Reverse glial glutamate uptake triggers neuronal cell death through extrasynaptic NMDA receptor activation

Evidence have accumulated that reverse glutamate uptake plays a key role in the pathophysiology of cerebral ischemia. Here, we investigated the effects of glial glutamate transporter dysfunction on neuronal survival using the substrate inhibitor of glutamate transporters, L-trans-pyrrolidine,2-4,dicarboxylate (PDC), that partly mimics reverse glutamate uptake. On mice primary cortical co-cultures of neurons and astrocytes, PDC treatment triggered an elevation of extracellular glutamate concentration, induced neuronal calcium influx and a massive NMDA receptor (NMDAR) mediated-neuronal death without having any direct agonist activity on NMDARs. We investigated the NMDAR subpopulation activated by PDC-induced glutamate release. PDC application led to the activation of both subtypes of NMDARs but the presence of astrocytes was required to activate NMDARs located extra-synaptically. Extrasynaptic NMDAR activation was also confirmed by the loss of neuronal mitochondrial membrane potential and the inhibition of pro-survival p-ERK signalling pathway. These data suggest that reverse glial glutamate uptake may trigger neuronal death through preferential activation of extrasynaptic NMDAR-related pathways.

5-HT1A receptors are differentially involved in the anxiolytic- and antidepressant-like effects of 8-OH-DPAT and fluoxetine in the rat

Fluoxetine, a selective serotonin reuptake inhibitor, shows moderate efficacy and potency in the rat forced swimming depression test and the shock-induced ultrasonic vocalization anxiety test, whereas the 5-HT(1A) receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) is highly efficient and potent in both models. Whereas the 5-HT(1A) receptor antagonist WAY 100,635 abolishes the effect of 8-OH-DPAT in both models, it only attenuates the antidepressant-like effect of fluoxetine. Pretreatment with the 5-HT-depleting agent parachlorophenylalanine attenuates the antidepressant-like effect of fluoxetine, but not that of 8-OH-DPAT. This suggests that the antidepressant-like effect of fluoxetine and 8-OH-DPAT results from indirect (via increased synaptic availability of 5-HT) and direct stimulation of postsynaptic 5-HT(1A) receptors, respectively; whereas the anxiolytic-like effect of fluoxetine is not mediated by 5-HT(1A) receptors. The data support the hypothesis that the antidepressant- and anxiolytic-like effect of 8-OH-DPAT is predominantly mediated by post- and presynaptic 5-HT(1A) receptors, respectively, and that 5-HT(1A) receptors are only partially involved in the antidepressant-like effect of fluoxetine.

Chronic L-DOPA treatment increases extracellular glutamate levels and GLT1 expression in the basal ganglia in a rat model of Parkinson's disease

There is growing experimental evidence for the implication of glutamate-mediated mechanisms both in the pathophysiology of Parkinson's disease and in the development of dyskinesias with long-term administration of L-3,4-dihydroxyphenylalanine (L-DOPA). However, the impact of this treatment on glutamate transmission in the basal ganglia has been poorly investigated. In this study, we examined the effects of 6-hydroxydopamine-induced lesion of nigral dopamine neurons with or without subsequent chronic L-DOPA treatment on several parameters of glutamate system function in the rat striatum and substantia nigra pars reticulata. All the lesioned animals treated with L-DOPA developed severe dyskinesias. Extracellular glutamate levels, measured by microdialysis in freely moving conditions, and gene expression of the glial glutamate transporter GLT1, assessed by in situ hybridization, were unaffected by dopamine lesion or L-DOPA treatment alone, but were both markedly increased on the lesion side of rats with subsequent L-DOPA treatment. No change in the expression of the vesicular glutamate transporters vGluT1 and vGluT2 was measured in striatum. These data show that chronic L-DOPA treatment leading to dyskinesias increases basal levels of glutamate function in basal ganglia. The L-DOPA-induced overexpression of GLT1 may represent a compensatory mechanism involving astrocytes to limit glutamate overactivity and subsequent toxic processes.

