Parkinson’s disease is a major neurocognitive disorder categorized by motor symptoms such as tremors, stiffness, and reduced coordination [13] . Parkinson’s disease pathology has been associated with the death of dopaminergic neurons as well as with the accumulation of α-Synuclein (α-Syn) protein structures within Lewy bodies protein deposits [14] . Degradation of neurons in the pathological process of Parkinson’s disease can also lead to dementia symptoms, which may be associated with the presence of Lewy bodies [15] .

1) Dopaminergic Neurons

Dopaminergic neurons are essential in the brain for emotional processes, such as those involved in mood and stress regulation as well as for voluntary movement processes [16] . Death of dopaminergic neurons, particularly those located in the substantia nigra, has been associated with the formation of Parkinson’s disease [16] . Damier et al. [17] investigated this phenomenon using brain samples of five patients who died of Parkinson’s disease as compared with brain samples from individuals with no neuropsychiatric disease of any kind. Based on charts created of the dopaminergic neurons in both the diseased and the healthy patients, it was determined that there was a 64% decrease in dopamine neurons in the diseased patients. These were concentrated in several areas, including the substantia nigra [17] .

2) Alpha-Synuclein (α-Syn) Protein Structures and Lewy Bodies

α-Syn accumulation has also been implicated in the formation of Parkinson’s disease [18] . Alpha-synuclein is a presynaptic protein, whose formation has been associated with a mutation in the SNCA gene [18] . The accumulations of α-Syn are partial constituents of Lewy bodies, which are neuronal inclusions in the brain associated with the development of Parkinson’s disease [19] [20] . Gruden et al. [21] exposed adult male C57Bl/6 mice to either a saline solution control, an α-Syn oligomeric aggregate solution, or an α-Syn oligomeric aggregate and fibril combined solution and were subsequently administered behavioral tests that measured Parkinsonian symptoms. Their results demonstrated that although the saline solution control and the α-Syn oligomeric aggregate solution demonstrated no significant difference in performance on the behavioral tests, the α-Syn oligomeric aggregate and fibril combined treatment demonstrated a significant reduction in locomotor activity [21] .

The association between glyphosate exposure and the formation of Parkinson’s disease is one of the most well-documented correlations among neurodegenerative disorders (e.g., [11] [12] ). Various studies have demonstrated glyphosate to be associated with the formation or exacerbation of Parkinsonian pathology.

For example, Hernández-Plata et al. [22] used male Sprague-Dawley rats to demonstrate the effect of acute glyphosate exposure on dopaminergic neurons and on the formation of visible symptoms associated with locomotion. Glyphosate was administered to the rats using six injections over two weeks at 50, 100, or 150 mg, as compared to a purely saline solution. Their results demonstrated significant decreases in locomotion immediately and extending into the two days following glyphosate exposure as compared with the saline solution [22] . It was also demonstrated that the D1 dopamine receptors had reduced binding in the nucleus accumbens as associated with decreased locomotion. The final significant discovery was that glyphosate exposure effects were absent at sixteen days following exposure, demonstrating a short-term effect [22] . However, this study did not investigate what impact repeated glyphosate exposure may have on the formation of long-term effects.

Pu et al. [23] examined adult male C57BL/6 mice who were exposed to Roundup Maxload at a 0.098% concentration or pure water control through their drinking water. Mice were also administered either MPTP, which produces dopaminergic neurotoxicity, or saline control, leading to a total of four study groups: water and saline, water and MPTP, glyphosate and saline, and glyphosate and MPTP. Their results demonstrated that, as expected, MPTP resulted in reduced dopamine transporter immunoreactivity as compared with the saline control. In addition to this, mice which had been exposed both to MPTP and glyphosate demonstrated a significant exacerbation in reduced dopamine transporter immunoreactivity [23] .

