Ethanol effects on harmaline-induced tremor and increase of cerebellar cyclic GMP

Harmaline toxicity on dorsal striatal neurons and its role in tremor

Harmaline is one of the β-carboline derivative compounds that is widely distributed in the food chain and human tissues. Harmine, a dehydrogenated form of harmaline, appeared to have a higher concentration in the brain, and appeared to be elevated in essential tremor (ET) and Parkinson's disease. Exogenous harmaline exposure in high concentration has myriad consequences, including inducing tremor, and causing neurodegeneration of Purkinje cells in the cerebellum. Harmaline-induced tremor is an established animal model for human ET, but its underlying mechanism is still controversial. One hypothesis posits that the inferior olive-cerebellum pathway is involved, and CaV3.1 T-type Ca2+ channel is a critical target of action. However, accumulating evidence indicates that tremor can be generated without disturbing T-type channels. This implies that additional neural circuits or molecular targets are involved. Using in vitro slice Ca2+-imaging and patch clamping, we demonstrated that harmaline reduced intracellular Ca2+ and suppressed depolarization-induced spiking activity of medium spiny striatal neurons (MSN), and this effect of harmaline can be partially attenuated by sulpiride (5 µM). In addition, the frequencies of spontaneous excitatory post-synaptic currents (sEPSCs) on MSNs were also significantly attenuated. Furthermore, the induced tremor in C57BL/6 J mice by harmaline injections (i.p. 12.5-18 mg/kg) was also shown to be attenuated by sulpiride (20 mg/kg). This series of experiments suggests that the dorsal striatum is a site of harmaline toxic action and might contribute to tremor generation. The findings also provide evidence that D2 signaling might be a part of the mechanism underlying essential tremor.

Cerebellar α6GABAA Receptors as a Therapeutic Target for Essential Tremor: Proof-of-Concept Study with Ethanol and Pyrazoloquinolinones

Ethanol has been shown to suppress essential tremor (ET) in patients at low-to-moderate doses, but its mechanism(s) of action remain unknown. One of the ET hypotheses attributes the ET tremorgenesis to the over-activated firing of inferior olivary neurons, causing synchronic rhythmic firings of cerebellar Purkinje cells. Purkinje cells, however, also receive excitatory inputs from granule cells where the α6 subunit-containing GABAA receptors (α6GABAARs) are abundantly expressed. Since ethanol is a positive allosteric modulator (PAM) of α6GABAARs, such action may mediate its anti-tremor effect. Employing the harmaline-induced ET model in male ICR mice, we evaluated the possible anti-tremor effects of ethanol and α6GABAAR-selective pyrazoloquinolinone PAMs. The burrowing activity, an indicator of well-being in rodents, was measured concurrently. Ethanol significantly and dose-dependently attenuated action tremor at non-sedative doses (0.4-2.4 g/kg, i.p.). Propranolol and α6GABAAR-selective pyrazoloquinolinones also significantly suppressed tremor activity. Neither ethanol nor propranolol, but only pyrazoloquinolinones, restored burrowing activity in harmaline-treated mice. Importantly, intra-cerebellar micro-injection of furosemide (an α6GABAAR antagonist) had a trend of blocking the effect of pyrazoloquinolinone Compound 6 or ethanol on harmaline-induced tremor. In addition, the anti-tremor effects of Compound 6 and ethanol were synergistic. These results suggest that low doses of ethanol and α6GABAAR-selective PAMs can attenuate action tremor, at least partially by modulating cerebellar α6GABAARs. Thus, α6GABAARs are potential therapeutic targets for ET, and α6GABAAR-selective PAMs may be a potential mono- or add-on therapy.

Is inferior olive central to the pathophysiology of essential tremor? No

Essential tremor (ET) represents one of the commonest movement disorder worldwide and is the most common tremor disorder. ET manifests with various combinations of motor and nonmotor symptoms. The clinical hallmark is a kinetic tremor of upper limbs. Historically, the pathogenesis of ET has been based on the hypothesis of an overactivity of the inferior olive (inferior olive hypothesis: IOH) where the inferior olive would act as the central pace-maker of ET, resulting in impaired electrophysiological discharges of the olivo-cerebellar tract. The absence of structural alterations in post-mortem studies of the inferior olive is a striking argument against the IOH. Furthermore, neuroimaging studies point towards the implication of the cerebello-thalamo-cerebral pathway rather than the IO, and the harmaline model which has been considered as an animal model of ET presents important weaknesses. By contrast, a series of experiments by Louis et al. have provided convincing evidence of impaired wiring of the Purkinje cell microcircuitry and progressive neurodegeneration of the cerebellar cortex. The Purkinje neuron appears as the primary culprit (Purkinjopathy). The cerebellar cortex hypothesis (CCH) has solid neuropathological signatures, unlike the purely physiological IOH. Rather than a dysregulatory electrophysiological disorder suggested by IOH, ET is a clinical-pathological entity similar to late onset neurodegenerative disorders such as Parkinson's disease or Alzheimer's disease. The CCH emphasizes the need to develop novel therapeutic strategies in order to maintain or promote the cerebellar reserve. The modern reconceptualization of ET in a genuine cerebellar disorder is cleaning the IOH to the light of histopathological studies. ET falls in the large basket of the neurodegenerative diseases and we have entered into a novel formulation of the disease pathogenesis with direct impacts on future therapies.

