DDT myoclonus: sites and mechanism of action

Brain regions and genes affecting myoclonus in animals

Myoclonus is defined as large-amplitude rhythmic movements. Brain regions underlying myoclonic jerks include brainstem, cerebellum, and cortex. Gamma-aminobutyric acid (GABA) appears to be the main neurotransmitter involved in myoclonus, possibly interacting with biogenic amines, opiates, acetylcholine, and glycine. Myoclonic jumping is a specific subtype seen in rodents, comprising rearing and hopping continuously against a wall. Myoclonic jumping can be seen in normal mouse strains, possibly as a result of simply being put inside a cage. Like other types, it is also triggered by changes in GABA, 5HT, and dopamine neurotransmission. Implicated brain regions include hippocampus and dorsal striatum, possibly with respect to D(1) dopamine, NMDA, and δ opioid receptors. There is reason to suspect that myoclonic jumping is underreported due to insufficient observations into mouse cages.

Myoclonic epilepsy with ragged-red fibers (MERRF): an immunohistochemical study of the brain

Myoclonic epilepsy with ragged-red fibers (MERRF) is a maternally inherited disorder of oxidative phosphorylation due to specific point mutations within the mitochondrial tRNA(Lys) gene. Mitochondrial dysfunction in the central nervous system (CNS) of patients with MERRF accounts for the neurological manifestations of the disease. Antibodies against subunits of complex I, III, IV and V of the respiratory chain were used to study the expression of these proteins in the frontal cortex, cerebellum and medulla from an autoptic case of MERRF. We found a selective decreased expression of subunit II of cytochrome c oxidase (COX-II) in these regions. Immunohistochemical abnormalities were more widespread than the lesions described by traditional histopathological techniques and made possible an attempt of explanation for the neurological symptoms of the patient.

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.

Therapeutic trial of milacemide in patients with myoclonus and other intractable movement disorders

We performed a therapeutic trial with the glycine precursor, milacemide, on 10 patients with intractable movement disorders. Six had myoclonus of various etiologies and one each had progressive supranuclear palsy, Filipino X-linked dystonia with parkinsonism, painful legs and moving toes, and stiff-person syndrome. Milacemide was initiated at a dose of 2,400 mg/day, orally, and increased gradually to a maximum of 4,800 mg/day. No clear-cut observable improvement occurred. There were no serious adverse effects.

Regional glycine receptor binding in the p,p'-DDT myoclonic rat model

Abnormal glycinergic neurotransmission has been implicated in the pathophysiology of DDT-induced myoclonus. To examine the role of glycine receptors in the DDT model, was measured [3H]strychnine receptor binding in brainstem and spinal cord in the rat after acute administration of DDT. The highest dose of DDT tested significantly increased both Bmax (20%) and Kd (57%) of glycine sites in spinal cord but not brainstem compared to vehicle-treated controls at 4 h. Lower DDT doses, which also induced myoclonus, had no significant effects on [3H]strychnine specific binding. In vitro, 10(-7) DDT did not displace [3H]strychnine binding in naive rat spinal cord, but higher doses could not be studied due to poor solubility of DDT under the assay conditions. These data suggest that only a maximal dose of DDT has significant though mixed effects on parameters of [3H]strychnine binding in spinal cord which are not correlated with the onset of myoclonus.

Reflex reticular myoclonus: relationship to some brainstem pathophysiological mechanisms

Two patients with reflex reticular myoclonus [RRM] were tested electrophysiologically and pharmacologically. In one of the cases the underlying disease was chronic Lyme borreliosis. In the other, the RRM attacks may have been associated with procarbazine therapy applied for Hodgkin's disease. No cortical lesion could be demonstrated either clinically or electrophysiologically [EEG, averaged EEg preceeding the jerks, SSEP]. An EMG analysis of the jerks revealed the shortest latency in the muscles innervated by the accessory nerve. The latencies became longer in a more rostral muscle [masseter], as well as in a more caudal one, the muscles innervated by the facial nerve were spared. it is presumed that the complete movement pattern of the myoclonus residues in the jerk generating structure. RRM in the described cases differs from the startle by sparing the facial nerve and from the Papio papio baboon non-epileptic myoclonus by the activating effect of physostigmine. A partial therapeutic effect was achieved with a serotonine precursor, but a GABAergic therapy proved to be the most effective.

A therapeutic trial of milacemide in myoclonus and the stiff-person syndrome

We investigated the therapeutic effects of milacemide in seven patients with myoclonus and three patients with the stiff-person syndrome in an open-label trial. Milacemide was initiated at 800 mg/day and was gradually increased to a maximum dosage of 2,400 mg/day. No significant improvement occurred in the 10 patients.

