Clare Loane

Serotonergic Neurons Mediate Dyskinesia Side Effects in Parkinson's Patients with Neural Transplants

Two patients with Parkinson’s disease, successfully treated with fetal tissue transplants more than a decade ago, developed troublesome involuntary movements, which could be treated with a serotonin receptor agonist. The Two Faces of Fetal Grafts Before stem cells, there were fetal grafts. Pioneering treatments performed in the 1990s in patients with Parkinson’s disease proved that the diseased brain could be repaired, at least for a while. Two of these patients received grafts, one in the putamen and the other in both the caudate and the putamen, of fetal midbrain tissue. For several years, the patients showed mild improvement but eventually were able to function well with no drugs. Recently, however, both have started to experience abnormal uncontrolled movements, which Politis and colleagues have determined are a result of an overabundance of serotonin-using neurons that developed from the graft. A serotonin agonist eliminates these dyskinesias. Brain imaging exposed what was happening in these patients’ brains. When imaged by positron emission tomography, radioactive tracers that tag dopaminergic neurons and that bind to the dopamine receptor showed that the dopamine neurons that decay during Parkinson’s disease were restored by the grafts. Another scan with an agent that binds to the serotonin transporter showed an abnormality; there seemed to be more serotonin neurons than usual. This presented a conundrum because dyskinesias in Parkinson’s disease are thought to be a result of dopamine, not serotonin, stimulation. The authors hypothesized that the explanation lies in the ability of the serotonin neurons to switch to a different neurotransmitter—to adopt dopamine as a so-called false transmitter, releasing it to cause dyskinesias. If this were the case, then desensitizing these serotonin neurons, and so inhibiting their activity, would reduce the dyskinesias. They tested this idea by giving the patients low doses of a serotonin receptor agonist called buspirone. Both patients responded by a sudden and almost complete resolution of the troublesome abnormal movements, suggesting that the excess serotonergic neurons had in fact been pumping out dopamine, causing the dyskinesias. The patients described here are only two of a larger number who received fetal neural tissue implants years ago. In some patients, the grafted cells survived, possibly as a result of stem cells within the graft, and were able to replace the function of the diseased dopamine cells, forming connections with the existing brain cells. Exploration of the long-term consequences of such replacement tissue, such as the atypical movements and their inhibition reported here, is important in that it will inform future treatments with grafts that consist of cells from other sources, such as bioengineered or stem cells. Troublesome involuntary movements in the absence of dopaminergic medication, so-called off-medication dyskinesias, are a serious adverse effect of fetal neural grafts that hinders the development of cell-based therapies for Parkinson’s disease. The mechanisms underlying these dyskinesias are not well understood, and it is not known whether they are the same as in the dyskinesias induced by l-dopa treatment. Using in vivo brain imaging, we show excessive serotonergic innervation in the grafted striatum of two patients with Parkinson’s disease, who had exhibited major motor recovery after transplantation with dopamine-rich fetal mesencephalic tissue but had later developed off-medication dyskinesias. The dyskinesias were markedly attenuated by systemic administration of a serotonin [5-hydroxytryptamine (5-HT)] receptor (5-HT1A) agonist, which dampens transmitter release from serotonergic neurons, indicating that the dyskinesias were caused by the serotonergic hyperinnervation. Our observations suggest strategies for avoiding and treating graft-induced dyskinesias that result from cell therapies for Parkinson’s disease with fetal tissue or stem cells.

Staging of serotonergic dysfunction in Parkinson's Disease: An in vivo 11C-DASB PET study

Thirty Parkinsons disease (PD) patients were divided into three equal groups according to their disease duration while 10 normal healthy volunteers matched for age and sex served as a control group. Striatal and extrastriatal serotonergic function was studied with (11)C-DASB PET, a marker of serotonin transporter availability. (11)C-DASB binding was correlated with disease disability and exposure to dopaminergic therapy. We found significant (11)C-DASB binding reductions in striatal, brainstem, and cortical regions in PD but no correlations were evident between (11)C-DASB binding and UPDRS scores, Hoehn &Yahr staging, disease duration and level of exposure to dopaminergic therapy. Our results suggest that progressive non-linear serotonergic dysfunction occurs in PD but it does not determine levels of disability. Additionally, chronic exposure to dopaminergic therapy does not appear to influence SERT binding.