Is the inferior olive central to essential tremor? Yes

We consider the question whether the inferior olive (IO) is required for essential tremor (ET). Much evidence shows that the olivocerebellar system is the main system capable of generating the widespread synchronous oscillatory Purkinje cell (PC) complex spike (CS) activity across the cerebellar cortex that would be capable of generating the type of bursting cerebellar output from the deep cerebellar nuclei (DCN) that could underlie tremor. Normally, synchronous CS activity primarily reflects the effective electrical coupling of IO neurons by gap junctions, and traditionally, ET research has focused on the hypothesis of increased coupling of IO neurons as the cause of hypersynchronous CS activity underlying tremor. However, recent pathology studies of brains from humans with ET and evidence from mutant mice, particularly the hotfoot17 mouse, that largely replicate the pathology of ET, suggest that the abnormal innervation of multiple Purkinje cells (PCs) by climbing fibers (Cfs) is related to tremor. In addition, ET brains show partial PC loss and axon terminal sprouting by surviving PCs. This may provide another mechanism for tremor. It is proposed that in ET, these three mechanisms may promote tremor. They all involve hypersynchronous DCN activity and an intact IO, but the level at which excessive synchronization occurs may be at the IO level (from abnormal afferent activity to this nucleus), the PC level (via aberrant Cfs), or the DCN level (via terminal PC collateral innervation).

Considerations Using Harmaline for a Primate Model of Tremor

Background

While harmaline has been used as a pharmacological model of essential tremor (ET) in rodents and pigs, less is known about the effects of this pharmacological treatment in awake-behaving non-human primates. In this study, we investigated the time-course, amplitude, frequency, and consistency of harmaline tremor in primates.

Methods

Three rhesus macaques were administered doses of harmaline ranging from 2-12 mg/kg (i.m.), and tremorous movements were quantified with accelerometers. One subject was also trained to perform a self-paced cued reaching task, with task engagement assessed under harmaline doses ranging from 2-8 mg/kg (i.m.).

Results

Whole-body tremors manifested within 30 minutes of threshold-dose administration, and peak oscillatory frequency ranged between 10-14 Hz. However, large differences in tremor intensity and intermittency were observed across individual subjects under similar dosing levels. Additionally, engagement with the reaching task was dependent on harmaline dose, with performance mostly unaffected at 2 mg/kg and with little task-engagement at 8 mg/kg.

Discussion

We provide a detailed assessment of factors that may underlie the heterogeneous responses to harmaline, and lay out important caveats towards the applicability of the behaving harmaline-tremoring non-human primate as a preclinical model for ET.

Highlights

The harmaline-primate is revisited for its potential as a preclinical model of tremor. Spontaneous tremor was heterogenous in amplitude across subjects despite similar harmaline doses, action tremors were not consistently observed, and performance on a behavioral task degraded with higher dosages.

Increased Purkinje Cell Complex Spike and Deep Cerebellar Nucleus Synchrony as a Potential Basis for Syndromic Essential Tremor. A Review and Synthesis of the Literature

We review advances in understanding Purkinje cell (PC) complex spike (CS) physiology that suggest increased CS synchrony underlies syndromic essential tremor (ET). We searched PubMed for papers describing factors that affect CS synchrony or cerebellar circuits potentially related to tremor. Inferior olivary (IO) neurons are electrically coupled, with the degree of coupling controlled by excitatory and GABAergic inputs. Clusters of coupled IO neurons synchronize CSs within parasagittal bands via climbing fibers (Cfs). When motor cortex is stimulated in rats at varying frequencies, whisker movement occurs at ~10 Hz, correlated with synchronous CSs, indicating that the IO/CS oscillatory rhythm gates movement frequency. Intra-IO injection of the GABA_A receptor antagonist picrotoxin increases CS synchrony, increases whisker movement amplitude, and induces tremor. Harmaline and 5-HT_2a receptor activation also increase IO coupling and CS synchrony and induce tremor. The hotfoot17 mouse displays features found in ET brains, including cerebellar GluRδ2 deficiency and abnormal PC Cf innervation, with IO- and PC-dependent cerebellar oscillations and tremor likely due to enhanced CS synchrony. Heightened coupling within the IO oscillator leads, through its dynamic control of CS synchrony, to increased movement amplitude and, when sufficiently intense, action tremor. Increased CS synchrony secondary to aberrant Cf innervation of multiple PCs likely also underlies hotfoot17 tremor. Deep cerebellar nucleus (DCN) hypersynchrony may occur secondary to increased CS synchrony but might also occur from PC axonal terminal sprouting during partial PC loss. Through these combined mechanisms, increased CS/DCN synchrony may plausibly underlie syndromic ET.

