Differential effects of corticostriatal and thalamostriatal deafferentation on expression of the glutamate transporter GLT1 in the rat striatum

This study compared the effects of the disruption of the two main presumably glutamatergic striatal inputs, the corticostriatal and thalamostriatal pathways, on GLT1 expression in the rat striatum, using in situ hybridization and immunohistochemistry. Unilateral ibotenate-induced thalamic lesion produced no significant changes in striatal GLT1 mRNA labeling and immunostaining as assessed at 5 and 12 days postlesion. In contrast, significant increases in both parameters were measured after bilateral cortical lesion by superficial thermocoagulation. GLT1 mRNA levels increased predominantly in the dorsolateral part of the striatum; there, the increases were significant at 5 (+84%), 12 (+101%), and 21 (+45%) but not at 35 days postlesion. GLT1 immunostaining increased significantly and homogeneously by 17-26% at 12 and 21 days postlesion. The increase in GLT1 expression at 12 days postlesion was further confirmed by western blot analysis; in contrast, a 36% decrease in glutamate uptake activity was measured at the same time point. These data indicate that striatal GLT1 expression depends on corticostriatal but not thalamostriatal innervation. Comparison of our results with previous data showing that cortical lesion by aspiration downregulates striatal GLT1 expression further suggests that differential changes in GLT1 expression, and thus presumably in glial cell function, may occur in the target striatum depending on the way the cortical neurons degenerate.     

Nigrostriatal denervation does not affect glutamate transporter mRNA expression but subsequent levodopa treatment selectively increases GLT1 mRNA and protein expression in the rat striatum

There is growing evidence that the loss of the nigrostriatal dopaminergic neurones induces an overactivity of the corticostriatal glutamatergic pathway which seems to be central to the physiopathology of parkinsonism. Moreover, glutamatergic mechanisms involving NMDA receptors have been shown to interfere with the therapeutical action of levodopa. Given the key role played by uptake processes in glutamate neurotransmission, this study examined the effects of nigrostriatal deafferentation and of levodopa treatment on the striatal expression of the glutamate transporters GLT1, GLAST and EAAC1 in the rat. No significant changes in striatal mRNA levels of these transporters were detected after either levodopa treatment (100 mg/kg; i.p., twice a day for 21 days) or unilateral lesion of the nigrostriatal pathway by intranigral 6-hydroxydopamine injection. In contrast, animals with the lesion subsequently treated with levodopa showed a selective increase (36%) in GLT1 mRNA levels in the denervated striatum versus controls. These animals also showed increased GLT1 protein expression, as assessed by immunostaining and western blotting. These data provide the first evidence that levodopa therapy may interfere with striatal glutamate transmission through change in expression of the primarily glial glutamate transporter GLT1. We further suggest that levodopa-induced GLT1 overexpression may represent a compensatory mechanism preventing neurotoxic accumulation of endogenous glutamate.     

N-Acetyl-aspartyl-glutamate in the rat dorsal striatum: topographical distribution and effect of sensorimotor cortex lesions

The dipeptide N-acetyl-aspartyl-glutamate (NAAG) has been proposed as putative neurotransmitter of some corticostriatal projections. To further explore this possibility, endogenous NAAG levels were measured in various microdissected striatal regions in normal animals and in those with bilateral lesion of sensorimotor cortex. In intact rats there was a rostro-caudal gradient for NAAG, with highest concentrations in the more caudal portions of the striatum without significant differences between the medial and lateral regions. Decortication induced no significant changes in peptide concentration in any of the striatal regions or in the respective crude synaptosomal (P2) fractions. However, decorticated animals showed a large degree of deafferentation as evidenced by a marked and significant decrease in [³H]glutamate uptake as well as in glutamate levels measured in striatal homogenates or in crude synaptosomal fractions. No changes in striatal dopamine levels were observed in lesioned animals. Thus, these findings are not in favor of the existence of corticostriatal projections arising from the sensorimotor cortex using NAAG as neurotransmitter.     