Depressive symptoms in PD correlate with higher 5-HTT binding in raphe and limbic structures

Background: Depression associated with Parkinson disease (PD) has a different symptom profile to endogenous depression. The etiology of depression in PD remains uncertain though abnormal serotonergic neurotransmission could play a role. Objective: To assess with PET serotonergic function via in vivo serotonin transporter (5-HTT) availability in antidepressant-naive patients with PD. Methods: Thirty-four patients with PD and 10 healthy matched control subjects had a clinical battery of tests including the patient-report Beck Depression Inventory–II (BDI-II), the clinician-report Hamilton Rating Scale for Depression (HRSD), and the structured clinical interview for DSM-IV Axis I Disorders (SCID-I). They underwent 11C-DASB PET, a selective in vivo marker of 5-HTT binding in humans. Results: BDI-II scores correlated with HRSD scores. Ten of 34 patients with PD (29.4%) had BDI-II and HRSD scores above the discriminative cutoff for PD depression though only half of these patients could be classed on SCID-I criteria as having an anxiety/mood disorder. Patients with PD with the highest scores for depression symptoms showed significantly raised 11C-DASB binding in amygdala, hypothalamus, caudal raphe nuclei, and posterior cingulate cortex compared to low score cases, while 11C-DASB binding values in other regions were similarly decreased in depressed and nondepressed patients with PD compared to healthy controls. Conclusion: Depressive symptoms in antidepressant-naive patients with PD correlate with relatively higher 5-HTT binding in raphe nuclei and limbic structures possibly reflecting lower extracellular serotonin levels. Our data are compatible with a key role of abnormal serotonergic neurotransmission contributing to the pathophysiology of PD depression and justify the use of agents acting on 5-HTT.

Serotonergic mechanisms responsible for levodopa-induced dyskinesias in Parkinson's disease patients.

Levodopa-induced dyskinesias (LIDs) are the most common and disabling adverse motor effect of therapy in Parkinsons disease (PD) patients. In this study, we investigated serotonergic mechanisms in LIDs development in PD patients using 11C-DASB PET to evaluate serotonin terminal function and 11C-raclopride PET to evaluate dopamine release. PD patients with LIDs showed relative preservation of serotonergic terminals throughout their disease. Identical levodopa doses induced markedly higher striatal synaptic dopamine concentrations in PD patients with LIDs compared with PD patients with stable responses to levodopa. Oral administration of the serotonin receptor type 1A agonist buspirone prior to levodopa reduced levodopa-evoked striatal synaptic dopamine increases and attenuated LIDs. PD patients with LIDs that exhibited greater decreases in synaptic dopamine after buspirone pretreatment had higher levels of serotonergic terminal functional integrity. Buspirone-associated modulation of dopamine levels was greater in PD patients with mild LIDs compared with those with more severe LIDs. These findings indicate that striatal serotonergic terminals contribute to LIDs pathophysiology via aberrant processing of exogenous levodopa and release of dopamine as false neurotransmitter in the denervated striatum of PD patients with LIDs. Our results also support the development of selective serotonin receptor type 1A agonists for use as antidyskinetic agents in PD.

Serotonin Neuron Loss and Nonmotor Symptoms Continue in Parkinson’s Patients Treated with Dopamine Grafts

