Chang-Joong Lee

Amygdala depotentiation and fear extinction

Auditory fear memory is thought to be maintained by fear conditioning-induced potentiation of synaptic efficacy, which involves enhanced expression of surface AMPA receptor (AMPAR) at excitatory synapses in the lateral amygdala (LA). Depotentiation, reversal of conditioning-induced potentiation, has been proposed as a cellular mechanism for fear extinction; however, a direct link between depotentiation and extinction has not yet been tested. To address this issue, we applied both ex vivo and in vivo approaches to rats in which fear memory had been consolidated. A unique form of depotentiation reversed conditioning-induced potentiation at thalamic input synapses onto the LA (T-LA synapses) ex vivo. Extinction returned the enhanced T-LA synaptic efficacy observed in conditioned rats to baseline and occluded the depotentiation. Consistently, extinction reversed conditioning-induced enhancement of surface expression of AMPAR subunits in LA synaptosomal preparations. A GluR2-derived peptide that blocks regulated AMPAR endocytosis inhibited depotentiation, and microinjection of a cell-permeable form of the peptide into the LA attenuated extinction. Our results are consistent with the use of depotentiation to weaken potentiated synaptic inputs onto the LA during extinction and provide strong evidence that AMPAR removal at excitatory synapses in the LA underlies extinction.

Maternal Stress Produces Learning Deficits Associated with Impairment of NMDA Receptor-Mediated Synaptic Plasticity

Stress in adulthood can have a profound effect on physiology and behavior, but the extent to which prolonged maternal stress affects the brain function of offspring when they are adult remains primarily unknown. In the present work, chronic immobilization stress to pregnant mice affected fetal growth and development. When pups born from stressed mice were reared to adulthood in an environment identical to that of nonstressed controls, several physiological parameters were essentially unaltered. However, spatial learning and memory was significantly impaired in the maternally stressed offspring in adulthood. Furthermore, electrophysiological examination revealed a significant reduction in NMDA receptor-mediated long-term potentiation in the CA1 area of hippocampal slices. Subsequent biochemical analysis demonstrated a substantial decrease in NR1 and NR2B subunits of the NMDA receptor in synapses of the hippocampus, and the interaction between these two subunits appeared to be reduced. These results suggest that prolonged maternal stress leads to long-lasting malfunction of the hippocampus, which extends to and is manifested in adulthood.

Induction mechanisms for L-LTP at thalamic input synapses to the lateral amygdala: requirement of mGluR5 activation.

&NA; L‐LTP (late‐phase long‐term potentiation) at thalamo‐amygdala synapses is thought to be critical for auditory fear conditioning, but it has not been clear what kinds of surface receptors and channels are involved in the induction phase of the L‐LTP. Here we report that the NMDA receptor antagonist D‐AP5 (50 &mgr;M), the L‐type calcium channel antagonist nifedipine (30 &mgr;M) and the metabotropic glutamate receptor 5 antagonist MPEP (10 &mgr;M) prevented L‐LTP induction when each antagonist was separately applied at saturating concentrations before and during repeated tetanus. By contrast, the mGluR1 antagonist CPCCOEt (80 &mgr;M) failed to show any effects on L‐LTP induction. Neither D‐AP5 nor MPEP produced any significant effects on potentiated synaptic responses when applied after L‐LTP had been established. Thus, our data suggest that NMDA receptors, L‐type calcium channels and mGluR5 are involved in L‐LTP induction in the thalamo‐amygdala pathway.

Cholinergic modulation of neural activity in the telencephalon of the zebrafish

In this study, we have assessed the effects of muscarinic receptor activation on neural activity in the telencephalon of the zebrafish. Electrically evoked field potentials via the stimulation of the posterior dorsal telencephalon were recorded in the anterior dorsal telencephalon of the ipsilateral hemisphere, with a peak latency of 20-30ms. The application of carbachol (1-25microM) and oxotremorine-M (0.25-60microM) suppressed the amplitude of field potentials in a dose-dependent manner. This suppressive effect was blocked by a muscarinic receptor antagonist, atropine (20microM), and an M1 receptor antagonist, pirenzepine (5microM), but not by the M2 receptor antagonists, methoctramine (5microM) and gallamine (5microM). Oxotremorine-M (1microM) also suppressed the induction of field potentials via the stimulation of the contralateral telencephalon and the medial fiber bundle, which was blocked by atropine (20microM). An acetylcholine esterase inhibitor, physostigmine (20-30microM), reduced the peak amplitude of the evoked field potentials, while increasing late bursting activity. Furthermore, a high dose of physostigmine (100microM) induced spontaneous bursting activity. These results demonstrated the cholinergic influence on the neural activity in the telencephalon of the zebrafish.

