Attenuation of excitotoxic cell swelling and GABA release by the GABA transport inhibitor SKF 89976A

Neuroprotective changes in degeneration-related gene expression in the substantia nigra following acupuncture in an MPTP mouse model of Parkinsonism: Microarray analysis

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the death of dopamine-generating cells in the substantia nigra (SN). Acupuncture stimulation results in an enhanced survival of dopaminergic neurons in the SN in Parkinsonism animal models. The present study investigated changes in gene expression profiles measured using whole transcript array in the SN region related to the inhibitory effects of acupuncture in a chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) Parkinsonism model. In this model, acupuncture stimulation at GB34 and LR3 attenuated the decrease in tyrosine hydroxylase in the SN region; stimulation at non-acupoints did not suppress this decrease. Gene array analysis revealed that 22 (10 annotated genes: Cdh1, Itih2, Mpzl2, Rdh9, Serping1, Slc6a13, Slc6a20a, Slc6a4, Tph2, and Ucma) probes that were up-regulated in MPTP animals relative to controls were exclusively down-regulated by acupuncture stimulation. In addition, 17 (two annotated genes: 4921530L21Rik and Gm13931) probes that were down-regulated in MPTP animals compared to controls were exclusively up-regulated by acupuncture stimulation. These findings indicate that the 39 probes (12 annotated genes) affected by MPTP and acupuncture may be responsible for the inhibitory effects of acupuncture on degeneration-related gene expression in the SN following damage induced by MPTP intoxication.

Spinal cord trauma and the molecular point of no return

A mechanical trauma to the spinal cord can be followed by the development of irreversible and progressive neurodegeneration, as opposed to a temporary or partially reversible neurological damage. An increasing body of experimental and clinical evidence from humans and animal models indicates that spinal cord injury may set in motion the development of disabling and at times fatal neuromuscular disorders, whose occurrence is not normally associated with any major environmental event. This outcome appears to be dependent on the co-occurrence of a particular form of mechanical stress and of a genetically-determined vulnerability. This increased vulnerability to spinal cord injury may depend on a change of the nature and of the timing of activation of a number of neuroprotective and neurodestructive molecular signals in the injured cord. Among the main determinants, we could mention an altered homeostasis of lipids and neurofilaments, an earlier inflammatory response and the failure of the damaged tissue to rein in oxidative damage and apoptotic cell death. These changes could force injured tissue beyond a point of no return and precipitate an irreversible neurodegenerative process. A better knowledge of the molecular signals activated in a state of increased vulnerability to trauma can inform future treatment strategies and the prediction of the neurological outcome after spinal cord injury.

Reversal or reduction of glutamate and GABA transport in CNS pathology and therapy

A dysfunction of amino acid neurotransmitter transporters occurs in a number of central nervous system disorders, including stroke, epilepsy, cerebral palsy and amyotrophic lateral sclerosis. This dysfunction can comprise a reversal of transport direction, leading to the release of neurotransmitter into the extracellular space, or an alteration in transporter expression level. This review analyses the role of glutamate and GABA transporters in the pathogenesis and therapy of a number of acute and chronic neurological disorders.

Sequential release of GABA by exocytosis and reversed uptake leads to neuronal swelling in simulated ischemia of hippocampal slices

GABA release during cerebral energy deprivation (produced by anoxia or ischemia) has been suggested either to be neuroprotective, because GABA will hyperpolarize neurons and reduce release of excitotoxic glutamate, or to be neurotoxic, because activation of GABA(A) receptors facilitates Cl- entry into neurons and consequent cell swelling. We have used the GABA(A) receptors of hippocampal area CA1 pyramidal cells to sense the rise of [GABA](o) occurring in simulated ischemia. Ischemia evoked, after several minutes, a large depolarization to approximately -20 mV. Before this "anoxic depolarization," there was an increase in GABA release by exocytosis (spontaneous IPSCs). After the anoxic depolarization, there was a much larger, sustained release of GABA that was not affected by blocking action potentials, vesicular release, or the glial GABA transporter GAT-3 but was inhibited by blocking the neuronal GABA transporter GAT-1. Blocking GABA(A) receptors resulted in a more positive anoxic depolarization but decreased cell swelling at the time of the anoxic depolarization. The influence of GABA(A) receptors diminished in prolonged ischemia because glutamate release evoked by the anoxic depolarization inhibited GABA(A) receptor function by causing calcium entry through NMDA receptors. These data show that ischemia releases GABA initially by exocytosis and then by reversal of GAT-1 transporters and that the resulting Cl- influx through GABA(A) receptor channels causes potentially neurotoxic cell swelling.

