The influence of glycine and related compounds on spinal cord injury-induced spasticity

GABAergic Mechanisms Can Redress the Tilted Balance between Excitation and Inhibition in Damaged Spinal Networks

Correct operation of neuronal networks depends on the interplay between synaptic excitation and inhibition processes leading to a dynamic state termed balanced network. In the spinal cord, balanced network activity is fundamental for the expression of locomotor patterns necessary for rhythmic activation of limb extensor and flexor muscles. After spinal cord lesion, paralysis ensues often followed by spasticity. These conditions imply that, below the damaged site, the state of balanced networks has been disrupted and that restoration might be attempted by modulating the excitability of sublesional spinal neurons. Because of the widespread expression of inhibitory GABAergic neurons in the spinal cord, their role in the early and late phases of spinal cord injury deserves full attention. Thus, an early surge in extracellular GABA might be involved in the onset of spinal shock while a relative deficit of GABAergic mechanisms may be a contributor to spasticity. We discuss the role of GABA A receptors at synaptic and extrasynaptic level to modulate network excitability and to offer a pharmacological target for symptom control. In particular, it is proposed that activation of GABA A receptors with synthetic GABA agonists may downregulate motoneuron hyperexcitability (due to enhanced persistent ionic currents) and, therefore, diminish spasticity. This approach might constitute a complementary strategy to regulate network excitability after injury so that reconstruction of damaged spinal networks with new materials or cell transplants might proceed more successfully.

Alteration of glycinergic receptor expression in lumbar spinal motoneurons is involved in the mechanisms underlying spasticity after spinal cord injury

Spasticity is a disabling motor disorder affecting 70% of people with brain and spinal cord injury. The rate-dependent depression (RDD) of the H reflex is the only electrophysiological measurement correlated with the degree of spasticity assessed clinically in spastic patients. Several lines of evidence suggest that the mechanism underlying the H reflex RDD depends on the strength of synaptic inhibition through GABA_A (GABA_AR) and glycine receptors (GlyR). In adult rats with spinal cord transection (SCT), we studied the time course of the expression of GABA_AR and GlyR at the membrane of retrogradely identified Gastrocnemius and Tibialis anterior motoneurons (MNs) 3, 8 and 16 weeks after injury, and measured the RDD of the H reflex at similar post lesion times. Three weeks after SCT, a significant decrease in the expression of GABA_A and GlyR was observed compared to intact rats, and the H-reflex RDD was much less pronounced than in controls. Eight weeks after SCT, GlyR values returned to normal. Simultaneously, we observed a tendency to recover normal RDD of the H reflex at higher frequencies. We tested whether an anti-inflammatory treatment using methylprednisolone performed immediately after SCT could prevent alterations in GABA_A/glycine receptors and/or the development of spasticity observed 3 weeks after injury. This treatment restored control levels of GlyR but not the expression of GABA_AR, and it completely prevented the attenuation of RDD. These data strongly suggest that alteration of glycinergic inhibition of lumbar MNs is involved in the mechanisms underlying spasticity after SCI.

Baclofen in the Therapeutic of Sequele of Traumatic Brain Injury: Spasticity

Traumatic brain injury (TBI) is an alteration in brain function, caused by an external force, which may be a hit on the skull, rapid acceleration or deceleration, penetration of an object, or shock waves from an explosion. Traumatic brain injury is a major cause of morbidity and mortality worldwide, with a high prevalence rate in pediatric patients, in which treatment options are still limited, not available at present neuroprotective drugs. Although the therapeutic management of these patients is varied and dependent on the severity of the injury, general techniques of drug types are handled, as well as physical and surgical. Baclofen is a muscle relaxant used to treat spasticity and improve mobility in patients with spinal cord injuries, relieving pain and muscle stiffness. Pharmacological support with baclofen is contradictory, because disruption of its oral administration may cause increased muscle tone syndrome and muscle spasm, prolonged seizures, hyperthermia, dysesthesia, hallucinations, or even multisystem organ failure. Combined treatments must consider the pathophysiology of broader alterations than only excitation/inhibition context, allowing the patient's reintegration with the greatest functionality.

Spinal glycine transporter-1 inhibition influences the micturition reflex in urethane-anesthetized rats

PURPOSE

Glycine is an inhibitory neurotransmitter in the central nervous system. So far, two types of glycine transporters (GlyTs), GlyT-1 and GlyT-2, have been cloned. The aim of this study is to investigate the effects of a selective GlyT-1 inhibitor that can increase endogenous glycine concentration on the micturition reflex in urethane-anesthetized rats.

