P2X7 receptor activation downmodulates Na(+)-dependent high-affinity GABA and glutamate transport into rat brain cortex synaptosomes.

Sodium-dependent high-affinity amino-acid transporters play crucial roles in terminating synaptic transmission in the central nervous system (CNS). However, there is lack of information about the mechanisms underlying the regulation of amino-acid transport by fast-acting neuromodulators, like ATP. Here, we investigated whether activation of the ATP-sensitive P2X7 receptor modulates Na(+)-dependent high-affinity γ-aminobutyric acid (GABA) and glutamate uptake into nerve terminals (synaptosomes) of the rat cerebral cortex. Radiolabeled neurotransmitter accumulation was evaluated by liquid scintillation spectrometry. The cell-permeant sodium-selective fluorescent indicator, SBFI-AM, was used to estimate Na(+) influx across plasma membrane. 2'(3')-O-(4-benzoylbenzoyl)ATP (BzATP, 3-300 µM), a prototypic P2X7 receptor agonist, concentration-dependently decreased [(3)H]GABA (14%) and [(14)C]glutamate (24%) uptake; BzATP decreased transport maximum velocity (Vmax) without affecting the Michaelis constant (Km) values. The selective P2X7 receptor antagonist, A-438079 (3 µM), prevented inhibition of [(3)H]GABA and [(14)C]glutamate uptake by BzATP (100 µM). The inhibitory effect of BzATP coincided with its ability to increase intracellular Na(+) and was mimicked by Na(+) ionophores, like gramicidin and monensin. Increases in intracellular Na(+) (with veratridine or ouabain) or substitution of extracellular Na(+) by N-methyl-D-glucamine (NMDG)(+) all decreased [(3)H]GABA and [(14)C]glutamate uptake and attenuated BzATP effects. Uptake inhibition by BzATP (100 µM) was also attenuated by calmidazolium, which selectively inhibits Na(+) currents through the P2X7 receptor pore. In conclusion, disruption of the Na(+) gradient by P2X7 receptor activation downmodulates high-affinity GABA and glutamate uptake into rat cortical synaptosomes. Interference with amino-acid transport efficacy may constitute a novel target for therapeutic management of cortical excitability.

Immobility responses between mouse strains correlate with distinct hippocampal serotonin transporter protein expression and function.

Mouse strain differences in immobility and in sensitivity to antidepressants have been observed in the forced swimming test (FST) and the tail suspension test (TST). However, the neurotransmitter systems and neural substrates that contribute to these differences remain unknown. To investigate the role of the hippocampal serotonin transporter (5-HTT), we measured baseline immobility and the immobility responses to fluoxetine (FLX) in the FST and the TST in male CD-1, C57BL/6, DBA and BALB/c mice. We observed strain differences in baseline immobility time, with CD-1 mice showing the longest and DBA mice showing the shortest. In contrast, DBA and BALB/c mice showed the highest sensitivity to FLX, whereas CD-1 and C57BL/6 mice showed the lowest sensitivity. Also we found strain differences in both the total 5-HTT protein level and the membrane-bound 5-HTT level (estimated by V max) as follows: DBA>BALB/c>CD-1=C57BL/6. The uptake efficiency of the membrane-bound 5-HTT (estimated by 1/K m) was highest in DBA and BALB/c mice and lowest in CD-1 and C57BL/6 mice. A correlation analysis of subregions within the hippocampus revealed that immobility time was negatively correlated with V max and positively correlated with K m in the hippocampus. Therefore a higher uptake capacity of the membrane-bound 5-HTT in the hippocampus was associated with lower baseline immobility and greater sensitivity to FLX. These results suggest that alterations in hippocampal 5-HTT activity may contribute to mouse strain differences in the FST and the TST.

Synaptic and sub-synaptic localization of amyloid-ß protein precursor in the rat hippocampus.

