Neurotransmitter systems are profoundly disrupted in progressive supranuclear palsy (PSP), contributing to the diverse motor, cognitive, and behavioral symptoms that characterize this 4R-tauopathy. Unlike Parkinson's disease, where dopaminergic loss dominates the clinical picture, PSP involves multi-system neurotransmitter deficits that reflect the distinctive pattern of neurodegeneration affecting subcortical structures, brainstem nuclei, and basal ganglia circuits["@litvan1997"]. Understanding these neurotransmitter disturbances is critical for developing symptomatic treatments and for differentiating PSP from related neurodegenerative disorders.
The neurotransmitter abnormalities in PSP result from tau pathology affecting specific neuronal populations that synthesize, store, release, or respond to particular neurotransmitters. The pattern of dysfunction provides mechanistic insight into the clinical features of PSP, including vertical supranuclear gaze palsy, axial rigidity, postural instability, cognitive impairment, and behavioral changes["@boxerman2020"].
The basal forebrain cholinergic system, comprising the nucleus basalis of Meynert and related structures, undergoes significant degeneration in PSP[@shinotoh2000]. These neurons project to the cerebral cortex and hippocampus and are essential for attention, learning, and memory. Post-mortem studies have demonstrated marked reduction in choline acetyltransferase (ChAT) activity in the frontal cortex and caudate nucleus of PSP patients, indicating presynaptic cholinergic dysfunction[@ruberg1985].
Neuroimaging studies using PET ligands such as [^11C]-PMP have confirmed reduced acetylcholinesterase (AChE) activity in the cortical and subcortical regions of living PSP patients[@bohnen2007]. The pattern of cholinergic loss differs from Alzheimer's disease, where the most severe deficits occur in the hippocampus and posterior cortex, reflecting the distinct regional vulnerability in PSP.
The cholinergic deficits in PSP contribute to cognitive impairment, particularly executive dysfunction and attention deficits. Cholinergic supplementation strategies have shown limited efficacy in PSP, likely because the deficits involve multiple neurotransmitter systems simultaneously.
The pedunculopontine nucleus (PPN), a major cholinergic nucleus in the brainstem, is particularly vulnerable in PSP[@jellinger1988]. Tau pathology in the PPN correlates with the gait and postural instability that characterize PSP. The PPN cholinergic neurons project to the thalamus and basal ganglia, modulating arousal and motor control.
Post-mortem studies have documented severe neuronal loss and tau pathology in the PPN of PSP patients[@zweig1988]. MRI studies have shown atrophy of the PPN region, and PET imaging has demonstrated reduced cholinergic marker binding. This dysfunction contributes to the early gait disturbance and falls that are hallmarks of PSP.
The external and internal segments of the globus pallidus (GPe and GPi) are heavily implicated in PSP pathophysiology[@albin1990]. These GABAergic output nuclei of the basal ganglia receive inhibitory input from the striatum and subthalamic nucleus and provide the major inhibitory projection to the thalamus and brainstem.
In PSP, tau pathology affects the striatal neurons that project to the globus pallidus, as well as the pallidal neurons themselves. The resulting GABAergic dysfunction leads to disinhibition of thalamocortical projections, contributing to the rigidity and axial symptoms that distinguish PSP from PD[@levy1997].
Neurochemical studies have demonstrated reduced GABA concentrations in the basal ganglia of PSP patients using magnetic resonance spectroscopy (MRS)[@drayer1987]. This reduction reflects both neuronal loss and impaired GABA synthesis. The GABAergic deficit contributes to the characteristic "cockroach gait" and neck rigidity in PSP.
Striatal GABAergic interneurons, including fast-spiking parvalbumin-positive cells and cholinergic tonically active neurons, are affected in PSP[@hardman1997]. These interneurons modulate the activity of medium spiny neurons that form the direct and indirect pathways of the basal ganglia. Dysfunction of these interneurons disrupts the finely tuned balance between movement facilitation and inhibition.
