in the hippocampus, but was less effective in the reduction of foraging activities. However, clozapine reversed all symptoms of CUMS-exposed mice including reduction of GFAP expression in the hippocampus and impaired foraging activity. Our findings of GFAP expression as a marker to validate the CUMS protocol provide further validation of our hypothesis, that the reduced food foraging is probably a new behavioural finding of depression in which the serotoninergic system could not be singly involved. Our study suggests that NMDA receptors, serotoninergic and dopaminergic systems are differentially involved in these food foraging behaviours. Our data suggest that the foraging test in rodents can be a useful tool to assess the ability of decision-making in depression.