Parkinson's disease (PD) is a progressive degenerative condition of the central nervous system, pathologically characterized by loss or degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). PD causes a widely variety of motor and non-motor symptoms (Soni et al., 2024, 2025). People generally pay attention to motor features, but ignore non-motor features. Subtle non-motor symptoms may even precede motor symptoms, and cognitive impairments were suggested to be a sign of prodromal PD (Darweesh et al., 2017). Working memory deficits are one of the most common cognitive impairments in PD, and may be apparent at very early stage of the disease.

Although degeneration in the dopaminergic system has long been regarded as the dominant reason for PD, there is also extensive evidence that the serotonergic system plays a crucial role in regulating extrapyramidal motor functions and some non-motor symptoms related to PD, such as cognitive impairments (Halliday et al., 2014; Huot and Fox, 2013; Ohno et al., 2015). 5-hydroxytryptamine (5-HT) is a bioactive substance synthesized from tryptophan, that is associated with emotional manifestation, cognitive function, and so on (Barnes and Sharp, 1999; Baumgarten and Grozdanovic, 1995). The effects of 5-HT are mediated through activation of 5-HT receptors which are typically divided into 7 families, encompassing at least 14 subtypes (Sharp and Barnes, 2020). Among them, 5-HT4 receptors have been proposed as a useful neural marker and therapeutic target for learning and memory (Madsen et al., 2011; Manuel-Apolinar et al., 2005; Meneses, 2015). Accumulated evidence has shown that 5-HT4 receptor agonists enhanced memory in various of cognitive behaviors (Haahr et al., 2013; Letty et al., 1997; Lo et al., 2014; Orsetti et al., 2003), and had therapeutic effect on Alzheimer's disease and amnesia (Baranger et al., 2017; Giannoni et al., 2013; Lalut et al., 2017; Lezoualc'h, 2007; Madsen et al., 2011; Maillet et al., 2004; Meneses, 1998; Ramirez et al., 2014; Spencer et al., 2004; Yahiaoui et al., 2016), indicating that 5-TH4 receptors are considered to be vital in cognitive function. In addition, quantitative autoradiography has shown that 5-HT4 receptors are widely expressed in brain areas anatomically linked to cognition (Jakeman et al., 1994).

The medial prefrontal cortex (mPFC) is one of the encephalic regions reported in relation with cognition, such as working memory, attentional processes, decision-making and goal-directed behavior (Vertes, 2004). The prelimbic cortex (PrL) is a major subregion within the mPFC in rodent, and is thought to have a high functional homology with the dorsolateral prefrontal cortex of primates, which plays a direct role in cognitive function, including working memory (Barbas, 2000; Groenewegen and Uylings, 2000; Kolb, 1984; Ongur and Price, 2000). For instance, the PrL specific lesions and PrL-infralimbic cortex lesions have been reported to produce obvious defects in delayed-response tasks (Delatour and Gisquet-Verrier, 1996, 2000; Dias and Aggleton, 2000). Gamma-aminobutyric acid (GABA)-ergic inhibition in the PrL impaired working memory in aging rodents (Gandy et al., 2023), while blocking excess tonic inhibition in the aged mPFC restored working memory (Banuelos et al., 2014). Moreover, activation of parvalbuminn neurons in the PrL improved working memory deficits in the mouse model of schizophrenia (Arime et al., 2024). The PrL receives high levels of serotonergic innervation from the raphe nuclei (O'Hearn and Molliver, 1984), and expresses moderate level of 5-HT4 receptors (Ohno et al., 2015). The PrL also receives afferent projections from other areas, such as the cortex, basal forebrain, amygdala, thalamus, hypothalamus, and midbrain (Hoover and Vertes, 2007). In addition, the PrL projects to the cortex, amygdala, ventral tegmental area (VTA), SNc, nucleus accumbens, midline thalamus and raphe nuclei and regulates their activity, and these projections also support the involvement of the PrL in cognitive function (Vertes, 2004).

From all the above, we speculate that cognitive impairment, especially working memory deficits, in PD might be connected with the 5-HT4 receptors in the PrL. However, to our knowledge, there was limited evidence to show the expression and function of PrL 5-HT4 receptors in parkinsonian animals. Therefore, in this study, we used 6-hydroxydopamine (6-OHDA)-lesioned rats as hemiparkinsonian model, and adopted a series of experiments to examine the effects of activation and blockade of PrL 5-HT4 receptors on cognitive behaviors, local field potential (LFP) in the PrL and monoamine levels in the limbic-related brain areas, and we also detected the changes in the expression of 5-HT4 receptor in the PrL of hemiparkinsonian rats.