While dopaminergic loss in PSP is less severe than in Parkinson's disease, the substantia nigra pars compacta (SNc) does exhibit significant pathology[@forno1982]. Tau-laden neurons in the SNc undergo degeneration, leading to reduced dopaminergic innervation of the striatum.
Unlike PD, where dopaminergic cell loss is relatively selective, PSP affects multiple brainstem nuclei, including the SNc. The resulting dopaminergic deficit contributes to parkinsonian features in PSP, but the pattern differs from PD. In PSP, dopamine loss is more uniform across striatal regions, whereas PD shows a gradient from posterior to anterior striatum[@brooks1987].
Post-mortem studies have documented 40-60% reduction in striatal dopamine content in PSP, compared to 80-90% reduction in PD[@agid1989]. This partial dopaminergic deficit explains the suboptimal response to levodopa in most PSP patients.
Dopamine receptor density is altered in PSP, with evidence of both pre-synaptic and post-synaptic dysfunction. PET studies using [^11C]-raclopride have demonstrated reduced dopamine D2 receptor binding in the striatum of PSP patients[@shinotoh1993]. This reduction reflects both neuronal loss and receptor downregulation.
The D2 receptor deficit may contribute to the cognitive and behavioral symptoms of PSP, as these receptors are abundant in the prefrontal cortex and limbic system. Dopamine D1 receptors are also affected, though generally to a lesser extent than D2 receptors.
The raphe nuclei, the primary source of serotonergic innervation to the forebrain, show tau pathology in PSP[@auger2015]. Both the dorsal raphe nucleus (DRN) and median raphe nucleus (MRN) contain tau-positive neurons, though the severity of involvement varies between individuals.
Neurochemical studies have demonstrated reduced serotonin metabolite (5-HIAA) concentrations in the cerebrospinal fluid of PSP patients[@sandyk1983]. This finding indicates decreased serotonergic turnover, reflecting either reduced serotonergic neuron numbers or impaired function.
Serotonergic dysfunction in PSP may contribute to depression, which is common in PSP though less frequent than in PD. Sleep disorders, which are prevalent in PSP, may also relate to raphe nucleus involvement, as these nuclei regulate sleep-wake cycles.
Post-mortem studies have identified alterations in serotonin receptor expression in PSP, including reduced 5-HT1A and 5-HT2A receptor binding in the frontal cortex[@cash1996]. These changes may reflect both direct neuronal loss and compensatory mechanisms.
The cortico-striatal glutamatergic pathway is affected in PSP through both direct and indirect mechanisms[@crossman1987]. Tau pathology in cortical layer V pyramidal neurons reduces excitatory input to the striatum. Additionally, postsynaptic striatal glutamate receptors may be downregulated in response to altered activity.
Magnetic resonance spectroscopy studies have demonstrated elevated glutamate levels in the basal ganglia of PSP patients[@federico1999]. This increase may reflect impaired glutamate clearance or compensatory upregulation. The glutamate excitotoxicity hypothesis suggests that elevated extracellular glutamate contributes to neurodegeneration through calcium influx and oxidative stress.
The subthalamic nucleus (STN), which provides excitatory glutamatergic input to the basal ganglia output nuclei, shows tau pathology in PSP[@revuelta2013]. The STN is hyperactive in PSP, contributing to excessive excitatory drive to the GPi and SNr.
Deep brain stimulation of the STN can improve motor symptoms in PSP, supporting the role of STN hyperactivity in the pathophysiology[@volkmann2018]. However, the benefits are less consistent than in PD, likely because PSP involves multiple neurotransmitter deficits beyond the glutamatergic system.
The neurotransmitter deficits in PSP must be understood in the context of basal ganglia circuitry. The direct pathway (facilitating movement) and indirect pathway (inhibiting movement) both require intact dopaminergic modulation[@delong1986]. In PSP, the combined loss of dopaminergic, GABAergic, and glutamatergic signaling disrupts the normal flow of information through these circuits.
