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Address for correspondence: Gireesh Gangadharan, Ph.D. Assistant Professor/ DBT-Ramalingaswami Fellow, Department of Cell and Molecular biology, Manipal School of Life Sciences, Manipal Academy of Higher Education, Karnataka 576104, India, OR , Phone: +91-8129066565
Affiliations
Department of Cell and Molecular Biology, Manipal School of Life Sciences, Manipal Academy of Higher Education, Manipal, Karnataka 576104, India
In addition to memory deficits, patients with Alzheimer's disease (AD) experience neuropsychiatric disturbances as well. Recent studies have suggested the association of obsessive-compulsive disorder (OCD) with the early stages of AD. However, there is a lack of understanding of the neurobiological underpinnings of compulsive-like behaviors at the neuronal circuit level and their relationship with AD.
METHODS
We have addressed this issue in an Aβ1-42 induced mouse model for AD and studied compulsive-like behaviors. Next, we compared the hippocampal and medial prefrontal cortex (mPFC) local field potential (LFP) pattern and coherence between these regions of control and AD mice. We also assessed the expression pattern of acetylcholine and glutamatergic signaling in these regions using quantitative PCR.
RESULTS
Here, our findings show that AD mice exhibit compulsive-like behaviors as evidenced by enhanced marble burying, nest building, and burrowing behaviors. Further, AD mice exhibited hippocampo-cortical circuit dysfunction, demonstrated by decreased power of rhythmic oscillations at the theta (4-12Hz) and gamma frequency (25-50Hz) in the hippocampus (HPC) and the mPFC, two functionally interconnected brain regions involved both in AD and compulsive behaviors. Importantly, coherence between the HPC and mPFC in the theta band of AD animals was significantly reduced. Furthermore, we found reduced cholinergic and glutamatergic neurotransmission in the HPC and mPFC of AD mice.
CONCLUSION
We conclude that the hippocampo-cortical functional alterations may play a significant role in mediating the compulsive-like behaviors observed in AD mice. These findings may help in understanding the underlying circuit mechanisms of OCD-like phenotypes associated with AD.
Emerging evidence indicates that the neurodegenerative pathology of Alzheimer disease (AD) can lead to neuropsychiatric disturbances at certain stages during the disease progression (
). Thus, neuropsychiatric symptoms (NPS) are being increasingly recognized as one of the key features of AD and related dementias. Recent studies have proposed a possible link between AD and obsessive-compulsive disorder (OCD) (
). However, the interaction between OCD and AD pathology is poorly understood. Structural and functional neuroimaging studies suggest that a wide range of brain regions and circuits, in particular the cortico-striato-thalamo-cortical circuit (CSTC), are implicated in the pathophysiology of OCD (
). These are also demonstrated in animal studies, for example, Sapap3- and Slitrk5-mutant mouse models of OCD exhibited defects in cortico-striatal synapses and OCD-like phenotypes (
) have suggested that the hippocampus (HPC) and medial prefrontal cortex (mPFC) play an essential role in compulsive behaviors. A recent study reported that the functional connectivity between HPC and mPFC is important for the regulation of invasive thoughts (
). The interaction between these two regions is regulated by neuronal oscillations, especially rhythmic oscillations at the theta frequency (4-12Hz) during behaviors (
Theta Oscillations in the Medial Prefrontal Cortex Are Modulated by Spatial Working Memory and Synchronize with the Hippocampus through Its Ventral Subregion.
). Several neurotransmitters, including acetylcholine, are known to regulate the fine-tuning of hippocampo-medial prefrontal cortex (HPC-mPFC) functional integration (
). Although dysfunction of neurotransmission in the CSTC circuits, including serotonin, dopamine, and glutamate systems are reported in the OCD condition (
These previous findings present a possibility that hippocampo-cortical function may be critically associated with compulsive behaviors in AD. We have tested this possibility by assessing and observing compulsive behavioral phenotype in Aβ1-42 induced AD mice. Furthermore, theta and gamma oscillations in the HPC and the mPFC were reduced in AD mice. Additionally, the coherence at the theta frequency oscillations between the HPC and mPFC neuronal circuit was disrupted in AD mice. In support of this, we found downregulation of acetylcholine and glutamatergic signaling in the hippocampo-cortical neuronal pathway. These results suggest that hippocampo-cortical neural circuit may play a significant role in the compulsive behaviors associated with AD.