Quinolinic acid stimulates synaptosomal glutamate release and inhibits glutamate uptake into astrocytes

Quinolinic acid (QA) is an endogenous neurotoxin involved in various neurological diseases, whose action seems to be exerted via glutamatergic receptors. However, the exact mechanism responsible for the neurotoxicity of QA is far from being understood. We have previously reported that QA inhibits vesicular glutamate uptake. In this work, investigating the effects of QA on the glutamatergic system from rat brain, we have demonstrated that QA (from 0.1 to 10mM) had no effect on synaptosomal L-[3H]glutamate uptake. The effect of QA on glutamate release in basal (physiological K+ concentration) or depolarized (40 mM KCl) conditions was evaluated. QA did not alter K+-stimulated glutamate release, but 5 and 10mM QA significantly increased basal glutamate release. The effect of dizolcipine (MK-801), a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptor on glutamate release was investigated. MK-801 (5 microM) did not alter glutamate release per se, but completely abolished the QA-induced glutamate release. NMDA (50 microM) also stimulated glutamate release, without altering QA-induced glutamate release, suggesting that QA effects were exerted via NMDA receptors. QA (5 and 10mM) decreased glutamate uptake into astrocyte cell cultures. Enhanced synaptosomal glutamate release, associated with inhibition of glutamate uptake into astrocytes induced by QA could contribute to increase extracellular glutamate concentrations which ultimately lead to overstimulation of the glutamatergic system. These data provide additional evidence that neurotoxicity of QA may be also related to disturbances on the glutamatergic transport system, which could result in the neurological manifestations observed when this organic acid accumulates in the brain.     

In Vivo Neurochemical Evaluation of Striatal Serotonergic Hyperinnervation in Rats Depleted of Dopamine at Infancy

Destruction of nigrostriatal dopamine (DA) neurons with 6-hydroxydopamine (6-OHDA) early in development results in hyperinnervation of striatum by the serotonergic afferents deriving from the dorsal raphe nucleus. We have used in vivo microdialysis to investigate the degree to which serotonergic neurotransmission in striatum is altered by this increase in the density of serotonin (5-HT) terminals. The effects of several manipulations known to influence 5-HT function on extracellular 5-HT and 5-hydroxyindoleacetic acid in striatum were compared in adult rats treated neonatally with 6-OHDA and in intact adult rats. Basal levels of 5-HT in extracellular fluid (ECF) of striatum were similar in neonatally DA-depleted rats and in intact rats. Perfusion with the 5-HT reuptake blocker, fluoxetine (100 microM), increased 5-HT in striatal ECF of neonatally DA-depleted rats to levels that were threefold greater than those achieved in intact rats. Likewise, K(+)-depolarization of the 5-HT terminals (100 mM in perfusate) or systemic administration of the 5-HT releaser, (+/-)-fenfluramine (10 mg/kg i.p.), increased the concentration of 5-HT in striatal ECF of neonatally DA-depleted rats to levels approximately threefold greater than those observed in striatum of intact rats. These findings indicate that the 5-HT hyperinnervation of striatum that takes place in rats depleted of DA at infancy is associated with an increased capacity for neurotransmitter release in this system. Concomitant increased in high-affinity 5-HT uptake may prevent the occurrence of any measurable changes in the resting concentration of 5-HT in striatal ECF.     

The 5-HT receptor antagonist M100,907 prevents extracellular glutamate rising in response to NMDA receptor blockade in the mPFC

We recently found that intracortical injection of the selective and competitive N-methyl-D-aspartate (NMDA) receptor antagonist 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP) impaired attentional performance in rats and blockade of 5-hydroxytryptamine (5-HT)2A receptors antagonized this effect. Here, we used the microdialysis technique in conscious rats to study the effect of CPP on extracellular glutamate (GLU) in the medial prefrontal cortex (mPFC) and the regulation of this effect by 5-HT2A receptors. Intraperitoneal injection of 20 mg/kg CPP increased extracellular GLU in the mPFC (201% of basal levels) but had no effect on 5-HT. Intracortical infusion of 100 microm CPP increased extracellular GLU (230% of basal values) and 5-HT (150% of basal values) in the mPFC, whereas 30 microm had no significant effect. The effect of 100 microm CPP on extracellular GLU was abolished by tetrodotoxin, suggesting that neuronal activity is required. Subcutaneous injection of 40 microg/kg M100,907 completely antagonized the effect of 100 microm cpp on extracellular GLU, whereas 10 microg/kg caused only partial attenuation. Likewise, intracortical infusion of 0.1 microm M100,907 completely reversed the increase of extracellular GLU induced by CPP. These findings show that blockade of NMDA receptors in the mPFC is sufficient to increase extracellular GLU locally. The increase of cortical extracellular GLU may contribute to CPP-induced cognitive deficits and blockade of 5-HT2A receptors may provide a molecular mechanism for reversing these deficits caused by dysfunctional glutamatergic transmission in the mPFC.     

