Analysis of the brain regions involved in myoclonus produced by intracerebral picrotoxin

A Low-Correlation Resting State of the Striatum during Cortical Avalanches and Its Role in Movement Suppression

During quiet resting behavior, involuntary movements are suppressed. Such movement control is attributed to cortico-basal ganglia loops, yet population dynamics within these loops during resting and their relation to involuntary movements are not well characterized. Here, we show by recording cortical and striatal ongoing population activity in awake rats during quiet resting that intrastriatal inhibition maintains a low-correlation striatal resting state in the presence of cortical neuronal avalanches. Involuntary movements arise from disturbed striatal resting activity through two different population dynamics. Nonselectively reducing intrastriatal γ-aminobutyric acid (GABA) receptor-A inhibition synchronizes striatal dynamics, leading to involuntary movements at low rate. In contrast, reducing striatal interneuron (IN)-mediated inhibition maintains decorrelation and induces intermittent involuntary movements at high rate. This latter scenario was highly effective in modulating cortical dynamics at a subsecond timescale. To distinguish intrastriatal processing from loop dynamics, cortex-striatum-midbrain cultures, which lack feedback to cortex, were used. Cortical avalanches in vitro were accompanied by low-correlated resting activity in the striatum and nonselective reduction in striatal inhibition synchronized striatal neurons similar to in vivo. Importantly, reduction of inhibition from striatal INs maintained low correlations in the striatum while reorganizing functional connectivities among striatal neurons. Our results demonstrate the importance of two major striatal microcircuits in distinctly regulating striatal and cortical resting state dynamics. These findings suggest that specific functional connectivities of the striatum that are maintained by local inhibition are important in movement control.

Why don’t neuronal “avalanches” in resting-state cortex cause involuntary movements? This study shows that a low-correlation striatal resting state suppresses such movements and explores mechanisms that disrupt this inhibition.

Even in the absence of apparent motor output, the brain produces a rich repertoire of neuronal activity patterns known as “resting state” activity. In the outer layer of the cortex, resting state patterns emerge as neuronal avalanches, precisely scale-invariant spatiotemporal bursts that often engage large populations of neurons. Little is known about how the brain suppresses involuntary movements during such activity. Here, we show that the striatum, which is part of the cortex-basal ganglia loop, maintains a low-correlation state during resting activity. By using a combination of in vivo and in vitro approaches with pharmacological manipulations, we demonstrate that the precise configuration of this low-correlation state effectively contributes to involuntary movements. Nonselective blockade of intra-striatal inhibition abolished the low-correlation striatal resting state, barely affected cortical avalanches, and led to involuntary movements at low rate. In contrast, selectively reducing striatal interneuron inhibition strongly affected cortical avalanches and triggered involuntary movements at high rate while maintaining a relatively decorrelated striatal resting state. Our results demonstrate the importance of different inhibitory striatal circuits in the suppression of involuntary movements and suggest that the precise spatiotemporal configuration of striatal activity plays an active role in the regulation of cortical resting state activity and motor control.

Pathophysiology of tic disorders

Tics are the defining symptom of Tourette syndrome and other tic disorders (TDs); however, they form only a part of their overall symptoms. The recent surge of studies addressing the underlying pathophysiology of tics has revealed an intricate picture involving multiple brain areas and complex pathways. The myriad of pathophysiological findings stem, at least partially, from the multifaceted properties of tics and the disorders that express them. Distinct brain pathways mediate the expression of tics, whereas others are involved in the generation of the premonitory urge, associated comorbidities, and other changes in brain state. Expression of these symptoms is controlled by additional networks underlying voluntary suppression by the patient or those reflecting overall behavioral state. This review aims to simplify the complex picture of tic pathophysiology by dividing it into these key components based on converging data from human and animal model studies. Thus, involvement of the corticobasal ganglia pathway and its interaction with motor, sensory, limbic, and executive networks in each of the components as well as their control by different neuromodulators is described. This division enables a focused definition of the neuronal systems involved in each of these processes and allows a better understanding of the pathophysiology of TDs as a whole.