MATERIALS AND METHODS
Animals
C57BL/6 male mice, aged 8-10-week-old and weighing 25-30g were used in this study. Animal handling and care was done in adherence to the Institutional animal ethical committee (IAEC), Kasturba Medical College (KMC), Manipal Academy of Higher Education (MAHE), Manipal, India (IAEC/KMC/05/ 2021, dated 23/01/2021). Briefly, four animals were housed per cage at a temperature of 22±2°C under a 12:12h light-dark schedule with light beginning at 6:30 A.M. Food and water were provided ad libitum.
Alzheimer’s disease induction in mice
Amyloid β Protein Fragment1-42 (A9810, Sigma, USA) was procured and Aβ1-42 oligomers (AβO) were prepared and injections were performed as previously described (
). A detailed description is provided in the Supplement.
Behavioral assays
All behavioral tests were performed during the daytime (9 AM to 5 PM). The mice were subjected to the following experiments: compulsive-like behavior assays (marble burying, nesting and, burrowing), spatial memory test, hole board assay, open field test, elevated plus maze, light-dark and forced swim test. Behavioral analysis was performed using the behavioral tracking system EthoVision XT 15 (Noldus, Netherlands). A detailed description of behavioural tests is provided in the Supplement.
In Vivo Local Field-Potential (LFP) Recordings and Data Analysis:
Hippocampal and mPFC LFP were recorded from control and AD mice using published protocols (
). Briefly, for the electrode implantation, mice were anesthetized as mentioned earlier and positioned on a stereotaxic apparatus and bregma and lambda oriented to be in the same horizontal surface. A Teflon-coated tungsten electrode (Cat No#796500, A-M Systems, USA) was unilaterally implanted into the hippocampal fissure (from bregma, AP -2.0 mm, ML ±1.2 mm, and DV -1.8 mm) and mPFC (from bregma, AP +1.7 mm, ML ±0.3 mm, and DV -2.0 mm) on the right hemisphere for LFP recording with grounding over the cerebellum. The reference electrode was implanted in the frontal lobe. LFP signals were amplified and digitized at a sampling frequency of 1,000 Hz. Data were acquired using an RHS stim/recording system (Intan Technologies, USA). The recording placements for hippocampal and mPFC LFP were verified by post-mortem histology (Nissl-staining) (Supplementary Figure S1).
The LFP data were analyzed offline in MATLAB (R2021a, MathWorks, Natick MA, USA) by the means of built-in and custom-written routines. The Welch periodogram technique was used to determine power spectral density using the pwelch.m function from the Signal Processing Toolbox (50% overlapping, 4 s Hamming windows), and the mean spectral power was calculated from artifact-free one-minute LFP recordings made from each animal. Time-frequency power decomposition was achieved using the spectrogram.m function. The coherence between the hippocampal and mPFC LFP was measured by magnitude-squared coherence using the mscohere.m function.
Immunohistochemistry and Real-time quantitative PCR
A detailed description of immunohistochemistry and real-time quantitative PCR is provided in the Supplement.
Statistics
Results are illustrated as mean ± SEM. Data were evaluated by the student’s t-test, with a significance level achieved when P<0.05, using Sigmaplot software (Systat Software).
RESULTS
AD mice exhibited compulsive-like behavior
As a first step, we aimed at characterizing the compulsive-like behaviors in Aβ1-42 induced AD mice. We tested the AD and control mice in three behavioral assays that measure stereotyped compulsive-like behavior in rodents: marble burying, nestlet shredding, and burrowing tests. In the marble burying test, AD mice buried a significantly greater number of marbles compared to the control group (Figure 1B, C) (Marbles buried, control: 10.50 1.11, AD: 17.50 0.71; p=0.0006, t=5.26, student’s t-test). In the nestlet shredding test, the AD mice demonstrated better performance in shredding and accomplished higher nest quality scores than the control mice (Figure 1D, E) (Nest ratings, control: 2.83 0.60, AD: 4.66 0.21; p=0.02, t=2.87, student’s t-test). Similarly, in the burrowing test, a significantly larger amount of food pellets was burrowed out of the tunnel by AD mice in comparison to the control (Figure 1F, G) (% of burrowed material, control: 12.76 5.94, AD: 88.77 6.96, p<0.0001, t=8.30, student’s t-test). Together, these findings demonstrated and confirmed compulsive-like behaviors in AD mice.
