Three glutamate transporters have been identified in rat, including astroglial transporters GLAST and GLT-1 and a neuronal transporter EAAC1. Here we demonstrate that inhibition of the synthesis of each glutamate transporter subtype using chronic antisense oligonucleotide administration, in vitro and in vivo, selectively and specifically reduced the protein expression and function of glutamate transporters. The loss of glial glutamate transporters GLAST or GLT-1 produced elevated extracellular glutamate levels, neurodegeneration characteristic of excitotoxicity, and a progressive paralysis. The loss of the neuronal glutamate transporter EAAC1 did not elevate extracellular glutamate in the striatum but did produce mild neurotoxicity and resulted in epilepsy. These studies suggest that glial glutamate transporters provide the majority of functional glutamate transport and are essential for maintaining low extracellular glutamate and for preventing chronic glutamate neurotoxicity.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that is characterized by selective upper and lower motor neuron degeneration, the pathogenesis of which is unknown. About 60%-70% of sporadic ALS patients have a 30%-95% loss of the astroglial glutamate transporter EAAT2 (excitatory amino acid transporter 2) protein in motor cortex and spinal cord. Loss of EAAT2 leads to increased extracellular glutamate and excitotoxic neuronal degeneration. Multiple abnormal EAAT2 mRNAs, including intron-retention and exon-skipping, have now been identified from the affected areas of ALS patients. The aberrant mRNAs were highly abundant and were found only in neuropathologically affected areas of ALS patients but not in other brain regions. They were found in 65% of sporadic ALS patients but were not found in nonneurologic disease or other disease controls. They were also detectable in the cerebrospinal fluid (CSF) of living ALS patients, early in the disease. In vitro expression studies suggest that proteins translated from these aberrant mRNAs may undergo rapid degradation and/ or produce a dominant negative effect on normal EAAT2 resulting in loss of protein and activity. These findings suggest that the loss of EAAT2 in ALS is due to aberrant mRNA and that these aberrant mRNAs could result from RNA processing errors. Aberrant RNA processing could be important in the pathophysiology of neurodegenerative disease and in excitotoxicity. The presence of these mRNA species in ALS CSF may have diagnostic utility.

Glutamate transport is critical for synaptic inactivation of glutamate and prevention of excitotoxicity. The following four glutamate transporters have been identified in human brain: EAAT1, EAAT2, EAAT3, and EAAT4. Deficient glutamate transport has been identified in the motor cortex and the spinal cord of tissue from amyotrophic lateral sclerosis (ALS) patients. The defect appears to be due to a selective loss of the astroglial specific glutamate transporter protein EAAT2. In these studies we sought to extend our understanding of glutamate transporters in ALS by examining the mRNA for each transporter subtype in ALS motor cortex. All tissue was matched for age and postmortem delay. There was no quantitative change in mRNA for EAAT1, EAAT2, or EAAT3 in ALS motor cortex, even in patients with a large loss of EAAT2 protein (95% decrease compared with control) and decreased tissue glutamate transport (73% decrease compared with control). These studies suggest that the dramatic abnormalities in EAAT2 may be due to translational or post-translational processes.

