Effects of Substantia Nigra pars compacta lesion on the behavioral sequencing in the 6-OHDA model of Parkinson’s disease

Sex-dependent behavioral effects of chronic nicotine during adolescence evaluated in young adult rats tested in Hole-Board

As one of the leading causes of death and serious illnesses, tobacco smoking remains a significant issue in modern societies. Many individuals smoke during adolescence, a trend that has been exacerbated by the prevalence of vaping among young people. In this context, studying the behavioral effects induced by nicotine administration in male and female rats, during the adolescent period, assumes great importance because it can help to better understand the dynamics underlying tobacco use in the two sexes. For this purpose, we employed 4 groups of rats, 2 male and 2 female groups, chronically treated with saline or nicotine 3 mg/kg i.p. for 30 days, spanning from postnatal day 30 to postnatal day 60. Utilizing quantitative analyses and T-pattern detection and analysis, our findings revealed a complex and multifaceted behavioral reorganization in adolescent rats subjected to chronic nicotine administration. Specifically, we observed an increase of anxiety in males and a reduction in females. The distinctive structural changes, induced by chronic nicotine in both sexes, have significant implications, from a translational perspective, for studies on nicotine dependence disorders.

The misfolding mystery: α-synuclein and the pathogenesis of Parkinson's disease

Neurodegenerative diseases such as Parkinson's disease (PD) are characterized by the death of neurons in specific areas of the brain. One of the proteins that is involved in the pathogenesis of PD is α-synuclein (α-syn). α-Syn is a normal protein that is found in all neurons, but in PD, it misfolds and aggregates into toxic fibrils. These fibrils can then coalesce into pathological inclusions, such as Lewy bodies and Lewy neurites. The pathogenic pathway of PD is thought to involve a number of steps, including misfolding and aggregation of α-syn, mitochondrial dysfunction, protein clearance impairment, neuroinflammation and oxidative stress. A deeper insight into the structure of α-syn and its fibrils could aid in understanding the disease's etiology. The prion-like nature of α-syn is also an important area of research. Prions are misfolded proteins that can spread from cell to cell, causing other proteins to misfold as well. It is possible that α-syn may behave in a similar way, spreading from cell to cell and causing a cascade of misfolding and aggregation. Various post-translational alterations have also been observed to play a role in the pathogenesis of PD. These alterations can involve a variety of nuclear and extranuclear activities, and they can lead to the misfolding and aggregation of α-syn. A better understanding of the pathogenic pathway of PD could lead to the development of new therapies for the treatment of this disease.

State-dependent alteration of respiration in a rat model of Parkinson's disease

Parkinson's disease (PD) is the second most frequent neurodegenerative disorder. Besides major deficits in motor coordination, patients may also display sensory and cognitive impairments, which are often overlooked despite being inherently part of the PD symptomatology. Amongst those symptoms, respiration, a key mechanism involved in the regulation of multiple physiological and neuronal processes, appears to be altered. Importantly, breathing patterns are highly correlated with the animal's behavioral states. This raises the question of the potential impact of behavioral state on respiration deficits in PD. To answer this question, we first characterized the respiratory parameters in a neurotoxin-induced rat model of PD (6-OHDA) across three different vigilance states: sleep, quiet waking and exploration. We noted a significantly higher respiratory frequency in 6-OHDA rats during quiet waking compared to Sham rats. A higher respiratory amplitude was also observed in 6-OHDA rats during both quiet waking and exploration. No effect of the treatment was noted during sleep. Given the relation between respiration and olfaction and the presence of olfactory deficits in PD patients, we then investigated the odor-evoked sniffing response in PD rats, using an odor habituation/cross-habituation paradigm. No substantial differences were observed in olfactory abilities between the two groups, as assessed through sniffing frequency. These results corroborate the hypothesis that respiratory impairments in 6-OHDA rats are vigilance-dependent. Our results also shed light on the importance of considering the behavioral state as an impacting factor when analyzing respiration.

Gastrointestinal dysmotility in rodent models of Parkinson's disease

Multiple studies describe prodromal, nonmotor dysfunctions that affect the quality of life of patients who subsequently develop Parkinson's disease (PD). These prodromal dysfunctions comprise a wide array of autonomic issues, including severe gastrointestinal (GI) motility disorders such as dysphagia, delayed gastric emptying, and chronic constipation. Indeed, strong evidence from studies in humans and animal models suggests that the GI tract and its neural, mainly vagal, connection to the central nervous system (CNS) could have a major role in the etiology of PD. In fact, misfolded α-synuclein aggregates that form Lewy bodies and neurites, i.e., the histological hallmarks of PD, are detected in the enteric nervous system (ENS) before clinical diagnosis of PD. The aim of the present review is to provide novel insights into the pathogenesis of GI dysmotility in PD, focusing our attention on functional, neurochemical, and molecular alterations in animal models.

