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Human Parkinson’s disease (PD) affects a variety of brain regions, leading to multiple motor and non-motor symptoms. Cognitive impairment is a disabling non-motor symptom, and affects approximately 25% of newly diagnosed PD patients (Ibarretxe-Bilbao et al., 2012). As the disease advances, up to 80% of PD patients without prior cognitive dysfunction develop mild cognitive impairment (PD-MCI) and dementia (Hely et al., 2008). Consequentially cognitive deficits have important implications in the disease management. Unfortunately, there are no effective therapeutic options available due to the incomplete understanding of underlying synaptic mechanisms leading to cognitive dysfunction.

In recent years, the field of movement disorders has evolved from the conventional idea that hippocampal dysfunction plays a minor role in PD, to one that now regards deficits in hippocampal synaptic plasticity to contribute significantly to memory loss in PD. This concept is supported by studies showing hippocampal atrophy in PD patients with impaired cognition (Camicioli et al., 2003; Bruck et al., 2004; Calabresi et al., 2013; Kandiah et al., 2014), and studies showing lower baseline volume in the CA1 hippocampal region accompanied by deficits in baseline attention in PD patients with MCI. Longitudinally, the decline in episodic memory appears to be associated with increasing atrophy of CA2-CA3 regions over 18 months (Foo et al., 2017). Thickness of hippocampal CA1 stratum pyramidale is also associated with episodic memory impairment in PD patients (Boentert, 2019). Importantly, in neurodegenerative diseases such as PD, synaptic transmission is negatively impacted prior to overt structural and behavioral abnormalities (Wishart et al., 2006; Raymond et al., 2011). Thus, exploration of changes in synaptic networks using preclinical models with a well-established timeline of motor deficits can provide insight into how relevant synaptic circuits are altered prior to and after motor symptoms appear.

In this study we leveraged the recently developed PD rat model with loss of function mutation in PARK6, the gene that encodes the protein PTEN-induced kinase 1 (PINK1) (Valente et al., 2004), to study hippocampal excitatory transmission. In humans, loss of function PINK1 mutations cause an autosomal recessive early-onset form of PD with clinical symptoms similar to idiopathic PD (Kasten et al., 2010). However, the mechanism by which PINK1 deficiency causes PD is unknown. Under physiological conditions, PINK1 is involved in many functions such as mitochondrial autophagy (Kane et al., 2014; Lazarou et al., 2015; Truban et al., 2017) and bioenergetics (Rango et al., 2020), maintenance of mitochondrial calcium homeostasis (Heeman et al., 2011), and misfolded protein clearance (Du et al., 2017), plus neuronal branching (Dagda et al., 2014), regulation of adaptive immunity (Matheoud et al., 2019) and neuroinflammation (Sliter et al., 2018). In a mouse model of Alzheimer’s disease, PINK1 overexpression rescued impairments in hippocampal LTP (Du et al., 2017). However, in PINK1 KO mice, no changes were detected in hippocampal LTP (Feligioni et al., 2016). The occurrence of both motor and non-motor phenotypes in PINK1 KO rats, specifically at ages prior to reported nigral cell loss, prompted our investigation of possible impairments in hippocampal synaptic transmission. Using PINK1 KO rats, we explored how loss of PINK1 impacts synaptic function at hippocampal CA3-CA1 synapses as this region represents the primary experimental model for the synaptic changes underlying learning and memory. We chose to perform these experiments in rats at 4 to 5 months of age because our laboratory previously found striatal circuit dysfunction at this age when motor symptoms begin to appear but prior to the age when dopaminergic neuronal loss becomes apparent (Creed et al., 2020).

The goal of this study was to determine whether hippocampal excitatory transmission in PINK1 KO rats is altered compared to WT rats at an age when motor and non-motor phenotypes are just appearing prior to reported nigral cell loss and when we have observed an increase in excitatory transmission in the dorsal striatum (Creed et al., 2020). We found no detectable changes in the strength of basal synaptic transmission, short-term presynaptic plasticity or LTP at CA3-CA1 synapses in PINK1 rats compared to WT rats. These results indicate that synaptic function is not significantly impacted by loss of PINK1 protein at this young adult age.