Harmaline tremor: underlying mechanisms in a potential animal model of essential tremor

BACKGROUND

Harmaline and harmine are tremorigenic β-carbolines that, on administration to experimental animals, induce an acute postural and kinetic tremor of axial and truncal musculature. This drug-induced action tremor has been proposed as a model of essential tremor. Here we review what is known about harmaline tremor.

METHODS

Using the terms harmaline and harmine on PubMed, we searched for papers describing the effects of these β-carbolines on mammalian tissue, animals, or humans.

RESULTS

Investigations over four decades have shown that harmaline induces rhythmic burst-firing activity in the medial and dorsal accessory inferior olivary nuclei that is transmitted via climbing fibers to Purkinje cells and to the deep cerebellar nuclei, then to brainstem and spinal cord motoneurons. The critical structures required for tremor expression are the inferior olive, climbing fibers, and the deep cerebellar nuclei; Purkinje cells are not required. Enhanced synaptic norepinephrine or blockade of ionic glutamate receptors suppresses tremor, whereas enhanced synaptic serotonin exacerbates tremor. Benzodiazepines and muscimol suppress tremor. Alcohol suppresses harmaline tremor but exacerbates harmaline-associated neural damage. Recent investigations on the mechanism of harmaline tremor have focused on the T-type calcium channel.

DISCUSSION

Like essential tremor, harmaline tremor involves the cerebellum, and classic medications for essential tremor have been found to suppress harmaline tremor, leading to utilization of the harmaline model for preclinical testing of antitremor drugs. Limitations are that the model is acute, unlike essential tremor, and only approximately half of the drugs reported to suppress harmaline tremor are subsequently found to suppress tremor in clinical trials.

Blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations in essential tremor: repeat observation in cases and controls in New York

Essential tremor (ET) is a widespread late-life neurological disease. Genetic and environmental factors are likely to play important etiological roles. Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing neurotoxin. Previously, elevated blood harmane concentrations were demonstrated in ET cases compared to controls, but these observations have all been cross-sectional, assessing each subject at only one time point. Thus, no one has ever repeat-assayed blood harmane in the same subjects twice. Whether the observed case-control difference persists at a second time point, years later, is unknown. The current goal was to reassess a sample of our ET cases and controls to determine whether blood harmane concentration remained elevated in ET at a second time point. Blood harmane concentrations were quantified by a well-established high-performance liquid chromatography method in 63 ET cases and 70 controls. A mean of approximately 6 yr elapsed between the initial and this subsequent blood harmane determination. The mean log blood harmane concentration was significantly higher in cases than controls (0.30 ± 0.61 g(-10)/ml versus 0.08 ± 0.55 g(-10)/ml), and the median value in cases was double that of controls: 0.22 g(-10)/ml versus 0.11 g(-10)/ml. The log blood harmane concentration was highest in cases with a family history of ET. Blood harmane concentration was elevated in ET cases compared to controls when reassessed at a second time point several years later, indicating what seems to be a stable association between this environmental toxin and ET.

An open-label, single-dose, crossover study of the pharmacokinetics and metabolism of two oral formulations of 1-octanol in patients with essential tremor

