Techniques: applications of the nerve-bouton preparation in neuropharmacology

Protein Kinase A Is Responsible for the Presynaptic Inhibition of Glycinergic and Glutamatergic Transmissions by Xenon in Rat Spinal Cord and Hippocampal CA3 Neurons

The effects of a general anesthetic xenon (Xe) on spontaneous, miniature, electrically evoked synaptic transmissions were examined using the "synapse bouton preparation," with which we can clearly evaluate pure synaptic responses and accurately quantify pre- and postsynaptic transmissions. Glycinergic and glutamatergic transmissions were investigated in rat spinal sacral dorsal commissural nucleus and hippocampal CA3 neurons, respectively. Xe presynaptically inhibited spontaneous glycinergic transmission, the effect of which was resistant to tetrodotoxin, Cd2+, extracellular Ca2+, thapsigargin (a selective sarcoplasmic/endoplasmic reticulum Ca2+-ATPase inhibitor), SQ22536 (an adenylate cyclase inhibitor), 8-Br-cAMP (membrane-permeable cAMP analog), ZD7288 (an hyperpolarization-activated cyclic nucleotide-gated channel blocker), chelerythrine (a PKC inhibitor), and KN-93 (a CaMKII inhibitor) while being sensitive to PKA inhibitors (H-89, KT5720, and Rp-cAMPS). Moreover, Xe inhibited evoked glycinergic transmission, which was canceled by KT5720. Like glycinergic transmission, spontaneous and evoked glutamatergic transmissions were also inhibited by Xe in a KT5720-sensitive manner. Our results suggest that Xe decreases glycinergic and glutamatergic spontaneous and evoked transmissions at the presynaptic level in a PKA-dependent manner. These presynaptic responses are independent of Ca2+ dynamics. We conclude that PKA can be the main molecular target of Xe in the inhibitory effects on both inhibitory and excitatory neurotransmitter release. SIGNIFICANCE STATEMENT: Spontaneous and evoked glycinergic and glutamatergic transmissions were investigated using the whole-cell patch clamp technique in rat spinal sacral dorsal commissural nucleus and hippocampal CA3 neurons, respectively. Xenon (Xe) significantly inhibited glycinergic and glutamatergic transmission presynaptically. As a signaling mechanism, protein kinase A was responsible for the inhibitory effects of Xe on both glycine and glutamate release. These results may help understand how Xe modulates neurotransmitter release and exerts its excellent anesthetic properties.

Depression of Synaptic N-methyl-D-Aspartate Responses by Xenon and Nitrous Oxide

In "synapse bouton preparation" of rat hippocampal CA3 neurons, we examined how Xe and N₂O modulate N-methyl-D-aspartate (NMDA) receptor-mediated spontaneous and evoked excitatory post-synaptic currents (sEPSC_NMDA and eEPSC_NMDA). This preparation is a mechanically isolated single neuron attached with nerve endings (boutons) preserving normal physiologic function and promoting the exact evaluation of sEPSC_NMDA and eEPSC_NMDA responses without influence of extrasynaptic, glial, and other neuronal tonic currents. These sEPSCs and eEPSCs are elicited by spontaneous glutamate release from many homologous glutamatergic boutons and by focal paired-pulse electric stimulation of a single bouton, respectively. The s/eEPSC_AMPA/KA and s/eEPSC_NMDA were isolated pharmacologically by their specific antagonists. Thus, independent contributions of pre- and postsynaptic responses could also be quantified. All kinetic properties of s/eEPSC_AMPA/KA and s/eEPSC_NMDA were detected clearly. The s/eEPSC_NMDA showed smaller amplitude and slower rise and 1/e decay time constant (τ _Decay) than s/eEPSC_AMPA/KA Xe (70%) and N₂O (70%) significantly decreased the frequency and amplitude without altering the τ _Decay of sEPSC_NMDA They also decreased the amplitude but increased the Rf and PPR without altering the τ _Decay of the eEPSC_NMDA These data show clearly that "synapse bouton preparation" can be an accurate model for evaluating s/eEPSC_NMDA Such inhibitory effects of gas anesthetics are primarily due to presynaptic mechanisms. Present results may explain partially the powerful analgesic effects of Xe and N₂O. SIGNIFICANCE STATEMENT: We could record pharmacologically isolated NMDA receptor-mediated spontaneous and (action potential-evoked) excitatory postsynaptic currents (sEPSC_NMDA and eEPSC_NMDA) and clearly detect all kinetic parameters of sEPSC_NMDA and eEPSC_NMDA at synaptic levels by using "synapse bouton preparation" of rat hippocampal CA3 neurons. We found that Xe and N₂O clearly suppressed both sEPSC_NMDA and eEPSC_NMDA. Different from previous studies, present results suggest that Xe and N₂O predominantly inhibit the NMDA responses by presynaptic mechanisms.