The net effect is increased inhibitory output from the basal ganglia to the thalamus and brainstem, manifesting as bradykinesia, rigidity, and postural instability. However, the specific pattern of neurotransmitter loss determines the relative contributions of different symptoms.
The brainstem nuclei providing modulatory inputs to the forebrain—the cholinergic PPN, serotonergic raphe, and noradrenergic locus coeruleus—collectively form a network regulating arousal, attention, and motor control[@jenkinson2006]. Tau pathology affecting multiple components of this network explains the multi-domain symptoms of PSP.
Current pharmacological approaches targeting neurotransmitter systems in PSP have shown limited efficacy. The following trials represent key therapeutic approaches:
| Agent/Approach | Target | Phase | NCT ID | Status |
|----------------|--------|-------|--------|--------|
| Tilavonemab (ABBV-8E12) | Tau antibody | Phase II | NCT02880910 | Completed - no significant benefit |
| Gosuranemab (BIIB092) | Tau antibody | Phase II | NCT02658916 | Terminated - futility |
| Lithium carbonate | GSK-3β/MAPT | Phase II | NCT05297202 | Recruiting |
| Davunetide (AL-108) | Tau phosphorylation | Phase II/III | NCT00422981 | Failed primary endpoint |
| CoQ10 (ubiquinone) | Mitochondrial function | Phase III | NCT00532571 | Completed - negative |
| Tolfenamic acid | GSK-3β | Phase I/II | NCT04253132 | Completed |
| AMX0035 | SOD1/TDP-43 | Phase II | NCT05358451 | Ongoing - CBS/PSP |
| NIO752 | Tau ASO | Phase I | NCT04539041 | Completed |
| LMTM (Lundbeck) | Tau aggregation | Phase III | NCT01689233 | Failed |
Neurotransmitter dysfunction in PSP can be monitored through several biomarker approaches:
The multi-system neurotransmitter dysfunction creates distinct clinical challenges:
Current pharmacological approaches to PSP target individual neurotransmitter systems with limited success[@ludolph2006]. Levodopa provides modest benefit in some patients but is generally less effective than in PD. Cholinesterase inhibitors have shown minimal cognitive benefits. Amantadine, which enhances dopaminergic transmission and has anti-glutamatergic properties, may offer temporary improvement in some patients.
Novel therapeutic approaches are being developed to address the multi-system neurotransmitter deficits in PSP[@boxer2017]. Tau-directed therapies may slow disease progression by reducing the underlying pathology. Gene therapy approaches aim to deliver neurotrophic factors to specific brain regions. Deep brain stimulation targeting multiple nodes of the affected network shows promise for symptom management.
The neurotransmitter profile of PSP differs from corticobasal syndrome (CBS) and Alzheimer's disease[@alexander2004]. While CBS shares some features with PSP, including GABAergic and cholinergic dysfunction, the regional distribution of deficits varies. AD shows more prominent cholinergic loss and different patterns of monoaminergic dysfunction.
Understanding these differences helps in differential diagnosis and in developing disease-specific therapeutic approaches. The neurotransmitter fingerprint of each disorder reflects the underlying pattern of tau pathology and neuronal vulnerability.
Neurotransmitter dysfunction in PSP is multi-system and reflects the widespread tau pathology affecting subcortical structures, brainstem nuclei, and basal ganglia circuits. The cholinergic, GABAergic, dopaminergic, serotonergic, and glutamatergic systems all contribute to the complex clinical phenotype of PSP. Understanding these disturbances provides insight into disease mechanisms and identifies potential therapeutic targets. Future treatments will likely need to address multiple neurotransmitter systems simultaneously to achieve meaningful clinical benefit.
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The following diagram shows the key molecular relationships involving Neurotransmitter Dysfunction in Progressive Supranuclear Palsy discovered through SciDEX knowledge graph analysis: