Pathophysiology: neurochemical aspects

A large body of imaging, spectroscopic and post-mortem studies investigating neuro-chemical aspects of GTS pathophysiology have been published to date. Given that ini-tial success of the dopaminergic antagonist haloperidol in reducing tic symptoms, these studies have mainly focused on the dopaminergic system. More recent works, however, have highlighted a role for other neurochemical systems in GTS. In the following sec-tion, an overview of neurochemical interactions in the basal ganglia in relation to GTS pathophysiology is presented.

2.5.1 Mico-Circuitry Of the Basal Ganglia

The basal ganglia are group of subcortical nuclei that act as a cohesive functional unit allowing the tight regulation of motor, cognitive and limbic functions. The main com-ponents of the basal ganglia are the Striatum (caudate and putamen), the Substantia Nigra (pars compacta — SNr and pars reticulata — SNr), the Subthalamic Nucleus (STN) and the Globus pallidus interna (GPi) and externa (GPe) (Figure2.4) [68]. With the exception of the excitatory glutamatergic (—) STN projections, all intrinsic and output projections of the basal ganglia areγ-Aminobutyric acid (GABA)ergic (—) and inhibitory. The striatum and the STN are the main input structures and they receive glutamatergic excitatory signals from the cerebral cortex, the brainstem, and the limbic system. On the other hand, the GPi/SNr complex is the main output system project-ing GABAergic inhibitory neurons onto the thalamic sub-nuclei which feed back to the cortex. The GPe and the SNc hold intrinsic functions and provide the striatum with important modulatory signals [69].

In current anatomical models of internal basal ganglia circuitry, two antagonistic striato-thalamic pathways exist. A direct and an indirect pathway are organized in a way that allows for the selection or inhibition of competing actions [70]. In this model, the striatum is linked to the GPi/SNr output complex via a monosynaptic direct pathway emanating from set of distinct GABAergic medium-sized spiny neurons (MSNs) within the striatum. In the direct pathway, excitatory cortical input to the striatum results in the release of inhibitory signals from the GPi/SNr to the thalamus. This releases the ’breaks’ off the thalamus allowing it to emit its excitatory signals that facilitate the execution of movement. The direct pathway can be summarized as follows:

Cortex—Striatum —SNr/GPi···· Thalamus —Cortex.

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The indirect pathway on the other hand, is polysynaptic where a different population of striatal MSNs delivers input to the GPi/SNr output complex via the GPe and its intercalated STN. In this case, cortical excitability of the striatum leads to a decrease in the tonic inhibition of the STN by the GPe. As such, the STN is free to release its glutamatergic excitatory signals to the output nuclei furthering the inhibition of the thalamus and leading to a net hypokinetic state. The indirect pathway can be summarized as follows:

Cortex — Striatum—GPe···· STN—SNr/GPi—Thalamus···· Cortex.

Overall, the indirect pathway leads to the inhibition of movement while the direct path-way leads to the facilitation of movement.

Macroscopically, early models of basal ganglia physiology have posited a "funneling and selection" function of its nuclei [71]. In the late 1980s, however, this model has been replaced by a segregated circuit model in which the basal ganglia are seen as components of five segregated and parallel re-entrant loops, over which information sent from specific cortical areas is processed topographically and is integrated within its internal circuitry [69,72]. These circuits fall into three major domains (motor, limbic and associative) and include the sensorimotor, oculomotor, dorsolateral prefrontal, lateral orbitofrontal, and anterior cingulate limbic loops [69,70,72] (Figure 2.4).

Disequilibrium in the natural mechanics of the inbound CSTC loops is a major factor in the emergence of both motor and non-motor features exhibited by various neuropsychi-atric and movement disorders [74,75]. Along this line, previous work has indicated that spatially focalized alterations in neurochemical ratios play a major role in the emergence of diverse motor, limbic and associative features (Figure2.5). Keeping in mind that GTS is not only centered on tics, where the majority of the patients exhibit a varied motor and behavioral symptomatology, early hypotheses have postulated that the expression of tics and its accompanying behavioral features is a result of atypical neurotransmis-sion within specific sub-territories of the basal ganglia, which consequentially lead to the aberrant integrative interplay of CSTC circuitry [76–78]. As the basal ganglia provide a mechanism for the selection of an action from a competing response [72,79], fundamen-tal alterations in the functional dynamics of CSTC circuitry could cause selection errors that are believed to lead to the expression of motor and non-motor features as exhibited in GTS [75,78].

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Figure 2.5: Selective dysfunction of basal ganglia subterritories. Utilizing local injections of the GABAergic antagonist bicuculline, a causal link between the neurochemical activity of specific functional subterritories of the basal ganglia and clin-ical manifestations observed in movement and behavioral disorders were demonstrated [75,80,81]. The authors showed that the emergence of symptoms associated with GTS (e.g. tics, stereotypy, attentional deficits) is dependent on the spatial location of the

neurochemical alteration. Figure retrieved from [75].