Figure 1(A-G) AD mice exhibited compulsive-like behavior. (A) Illustration of intra-hippocampal injection of amyloid beta 1-42 (B) Representative images of the marble burying test (red arrows for 2/3 buried after test) (C) AD mice displayed a greater number of marbles buried (D) Representative images of the nest building test (E) AD mice built nests of better quality (F) Representative images of the burrowing test (G) AD mice burrowed a higher percentage of food pellets. (H-I) AD mice displayed decreased novelty-seeking behavior (H) Number of head dips (I) time spent in head dipping during the hole board test. (J) AD mice displayed depressive like behavior. AD mice remained immobile for a longer duration. Values represent mean ± SEM. Control n=6, AD n=6; *P < 0.05, **P < 0.01 and ***P < 0.001.
Individual differences in novelty-seeking behavior but not in anxiety response to a new environment can predict nicotine consumption in adolescent C57BL/6 mice.
). The AD mice displayed a significant reduction in the number and duration of head dips (Figure 1H) (number of head dips, control: 21.00 1.26, AD: 9.33 1.20, p<0.0001, t=6.68; time spent dipping (Figure 1I), control: 10.64 0.93, AD: 5.64 0.53, p=0.001, t=4.62, student’s t-test). These findings suggested decreased novelty-seeking behavior in AD mice.
AD mice displayed depressive-like behavior
OCD is frequently accompanied by multiple disorders, with depression being the most common comorbidity (
). To evaluate depression-like symptoms in AD mice, we performed the forced swim test, one of the most widely used behavioral paradigm for the investigation of depressive-like state in rodents. The FST draws on the notion that an animal placed in a container filled with water would struggle to escape at first, but gradually become immobile, which might be read as a symptom of behavioural despair. AD mice spent significantly less time in the tank trying to escape (mobility, climbing, and swimming) (Figure 1J) in comparison with the control group (mobile time, control: 92.33 14.15, AD: 50.83 6.26, p=0.03, t=2.68, student’s t-test). These results indicated depressive-like behavior in AD mice.
Unchanged anxiety levels in AD mice
The light-dark transition test and EPM were conducted to evaluate whether anxiety was comorbid with compulsive-like behaviors. The transition from dark to light (Figure 2A) (control: 7.66 1.25, AD: 8.00 1.46, p=0.86, t=0.17) and the total time spent in the light chamber (Figure 2B) (control: 96.16 24.09, AD: 103.5023.04, p=0.83, t=0.22, student’s t-test) remained similar between the AD and control groups. Similarly, no significant difference in the percentage of open arm entries (Figure 2C) (control: 40.40 3.59, AD: 42.35 3.79; p=0.71, t=0.37), total entries (Figure 2D) (control: 19.50 2.10, AD: 17.66 ; p=0.58, t=0.55), and total distance travelled (Figure 2E) (control: 1171.70 80.34, AD: 985.42 114.27; p=0.21, t=1.33, student’s t-test) were observed in EPM assay. These results indicated that Aβ1-42 induced AD mice do not exhibit anxiety-like behavior.
Figure 2AD mice displayed (A-E) intact anxiety like behavior in (A-B) light-dark, (C-E) elevated plus maze test and (F-H) locomotor activity in open field test. (A) Number of transitions to light compartment (B) Time spent in light compartment. (C) Percentage of open arm entries and (D) Total number of entries (E) Total distance travelled (F) Number of times mice entered the center of the open field. (G) Distance travelled across 5 min time bins in the 60 min (H) Total distance travelled in the open field during 60 min window revealing no significant alterations in the locomotor activity. Values represent mean ± SEM. Control n=6, AD n=6; n.s., not significant.
To rule out the possibility of any potential locomotion-related abnormalities interfering with the behavioral assays, we performed the open field test for a period of 1 hour and found no difference in the distance traveled and the number of entries into the center between the groups (Figure 2F-H) (frequency of entries, control: 47.00 17.94, AD: 61.83 16.02, p=0.55, t=0.61, total distance moved, control: 8186.40 1230.67 cm, AD: 9617.07 987.54 cm, p=0.38, t=0.90, student’s t-test).