Mutations in the gene for Cu/Zn superoxide dismutase (SOD1) have been detected in some families with an autosomal dominant form ofamyotrophic lateral sclerosis; these mutations appear to reduce the activity of this enzyme. To determine whether decreased SOD activity could contribute to motor neuron loss, SODi was inhibited chronically with either antisense oligodeoxynucleotides or diethyldithiocarbamate in spinal cord organotypic cultures. Chronic inhibition of SOD resulted in the apoptotic degeneration of spinal neurons, including motor neurons, over several weeks. Motor neuron loss was markedly potentiated by the inhibition of glutamate transport.In this paradigm, motor neuron toxicity could be entirely prevented by the antioxidant N-acetylcysteine and, to a lesser extent, by the non-N-methyl-D-aspartate glutamate receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-SH-2,3-benzodiazepine hydrochloride. These data support the hypothesis that the loss ofmotor neurons in familial amyotrophic lateral sclerosis could be due to a reduction in SODi activity, possibly potentiated by inefficient glutamate transport.Amyotrophic lateral sclerosis (ALS) is a motor neuron disease characterized clinically by progressive weakness, wasting of muscles, and spasticity due to the slow loss of lower motor neurons in the spinal cord and in the neocortex. Familial ALS represents 5-10o of all cases and is virtually indistinguishable clinically from the more common sporadic form. Mutations in the Cu/Zn superoxide dismutase (SOD1) gene on chromosome 21 have been detected in some families with the autosomal dominant form of familial AML (1, 2). Initial studies indicate that these mutations reduce the activity of SODi (2,3). This protein is a homodimeric metalloenzyme that catalyzes the dismutation of the superoxide anion to oxygen and hydrogen peroxide (4). Excessive levels of oxygen radicals, such as superoxide anion, have been implicated in neuronal injury, either directly or through the formation of more reactive oxygen species such as hydroxyl radicals (5-7). However, it has not been established that a chronic reduction in SODi activity diminishes the viability of spinal motor neurons. To investigate this, we have developed a model of slow toxicity in cultured organotypic spinal cord slices which combines the advantages of long-term survival with partially preserved synaptic connections (8). In this model, SOD activity was inhibited by the use of either antisense oligodeoxynucleotides (ODNs) or metal-chelating agents such as diethyldithiocarbamate (DDC), which has been shown to potentiate oxygen radical-induced toxicity in acute preparations (5). METHODSOrganotypic Spinal Cord Cultures. Eight-day-old neonatal rat pups were decapitated, and the spinal cords were rapidly harvested and cultured (8). Culture medium, including any added pharmacological agent, was changed twice weekly. With this technique, >95% of the explants can be maintained in culture for >3 months with excellent organotypic cellular organization...

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The presence of prolactin (PRL) mRNA in the mammary gland, placenta, and pituitary gland of lactating and pregnant rats was investigated by polymerase chain reaction (PCR). Polyadenylated RNA was prepared from total RNA samples by oligo(dT)-cellulose chromatography, and complementary cDNAs were synthesized. A standardized amount of cDNA from each sample was used as the template in a Taq PCR under high-stringency conditions. PCR amplified a signal with the predicted size of approximately 375 bp in mammary and pituitary glands of lactating and pregnant rats, and in placentae of pregnant rats. This band specifically hybridized with a probe overlapping the entire sequence of the mature rat (r) PRL mRNA in Southern blot analysis. When the rPRL-specific primers were used, PCR revealed no signal in the liver or in lactating mammary gland explants cultured in vitro for 48 h, while the same cDNA preparations gave strong signals for beta-actin. The viability of the mammary gland explants was also suggested by their ability to secrete immunoreactive casein in vitro. PRL mRNA was localized in the epithelium of alveoli and ducts of the lactating mammary gland by in situ hybridization. These data provide evidence that the PRL gene is expressed in the mammary gland of pregnant and lactating rats, and suggest that the mammary gland might contribute to PRL in milk by de novo synthesis. Thus, while the placenta is an exogenous source of PRL-like activities for the fetus in utero, the mammary gland might take over this function after birth.

Polyamines positively modulate the activity of the N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors. The concentration of polyamines in the brain increases in certain pathological conditions, such as ischemia and brain trauma, and these compounds have been postulated to play a role in excitotoxic neuronal death. In primary cultures of rat cerebellar granule neurons, exogenous application of the polyamines spermidine and spermine (but not putrescine) potentiated the delayed neurotoxicity elicited by NMDA receptor stimulation with glutamate. Furthermore, both toxic and nontoxic concentrations of glutamate stimulated the activity of ornithine decarboxylase (ODC)--the key regulatory enzyme in polyamine synthesis--and increased the concentration of ODC mRNA in cerebellar granule neurons but not in glial cells. Glutamate-induced ODC activation but not neurotoxicity was blocked by the ODC inhibitor difluoromethylornithine. Thus, high extracellular polyamine concentrations potentiate glutamate-triggered neuronal death, but the glutamate-induced increase in neuronal ODC activity may not play a determinant role in the cascade of intracellular events responsible for delayed excitotoxicity.

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