Additional research combined investigations of pure glyphosate and glyphosate herbicide exposure on formation of Parkinsonian abnormalities by using adult female Sprague-Dawley rats exposed both to 98% pure glyphosate as well as Roundup herbicide containing glyphosate [24] . The target for investigation in this study was dopaminergic release in the rat striatum. Behavior and motor performance were assessed following exposure, as well as striatal dopamine release. The investigator’s results demonstrated that rats treated with glyphosate exhibited significantly poorer performance than their control counterparts on rotarod and Bonferroni motor tests up to 24 hours following treatment [24] . In addition to this, an unusual finding was discovered in that a significant increase in release of dopamine in the rat striatum was noted for both the pure glyphosate and the Roundup treatment groups [24] . While this may seem contradictory, a study conducted by Zhang et al. [25] , who used electrophysiological recordings of a Parkinsonian primate, demonstrated dopamine overflow similar to the finding of Costas-Ferreria et al. [24] . In addition to this, Costas-Ferreria et al. [26] conducted a further study investigating this unusual phenomenon and, using adult female Sprague-Dawley rats exposed to glyphosate intrastriatally, demonstrated that glyphosate’s influence on increasing dopamine levels through modification of the dopaminergic transporter is mediated by voltage-dependent calcium levels.

An epidemiological study of deceased individuals in Washington state, where residential pesticide exposure was analyzed using spatial analyses and a crop-exposure matrix, was informative as well [27] . Here, mapping demonstrated the correlation between Parkinson’s disease and spatial land-use data of glyphosate and Paraquat. Results demonstrated that Paraquat exposure was not correlated significantly with premature death. Conversely, glyphosate was correlated with Parkinson’s disease premature death, with the link directly correlated with an increase in glyphosate exposure [27] .

Diverse literature exists that correlates Parkinson’s disease formation with exposure to glyphosate and glyphosate-containing herbicides [22] - [24] [26] [27] . This literature ranges from laboratory research to epidemiological studies investigating pure glyphosate exposure and glyphosate-containing Roundup exposure. Both factors demonstrated significant symptoms in test subjects. In addition to this, impacts were implicated in the death of dopaminergic neurons. Research had previously been conducted investigating the correlation between α-Syn associated with Parkinsonian symptoms and glyphosate exposure, demonstrating mixed results; however, these studies did not meet the criteria for this review as they were published before 2014.

Alzheimer’s disease is another major neurocognitive disorder characterized by detrimental physiological and psychological abnormalities. These include death of the neurons in the brain as well as severe loss of memory and cognition ( [28] , pp. 602-605). Ultimately, brain damage becomes so severe that it results in fatality. The primary brain abnormalities that have been demonstrated to be associated with the formation of amyloid-beta plaques leading to the formation of Alzheimer’s disease include the amyloid-beta precursor protein, the APOe4 mutation of the APOE gene, and tumor necrosis factor α (TNFα) [29] - [31] .

1) Amyloid Precursor Protein (APP)

The APP is essential in the proper functioning of neurons, including the formation of synapses and cellular regulation [29] . The irregular functions of this protein can cause disruption in protein transport and neurotoxic effects on neuronal cells [29] . One abnormality in the APP associated with the formation of Alzheimer’s disease is a particular mutation of the protein called the amyloid-beta precursor protein [32] . This leads to the formation and deposition of extracellular amyloid-beta peptide plaques by giving rise to changes in amyloid-beta cleavage [32] [33] . These changes result in cleavage by β- and γ-secretases rather than the normal α- and γ-secretases [33] . Masuda et al. [34] examined the toxicity of the APP using the mouse model App-KI. Their findings were consistent with behavioral abnormalities associated with individuals suffering from Alzheimer’s disease. As compared with the wild-type mice, the APP-KI mouse model demonstrated deficits in spatial learning and attention as well as place avoidance [34] .

2) Apolipoprotein Gene (APOE)

In addition to the APP, the apolipoprotein gene (APOE) has also been linked to the formation of Alzheimer’s disease. Certain mutations of the APOE gene have been implicated in exacerbated formation of the amyloid-beta precursor protein [30] . The APOe4 mutation of this gene has been associated with increased deposition of amyloid-beta peptide plaques, indicating regulatory dysfunction associated with the APP [30] . This occurs due to alterations in lipid transportation, as seen in the APOe4 mutation but not the APOe3 and APOe2 mutations [35] . In another investigation [36] , the impact of the e4 allele on the APOE gene associated with cognitive decline in mice that mimicked Alzheimer’s disease pathogenesis was explored. The results demonstrated significantly poorer performance in APOe4 mice on the radial arm maze as associated with the use of working memory, but not on the Morris Water Maze [36] . Similarly, Schmitt et al. [37] examined the role of the APOe4 gene in causing deterioration of cognitive function in knock-in APOe4 mice compared to knock-in APOe3 mice. Their results demonstrated a significant decrease in cognitive flexibility of APOe4 mice at the 6-month-old mark compared to APOe3 mice.