Existing therapeutic options for management of essential tremor are frequently limited by poor efficacy and adverse effects. Likely the most potent tremor suppressant used is ethanol, although its use is prohibitive due to a brief therapeutic window, and the obvious implications of excessive alcohol use. Longer-chain alcohols have been shown to suppress tremor in harmaline animal models, and appear to be safe and well tolerated in 2 prior studies in humans. Here we report on the findings of a phase I/II study of 1-octanol designed to explore pharmacokinetics, efficacy, and safety. The most significant finding was the identification of octanoic acid as the product of rapid 1-octanol metabolism. Furthermore, the temporal profile of efficacy closely matches the plasma concentration of octanoic acid. Therefore, these findings identify a novel class of compound (e.g., carboxylic acids) with tremor suppressive properties in ET. Administration of 1-octanol also appears to be safe based on various measures collected. Essential tremor (ET) is the most common tremor disorder, with tremors occurring during static posturing or movement. These tremors are known to briefly improve in many cases after alcohol (ethanol) consumption. Two previous studies of a longer chain alcohol, 1-octanol, have demonstrated longer duration tremor-suppressive effects without the occurrence of intoxication. The aim of this study was to characterize the pharmacokinetics of 1-octanol and its primary metabolite octanoic acid using two formulations, along with additional safety and efficacy measures. Participants with proven ethanol-responsive ET were recruited into 1 of 2 parts: (part A) a dose escalation study (1-64 mg/kg; n = 4), and (part B) a fixed dose (64 mg/kg; n = 10) balanced, open-label crossover design. Two participants in part B then completed an exploratory part C evaluating 128 mg/kg.Plasma samples were collected at 10 intervals during a 6-hour period postingestion. Efficacy was assessed using spirography, whereas safety was assessed with electrocardiograms, vital signs, adverse effects surveys, and an intoxication assessment. Plasma concentrations of 1-octanol were detectable at low levels whereas octanoic acid (OA) concentrations were approximately 100-fold higher. The half-life of OA was 87.6 minutes. This was matched by a clinical reduction in tremor severity of 32% at 90 minutes, assessed using spirography. The safety profile was favorable, with the most commonly reported adverse effect being dysgeusia (38%). Early detection and higher plasma concentrations of OA are a product of rapid metabolism of 1-octanol.OA pharmacokinetics mirrored the timing of clinical improvement. These findings provide preliminary evidence for a new class of compound that may be effective in the treatment of ET.

The effect of memantine in harmaline-induced tremor and neurodegeneration

Essential tremor (ET) is one of the most common and most disabling movement disorders among adults. The drug treatment of ET remains unsatisfactory. Additional therapies are required for patients with inadequate response or intolerable side effects. The current study aims to investigate the anti-tremogenic and neuroprotective effects of memantine (NMDA receptor antagonist) on the harmaline model of transient action tremor. The effects of memantine were further compared with ethanol. Three separate groups of male Wistar rats were injected either with saline, ethanol (1.5 gr/kg), or memantine (5 mg/kg) 15 min prior to a single intraperitoneal injection of harmaline (20 mg/kg). Tremor and locomotion were evaluated by a custom-built tremor and locomotion analysis system. After 24 h of harmaline injection, cellular viability, and apoptosis were assessed using crystal violet staining, and caspase-3 immunostaining, respectively. Harmaline caused neuronal cell loss and caspase-3 mediated apoptosis in cerebellar granular and purkinje cells as well as the inferior olivary neurons. Despite a reduction in tremor intensity and duration with ethanol, this compound resulted in cell loss in cerebellum and olivary nucleus. Memantine exhibited neuroprotective efficacy on cerebellar and inferior olivary neurons albeit weaker anti-tremor effect compared to ethanol. In conclusion, anti-tremogenic and neuroprotective effects do not necessarily overlap. Memantine is a potential treatment for ET particularly given its neuroprotective efficacy.

Beta-carboline alkaloids and essential tremor: exploring the environmental determinants of one of the most prevalent neurological diseases

Essential tremor (ET) is among the most prevalent neurological diseases, yet its etiology is not well understood. Susceptibility genotypes undoubtedly underlie many ET cases, although no genes have been identified thus far. Environmental factors are also likely to contribute to the etiology of ET. Harmane (1-methyl-9H-pyrido[3,4-beta]indole) is a potent, tremor-producing beta-carboline alkaloid, and emerging literature has provided initial links between this neurotoxin and ET. In this report, we review this literature. Two studies, both in New York, have demonstrated higher blood harmane levels in ET cases than controls and, in one study, especially high levels in familial ET cases. Replication studies of populations outside of New York and studies of brain harmane levels in ET have yet to be undertaken. A small number of studies have explored several of the biological correlates of exposure to harmane in ET patients. Studies of the mechanisms of this putative elevation of harmane in ET have explored the role of increased dietary consumption, finding weak evidence of increased exogenous intake in male ET cases, and other studies have found initial evidence that the elevated harmane in ET might be due to a hereditarily reduced capacity to metabolize harmane to harmine (7-methoxy-1-methyl-9H-pyrido[3,4-beta]-indole). Studies of harmane and its possible association with ET have been intriguing. Additional studies are needed to establish more definitively whether these toxic exposures are associated with ET and are of etiological importance.

Relationship between blood harmane and harmine concentrations in familial essential tremor, sporadic essential tremor and controls

INTRODUCTION

Harmane, a potent tremor-producing β-carboline alkaloid, may play a role in the etiology of essential tremor (ET). Blood harmane concentrations are elevated in ET cases compared with controls yet the basis for this elevation remains unknown. Decreased metabolic conversion (harmane to harmine) is one possible explanation. Using a sample of >500 individuals, we hypothesized that defective metabolic conversion of harmane to harmine might underlie the observed elevated harmane concentration in ET, and therefore expected to find a higher harmane to harmine ratio in familial ET than in sporadic ET or controls.