Several lines of evidence have suggested a strong role for dopamine in GTS pathophysiol-ogy [31,89]. Dopaminergic dysfunction in GTS is supported by(a)initial clinical obser-vations of improvements in tic-like symptoms following the administration of dopamin-ergic antagonists (pimozide, fluphenazine, haloperidol, risperidone, aripiprazole), syn-thesis blockers (α-methyl-para-tyrosine) or monoamine depletion drugs (tetrabenazine);

and the exacerbation of symptoms following the administration of dopaminergic stimu-lants (L-DOPA, central stimustimu-lants) [20,90–94]; (b) results from varied nuclear imaging studies that show alterations in dopamine transporter and receptor function in stri-atal and extra-stristri-atal regions [95–110]; (c) increased dopamine concentrations in cere-brospinal fluid [89, 111] and (d) altered dopamine levels as revealed by post-mortem studies [100, 110, 112]. Given this data, multiple dopaminergic hypotheses have thus been posited on the role of dopamine in GTS pathophysiology. These hypotheses include:

(a) dopaminergic hyper-innervation within the striatum; (b) a presynaptic abnormality in aromatic L-amino acid dihydroxyphenylalanine (DOPA) decarboxylase, which is in-volved in the catalysis of L-DOPA to dopamine; (c) super-sensitivity of postsynaptic

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2.5.3 GABA

GABA is the primary inhibitory neurotransmitter in the human brain. There are two main types of GABAergic neurons that include (a) projections neurons and (b) in-terneurons [122]. Although the majority of GABAergic striatal neurons are of the spiny projection type (i.e. MSNs), striatal GABAergic interneurons produce a strong inhibitory influence over MSNs, regulating their output to the GPi and the GPe in the direct and indirect pathways, respectively [123].

Considering that GTS is in essence a disorder exhibiting pathological mechanisms of inhibitory control, various authors have posited that the primary inhibitory neurotrans-mitter GABA may have an important role in GTS pathophysiology [31]. For example, the burst-like disinhibition of thalamo-cortical projections which would ultimately facil-itate hyperkinetic behaviour (ie. tics), could be a result of alterations in GABAergic striatal projection neurons (ie. MSNs) or striatal as well cortical interneurons. Given that pallidal GABAergic neurons exhibit a potent influence over dopaminergic neurons of the substantia nigra, abnormalities in GABAergic pallidal projection could also have an important role in GTS pathophysiology.

Although preliminarypost mortem work investigating GABA levels in various brain re-gions failed to find any significant differences between patients and controls [124], more recent work has demonstrated that GABA does exhibit abnormalities in GTS. Utilizing unbiased immunocytochemical techniques, Kalanithiet al. [125] demonstrated profound alterations in the density and distribution of a specific type of GABAergic interneurons.

Specifically, higher densities of parvalbumin-positive GABAergic interneurons were found in the GPi, whereas lower densities were observed in the GPe, caudate and putamen.

The authors were able to replicate and extend their work in another study, in which they showed that different subtypes of interneurons also exhibit reductions [126]. In another study, Lerneret al. [127] utilized Positron Emission Tomography with injection of the ra-dioligand [¹¹C]flumazenil to visualize the distribution ofGABAAreceptors. The authors demonstrated that patients with GTS exhibit widespread abnormalities in the GABAer-gic system, as they observed(a)reductions in the binding capacity ofGABA_Areceptors in the ventral striatum, thalamus, amygdala and right insula, and (b) reductions in the substantia nigra, periaqueductal gray, posterior cingulate cortex and cerebellum. More recent work utilizing Magnetic Resonance Spectroscopy expanded on these findings by demonstrating GABAergic reductions in(a)striatal, cingulate and sensorimotor regions in pediatric sample [128,129], and(b) elevations in the supplementary motor area in an adult sample [130]. In general, this work provides strong support for an abnormality in GABAergic neurotransmission in GTS.

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2.5.4 Glutamate

Given its wide distribution as the brain’s primary excitatory neurotransmitter and its essential role in the normal mechanics of CSTC circuitry, a pathophysiological role for glutamatergic neurotransmission in GTS has recently been postulated [131]. A role for glutamate in GTS pathophysiology can be indirectly drawn from the fact that (a) glu-tamate and GABA exhibit close metabolic links, in which the penta-carbon skeleton of glutamate acts a precursor for the synthesis of GABA and(b) glutamate and dopamine exhibit extensive interactions at different levels of cortico-striato-thalamo-cortical cir-cuitry [131] (Figure2.6). With respect to the interactions of glutamate and dopamine, it has been suggested that the co-transmission of both neurotransmitters is possible in cen-tral monoaminergic neurons such as in the vencen-tral tegmental area (VTA) [132]. Second, mesocortical dopaminergic inputs from the VTA are able to directly and indirectly alter pyramidal neuron excitability in prefrontal cortical regions [133]. Third, descending cor-tical glutamatergic afferents, modulate dopaminergic VTA and SNc neurons which feed back to the striatal matrisomes and striosomes, respectively [134, 135]. Fourth, corti-cal glutamatergic afferents and dopaminergic SNc projections converge towards striatal MSNs, where dopamine is able to modulate glutamatergic neurotransmission depending on the type of receptor it targets.