Glial soluble factors regulate the activity and expression of the neuronal glutamate transporter EAAC1: implication of cholesterol

A co-ordinated regulation between neurons and astrocytes is essential for the control of extracellular glutamate concentration. Here, we have investigated the influence of astrocytes and glia-derived cholesterol on the regulation of glutamate transport in primary neuronal cultures from rat embryonic cortices. Glutamate uptake rate and expression of the neuronal glutamate transporter EAAC1 were low when neurons were grown without astrocytes and neurons were unable to clear extracellular glutamate. Treatment of the neuronal cultures with glial conditioned medium (GCM) increased glutamate uptake Vmax, EAAC1 expression and restored the capacity of neurons to eliminate extracellular glutamate. Thus, astrocytes up-regulate the activity and expression of EAAC1 in neurons. We further showed that cholesterol, present in GCM, increased glutamate uptake activity when added directly to neurons and had no effect on glutamate transporter expression. Furthermore, part of the GCM-induced effect on glutamate transport activity was lost when cholesterol was removed from GCM (low cholesterol-GCM) and was restored when cholesterol was added to low cholesterol-GCM. This demonstrates that glia-derived cholesterol regulates glutamate transport activity. With these experiments, we provide new evidences for neuronal glutamate transport regulation by astrocytes and identified cholesterol as one of the factors implicated in this regulation.

Discriminative stimulus properties of the selective norepinephrine reuptake inhibitor, reboxetine, in rats

RATIONALE

Although drug discrimination procedures have proven difficult to apply to antidepressant agents, we recently characterized discriminative stimulus properties of the selective serotonin (5-HT) reuptake inhibitor, citalopram, in rats. However, discriminative stimulus properties of selective norepinephrine (NE) reuptake inhibitors remain to be evaluated.

OBJECTIVE

We determined the potential discriminative stimulus properties of the highly selective NE reuptake inhibitor and antidepressant, reboxetine.

METHODS

Employing a two-lever discrimination procedure, rats were trained to discriminate reboxetine (2.5 mg/kg, IP) from saline. In parallel, the influence of reboxetine (2.5 mg/kg) upon dialysate levels of monoamines in frontal cortex and dorsal hippocampus of freely moving rats was determined.

RESULTS

After 54+/-10 training sessions, reboxetine elicited robust stimulus recognition, fully generalizing to itself with an ED50 of 1.2 mg/kg. Two further NE reuptake inhibitors, desipramine (5.3) and maprotiline (1.8), as well as the 5-HT/NE reuptake inhibitor, venlafaxine (1.0), likewise generalized. In contrast, the 5-HT reuptake inhibitors, paroxetine, citalopram and sertraline, and the DA reuptake inhibitors, GBR12935 and bupropion, did not show significant generalization. Reboxetine markedly increased dialysate levels of NE, but not 5-HT, in frontal cortex and hippocampus. Dopamine (DA) levels were also (though less markedly) enhanced in frontal cortex.

CONCLUSION

In parallel with an elevation in extracellular levels of NE, the selective NE reuptake inhibitor, reboxetine, elicits a specific discriminative stimulus in rats.

Serotonin(2C) receptors tonically suppress the activity of mesocortical dopaminergic and adrenergic, but not serotonergic, pathways: a combined dialysis and electrophysiological analysis in the rat

The present study evaluated, via a combined electrophysiological and dialysis approach, the potential influence of serotonin (5-HT)(2C) as compared to 5-HT(2A) and 5-HT(2B) receptors on dopaminergic, adrenergic, and serotonergic transmission in frontal cortex (FCX). Whereas the selective 5-HT(2A) antagonist MDL100,907 failed to modify extracellular levels of dopamine (DA), noradrenaline (NA) or 5-HT simultaneously quantified in single dialysate samples of freely-moving rats, the 5-HT(2B)/5-HT(2C) antagonist SB206,553 dose-dependently increased levels of DA and NA without affecting those of 5-HT. This action was attributable to 5-HT(2C) receptor blockade inasmuch as the selective 5-HT(2C) antagonist SB242,084 likewise increased FCX levels of DA and NA, whereas the selective 5-HT(2B) antagonist SB204,741 was ineffective. Further, the preferential 5-HT(2C) receptor agonist Ro60-0175 dose-dependently depressed FCX levels of DA. The suppressive influence of 5-HT(2C) receptors on DA release was also expressed on mesolimbic and nigrostriatal dopaminergic pathways, in that levels of DA in nucleus accumbens and striatum were likewise reduced by Ro60-0175 and elevated, though less markedly, by SB206,553. In line with the above findings, Ro60-0175 dose-dependently decreased the firing rate of ventrotegmental dopaminergic and locus coeruleus (LC) adrenergic perikarya, whereas their activity was dose-dependently enhanced by SB206,553. Furthermore, SB206,553 transformed the firing pattern of ventrotegmental dopaminergic neurons into a burst mode. In contrast to SB206,553, MDL100,907 had little affect on the firing rate of dopaminergic or adrenergic neurons. In conclusion, as compared to 5-HT(2A) and 5-HT(2B) receptors, 5-HT(2C) receptors exert a tonic, suppressive influence on the activity of mesocortical - as well as mesolimbic and nigrostriatal - dopaminergic pathways, likely via indirect actions expressed at the level of their cell bodies. Frontocortical adrenergic, but not serotonergic, transmission is also tonically suppressed by 5-HT(2C) receptors.