METHODS

Blood harmane and harmine concentrations were quantified by high performance liquid chromatography.

RESULTS

There were 78 familial ET cases, 187 sporadic ET cases, and 276 controls. Blood harmane and harmine concentrations were correlated with one another (Spearman's r=0.24, p<0.001). The mean (±SD) harmane/harmine ratio=23.4±90.9 (range=0.1-987.5). The harmane/harmine ratio was highest in familial ET (46.7±140.4), intermediate in sporadic ET (28.3±108.1), and lowest in controls (13.5±50.3) (p=0.03). In familial ET cases, there was no association between this ratio and tremor severity (Spearman's r=0.08, p=0.48) or tremor duration (Spearman's r=0.14, p=0.24).

CONCLUSION

The basis for the elevated blood harmane concentration, particularly in familial ET, is not known, although the current findings (highest harmane/harmine ratio in familial ET cases) lends support to the possibility that it could be the result of a genetically-driven reduction in harmane metabolism.

GABAergic dysfunction in essential tremor: an 11C-flumazenil PET study

Essential tremor is the most common movement disorder, but the underlying pathophysiology is not well understood. A primary overactivity of cerebellothalamic output pathways is the most conspicuous finding, as indicated by animal and human studies. It has been argued that this overactivity may be due to impaired central inhibition, and converging evidence points toward a potential role of gamma-aminobutyric acid (GABA) dysfunction in tremor generation.

METHODS

Using (11)C-flumazenil and PET, we calculated the distribution volume, an index of availability of benzodiazepine receptor sites of the GABA(A) complex, in a group of 8 patients with bilateral essential tremor, as compared with 11 healthy controls.

RESULTS

Significant increases in binding of (11)C-flumazenil at the benzodiazepine receptor site of the GABA(A) receptor in the cerebellum, the ventrolateral thalamus, and the lateral premotor cortex were identified in the essential tremor group.

CONCLUSION

Essential tremor is associated with reduced GABAergic function and increased availability of benzodiazepine receptor sites in brain regions implicated specifically in tremor genesis. This finding is thought to reflect overactivity of cerebellothalamic circuits and, hence, lends support to the "GABA hypothesis" of essential tremor.

Environmental epidemiology of essential tremor

BACKGROUND

Essential tremor (ET) is one of the most common neurological disorders. Despite this, the disease mechanisms and etiology are not well understood. While susceptibility genotypes undoubtedly underlie many ET cases, no ET genes have been identified thus far. As with many other progressive, degenerative neurological disorders, it is likely that environmental factors contribute to the etiology of ET. Environmental epidemiology is the study in specific populations or communities of the effect on human health of physical, biologic and chemical factors in the external environment. The purpose of this article is to review current knowledge with regards to the environmental epidemiology of ET.

RESULTS

As will be discussed, a series of preliminary case-control studies in recent years has begun to explore several candidate toxins/exposures, including harmane (1-methyl-9H-pyrido[3,4-b]indole), lead and agricultural exposures/pesticides.

CONCLUSIONS

While several initial results are promising, as will be discussed, additional studies are needed to more definitively establish whether these exposures are associated with ET and if they are of etiological importance.

Elevated blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations in essential tremor

Essential tremor (ET) is a widespread late-life neurological disease. Genetic and environmental factors likely play an etiological role. Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing neurotoxin. In 2002, we demonstrated elevated blood harmane concentrations in an initial sample of 100 ET cases compared to 100 controls. Between 2002 and 2007, we assembled a new and larger sample of ET cases and controls. We now attempt to replicate our previous findings. Cases and controls were frequency-matched on age, gender, and race. Blood harmane concentrations were quantified by high-performance liquid chromatography. Subjects comprised 150 ET cases and 135 controls (mean age 65.3+/-15.5 vs. 65.5+/-14.2 years, p=0.94). Mean log blood harmane concentration was approximately 50% higher in cases than controls (0.50+/-0.54g(-10)/ml vs. 0.35+/-0.62g(-10)/ml, p=0.038). In a logistic regression analysis, log blood harmane concentration was associated with ET (OR(adjusted) 1.56, 95% CI 1.01-2.42, p=0.04), and odds of ET was 1.90 (95% CI 1.07-3.39, p=0.029) in the highest versus lowest log blood harmane tertile. Log blood harmane was highest in ET cases with familial ET (0.53+/-0.57g(-10)/ml), intermediate in cases with sporadic ET (0.43+/-0.45g(-10)/ml) and lowest in controls (0.35+/-0.62g(-10)/ml) (test for trend, p=0.026). Blood harmane appears to be elevated in ET. The higher concentrations in familial ET suggests that the mechanism may involve genetic factors.