3) Tumor Necrosis Factor α (TNF-α)

In addition to the APP and the APOe4 gene, TNF-α has been associated with the formation of Alzheimer’s disease [31] . TNF-α has been implicated in the formation and deposition of amyloid-beta plaques as well as inflammation in the brain and the formation of neurofibrillary tangles, all of which have been implicated in Alzheimer’s disease pathogenesis [31] . For example, McAlpine et al. [38] investigated the impact of blocking TNF-α in 3xTgAD mice models that demonstrated Alzheimer’s disease pathogenesis. Their results demonstrated that blocking of the TNF-α reduced the number of amyloid-beta plaques detected. Thus, it has been recognized that TNF-α is another major contributor to Alzheimer’s disease pathogenesis.

The association between glyphosate exposure and Alzheimer’s disease has only recently been evaluated, and thus, it is not currently well documented. However, it is important to note that a few recent studies have found correlations implicating the potential for glyphosate exposure to impact the formation of Alzheimer’s disease risk factors.

For example, Winstone et al. [39] conducted a study in which non-transgenic C57BL/6J mouse models were orally administered varying levels of pure glyphosate (125, 250, and 500 mg/kg/day) prepared at 0.107 g/L in 1.89 M sodium hydroxide. Glyphosate levels were measured in urine samples collected from the mice as well as in brain tissue using UPLC-MS and in blood plasma to assess an increase in TNF-α. The results of their study demonstrated that glyphosate could cross the blood-brain barrier, making Winstone et al. the first to use an in vivo model to demonstrate this phenomenon. In addition to this, a significant increase in the expression of TNF-α in the APP/PS1 primary cortical neurons was demonstrated, a phenomenon associated with cellular death as well as the increase in Aβ_40-42 as associated with the formation of Alzheimer’s disease [39] .

Similarly, the effects of glyphosate exposure on producing oxidative stress in dopaminergic human neuroblastoma SH-SY5Y cells were evaluated [40] . Within this study, the effect of glyphosate exposure on changing the folding of various proteins was assessed, demonstrating a modification of several genes associated with normal neuronal development and functioning [40] . Among the proteins evaluated was the APP, but it was not found to undergo a protein alteration following exposure and thus demonstrated no potential for gene alteration. However, it was discovered that pro-inflammatory IL6 and TNF-α were significantly upregulated [40] . This, as previously demonstrated, can be correlated with the potential for formation of Alzheimer’s disease.

Recently, an epidemiological study was conducted to evaluate glyphosate exposure in humans and assessed the presence of behaviors associated with Alzheimer’s disease using an Alzheimer’s disease memory test. In this study, Hsiao et al. [41] conducted a human subjects investigation in which a representative sample of the population had urinary samples analyzed for glyphosate. Subsequently, participants 60 years and older were subjected to a variety of tests including the Consortium to Establish a Registry for Alzheimer's Disease Word List Memory Test (CERAD-WLT) [41] . The results of their study demonstrated a significant negative correlation between urinary glyphosate levels and the test scores obtained on the CERAD-WLT, suggesting a relationship between cognitive decline and greater long-term exposure to glyphosate. Ultimately, this finding indicates a possible implication for the association of glyphosate exposure with formation of Alzheimer’s disease pathogenesis in humans.

In summary, research on the impact of exposure to glyphosate and glyphosate-containing products in the formation of Alzheimer’s disease is clearly still deficient. Recent studies have implicated a possible correlation between exposure and the upregulation of TNFα, an Alzheimer’s disease pathogenesis factor. However, no literature was found that examined the correlation between glyphosate exposure and the mutation of APOe4 or between glyphosate exposure and APP. Since a potential correlation to Alzheimer’s disease has been suggested by the limited sources available, a greater body of literature needs to be accumulated that investigates the correlation between factors influencing amyloid-beta plaque formation (APP, APOe4, & TNF-α) and glyphosate exposure, with the goal of eliminating irrelevant factors.

Convulsions and seizures have much broader pathological causes as compared with Parkinson’s and Alzheimer’s disease. Seizures are most often associated with the condition of epilepsy [42] . However, other triggers have been associated with the formation of seizures, including high fever, infection or illness, certain medications or drugs, and a head trauma or brain injury [42] . One of the major neurotransmitter systems in the brain associated with seizure behavior is the GABAergic system.