Buspirone modulates basal and fluoxetine-stimulated dialysate levels of dopamine, noradrenaline and serotonin in the frontal cortex of freely moving rats: activation of serotonin1A receptors and blockade of alpha2-adrenergic receptors underlie its actions

The serotonin1A receptor partial agonist, buspirone, also displays antagonist properties at D2 receptors and is metabolized to the alpha2-adrenergic receptor antagonist, 1-(2-pyrimidinyl-piperazine). Herein, we examined mechanisms underlying the influence of buspirone alone, and in association with the serotonin reuptake inhibitor, fluoxetine, upon extracellular levels of serotonin, dopamine and noradrenaline simultaneously quantified in the frontal cortex of freely moving rats. Buspirone (0.01-2.5 mg/kg, s.c.) dose-dependently decreased dialysate levels of serotonin (-50%), and increased those of dopamine (+100%) and noradrenaline (+140%). The reduction by buspirone of serotonin levels was abolished by the serotonin1A receptor antagonist, WAY 100,635 (0.16), which did not, however, modify its influence upon dopamine and noradrenaline. In contrast to buspirone, the serotonin reuptake inhibitor, fluoxetine (10.0), increased frontocortical levels of serotonin (+ 120%), dopamine (+55%) and noradrenaline (+90%). Buspirone dose-dependently (0.01-2.5) decreased the induction by fluoxetine of serotonin levels yet potentiated (three-fold) its elevation of dopamine and noradrenaline levels. The serotonin1A agonist, 8-hydroxy-2-(di-n-propyl-amino)-tetralin (0.16), mimicked the action of buspirone in reducing resting levels of serotonin (-60%) and in enhancing those of dopamine (+135%) and noradrenaline (+165%). Like buspirone, it attenuated the influence of fluoxetine upon serotonin levels, yet facilitated its influence upon dopamine and noradrenaline levels. In contrast, WAY 100,635 selectively potentiated the increase in levels of serotonin (two-fold) versus dopamine and noradrenaline elicited by fluoxetine. Further, WAY 100,635 abolished the inhibitory influence of buspirone upon fluoxetine-induced serotonin release, but only partly interfered with its potentiation of fluoxetine-induced increases in dopamine and noradrenaline levels. The D2/D3 receptor antagonist, raclopride (0.16), increased basal dopamine (+60%) levels but little influenced those of serotonin and noradrenaline, and failed to modify the action of fluoxetine. The alpha2-adrenergic receptor antagonist, 1-(2-pyrimidinyl-piperazine) (2.5), which did not modify resting levels of serotonin, markedly increased those of dopamine (+90%) and noradrenaline (+190%) and potentiated (two-fold) the increases in dialysate levels of dopamine, noradrenaline and serotonin provoked by fluoxetine. Further, the alpha2-adrenergic receptor agonist, S18616, attenuated the enhancement by buspirone of the fluoxetine-induced increase in levels of dopamine and noradrenaline. In conclusion, the inhibitory influence of buspirone upon resting and fluoxetine-stimulated serotonin levels reflects its agonist properties at serotonin1A autoreceptors. The facilitatory influence of buspirone upon resting and fluoxetine-stimulated dopamine and noradrenaline levels may also involve its serotonin1A properties. However, its principal mechanism of action in this respect is probably the alpha2-adrenergic antagonist properties of its metabolite, 1-(2-pyrimidinyl-piperazine). The present observations are of significance to experimental and clinical studies of the influence of buspirone upon depressive states, alone and in association with antidepressant agents.

S-16924, a novel, potential antipsychotic with marked serotonin1A agonist properties. IV. A drug discrimination comparison with clozapine

