Serotonin receptors in the dorsal hippocampus regulate working memory and long-term habituation in the hemiparkinsonian rats.

Although the dorsal hippocampus (dHPC) and serotonin (5-HT) receptor are involved in cognition, their roles in cognitive impairments in Parkinson' disease (PD) are still unclear. In the present study, the effects of the 5-HT receptor agonist 8-OH-DPAT and antagonist WAY100635 administrated into the dHPC of rats were assessed in T-maze rewarded alternation test for working memory and in hole-board test for long-term habituation. Unilateral 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle in rats impaired working memory and long-term habituation, decreased dopamine (DA) levels in the medial prefrontal cortex (mPFC), dHPC and ventral hippocampus (vHPC), and decreased the mean density of 5-HT receptors and co-localization of 5-HT receptor and excitatory amino acid carrier 1-immunoreactive (EAAC1-ir) neurons in the dHPC compared to sham-operated rats. Activation of dHPC 5-HT receptors by local infusion of 8-OH-DPAT impaired working memory and long-term habituation in both sham-operated and the 6-OHDA-lesioned rats. Furthermore, blockade of dHPC 5-HT receptors by WAY100635 improved the memories in the 6-OHDA-lesioned rats, but had no effects in sham-operated rats. Additionally, dHPC injection of 8-OH-DPAT decreased noradrenaline (NA) levels, increased 5-HT levels in the mPFC, dHPC and vHPC in sham-operated and lesioned rats, while WAY100635 increased DA and NA levels only in lesioned rats. The results of the present study suggest that dHPC 5-HT receptors regulate cognitive impairments in PD by changes of monoamines in the related brain regions.