1) GABAergic System

Gamma-aminobutyric acid (GABA) is one of the primary neurotransmitters in the body and serves as an inhibitory neurotransmitter both in the brain and the spinal cord [43] . It has been associated with a variety of different neurologic and psychiatric disorders, including seizures and epilepsy [43] . According to Macdonald et al. [44] , mutations in the GABA-A receptor have been correlated with the formation of epilepsy. In addition to this, Petroff et al. [45] , who investigated seizure control in patients with complex partial seizures, correlated low seizure control with low levels of GABA neurotransmitter in the brain.

The correlation between glyphosate exposure and seizures is incredibly novel. Several case studies have previously investigated the correlation between glyphosate and seizure behavior, with unclear results. Naraine et al. [46] were the first to demonstrate a glyphosate and seizure correlation in the laboratory using the C. elegans model.

Recently, a study was conducted investigating seizures in 464 individuals who ingested one or more mouthfuls of pesticide with the intention of committing suicide [47] . These pesticides included the following categories: glufosinate ammonium, pyrethroid, glyphosate, paraquat, organophosphate, and others. The highest number of seizures was seen in those who ingested glufosinate ammonium (31.5% instance of seizures), while only a 5.4% instance of seizures was demonstrated in those who ingested glyphosate [47] . Thus, it was unclear from this study whether a strong correlation existed between glyphosate exposure and the formation of seizures.

In a population epidemiological study conducted by Requena et al. [48] , people living with epilepsy, as compared with control subjects, were investigated for pesticide exposure, including pesticides such as paraquat, diquat, glyphosate, glufosinate, and lufenuron. Their results demonstrated that areas with high pesticide concentrations were correlated with epilepsy incidence and risk. However, it is important to note that the study needed to elaborate on which pesticides contribute more than others. Thus, it cannot be determined if glyphosate was a prominent contributor to this result.

In a recent study [46] , the goal was to examine convulsant behavior in C. elegans models following exposure to glyphosate-containing Roundup using an electroshock convulsion assay. Results from this study demonstrated that a 0.1 mM concentration of glyphosate followed by the electroshock assay resulted in convulsions in the C. elegans models and a non-recovery phenotype. In addition, it was noted that introducing a GABA-A antagonist with a subeffective glyphosate dose resulted in a threefold increase in nonrecovery [46] . Of note, when the C. elegans were administered the antiepileptic drug sodium valproate; however, it was noted that the duration of seizures was significantly reduced, and every worm demonstrated recovery. The GABAergic mechanism, which has been implicated in seizure pathology of humans, was thus implicated as the potential pathway affected by the glyphosate exposure and subsequent convulsion assay [46] .

All research prior to the study reported above had been conducted to investigate the correlation between seizures and glyphosate exposure but were unable to demonstrate distinct results. As with any epidemiological study, studies attempting to draw correlations from neighborhood or work-related glyphosate exposure to subsequent population increases in seizure formation could have been subject to confounding variables associated with other toxins or lifestyle choices relevant to the area studied. With the research conducted by Naraine et al. [46] , more unambiguous evidence now exists for a correlation between seizures and glyphosate-containing herbicide exposure. Further research should seek to confirm this correlation, using multiple different animal models. In addition, research should be conducted on pure glyphosate to determine whether the glyphosate or another component in the Roundup herbicide resulted in the seizure behavior.

Unlike the previously discussed neurological disorders, the pathology of autism spectrum disorder is still not well known. However, it has been suggested that a combination of genetic and environmental factors, especially ones implicating prenatal exposure, can be associated with the formation of autism. In addition, associations have been made with biological mechanisms, including the Wnt/Ca²⁺ pathway, corpus callosum agenesis, and proinflammatory cytokines.

1) Wnt/Ca²⁺ Pathway

The Wnt/Ca²⁺ pathway plays critical roles in the development of humans, ranging from effects on the formation of the head and neuron patterns as well as different bodily organs [49] . Abnormalities in the Wnt/Ca²⁺ system could cause major effects on neuronal signaling and have been implicated in the formation of autism spectrum disorder [49] . For example, Sowers et al. [50] demonstrated that Prickle2 mutant mice, a mutation that leads to a mutation in the Wnt signaling protein, displayed abnormalities associated with autism spectrum disorder including alterations in learning and social behavior. In addition, the investigators reported a decrease in the size of synaptic currents.