The proposed role of neurotransmitter receptors in the pathophysiology of human myoclonic disorders

The hypothesis that central neurotransmitter receptor abnormalities are the basis of human myoclonic disorders is novel. Receptor abnormalities in any of several different neurotransmitter systems implicated in myoclonus may be genetic or the consequence of various brain injuries. These abnormalities might define pharmacologic subgroups of possible relevance to the clinical, neurophysiologic, and pathologic heterogeneity of myoclonus. Receptor abnormalities may be the primary pathophysiologic defect, involving the surface recognition site or effector-transducer mechanisms in the defect, involving the surface recognition site or effector-transducer mechanisms in the post-synaptic cell. Alternatively, changes in receptor density or affinity may be adaptive (recovery of function) or maladaptive (supersensitivity, subsensitivity, dysregulation). Drug treatments, then, could be targeted at the receptor changes, reversing abnormalities and enhancing compensatory mechanisms. Current therapy may inadvertently have such an effect. Polytherapy may be justified to target more than one component of the "pharmacologic receptor". Identification of receptor abnormalities in human post-mortem brain may have diagnostic and therapeutic significance. New advances in the pharmacologic selectivity of receptor agonists and antagonists and in the measurement of receptors should be applied to the problem of myoclonus.

Milacemide increases 5-hydroxytryptamine and dopamine levels in rat brain--possible mechanisms of milacemide antimyoclonic property in the p,p'-DDT-induced myoclonus

Milacemide, a glycine prodrug that is able to enter the brain readily, has been shown to have an antimyoclonic property in the p,p'-DDT-induced myoclonus syndrome. Milacemide increased regional 5-HT and dopamine and decreased 5-HIAA, DOPAC and HVA levels in naive rats. In p,p'-DDT-treated rats, 5-HT levels were unchanged at the time the rats experienced spontaneous myoclonus in all brain regions except in the striatum, where it increased. 5-HIAA levels increased but did not reach significant levels except in the striatum. Dopamine, DOPAC, HVA and norepinephrine were unchanged. When rats were treated concurrently with both p,p'-DDT and milacemide, regional 5-HT levels were increased and NE levels in the brainstem and cerebellum decreased. Depletion of brain serotonin by pretreatment with PCPA or with 5,7-DHT, or blocking 5-HT receptors with different 5-HT antagonists, failed to eliminate the antimyoclonic property of milacemide. This antimyoclonic effect of milacemide may be mediated through other mechanisms besides its ability to increase brain 5-HT levels. Possible mechanisms to be considered are its antiepileptic property, and its ability to increase brain glycine levels. Milacemide may have potential for therapeutic trials in patients with myoclonus.

Glycine involvement in DDT-induced myoclonus

The DDT syndrome in rats consists of tremor, myoclonus, running seizures, hyperthermia, episodic boxing, and excessive grooming. DDT did not change whole-brain glycine levels when the rats had stimulus-sensitive myoclonus, spontaneous myoclonus, or seizures. However, regional analysis showed a decrease in glycine levels in the pons and medulla initially, but they rose again despite worsening of the myoclonus. Glycine given intraventricularly and the glycine prodrug, milacemide, given intraperitoneally suppressed DDT-induced myoclonus. A dose of milacemide that prevented DDT-induced myoclonus caused a significant increase in glycine levels in cortex, septum accumbens, cerebellum, striatum, hippocampus diencephalon, midbrain, pons, and medulla. The increase was most marked in the forebrain structures. There was no change in serine levels in these areas. These data suggest that the glycinergic system may be playing an important role in the manifestation of DDT-induced myoclonus.

Effect of inferior olive lesion on seizure threshold in the rat

Cerebellar stimulation has been associated with anticonvulsant activity in several experimental seizure models. We examined the effect of destruction of cerebellar climbing fibers, by systemic administration of 3-acetylpyridine (3AP) or electrothermic lesion of the inferior olive, on seizures produced by various chemical convulsants in rats. We found that inferior olive lesioned rats had lower threshold to seizures induced by strychnine and brucine, both glycine antagonists. The dose response curve for strychnine seizure was shifted 2.5 times to the left in 3AP lesioned rats. No difference in seizure threshold was seen when picrotoxin, bicuculline or pentylenetetrazole PTZ) were used to produce seizures. Abnormal motor behavior (AMB) including myoclonus, backward movement and hyperextension, produced by all of the convulsants tested, was significantly aggravated in 3AP pretreated rats. The inferior olive-climbing fiber projection to the cerebellum appears to modulate seizures induced by inhibition of glycinergic neurotransmission.

