In vivo regulation of an Aplysia glutamate transporter, ApGT1, during long-term memory formation

Neuronal Loss of the Glutamate Transporter GLT-1 Promotes Excitotoxic Injury in the Hippocampus

GLT-1, the major glutamate transporter in the mammalian central nervous system, is expressed in presynaptic terminals that use glutamate as a neurotransmitter, in addition to astrocytes. It is widely assumed that glutamate homeostasis is regulated primarily by glutamate transporters expressed in astrocytes, leaving the function of GLT-1 in neurons relatively unexplored. We generated conditional GLT-1 knockout (KO) mouse lines to understand the cell-specific functions of GLT-1. We found that stimulus-evoked field extracellular postsynaptic potentials (fEPSPs) recorded in the CA1 region of the hippocampus were normal in the astrocytic GLT-1 KO but were reduced and often absent in the neuronal GLT-1 KO at 40 weeks. The failure of fEPSP generation in the neuronal GLT-1 KO was also observed in slices from 20 weeks old mice but not consistently from 10 weeks old mice. Using an extracellular FRET-based glutamate sensor, we found no difference in stimulus-evoked glutamate accumulation in the neuronal GLT-1 KO, suggesting a postsynaptic cause of the transmission failure. We hypothesized that excitotoxicity underlies the failure of functional recovery of slices from the neuronal GLT-1 KO. Consistent with this hypothesis, the non-competitive NMDA receptor antagonist MK801, when present in the ACSF during the recovery period following cutting of slices, promoted full restoration of fEPSP generation. The inclusion of an enzymatic glutamate scavenging system in the ACSF conferred partial protection. Excitotoxicity might be due to excess release or accumulation of excitatory amino acids, or to metabolic perturbation resulting in increased vulnerability to NMDA receptor activation. Previous studies have demonstrated a defect in the utilization of glutamate by synaptic mitochondria and aspartate production in the synGLT-1 KO in vivo, and we found evidence for similar metabolic perturbations in the slice preparation. In addition, mitochondrial cristae density was higher in synaptic mitochondria in the CA1 region in 20–25 weeks old synGLT-1 KO mice in the CA1 region, suggesting compensation for loss of axon terminal GLT-1 by increased mitochondrial efficiency. These data suggest that GLT-1 expressed in presynaptic terminals serves an important role in the regulation of vulnerability to excitotoxicity, and this regulation may be related to the metabolic role of GLT-1 expressed in glutamatergic axon terminals.

Functional Characterization of a Vesicular Glutamate Transporter in an Interneuron That Makes Excitatory and Inhibitory Synaptic Connections in a Molluscan Neural Circuit

Understanding circuit function requires the characterization of component neurons and their neurotransmitters. Previous work on radula protraction in the Aplysia feeding circuit demonstrated that critical neurons initiate feeding via cholinergic excitation. In contrast, it is less clear how retraction is mediated at the interneuronal level. In particular, glutamate involvement was suggested, but was not directly confirmed. Here we study a suspected glutamatergic retraction interneuron, B64. We used the representational difference analysis (RDA) method to successfully clone an Aplysia vesicular glutamate transporter (ApVGLUT) from B64 and from a glutamatergic motor neuron B38. Previously, RDA was used to characterize novel neuropeptides. Here we demonstrate its utility for characterizing other types of molecules. Bioinformatics suggests that ApVGLUT is more closely related to mammalian VGLUTs than to Drosophila and Caenorhabditis elegans VGLUTs. We expressed ApVGLUT in a cell line, and demonstrated that it indeed transports glutamate in an ATP and proton gradient-dependent manner. We mapped the ApVGLUT distribution in the CNS using in situ hybridization and immunocytochemistry. Further, we demonstrated that B64 is ApVGLUT positive, supporting the idea that it is glutamatergic. Although glutamate is primarily an excitatory transmitter in the mammalian CNS, B64 elicits inhibitory PSPs in protraction neurons to terminate protraction and excitatory PSPs in retraction neurons to maintain retraction. Pharmacological data indicated that both types of PSPs are mediated by glutamate. Thus, glutamate mediates the dual function of B64 in Aplysia. More generally, our systematic approaches based on RDA may facilitate analyses of transmitter actions in small circuits with identifiable neurons.