METHODS

Continuous cystometrograms (0.04 ml/min) were performed in female Sprague-Dawley rats (232-265 g) under urethane anesthesia. After stable micturition cycles were established, ALX5407, a selective GlyT-1 inhibitor, was administered intrathecally or intracerebroventricularly to evaluate changes in bladder activity. Cystometric parameters were recorded and compared before and after drug administration.

RESULTS

Intrathecal administration of ALX5407 (1, 3, 10 and 30 μg) increased intercontraction intervals at doses of 3 μg or higher in a dose-dependent fashion. Intrathecal administration of ALX5407 (1, 3, 10 and 30 μg) also increased pressure threshold at doses of 3 μg or higher in a dose-dependent fashion. However, when ALX5407 (1, 3, 10 and 30 μg) was administered intracerebroventricularly, there were no significant changes in intercontraction intervals, pressure threshold, maximum voiding pressure or baseline pressure or post-void residual urine volume at any doses tested.

CONCLUSION

The results of our study indicate that GlyT-1 plays an important role in the modulation of micturition. Furthermore, these findings indicate that in urethane-anesthetized rats suppression of GlyT-1 can inhibit the micturition reflex at the spinal cord level. Thus, GlyT-1 could be a potential target for the treatment of bladder dysfunction such as overactive bladder.

Bronchial epilepsy or broncho-pulmonary hyper-excitability as a model of asthma pathogenesis

Over the last 20 years, the prevalence of asthma has nearly doubled in industrialized countries. A similar increase has been predicted for the next two decades. Asthma is major illness in terms of morbidity and suffering, asthma is the leading cause of hospitalizations in children under 15 years of age. According to many top experts, asthma is correctly characterized as a syndrome rather than disease. This lack of definition for asthma makes the search for a cause, prevention and potential cure elusive. Episodic airway obstruction and reversible bronchial hyperresponsiveness to non-specific irritants are the major symptoms of asthma. Airway inflammation is now widely accepted as the key factor underlying the pathogenesis of asthma. However, many patients show no signs of inflammation, yet they still have severe airflow limitation and asthma symptoms. The primary clinical symptoms of asthma are attacks of shortness of breath, wheezing, and coughing resulting from excessive and inappropriate constriction of the airway smooth muscle. Our research suggests a possible epileptic or hyper-excitatory condition of bronchial system in asthma pathogenesis. The paroxysmal, spasmodic character of asthma attacks may be similar to seizures. We propose a unified pathogenetic mechanism of asthma as a syndrome of inducible or genetically predisposed membrane hyper-excitability (bronchial epilepsy).

Validation of a fluorescent imaging plate reader membrane potential assay for high-throughput screening of glycine transporter modulators

A fluorescent imaging plate reader (FLIPR) membrane potential (V(m)) assay was evaluated for pharmacological characterization and high-throughput screening (HTS) of rat glycine transporter type 2 (rGlyT(2)) in a stable rGlyT(2)-HEK cell line. Data show that glycine activation of rGlyT(2) consistently results in a concentration-dependent V(m) response on the FLIPR that is blocked by the potent and selective GlyT(2) antagonist 4-benzyloxy-3,5-dimethoxy-N-[1-dimethylamino-cyclopentyl)methyl]-benz-amide (Org-25543). Agonist and antagonist pharmacologies match those reported using conventional [(3)H]glycine uptake assays and electrophysiology. The glycine response is dependent on buffer ionic composition consistent with GlyT(2) physiology. Assay signal-to-background and coefficient of variation meets sufficient statistical criteria to conduct HTS. The results of a screen of the chemical inventory demonstrate that the assay is able to successfully identify and confirm GlyT(2) inhibitors. The advantages of this assay are its homogeneity, compatibility with both 96- and 384-well formats, and lack of radioactivity usage. Thus, the authors conclude that a fluorescence-based V(m) assay on FLIPR is a viable approach for identification and pharmacological profiling of small molecule modulators of the electrogenic transporter rGlyT(2).

Structure, function and regulation of glycine neurotransporters

Glycine exerts multiple functions in the central nervous system, as an inhibitory neurotransmitter through activation of specific, Cl--permeable, ligand-gated ionotropic receptors and as an obligatory co-agonist with glutamate on the activation of N-methyl-D-aspartate (NMDA) receptors. In some areas of the central nervous system, glycine seems to be co-released with gamma-aminobutyric acid (GABA), the main inhibitory amino acid neurotransmitter. The synaptic action of glycine ends by active recapture through sodium- and chloride-coupled glycine transporters located in glial and neuronal plasma membranes, whose structure-function relationship is being studied. The trafficking and plasma membrane expressions of these proteins are controlled by regulatory mechanisms. Glycine transporter inhibitors may find application in the treatment of muscle tone defects, epilepsy, schizophrenia, pain and neurodegenerative disorders. This review deals on recent progress on localization, transport mechanisms, structure, regulation and pharmacology of the glycine transporters (GLYTs).