Dopamine-rich fetal tissue grafts relieve motor but not nonmotor symptoms in PD patients, requiring additional therapeutic interventions to treat continuing serotonergic denervation. Imaging Cell Therapy in Parkinson’s Disease Parkinson’s disease (PD) is the second most common neurodegenerative disorder affecting about 1 person in every 500 in the United States. A pioneering treatment involving transplantation of dopamine-rich fetal grafts in the brains of patients with PD was initiated nearly 2 decades ago. The goal was to restore dopaminergic neurons in the nigrostriatal pathway that are selectively lost in PD and hence improve motor performance. Although there have been inconsistent results among different trials, some PD patients showed encouraging clinical improvements in motor performance but no improvement in nonmotor symptoms. PD causes not only severe motor symptoms associated with dopamine loss but also nonmotor symptoms such as depression, fatigue, visual hallucinations, and sleep problems that have a significant negative impact on quality of life. Given the importance of nonmotor symptoms in PD, Politis et al. decided to use sophisticated brain imaging techniques to investigate why three PD patients transplanted with fetal grafts 13 to 16 years previously still exhibited nonmotor symptoms even as their motor symptoms improved. When these patients were imaged by positron emission tomography, radioactive tracers that tag dopamine neurons and receptors showed that dopamine neuronal function was restored by the fetal grafts. Also, the principal site of synthesis of another key neurotransmitter, called norepinephrine, was unaffected in these patients. But another scan with an agent that binds to the serotonin transporter and measures the integrity of serotonin-producing neurons showed that there were far fewer serotonin neurons than usual in brain areas related to the regulation of sleep, arousal, feeding, satiety, mood, and emotion. These findings indicate that for more complete, long-term symptomatic relief of both motor and nonmotor symptoms in PD, dopamine neuron replacement with fetal or stem cells will need to be combined with other therapeutic approaches such as additional grafts of serotonin neurons in specific brain areas to relieve nonmotor symptoms by restoring serotonin neurotransmission. Cell therapy studies in patients with Parkinson’s disease (PD) have been confined to intrastriatal transplantation of dopamine-rich fetal mesencephalic tissue in efforts to improve motor performance. Although some PD patients receiving the dopamine-rich grafts showed improvements in motor symptoms due to replacement of dopaminergic neurons, they still suffered from nonmotor symptoms including depression, fatigue, visual hallucinations, and sleep problems. Using functional imaging and clinical evaluation of motor and nonmotor symptoms in three PD patients transplanted with intrastriatal fetal grafts 13 to 16 years previously, we assessed whether reestablishment of dopaminergic neuronal networks is sufficient to improve a broad range of symptoms. At 13 to 16 years after transplantation, dopaminergic innervation was restored to normal levels in basal ganglia and preserved in a number of extrabasal ganglia areas. These changes were associated with long-lasting relief of motor symptoms. Then, we assessed the integrity of their serotonergic and norepinephrine neuronal systems using [11C]DASB {[11C]3-amino-4-(2-dimethylaminomethylphenylthio) benzonitrile} positron emission tomography (PET) and 18F-dopa PET, respectively. 18F-Dopa uptake in the locus coeruleus was within the normal range. In contrast, [11C]DASB uptake in the raphe nuclei and regions receiving serotonergic projections was markedly reduced. These results indicate ongoing degeneration of serotonergic raphe nuclei and their projections to regions involved in the regulation of sleep, arousal, feeding, satiety, mood, and emotion. Our findings indicate that future cell-based therapies using fetal tissue or stem cells in PD patients may require additional grafts of serotonergic neurons to relieve nonmotor symptoms by restoring serotonergic neurotransmission in specific cerebral targets.

Excessive hoarding in Parkinson's disease†

Hoarding is seen in several psychiatric conditions, but has not been specifically assessed in Parkinsons disease (PD). This study investigates hoarding tendency amongst patients with PD, and its association with impulsive‐compulsive spectrum behaviors (ICBs). We compare clinical features, measures of hoarding, impulse buying, self‐control, obsessive‐compulsive symptoms, depression, and anxiety in 39 patients with PD with ICBs (PD + ICB), 61 patients with PD without ICBs (PD − ICB), and 50 healthy controls. A much higher proportion of PD + ICB (27.8%) than PD − ICB (3.5%) were hoarders (P = 0.001). 6% of healthy controls were hoarders. Compulsive shoppers scored higher than other varieties of ICB on excessive acquisition measures. Hoarding correlated positively with impulsive buying, obsessive‐compulsive symptoms, PD duration, and negatively with self‐control measures. Using multivariate regression analyzes, the presence of ICBs and measures of impulsive buying were the only variables independently associated with hoarding in PD. The association of hoarding with other ICBs and low trait impulse control suggests that excessive hoarding is related to the spectrum of impulsive behaviors in PD.