Rodent models of tremor

This review focuses on rodent models of tremor, particularly those induced by pharmacological agents. Harmaline is one of the most frequently used tremor-generating drugs and harmaline-induced tremor is regarded as a model of essential tremor. Harmaline acts on inferior olive neurons, causing enhanced neuronal synchrony and rhythmicity in the olivocerebellar system. In addition, it selectively induces cerebellar Purkinje cell death, speculatively because of excessive glutamate release from nerve terminals of the olivocerebellar system onto Purkinje cells. Systemic administration of cholinomimetics can also produce generalized tremor, and muscarinic receptors on striatal neurons are thought to be the best candidate for the tremor-generating mechanism. On the other hand, dopaminergic neurotoxins, which are used in models of parkinsonism, have yet to be used for experimental analysis of tremor, because tremors induced by dopamine depletion in rodents are less remarkable than those induced by harmaline or cholinomimetics. Recently developed gamma-aminobutyric acid (GABA)(A) receptor alpha-1 subunit knockout mice exhibit postural and kinetic tremors, and clearly reproduce the features of essential tremors. Although from a phenomenological point of view, rodent models of tremor cannot entirely mimic human tremor disorders, they have useful advantages in the analysis of pathophysiological mechanisms underlying tremor. Development of convenient and reproducible methods for evaluating rodent tremor is therefore recommended.

Carbenoxolone and mefloquine suppress tremor in the harmaline mouse model of essential tremor

Excessive olivo-cerebellar synchrony is implicated in essential tremor. Because synchrony in some networks is mediated by gap junctions, we examined whether the gap junction blockers heptanol, octanol, carbenoxolone, and mefloquine suppress tremor in the mouse harmaline model, and performed an open-treatment clinical study of mefloquine for essential tremor. Digitized motion was used to quantify tremor in mice administered harmaline, 20 mg/kg s.c. In mice the broad-spectrum gap junction blockers heptanol, octanol (350 mg/kg i.p. each), and carbenoxolone (20 mg/kg) suppressed harmaline tremor. Mefloquine (50 mg/kg), which blocks gap junctions containing connexin 36, robustly suppressed harmaline tremor. Glycyrrhizic acid (related to carbenoxolone) and chloroquine (related to mefloquine), which do not block gap junctions, failed to suppress harmaline tremor in mice. Clinically, tremor was assessed with standard rating scales, and subjects asked to take 62.5, 125, and 250 mg mefloquine weekly for 12 weeks at each dose. None of the four human subjects showed a meaningful tremor reduction with mefloquine, likely because clinical levels were below those required for efficacy. In view of recent genetic evidence, the anti-tremor mechanism of these compounds is uncertain but may represent a novel therapeutic target, possibly involving gap junctions other than those containing connexin 36.

Benefits and risks of pharmacological and surgical treatments for essential tremor: disease mechanisms and current management

Essential tremor (ET) is a neurological disease (and possibly a family of diseases) whose most recognisable feature is an action tremor of the hands and occasionally of the voice and head. Current data support the view that CNS gamma-amino-butyric acid (GABA)-ergic mechanisms may underlie ET and that the tremor may be further modulated by peripheral (muscle) adrenoreceptors. Potential pharmacotherapeutic options, targeted to influence the activity of the neurotransmitter GABA within the CNS and the peripheral adrenergic receptors, are part of the current armamentarium to treat ET. As such, primidone and propranolol remain the mainstays of the therapy for ET. Intramuscular injections of botulinum toxin A may play a role in the treatment of voice and head tremor. Surgical options, which are reserved for patients with severe, medically-refractory tremor, provide adequate tremor control in the majority of patients. As with other progressive neurological disorders of late life, the ability to use neuroprotective medications to intervene in the developing disease to either slow or halt the progression of the pathological process, would involve an understanding of underlying disease mechanisms. The understanding of these mechanisms in ET is limited and further study of these mechanisms is critical for the development of such therapies.

Blood harmane concentrations and dietary protein consumption in essential tremor

BACKGROUND

Beta-carboline alkaloids (e.g., harmane) are highly tremorogenic chemicals. Animal protein (meat) is the major dietary source of these alkaloids. The authors previously demonstrated that blood harmane concentrations were elevated in patients with essential tremor (ET) vs controls. Whether this difference is due to greater animal protein consumption by patients or their failure to metabolize harmane is unknown.

OBJECTIVE

The aim of this study was to determine whether patients with ET and controls differ with regard to 1) daily animal protein consumption and 2) the correlation between animal protein consumption and blood harmane concentration.