Inhibitory effect of intrathecal glycine on the micturition reflex in normal and spinal cord injury rats

We examined the influence of lumbosacral glycinergic neurons on the spinobulbospinal and spinal micturition reflexes. Female rats were divided into intact rats, rats with acute injury to the lower thoracic spinal cord (SCI), and rats with chronic SCI. Under urethane anesthesia, isovolumetric cystometry was performed in each group before and after intrathecal (IT) injection of glycine or strychnine into the lumbosacral cord level. The glutamate and glycine levels of the lumbosacral cord were measured after injection of glycine or strychnine in intact and chronic SCI rats. Expression of strychnine-sensitive glycine receptor alpha-1 (GlyR alpha1) mRNA in the lumbosacral cord was also assessed in both rats. In chronic SCI rats, the interval and amplitude of bladder contractions were shorter and smaller when compared with intact rats. IT glycine (0.1-100 microg) prolonged the interval and decreased the amplitude of bladder contractions in both intact rats and chronic SCI rats. IT strychnine (0.01-10 microg) elevated the baseline pressure in intact rats and induced bladder contraction in acute SCI rats. On amino acid analysis, IT glycine (0.01-100 microg) decreased the glutamate level of the lumbosacral cord in intact rats, but not in chronic SCI rats. The glycine level of the lumbosacral cord was 54% lower in chronic SCI rats when compared with intact rats, while the GlyR alpha1 mRNA level did not change after SCI. These results suggest that glycinergic neurons may have an important inhibitory effect on the spinobulbospinal and spinal micturition reflexes at the level of the lumbosacral cord.

Intrathecal pertussis toxin induces thermal hyperalgesia: involvement of excitatory and inhibitory amino acids

Intrathecal pertussis toxin injection has been used as a neuropathic pain model. In the present study, its effects on cerebrospinal fluid biochemistry and nociceptive behavioral expression were examined in rats. Cerebrospinal fluid dialysate samples were collected and pertussis toxin was injected using an intrathecally implanted dialysis loop catheter; samples were collected and hyperalgesia behavior was noted every 2 days for 8 days after pertussis toxin injection. Pertussis toxin injection induced thermal hyperalgesia which peaked between day 2 and 4; no cold allodynia was observed. Pertussis toxin at all doses tested (0.5, 1, or 2 microg) also induced a significant increase in cerebrospinal fluid concentrations of aspartate and glutamate between days 2 and 8, while level of the inhibitory amino acid glycine were significantly decreased by the two higher doses of pertussis toxin. Intrathecal administration of the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosponovaleric acid (10 microg) or glycine (200 microg), inhibited pertussis toxin-induced thermal hyperalgesia. Pertussis toxin injection had no effect on serine, glutamine, and taurine concentrations. These results show that intrathecal pertussis toxin injection induces thermal hyperalgesia and it is associated with an increasing of excitatory and a decreasing of inhibitory amino acids release in the spinal cord.

Substrate-induced conformational changes of extracellular loop 1 in the glycine transporter GLYT2

The neurotransmitter glycine is removed from the synaptic cleft by two Na(+)-and Cl(-)-dependent transporters, the glial (GLYT1) and neuronal (GLYT2) glycine transporters. GLYT2 lacks a conserved cysteine in the first hydrophilic loop (EL1) that is reactive to [2-(trimethylammonium)ethyl] methanethiosulfonate (MTSET) in related transporters. A chimeric GLYT2 (GLYT2a-EL1) that contains GLYT1 sequences in this region, including the relevant cysteine, was sensitive to the reagent, and its sensitivity was decreased by co-substrates. We combined cysteine-specific biotinylation to detect transporter-reagent interactions with MTSET inactivation assays and temperature dependence analysis to study the mechanism by which Cl(-), Na(+), and glycine reduce methanethiosulfonate reagent inhibition. We demonstrate a Na(+) protective effect rather than an increased susceptibility to the reagent exerted by Li(+), as reported for the serotonin transporter. The different inhibition, protection, and reactivation properties between GLYT2a-EL1 and serotonin transporter suggest that EL1 is a source of structural heterogeneity involved in the specific effect of lithium on serotonin transport. The protection by Na(+) or Cl(-) on GLYT2a-EL1 was clearly dependent on temperature, suggesting that EL1 is not involved in ion binding but is subjected to ion-induced conformational changes. Na(+) and Cl(-) were required for glycine protection, indicating the necessity of prior ion interaction with the transporter for the binding of glycine. We conclude that EL1 acts as a fluctuating hinge undergoing sequential conformational changes during the transport cycle.

