Alterations in dopamine system and in its connectivity with serotonin in a rat model of Alzheimer's disease.

Dopamine pathways alterations are reported in Alzheimer's disease. However, it is difficult in humans to establish when these deficits appear and their impact in the course of Alzheimer's disease. In the TgF344-Alzheimer's disease rat model at the age of 6 months, we showed a reduction in release of striatal dopamine due to serotonin 5HT-receptor blockade, in the absence of alterations in 5HT-receptor binding, suggesting a reduction in 5HT-receptor-dopamine system connectivity. In addition, a functional hypersensitivity of postsynaptic dopamine D-receptors and D-autoreceptors was also reported without any change in D-receptor density and in the absence of amyloid plaques or overexpression of the 18 kDa translocator protein (an inflammatory marker) in areas of the dopamine system. Citalopram, a selective serotonin reuptake inhibitor, induced functional 5HT-receptor-D-receptor connectivity changes but had no effect on D-autoreceptor hypersensitivity. In older rats, dopamine cell bodies overexpressed translocator protein and dopamine projection sites accumulated amyloid. Interestingly, the 5HT-receptor density is decreased in the accumbens subdivisions and the substantia nigra pars compacta. This reduction in the striatum is related to the astrocytic expression of 5HT-receptor. Our results indicate that both serotonin/dopamine connectivity and dopamine signalling pathways are dysregulated and potentially represent novel early diagnostic and therapeutic avenues.