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Previously, we found that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson’s disease (PD) model mice (PD mice) showed facilitation of hippocampal memory extinction via reduced cyclic adenosine monophosphate (cAMP)/cAMP-dependent response element-binding protein (CREB) signaling, which may cause cognitive impairment in PD. Serotonergic neurons in the median raphe nucleus (MnRN) project to the hippocampus, and functional abnormalities have been reported. In the present study, we investigated the effects of the serotonin 5-HT4 receptor (5-HT4R) agonists prucalopride and velusetrag on the facilitation of memory extinction observed in PD mice. Both 5-HT4R agonists restored facilitation of contextual fear extinction in PD mice by stimulating the cAMP/CREB pathway in the dentate gyrus of the hippocampus. A retrograde fluorogold-tracer study showed that γ-aminobutyric acid-ergic (GABAergic) neurons in the reticular part of the substantia nigra (SNr), but not dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNpc), projected to serotonergic neurons in the MnRN, which are known to project their nerve terminals to the hippocampus. It is possible that the degeneration of the SNpc DAergic neurons in PD mice affects the SNr GABAergic neurons, and thereafter, the serotonergic neurons in the MnRN, resulting in hippocampal dysfunction. These findings suggest that 5HT4R agonists could be potentially useful as therapeutic drugs for treating cognitive deficits in PD.

Cognitive impairment often occurs in Parkinson's disease (PD), but the mechanism of onset remains unknown. Recently, we reported that PD model mice produced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) show facilitation of hippocampal memory extinction, which may be the cause of cognitive impairment in PD. When we examined the cAMP/CREB signaling in the hippocampus, decreased levels of cAMP and phosphorylated CREB were observed in the dentate gyrus (DG) of MPTP-treated mice. Administration of rolipram improved the memory deficits with concomitant recovery of cAMP and phosphorylated CREB levels, suggesting that reduced cAMP/CREB signaling in the DG leads to cognitive impairment in MPTP-treated mice.

Bilateral caudal Me5 lesions alter the nocturnal properties of food and water intake and locomotor activity in mice and decrease the mRNA expression level of orexin in the perifornical area during the dark phase. These results suggest that Me5 activity may influence the nocturnal properties of feeding and its related behaviors by adjusting the activity of orexin neurons in the perifornical area.