The neurobiological effects of senescence on dopaminergic system: A comprehensive review

Over time, the body undergoes a natural, multifactorial, and ongoing process named senescence, which induces changes at the molecular, cellular, and micro-anatomical levels in many body systems. The brain, being a highly complex organ, is particularly affected by this process, potentially impairing its numerous functions. The brain relies on chemical messengers known as neurotransmitters to function properly, with dopamine being one of the most crucial. This catecholamine is responsible for a broad range of critical roles in the central nervous system, including movement, learning, cognition, motivation, emotion, reward, hormonal release, memory consolidation, visual performance, sexual drive, modulation of circadian rhythms, and brain development. In the present review, we thoroughly examine the impact of senescence on the dopaminergic system, with a primary focus on the classic delimitations of the dopaminergic nuclei from A8 to A17. We provide in-depth information about their anatomy and function, particularly addressing how senescence affects each of these nuclei.

Role of the basal ganglia in innate and learned behavioural sequences

Integrating individual actions into coherent, organised behavioural units, a process called chunking, is a fundamental, evolutionarily conserved process that renders actions automatic. In vertebrates, evidence points to the basal ganglia - a complex network believed to be involved in action selection - as a key component of action sequence encoding, although the underlying mechanisms are only just beginning to be understood. Central pattern generators control many innate automatic behavioural sequences that form some of the most basic behaviours in an animal's repertoire, and in vertebrates, brainstem and spinal pattern generators are under the control of higher order structures such as the basal ganglia. Evidence suggests that the basal ganglia play a crucial role in the concatenation of simpler behaviours into more complex chunks, in the context of innate behavioural sequences such as chain grooming in rats, as well as sequences in which innate capabilities and learning interact such as birdsong, and sequences that are learned from scratch, such as lever press sequences in operant behaviour. It has been proposed that the role of the striatum, the largest input structure of the basal ganglia, might lie in selecting and allowing the relevant central pattern generators to gain access to the motor system in the correct order, while inhibiting other behaviours. As behaviours become more complex and flexible, the pattern generators seem to become more dependent on descending signals. Indeed, during learning, the striatum itself may adopt the functional characteristics of a higher order pattern generator, facilitated at the microcircuit level by striatal neuropeptides.

The hole-board apparatus in the study of anxiety

Anxiety disorders pose a significant challenge in contemporary society, and their impact in terms of social and economic burden is overwhelming. Behavioral research conducted on animal subjects is crucial for comprehending these disorders and, from a translational standpoint, for introducing innovative therapeutic approaches. In this context, the Hole-Board apparatus has emerged as a widely utilized test for studying anxiety-related behaviors in rodents. Although a substantial body of literature underscores the utility and reliability of the Hole-Board in anxiety research, recent decades have witnessed a range of studies that have led to uncertainties and misinterpretations regarding the validity of this behavioral assay. The objective of this review is twofold: firstly, to underscore the utility and reliability of the Hole-Board assay, and concurrently, to examine the underlying factors contributing to potential misconceptions surrounding its utilization in the study of anxiety and anxiety-related behaviors. We will present results from both conventional quantitative analyses and multivariate approaches, while referencing a comprehensive collection of studies conducted using the Hole-Board.

Restraint Stress Exacerbates Apoptosis in a 6-OHDA Animal Model of Parkinson Disease

Activation of the apoptotic pathway has been associated with promoting neuronal cell death in the pathophysiology of Parkinson disease (PD). Nonetheless, the mechanisms by which it may occur remain unclear. It has been suggested that stress-induced oxidation and potential apoptosis may play a major role in the progression of PD. Thus, in this study, we aimed to investigate the effect of subchronic restraint stress on striatal dopaminergic activity, iron, p53, caspase-3, and plasmatic acetylcholinesterase (AChE) levels in male Wistar rat model of PD induced by administration of 6-hydroxydopamine (6-OHDA) in the medial forebrain bundle (MFB). The obtained results showed that restraint stress exacerbates motor coordination deficits and anxiety in animals treated with 6-OHDA in comparison to animals receiving saline, and it had no effect on object recognition memory. On another hand, 6-OHDA decreased dopamine (DA) levels, increased iron accumulation, and induced overexpression of the pro-apoptotic factors caspase-3, p53, and AChE. More interestingly, post-lesion restraint stress exacerbated the expression of caspase-3 and AChE without affecting p53 expression. These findings suggest that subchronic stress may accentuate apoptosis and may contribute to DA neuronal loss in the striatal regions and possibly exacerbate the progression of PD.