Abnormal neuronal activity in Tourette syndrome and its modulation using deep brain stimulation

Tourette syndrome (TS) is a common childhood-onset disorder characterized by motor and vocal tics that are typically accompanied by a multitude of comorbid symptoms. Pharmacological treatment options are limited, which has led to the exploration of deep brain stimulation (DBS) as a possible treatment for severe cases. Multiple lines of evidence have linked TS with abnormalities in the motor and limbic cortico-basal ganglia (CBG) pathways. Neurophysiological data have only recently started to slowly accumulate from multiple sources: noninvasive imaging and electrophysiological techniques, invasive electrophysiological recordings in TS patients undergoing DBS implantation surgery, and animal models of the disorder. These converging sources point to system-level physiological changes throughout the CBG pathway, including both general altered baseline neuronal activity patterns and specific tic-related activity. DBS has been applied to different regions along the motor and limbic pathways, primarily to the globus pallidus internus, thalamic nuclei, and nucleus accumbens. In line with the findings that also draw on the more abundant application of DBS to Parkinson's disease, this stimulation is assumed to result in changes in the neuronal firing patterns and the passage of information through the stimulated nuclei. We present an overview of recent experimental findings on abnormal neuronal activity associated with TS and the changes in this activity following DBS. These findings are then discussed in the context of current models of CBG function in the normal state, during TS, and finally in the wider context of DBS in CBG-related disorders.

Corticostriatal interactions in the generation of tic-like behaviors after local striatal disinhibition

The pathophysiology of the tics that define Gilles de la Tourette syndrome (TS) is not well understood. Local disinhibition within the striatum has been hypothesized to play a pathogenic role. In support of this, experimental disinhibition by local antagonism of GABA-A receptors within the striatum produces tic-like phenomenology in monkey and rat. We replicated this effect in mice via local picrotoxin infusion into the dorsal striatum. Infusion of picrotoxin into sensorimotor cortex produced similar movements, accompanied by signs of behavioral activation; higher-dose picrotoxin in the cortex produced seizures. Striatal inhibition with local muscimol completely abolished tic-like movements after either striatal or cortical picrotoxin, confirming their dependence on the striatal circuitry; in contrast, cortical muscimol attenuated but did not abolish movements produced by striatal picrotoxin. Striatal glutamate blockade eliminated tic-like movements after striatal picrotoxin, indicating that glutamatergic afferents are critical for their generation. These studies replicate and extend previous work in monkey and rat, providing additional validation for the local disinhibition model of tic generation. Our results reveal a key role for corticostriatal glutamatergic afferents in the generation of tic-like movements in this model.

Motor tics evoked by striatal disinhibition in the rat

Motor tics are sudden, brief, repetitive movements that constitute the main symptom of Tourette syndrome (TS). Multiple lines of evidence suggest the involvement of the cortico-basal ganglia system, and in particular the basal ganglia input structure-the striatum in tic formation. The striatum receives somatotopically organized cortical projections and contains an internal GABAergic network of interneurons and projection neurons' collaterals. Disruption of local striatal GABAergic connectivity has been associated with TS and was found to induce abnormal movements in model animals. We have previously described the behavioral and neurophysiological characteristics of motor tics induced in monkeys by local striatal microinjections of the GABAA antagonist bicuculline. In the current study we explored the abnormal movements induced by a similar manipulation in freely moving rats. We targeted microinjections to different parts of the dorsal striatum, and examined the effects of this manipulation on the induced tic properties, such as latency, duration, and somatic localization. Tics induced by striatal disinhibition in monkeys and rats shared multiple properties: tics began within several minutes after microinjection, were expressed solely in the contralateral side, and waxed and waned around a mean inter-tic interval of 1-4 s. A clear somatotopic organization was observed only in rats, where injections to the anterior or posterior striatum led to tics in the forelimb or hindlimb areas, respectively. These results suggest that striatal disinhibition in the rat may be used to model motor tics such as observed in TS. Establishing this reliable and accessible animal model could facilitate the study of the neural mechanisms underlying motor tics, and the testing of potential therapies for tic disorders.

Animal models recapitulating the multifactorial origin of Tourette syndrome

Tourette Syndrome (TS) is a neurological disorder characterized by motor and phonic tics affecting approximately 1% of the pediatric population. Behavioral comorbidities often include obsessive-compulsive behavior and impaired attention. The neurobiological substrates associated with TS generally entail abnormalities in neurotransmitter circuitry regulating basal ganglia activity. The neurotransmitters most often associated with TS are dopamine, serotonin, and GABA. TS origin roots in genetic predisposing factors, and environmental variables favoring tic onset and exacerbation. Among the latter, repeated infections with group A beta-hemolytic Streptococcus and psychosocial stressors encountered during development have been proposed to constitute likely susceptibility factors. In this chapter, we describe how this clinical/epidemiological knowledge has been translated into animal models of TS. Specifically, we review several studies attempting to reproduce TS-like symptoms (tics and behavioral stereotypies) and comorbidities (impaired attention, increased locomotion, and perseverative responding) in laboratory rodents. Additionally, we discuss studies in which the genetic and environmental predisposing factors have been modeled in experimental subjects. Ultimately, we propose a unifying perspective recapitulating dependent and independent variables in the preclinical study of TS and discuss its potential theoretical and heuristic implications.