2) Agenesis of the Corpus Callosum

Agenesis of the corpus callosum is a congenital cerebral malformation in which there is either a complete or a partial deficiency in corpus callosum brain matter [51] . This malformation has been associated with a variety of different factors, including maternal alcohol use, phenylketonuria, Chiari II malformation, and several genetic and chromosomal factors [51] . Past research has demonstrated that among individuals with brain malformations, including agenesis of the corpus callosum, autism screening was positive for 45% of children, 35% of adolescents, and 18% of adults in the study [52] .

3) Proinflammatory Cytokines

Proinflammatory cytokines are found in the human immune system following the activation of macrophages [53] . Further, they include IL-1β, IL-6, and TNF-α and are released to increase the inflammatory reaction of the body [53] . Last, both IL-6 and TNF-α, previously discussed in correlation with amyloid-beta plaques and Alzheimer’s disease, are located within the nervous system [53] . More recently, research discovered that patients with autism spectrum disorder demonstrated upregulation of both IL-6 and TNF-α [54] . In addition to this, maternal stress, including exposure to infections and pollution, has been associated with prenatal complications leading to the upregulation of proinflammatory cytokines and the formation of autism spectrum disorder in the child [55] .

Since autism spectrum disorder has been associated with environmental factors in conjunction with genetic abnormalities (see, [50] ), glyphosate has been investigated as a potential exacerbator of autism. Several studies have been conducted which link glyphosate exposure with various autism risk factors.

For example, research was conducted to investigate glyphosate exposure associated with the formation of autism spectrum disorder in both mother mice and juvenile offspring [56] . In the study, pregnant mice were administered 1% Roundup in drinking water over the span of E5 to P21 (pup weaning). The results demonstrated that prenatal exposure to Roundup mimics the behavior of autism spectrum disorder in newborn pups. In addition, it was discovered that the sHE protein, which has been associated with autism spectrum behavior, was upregulated in juveniles exposed to Roundup prenatally as compared with the control group. Finally, a decrease in parvalbumin immunoreactivity was noted in the prenatally exposed juveniles compared to the control group [56] .

A follow-up study by the same research group was conducted but Roundup was replaced with pure glyphosate and examined in pregnant mice [57] . A formula of 0.098% glyphosate was administered to the pregnant ddY mice (embryo 9 - 10 weeks old) between E5 and P21 (weaning). When this study was compared to the previous report [56] , the results suggested similarities between the two, in which juveniles exhibited autism spectrum behaviors following exposure as compared with the control group. Thus, based on the combined results from both studies, it is likely that the glyphosate itself, and not any other ingredient in the Roundup herbicide contributed to the formation of autism-like behaviors in the juveniles.

Conducted in California, a relevant population epidemiological study investigated individuals who were born in the area under consideration and had been diagnosed while residing there [58] . Using an open-source geocoder called CA-PUR in combination with a land use survey, the investigation included examination of the application of such pesticides as glyphosate, chlorpyrifos, diazinon, acephate, malathion, permethrin, bifenthrin, methyl bromide, imidacloprid, avermectin, and myclobutanil both individually and in multi-pesticide models. Results demonstrated a significantly greater number of males with autism spectrum disorder as compared with females, and the mothers of those individuals were relatively older [58] . In addition, it was demonstrated that the odds of children developing autism spectrum disorder in the first year were significantly increased for most of the pesticides, including glyphosate. Last, for the combination of autism spectrum disorder and intellectual disability, increased odds were significantly marked in individuals exposed to glyphosate and permethrin [58] .

Coullery et al. [59] examined the development of hippocampal neurons associated with exposure to glyphosate, particularly in the Wnt5a pathway, which, as previously discussed, has been implicated in the formation of autism spectrum disorder. As the authors noted, the Wnt5a pathway is particularly associated with axonal branching, sympathetic neuron growth, as well as other types of neuronal development. Hippocampal pyramidal neuron cell cultures were prepared from Wistar rats followed by exposure to glyphosate, and these cultures contained recombinant protein Wnt5a. Results demonstrated that the glyphosate did not kill any of the neurons but did affect the polarization of the neurons and neuronal development such as delayed axon differentiation and dendrite growth in the Wnt5a pathway [59] .