Glial transporters for glutamate, glycine, and GABA III. Glycine transporters

Glial cells possess transport systems for the three major amino acid neurotransmitters glutamate, gamma-aminobutyric acid (GABA) and glycine, involved in the arrest of neurotransmission mediated by these compounds. Two glycine transporters have been cloned: GLYT1, mainly expressed by glial cells and shown to colocalize with NMDA receptors, and GLYT2, exclusively expressed by neurons and colocalized with the inhibitory glycine receptors. The way in which the regulation of extracellular glycine concentration by glial glycine transporters affects physiological and pathological conditions is discussed. The presence, differential pharmacology and specific regulation of glycine transporters in glial cells strongly support an important role for glia in the modulation of both, excitatory and inhibitory neurotransmission.

Transmembrane domain III plays an important role in ion binding and permeation in the glycine transporter GLYT2

The neuronal glycine transporter GLYT2 takes up glycine from the extracellular space by an electrogenic process where this neurotransmitter is co-transported with sodium and chloride ions. We report in this paper that tyrosine at position 289 of GLYT2a is crucial for ion coupling, glycine affinity and sodium selectivity, stressing the essential role played by this residue of transmembrane domain III in the mechanism of transport. Substitution to tryptophan (Y289W), phenylalanine (Y289F), or serine (Y289S), renders transporters unable to catalyze glycine uptake. Measurements of glycine evoked steady-state currents in transfected HEK-293 cells reveal EC(50) values for glycine 17-fold (Y289F) and 45-fold (Y289S) higher than that of the wild type transporter. Sodium dependence is severely altered in tyrosine 289 mutants, both at the level of apparent affinity and cooperativity, with the more dramatic change corresponding to the less conservative substitution (Y289S). Accordingly, sodium selectivity is gradually lost in Y289F and Y289S mutants, and chloride dependence of glycine evoked currents is markedly decreased in Y289F and Y289S mutants. In the absence of three-dimensional information from these transporters, these results provide experimental evidence supporting the hypothesis of transmembrane domain III being part of a common permeation pathway for substrate and co-transported ions.

Inhibitory casting decreases a vibratory inhibition index of the H-reflex in the spastic upper limb

OBJECTIVE

To test the hypothesis that application of an inhibitory cast to the spastic upper limb will decrease a vibratory inhibition index (VII) of the H-reflex in the spastic upper limb.

DESIGN

Prospective, nonrandomized, open-label trial.

SETTING

University tertiary care center.

PARTICIPANTS

Eight adults with upper limb spasticity.

INTERVENTION

Fiberglass cast application spanning the wrist to the upper arm.

MAIN OUTCOME MEASURE

The amplitude of the H-reflex with and without continuous 60Hz vibration to the tendon of the flexor carpi radialis was measured, and the VII was calculated using the formula: [H-reflex amplitude (vibrated)/H-reflex amplitude (control)] x 100%.

RESULTS

Mean VII decreased from baseline (70.7) on day 1 (67.6, p = .699), day 2 (55.9, p =.066), and day 3 (43.5, p = .033) of casting, and increased on day 4 (89.9, p = .146) after removal of the cast.

CONCLUSION

Findings lend support to the idea that during application of an inhibitory cast motor neuron excitability is decreased in the spastic upper limb.

Characterization of glycine transport in cultured Müller glial cells from the retina

Rapid termination of the synaptic action of glutamate (Glu) and glycine (Gly) is achieved by uptake into the presynaptic terminal and glial cells. In the vertebrate CNS, Gly acts both as an inhibitory neurotransmitter and as a Glu modulator or coagonist at postsynaptic N-methyl-D-aspartate (NMDA) receptors. We have previously described NMDA receptors in Müller cells of chick retina coupled to the phosphoinositide cascade, the entry of calcium, and the activation of protein kinase C (PKC; López-Colomé et al. Glia 9:127-135, 1993). A colocalization of Gly transporters and NMDA receptors has been reported in brain tissue (Smith et al. Neuron 8:927-936, 1992); since the concentration of Gly could participate in the modulation of Glu excitatory transmission in the vertical pathways of the retina, transport of Gly in monolayer cultures of Müller cells was studied. Gly transport was found pH-sensitive with an optimum at pH 7.4. Kinetic analysis of the saturation curve for Gly within a concentration range of 0.01-2 mM, revealed two components of transport: a low-affinity system with Km = 1.7 mM, Vmax = 30 nmol/10 min/mg protein, and a high-affinity one with a Km = 27 microM, Vmax = 3 nmol/10 min/mg protein. Both systems were Na+ -dependent; the high-affinity system proved also dependent on external Cl- and was inhibited by sarcosine, characteristic of GLYT1 transporters. The inhibition of low-affinity uptake by 2-(methylamino)isobutyric acid (MeAIB) and 2-aminoisobutyric acid (AIB) suggests the presence of transport system A in Müller cells. The process is energy-requiring, since Gly transport was decreased by metabolic inhibitors. Data obtained are in keeping with a modulatory role for Müller glia on excitatory transmission in the retina.