Amyloid-ß protein precursor (AßPP) is a large transmembrane protein highly expressed in the central nervous system and cleavage of it can produce amyloid-ß peptides (Aß) involved in synaptic dysfunction and loss associated with cognitive impairment in Alzheimer's disease (AD). Surprisingly, little is known about the synaptic and sub-synaptic distribution of AßPP in different types of nerve terminals. We used total, synaptic, sub-synaptic, and astrocytic membrane preparations obtained from the hippocampus of adult rats to define the localization of AßPP, using two different antibodies against different AßPP epitopes. Western blot analysis revealed that AßPP was not significantly enriched in synaptosomal as compared to total membranes. Within synapses, AßPP immunoreactivity was more abundant in pre- (60 ± 4%) than post- (30 ± 5%) or extra-synaptic fractions (10 ± 2%). Immunocytochemical analysis of purified nerve terminals indicated that AßPP was more frequently associated with glutamatergic (present in 31 ± 4% of glutamatergic terminals) rather than with GABAergic (16 ± 3%) or cholinergic terminals (4 ± 1%, n = 4). We also observed a general lack of co-localization of AßPP and GFAP immunoreactivities in the hippocampus of sections of adult rat brain, albeit we could detect the presence of AßPP in gliosomes (vesicular specializations of astrocytic membranes), suggesting that AßPP has a heterogeneous localization restricted to certain regions of astrocytes. These results provide the first direct demonstration that AßPP is mostly distributed among glutamatergic rather than GABAergic or cholinergic terminals of the adult rat hippocampus, in remarkable agreement with the particular susceptibility to dysfunction and degeneration of glutamatergic synapses in early AD.

Mechanisms underlying the impairment of hippocampal long-term potentiation and memory in experimental Parkinson's disease.

Although patients with Parkinson's disease show impairments in cognitive performance even at the early stage of the disease, the synaptic mechanisms underlying cognitive impairment in this pathology are unknown. Hippocampal long-term potentiation represents the major experimental model for the synaptic changes underlying learning and memory and is controlled by endogenous dopamine. We found that hippocampal long-term potentiation is altered in both a neurotoxic and transgenic model of Parkinson's disease and this plastic alteration is associated with an impaired dopaminergic transmission and a decrease of NR2A/NR2B subunit ratio in synaptic N-methyl-d-aspartic acid receptors. Deficits in hippocampal-dependent learning were also found in hemiparkinsonian and mutant animals. Interestingly, the dopamine precursor l-DOPA was able to restore hippocampal synaptic potentiation via D1/D5 receptors and to ameliorate the cognitive deficit in parkinsonian animals suggesting that dopamine-dependent impairment of hippocampal long-term potentiation may contribute to cognitive deficits in patients with Parkinson's disease.

A novel radioligand for glycine transporter 1: characterization and use in autoradiographic and in vivo brain occupancy studies.

INTRODUCTION: In an effort to develop agents to test the NMDA hypofunction hypothesis of schizophrenia, benchmark compounds from a program to discover potent, selective, competitive glycine transporter 1 (GlyT1) inhibitors were radiolabeled in order to further study the detailed pharmacology of these inhibitors and the distribution of GlyT1 in brain. We here report the in vitro characterization of [35S](S)-2-amino-4-chloro-N-(1-(4-phenyl-1-(propylsulfonyl)piperidin-4-yl)ethyl)benzamide ([35S]ACPPB), a radiotracer developed from a potent and selective non-sarcosine-derived GlyT1 inhibitor, its use in autoradiographic studies to localize (S)-2-amino-6-chloro-N-(1-(4-phenyl-1-(propylsulfonyl)piperidin-4-yl)ethyl)benzamide (ACPPB) binding sites in rat and rhesus brain and for in vivo occupancy assays of competitive GlyT1 inhibitors. METHODS: Functional potencies of unlabeled compounds were characterized by [14C]glycine uptake into JAR (human placental choriocarcinoma) cells and synaptosomes. Radioligand binding studies were performed with tissue homogenates. Autoradiographic studies were performed on tissue slices. RESULTS: ACPPB is a potent (Kd=1.9 nM), selective, GlyT1 inhibitor that, when radiolabeled with [35S], is a well-behaved radioligand with low nondisplaceable binding. Autoradiographic studies of rat and rhesus brain slices with this ligand showed that specific binding sites were plentiful and nonhomogeneously distributed, with high levels of binding in the brainstem, cerebellar white matter, thalamus, cortical white matter and spinal cord gray matter. In vivo studies demonstrate displaceable binding of [35S]ACPPB in rat brain tissues following iv administration of this radioligand. CONCLUSIONS: This is the first report of detailed anatomical localization of GlyT1 using direct radioligand binding, and the first demonstration that an in vivo occupancy assay is feasible, suggesting that it may also be feasible to develop positron emission tomography tracers for GlyT1.