Impaired spatial memory in AD mice
The early clinical indication of AD is loss of spatial memory (
). To confirm any memory deficits in AD mice, we performed the object location recognition test. During the familiarization phase, both control and AD mice exhibited similar preference percentages towards both objects (Figure 3A). In the test phase, the control mice were able to recognize the displaced object and showed more preference for the displaced object. On the other hand, AD mice failed to distinguish between the two objects and showed reduced preference for the displaced object (Figure 3B) (control: 57.21 0.47; AD: 47.401.71, p=0.001, t=5.49, student’s t-test). This data suggested that intrahippocampal injections of Aβ1-42 impaired spatial memory.
Figure 3(A-B) AD mice exhibited impaired spatial recognition memory (A) Control and AD mice showed similar preference toward object 1 and object 2 in the familiarization phase (B) AD mice showed significant impairments in remembering the original location of the displaced object. Values represent mean ± SEM. Control n=6, AD n=6; **P < 0.01.
). Thus, understanding functional changes in the hippocampo-cortical neural circuit of AD mice could help to identify potential circuit mechanisms for compulsive behaviors associated with AD. Therefore, we recorded hippocampal LFP to assess any rhythmic alterations that could be present in AD mice. In the power spectral analysis, AD mice demonstrated a significant reduction in the hippocampal theta power (4 – 12 Hz) compared to the control group. However, the gamma (25 – 50 Hz) frequency band in the HPC did not show a statistically significant difference even though the power of gamma was reduced in AD mice (Figure 4D) (Theta power, control: 6.10⨯10-6 9.32⨯10-7, AD: 2.82⨯10-6 4.45⨯10-7; p=0.009, t=3.17; gamma power, control: 1.77⨯10-6 6.01⨯10-7, AD: 7.04⨯10-7 1.25⨯10-7; p=0.11, t=1.73, student’s t-test). The mPFC power spectral analysis of the AD mice showed significant reduction in both theta and gamma frequencies compared to the control group (Figure 4E) (Theta power, control: 9.98⨯10-7 1.45⨯10-7, AD: 5.81⨯10-7 7.05⨯10-8; p=0.02, t=2.58; gamma power, control: 2.66⨯10-7 3.80⨯10-8, AD: 1.04⨯10-7 9.50⨯10-9; p=0.002, t=4.13, student’s t-test). We also examined the coherence between the HPC and mPFC and observed a significantly reduced coherence in the theta band of the AD animals (Figure 4F) (Theta coherence, control: 0.65 0.02, AD: 0.55 0.01; p=0.01, t=3.09; gamma coherence control: 0.16 0.007, AD: 0.21 0.03; p=0.15, t=1.52, student’s t-test). Taken together, these data suggested that the functional interaction between HPC and mPFC is reduced in Aβ1-42 induced AD mice.
Figure 4(A-E) Attenuated theta and gamma band oscillations in the HPC and mPFC of AD mice. (A) Illustrate showing in vivo LFP recordings in the HPC and mPFC. (B) Representative oscillatory traces in the HPC and mPFC of control and AD mice under awake state. (C) Representative power spectrogram of control and AD mice (D-E) The oscillatory power at θ and γ band was reduced in (D) HPC and (E) mPFC of AD mice. (F) Impaired theta coherence between the HPC and mPFC of AD mice. (n = 6 control, n = 6 AD). Values represent mean ± SEM. Control n=6, AD n=6; *P < 0.05, **P < 0.01 and n.s., not significant.
Reduced cholinergic and glutamatergic neurotransmission in the HPC and mPFC of AD mice
The disrupted LFP oscillations found in the HPC and mPFC of AD mice may indicate dysfunction of the hippocampo-cortical neural circuit. Cholinergic deficits are suggested as a significant contributor to the neuropsychiatric aspect of AD (
Attenuation of Compulsive-Like Behavior Through Positive Allosteric Modulation of α4β2 Nicotinic Acetylcholine Receptors in Non-Induced Compulsive-Like Mice.