Amitriptyline Decreases GABAergic Transmission in Basal Forebrain Neurons Using an Optogenetic Model of Aging

The antidepressant drug amitriptyline is used in the treatment of clinical depression and a variety of neurological conditions such as anxiety, neuropathic pain disorders and migraine. Antidepressants are associated with both therapeutic and untoward effects, and their use in the elderly has tripled since the mid-1990s. Because of this widespread use, we are interested in testing the acute effects of amitriptyline on synaptic transmission at therapeutic concentrations well below those that block voltage-gated calcium channels. We found that 3 μM amitriptyline reduced the frequency of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) and reduced quantal content in mice at ages of 7-10 mo. and 23-25 mo., suggesting a presynaptic mechanism of action that does not diminish with age. We employed a reduced synaptic preparation of the basal forebrain (BF) and a new optogenetic aging model utilizing a bacterial artificial chromosome (BAC) transgenic mouse line with stable expression of the channelrhodopsin-2 (ChR2) variant H134R specific for GABAergic neurons [VGAT-ChR2(H134R)-EYFP]. This model enables optogenetic light stimulation of specific GABAergic synaptic terminals across aging. Age-related impairment of circadian behavior was used to confirm predictable age-related changes associated with this model. Our results suggest that low concentrations of amitriptyline act presynaptically to reduce neurotransmitter release and that this action is maintained during aging.

Ethanol induces persistent potentiation of 5-HT3 receptor-stimulated GABA release at synapses on rat hippocampal CA1 neurons

Several studies have shown that ethanol (EtOH) can enhance the activity of GABAergic synapses via presynaptic mechanisms, including in hippocampal CA1 neurons. The serotonin type 3 receptor (5-HT3-R) has been implicated in the neural actions of ethanol (EtOH) and in modulation of GABA release from presynaptic terminals. In the present study, we investigated EtOH modulation of GABA release induced by 5-HT3-R activation using the mechanically isolated neuron/bouton preparation from the rat CA1 hippocampal subregion. EtOH application before and during exposure to the selective 5-HT3 receptor agonist, m-chlorophenylbiguanide (mCPBG) potentiated the mCPBG-induced increases in the peak frequency and charge transfer of spontaneous GABAergic inhibitory postsynaptic currents. Interestingly, the potentiation was maintained even after EtOH was removed from the preparation. A protein kinase A inhibitor reduced the magnitude of EtOH potentiation. Fluorescent Ca2+ imaging showed that Ca2+ transients in the presynaptic terminals increased during EtOH exposure. These findings indicate that EtOH produces long-lasting potentiation of 5-HT3-induced GABA release by modulating calcium levels, via a process involving cAMP-mediated signaling in presynaptic terminals.

Astrocytic pyruvate dehydrogenase kinase-2 is involved in hypothalamic inflammation in mouse models of diabetes

Hypothalamic inflammation plays an important role in disrupting feeding behavior and energy homeostasis as well as in the pathogenesis of obesity and diabetes. Here, we show that pyruvate dehydrogenase kinase (PDK)-2 plays a role in hypothalamic inflammation and its sequelae in mouse models of diabetes. Cell type-specific genetic ablation and pharmacological inhibition of PDK2 in hypothalamic astrocytes suggest that hypothalamic astrocytes are involved in the diabetic phenotype. We also show that the PDK2-lactic acid axis plays a regulatory role in the observed metabolic imbalance and hypothalamic inflammation in mouse primary astrocyte and organotypic cultures, through the AMPK signaling pathway and neuropeptidergic circuitry governing feeding behavior. Our findings reveal that PDK2 ablation or inhibition in mouse astrocytes attenuates diabetes-induced hypothalamic inflammation and subsequent alterations in feeding behavior.

Hypothalamic inflammation is involved in the pathogenesis of diabetes. The underlying mechanisms are unclear. Here, the authors show that astrocytic PDK2 ablation or inhibition attenuates hypothalamic inflammation in mouse models of diabetes.

Efficacy and pharmacokinetics evaluation of 4-(2-chloro-4-fluorobenzyl)-3-(2-thienyl)-1,2,4-oxadiazol-5(4H)-one (GM-90432) as an anti-seizure agent

Epilepsy is a common chronic neurological disease characterized by recurrent epileptic seizures. A seizure is an uncontrolled electrical activity in the brain that can cause different levels of behavior, emotion, and consciousness. One-third of patients fail to receive sufficient seizure control, even though more than fifty FDA-approved anti-seizure drugs (ASDs) are available. In this study, we attempted small molecule screening to identify potential therapeutic agents for the treatment of seizures using seizure-induced animal models. Through behavioral phenotype-based screening, 4-(2-chloro-4-fluorobenzyl)-3-(2-thienyl)-1,2,4-oxadiazol-5(4H)-one (GM-90432) was identified as a prototype. GM-90432 treatment effectively decreased seizure-like behaviors in zebrafish and mice with chemically induced seizures. These results were consistent with decreased neuronal activity through immunohistochemistry for pERK in zebrafish larvae. Additionally, electroencephalogram (EEG) analysis revealed that GM-90432 decreases seizure-specific EEG events in adult zebrafish. Moreover, we revealed the preferential binding of GM-90432 to voltage-gated Na⁺ channels using a whole-cell patch clamp technique. Through pharmacokinetic analysis, GM-90432 effectively penetrated the blood-brain barrier and was distributed into the brain. Taken together, we suggest that GM-90432 has the potential to be developed into a new ASD candidate.