3-Acetylpyridine lesions and four serotonergic behavioral syndromes in the rat

We studied the effect of 3-acetylpyridine (3-AP) lesions on the serotonergic-myoclonic syndromes evoked by quipazine (QP), 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), fenfluramine (FF), and p-chloroamphetamine (PCA) in the adult rat. Eleven behaviors were scored from videotapes by an observer blind to drug status. In unlesioned rats, drugs could be differentiated by forelimb and axial myoclonus, pivoting and backing. All drugs significantly suppressed rearing. 3-AP produced a lasting action-enhanced body tremor which differed from axial myoclonus in its vertical direction and rhythmicity. 3-AP lesions modified the effect of drugs on several behaviors, increasing axial (QP, FF, PCA) and forelimb (5-MeO-DMT, FF, PCA) myoclonus and decreasing locomotor score. Prior lesions with 5,7-dihydroxytryptamine did not prevent the effect of 3-AP or any behaviors of the serotonin syndrome, but had a slight effect on the magnitude of forelimb myoclonus, head weaving, and hunching induced by some drugs. Neither lesion abolished or reduced myoclonus. These data suggest that intact 5-HT terminals are not requisite for the tremorogenic and cytotoxic effect of 3-AP. To the extent that chemical lesions with 3-AP are selective for the inferior olive (IO), the role of the IO in myoclonus in several 5-HT rodent myoclonic models appears to be regulatory rather than stimulatory.

Distribution of biogenic amines and their catabolites in brains from patients with Alzheimer's disease

Norepinephrine, epinephrine, dopamine, serotonin and their major catabolites were measured in 17 regions of the left hemisphere of two brains obtained from two brothers with Alzheimer's disease with very early onset. The clinical diagnosis was confirmed by histological examination of the right hemispheres and brain stems. The quantitative data were compared with our values in normal brains. In the patient suffering from the less severe dementia, there was a severe reduction of the serotonin concentration in all examined neocortical areas and its concentration was even below the detection limit in the nucleus amygdalis, hippocampus, caudate nucleus, putamen, globus pallidus and substantia nigra. In the other patient, who suffered from a more pronounced dementia with myoclonus, the serotonin concentration was below the detection limit in all examined structures. In contrast with these findings, the noradrenergic, adrenergic and dopaminergic systems appeared to be relatively unaffected by the disease process. Focusing our attention on the nuclei wherein the monoamine transmitter systems originate, it appeared that neuronal losses and neurofibrillary tangles clearly predominated in the substantia grisea subependymalis, the nucleus centralis superior and the nucleus raphe dorsalis, origin of the main serotonergic system. The serotonin deficiency sheds light on possible mechanisms of myoclonus in Alzheimer's disease.

Kainic acid-induced limbic seizures in cats: some reflections on sleep-epilepsy interactions

Sleep-epilepsy interactions were investigated in a model of temporal lobe seizures induced in cats by intra-amygdaloid kainic acid (KA) microinjections. We found that limbic status epilepticus disrupted sleep for 2 or 3 days after injection. Sleep, in turn, modulated the frequency of interictal discharges. However, such modulation was variable depending on the time elapsed since KA injection. For this and other reasons (such as the occurrence of subclinical seizures during paradoxical sleep), we postulate a dual effect--facilitatory or inhibitory--of paradoxical sleep on limbic epilepsy. A role in seizure induction for bulbopontine structures is proposed on the basis of seizure precipitation during phasic paradoxical sleep. Propagated limbic seizures and paradoxical sleep without atonia displayed similar behavioral patterns. This fact and the possibility that a seizure may substitute for paradoxical sleep, lead us to think that limbic seizures and paradoxical sleep subserve similar functions. One of them might be the elimination of a potentially neurotoxic endogenous product.

Urea-induced myoclonus: medullary glycine antagonism as mechanism of action

Stimulus sensitive myoclonus is a prominent symptom of uremia in both man and animals. Intravenous injection of urea into cats had been previously reported to produce spike and sharp wave electrical discharges in the medullary reticular formation which correlated with the myoclonic movements. In the present investigations, intraperitoneal injections of 2 g/kg urea every 15 minutes for 4 injections produced myoclonus in rats accompanied by brain urea concentrations of 6.8 X 10(-2)M, which is sevenfold higher than normal. 10(-2) and 10(-1) M urea significantly reduced 3H-strychnine binding to rat medulla membranes by 30% and 43% respectively. Urea inhibition of 3H-strychnine binding was reversible and binding kinetics revealed that 10(-1)M urea decreased Bmax by 65% with no effect on the affinity. Brain glycine levels did not change after urea injections and urea had no effect on synaptosomal uptake of 3H-glycine. Urea did not alter 3H-GABA, 3H-glutamate and 3H-QNB receptor binding but decreased 3H-diazepam receptor binding in the medulla. Mannitol also reduced 3H-diazepam binding but had no effect on 3H-strychnine binding. Stereotaxic injection of the glycine receptor antagonist, strychnine, into the rat medullary reticular formation produced myoclonus, whereas Ro 15-1788, a benzodiazepine antagonist, had no effect. Urea may produce myoclonus by blockade of glycine receptors in the medullary reticular formation.