Myoclonus and extrapyramidal diseases

Parkinsonism or dystonia are associated with myoclonus in several extrapyramidal diseases. Although the latter symptom is not always prominent, stimulus-sensitive, distal, or focal reflex myoclonus is frequently observed. This review will consider the clinical and electrophysiological features of myoclonus in Parkinson's disease, multiple system atrophy, corticobasal degeneration, progressive supranuclear palsy, Huntington's disease, dentatorubral-pallidoluysian atrophy, Lewy body dementia, and myoclonus with dystonia. The evidence of a long-latency reflex response, the presence of giant somatosensory evoked potentials, and the demonstration of a back-averaged premyoclonus focal cortical EEG activity often lead to classify myoclonus as a cortical phenomenon. However, a subcortical origin cannot always be ruled out.

Myoclonic tremulous movements in multiple system atrophy are a form of cortical myoclonus

We prospectively studied the clinical and electrophysiological features of myoclonic tremulous movements in patients with multiple system atrophy (MSA). Among 42 consecutive patients, 12 MSA-p (parkinsonian type) and 3 MSA-c (cerebellar type) patients showed small-amplitude myoclonic movements. These movements occurred in the distal part of the arms and fingers, particularly in posture or during voluntary movements. We conducted detailed electrophysiological studies in 11 patients. Electromyographic recordings showed irregular bursts of brief duration often synchronously involving the antagonistic muscles. Somatosensory evoked responses were slightly enlarged in some patients. Long-latency reflexes were enhanced in 7 patients. A jerk-locked averaging technique revealed premyoclonic cortical potentials in 9 patients. These findings suggest that small-amplitude myoclonic movements in MSA are a form of cortical myoclonus.

Antimyoclonic effect of gabapentin in a posthypoxic animal model of myoclonus

The antimyoclonic property of the novel antiepileptic drug, gabapentin (1-(aminomethyl) cyclohexane acetic acid), was tested in cardiac arrest-and p,p'-DDT(1,1,1-trichloro-2,2-bis (p-chlorophenyl)ethane)-induced animal models of myoclonus. Gabapentin dose-dependently attenuated myoclonus in posthypoxic rats for more than 3 h. The drug was also found to be effective in controlling the early stages of seizures following the anoxic insult. In contrast, the drug was ineffective in controlling either myoclonus or seizures in p,p'-DDT-treated animals. These results suggest that gabapentin can be used used as an effective therapeutic agent in an acute hypoxia/ischemia-induced neurological disorder. The data further indicate that distinct neurological mechanisms may be operating in the expression of myoclonus among posthypoxic and p,p'-DDT-induced animal models.

Association between brain indole levels and severity of posthypoxic myoclonus in rats

We have previously reported the presence of posthypoxic, audiogenic myoclonus in rats after cardiac arrest and the ability of the 5-HT precursor, 5-HTP, to attenuate these muscle jerks. In addition, we have recently shown that 5-HT2 and 5-HT3 agonists can reduce the severity of myoclonus in these animals, suggesting a deficiency in serotonergic neurotransmission. In the present study, the levels of 5-HTP, 5-HT, and 5-HIAA were measured in seven regions of the brain in myoclonic and normal rats to identify the areas of the brain in which a serotonergic dysfunction resides. Similar to previous studies, we observed pronounced posthypoxic, audiogenic myoclonus 3 and 14 days after resuscitation from cardiac arrest, with a resolution of the abnormal movements by 45 days postarrest. HPLC measurements revealed significant changes in indole levels in the following areas of the brain: cortical 5-HIAA, striatal 5-HT, striatal 5-HIAA, hippocampal 5-HT, mesencephalic 5-HIAA, myelencephalic 5-HT, myelencephalic 5-HIAA, cerebellar 5-HTP, and cerebellar 5-HT. The changes in striatal 5-HT, cortical 5-HIAA, and mesencephalic 5-HIAA appear most relevant to the pathophysiology of posthypoxic myoclonus because regression analyses showed significant correlations between the myoclonus scores of the animals and the levels of these indoles. Based on the observed pattern of results, we postulate a dysfunction in serotonergic lateral (cortical) and far lateral (extrapyramidal) ascending pathways in posthypoxic myoclonus.