Menthol facilitates excitatory and inhibitory synaptic transmission in rat medullary dorsal horn neurons

Menthol, which acts as an agonist for transient receptor potential melastatin 8 (TRPM8), has complex effects on nociceptive transmission, including pain relief and hyperalgesia. Here, we addressed the effects of menthol on spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs, respectively) in medullary dorsal horn neurons, using a whole-cell patch-clamp technique. Menthol significantly increased sEPSC frequency, in a concentration-dependent manner, without affecting current amplitudes. The menthol-induced increase in sEPSC frequency could be completely blocked by AMTB, a TRPM8 antagonist, but was not blocked by HC-030031, a transient receptor potential ankyrin 1 (TRPA1) antagonist. Menthol still increased sEPSC frequency in the presence of Cd²⁺, a general voltage-gated Ca²⁺ channel blocker, suggesting that voltage-gated Ca²⁺ channels are not involved in the menthol-induced increase in sEPSC frequency. However, menthol failed to increase sEPSC frequency in the absence of extracellular Ca²⁺, suggesting that TRPM8 on primary afferent terminals is Ca²⁺ permeable. On the other hand, menthol also increased sIPSC frequency, without affecting current amplitudes. The menthol-induced increase in sIPSC frequency could be completely blocked by either AMTB or CNQX, an AMPA/KA receptor antagonist, suggesting that the indirect increase in excitability of inhibitory interneurons may lead to the facilitation of spontaneous GABA and/or glycine release. The present results suggested that menthol exerts analgesic effects, via the enhancement of inhibitory synaptic transmission, through central feed-forward neural circuits within the medullary dorsal horn region.

Effects of nitrous oxide on glycinergic transmission in rat spinal neurons

We investigated the effects of nitrous oxide (N₂O) on glycinergic inhibitory whole-cell and synaptic responses using a "synapse bouton preparation," dissociated mechanically from rat spinal sacral dorsal commissural nucleus (SDCN) neurons. This technique can evaluate pure single- or multi-synaptic responses from native functional nerve endings and enable us to accurately quantify how N₂O influences pre- and postsynaptic transmission. We found that 70 % N₂O enhanced exogenous glycine-induced whole-cell currents (I_Gly) at glycine concentrations lower than 3 × 10^-5 M, but did not affect I_Gly at glycine concentrations higher than 10^-4 M. N₂O did not affect the amplitude and 1/e decay-time of both spontaneous and miniature glycinergic inhibitory postsynaptic currents recorded in the absence and presence of tetrodotoxin (sIPSCs and mIPSCs, respectively). The decrease in frequency induced by N₂O was observed in sIPSCs but not in mIPSCs, which was recorded in the presence of both tetrodotoxin and Cd²⁺, which block voltage-gated Na⁺ and Ca²⁺ channels, respectively. N₂O also decreased the amplitude and increased the failure rate and paired-pulse ratio of action potential-evoked glycinergic inhibitory postsynaptic currents. N₂O slightly decreased the Ba²⁺ currents mediated by voltage-gated Ca²⁺ channels in SDCN neurons. We found that N₂O suppresses glycinergic responses at synaptic levels with presynaptic effect having much more predominant role. The difference between glycinergic whole-cell and synaptic responses suggests that extrasynaptic responses seriously modulate whole-cell currents. Our results strongly suggest that these responses may thus in part explain analgesic effects of N₂O via marked glutamatergic inhibition by glycinergic responses in the spinal cord.

Extracellular pH modulation of excitatory synaptic transmission in hippocampal CA3 neurons

In this study, the effect of extracellular pH on glutamatergic synaptic transmission was examined in mechanically dissociated rat hippocampal CA3 pyramidal neurons using a whole-cell patch-clamp technique under voltage-clamp conditions. Native synaptic boutons were isolated without using any enzymes, using a so-called "synapse bouton preparation," and preserved for the electrical stimulation of single boutons. Both the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) were found to decrease and increase in response to modest acidic (~pH 6.5) and basic (~pH 8.5) solutions, respectively. These changes in sEPSC frequency were not affected by the addition of TTX but completely disappeared by successive addition of Cd²⁺. However, changes in sEPSC amplitude induced by acidic and basic extracellular solutions were not affected by the addition of neither TTX nor Cd²⁺. The glutamate-induced whole-cell currents were decreased and increased by acidic and basic solutions, respectively. Acidic pH also decreased the amplitude and increased the failure rate (R_f) and paired-pulse rate (PPR) of glutamatergic electrically evoked excitatory postsynaptic currents (eEPSCs), while a basic pH increased the amplitude and decreased both the R_f and PPR of eEPSCs. The kinetics of the currents were not affected by changes in pH. Acidic and basic solutions decreased and increased voltage-gated Ca²⁺ but not Na⁺ channel currents in the dentate gyrus granule cell bodies. Our results indicate that extracellular pH modulates excitatory transmission via both pre- and postsynaptic sites, with the presynaptic modulation correlated to changes in voltage-gated Ca²⁺ channel currents.NEW & NOTEWORTHY The effects of external pH changes on spontaneous, miniature, and evoked excitatory synaptic transmission in CA3 hippocampal synapses were examined using the isolated nerve bouton preparation, which allowed for the accurate regulation of extracellular pH at the synapses. Acidification generally reduced transmission, partly via effects on presynaptic Ca²⁺ channel currents, while alkalization generally enhanced transmission. Both pre- and postsynaptic sites contributed to these effects.