On the other hand, direct evidence of glutamatergic abnormalities in GTS is drawn from pathophysiological and genomic studies. Pathophysiologically, early postmortem studies demonstrated a reduction in glutamate levels in the GPi, GPe, and SNr [124]. This evidence correlates with volumetric MRI analysis that indicated a reduction in the size of the left GP [136], although a direct link was never made. Moreover, two genetic studies have also highlighted a role for glutamate in GTS. One large multigenerational family genome scan identified a role for 5p13 - an area that overlaps with glial glutamate transporter gene1 [137]. Another genome scan using sibling pairs identified a missense variant in E219D, a highly conserved residue that results in increased glutamate uptake [138,139]. Additionally, inference can also be drawn from studies that investigated the role of glutamate in OCD since there is a high degree of pathophysiological overlap. Other genetic studies also implicated SLC1A1 gene (glutamate transporter) and GRIN2B gene (glutamate related gene expressing a subunit of the NMDA glutamate receptor) in OCD [140–142], which exhibits a phenomenological overlap with GTS pathophysiology.

In a Magnetic Resonance Spectroscopy study conducted on an adolescent patient sample, DeVitoet al. [143] did not find any differences in the concentration of glutamate in vari-ous brain regions. However, the authors demonstrated reduced levels of N-acetylaspartate and choline in the left putamen, and reduced creatine levels in the putamen bilaterally.

Nevertheless, this study had several limitations as (a) 50% of their patient group was

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sedated using chloral hydrate and this was not accounted for; (b) the impact of comor-bidities and medication was not included in the analysis; (c) only a male population was investigated. In conclusion, the authors suggested that further studies are needed to replicate their results and to investigate the impact of comorbidity and the effect of psychotropic medication on metabolite levels.

In conclusion, though insufficient, some published work points to a role for glutamate in GTS. It is not known whether GTS patients exhibit a hyper- or a hypo-glutamatergic state. For example, tics can be induced either by (a) excess dopaminergic stimulation of the striatum or by (b)excess cortical glutamatergic input onto the striatum. In both of these scenarios, hyperkinetic effects could be induced [131]. Interrogating the role of glutamate in GTS will possibly have direct implications on novel interventional strategies [131].

2.5.5 Other neurotranmistters

Considering that typical motor and non-motor behaviour is driven by strict spatio-temporal interactions between various neurochemical systems, it is possible that neu-rotransmitter systems other than the primary excitatory, inhibitory and modulatory systems may be involved. Though preliminary, some work has indicated links between the cholinergic, serotonergic and histaminergic neurotransmitter systems and GTS. The strongest possible link is related to the cholinergic system. Initial immunocytochemical work indicated that patients with GTS exhibit alterations in the density and distribution of cholinergic interneurons [126], which may have drastic downstream consequences on the GABAergic projection and interneuron populations that they influence. This notion was demonstrated by the targeted ablation of striatal cholinergic neurons in a rat model, in which tic-like stereotypies were induced following acute stress of d-amphetamine chal-lenges [144].

Studies investigating the role of serotonin in GTS have revealed: (a)reductions of sero-tonin metabolite levels in the cerebrospinal fluid [111, 145, 146] and the basal ganglia [124] in some but not all patients;(b) normal metabolite levels in the cortex [112];(c) a decrease in postmortem density of 5HT-1A receptor levels in frontal and occipital areas [110]; (d) a reduction in the binding capacity of 5HT transporters in association with OCD [147]; (iv) a negative correlation between vocal tics and 5HT transporter binding potential in the midbrain and thalamic areas [148]. Other nuclear imaging studies in-vestigating serotonin receptor binding exhibited variables results, where some showed no change [108] to increases in multiple regions [149]. In summary, these findings indicate

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that patients with GTS plus comorbid OCD may exhibit an abnormality in serotonergic signalling.

Related to the histaminergic system, thoughin vivoinvestigations of histaminergic levels in have not yet been explored in GTS, genetic findings have indicated that dysregula-tions in histaminergic neurotransmission may represent a rare cause for tourette syn-drome [31, 150]. In general, proposals for the underlying mechanism of abnormality have included most neurochemical systems, however, the strongest evidence published to date indicates abnormalities in dopamine and GABA. Further cross-sectional and longi-tudinalin vivo investigations of other neurochemical systems may further elucidate the pathophysiological mechanisms of GTS.