Plant-Derived Neurotoxic Amino Acids (β-N-Oxalylamino-l-Alanine and β-N-Methylamino-l-Alanine): Effects on Central Monoamine Neurons

In the present study the subacute effects of beta-N-oxalylamino-L-alanine (BOAA) and beta-N-methylamino-L-alanine (BMAA) on CNS monoamine neurons in rats were investigated following intracisternal injections or local intracerebral administration into substantia nigra. In vitro effects of BOAA and BMAA on high-affinity synaptosomal uptake of dopamine (DA), noradrenaline (NA), and serotonin (5-HT) were also examined. Intracisternal administration of BMAA decreased NA levels in hypothalamus, whereas no effects were seen on DA or 5-HT levels. Following intranigral injections of BOAA, NA levels tended to decrease in several regions, whereas the DA levels and the levels of DA metabolites were unaffected in all regions analyzed. Loss of tyrosine hydroxylase (TH) immunoreactivity in the intranigral injection sites and the presence of TH-immunoreactive pyknotic neurons near the borders of the injection sites were observed following both BOAA and BMAA treatments. Furthermore, substance P-immunoreactive terminals in substantia nigra pars reticulata were also found to have disappeared within the lesioned area following either BOAA or BMAA injections. Incubations with both BOAA and BMAA (10(-5) M) reduced high-affinity [3H]NA uptake in cortical synaptosomes to 69% and 41% of controls, respectively, whereas the striatal high-affinity [3H]DA uptake and the cortical high-affinity [3H]5-HT uptake were unaffected by BOAA or BMAA. The results demonstrate that both BOAA and BMAA can affect central monoamine neurons, although the potency and specificity of these substances on monoamine neurons when administered acutely into cerebral tissue or liquor cerebri seem to be low. However, the in vitro studies indicate selective effects of both compounds on NA neurons in synaptosomal preparations.     

Altered astrocyte glutamate transporter regulation of hypothalamic neurosecretory neurons in heart failure rats

Neurohumoral activation, which includes augmented plasma levels of the neurohormone vasopressin (VP), is a common finding in heart failure (HF) that contributes to morbidity and mortality in this disease. While an increased activation of magnocellular neurosecretory cells (MNCs) and enhanced glutamate function in HF is well documented, the precise underlying mechanisms remain to be elucidated. Here, we combined electrophysiology and protein measurements to determine whether altered glial glutamate transporter function and/or expression occurs in the hypothalamic supraoptic nucleus (SON) during HF. Patch-clamp recordings obtained from MNCs in brain slices show that pharmacological blockade of astrocyte glutamate transporter 1 (GLT1) function [500 μM dihydrokainate (DHK)], resulted in a persistent N-methyl-D-aspartate receptor (NMDAR)-mediated inward current (tonic I(NMDA)) in sham rats, an effect that was significantly smaller in MNCs from HF rats. In addition, we found a diminished GLT1 protein content in plasma membrane (but not cytosolic) fractions of SON punches in HF rats. Conversely, astrocyte GLAST expression was significantly higher in the SON of HF rats, while nonselective blockade of glutamate transport activity (100 μM TBOA) evoked an enhanced tonic I(NMDA) activation in HF rats. Steady-state activation of NMDARs by extracellular glutamate levels was diminished during HF. Taken together, these results support a shift in the relative expression and function of two major glial glutamate transporters (from GLT1 to GLAST predominance) during HF. This shift may act as a compensatory mechanism to preserve an adequate basal glutamate uptake level in the face of an enhanced glutamatergic afferent activity in HF rats.     