Strychnine-insensitive glycine site antagonists attenuate a cardiac arrest-induced movement disorder

Male Sprague-Dawley rats underwent experimentally induced cardiac arrest and resuscitation, subsequently exhibiting involuntary jerking movements (myoclonus) with salient features similar to the human form of the disorder. The novel strychnine-insensitive glycine site antagonists ACEA-1011 (5-chloro-7-trifluoromethyl-1,2,3,4-tetrahydroquinoxaline-2,3,-dio ne) and ACEA-1021 (5-nitro-6,7-dichloro-quinoxalinedione) significantly attenuated the myoclonus in cardiac-arrested rats. (+)-HA-966, (+/-)-HA-966 (3-amino-1-hydroxy-2-pyrrolidinone), and felbamate (2-phenyl-1,3-propanediol dicarbamate) were also effective. Although the drugs vary in their selectivity for strychnine-insensitive glycine sites, they all possess antagonist activity at these sites. Vehicle injections (saline, dimethyl sulfoxide, water) were without effect and no obvious side effects were observed with any of the ligands tested in this study. Since hyperexcitability in the central nervous system is thought to underlie myoclonus, the attenuation of excitatory amino acid neurotransmission through antagonism of strychnine-insensitive glycine sites provides a logical mechanism of action for the antimyoclonic effects observed herein.

Neuropsychiatric disorders, myoclonus, and dystonia in calcification of basal ganglia pathways

Two cases of basal ganglia calcification involving the globus pallidus are presented. Both patients had cognitive dysfunction, temporal lobe-like symptoms (including amnestic state, perceptual distortions, or complex visual hallucinations), and myoclonus. Patient 1 manifested depression, auditory hallucinations, anxiety, paranoia, and postural tremor; patient 2 manifested multifocal dystonia with dystonic tremor. These cases supplement other reports of psychotic features and dementia associated with pallidal pathology. Additionally, the phenomena encountered in these cases are considered in light of recent advances in our understanding of basal ganglia functional pathways. These cases afford a potential pathophysiological window to the possible role of the globus pallidus in these neuropsychiatric conditions. In concert with other recent findings, these cases suggest specific pathway involvement in hallucinations, paranoia, depression, myoclonus, and dystonia. Further research will indicate if these pathways play a role in schizophrenia, mood disorders, and anxiety disorders.

Electrophysiological study of dyskinesia produced by microinjection of picrotoxin into the striatum of the rat

The neuronal mechanism of dyskinesia of the contralateral forelimb produced by microinjection of picrotoxin (PTX), a GABA antagonist, into the caudoputaminal unit (CPU) was analyzed electrophysiologically. PTX injection resulted in production of sharp, negative field potentials with or without preceding or succeeding positivity in the CPU and cerebral cortex (CX). These spike potentials first appeared in the CPU, thereafter within the CX, and were finally accompanied by dyskinesia indicated by electromyogram activity of the forelimb in association with fully developed CX spike potentials. Dyskinesia disappeared reversibly by cooling, and irreversibly by ablation, of the CX, indicating the crucial participation of the CX in the manifestation of dyskinesia. CPU spike potentials remained even after isolation of the CPU from its major afferents. Grouped discharges of extracellularly recorded single-unit spikes were observed in the CPU in association with the development of CPU spike potentials. Single-unit spikes of the pars reticulata of the substantia nigra were inhibited, and those of the ventromedial nucleus (VM) of the thalamus were excited in association with CPU spike potentials. We therefore propose that the primary origin of the dyskinesia is the CPU and that the inhibitory CPU efferent, as well as the nigrothalamic, pathway plays a basic role in the manifestation of the dyskinesia.

Therapeutic trial with glycine in myoclonus

We investigated the therapeutic effects of glycine in seven patients with various forms of myoclonus. The initial phase was an open label trial. If benefit was seen in any patient, a double-blind substitution of placebo was carried out to determine if the benefit was due, in fact, to glycine. The dosage of glycine was initiated at 600 mg/day and was increased gradually until a maximum dosage of 6,000 mg/day was reached. This dosage was maintained for at least 6 weeks before lack of efficacy was declared. No improvement was seen in four patients. One patient reported improvement, but he discontinued the drug because of adverse effects encountered with the use of a concomitant medication and before he could be tested in a double-blind crossover phase. Two other patients also noticed improvement, but these improvements were not validated in the crossover phase. There were no adverse effects associated with glycine. Plasma glycine levels peaked at 30 min and returned to normal 1.5-h after 1 g of glycine.p.o. Cerebrospinal fluid (CSF) glycine levels did not change during treatment, suggesting inadequate penetration into the central nervous system of glycine at the dosage used.