METHODS

Data on current diet were collected with a semiquantitative food frequency questionnaire and daily calories and consumption of animal protein and other food types was calculated. Blood harmane concentrations were log-transformed (logHA).

RESULTS

The mean logHA was higher in 106 patients than 161 controls (0.61 +/- 0.67 vs 0.43 +/- 0.72 g(-10)/mL, p = 0.035). Patients and controls consumed similar amounts of animal protein (50.2 +/- 19.6 vs 49.4 +/- 19.1 g/day, p = 0.74) and other food types (animal fat, carbohydrates, vegetable fat) and had similar caloric intakes. In controls, logHA was correlated with daily consumption of animal protein (r = 0.24, p = 0.003); in patients, there was no such correlation (r = -0.003, p = 0.98).

CONCLUSIONS

The similarity between patients and controls in daily animal protein consumption and the absence of the normal correlation between daily animal protein consumption and logHA in patients suggests that another factor (e.g., a metabolic defect) may be increasing blood harmane concentration in patients.

Harmaline-induced tremor as a potential preclinical screening method for essential tremor medications

No preclinical method to evaluate potential new medications for essential tremor (ET) is available currently. Although harmaline tremor is a well known animal model of ET, it has not found utility as a preclinical drug screen and has not been validated with anti-ET medications. We measured harmaline tremor in rats (10 mg/kg s.c.) and mice (20 mg/kg s.c.) with a load sensor under the cage floor and performed spectral analysis on 20-minute epochs. The motion power over the tremor frequency bandwidth (8-12 Hz in rats; 10-16 Hz in mice) was divided by the motion power over the full motion frequency range (0-15 Hz in rats; 0-34 Hz in mice). The use of these measures greatly reduced data variability, permitting experiments with small sample sizes. Three drugs that suppress ET (propranolol, ethanol, and octanol) all significantly suppressed harmaline-induced tremor. We propose that, with this methodology, harmaline-induced tremor may be useful as a preclinical method to identify potential medications for ET.

Vagus nerve stimulation inhibits harmaline-induced tremor

Excessive olivo-cerebellar burst-firing occurs during harmaline-induced tremor. This system receives rich sensory inputs, including visceral. We hypothesized that electrical vagus nerve stimulation (VNS) would suppress harmaline tremor, as measured with digitized motion power in the rat. Cervical vagus nerve stimulation suppressed power in the 8-12-Hz tremor range by 40%, whereas sham stimulation was ineffective. This study raises the possibility that activation of various sensory modalities, as well as visceral, may reduce tremor.

Baclofen attenuates harmaline induced tremors in rats

Recent experimental and clinical studies clearly suggest the role of gamma-aminobutyric acid (GABA) in the pathogenesis of tremors. The present study was undertaken to investigate the effect of baclofen, a GABA B receptor agonist on harmaline induced tremors. Four groups of female Wistar rats weighing 100+/-15 g were injected with harmaline (10 mg/kg, intraperitoneally) for inducing experimental tremors. The animals in groups 2, 3 and 4 were given baclofen by gavage at doses of 2.5, 5 and 10 mg/kg, respectively, half an hour before harmaline administration, whereas, the rats in group 1 served as control and received water. The latency of onset, intensity and duration of tremor and electromyographic (EMG) responses were recorded. Treatment with baclofen resulted in a dose dependent decrease in the intensity of tremor. Our EMG study also revealed a significant decrease in the amplitude of tremors in baclofen treated rats. A highly significant increase in latency of onset of tremor was observed in the rats treated with high dose (10 mg/kg) of baclofen only. This study clearly suggests beneficial effects of baclofen in harmaline induced tremors.

Suppression of harmaline-induced tremor in rats by vagus nerve stimulation

We studied whether vagus nerve stimulation could suppress tremor in the harmaline tremor model in the rat. Animals were chronically implanted with helical leads around the left vagus nerve and a disk-shaped electrode positioned subcutaneously in the dorsal neck. Harmaline-induced tremor was recorded on a physiograph while each animal received a sequence of five 20-minute trials. Each trial consisted of five minutes of pre-stimulation baseline, five minutes of vagus nerve stimulation, and ten minutes of post-stimulation. Vagus nerve stimulation significantly suppressed harmaline-induced tremor. The suppressive effect was present within the first minute of stimulation and was reproducible across the five trials of the study. The results of this study suggest that the central generator or expression of tremor in the harmaline animal model can be suppressed by vagus nerve stimulation.