Xenon modulates the GABA and glutamate responses at genuine synaptic levels in rat spinal neurons

Effects of xenon (Xe) on whole-cell currents induced by glutamate (Glu), its three ionotropic subtypes, and GABA, as well as on the fast synaptic glutamatergic and GABAergic transmissions, were studied in the mechanically dissociated "synapse bouton preparation" of rat spinal sacral dorsal commissural nucleus (SDCN) neurons. This technique evaluates pure single or multi-synapse responses from native functional nerve endings and enables us to quantify how Xe influences pre- and postsynaptic transmissions accurately. Effects of Xe on glutamate (Glu)-, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-, kainate (KA)- and N-methyl-d-aspartate (NMDA)- and GABA_A receptor-mediated whole-cell currents were investigated by the conventional whole-cell patch configuration. Excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) were measured as spontaneous (s) and evoked (e) EPSCs and IPSCs. Evoked synaptic currents were elicited by paired-pulse focal electric stimulation. Xe decreased Glu, AMPA, KA, and NMDA receptor-mediated whole-cell currents but did not change GABA_A receptor-mediated whole-cell currents. Xe decreased the frequency and amplitude but did not affect the 1/e decay time of the glutamatergic sEPSCs. Xe decreased the frequency without affecting the amplitude and 1/e decay time of GABAergic sIPSCs. Xe decreased the amplitude and increased the failure rate (Rf) and paired-pulse ratio (PPR) without altering the 1/e decay time of both eEPSC and eIPSC, suggesting that Xe acts on the presynaptic side of the synapse. The presynaptic inhibition was greater in eEPSCs than in eIPSCs. We conclude that Xe decreases glutamatergic and GABAergic spontaneous and evoked transmissions at the presynaptic level. The glutamatergic presynaptic responses are the main target of anesthesia-induced neuronal responses. In contrast, GABAergic responses minimally contribute to Xe anesthesia.

Xenon modulates synaptic transmission to rat hippocampal CA3 neurons at both pre- and postsynaptic sites

KEY POINTS

Xenon (Xe) non-competitively inhibited whole-cell excitatory glutamatergic current (I_Glu ) and whole-cell currents gated by ionotropic glutamate receptors (I_AMPA , I_KA , I_NMDA ), but had no effect on inhibitory GABAergic whole-cell current (I_GABA ). Xe decreased only the frequency of glutamatergic spontaneous and miniature excitatory postsynaptic currents and GABAergic spontaneous inhibitory postsynaptic currents without changing the amplitude or decay times of these synaptic responses. Xe decreased the amplitude of both the action potential-evoked excitatory and the action potential-evoked inhibitory postsynaptic currents (eEPSCs and eIPSCs, respectively) via a presynaptic inhibition in transmitter release. We conclude that the main site of action of Xe is presynaptic in both excitatory and inhibitory synapses, and that the Xe inhibition is much greater for eEPSCs than for eIPSCs.

ABSTRACT

To clarify how xenon (Xe) modulates excitatory and inhibitory whole-cell and synaptic responses, we conducted an electrophysiological experiment using the 'synapse bouton preparation' dissociated mechanically from the rat hippocampal CA3 region. This technique can evaluate pure single- or multi-synapse responses and enabled us to accurately quantify how Xe influences pre- and postsynaptic aspects of synaptic transmission. Xe inhibited whole-cell glutamatergic current (I_Glu ) and whole-cell currents gated by the three subtypes of glutamate receptor (I_AMPA , I_KA and I_NMDA ). Inhibition of these ionotropic currents occurred in a concentration-dependent, non-competitive and voltage-independent manner. Xe markedly depressed the slow steady current component of I_AMPA almost without altering the fast phasic I_AMPA component non-desensitized by cyclothiazide. It decreased current frequency without affecting the amplitude and current kinetics of glutamatergic spontaneous excitatory postsynaptic currents and miniature excitatory postsynaptic currents. It decreased the amplitude, increasing the failure rate (Rf) and paired-pulse rate (PPR) without altering the current kinetics of glutamatergic action potential-evoked excitatory postsynaptic currents. Thus, Xe has a clear presynaptic effect on excitatory synaptic transmission. Xe did not alter the GABA-induced whole-cell current (I_GABA ). It decreased the frequency of GABAergic spontaneous inhibitory postsynaptic currents without changing the amplitude and current kinetics. It decreased the amplitude and increased the PPR and Rf of the GABAergic action potential-evoked inhibitory postsynaptic currents without altering the current kinetics. Thus, Xe acts exclusively at presynaptic sites at the GABAergic synapse. In conclusion, our data indicate that a presynaptic decrease of excitatory transmission is likely to be the major mechanism by which Xe induces anaesthesia, with little contribution of effects on GABAergic synapses.