Frontiers in Behavioral Neuroscience.2017; 10: 244
). This evidence prompted us to examine whether the muscarinic/nicotinic cholinergic and glutamatergic neurotransmission in the HPC and mPFC of AD mice were altered. Using real-time PCR, we examined the expression patterns of the genes for muscarinic (CHRM1, CHRM3, CHRM5), nicotinic (CHRNA4, CHRNA7), PLCβ1, and glutamate (GRIN2B) in the dorsal hippocampus (dHPC), ventral hippocampus (vHPC) and medial prefrontal cortex (mPFC) of control and AD mice. The results revealed a statistically significant reduction in the expression patterns of all the genes except CHRM5 gene in the dHPC and mPFC, and CHRNA4 in vHPC (Figure 5) (dHPC: CHRM1: 0.55 0.04, p=0.002, t=9.54; CHRM3: 0.56 0.07, p=0.009, t=5.95; CHRM5: 0.60 0.13, p=0.06, t=2.89; CHRNA4: 0.61 0.11, p=0.04, t=3.27; CHRNA7: 0.53 0.03, p=0.001, t=11.66; PLCβ1: 0.73 0.04, p=0.009, t=5.96; GRIN2B: 0.50 0.12, p=0.02, t=4.08; vHPC: CHRM1: 0.47 0.07, p=0.006, t=6.76; CHRM3: 0.45 0.06, p=0.004, t=7.75; CHRM5: 0.35 0.07, p=0.03, t=8.73; CHRNA4: 0.68 0.24, p=0.29, t=1.27; CHRNA7: 0.47 0.07, p=0.006, t=6.91; PLCβ1: 0.60 0.06, p=0.007, t=6.40; GRIN2B: 0.58 0.03, p=0.001, t=11.66; mPFC: CHRM1: 0.71 0.08, p=0.04, t=3.36; CHRM3: 0.13 0.03, p=0.0002, t=23.48; CHRM5: 0.90 0.13, p=0.53, t=0.69; CHRNA4: 0.39 0.06, p=0.002, t=9.27; CHRNA7: 0.40 0.04, p=0.001, t=12.17; PLCβ1: 0.75 0.07, p=0.04, t=3.39; GRIN2B: 0.69 0.05, p=0.01, t=5.54, student’s t-test). Overall, our molecular experiments revealed that acetylcholine and glutamatergic signalling pathways are altered in the HPC-mPFC neuronal circuit in Aβ1-42 induced AD mice.
Figure 5(A-M) Reduced acetylcholine and glutamatergic neurotransmission in the HPC and mPFC of AD mice (A) Pictorial representation of the brain regions used for the quantitative PCR analysis. AD mice showed downregulation of muscarinic (M1, M3, M5), nicotinic (α4, α7), PLCβ1, and glutamate (NR2B) receptor gene expression in the (B-E) dHPC, (F-I) vHPC and (J-M) mPFC. Values represent mean ± SEM. Control n=5, AD n=4; *P < 0.05, **P < 0.01, ***P < 0.001 and n.s., not significant.
The mice were sacrificed on the tenth day after the last injection to assess amyloid deposition. Hippocampus sections were immunostained for 6E10 positive plaques. Confocal imaging of the immunostained sections revealed clear and widespread depositions of amyloid beta plaques in the hippocampus of AD mouse brain (Figure 6). These results suggest that intra-hippocampal injections of oligomeric assemblies of Aβ1-42 were able to induce amyloid-beta deposition.
Figure 6(A-D) Immunostaining of amyloid-β protein deposition in the mouse hippocampus 10 days after the injection of Aβ1-42. Representative HPC sections of Aβ1-42 or vehicle injected mice stained by 6E10 antibody (A-B) Lower magnification (10x) Scale bar, 250 μm and (C-D) higher magnification (63x) Scale bar, 25 μm.
). The current study revealed that Aβ1-42 induced AD mice exhibit a compulsive-like phenotype coupled with a depressive-like state, and also show impaired spatial memory. Additionally, deficits in novelty-seeking, but not anxiogenic-like, behaviors were observed. These behavioral outcomes were accompanied by impaired rhythmic oscillations along with dowregulated expression of acetylcholine and glutamatergic neurotransmission in the HPC-mPFC neuronal circuit. This study is the first to reveal a potential link between the dysfunction of the HPC-mPFC neuronal circuit and compulsive behavior in an AD mouse model.
AD pathology is accompanied by multiple neurocognitive symptoms during the course of disease progression, and is also known to share similarities with OCD pathology (
Studies for Improving a Rat Model of Alzheimer’s Disease: Icv Administration of Well-Characterized β-Amyloid 1-42 Oligomers Induce Dysfunction in Spatial Memory [no. 11].