Structural analyses in the study of behavior: From rodents to non-human primates

The term "structure" indicates a set of components that, in relation to each other, shape an organic complex. Such a complex takes on essential connotations of functionally unitary entity resulting from the mutual relationships of its constituent elements. In a broader sense, we can use the word "structure" to define the set of relationships among the elements of an emergent system that is not determined by the mere algebraic sum of these elements, but by the interdependence relationships of these components from which the function of the entire structure itself derives. The behavior of an integrated living being can be described in structural terms via an ethogram, defined as an itemized list of behavioral units. Akin to an architectural structure, a behavioral structure arises from the reciprocal relationships that the individual units of behavior establish. Like an architectural structure, the function of the resulting behaving complex emerges from the relationships of the parts. Hence, studying behavior in its wholeness necessitates not only the identification of its constitutive units in their autarchic individuality, but also, and importantly, some understanding of their relationships. This paper aimed to critically review different methods to study behavior in structural terms. First, we emphasized the utilization of T-pattern analysis, i.e., one of the most effective and reliable tools to provide structural information on behavior. Second, we discussed the application of other methodological approaches that are based on the analysis of transition matrices, such as hierarchical clustering, stochastic analyses, and adjusted residuals. Unlike T-pattern analysis, these methods allow researchers to explore behavioral structure beyond its temporal characteristics and through other relational constraints. After an overview of how these methods are used in the study of animal behavior, from rodents to non-human primates, we discussed the specificities, advantages and challenges of each approach. This paper could represent a useful background for all scientists who intend to study behavior both quantitatively and structurally, that is in terms of the reciprocal relationships that the various units of a given behavioral repertoire normally weave together.

A Systematic Review of the Use of T-Pattern and T-String Analysis (TPA) With Theme: An Analysis Using Mixed Methods and Data Mining Techniques

In recent years, research interest in human and non-human behavioral analysis has increased significantly. One key element in the resulting studies is the use of software that facilitates comparative analysis of behavioral patterns, such as using T-Pattern and T-String analysis -TPA- with THEME. Furthermore, all these studies use mixed methods research. Results from these studies have indicated a certain amount of similarity between the biological, temporal, and spatial patterns of human social interactions and the interactions between the contents of their constituent cells. TPA has become an important, widely-used technique in applied behavioral science research. The objectives of the current review were: (1) To identify the results of research over the last 4 years related to the concepts of T-Pattern, TPA, and THEME, since it is in this period in which more publications on these topics have been detected (2) To examine the key concepts and areas in the selected articles with respect to those concepts, applying data and text mining techniques. The results indicate that, over the last 4 years, 20% of the studies were laboratory focused with non-humans, 18% were in sports environments, 9% were in psychological therapy environments and 9% were in natural human contexts. There were also indications that TPA is beginning to be used in workplace environments, which is a very promising setting for future research in this area.

Keeping Track of the Genealogy of Heterozygotes Using Epigenetic Reference Codes and Breeding Tables

Studying neurobehavioral consequences of the hypofunctional dopamine transporter (DAT) across several generations entails the need to monitor allelic transmission to offspring, taking into account both maternal and paternal inheritance. Since each type of heterozygote expresses differential phenotypes, based on lineage of inheritance for wild and mutated alleles (from male or female ancestors), it is important to track transgenerational epigenetic effects. We deemed it essential to assign specific abbreviations identifying their characteristics. Therefore, we devised a Mendelian-inspired table to keep track of these. Starting from two progenitors (WT and KO) we named resulting heterozygous progenies MAT and PAT to differentiate them based on inheritance of the wild allele (from the mother or father). Tracing subsequent generations, similar logic has been followed: if coupling HET dams with KO males, initials “M” [(grand)maternal] and “P” [(grand)paternal] are kept, but “AT” is turned into “IX” (MIX and PIX), while if breeding HETs with WTs, “M” is changed to “W” resembling an upside down “M” and “P” to “S” for “sperm” (WAT and SOT). To underline the development within “hyperdopaminergic-uterus” a central letter “U” is added (MUX, PUX, and QULL), while a Greek initial (μAT, μIX, and νIX) underlines the uterine-worsened origin of the allele. In HET × HET breeding (GIX and DIX), the mutated allele can be inherited from both sides of the genealogical line. However, when the mother is MAT, wild and mutated alleles encounter for the first time, causing putative anomalies in the progeny. Replacing dam with a second-generation female (MIX and MUX) may mitigate epigenetic effects on third-generation offspring; therefore suffixes (“-f,” “-fu,” “-ϕ,” and “-ϕu”) emphasize that subsequent-generation dams imply that the alleles already encountered in HET (rather than WT) grand-dams.