Effects of cenobamate (YKP3089), a newly developed anti-epileptic drug, on voltage-gated sodium channels in rat hippocampal CA3 neurons

New, more effective pharmacologic treatments for epilepsy are needed, as a substantial portion of patients (>30%) are refractory to currently available anti-epileptic drugs. Cenobamate (YKP3089) is an investigational anti-epileptic drug in clinical development. Two completed adequate and well-controlled studies demonstrated a significant reduction in focal seizures with cenobamate in patients with epilepsy. In this study, we characterized the effects of cenobamate on voltage-gated Na⁺ channels in acutely isolated rat hippocampal CA3 neurons using a whole-cell patch-clamp technique. While cenobamate had little effect on the peak component of transient Na⁺ current (I_NaT) induced by brief depolarizing step pulses, it potently inhibited the non-inactivating persistent component of I_Na (I_NaP). In addition, cenobamate potently inhibited the current by slow voltage-ramp stimuli. Cenobamate significantly shifted the steady-state fast inactivation relationship toward a hyperpolarizing range, indicating that cenobamate binds to voltage-gated Na⁺ channels at the inactivated state with a higher affinity. Cenobamate also accelerated the development of inactivation and retarded recovery from inactivation of voltage-gated Na⁺ channels. In current clamp experiments, cenobamate hyperpolarized membrane potentials in a concentration-dependent manner, and these effects were mediated by inhibiting the I_NaP. Cenobamate also increased the threshold for generation of action potentials, and decreased the number of action potentials elicited by depolarizing current injection. Given that the I_NaP plays a pivotal role in the repetitive and/or burst generation of action potentials, the cenobamate-mediated preferential blockade of I_NaP might contribute to anti-epileptic activity.

Cocaine Directly Inhibits α6-Containing Nicotinic Acetylcholine Receptors in Human SH-EP1 Cells and Mouse VTA DA Neurons

Alpha6-containing nicotinic acetylcholine receptors are primarily found in neurons of the midbrain dopaminergic (DA) system, suggesting these receptors are potentially involved in drug reward and dependence. Here, we report a novel effect that cocaine directly inhibits α6N/α3Cβ2β3-nAChR (α6*-nAChRs) function. Human α6*-nAChRs were heterologously expressed within cells of the SH-EP1 cell line for functional characterization. Mechanically dissociated DA neurons from mouse ventral tegmental area (VTA) were used as a model of presynaptic α6*-nAChR activation since this method preserves terminal boutons. Patch-clamp recordings in whole-cell configuration were used to measure α6*-nAChR function as well as evaluate the effects of cocaine. In SH-EP1 cells containing heterologously expressed human α6*-nAChRs, cocaine inhibits nicotine-induced inward currents in a concentration-dependent manner with an IC₅₀ value of 30 μM. Interestingly, in the presence of 30 μM cocaine, the maximal current response of the nicotine concentration-response curve is reduced without changing nicotine's EC₅₀ value, suggesting a noncompetitive mechanism. Furthermore, analysis of whole-cell current kinetics demonstrated that cocaine slows nAChR channel activation but accelerates whole-cell current decay time. Our findings demonstrate that cocaine-induced inhibition occurs solely with bath application, but not during intracellular administration, and this inhibition is not use-dependent. Additionally, in Xenopus oocytes, cocaine inhibits both α6N/α3Cβ2β3-nAChRs and α6M211L/α3ICβ2β3-nCAhRs similarly, suggesting that cocaine may not act on the α3 transmembrane domain of chimeric α6N/α3Cβ2β3-nAChR. In mechanically isolated VTA DA neurons, cocaine abolishes α6*-nAChR-mediated enhancement of spontaneous inhibitory postsynaptic currents (sIPSCs). Collectively, these studies provide the first evidence that cocaine directly inhibits the function of both heterologously and naturally expressed α6*-nAChRs. These findings suggest that α6*-nAChRs may provide a novel pharmacological target mediating the effects of cocaine and may underlie a novel mechanism of cocaine reward and dependence.

Intra-axonal Ca2+ mobilization contributes to triphenyltin-induced facilitation in glycinergic transmission of rat spinal neurons

Triphenyltin (TPT) is an organotin compound causing environmental hazard to many wild creatures. Our previous findings show that TPT increases of the frequency of spontaneous glycinergic inhibitory postsynaptic currents (sIPSCs) in rat spinal neurons without changing the amplitude and 1/e decay time. In our study, the effects of 2-aminoethoxydiphenyl borate (2-APB), dantrolene sodium, and thapsigargin on sIPSC frequency were examined to reveal the contribution of intra-axonal Ca²⁺ mobilization by adding TPT. 2-APB considerably attenuated the TPT-induced facilitation of sIPSC frequency while dantrolene almost completely masked the TPT effects, suggesting that the TPT-induced synaptic facilitation results from the activation of both IP₃ and ryanodine receptors on endoplasmic reticulum (ER) membrane, though inositol triphosphate (IP₃) receptor is less sensitive to TPT. Thapsigargin itself significantly increased the sIPSC frequency without affecting the current amplitude and decay time. Successive addition of TPT could not further increase the sIPSC frequency in the presence of thapsigargin, indicating that thapsigargin completely masked the facilitatory action of TPT. Results suggest that TPT activates the IP₃ and ryanodine receptors while TPT inhibits the Ca²⁺-pump of ER membranes, resulting in the elevation of intra-axonal Ca²⁺ levels, leading to the increase of spontaneous glycine release from synaptic vesicles.