The novel benzodioxopyrrolidine (S-16924) displays a clozapine-like profile of interaction with multiple monoaminergic receptors, in addition to potent agonist activity at serotonin (5-HT)1A receptors. S-16924 (2.5 mg/kg i.p.) and clozapine (5.0 mg/kg i.p.) generated robust discriminative stimuli (DS) and displayed full mutual generalization. The D4 antagonists L-745,870 and S-18126, the D1/D5 antagonist SCH-39166, and the D3 antagonist S-14297 showed at most partial generalization to S-16924 and clozapine. The D2/D3 antagonist raclopride fully generalized to S-16924, but only partially generalized to clozapine. The 5-HT2A antagonist MDL-100, 907 fully generalized to S-16924 and two further 5-HT2A antagonists, fananserin and SR-46349, showed partial generalization. However, MDL-100,907, fananserin, and SR-46349 showed less pronounced generalization to clozapine. Similarly, the 5-HT2C antagonists SB-200,646 and SB-206,553 more markedly generalized to S-16924 than to clozapine. The 5-HT1A receptor agonist (+/-)-8-dihydroxy-2-(di-n-propylamino) tetralin generalized fully to S-16924 but not to clozapine. Full generalization was obtained to both S-16924 and clozapine for the clozapine congeners, olanzapine and quetiapine. In distinction, the benzisoxazole, risperidone, and the phenylindole, sertindole, weakly generalized to S-16924 and clozapine. However, the benzisoxazole ziprasidone, which possesses 5-HT1A agonist properties, generalized fully to S-16924 but not to clozapine. Finally, the muscarinic antagonist scopolamine generalized fully to clozapine and partially to S-16924. In conclusion, S-16924 and clozapine display both communalities and differences in their "compound" DS; this likely reflects their respective complex patterns of interaction with multiple monoaminergic receptors. Although no specific receptor was identified as underlying the clozapine DS, 5-HT1A agonist as well as D2 and 5-HT2A/2C antagonist properties contribute to the S-16924 DS.

The role of 5-HT receptor subtypes in the anxiolytic effects of selective serotonin reuptake inhibitors in the rat ultrasonic vocalization test

We evaluated whether the anxiolytic effects of selective serotonin reuptake inhibitors (SSRIs) in the rat ultrasonic vocalization (USV) test are preferentially mediated by (indirect) activation of 5-HT1A, 5-HT1B/1D, 5-HT2A, 5-HT3 or 5-HT4 receptors. The SSRIs, paroxetine (ED50 in mg/kg, IP: 6.9), citalopram (6.5), fluvoxamine (11.7) and fluoxetine (> 30), dose dependently reduced shock-induced USV. The effects of paroxetine (3.0 mg/kg, IP) were not blocked by the selective 5-HT1A receptor antagonist, WAY-100635 (3.0 mg/kg, IP), the 5-HT1B/1D receptor antagonist, GR 127935 (30 mg/kg, IP), the nonselective 5-HT2A receptor antagonists, ritanserin (3.0 mg/kg, IP) and ketanserin (1.0 mg/kg, IP), the 5-HT3 receptor antagonist, ondansetron (0.1 mg/kg, IP), or the 5-HT4 receptor antagonist, GR 125487D (3.0 mg/kg, SC). In contrast, the selective 5-HT2A receptor antagonist, MDL 100,907 (0.1 mg/kg, IP), completely prevented the paroxetine-induced reduction of USV. Under similar conditions, WAY-100635 blocked the anxiolytic-like effects of the selective 5-HT1A receptor agonist, 8-OH-DPAT [(+/-)-8-hydroxy-2-(di-n-propylamino)tetralin, 1.0 mg/kg, IP], and ritanserin, ketanserin, and MDL 100,907 blocked the anxiolytic-like effects of the mixed 5-HT2A/2C receptor agonist, DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, 3.0 mg/kg, IP]. WAY-100635 (1.0 mg/kg, IP) in combination with ritanserin (3.0 mg/kg, IP), but not ondansetron (0.1 mg/kg, IP), GR 125487D (3.0 mg/kg, SC), or GR 127935 (30 mg/kg, IP), attenuated the USV reducing effects of paroxetine. Although the results suggest that selective stimulation of 5-HT1A and 5-HT2A receptors produces a decrease of USV, we postulate that only 5-HT2A receptors play a pivotal role in the effects of SSRIs in this model of anxiety.

WAY 100,635 enhances both the 'antidepressant' actions of duloxetine and its influence on dialysate levels of serotonin in frontal cortex

The mixed serotonin and noradrenaline reuptake inhibitor, duloxetine, (5.0 mg/kg, s.c.), increased levels of serotonin (220%), dopamine (180%) and noradrenaline (470%) in individual dialysates of frontal cortex of freely moving rats. Its influence on serotonin, but not dopamine or noradrenaline, levels was enhanced by the 5-HT1A receptor antagonist, WAY 100,635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide 3HCl) (0.16 mg kg(-1), s.c). In the forced swimming test, although duloxetine was inactive alone, it dose dependently reduced immobility in the presence of WAY 100,635. Thus, blockade of 5-HT1A (auto)receptors selectively facilitates the influence of duloxetine on serotonin levels in the frontal cortex in rats and, in the forced swimming model, enhances its 'antidepressant' properties in parallel.