First, we investigated the strength of synaptic transmission at CA3-CA1 synapses by measuring maximum transmission using I/O curves and found no genotype differences. Similar results were observed in PINK1 KO mice where no change was observed in I/O curves at CA3-CA1 synapses in two and six-month-old animals. However, the frequency, but not amplitude, of spontaneous excitatory postsynaptic currents (sEPSCs) was increased at CA3-CA1 synapses at 6 months of age in PINK1 KO mice (Feligioni et al., 2016). This finding is similar to our finding of increased frequency and amplitude of spontaneous EPSCs recorded from striatal spiny projection neurons in PINK1 KO rats at 4 months of age, in the absence of a change in the corticostriatal I/O curve (Creed et al., 2020). In contrast to PINK1 KO rats, no difference was observed in PINK1 KO mice in the frequency or amplitude of spontaneous EPSCs or miniature EPSCs recorded from spiny projection neurons in dorsal striatum (Madeo et al., 2014). Further investigations of CA1 pyramidal neurons in PINK1 KO rats using whole-cell patch clamp is needed in future experiments to investigate whether there is a similar increase in frequency of spontaneous EPSCs, or other alterations in synaptic transmission not observed in the current study.

To determine if there was an alteration in presynaptic release probability, we measured the PPR and did not find any statistical difference between genotypes at any inter-stimulus interval. A similar result was reported in an analysis of PINK1 KO mice (Feligioni et al., 2016). Specifically, no changes in PPR between PINK1 KO and WT mice at 2 and 6 months of age was observed, although an increase in the frequency of the spontaneous EPSCs was found. The authors speculated that this effect on spontaneous EPSCs might be linked to increased presynaptic accumulation of alpha-synuclein. Previous studies from our lab and others reported the spontaneous appearance of proteinase K-resistant α-synuclein-immunoreactive aggregates in various brain regions of PINK1 KO rats, including cortex, thalamus, striatum, and ventral midbrain (Grant et al., 2015; Creed and Goldberg, 2018). Recently, using slice electrophysiology, we reported increased glutamate transmission onto dorsal striatum spiny projection neurons in PINK1 KO rats at 4 months of age (Creed et al., 2020). α-synuclein plays a vital role in the presynaptic mobilization of the reserve pool of neurotransmitter vesicles, not only for dopamine but also for glutamate (Gureviciene et al., 2007). Our inability to detect changes in short-term synaptic plasticity in acute hippocampal slices of PINK1 KO rats may be related to the apparent lack of α-synuclein pathology in this area at this age.

We found no impact of the loss of PINK1 on LTP induced either by strong or weak TBS at CA3-CA1 synapses. This suggests that these synapses do not differ in their ability to undergo long-term plasticity between the two genotypes, and there was no saturation effect from strong TBS. In transgenic mice expressing a 120 amino acid truncated form of α-synuclein, there is a reduction in striatal dopamine levels and impaired ability to generate hippocampal CA1 LTP (Tofaris et al., 2006; Costa et al., 2012). In both homozygous and heterozygous PINK1 KO mice, evoked dopamine release was decreased, leading to impaired corticostriatal LTP (Kitada et al., 2007; Madeo et al., 2014). Consistent with this, we have previously reported decreased dopamine tone in PINK1 KO rats at corticostriatal synapses at 4 months of age and decreased striatal dopamine levels at age 12 months compared to 4 months, measured using in vivo microdialysis (Creed et al., 2019; Creed et al., 2020). Because bidirectional plasticity is critical for normal hippocampus dependent learning and memory (Wang et al., 2003), it will be important to determine in future studies whether LTD is intact at hippocampal excitatory synapses in PINK1 KO rats, and how loss of PINK1 during the aging process alters long-term plasticity that contributes to learning and memory.

In conclusion, this short report is the first hippocampal synaptic physiology study to investigate the impact of PINK1 deficiency in acute brain slices from PINK1 KO rats. By recording extracellular dendritic fEPSPs, we showed no early functional changes in mechanisms of short- and long-term plasticity as well as the strength of basal synaptic transmission at CA3-CA1 hippocampal synapses. These results demonstrate that loss of PINK1 does not alter hippocampal synaptic plasticity at onset of both motor and non-motor phenotypes.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

The animal study was reviewed and approved by the UAB Institutional Animal Care and Use Committee.

AM conceptualized the study, performed the majority of the experiments and analyses, made the figures, and wrote the first draft of the manuscript. MB and RC performed some experiments, updated the figures, performed the statistics, and edited the manuscript. LM, AA, and MG conceptualized the study, reviewed, and edited the manuscript. All authors approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.