Housing conditions modulate escitalopram effects on antidepressive-like behaviour and brain neurochemistry

Despite limited understanding of the pathophysiology of depression and the underlying mechanisms mediating antidepressant effects, there are several efficient treatments. The anhedonia symptoms of depression are characterized by decreased motivation and drive and imply possible malfunctioning of the mesolimbic dopamine system, whereas cognitive deficits might reflect decreased plasticity in hippocampus. In female Flinders Sensitive Line (FSL) rats, a model of depression, we compared the effects of three long-term antidepressant treatments: voluntary running, escitalopram and the combination of both on antidepressant-like behaviour in the Porsolt swim test (PST), and on regulation of mRNA for dopamine and neuropeptides in striatal dopamine pathways and brain-derived neurotrophic factor (BDNF) in hippocampus. Escitalopram diet attenuated running behaviour in FSL rats but not in non-depressed controls rats. In the PST the running group had increased climbing activity (noradrenergic/dopaminergic response), whereas the combination of escitalopram and running-wheel access increased swimming (serotonergic response). Running elevated mRNA for dynorphin in caudate putamen and BDNF in hippocampus. The combined treatment down-regulated D1 receptor and enkephalin mRNA in accumbens. Escitalopram alone did not affect behaviour or mRNA levels. We demonstrate a novel behavioural effect of escitalopram, i.e. attenuation of running in 'depressed' rats. The antidepressant-like effect of escitalopram was dependent on the presence of a running wheel, but not actual running indicating that the environment influenced the antidepressant effect of escitalopram. Different patterns of mRNA changes in hippocampus and brain reward pathways and responses in the PST by running and escitalopram suggest that antidepressant-like responses by running and escitalopram are achieved by different mechanisms.