Glucose increases synaptic transmission from vagal afferent central nerve terminals via modulation of 5-HT3 receptors

Acute hyperglycemia has profound effects on vagally mediated gastrointestinal functions. We have reported recently that the release of glutamate from the central terminals of vagal afferent neurons is correlated directly with the extracellular glucose concentration. The present study was designed to test the hypothesis that 5-HT(3) receptors present on vagal afferent nerve terminals are involved in this glucose-dependent modulation of glutamatergic synaptic transmission. Whole-cell patch-clamp recordings were made from neurons of the nucleus tractus solitarius (NTS) in thin rat brainstem slices. Spontaneous and evoked glutamate release was decreased in a concentration-dependent manner by the 5-HT(3) receptor selective antagonist, ondansetron. Alterations in the extracellular glucose concentration induced parallel shifts in the ondansetron-mediated inhibition of glutamate release. The changes in excitatory synaptic transmission induced by extracellular glucose concentration were mimicked by the serotonin uptake inhibitor, fenfluramine. These data suggest that glucose alters excitatory synaptic transmission within the rat brainstem via actions on tonically active 5-HT(3) receptors, and the number of 5-HT(3) receptors on vagal afferent nerve terminals is positively correlated with the extracellular glucose concentration. These data indicate that the 5-HT(3) receptors present on synaptic connections between vagal afferent nerve terminals and NTS neurons are a strong candidate for consideration as one of the sites where glucose acts to modulate vagovagal reflexes.     

Rat brain monoamine and serotonin S2 receptor changes during pregnancy

The concentrations of noradrenaline (NA), dopamine (DA), serotonin (5-HT), and their metabolites were determined in 5 brain areas of non-pregnant, 15 and 20 day pregnant and 4 day post-partum rats. Striatal 5-HT content was significantly lower in 15 and 20 day pregnant rats than in estrous controls. A significant decrease in striatal and frontal cortex 5-hydroxyindole-3-acetic acid (5-HIAA) concentration was observed in 15 day pregnant rats. Significant increases in hypothalamic and hippocampal NA levels were observed at 4 days post-partum. Frontal cortex serotonin S2 receptorKd was reduced in 4 day post-partum rats. There was no significant change in S2 receptorB _max during pregnancy. Levels of progesterone were negatively correlated with striatal DA, homovanillic acid (HVA), 5-HT, and 5-HIAA levels, hypothalamic DA, hippocampal 5-HT, and frontal cortex 5-HIAA values as well as striatal HVA to DA, and HVA to 3,4-dihydroxyphenylacetic acid (DOPAC) ratios and amygdaloid HVA to DOPAC ratios. The limbic neurotransmitter changes might possibly contribute to mood changes which occur during pregnancy and post-partum.     

A Pressurized Nitrogen Counterbalance to Cortical Glutamatergic Pathway Stimulation

Previous microdialysis studies performed in rats have revealed a decrease of striatal dopamine and glutamate induced by nitrogen narcosis. We sought to establish the hypothetical role of the glutamatergic corticostriatal pathway because of the glutamate deficiency which occurs in the basal ganglia in this hyperbaric syndrome. Retrodialysis with 1 mM of Saclofen and 100 mM of KCl in the prefrontal cortex under normobaric conditions led to an increase in striatal levels of glutamate by 95.2% and no changes in dopamine levels. Under 3 MPa of nitrogen and with the infusion, the rate of striatal glutamate decreased by 51.3%, to a greater extent than under pressurised nitrogen alone (−23.8%). The rate of dopamine decreased, which also occurred under pressurised nitrogen (−36.9 and −31.4%, respectively). In conclusion, the function of the corticostriatal pathway is affected by nitrogen under pressure. This suggests that the nitrogen-induced break point seems to be located at the glutamatergic striatopetal neurons.     

Chronic Stress and Lithium Treatments Alter Hippocampal Glutamate Uptake and Release in the Rat and Potentiate Necrotic Cellular Death After Oxygen and Glucose Deprivation

This study was undertaken to evaluate the effects of chronic variate stress and lithium treatment on glutamatergic activity and neuronal vulnerability of rat hippocampus. Male Wistar rats were simultaneously treated with lithium and submitted to a chronic variate stress protocol during 40?days, and afterwards the hippocampal glutamatergic uptake and release, measured in slices and synaptosomes, were evaluated. We observed an increased synaptosomal [(3)H]glutamate uptake and an increase in [(3)H]glutamate stimulated release in hippocampus of lithium-treated rats. Chronic stress increased basal [(3)H]glutamate release by synaptosomes, and decreased [(3)H]glutamate uptake in hippocampal slices. When evaluating cellular vulnerability, both stress and lithium increased cellular death after oxygen and glucose deprivation (OGD). We suggest that the manipulation of glutamatergic activity induced by stress may be in part responsible for the neuroendangerment observed after stress exposure, and that, in spite of the described neuroprotective effects of lithium, it increased the neuronal vulnerability after OGD.     