Dopaminergic modulation of the circadian activity and sociability: Dissecting parental inheritance versus maternal styles as determinants of epigenetic influence

In mammalians, social life and circadian rhythms find their neurobiological basis in a network that includes the dopaminergic system. The malfunctioning of dopamine pathways can lead to various disorders such as Attention-Deficit/Hyperactivity and Obsessive/compulsive disorders. A useful research approach is to exploit animal models that carry a functional silencing of SLC6A3 gene, encoding the dopamine transporter (DAT). Hyperactivity, working memory deficits, and asocial tendencies are core features in truncated-DAT rats, for example. We investigated how inheritance and maternal caring style influence circadian rhythms and social behaviours in DAT heterozygous (HET) rats, belonging to four groups: Mat-P, Mat-M, Mix-P, and Mix-M (Mat label stands for care from wild-type dam, Mix label stands for care by heterozygous dam; M label stands for maternal wild-DAT and P label stands for paternal wild-DAT). In Experiment 1, we monitored 24/7 the spontaneous locomotor activity of peri-adolescent subjects. Hyperactivity occurred only in P-asset subjects (with maternal-origin truncated-DAT allele) at specific bins of the day. In Experiment 2, we observed social interactions of the same rats. Mix-M subjects (raised by HET dams and/or inheriting the wild-DAT allele from mothers) tend to interact with all rats; Mat-P (cared by WT dams and/or inheriting the truncated-DAT allele from mothers) seem to be ignored, when acting as stimulus subjects. Overall, results confirm complex modulations for circadian cycle and social life: flexible DAT expression in HET subjects depends on epigenetic combinations of parental inheritance and early experiential factors. Once confirmed, these data could shed light on trans-generational contributions to dopaminergic-related disorders.

Blocking NK1 receptors disrupts the sequential and temporal organization of chain grooming in rats

The basal ganglia are a group of sub-cortical structures believed to play a critical role in action selection and sequencing. The striatum is the largest input structure of the basal ganglia and contains the neuropeptide substance P in abundance. Recent computational work has suggested that substance P could play a critical role in action sequence performance and acquisition, but this has not been tested experimentally before. The aim of the present study was to test how blocking substance P's main NK1-type receptors affected the sequential and temporal organization of spontaneous behavioral patterns. We did this in rats by focusing on the grooming chain, an innate and highly stereotyped ordered sequence. We performed an open field experiment in which the NK1 receptor antagonist L-733,060 was injected intraperitoneally in rats at two doses (2 and 4 mg/kg/ml), in a within-subject counterbalanced design. We used first order transition probabilities, Variable Length Markov Models, entropy metrics and T-pattern analysis to evaluate the effects of L-733,060 on sequential and temporal aspects of spontaneously ordered behavioral sequences. Our results suggest that blocking NK1 receptors made the transitions between the grooming chain elements significantly more variable, the transition structure of the grooming bouts simpler, and it increased the probability of transitioning from active to inactive states. Overall, this suggest that blocking substance P receptors led to a general break down in the fluency of spontaneous behavioral sequences, suggesting that substance P could be playing a key role in the implementation of sequential patterns.

Effects of Different Anxiety Levels on the Behavioral Patternings Investigated through T-pattern Analysis in Wistar Rats Tested in the Hole-Board Apparatus

The Hole-Board is an ethologically based tool for investigating the anxiety-related behavior of rats following manipulation of the central anxiety level. The present paper aims to assess behavioral patterning following pharmacological manipulation of emotional assets in Wistar rats tested in this experimental apparatus. For this purpose, the behavior of three groups of rats injected with saline, diazepam or FG7142 was evaluated using conventional quantitative and multivariate T-pattern analyses. The results demonstrate that quantitative analyses of individual components of the behavior, disjointed from the comprehensive behavioral structure, are of narrow utility in the understanding of the subject’s emotional condition. Among the components of the behavioral repertoire in rodents tested in the Hole-Board, Edge-Sniff and Head-Dip represent the most significant ones to rate anxiety level. They are characterized by a strong bivariate relationship and are also firmly part of the behavioral architecture, as revealed by the T-pattern analysis (TPA), a multivariate technique able to detect significant relationships among behavioral events over time. Edge-Sniff → Head-Dip sequences, in particular, are greatly influenced by the level of anxiety: barely detectable in control animals, they completely disappear in subjects with a reduced level of anxiety and are present in almost 25% of the total of T-patterns detected in subjects whose anxiety level increased.