Alpha2-adrenergic receptor blockade markedly potentiates duloxetine- and fluoxetine-induced increases in noradrenaline, dopamine, and serotonin levels in the frontal cortex of freely moving rats

Evidence exists that a reinforcement in monoaminergic transmission in the frontal cortex (FCX) is associated with antidepressant (AD) properties. Herein, we examined whether blockade of alpha2-adrenergic receptors modified the influence of monoamine reuptake inhibitors on FCX levels of serotonin (5-HT), noradrenaline (NAD), and dopamine (DA). The selective alpha2-adrenergic receptor agonist S 18616 (0.16 mg/kg, s.c.) suppressed extracellular levels of NAD, DA, and 5-HT (by 100, 51, and 63%, respectively) in single dialysates of FCX of freely moving rats. In contrast, the selective alpha2-adrenergic receptor antagonists atipamezole (0.16 mg/kg, s.c.) and 1-(2-pyrimidinyl)piperazine (1-PP; 2.5 mg/kg, s.c.) increased levels of NAD (by 180 and 185%, respectively) and DA (by 130 and 90%, respectively), without affecting 5-HT levels. Duloxetine (5.0 mg/kg, s.c.), a mixed inhibitor of 5-HT and NAD reuptake, and fluoxetine (10.0 mg/kg, s.c.), a selective 5-HT reuptake inhibitor, both increased levels of 5-HT (by 150 and 120%, respectively), NAD (by 400 and 100%, respectively), and DA (by 115 and 55%, respectively). Atipamezole (0.16 mg/kg, s.c.) markedly potentiated the influence of duloxetine and fluoxetine on levels of 5-HT (by 250 and 330%, respectively), NAD (by 1,030 and 215%, respectively), and DA (by 370 and 170%, respectively). 1-PP similarly potentiated the influence of duloxetine on 5-HT, NAD, and DA levels (by 290, 1,320, and 600%, respectively). These data demonstrate that alpha2-adrenergic receptors tonically inhibit NAD and DA and phasically inhibit 5-HT release in the FCX and that blockade of alpha2-adrenergic receptors strikingly potentiates the increase in FCX levels of 5-HT, NAD, and DA elicited by reuptake inhibitors. Concomitant alpha2-adrenergic receptor antagonism and inhibition of monoamine uptake may thus provide a mechanism allowing for a marked increase in the efficacy of AD agents.

Evidence that dopamine D3 receptors participate in clozapine-induced hypothermia

In analogy to the dopamine D3 receptor agonist, (+)-7-OH-DPAT (7-hydroxy-2-(di-n-propylamino)tetralin) (0.01-0.63 mg/kg s.c.), clozapine dose-dependently (0.63-40.0 mg/kg s.c.) elicited hypothermia in rats. Haloperidol and raclopride, mixed dopamine D2/D3 receptor antagonists, failed, in contrast, to modify core temperature. Further, they dose-dependently inhibited the action of clozapine with inhibitory dose50 values (ID50) of 0.3 mg/kg s.c., in each case. The preferential dopamine D3 versus D2 receptor antagonist, (+)-AJ 76 (cis-(+)-5-methoxy-1-methyl-2-(n-propylamino)tetralin) (ID50 = 2.8), and the selective dopamine D3 versus D2 receptor antagonist, (+/-)-S 11566 ((+/-)(-)[7-(N,N-dipropylamino)-5,6,7,8-tetrahydro-naphtho(2,3b) dihydro,2,3-furane]) (ID50 = 1.6) likewise blocked the action of clozapine without reducing core temperature alone. The action of (+/-)-S 11566 was stereospecific in that its active eutomer, (+)-S 14297 (ID50 = 1.0), also inhibited the action of clozapine whereas its inactive distomer, (-)-S 17777 (ID50 > 10.0), was not effective. Antagonist potency for blockade of clozapine-induced hypothermia correlated powerfully both with potency for blockade of (+)-7-OH-DPAT-induced hypothermia (r = 0.98) and with affinity at cloned human dopamine D3 receptors transfected into Chinese hamster ovary (CHO) cells (r = 0.92). In conclusion, these data suggest that dopamine D3 receptors may be involved in the induction of hypothermia by clozapine in the rat.

Acute free perforation as first sign of Crohn's disease

Free perforation with generalized peritonitis remains a rare manifestation of regional enteritis. Most of these perforations occur as complications of recurrent, well-diagnosed regional enteritis. A review of the literature reveals only 14 recorded cases of free perforation in patients without any previous abdominal complaints. The present report describes two more cases. Both of them were treated by resection of the involved segment with immediate anastomosis.