Calcium channel subtypes on glutamatergic mossy fiber terminals synapsing onto rat hippocampal CA3 neurons

The current electrophysiological study investigated the functional roles of high- and low-voltage-activated Ca2+ channel subtypes on glutamatergic small mossy fiber nerve terminals (SMFTs) that synapse onto rat hippocampal CA3 neurons. Experiments combining both the “synapse bouton” preparation and single-pulse focal stimulation technique were performed using the conventional whole cell patch configuration under voltage-clamp conditions. Nifedipine, at a high concentration, and BAY K 8644 inhibited and facilitated the glutamatergic excitatory postsynaptic currents (eEPSCs) that were evoked by 0.2-Hz stimulation, respectively. However, these drugs had no effects on spontaneous EPSCs (sEPSCs). Following the use of a high stimulation frequency of 3 Hz, however, nifedipine markedly inhibited eEPSCs at the low concentration of 0.3 µM. Moreover, ω-conotoxin GVIA and ω-agatoxin IVA significantly inhibited both sEPSCs and eEPSCs. Furthermore, SNX-482 slightly inhibited eEPSCs. R(−)-efonidipine had no effects on either sEPSCs or eEPSCs. It was concluded that glutamate release from SMFTs depends largely on Ca2+ entry through N- and P/Q-type Ca2+ channels and, to a lesser extent, on R-type Ca2+ channels. The contribution of L-type Ca2+ channels to eEPSCs was small at low-firing SMFTs but more significant at high-firing SMFTs. T-type Ca2+ channels did not appear to be involved in neurotransmission at SMFTs.

NEW & NOTEWORTHY Action potential-evoked glutamate release from small mossy fiber nerve terminals (SMFTs) that synapse onto rat hippocampal CA3 neurons is regulated by high-threshold but not low-threshold Ca2+ channel subtypes. The functional contribution mainly depends on N- and P/Q-type Ca2+ channels and, to a lesser extent, on R-type Ca2+ channels. However, in SMFTs stimulated at a high 3-Hz frequency, L-type Ca2+ channels contributed significantly to the currents. The present results are consistent with previous findings from fluorometric studies of large mossy fiber boutons.

Effects of triphenyltin on glycinergic transmission on rat spinal neurons

Glycine is a fast inhibitory transmitter like γ-aminobutyric acid in the mammalian spinal cord and brainstem, and it is involved in motor reflex, nociception, and neuronal development. Triphenyltin (TPT) is an organometallic compound causing environmental hazard to many wild creatures. Our previous findings show that TPT ultimately induces a drain and/or exhaustion of glutamate in excitatory presynaptic nerve terminals, resulted in blockage of neurotransmission as well as methylmercury. Therefore, we have investigated the neurotoxic mechanism how TPT modulates inhibitory glycinergic transmission in the synaptic bouton preparation of rat isolated spinal neurons using a patch clamp technique. TPT at environmentally relevant concentrations (3-300 nM) significantly increased the number of frequency of glycinergic spontaneous and miniature inhibitory postsynaptic currents (sIPSC and mIPSC) without affecting the current amplitude and decay time. The TPT effects were also observed in external Ca²⁺-free solution containing tetrodotoxin (TTX) but removed in Ca²⁺-free solution with both TTX and BAPTA-AM (Ca²⁺ chelator). On the other hand, the amplitude of glycinergic evoked inhibitory postsynaptic currents (eIPSCs) increased with decreasing failure rate (Rf) and paired pulse ratio (PPR) in the presence of 300 nM TPT. At a high concentration (1 µM), TPT completely blocked eIPSCs after a transient facilitation. Overall, these results suggest that TPT directly acts transmitter-releasing machinery in glycinergic nerve terminals. Effects of TPT on the nerve terminals releasing fast transmitters were greater in the order of glycinergic > glutamatergic > GABAergic ones. Thus, TPT is supposed to cause a strong synaptic modulations on glycinergic neurotransmission in wild creatures.