Protection of malonate-induced GABA but not dopamine loss by GABA transporter blockade in rat striatum

Previous work has shown that overstimulation of GABA(A) receptors can potentiate neuronal cell damage during excitotoxic or metabolic stress in vitro and that GABA(A) antagonists or GABA transport blockers are neuroprotective under these situations. Malonate, a reversible succinate dehydrogenase/mitochondrial complex II inhibitor, is frequently used in animals to model cell loss in neurodegenerative diseases such as Parkinson's and Huntington's diseases. To determine if GABA transporter blockade during mitochondrial impairment can protect neurons in vivo as compared with in vitro studies, rats received a stereotaxic infusion of malonate (2 micromol) into the left striatum to induce a metabolic stress. The nonsubstrate GABA transport blocker, NO711 (20 nmol) was infused in some rats 30 min before and 3 h following malonate infusion. After 1 week, dopamine and GABA levels in the striata were measured. Malonate caused a significant loss of striatal dopamine and GABA. Blockade of the GABA transporter significantly attenuated GABA, but not dopamine loss. In contrast with several in vitro reports, GABA(A) receptors were not a downstream mediator of protection by NO711. Intrastriatal infusion of malonate (2 micromol) plus or minus the GABA(A) receptor agonist muscimol (1 micromol), the GABA(A) Cl- binding site antagonist picrotoxin (50 nmol) or the GABA(B) receptor antagonist saclofen (33 nmol) did not modify loss of striatal dopamine or GABA when examined 1 week following infusion. These data show that GABA transporter blockade during mitochondrial impairment in the striatum provides protection to GABAergic neurons. GABA transporter blockade, which is currently a pharmacological strategy for the treatment of epilepsy, may thus also be beneficial in the treatment of acute and chronic conditions involving energy inhibition such as stroke/ischemia or Huntington's disease. These findings also point to fundamental differences between immature and adult neurons in the downstream involvement of GABA receptors during metabolic insult.

Attenuation of malonate toxicity in primary mesencephalic cultures using the GABA transport blocker, NO-711

Cultured rat mesencephalic neurons were used to assess the effects of gamma-aminobutyric acid (GABA) transport blockers on toxicity caused by malonate, a reversible, competitive inhibitor of succinate dehydrogenase. Previous studies utilizing an ex vivo chick retinal preparation have shown that GABA release and cell swelling are early consequences of acute energy impairment and that GABA transport blockers attenuate this toxicity. The present results demonstrate that the nonsubstrate GABA transport blocker, NO-711 (1 nM-1 microM), dose-dependently protected cultured mesencephalic dopamine (DA) and GABA neurons from malonate-induced toxicity. Similar protection was demonstrated with nipecotic acid (1 mM) and SKF89976A (100 nM), substrate and nonsubstrate GABA transport blockers, respectively. These compounds by themselves produced no signs of toxicity, although nipecotic acid caused a long-term decrease in GABA uptake not associated with toxicity. Compounds which decrease intracellular reactive oxygen species (ROS) are protective in this model, but NO-711 did not prevent the rise in intracellular ROS induced by malonate, indicating its protective effects were downstream of ROS production. Supplementation of malonate treated cultures with the GABA(A) agonist, muscimol (10 microM), increased the toxicity toward the DA and GABA neuron populations. Antagonists at the GABA(A) and glycine receptors provided partial protection to both the GABA and DA neurons. These findings suggest that the GABA transporter, GABA(A), and/or glycine channels contribute to cell damage associated with energy impairment in this model.

Neurons in layer III of the entorhinal cortex. A role in epileptogenesis and epilepsy?

A preferential lesion of neurons in layer III of the entorhinal cortex (EC) is often observed in patients suffering from temporal lobe epilepsy and in several animal models of the disease. This lesion is duplicated in rats by a focal, intra-entorhinal injection of the "indirect" excitotoxin aminooxyacetic acid (AOAA), providing a model that can be used to study the mechanisms underlying seizure-induced cell death and epilepsy. Doomed neurons in the EC and in several associated limbic structures show pathological changes within hours after the AOAA injection, but GABAergic neurons in layer III of the EC are quite resistant. This pattern of neuron loss eventually results in hippocampal and entorhinal hyperexcitability. Notably, the seizure-induced death of layer III neurons in the EC can be attenuated by eliminating the prominent excitatory input from the presubiculum. Taken together, these results suggest opportunities to target parahippocampal structures for the treatment of temporal lobe epilepsy.

Influence of gamma-aminobutyric acid on retinal cells excitotoxicity upon glucose deprivation

The role of extracellular endogenous gamma-aminobutyric acid (GABA) in rescuing retinal cells in culture from the decrease in viability induced by Glu under metabolic inhibition is analyzed. Glutamate (10 microM-10 mM) dose-dependently decreased the intracellular GABA content, but increased the extracellular accumulation of GABA. In the absence of glucose, Glu (10-100 microM) decreased the intracellular GABA (2-fold), whereas the extracellular accumulation of GABA was increased by about 4-fold. Glu-mediated decrement in cell survival was not affected by inhibiting the GABA(A) receptors with bicuculline (1 or 10 microM) or by blocking the Na+ -dependent release of GABA with 1-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid (SKF89976-A). Data suggest a non-protective role of endogenous GABA release after metabolic deprivation of retinal cells submitted to Glu.