Extracellular Striatal Dopamine and Glutamate After Decortication and Kainate Receptor Stimulation, as Measured by Microdialysis

Abstract: Disruption of corticostriatal glutamate input in the striatum decreased significantly extracellular striatal glutamate and dopamine levels. Local administration of 300 µ M concentration of excitatory receptor agonist kainic acid increased significantly extracellular striatal dopamine in intact freely moving rats. These findings support the hypothesis that glutamate exerts a tonic facilitatory effect on striatal dopamine release. The effect of kainic acid on extracellular striatal glutamate concentration in intact rats was a biphasic increase. The first glutamate increase can be explained by stimulation of presynaptic kainate receptors present on corticostriatal glutamatergic nerve terminals; the second increase is probably the result of a continuous interaction of the different striatal neurotransmitters after disturbance of their balance. Release of dopamine and glutamate was modulated differently in the intact striatum and in the striatum deprived of corticostriatal input. Dopamine release in the denervated striatum after kainate receptor stimulation was significantly lower than in intact striatum, confirming the so-called cooperativity between glutamate and kainic acid. Loss of presynaptic kainate receptors on the glutamatergic nerve terminals after decortication resulted in a loss of effect of kainic acid on glutamate release in denervated striatum. Aspartate showed no significant changes in this study.     

Anticonvulsant action of hippocampal dopamine and serotonin is independently mediated by D and 5-HT receptors

The present microdialysis study evaluated the anticonvulsant activity of extracellular hippocampal dopamine (DA) and serotonin (5-HT) with concomitant assessment of the possible mutual interactions between these monoamines. The anticonvulsant effects of intrahippocampally applied DA and 5-HT concentrations were evaluated against pilocarpine-induced seizures in conscious rats. DA or 5-HT perfusions protected the rats from limbic seizures as long as extracellular DA or 5-HT concentrations ranged, respectively, between 70-400% and 80-350% increases compared with the baseline levels. Co-perfusion with the selective D(2) blocker remoxipride or the selective 5-HT(1A) blocker WAY-100635 clearly abolished all anticonvulsant effects. These anticonvulsant effects were mediated independently since no mutual 5-HT and DA interactions were observed as long as extracellular DA and 5-HT levels remained within these protective ranges. Simultaneous D(2) and 5-HT(1A) receptor blockade significantly aggravated pilocarpine-induced seizures. High extracellular DA (> 1000% increases) or 5-HT (> 900% increases) concentrations also worsened seizure outcome. The latter proconvulsive effects were associated with significant increases in extracellular glutamate (Glu) and mutual increases in extracellular monoamines. Our results suggest that, within a certain concentration range, DA and 5-HT contribute independently to the prevention of hippocampal epileptogenesis via, respectively, D(2) and 5-HT(1A) receptor activation.     

Effects of Ionotropic Excitatory Amino Acid Receptor Antagonists on Glutamate Transport and Transport-Mediated Changes in Extracellular Excitatory Amino Acids in the Rat Striatum

Abstract: This study examined the effects of intrastriatal administration of ionotropic excitatory amino acid receptor antagonists on biochemical markers of excitatory amino acid transmission in the rat striatum. High-affinity glutamate uptake was measured ex vivo on striatal homogenates 15 min after the local administration of either 6,7-dinitroquinoxaline-2,3-dione (DNQX), a non-NMDA receptor antagonist, or dl -2-amino-5-phosphonopentanoic acid (AP5), a competitive NMDA antagonist, at various doses (10–500 pmol injected). DNQX induced a dose-dependent increase in glutamate uptake rate, related to an increase in the V _max of the transport process, whereas no significant change in glutamate uptake was detected after AP5 administration. Similar results were obtained from animals subjected to excitotoxic lesion of striatal neurons by kainate administration 15 days before the injection of DNQX or AP5. In a parallel series of experiments using in vivo microdialysis we showed that DNQX (10⁻⁵ M ) in the dialysis probe diminished by ∼30–40% the increases in the concentrations of glutamate and aspartate elicited by l - trans -pyrrolidine-2,4-dicarboxylic acid (1 m M ). These data suggest that presynaptic glutamate transmission in the rat striatum may undergo facilitatory autoregulatory processes involving ionotropic non-NMDA receptors and highlight the view that transporters for glutamate may be potent regulatory sites for glutamatergic transmission.     