Possible Contribution of T-pattern Detection and Analysis to the Study of the Behavioral Correlates of Afferent Inhibition

A pivotal tenet in modern behavioral sciences is that the study of behavior, in its most intimate structure, necessarily deals with time and, for this reason, behavioral dynamics are not intuitively perceivable and/or detectable (Eibl-Eibesfeldt, 1970). In reality, the possibility to describe a given behavior in terms of its structural/temporal features makes available new and detailed information otherwise unavailable. The aim of the present paper is to discuss the possible application of T-pattern detection and analysis, i.e., a multivariate approach specifically developed to describe the temporal structure of behavior, to the study of an important and still scantly investigated issue, namely the detection of behavioral correlates of the neurophysiological phenomenon known as afferent inhibition.

Stereotyped, automatized and habitual behaviours: are they similar constructs under the control of the same cerebral areas?

Comprehensive knowledge about higher executive functions of motor control has been covered in the last decades. Critical goals have been targeted through many different technological approaches. An abundant flow of new results greatly progressed our ability to respond at better-posited answers to look more than ever at the challenging neural system functioning. Behaviour is the observable result of the invisible, as complex cerebral functioning. Many pathological states are approached after symptomatology categorisation of behavioural impairments is achieved. Motor, non-motor and psychiatric signs are greatly shared by many neurological/psychiatric disorders. Together with the cerebral cortex, the basal ganglia contribute to the expression of behaviour promoting the correct action schemas and the selection of appropriate sub-goals based on the evaluation of action outcomes. The present review focus on the basic classification of higher motor control functioning, taking into account the recent advances in basal ganglia structural knowledge and the computational model of basal ganglia functioning. We discuss about the basal ganglia capability in executing ordered motor patterns in which any single movement is linked to each other into an action, and many actions are ordered into each other, giving them a syntactic value to the final behaviour. The stereotypic, automatized and habitual behaviour's constructs and controls are the expression of successive stages of rule internalization and categorisation aimed in producing the perfect spatial-temporal control of motor command.

Recurring sequences of multimodal non-verbal and verbal communication during a human psycho-social stress test: A temporal pattern analysis

BACKGROUND

The Trier Social Stress Test (TSST) is a widely used protocol to study human psycho-social stress responses. Quantitative reports of non-verbal behaviors have been carried out by means of the Ethological Coding System for Interviews (ECSI). However, no data have described whether and how non-verbal and verbal behaviors take part in the composition of multimodal sequences of communication during the test.

METHOD

Five non-verbal ECSI categories and four verbal behaviors related with communication were included in the Ethogram. A focal sampling was employed to ensure a high temporal resolution of the behavioral annotation. T-Pattern Analysis was employed to detect statistically-grounded behavioral sequences.

RESULTS

As a first step, frequency, overall duration and mean time length were reported for each component of the Ethogram. Besides, T-Pattern Analysis revealed that communication during TSST is organized according to a complex temporal patterning. We found 51 different sequences (T-patterns): 8 T-patterns included exclusively non-verbal behaviors; 17 T-patterns included verbal behaviors and 26 T-patterns encompassed mixed non-verbal and verbal behaviors. T-patterns were discussed depending on their putative functional meaning since non-verbal behaviors almost did not overlap within patterns.

CONCLUSIONS

The implementation of an Ethogram including non-verbal and verbal components highlights the multimodal human communication in TSST. T-Pattern Analysis unveils the real-time interplay among these components. In this study results are discussed according to Jakobson's six constitutive factors of communication.