Lipopolysaccharide inhibits induction of long-term potentiation and depression in the rat hippocampal CA1 area

We examined the effects of lipopolysaccharide, a bacterial endotoxin, on synaptic plasticity in the rat hippocampal CA1 area in vitro. Lipopolysaccharide suppressed the induction of long-term potentiation elicited by tetanic stimulation and long-term depression, elicited by low-frequency stimulation of Schaffer collateral-commissural fibres at 10 and 50 microg/ml, respectively. Lipid A (1 microg/ml), the biologically active component of lipopolysaccharide, mimicked the effects of 10 microg/ml lipopolysaccharide on long-term potentiation and depression. Nifedipine, an L-type voltage-sensitive Ca(2+) channel antagonist, did not influence the induction of long-term potentiation and depression, whereas a high concentration of extracellular calcium enabled long-term potentiation induction in the presence of 10 microg/ml lipopolysaccharide. The NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (APV, 50 microM), nifedipine (10 microM) or lipopolysaccharide (10 or 50 microg/ml) partially reduced the magnitude of tetraethylammonium-induced long-term potentiation. Nifedipine combined with lipopolysaccharide completely blocked tetraethylammonium-induced long-term potentiation. Whole-cell voltage clamp recordings showed that lipopolysaccharide suppressed NMDA receptor-mediated excitatory postsynaptic currents (EPSCs). Our results indicate that lipopolysaccharide acutely modifies synaptic plasticity by blocking Ca(2+) entry through NMDA receptors, suggesting a possible mechanism for the amnesic action of bacterial infection.

Increased cell proliferation and neural activity by physostigmine in the telencephalon of adult zebrafish

Physostigmine, an acetylcholinesterase inhibitor, is known to affect the brain function in various aspects. This study was conducted to test whether physostigmine affects cell proliferation in the telencephalon of zebrafish. BrdU-labeled cells was prominently observed in the ventral zone of the ventral telencephalon of zebrafish. The increased number of BrdU- and proliferating cell nuclear antigen-labeled cells were shown in zebrafish treated with 200μM physostigmine, which was inhibited by pretreatment with 200μM scopolamine. iNOS mRNA expression was increased in the brain of zebrafish treated with 200μM physostigmine. Consistently, aminoguanidine, an iNOS inhibitor, attenuated the increase in the number of BrdU-labeled cells by physostigmine treatment. Zebrafish also showed seizure-like locomotor activity characterized by a rapid and abrupt movement during a 30min treatment with 200μM physostigmine. Neural activity in response to an electrical stimulus was increased in the isolated telencephalon of zebrafish continuously perfused with 200μM physostigmine. None of the number of BrdU-labeled cells, neural activity, or locomotor activity was affected by treatment with 20μM physostigmine. These results suggest that 200μM physostigmine increased neural activity and induced cell proliferation via nitric oxide production in zebrafish.

Hypoxia-Induced Neuroinflammation and Learning–Memory Impairments in Adult Zebrafish Are Suppressed by Glucosamine

This study investigated changes in neuroinflammation and cognitive function in adult zebrafish exposed to acute hypoxia and protective effects of glucosamine (GlcN) against hypoxia-induced brain damage. The survival rate of zebrafish following exposure to hypoxia was improved by GlcN pretreatment. Moreover, hypoxia-induced upregulation of neuroglobin, NOS2α, glial fibrillary acidic protein, and S100β in zebrafish was suppressed by GlcN. Hypoxia stimulated cell proliferation in the telencephalic ventral domain and in cerebellum subregions. GlcN decreased the number of bromodeoxyuridine (BrdU)-positive cells in the telencephalon region, but not in cerebellum regions. Transient motor neuron defects, assessed by measuring the locomotor and exploratory activity of zebrafish exposed to hypoxia recovered quickly. GlcN did not affect hypoxia-induced motor activity changes. In passive avoidance tests, hypoxia impaired learning and memory ability, deficits that were rescued by GlcN. A learning stimulus increased the nuclear translocation of phosphorylated cAMP response element binding protein (p-CREB), an effect that was greatly inhibited by hypoxia. GlcN restored nuclear p-CREB after a learning trial in hypoxia-exposed zebrafish. The neurotransmitters, γ-aminobutyric acid and glutamate, were increased after hypoxia in the zebrafish brain, and GlcN further increased their levels. In contrast, acetylcholine levels were reduced by hypoxia and restored by GlcN. Acetylcholinesterase inhibitor physostigmine partially reversed the impaired learning and memory of hypoxic zebrafish. This study represents the first examination of the molecular mechanisms underlying hypoxia-induced memory and learning defects in a zebrafish model. Our results further suggest that GlcN-associated hexosamine metabolic pathway could be an important therapeutic target for hypoxic brain damage.

Ginsenoside Rg 1 Reduced Spontaneous Epileptiform Discharges and Behavioral Seizure in the Zebrafish

Epileptifrom discharges were induced in the telencephalon of the adult zebrafish via perfusion with pentylenetetrazole (PTZ), bicuculline methiodide, kainic acid-treated artificial cerebrospinal fluid (aCSF), and Mg 2+ -free aCSF. Ginseng total saponin [GTS (50 ㎍/ml)] was shown to attenuate the occurrence rate of epilpetiform discharges by 50-75%, compared to the control. Ginsenoside Rg₁ (130 ㎛) reduced the epileptiform discharges in the isolated telenephalon and delayed the occurrence of behavioral seizures observed from the adult zebrafish placed in the PTZ (10 mM)-containing aquarium water. However, Re was not effective in the suppression of epileptiform discharges and behavioral seizures. These results indicate that Rg1 may be useful in the control of epileptiform discharges and effective in controlling behavioral seizures, and that the zebrafish can be used as a model animal for the testing of potential anticonvulsant drugs.