Serotonergic Dysfunction in Parkinson's Disease and Its Relevance to Disability

Growing evidence suggests that Parkinsons disease is not solely affecting the dopaminergic system. Results from biochemical, animal, postmortem, and functional imaging studies have revealed that other neurotransmitter systems are affected as well, including the serotonergic system. With the use of in vivo positron emission tomography functional imaging, it has been shown that serotonergic terminals are affected at a varying, nonlinear degree starting early in the clinical course of Parkinsons disease. Tremor and the majority of nonmotor symptoms do not seem to respond adequately to dopaminergic medication. Recent studies suggest that serotonergic dysfunction has a direct relevance to Parkinsons disease symptoms, the so-called nonmotor symptoms, including depression, fatigue, weight changes, and visual hallucinations. These in vivo findings indicate that agents acting on the serotonergic system could help towards alleviating these symptoms. This paper aims to review the current literature and to highlight the need for further in vivo investigations.

Serotonergic mediated body mass index changes in Parkinson's disease.

More than 50% of patients with Parkinsons disease (PD) are expected to show abnormalities with their weight in a process that starts several years before the diagnosis. The serotonergic (5-HT) system has been proposed to regulate appetite and the 5-HT transporter (SERT) is a key modulator of 5-HT metabolism. Here, we hypothesized that a dysfunctional 5-HT system could be responsible for alterations of weight in PD and we sought to investigate this in vivo. Thirty four PD patients had Body Mass Index (BMI) changes monitored over a 12-month period and one positron emission tomography (PET) brain scan with (11)C-DASB, a selective marker of SERT availability, during their second clinical assessment. Results were compared with those of a group of 10 normal controls. Half (17) of the PD patients showed abnormal BMI changes over the 12-month period; 12 lost while 5 gained weight. PD patients with abnormal BMI changes showed significantly raised (11)C-DASB binding in rostral raphe nuclei, hypothalamus, caudate nucleus and ventral striatum compared to cases with no significant BMI changes. (11)C-DASB binding in other regions was similarly decreased in the PD BMI subgroups compared to normal controls. BMI gainers showed significantly raised (11)C-DASB binding in anterior cingulate cortex (ACC) compared to BMI losers. Our findings suggest that abnormal BMI changes over a 12-month period are linked with relatively raised SERT availability in PD on an overall background of decreased 5-HT function. The regions implicated are the rostral raphe nuclei and its connections to limbic and cognitive areas. It is conceivable that 5-HT agents could help alleviate abnormal changes in BMI in PD.

The role of pallidal serotonergic function in Parkinson's disease dyskinesias: a positron emission tomography study.

We have investigated the role of globus pallidus (GP) serotonergic terminals in the development of levodopa-induced dyskinesias (LIDs) in Parkinsons disease (PD). We studied 12 PD patients without LIDs, 12 PD patients with LIDs, and 12 healthy control subjects. We used (11)C-DASB positron emission tomography (PET), a marker of serotonin transporter availability, and (11)C-raclopride PET to measure changes in synaptic dopamine levels following levodopa administration. PD patients without LIDs showed a significant reduction of GP serotonin transporter binding compared with healthy controls although this was within the normal range in PD patients with LIDs. Levels of GP serotonin transporter binding correlated positively with severity of dyskinesias. (11)C-raclopride PET detected a significant rise in GP synaptic dopamine levels of patients with LIDs after a levodopa challenge but not in patients with a stable response. Our findings indicate that LIDs in PD are associated with higher GP serotonergic function. This increased serotonin function may result in further dysregulation of thalamocortical signals and so promote the expression of dyskinesias.

Ambient particulate matter and its potential neurological consequences.

Abstract Exposure to ambient air pollution has been consistently associated with respiratory and cardiovascular disease. However, the neurological effects of air pollution have received little attention. It is suggested that the components of air pollution, such as particulate matter (PM) and specifically ultrafine particulate matter (UFP), may have the potential to extend beyond pulmonary organs to the central nervous system (CNS) and, ultimately, the brain. The transport mechanisms are not clear, although at least four possible routes have been proposed implicating PM and UFP in neurological disease processes. A limited number of studies have been undertaken to assess the role of PM and UFP in CNS diseases, including migraine, headache, stroke, Alzheimer’s disease, and Parkinson’s disease. Considering the high prevalence of such CNS diseases, along with the frequent and increasing exposure to ambient air pollution, it is important to highlight possible associations with regards to preventative, monitoring, and control measures. This article aimed to review the literature in relation to translocation routes of PM and UFP and their potential role in neurological disease processes.