The neurology and enterology of equine grass sickness: a review of basic mechanisms

Autonomic dysfunction constitutes a prominent clinical feature of equine grass sickness (EGS). Significant injury to the nervous control of the alimentary system is life threatening, partly because of dysphagia but also because of the failure of the unique regulatory mechanisms in equine digestion involving water and electrolyte balance. The neuropathology also indicates the presence of a somatic polyneuropathy. The morphological features of EGS are similar to those of excitotoxic neuronal degeneration, which resembles neuronal apoptosis. It is difficult to ascertain from published accounts the degree of damage to central neurones: the distribution is well documented and selective but the proportion of damage is poorly quantified. If lesions involve a significant number of regulatory neurones they should produce functional deficits. Any clinical assessment of horses, especially those with chronic EGS, should include a thorough neurological examination. Although this will not necessarily improve the outcome of the case, it may enable the rational selection of animals with a reasonable prognosis for recovery which is partly determined by the extent of CNS lesions. The evidence supports the following pathogenesis. There is an initial lesion in the enteric nervous system of susceptible horses. In the acute form of EGS, massive enteric neuronal damage occurs first functionally, then structurally leading to generalized alimentary smooth muscle atony, enhanced secretions and altered fluid fluxes. Severe distension of the stomach and small intestines rapidly develops, which augments the intestinal ileus by intersegmental inhibitory reflexes and causes colic and dehydration. In subacute cases, failure of intestinal bicarbonate buffer together with alimentary stasis rapidly reduces caecal-colonic fermentation. Thus the osmolality of large intestinal digesta reduces and water travels out of the bowel along osmotic gradients. Water returns to the circulation, but is eventually lost in the gastric and small intestinal secretions. The observation that pathological lesions may not be seen in the prevertebral ganglia within the first few days of acute cases supports the view that a functional deficit precedes structural lesions which may be secondary to a retrograde degeneration. It is therefore possible to resolve the observations that less damage may be seen in prevertebral ganglia and elsewhere in peracute and acute cases with the more common finding that greater neuronal damage is present in acute than in chronic cases. These different observations are probably time dependent. Chronic EGS occurs when there is less initial enteric nerve damage which may lead to less secondary prevertebral ganglionic pathology, and more time for functional and structural compensatory mechanisms to develop. Denervation hypersensitivity develops at target sites both in the gut and in peripheral somatic nerves which may account, in part, for the clinical signs of patchy sweating and muscle tremors. Raised circulating adrenaline levels may also account for generalized sweating, may contribute to gastrointestinal atony and may affect pacemakers at the pelvic flexure. Many of the features of EGS make worthwhile the re-investigation of Clostridium botulinum Group III toxins, which are known to prevent vesicular exocytosis, stimulate neurosecretion, produce neuronal chromatolysis and inhibit neutrophil migration. Also, evidence from other species suggests that increased nitrergic neuronal activity can account for many of the clinical signs of EGS, namely dysphagia, generalized ileus, gastric dilatation, sweating, peripheral vasodilatation, tachycardia, salivary hypersecretion, muscle wastage and cachexia.

The intact isolated (ex vivo) retina as a model system for the study of excitotoxicity

Excitotoxicity is defined as a mode of neural cell death triggered by overactivation of receptors for the amino acid transmitter glutamate. There is considerable evidence that excitotoxicity is responsible for cell death in several neuropathological states, including some retinal diseases. The isolated retina, particularly from chick embryos, has been used extensively as an experimental system to characterize this process. This paper summarizes the use of isolated retina as a model system for studies of excitotoxicity from a theoretical and methodological point of view, and reviews results obtained from studies utilizing this system.

Activity at the GABA transporter contributes to acute cellular swelling produced by metabolic impairment in retina

The role of the GABA transporter in acute toxicity in chick retina due to metabolic inhibition was investigated by the use of several substrate (nipecotic acid, THPO) and nonsubstrate (SKF 89976A, NO711) GABA transport inhibitors. Metabolic stress-induced acute toxicity in the retina is characterized by swelling of distinct populations of retinal neurons and selective release of GABA into the medium. Inhibitor concentrations were based on that needed to attenuate 14C-GABA uptake at its approximate KM concentration by > or = 70%. Under basal conditions, substrate, but not nonsubstrate, inhibitors increased extracellular GABA, but did not cause histological swelling per se. Under conditions of glycolytic inhibition, nonsubstrate, but not substrate, inhibitors significantly attenuated acute toxicity. Metabolic stress-induced acute toxicity was not altered by the GABA agonist muscimol, nor did muscimol reverse the protective effects of nonsubstrate transport inhibitors, suggesting that an increase in extracellular GABA during metabolic stress was not a component of the acute phase of toxicity. The results indicate that during metabolic inhibition, activity at the GABA transporter contributes to acute cellular swelling.