4,5-Dichloro-2-octyl-4-isothiazolin-3-one (DCOIT) modifies synaptic transmission in hippocampal CA3 neurons of rats

4,5-Dichloro-2-octyl-4-isothiazolin-3-one (DCOIT) is an alternative to organotin antifoulants, such as tributyltin and triphenyltin. Since DCOIT is found in harbors, bays, and coastal areas worldwide, this chemical compound may have some impacts on ecosystems. To determine whether DCOIT possesses neurotoxic activity by modifying synaptic transmission, we examined the effects of DCOIT on synaptic transmission in a 'synaptic bouton' preparation of rat brain. DCOIT at concentrations of 0.03-1 μM increased the amplitudes of evoked synaptic currents mediated by GABA and glutamate, while it reduced the amplitudes of these currents at 3-10 μM. However, the currents elicited by exogenous applications of GABA and glutamate were not affected by DCOIT. DCOIT at 1-10 μM increased the frequency of spontaneous synaptic currents mediated by GABA. It also increased the frequency of glutamate-mediated spontaneous currents at0.3-10 μM. The frequencies of miniature synaptic currents mediated by GABA and glutamate, observed in the presence of tetrodotoxin under external Ca²⁺-free conditions, were increased by 10 μM DCOIT. With the repetitive applications of DCOIT, the frequency of miniature synaptic currents mediated by glutamate was not increased by the second and third applications of DCOIT. Voltage-dependent Ca²⁺ channels were not affected by DCOIT, but DCOIT slowed the inactivation of voltage-dependent Na⁺ channels. These results suggest that DCOIT increases Ca²⁺ release from intracellular Ca²⁺ stores, resulting in the facilitation of both action potential-dependent and spontaneous neurotransmission, possibly leading to neurotoxicity.

Short-term facilitation and depression of transmitter release at amphibian sympathetic ganglionic cells - Mathematical/computational modeling

There have been few investigations of the short-term plasticity of synaptic transmission at amphibian sympathetic ganglionic cells where the frequency of miniature excitatory postsynaptic potentials is too low to measure an accurate quantum size. This has made it difficult to investigate the mechanism of synaptic transmission at the ganglionic cells by quantal analysis. A theoretical equation, therefore, is proposed. This equation is based on the premise that transmitter release is due to the product of two factors: intracellular calcium ([Ca²⁺]_i) and acetylcholine (ACh), which is a readily releasable transmitter. The equation accounts for the mechanism of synaptic facilitation and depression of transmitter release at the ganglionic cells in the paired-pulse experiments. The purpose of the present experiment is to investigate whether the equation accounts for the mechanism of short-term plasticity of synaptic transmission produced by a train of pulses at the ganglionic cells. Trains of excitatory postsynaptic current (EPSC) were recorded, and the ratios of the n^th EPSC induced by the n^th pulse to the initial EPSC were analyzed by the equation. The results indicated that the mechanism of short-term facilitation and depression was interpreted by the equation, which met the following two requirements: [Ca²⁺]_i consisting of two components of residual Ca²⁺ and the mobilization rate of ACh which accelerated as stimulus frequencies increased. The findings were consistent with those clarified by the quantal analysis. It is suggested that the theoretical equation is also useful for the investigation of the effect of chemical substances on synaptic transmission.

Presynaptic control of inhibitory neurotransmitter content in VIAAT containing synaptic vesicles

In mammals, fast inhibitory neurotransmission is carried out by two amino acid transmitters, γ-aminobutyric acid (GABA) and glycine. The higher brain uses only GABA, but in the spinal cord and brain stem both GABA and glycine act as inhibitory signals. In some cases GABA and glycine are co-released from the same neuron where they are co-packaged into synaptic vesicles by a shared vesicular inhibitory amino acid transporter, VIAAT (also called vGAT). The vesicular content of all other classical neurotransmitters (eg. glutamate, monoamines, acetylcholine) is determined by the presence of a specialized vesicular transporter. Because VIAAT is non-specific, the phenotype of inhibitory synaptic vesicles is instead predicted to be dependent on the relative concentration of GABA and glycine in the cytosol of the presynaptic terminal. This predicts that changes in GABA or glycine supply should be reflected in vesicle transmitter content but as yet, the mechanisms that control GABA versus glycine uptake into synaptic vesicles and their potential for modulation are not clearly understood. This review summarizes the most relevant experimental data that examines the link between GABA and glycine accumulation in the presynaptic cytosol and the inhibitory vesicle phenotype. The accumulated evidence challenges the hypothesis that vesicular phenotype is determined simply by the competition of inhibitory transmitter for VIAAT and instead suggest that the GABA/glycine balance in vesicles is dynamically regulated.

Effects of propofol on glycinergic neurotransmission in a single spinal nerve synapse preparation

The effects of the intravenous anesthetic, propofol, on glycinergic transmission and on glycine receptor-mediated whole-cell currents (IGly) were examined in the substantia gelatinosa (SG) neuronal cell body, mechanically dissociated from the rat spinal cord. This "synaptic bouton" preparation, which retains functional native nerve endings, allowed us to evaluate glycinergic inhibitory postsynaptic currents (IPSCs) and whole-cell currents in a preparation in which experimental solution could rapidly access synaptic terminals. Synaptic IPSCs were measured as spontaneous (s) and evoked (e) IPSCs. The eIPSCs were elicited by applying paired-pulse focal electrical stimulation, while IGly was evoked by a bath application of glycine. A concentration-dependent enhancement of IGly was observed for ≥10µM propofol. Propofol (≥3µM) significantly increased the frequency of sIPSCs and prolonged the decay time without altering the current amplitude. However, propofol (≥3µM) also significantly increased the mean amplitude of eIPSCs and decreased the failure rate (Rf). A decrease in the paired-pulse ratio (PPR) was noted at higher concentrations (≥10µM). The decay time of eIPSCs was prolonged only at the maximum concentration tested (30µM). Propofol thus acts at both presynaptic glycine release machinery and postsynaptic glycine receptors. At clinically relevant concentrations (<1μM) there was no effect on IGly, sIPSCs or eIPSCs suggesting that at anesthetic doses propofol does not affect inhibitory glycinergic synapses in the spinal cord.