Activity-Dependent Plasticity of Astroglial Potassium and Glutamate Clearance

Recent evidence has shown that astrocytes play essential roles in synaptic transmission and plasticity. Nevertheless, how neuronal activity alters astroglial functional properties and whether such properties also display specific forms of plasticity still remain elusive. Here, we review research findings supporting this aspect of astrocytes, focusing on their roles in the clearance of extracellular potassium and glutamate, two neuroactive substances promptly released during excitatory synaptic transmission. Their subsequent removal, which is primarily carried out by glial potassium channels and glutamate transporters, is essential for proper functioning of the brain. Similar to neurons, different forms of short- and long-term plasticity in astroglial uptake have been reported. In addition, we also present novel findings showing robust potentiation of astrocytic inward currents in response to repetitive stimulations at mild frequencies, as low as 0.75 Hz, in acute hippocampal slices. Interestingly, neurotransmission was hardly affected at this frequency range, suggesting that astrocytes may be more sensitive to low frequency stimulation and may exhibit stronger plasticity than neurons to prevent hyperexcitability. Taken together, these important findings strongly indicate that astrocytes display both short- and long-term plasticity in their clearance of excess neuroactive substances from the extracellular space, thereby regulating neuronal activity and brain homeostasis.

Transcriptional changes following long-term sensitization training and in vivo serotonin exposure in Aplysia californica

We used Aplysia californica to compare the transcriptional changes evoked by long-term sensitization training and by a treatment meant to mimic this training, in vivo exposure to serotonin. We focused on 5 candidate plasticity genes which are rapidly up-regulated in the Aplysia genus by in vivo serotonin treatment, but which have not yet been tested for regulation during sensitization: CREB1, matrilin, antistasin, eIF3e, and BAT1 homolog. CREB1 was rapidly up-regulated by both treatments, but the regulation following training was transient, falling back to control levels 24 hours after training. This suggests some caution in interpreting the proposed role of CREB1 in consolidating long-term sensitization memory. Both matrilin and eIF3e were up-regulated by in vivo serotonin but not by long-term sensitization training. This suggests that in vivo serotonin may produce generalized transcriptional effects that are not specific to long-term sensitization learning. Finally, neither treatment produced regulation of antistasin or BAT1 homolog, transcripts regulated by in vivo serotonin in the closely related Aplysia kurodai. This suggests either that these transcripts are not regulated by experience, or that transcriptional mechanisms of memory may vary within the Aplysia genus.

Pharmacological evidence that D-aspartate activates a current distinct from ionotropic glutamate receptor currents in Aplysia californica

D-Aspartate (D-Asp) activates a nonspecific cation current of unknown identity independent of L-glutamate (L-Glu) in neurons of Aplysia californica. Whole-cell voltage clamp studies were conducted using primary cultures of Aplysia buccal S cluster (BSC) neurons to characterize these receptor channels pharmacologically. The N-methyl-D-aspartate receptor (NMDAR) coagonist glycine potentiated D-Asp currents only at -30 mV, while D-serine did not potentiate D-Asp currents at any amplitude. Portions of D-Asp currents were blocked by the L-Glu antagonists kynurenate, DL-2-amino-5-phosphonopentanoic acid (APV), (2S,3R)-1-(phenanthren-2-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA), and 1,3-dihydro-5-[3-[4-(phenylmethyl)-1-2H-benzimidazol-2-one (TCS46b), suggesting that L-Glu channels, particularly NMDAR-like channels, may partially contribute to D-Asp whole-cell currents. In contrast, L-Glu currents were unaffected by APV, and showed greater block by kynurenate, suggesting that D-Asp and L-Glu act, in part, at different sites. The excitatory amino acid transport blocker DL-threo-b-Benzyloxyaspartic acid (TBOA) blocked a fraction of D-Asp currents, suggesting that currents associated with these transporters also contribute. Non-NMDA L-GluR antagonists that preferentially block alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate receptors significantly increased D-Asp currents, suggesting a possible allosteric potentiating effect of these antagonists on D-Asp receptors. L-Glu-induced currents were significantly reduced in the presence of bath-applied D-Asp, whereas bath-applied L-Glu had no effect on D-Asp-induced currents. The mixed effects of these agents on D-Asp-induced currents in Aplysia illustrate that the underlying channels are not uniformly characteristic of any known agonist associated channel type.