Oxytocin regulates changes of extracellular glutamate and GABA levels induced by methamphetamine in the mouse brain

Oxytocin (OT), a neurohypophyseal neuropeptide, affects adaptive processes of the central nervous system. In the present study, we investigated the effects of OT on extracellular levels of glutamate (Glu) and γ-aminobutyric acid (GABA) induced by methamphetamine (MAP) in the medial prefrontal cortex (mPFC) and dorsal hippocampus (DHC) of freely moving mice, using in vivo microdialysis coupled to high-performance liquid chromatography and fluorescence detection. The results showed that OT had no effect on basal Glu levels, but attenuated MAP-induced Glu increase in the mPFC and decrease in the DHC. OT increased the basal levels of extracellular GABA in mPFC and DHC of mice, and inhibited the MAP-induced GABA decrease in DHC. Western blot results indicated that OT significantly inhibited the increased glutamatergic receptor (NR1 subunit) levels in the PFC after acute MAP administration, whereas OT further enhanced the elevated levels of glutamatergic transporter (GLT1) induced by MAP in the hippocampus of mice. Atosiban, a selective inhibitor of OT receptor, antagonized the effects of OT. The results provided the first neurochemical evidence that OT, which exerted its action via its receptor, decreased Glu release induced by MAP, and attenuated the changes in glutamatergic neurotransmission partially via regulation of NR1 and GLT1 expression. OT-induced extracellular GABA increase also suggests that OT acts potentially as an inhibitory neuromodulator in mPFC and DHC of mice.     

The 5-HT(1A) receptor agonist 8-OH-DPAT prevents prefrontocortical glutamate and serotonin release in response to blockade of cortical NMDA receptors

We studied the role of 5-HT(1A) receptors in controlling the release of glutamate (GLU) in the medial prefrontal cortex (mPFC) of conscious rats with the in vivo microdialysis technique. The effect of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin infused in the prefrontal cortex was examined under basal conditions and on the rise of extracellular GLU (+106%) induced by co-infusion of the competitive N-methyl-d-aspartate receptor antagonist 3-[(R)-2-carboxypiperazin-4yl]-propyl-1-phosphonic acid (CPP). 8-OH-DPAT (0.3 and 3 microm) had no effect on basal extracellular GLU, but the higher concentration completely abolished the rise of extracellular GLU induced by CPP. CPP also increased extracellular serotonin (5-HT) in the mPFC (+50%) and this effect was antagonized by 3 microm 8-OH-DPAT which, by itself, had no effect on basal 5-HT release. The effects of 8-OH-DPAT on extracellular GLU and 5-HT were reversed by the 5-HT(1A) receptor antagonist WAY100 635 (100 microm), indicating a selective involvement of 5-HT(1A) receptors. WAY100 635 had no effect by itself. These results show that the stimulation of cortical 5-HT(1A) receptors prevents the CPP-evoked rise of extracellular GLU and 5-HT and suggest that these effects may contribute to the ability of intracortical 8-OH-DPAT to counteract cognitive deficits caused by the blockade of NMDA receptors.     

Evidence that [3H]Dopamine Is Taken Up and Released from Nondopaminergic Nerve Terminals in the Rat Substantia Nigra In Vitro

