Byung-Ju Choi

Signaling pathways of bisphenol A–induced apoptosis in hippocampal neuronal cells: Role of calcium-induced reactive oxygen species, mitogen-activated protein kinases, and nuclear factor–κB

In the present study, we investigated the neurotoxicity of bisphenol A [BPA; 2,2‐bis‐(4 hydroxyphenyl) propane] and the underlying mechanisms of action in mouse hippocampal HT‐22 cells. BPA, known to be a xenoestrogen, is used in the production of water bottles, cans, and teeth suture materials. BPA‐treated HT‐22 cells showed lower cell viability than did controls at concentrations of BPA over 100 μM. BPA induced apoptotic cell death as indicated by staining with Hoechst 33258, costaining with Annexin V/propidium iodide, and activation of caspase 3. BPA regulated the generation of reactive oxygen species (ROS) by increasing intracellular calcium. BPA activated phosphorylation of extracellular signal–regulated kinase (ERK) and c‐Jun N‐terminal kinase (JNK), and nuclear translocation of nuclear factor (NF)‐κB. Pretreatment with specific inhibitors for calcium, ROS, ERK, and JNK decreased BPA‐induced cell death; however, inhibitor for NF‐κB increased BPA‐induced cell death. The results suggest that calcium, ROS, ERK, and JNK are involved in BPA‐induced apoptotic cell death in HT‐22 cells. In contrast, an NF‐κB cascade was activated for survival signaling after BPA treatment.

Modified iterative Landweber method in electrical capacitance tomography

Electrical capacitance tomography (ECT) is a non-invasive imaging technique that aims at visualization of the cross-sectional permittivity distribution of a dielectric object based on the measured capacitance. In this paper, we propose a modified iterative Landweber method to accelerate the convergence rate and enhance the quality of the reconstructed image. In doing so, an additional acceleration term is added to the conventional iterative Landweber method and the optimal step length is determined analytically.

The Na+/H+ exchanger is a major pH regulator in GABAergic presynaptic nerve terminals synapsing onto rat CA3 pyramidal neurons

The effects of pHi on GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were studied in mechanically dissociated CA3 pyramidal neurons, by use of ammonium prepulse and whole‐cell patch‐clamp techniques, under the voltage‐clamp condition. NH4Cl itself, which is expected to alkalinize pHi, increased GABAergic mIPSC frequency in a concentration‐dependent manner. In contrast, NH4Cl decreased mIPSC frequency, either in the presence of 200 µm Cd2+ or in Ca2+‐free external solution, suggesting that intraterminal alkalosis decreased GABAergic mIPSC frequency while [NH4+] itself may activate Ca2+ channels by depolarizing the terminal. On the other hand, GABAergic mIPSC frequency was greatly increased immediately after NH4Cl removal, a condition expected to acidify pHi, and recovered to the control level within 2 min after NH4Cl removal. This explosive increase in mIPSC frequency observed after NH4Cl removal was completely eliminated after depletion of Ca2+ stores with 1 µm thapsigargin in the Ca2+‐free external solution, suggesting that acidification increases in intraterminal Ca2+ concentration via both extracellular Ca2+ influx and Ca2+ release from the stores. However, the acidification‐induced increase in mIPSC frequency had not recovered by 10 min after NH4Cl removal either in the Na+‐free external solution or in the presence of 10 µm 5‐(N‐ethyl‐N‐isopropyl)‐amiloride (EIPA), a specific Na+/H+ exchanger (NHE) blocker. The present results suggest that NHEs are major intraterminal pH regulators on GABAergic presynaptic nerve terminals, and that the NHE‐mediated regulation of pHi under normal physiological or pathological conditions might play an important role in the neuronal excitability by increasing inhibitory tones.

Alpha neurofeedback improves the maintaining ability of alpha activity

The effects of &agr;-neurofeedback (ANF) on electroencephalographic &agr;-activity were investigated. Each session consisted of a 2.5-min eye-opened state and 17.5-min of ANF, which was divided into 16 1.25-min bins. Alpha amplitudes were gradually increased as the session was repeated. The maximum value at the start of ANF gradually decreased as time passed, but the slowdown of &agr;-activity during each session was decreased as the session was repeated. The correlation between &agr;-activity at the end of ANF and at the following sessions eye-opened state was highly significant. These results showed that ANF enhances the ability of &agr;-activity to maintain itself rather than the increase of &agr;-amplitude during intrasession and that the maintained &agr;-activity during former training remained until the next session.