Circadian regulation of ion channels and their functions

Ion channels are the gatekeepers to neuronal excitability. Retinal neurons of vertebrates and invertebrates, neurons of the suprachiasmatic nucleus (SCN) of vertebrates, and pinealocytes of non-mammalian vertebrates display daily rhythms in their activities. The interlocking transcription-translation feedback loops with specific post-translational modulations within individual cells form the molecular clock, the basic mechanism that maintains the autonomic approximately 24-h rhythm. The molecular clock regulates downstream output signaling pathways that further modulate activities of various ion channels. Ultimately, it is the circadian regulation of ion channel properties that govern excitability and behavior output of these neurons. In this review, we focus on the recent development of research in circadian neurobiology mainly from 1980 forward. We will emphasize the circadian regulation of various ion channels, including cGMP-gated cation channels, various voltage-gated calcium and potassium channels, Na(+)/K(+)-ATPase, and a long-opening cation channel. The cellular mechanisms underlying the circadian regulation of these ion channels and their functions in various tissues and organisms will also be discussed. Despite the magnitude of chronobiological studies in recent years, the circadian regulation of ion channels still remains largely unexplored. Through more investigation and understanding of the circadian regulation of ion channels, the future development of therapeutic strategies for the treatment of sleep disorders, cardiovascular diseases, and other illnesses linked to circadian misalignment will benefit.

Post-translational regulation of an Aplysia glutamate transporter during long-term facilitation

Regulation of glutamate transporters accompanies plasticity of some glutamatergic synapses. The regulation of glutamate uptake at the Aplysia sensorimotor synapse during long-term facilitation (LTF) was investigated. Previously, increases in levels of ApGT1 (Aplysia glutamate transporter 1) in synaptic membranes were found to be related to long-term increases in glutamate uptake. In this study, we found that regulation of ApGT1 during LTF appears to occur post-translationally. Serotonin (5-HT) a transmitter that induces LTF did not increase synthesis of ApGT1. A pool of ApGT1 appears to exist in sensory neuron somata, which is transported to the terminals by axonal transport. Blocking the rough endoplasmic reticulum-Golgi-trans-Golgi network (TGN) pathway with Brefeldin A prevented the 5-HT-induced increase of ApGT1 in terminals. Also, 5-HT produced changes in post-translational modifications of ApGT1 as well as changes in the levels of an ApGT1-co-precipitating protein. These results suggest that regulation of trafficking of ApGT1 from the vesicular trafficking system (rough endoplasmic reticulum-Golgi-TGN) in the sensory neuron somata to the terminals by post-translational modifications and protein interactions appears to be the mechanism underlying the increase in ApGT1, and thus, glutamate uptake during memory formation.

Astroglial and cognitive effects of chronic cerebral hypoperfusion in the rat

The permanent occlusion of common carotid arteries (2VO) causes a significant reduction of cerebral blood flow (hypoperfusion) in rats and constitutes a well established experimental model to investigate neuronal damage and cognitive impairment that occurs in human ageing and Alzheimer's disease. In the present study, we evaluated two astroglial proteins--S100B and glial fibrillary acidic protein (GFAP)--in cerebral cortex and hippocampus tissue, glutamate uptake and glutamine synthetase activity in hippocampus tissue, as well as S100B in cerebrospinal fluid. Cognition, as assessed by reference and working spatial memory protocols, was also investigated. Adult male Wistar rats were submitted to 10 weeks of chronic cerebral hypoperfusion by the 2VO method. A significant increase of S100B and GFAP in hippocampus tissue was observed, as well a significant decrease in glutamate uptake. Interestingly, we observed a decrease in S100B in cerebrospinal fluid. As for the cognitive outcome, there was an impairment of both reference and working spatial memory in the water maze; positive correlation between cognitive impairment and glutamate uptake decrease was evidenced in hypoperfused rats. These data support the hypothesis that astrocytes play a crucial role in the mechanisms of experimental neurodegeneration and that hippocampal pathology arising after chronic hypoperfusion gives rise to memory deficits.