In 2020, Coullery et al. conducted a second study which also investigated the association of glyphosate exposure in the Wnt5a-CaMKII pathway [60] . This study investigated the ingestion of glyphosate as compared with administration directly to the neuronal cells, and it combined assessment of the Wnt5a-CaMKII pathway with behavioral tests. Pregnant female Wistar rats were administered either glyphosate in 25 mg/kg or 35 mg/kg as compared to a control, and pups were subjected either to a behavioral test, the Morris Water Maze, or the Conditioned Fear Test [60] . In addition, the activity of the Wnt5a-CaMKII pathway was analyzed from hippocampal homogenates using electrophoresis and Western Blot. Their results demonstrated that glyphosate-exposed pups spent less time in the target quadrant of the probe trial of the Morris Water Maze and displayed impaired contextual fear. In addition, it demonstrated that exposure to the higher dose of glyphosate resulted in decreased expression of Wnt5a, which forms synaptic structures, and an overall downregulation of Wnt-CaMKII signaling [59] . Since Wnt is correlated with the formation of autism spectrum disorder, these two studies conducted by Coullery et al. suggest that glyphosate plays a significant role in altering the Wnt pathway and associated neurons, which can be further correlated with the potential of glyphosate to impact autism spectrum disorder formation.

Other research models have also been utilized. For example, Alarcón et al. investigated the Wnt system by studying glyphosate exposure in prepubertal ewe lambs [61] . Female Friesian lambs were exposed to either a saline solution or a commercial glyphosate formula, with no exposure in the pasture and drinking water as controlled variables. Blood samples and uterine transverse sections were collected from the lambs and analyzed. Their results revealed that the ewe lambs who ingested glyphosate demonstrated that Wnt5a and Wnt7a expression were significantly decreased [61] . As mentioned previously, the Wnt pathway has been associated with the formation of autism spectrum disorder, and Alacrón et al. [61] demonstrated that glyphosate not only affected the Wnt5a pathway, as demonstrated by Coullery et al. [60] but also caused alterations to the Wnt7a pathway.

Thus, a growing body of literature is being accumulated that correlates glyphosate and glyphosate-containing herbicide exposure with autism spectrum disorder. In particular, the research mentioned above has suggested that prenatal exposure to glyphosate is significant in determining the impact of glyphosate on the formation of autism in an individual. Glyphosate was positively correlated with alterations in the Wnt pathway and upregulation of proinflammatory cytokines, which are both implicated in prenatal development as associated with pregnant mother exposure. No research has been found that links glyphosate exposure with agenesis of the corpus callosum.

More recently, the association between the gut microbiome and the brain system has become a significant focus in research regarding neurodegenerative disorders. This system, known as the gut-brain axis, connects the enteric nervous system and central nervous system physically but also includes connections with other systems, including the endocrine system and the immune system [62] . The gut-brain axis has been associated with influences both on emotional health, including state and regulation, as well as on physical brain health, including neuronal growth and cognitive function [62] . In addition to this, the gut-brain axis is associated with the release of metabolites including neurotransmitters and hormones which have been further associated with neurodegenerative disorders such as Parkinson’s and Alzheimer’s disease [62] . A growing body of literature has been released that has begun to examine the association of the gut-brain axis with the ingestion of herbicides such as those containing glyphosate.

A relevant example can be found in the report of a study investigating the microbiome, brain plasticity and behavior of pregnant female rats following exposure to pure glyphosate and Roundup [63] . The pregnant Sprague-Dawley rats were examined for changes in behavior, and brain, synapse, and fecal microbiome analyses were conducted. The results demonstrated that the rats who ingested both the pure glyphosate and the glyphosate formula in Roundup experienced changes in maternal licking behavior and neurogenesis, with effects including a greater number of immature neurons in the dentate gyrus and alterations to synaptophysin [63] . In addition to this, results showed that Roundup exposure caused a significant decrease in Firmicutes bacteria and a significant increase in Bacteroidetes bacteria. However, this result was not demonstrated for the formulation of pure glyphosate, which indicates that either another chemical besides glyphosate or the interaction between glyphosate and the other chemicals in the formulation contributed to this result [63] . As these investigators noted, it also shows the potential for Roundup to contribute to neurodegenerative diseases as well as psychiatric disorders, as any dysregulation of the microbiota can result in significant changes in the function of the central nervous system.