Presynaptic glycine receptors facilitate spontaneous glutamate release onto hilar neurons in the rat hippocampus

Although glycine receptors are found in most areas of the brain, including the hippocampus, their functional significance remains largely unknown. In the present study, we have investigated the role of presynaptic glycine receptors on excitatory nerve terminals in spontaneous glutamatergic transmission. Spontaneous EPSCs (sEPSCs) were recorded in mechanically dissociated rat dentate hilar neurons attached with native presynaptic nerve terminals using a conventional whole‐cell patch recording technique under voltage‐clamp conditions. Exogenously applied glycine or taurine significantly increased the frequency of sEPSCs in a concentration‐dependent manner. This facilitatory effect of glycine was blocked by 1 μM strychnine, a specific glycine receptor antagonist, but was not affected by 30 μM picrotoxin. In addition, Zn2+ (10 μM) potentiated the glycine action on sEPSC frequency. Pharmacological data suggested that the activation of presynaptic glycine receptors directly depolarizes glutamatergic terminals resulting in the facilitation of spontaneous glutamate release. Bumetanide (10 μM), a specific Na‐K‐2C co‐transporter blocker, gradually attenuated the glycine‐induced sEPSC facilitation, suggesting that the depolarizing action of presynaptic glycine receptors was due to a higher intraterminal Cl− concentration. The present results suggest that presynaptic glycine receptors on excitatory nerve terminals might play an important role in the excitability of the dentate gyrus‐hilus‐CA3 network in physiological and/or pathological conditions.

Adenosine A1 receptors inhibit GABAergic transmission in rat tuberomammillary nucleus neurons

The adenosinergic modulation of GABAergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) was investigated in mechanically dissociated rat tuberomammillary nucleus (TMN) neurons using a conventional whole‐cell patch clamp technique. Adenosine (100 μM) reversibly decreased mIPSC frequency without affecting the current amplitude, indicating that adenosine acts presynaptically to decrease the probability of spontaneous GABA release. The adenosine action on GABAergic mIPSC frequency was completely blocked by 1 μM DPCPX, a selective A1 receptor antagonist, and mimicked by 1 μM CPA, a selective A1 receptor agonist. This suggests that presynaptic A1 receptors were responsible for the adenosine‐mediated inhibition of GABAergic mIPSC frequency. CPA still decreased GABAergic mIPSC frequency even either in the presence of 200 μM Cd2+, a general voltage‐dependent Ca2+ channel blocker, or in the Ca2+‐free external solution. However, the inhibitory effect of CPA on GABAergic mIPSC frequency was completely occluded by 1 mM Ba2+, a G‐protein coupled inwardly rectifying K+ (GIRK) channel blocker. In addition, the CPA‐induced decrease in mIPSC frequency was completely occluded by either 100 μM SQ22536, an adenylyl cyclase (AC) inhibitor, or 1 μM KT5720, a specific protein kinase A (PKA) inhibitor. The results suggest that the activation of presynaptic A1 receptors decreases spontaneous GABAergic transmission onto TMN neurons via the modulation of GIRK channels as well as the AC/cAMP/PKA signal transduction pathway. This adenosine A1 receptor‐mediated modulation of GABAergic transmission onto TMN neurons may play an important role in the fine modulation of the excitability of TMN histaminergic neurons as well as the regulation of sleep‐wakefulness.

Multiple effects of bisphenol A, an endocrine disrupter, on GABAA receptors in acutely dissociated rat CA3 pyramidal neurons

Bisphenol A (BPA), an endocrine disrupter, is contained in cans, polycarbonate bottles and some dental sealants. While the toxicological effects of BPA on the endocrine system have been extensively studied, its action on the central nervous system is poorly understood. Herein, we report the effects of BPA on GABA-induced currents (I(GABA)), using a conventional whole-cell patch clamp technique from acutely isolated rat CA3 pyramidal neurons. By itself, BPA concentration-dependently elicited the membrane current, which was significantly blocked by bicuculline, a selective GABA(A) receptor antagonist. BPA potentiated the peak I(GABA) induced by lower concentrations of GABA (<10 microM) in a concentration-dependent manner. The extent of BPA-induced potentiation of I(GABA) was significantly reduced by either diazepam or ethanol, allosteric modulators of GABA(A) receptors. BPA, however, inhibited the peak I(GABA) induced by higher concentrations of GABA (>30 microM), and accelerated the desensitization rate of I(GABA). BPA also greatly inhibited the steady state I(GABA) induced by higher concentrations of GABA (>30 microM) in a noncompetitive manner. In addition, BPA affected synaptic GABA(A) receptors as it decreased the amplitude of GABAergic miniature inhibitory postsynaptic currents in a concentration-dependent manner. Considering its complex modulatory effects on GABA(A) receptors, BPA might have potential toxicological effects on the central nervous system.