), are suggested to be responsible for compulsive-like behaviors . By HPC specific injection of β-amyloid, we demonstrated that compulsive behaviors are linked to the pathology of AD. Recently, there has been evidence of compulsive behavior in multiple animal models of AD (
). In accordance with this, the current study shows an increase in marble-burying, nest-building, and burrowing behaviors. These findings are suggestive of the use of compulsive behaviors in AD mice as an additional tool for translational assessments of neurocognitive behaviors. It might also be useful in assessing potential risk factors for the disease. Moreover, we noticed reduced novelty-seeking behavior in AD mice. It is known that novelty-seeking behavior is impaired in individuals with OCD (
). Although anxiety is comorbid with OCD patients, our current AD model shows intact anxiety behaviors. Similarly, AD mice show no difference in locomotion and entries into the center in the open field experiment. These observations exclude the possibility of the impact of anxiety and hyperactivity on the compulsive behaviors exhibited by Aβ1-42 induced AD mice. Taken together, our behavioral results suggest that AD and OCD status share some overlapping cognitive symptoms that are consistent with recent clinical observations (
). For instance, functional connectivity deficits in cortico-striatal brain networks have been demonstrated to be driven by soluble Aβ neurotoxicity in a mature transgenic mice model of amyloidosis (
). Furthermore, in APP/PS1 mice that exhibit compulsive-like behavior as early as 3-5 months of age, structural and functional alterations to cortico-striatal synchronization have been reported (
) in the pathology of OCD. A study using SPRED2 knockout mice revealed that dysfunction of the thalamo-amygdala circuit also contributes to the pathogenesis of OCD (
), in the pathophysiology of OCD. A functional connection exists between the HPC and mPFC and their communication is reflected in oscillatory coherence (
). It has been suggested that disturbances in the oscillatory interaction between HPC and mPFC contribute to cognitive deficits in neuropsychiatric and neurodegenerative disorders (
). However, the link between the alterations of oscillatory interactions within the hippocampo-cortical circuit and compulsive behavior seen in AD still remain unclear. In the present study, we noticed a considerable decrease in the power of theta and gamma band frequencies in the HPC and mPFC of AD mice under awake condition. Both the regions exhibited a statistically significant difference in oscillation power, with the exception of hippocampal gamma oscillation. Notably, we observed impairment in the coherence at the theta frequency in the HPC-mPFC neuronal circuit. A reorganization of hippocampal and cortical networks is well documented in AD patients and animal models. This reorganization is triggered by an imbalance between excitation and inhibition, resulting in impaired network activity (
). Taken together, the disrupted LFP oscillations found in the HPC and mPFC and the impaired theta coherence between these regions in AD mice may indicate dysfunction of the hippocampo-cortical circuit.
Accumulating evidence suggests that impairment of neurotransmitter systems could contribute to the dysfunction of neuronal networks and lead to an imbalance of inhibition/excitation, which profoundly affects cognition. AD-associated dementia is known to be associated with the dysregulation of cholinergic (
Increased medial thalamic choline found in pediatric patients with obsessive-compulsive disorder versus major depression or healthy control subjects: a magnetic resonance spectroscopy study.
). Additionally, nicotine augmentation has been shown to improve clinical symptoms of OCD in recent studies, where patients with treatment-resistant OCD exhibited partial or no response to multiple selective serotonin reuptake inhibitors (SSRIs) (
). The gene GRIN2B that codes for the NR2B subunit of glutamate receptors in specific, is highly expressed within the striatum and PFC, and has been shown to be associated with predisposition to OCD (
Attenuation of Compulsive-Like Behavior Through Positive Allosteric Modulation of α4β2 Nicotinic Acetylcholine Receptors in Non-Induced Compulsive-Like Mice.