Another study, reported by Del Castilo et al. [64] , investigated low-dose glyphosate-containing herbicide exposure on the gut microbiome and behavior of mice. In order to demonstrate the effects of chronic exposure as compared with acute exposure, the mice were exposed to Roundup from pregnancy to adulthood. Analyses were conducted which analyzed gut microbiome, intestinal alterations, and behavioral changes. The relevant results demonstrated an alteration in behavior with Roundup exposure, including increased behavioral repetitions and decreased social interest, especially in male mice [64] . This finding suggests a correlation with autism spectrum disorder, as previously discussed, since these behavioral alterations are consistent with the formation of autism. Of considerable import, significant alterations in bacterial levels such as Proteobacteria, Desulfobacterota, and Bacteroideta. In addition, other gut changes were demonstrated, including increased mucus cells, lymphocyte differentiation, and alterations of adhesion molecules [64] . Thus, this research indicates a correlation between glyphosate-containing herbicide exposure, alterations to the gut microbiome, and the formation of autism spectrum disorder, and it could also, as previously suggested by Dechartres et al. [63] , indicate the pathway through which individuals develop other neurodegenerative disorders.

In a similar study conducted using an avian model the impact of glyphosate-containing herbicides on gut microbiome and reproductive hormones was assessed [65] . Japanese quails fed either Roundup-treated organic feed or just the organic feed as a control were analyzed for glyphosate residue in the liver as well as subjected to fecal microbiome analysis and oxidative stress analysis. The research also included consideration of the impact of long-term exposure, subjecting the poultry to Roundup over the course of 52 weeks, as compared to an acute exposure model. Results showed no evidence of damage associated with oxidative stress and no effect associated with acetylcholinesterase activity, which was novel compared to rodent models. However, dysbiosis of the gut microbiome was significantly noted, with a particular decrease in the growth of bacteria such as Firmicutes and an increase in Actinobacteria [65] . Further, the change was especially notable with increases in age and in the female poultry [65] . Therefore, although this study did not discover any correlation with the neurological system directly, nor was it able to link Roundup exposure with alterations to neurotransmitter systems such as that which is associated with acetylcholinesterase, it did demonstrate, as with previously listed studies, a significant alteration to the gut microbiome as associated with the Roundup exposure, which has been further associated with central nervous system alterations.

In another study of low-dose glyphosate-based herbicide exposure as well as pure glyphosate exposure, the impact of such exposure on Sprague-Dawley female rats and both male and female pups [66] . Females were exposed to glyphosate-based herbicide or pure glyphosate through drinking water from gestation to weaning, and male and female pups were exposed to the herbicide in utero and through drinking their mother’s milk. Metabolomics was analyzed through urine and feces samples. Results demonstrated that glyphosate-based herbicide exposure and the pure glyphosate exposure resulted in alterations in metabolomic biomarkers in both the females and the pups as investigated in urine samples [66] . This included an increase in homocysteine which suggests a reduction in the activity of Prevotella bacteria. This finding is significant because a reduction in microbiota such as Prevotella has been implicated in the formation of autism spectrum disorder [66] .

Last, Mesnage et al. [67] investigated the impact of glyphosate or Roundup exposure on the gut microbiota system and serum metabolome, investigating in particular the shikimate pathway. Sprague-Dawley female rats were treated for 90 days with pure glyphosate, Roundup, or control and were subsequently investigated with metabolomics, shikimic acid, and bacterial growth analysis, as well as shotgun metagenomics. Their results demonstrated through metabolomic studies that glyphosate exposure resulted in metabolite changes, which implicated an inhibitory reaction with the shikimate pathway of the gut microbiome [67] . This, as previously mentioned, further suggests a correlation between glyphosate exposure and microbiome alterations that can have greater impacts on the nervous systems and ultimately the formation of neurodegenerative disorders.

The research discussed above demonstrates correlations between Roundup exposure and alterations to the gut microbiome, suggesting significant implications for the gut-brain axis and the formation of neurologic disorders. The most well-documented association between the gut-brain axis and the formation of a previously mentioned neurological disorder, as discussed in the above literature, was the correlation with autism spectrum disorder. The association between the gut-brain axis, glyphosate exposure, and neurodegenerative disorders such as Alzheimer’s disease and Parkinson’s disease was less direct. However, since changes in the gut microbiome were demonstrated in correlation with glyphosate and glyphosate-containing herbicides and gut microbiome alterations have been associated with changes in the central nervous system, this suggests the gut microbiome is a potential pathway through which glyphosate and glyphosate-containing herbicides impact the formation of neurodegenerative disorders.