GABAB receptor-mediated presynaptic inhibition of glycinergic transmission onto substantia gelatinosa neurons in the rat spinal cord

&NA; The GABAB receptor‐mediated presynaptic inhibition of glycinergic transmission was studied from young rat substantia gelatinosa (SG) neurons using a conventional whole‐cell patch clamp technique. Action potential‐dependent glycinergic inhibitory postsynaptic currents (IPSCs) were recorded from SG neurons in the presence of 3 mM kynurenic acid and 10 μM SR95531. In these conditions, baclofen (30 μM), a selective GABAB receptor agonist, greatly reduced the amplitude of glycinergic IPSCs and increased the paired‐pulse ratio. Such effects were completely blocked by 3 μM CGP55845, a selective GABAB receptor antagonist, indicating that the activation of presynaptic GABAB receptors decreases glycinergic synaptic transmission. Glycinergic IPSCs were largely dependent on Ca2+ influxes passing through presynaptic N‐ and P/Q‐type Ca2+ channels, and these channels contributed equally to the baclofen‐induced inhibition of glycinergic IPSCs. However, the baclofen‐induced inhibition of glycinergic IPSCs was not affected by either 100 μM SQ22536, an adenylyl cyclase inhibitor, or 1 mM Ba2+, a G‐protein coupled inwardly rectifying K+ channel blocker. During the train stimulation (10 pulses at 20 Hz), which caused a marked synaptic depression of glycinergic IPSCs, baclofen at a 30 μM concentration completely blocked glycinergic synaptic depression, but at a 3 μM concentration it largely preserved glycinergic synaptic depression. Such GABAB receptor‐mediated dynamic changes in short‐term synaptic plasticity of glycinergic transmission onto SG neurons might contribute to the central processing of sensory signals.

Serotoninergic modulation of GABAergic synaptic transmission in developing rat CA3 pyramidal neurons

Serotoninergic modulation of GABAergic mIPSCs was investigated in immature (postnatal 12–16‐days old) rat CA3 pyramidal neurons using a conventional whole‐cell patch clamp technique. Serotonin or 5‐hydroxytryptamine (5‐HT) (10 μmol/L) transiently and explosively increased mIPSC frequency with a small increase in the current amplitude. However, 5‐HT did not affect the GABA‐induced postsynaptic currents, indicating that 5‐HT acts presynaptically to facilitate the probability of spontaneous GABA release. The 5‐HT action on GABAergic mIPSC frequency was completely blocked by 100 nmol/L MDL72222, a selective 5‐HT3 receptor antagonist, and mimicked by mCPBG, a selective 5‐HT3 receptor agonist. The 5‐HT action on GABAergic mIPSC frequency was completely occluded either in the presence of 200 μmol/L Cd2+ or in the Na+‐free external solution, suggesting that the 5‐HT3 receptor‐mediated facilitation of mIPSC frequency requires a Ca2+influx passing through voltage‐dependent Ca2+channels from the extracellular space, and that presynaptic 5‐HT3 receptors are less permeable to Ca2+. The 5‐HT action on mIPSC frequency in the absence or presence of extracellular Na+ gradually increased with postnatal development. Such a developmental change in the 5‐HT3 receptor‐mediated facilitation of GABAergic transmission would play important roles in the regulation of excitability as well as development in CA3 pyramidal neurons.

Differential pharmacological properties of GABAA receptors in axon terminals and soma of dentate gyrus granule cells

Although it has been well established that GABAA receptors are molecular targets of a variety of allosteric modulators, such as benzodiazepines, the pharmacological properties of presynaptic GABAA receptors are poorly understood. In this study, the effects of diazepam and Zn2+ on presynaptic GABAA receptors have been investigated by measuring the GABAA receptor‐mediated facilitation of spontaneous glutamate release in mechanically dissociated rat CA3 pyramidal neurons. Diazepam significantly enhanced the muscimol‐induced facilitation (particularly at submicromolar concentrations) of spontaneous glutamate release and shifted the concentration–response relationship for muscimol toward the left, whereas Zn2+ (≤ 100 μM) had little effect on the muscimol‐induced facilitation of spontaneous glutamate release. In contrast, Zn2+ significantly suppressed the muscimol‐induced currents mediated by GABAA receptors expressed on dentate gyrus granule cells, which are parent neurons of mossy fibers, whereas the effect of diazepam on GABAA receptors expressed on dentate gyrus granule cells was lesser than that on presynaptic GABAA receptors. The results suggest that the pharmacological properties of GABAA receptors differ considerably between presynaptic (axon terminals) and somatic regions in the same granule cell and that presynaptic GABAA receptors should be considered as one of the important pharmacological targets of many drugs affecting GABAA receptors.