Operant conditioning of H-reflex increase in spinal cord--injured rats

Operant conditioning of the spinal stretch reflex or its electrical analog, the H-reflex, is a new model for exploring the mechanisms of long-term supraspinal control over spinal cord function. Primates and rats can gradually increase (HRup conditioning mode) or decrease (HRdown conditioning mode) the H-reflex when reward is based on H-reflex amplitude. An earlier study indicated that HRdown conditioning of the soleus H-reflex in rats is impaired following contusion injury to thoracic spinal cord. The extent of impairment was correlated with the percent of white matter lost at the injury site. The present study investigated the effects of spinal cord injury on HRup conditioning. Soleus H-reflexes were elicited and recorded with chronically implanted electrodes from 14 rats that had been subjected to calibrated contusion injuries to the spinal cord at T8. At the lesion epicenter, 12-39% of the white matter remained. After control-mode data were collected, each rat was exposed to the HRup conditioning mode for 50 days. Final H-reflex amplitudes after HRup conditioning averaged 112% (+/-22% SD) of control. This value was significantly smaller than that for 13 normal rats exposed to HRup conditioning, in which final amplitude averaged 153% (+/-51%) SD of control. As previously reported for HRdown conditioning after spinal cord injury, success was inversely correlated with the severity of the injury as assessed by white matter preservation and by time to return of bladder function. HRup and HRdown conditioning are similarly sensitive to injury. These results further demonstrate that H-reflex conditioning is a sensitive measure of the long-term effects of injury on supraspinal control over spinal cord functions and could prove a valuable measure of therapeutic efficacy.

Morphine and dextrorphan lose antinociceptive activity but exhibit an antispastic action in chronic spinal rats

Within 3-4 weeks after spinal transection, morphine-induced antinociception, assessed with the tail flick reflex in rats, is profoundly reduced. The cause of this decrement is unknown. The present studies were conducted to determine whether this phenomenon reflects a general loss in opiate activity or a selective decline in opiate antinociception. This was accomplished by assessing the effect of morphine on two different responses, the tail flick reflex and the hindlimb spasticity that develops in chronic spinal rats. Because excitatory amino acid antagonists are also antinociceptive in acute spinal rats, the effect of one such drug, dextrorphan, on these two behaviors was also evaluated in chronic spinal animals. The antinociceptive and antispastic effect of subcutaneous (6 mg/kg) and intrathecal (5 micrograms) morphine injections were assessed in intact and chronic (21-28 days) spinal rats, whereas the effect of subcutaneous (25 and 40 mg/kg) and intrathecal (350 micrograms) dextrorphan was assessed in acute (1 day) and chronic spinal rats. The antinociceptive effect of both drugs was significantly reduced in chronic spinal animals, relative to saline controls. However, each drug treatment produced a significant antispastic effect in the same animals, indicating a selective decline in opiate action. This outcome also suggests that excitatory amino acid antagonists may be useful as adjunct antispastic agents.

Reduction in thermal hyperalgesia by intrathecal administration of glycine and related compounds

We have previously shown in animal models that enhanced segmental glycine release is produced by neuroaugmentation techniques commonly used to control pain in humans. Our current hypothesis is that glycine administered intrathecally reduces the pain response evoked by the hotplate analgesia meter method. Neuropathic rats created by unilateral partial ligation of the sciatic nerve were treated with intrathecal infusion of glycine, strychnine, MK-801, or 5-7 DKA at 0.1 mumol for 2 hours at a rate of 10 microliters/min. Time required for limb withdrawal at 42 degrees C was significantly increased after glycine administration but not altered by strychnine, a specific glycine receptor antagonist. Administration of the NMDA receptor antagonist, MK-801, blocked the influence of glycine, with a less obvious antagonistic response from 5.7 DKA. Our results provide evidence that glycine and related compounds significantly modify thermal hyperalgesia, and may operate primarily through the NMDA receptor complex.