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

Gabapentin-induced Myoclonus in End-stage Renal Disease

PURPOSE

We analyzed the occurrence and clinical features of myoclonus in patients with end-stage renal disease (ESRD) who were treated with gabapentin (GBP).

METHODS

We reviewed the medical records of patients with ESRD who were treated with GBP and hospitalized during an 18-month period and analyzed clinical details such as type of myoclonus, doses of GBP, electroencephalographic (EEG) findings, and relation of symptoms to GBP exposure and dosage.

RESULTS

Three of 71 patients had myoclonus with GBP doses ranging from 9 mg/kg to 20 mg/kg and within 4 months of treatment onset. Myoclonus was characterized as multifocal, involving all extremities in the three patients. EEG did not show epileptiform discharges with the myoclonus. Myoclonus resolved in the three individuals within 4-15 days after GBP was discontinued.

CONCLUSIONS

GBP increases the risk of myoclonus in ESRD. Myoclonus in these individuals was more disabling than that in patients with normal renal function, and discontinuation of GBP is required to restore normal function.

Glycine increases arterial pressure and augments NMDA-induced pressor responses in the dorsomedial and ventrolateral medulla of cats

The present study is designed to determine and characterize two neurobiological events. Firstly, we investigated whether increases of systemic arterial pressure (SAP) and sympathetic vertebral nerve activity (VNA) produced by microinjection of glycine (Gly) in the dorsomedial (DM) or rostral ventrolateral medulla (RVLM) are mediated by pressor neurons in DM or RVLM. Secondly, we assessed whether simultaneous microinjections of Gly and N-methyl-D-aspartate (NMDA) in DM or RVLM potentiate the NMDA-pressor effects. Changes in SAP and VNA were recorded in 33 cats under alpha-chloralose and urethane anesthesia. Microinjection of sodium glutamate (Glu, 0.25 M, 30 nl) or Gly (1.0 M, 30 nl) into the DM or RVLM increased SAP and VNA in similar magnitude. Latencies of changes in SAP and VNA induced by Gly, however, were longer (3 s) than those induced by Glu. Prior microinjection of the following antagonists blocked the Gly-induced pressor responses: 2-amino-5-phosphonopentanoate (AP-5, 25 mM, 30 nl), a specific NMDA receptor antagonist; or glutamate diethyl ester (GDEE, 0.5 M, 30 nl), a quisqualate receptor antagonist; or kynurenic acid (KYN, 10 mM, 30 nl), a broad spectrum competitive Glu antagonist. Prior treatment with strychnine (3 mM, 30 nl), a specific Gly antagonist, also blocked the Gly-induced pressor responses. Since Gly is believed to be an inhibitory neurotransmitter, these data suggest that Gly may produce pressor actions via an inhibition on specific inhibitory neurons synapsing with the pressor neurons. NMDA (0.1 M, 30 nl) and Gly (1.0 M, 30 nl) microinjected simultaneously in DM or RVLM produced a greater pressor action than NMDA alone. This potentiation was blocked by KYN, another known antagonist for such potentiation, but was only partially blocked by strychnine.

Phenotypic heterogeneity and disease course in three murine strains with mutations in genes encoding for alpha 1 and beta glycine receptor subunits

Impaired glycinergic inhibition causes human hyperekplexia, and may be involved in the pathogenesis of movement disorders associated with uremia, spinal cord lesions, DDT poisoning, and tetanus. Three autosomal recessive mutant mouse strains with single-gene mutations affecting either the alpha 1 (spasmodic and oscillator) or beta (spastic) subunits of the glycine receptor were studied. Serial videotaped examinations assessed the severity of hyperkinetic features. Homozygote oscillator mice appeared normal until postnatal day (P) 11-14, when decreased exploratory movements, spastic gait, stimulus-induced myoclonic bouts, rigidity, and tremor were noticeable. All symptoms gradually worsened until death by P21-P23. In contrast, spastic and spasmodic mice were most severely affected by the 3rd-5th week of life and had a lessening of symptom severity in adulthood. Within each mutant strain, there was marked interanimal variation of severity of the other motor abnormalities, possibly because of stochastic variability in developmental processes. These mutants represent good animal models for elucidation of molecular and cellular issues regarding the glycine receptor and for the study of pathogenetic mechanisms of movement disorders.

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.

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.

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.

Anticonvulsant drug action and regional neurotransmitter amino acid changes

The role played by the inhibitory transmitters, GABA, glycine and taurine, and by excitatory (aspartate/glutamate) antagonists in mediating anticonvulsant action will be documented. This study provides examples of one anticonvulsant compound that affects glycine metabolism (milacemide), and another that affects aspartate metabolism (beta-methylene-aspartate). Beta-Methylene-aspartate, a selective inhibitor of glutamate-aspartate transaminase activity, protects against sound-induced seizures in audiogenic DBA/2 mice, with an ED50 value of 1.9 mumoles (icv; clonic phase). Forebrain and cerebellar aspartate, glutamate and GABA levels are reduced by 15-30% following the administration of beta-methylene-aspartate. Milacemide, a glycinamide derivative with experimental and clinical anticonvulsant activity, is ineffective against sound-induced seizures in DBA/2 mice. Following the ip administration of milacemide (100 mg/kg; 3 hours) there were significant increases in rat brain glycine levels in the cerebellum (+137%), cortex (+45%) and hippocampus (+59%).

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.

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.