Comparison between Tail Suspension Swing Test and Standard Rotation Test in Revealing Early Motor Behavioral Changes and Neurodegeneration in 6-OHDA Hemiparkinsonian Rats

The unilateral 6-hydroxydopamine (6-OHDA) model of Parkinson’s disease (PD) is one of the most commonly used in rodents. The anatomical, metabolic, and behavioral changes that occur after severe and stable 6-OHDA lesions have been extensively studied. Here, we investigated whether early motor behavioral deficits can be observed in the first week after the injection of 6-OHDA into the right substantia nigra pars compacta (SNc), and if they were indicative of the severity of the dopaminergic (DAergic) lesion in the SNc and the striatum at different time-points (day 1, 3, 5, 7, 14, 21). With this aim, we used our newly modified tail suspension swing test (TSST), the standard rotation test (RT), and immunohistochemical staining for tyrosine hydroxylase (TH). The TSST, but not the standard RT, revealed a spontaneous motor bias for the 6-OHDA-lesioned rats from the day 1 post-surgery. Both tests detected the motor asymmetry induced by (single and repeated) apomorphine (APO) challenges that correlated, in the first week, with the DAergic neuronal degeneration. The described TSST is fast and easy to perform, and in the drug-free condition is useful for the functional assessment of early motor asymmetry appearing after the 6-OHDA-lesion in the SNc, without the confounding effect of APO challenges.

Effects of chronic nicotine on the temporal structure of anxiety-related behavior in rats tested in hole-board

The present study aimed to assess the behavioral effects of chronic treatments of different doses of nicotine by using both quantitative and multivariate T-pattern analysis (TPA), which can reveal hidden behavioral structures, in Sprague-Dawley rats tested in the hole-board apparatus. To this purpose, nicotine ditartrate was administered at the doses of 0.1, 0.5 and 1 mg/kg i.p., three times per day, for 14 consecutive days. As to quantitative evaluations, we observed significant reductions in the mean durations and mean frequencies of walking, climbing, immobile-sniffing and rearing in comparison to control. A significant reduction of edge-sniff and head-dip mean frequencies was also detected for all the doses tested. TPA revealed an increase in the number and the mean length of different T-patterns induced by the three doses of nicotine. On the other hand, a significant reduction of the mean occurrences of T-patterns was revealed. Overall, our results obtained by using both quantitative and T-pattern analyses indicate that chronic nicotine induces an anxiety condition characterized by a behavioral re-organization orbiting around the two main components of hole exploration, that is, head-dip and edge-sniff. A better understanding of the link between nicotine and anxiety might help to find new therapies for smoking cessation.

T-patterns in the study of movement and behavioral disorders

Aim of the present review is to offer an outline of the application of T-pattern analysis (TPA) in the study of neurological disorders characterized by anomalies of movement and, more in general, of behavior. TPA is a multivariate technique to detect real time patterns of behavior on the basis of statistically significant constraints among the events in sequence. TPA is particularly suitable to analyse the structure of behavior. The application of TPA to study movement and behavioral disorders is able to offer, with a high level of detail, hidden characteristics of behavior otherwise impossible to detect. For its intrinsic features, TPA is completely different not only from quantitative evaluations of behavior such as assessments of frequencies, durations, percent distributions etc. of individual behavioral components, but also from the largest extent of multivariate approaches based, for instance, on the analysis of transition matrices. Various applications of TPA in the study of behavior in human patients and in animal models of neurological disorders are discussed. TPA is a suitable tool to study the movement and behavioral disorders. This review represents a useful background for researchers, therapists, physicians etc. who intend to use this technique.

A secret that underlies Parkinson's disease: The damaging cycle

Parkinson's disease (PD) is a movement disorder, and its common characteristics include the loss of dopaminergic neurons and the accumulation of a special type of cytoplasmic inclusions called Lewy bodies in the substantia nigra pars compacta, which are more prevalent in the elderly. However, the pathophysiology of PD is still elusive. In this review, we summarized five common factors involved in PD, namely, (i) oxidative stress, (ii) mitochondrial dysfunction, (iii) inflammation, (iv) abnormal α-synuclein, and (v) endogenous neurotoxins, and proposed a hypothesis involving a damaging cycle. Oxidative stress-triggered aldehydes react with biogenic amines to produce endogenous neurotoxins. They cause mitochondrial dysfunction and the formation of inflammasomes, which induce the activation of neuroglial cells and the infiltration of T lymphocytes. The synergistic effect of these processes fosters chronic inflammation and α-synuclein aggregation and further exacerbates the impact of oxidative stress to establish a damaging cycle that eventually results in the degeneration of dopaminergic neurons. This damaging cycle provides an explanation of progressive neuronal death during the pathogenesis of PD and provides new potential targets beneficial for developing new drugs and approaches for clinical neuroprotection.