Frontiers in Behavioral Neuroscience.2017; 10: 244
). In light of this, the present study targeted assessing the possible changes in the expression patterns of muscarinic acetylcholine receptors (M1, M3, M5), nicotinic acetylcholine receptors (α4, α7), and glutamate (NR2B) receptors in the dorsal and ventral hippocampus as well as mPFC of AD mice. We also assessed the expression of PLCβ1, an important enzyme involved in the acetylcholine signaling pathway. Interestingly, regulation of theta oscillations by the cholinergic-PLC signaling pathway plays a major role in hippocampal functions and diverse behaviors (
). Our molecular experiments clearly revealed a downregulation in the expression patterns of both cholinergic and glutamate transmission receptors in the HPC and mPFC of AD mice compared to the control mice. These results explain the impairment in the coherence at theta frequency in the HPC-mPFC neuronal circuit in AD mice. Altogether, these results indicate that therapies involving muscarinic and nicotinic acetylcholine receptors could be an alternative approach for attenuating compulsive-like behaviors associated with AD, thereby posing important translational potential. Future studies are expected to establish neural mechanisms underlying how the interaction of different neurotransmitters affect compulsive behaviours in the AD state. For instance, the implication of the brain serotonin system in OCD has been a leading hypothesis in the recent decades, primarily attributable to the relatively high effectiveness of the treatment of SSRIs (
Functional connectivity of the raphe nucleus as a predictor of the response to selective serotonin reuptake inhibitors in obsessive-compulsive disorder [no. 12].
). Although the studies are limited, there is convincing evidence of abnormalities in central norepinephrine (NE) function in patients with compulsive disorders (
Interestingly, growing evidence suggests that neuro-inflammation could drive the pathogenic process in AD. The analysis of Alzheimer's patients with mild cognitive impairment further supports early and significant involvement of inflammation in disease pathogenesis (
). Specifically in AD, it has been shown that there is an elevated expression of inflammatory mediators in the vicinity of Aβ peptide deposits and neurofibrillary tangles, suggesting the relationship between neuroinflammation and neurodegeneration (
). Additional studies are required to define if there is a relationship between neuroinflammation in AD and compulsive behaviors.
In summary, our findings demonstrate perturbations to the physiological and molecular interactions between HPC and mPFC in the Aβ1-42 induced mouse model of AD. It further provides a strong impetus for exploring the potential role of other brain regions outside the cortico-striatal circuit in AD-associated compulsive-like behaviour. Future studies using behavioral state-dependent and targeted circuit manipulations are required to assess how the hippocampo-medial prefrontal cortex neuronal circuit dysfunction can play a role in compulsive-like behaviors during AD condition.
ACKNOWLEDGMENTS AND DISCLOSURE
This work was supported by Science and Engineering Research Board (SERB), India (No. CRG/2020/004205) and Ramalingaswami Re-entry Fellowship, Department of Biotechnology, India (No. BT/RLF/Re-863 entry/49/2018) to GG. ABS was supported by the Innovation in Science Pursuit for Inspired Research (INSPIRE) fellowship, Department of Science and Technology, India (IF190783). SFM was supported by Dr. TMA Pai Ph.D. scholarship, Manipal Academy of Higher Education (MAHE), Manipal, India. The authors thank Manipal School of Life Sciences, Manipal Academy of Higher Education (MAHE), and Manipal for the infrastructure and support. The authors report no biomedical financial interests or potential conflicts of interest.
Theta Oscillations in the Medial Prefrontal Cortex Are Modulated by Spatial Working Memory and Synchronize with the Hippocampus through Its Ventral Subregion.
Individual differences in novelty-seeking behavior but not in anxiety response to a new environment can predict nicotine consumption in adolescent C57BL/6 mice.
Attenuation of Compulsive-Like Behavior Through Positive Allosteric Modulation of α4β2 Nicotinic Acetylcholine Receptors in Non-Induced Compulsive-Like Mice.
Frontiers in Behavioral Neuroscience.2017; 10: 244
Studies for Improving a Rat Model of Alzheimer’s Disease: Icv Administration of Well-Characterized β-Amyloid 1-42 Oligomers Induce Dysfunction in Spatial Memory [no. 11].
Increased medial thalamic choline found in pediatric patients with obsessive-compulsive disorder versus major depression or healthy control subjects: a magnetic resonance spectroscopy study.
Functional connectivity of the raphe nucleus as a predictor of the response to selective serotonin reuptake inhibitors in obsessive-compulsive disorder [no. 12].
G.G., A.B.S, S.K. and S.F.M conceived the project and designed all experiments. A.B.S. performed all behavioral, electrophysiological, and immunohistochemistry experiments, S.F.M performed molecular experiments and analysis, S.K. performed behavioral and electrophysiological analysis. G.G., A.B.S, S.K., and S.F.M wrote the manuscript.
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