Potassium chloride (25 mM) and (+)-amphetamine (100 microM) both stimulated the release of radioactivity from slices of substantia nigra preincubated with [3H]3,4-dihydroxyphenylethylamine [( 3H]dopamine). Potassium chloride (25 mM) released radioactivity from slices of both zona compacta and zona reticulata. Prior 6-hydroxydopamine (6-OHDA) lesions of one nigrostriatal pathway did not reduce the spontaneous release of radioactivity, or the potassium chloride- or amphetamine-induced release of radioactivity from slices of nigra ipsilateral to the lesion after preincubation with [3H]dopamine. The accumulation of radioactivity following incubation of nigral slices from 6-OHDA-lesioned animals with [3H]dopamine was increased when compared to uptake into slices from intact tissue. In synaptosomal preparations of striatum, nomifensine but not desipramine or fluoxetine inhibited [3H]dopamine uptake. In contrast, nomifensine, desipramine, and fluoxetine all inhibited [3H]dopamine uptake in nigral synaptosomal preparations. Following 6-OHDA lesions of one nigrostriatal pathway the uptake of [3H]dopamine into nigral synaptosomal preparations was unchanged but uptake into striatal preparations was substantially decreased. In contrast, bilateral electrolesions of the dorsal and medial raphe nuclei reduced [3H]dopamine uptake into nigral preparations but not into striatal synaptosomes. The uptake of [3H]5-hydroxytryptamine ([3H]5-HT) into synaptosomal preparations of substantia nigra was abolished by fluoxetine and reduced by desipramine, but was unaffected by nomifensine. In contrast, fluoxetine, desipramine, and nomifensine all inhibited [3H]5-HT uptake into striatal synaptosomal preparations. Following 6-OHDA lesions of one nigro-striatal pathway the uptake of [3H]5-HT into nigral synaptosomal preparations was unchanged but uptake into striatal preparations was reduced.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antagonism of Serotonin 5-HT1A Receptors Potentiates the Increases in Extracellular Monoamines Induced by Duloxetine in Rat Hypothalamus

Abstract: In the current study we examined the effects of coadministration of a serotonin 5-HT_1A antagonist, (±)-1-(1 H -indol-4-yloxy)-3-(cyclohexylamino)-2-propanol maleate (LY 206130), and a dual 5-HT and norepinephrine (NE) uptake inhibitor, duloxetine, on extracellular levels of NE, 5-HT, dopamine (DA), 5-hydroxyindoleacetic acid, and 3,4-dihydroxyphenylacetic acid in rat hypothalamus microdialysates. LY 206130 (3.0 mg/kg, s.c.) alone significantly increased NE and DA levels by 60 and 34%, respectively, without affecting 5-HT levels. Duloxetine administration at 4.0 mg/kg, i.p. alone produced no significant changes in levels of 5-HT, NE, or DA. In contrast, when LY 206130 and duloxetine were coadministered at 3.0 mg/kg, s.c. and 4.0 mg/kg, i.p., respectively, 5-HT, NE, and DA levels increased to 5.7-, 4.8-, and threefold over their respective basal levels. These data demonstrate that antagonism of somatodendritic 5-HT_1A autoreceptors and concomitant inhibition of 5-HT and NE uptake with duloxetine may promote synergistic increases in levels of extracellular 5-HT, NE, and DA in hypothalamus of conscious, freely moving rats.     

Extracellular Glutamate Levels Are Chronically Elevated in the Brains of LP-BM5-Infected Mice: A Mechanism of Retrovirus-Induced Encephalopathy

Abstract: Mice infected with the LP-BM5 leukemia retrovirus mixture develop a progressive immunodeficiency with associated behavioral, histological, and neurochemical alterations consistent with glutamatergic hyperactivation. To gain insight into the contribution of excitatory amino acids to the neurodegeneration observed in these mice, their concentrations were measured in the CSF and striatal microdialysates. Glutamate concentrations were significantly elevated in CSF but not plasma as early as 4 weeks postinoculation. Steady-state glutamate levels in striatal microdialysates were increased threefold and could be reduced 40% by application of l -α-aminoadipate, an inhibitor of microglial glutamate transport. Stimulation of infected mice with KCl/ l - trans -2,4-pyrrolidine dicarboxylate further increased glutamate levels 170–270% above those evoked in control mice. Tetrodotoxin suppressed the depolarization-evoked increase in glutamate by 88% in control mice, but it had only negligible effects in 40% of infected mice. Analysis of glutamate transport and catabolism suggests that abnormal astrocytic function does not contribute to the increase in basal extracellular glutamate levels. These findings are the first direct evidence that infection with an immunodeficiency-inducing retrovirus leads to a chronic elevation of extracellular free glutamate levels in the brain, which contributes to the neurodegenerative and cognitive deficits observed in these mice.