Animal models of tremor

Animal models of tremor have been widely used in experimental neurology, because they are an indispensable requirement for understanding the pathophysiology of human tremor disorders and the development of new therapeutic agents. This review focuses on three approaches to produce tremor in animals (application of tremorgenic drugs, experimental central nervous system lesions, study of genetic mutants) and their use in simulating tremor syndromes of humans. Whereas harmaline induces a postural/kinetic tremor in animals that shares some features with human essential tremor/enhanced physiological tremor, MPTP tremor is the best model available for rest tremor in people. The tremor following experimental lesion of the ventromedial tegmentum in primates closely resembles Holmes tremor in humans, whereas cerebellar intention tremor is mimicked by cooling of the lateral cerebellar nuclei. The "campus syndrome," discovered in a breed of Pietrain pigs, might be a useful model of human orthostatic tremor. However, no animal model has yet been generated that exactly recreates all features of any of the known tremor disorders in humans. Problems encountered when comparing tremor in animals and humans include differing tremor frequencies and the uncertainty, if specific transmitter abnormalities/central nervous system lesions seen in animal tremor models are characteristic for their human counterparts. The search for adequate tremor models continues.

Functional imaging of tremor

We review the insight that has been obtained from positron emission tomography (PET) and magnetic resonance imaging (MRI) activation studies on the functional anatomy underlying different forms of tremor. The effects of treatment, both pharmacologic and interventional approaches, on the dysfunctional cerebral systems involved in tremor, also are considered. Finally, we speculate on the neuropharmacologic basis of different types of tremor based on available in vivo PET evidence.

Animal models of action tremor

This article is devoted to animal models of tremors that emerge from lesions in the Guillain-Mollaret triangle. Cerebellar intention tremor is caused by lesions in the brachium conjunctivum or in the interpositus nucleus, possibly in combination with damage to the dentate nucleus. Impaired feed-forward motor control delays the braking of rapid movements, resulting in target overshoot and subsequent oscillation. Transcortical and transcerebellar sensorimotor loops undergo oscillation at a frequency that depends on the mechanical properties of the limb and the length of the sensorimotor loop (mechanical reflex oscillation). The crescendo quality of intention tremor may be a result of amplification of tremor in reverberating brain stem-cerebellar or thalamocortical loops. So-called rubral or midbrain tremor is caused by a combination of damage to the brachium conjunctivum and nigrostriatal pathways in the vicinity of the red nucleus. Secondary compensatory changes in the motor system are probably involved because midbrain tremor in people usually begins weeks or months after a midbrain stroke or trauma. Harmaline causes enhanced neuronal synchrony and rhythmicity in the inferior olive; this animal model, although as yet unproven, is the most popular one for essential tremor (ET). Additional studies in laboratory animals are needed to define the seemingly universal involvement of the cerebellum and ventrolateral thalamus (ventralis intermedius [Vim]) in virtually all human tremor disorders.

The effect of theophylline on essential tremor: the possible role of GABA

The chronic administration of theophylline was studied in twenty patients with essential tremor in a double-blind cross-over trial. The tremor was improved significantly after four weeks of treatment. In mice the chronic administration of theophylline was compared with propranolol on the modulation by adenosine, 5-HT, (-)isoprenaline or GABA of NMDA-induced depolarisation of neocortical slices. Adenosine depolarisation was abolished by two-weeks treatment with theophylline but not propranolol. Potentiation by (-)isoprenaline of NMDA responses was reduced by theophylline (100 mg/kg/day) and propranolol treatment (25 mg/kg/day), but a lower dose of propranolol further increased it. The enhancement by 5-HT of NMDA-induced depolarisation was unaffected by the pretreatment with theophylline, while the higher dose of propranolol blocked it. GABA caused no significant change of NMDA depolarisation in control slices, but after theophylline treatment (100 mg/kg/day) and propranolol administration at both doses it significantly potentiated NMDA depolarisation. The enhancement of GABA sensitivity might be an important common factor in decreasing the essential tremor after propranolol and theophylline treatment.

The benzodiazepine antagonist Ro 15-1788 reverses the effect of methyl-beta-carboline-3-carboxylate but not of harmaline on cerebellar cGMP and motor performance in mice

The cerebellar cGMP level in mice was decreased in a dose-dependent manner 30 min after diazepam (ED50 = 2 mg/kg p.o.). This effect was reversed by the specific benzodiazepine antagonist Ro 15-1788. Methyl-beta-carboline-3-carboxylate (beta-CCM) and harmaline increased cGMP. Ro 15-1788 dose dependently counteracted the beta-CCM- but not the harmaline-induced increase in cGMP. In the horizontal wire test Ro 15-1788 antagonized the impairment of motor performance induced by beta-CCM, but not that induced by harmaline. These findings further support the view that harmaline in contrast to beta-carboline-3-carboxylates does not act through benzodiazepine receptors, and that Ro 15-1788 antagonizes only those convulsants and stimulants that act through specific benzodiazepine receptors.