Nitrous oxide directly inhibits action potential-dependent neurotransmission from single presynaptic boutons adhering to rat hippocampal CA3 neurons

We evaluated the effects of N2O on synaptic transmission using a preparation of mechanically dissociated rat hippocampal CA3 neurons that allowed assays of single bouton responses evoked from native functional nerve endings. We studied the effects of N2O on GABAA, glutamate, AMPA and NMDA receptor-mediated currents (IGABA, IGlu, IAMPA and INMDA) elicited by exogenous application of GABA, glutamate, (S)-AMPA, and NMDA and spontaneous, miniature, and evoked GABAergic inhibitory and glutamatergic excitatory postsynaptic current (sIPSC, mIPSC, eIPSC, sEPSC, mEPSC and eEPSC) in mechanically dissociated CA3 neurons. eIPSC and eEPSC were evoked by focal electrical stimulation of a single bouton. Administration of 70% N2O altered neither IGABA nor the frequency and amplitude of both sIPSCs and mIPSCs. In contrast, N2O decreased the amplitude of eIPSCs, while increasing failure rates (Rf) and paired-pulse ratios (PPR) in a concentration-dependent manner. On the other hand, N2O decreased IGlu, IAMPA and INMDA. Again N2O did not change the frequency and amplitude of either sEPSCs of mEPSCs. N2O also decreased amplitudes of eEPSCs with increased Rf and PPR. The decay phases of all synaptic responses were unchanged. The present results indicated that N2O inhibits the activation of AMPA/KA and NMDA receptors and also that N2O preferentially depress the action potential-dependent GABA and glutamate releases but had little effects on spontaneous and miniature releases.

Modifications of excitatory and inhibitory transmission in rat hippocampal pyramidal neurons by acute lithium treatment

The acute effects of high-dose Li(+) treatment on glutamatergic and GABAergic transmissions were studied in the "synaptic bouton" preparation of isolated rat hippocampal pyramidal neurons by using focal electrical stimulation. Both action potential-dependent glutamatergic excitatory and GABAergic inhibitory postsynaptic currents (eEPSC and eIPSC, respectively) were dose-dependently inhibited in the external media containing 30-150 mM Li(+), but the sensitivity for Li(+) was greater tendency for eEPSCs than for eIPSCs. When the effects of Li(+) on glutamate or GABAA receptor-mediated whole-cell responses (IGlu and IGABA) elicited by an exogenous application of glutamate or GABA were examined in the postsynaptic soma membrane of CA3 neurons, Li(+) slightly inhibited both IGlu and IGABA at the 150 mM Li(+) concentration. Present results suggest that acute treatment with high concentrations of Li(+) acts preferentially on presynaptic terminals, and that the Li(+)-induced inhibition may be greater for excitatory than for inhibitory transmission.

Morphine disinhibits glutamatergic input to VTA dopamine neurons and promotes dopamine neuron excitation

One reported mechanism for morphine activation of dopamine (DA) neurons of the ventral tegmental area (VTA) is the disinhibition model of VTA-DA neurons. Morphine inhibits GABA inhibitory neurons, which shifts the balance between inhibitory and excitatory input to VTA-DA neurons in favor of excitation and then leads to VTA-DA neuron excitation. However, it is not known whether morphine has an additional strengthening effect on excitatory input. Our results suggest that glutamatergic input to VTA-DA neurons is inhibited by GABAergic interneurons via GABAB receptors and that morphine promotes presynaptic glutamate release by removing this inhibition. We also studied the contribution of the morphine-induced disinhibitory effect on the presynaptic glutamate release to the overall excitatory effect of morphine on VTA-DA neurons and related behavior. Our results suggest that the disinhibitory action of morphine on presynaptic glutamate release might be the main mechanism for morphine-induced increase in VTA-DA neuron firing and related behaviors.

DOI:http://dx.doi.org/10.7554/eLife.09275.001

Morphine is one of the most commonly used drugs for the treatment of severe pain. It is derived from opium, which is extracted from poppies, and binds to the same receptors in the brain as the body's own naturally produced painkillers. As well as providing pain relief, morphine can act directly on the brain's reward system to trigger a state of euphoria, and can therefore be highly addictive.

One of the key components of the brain's reward circuit that morphine affects is called the ventral tegmental area (VTA). The activity of the VTA is regulated by the combined efforts of two groups of cells: excitatory glutamatergic neurons that increase VTA activity and inhibitory interneuronsthat reduce the activity of the VTA.

Morphine inhibits the interneurons, thereby allowing the glutamatergic neurons to activate the VTA. But does morphine also strengthen this excitatory input directly? By examining the effects of morphine on individual VTA neurons, Chen et al. show that the drug does indeed enhance the activity of the glutamatergic neurons. However, it does so indirectly by inhibiting another group of interneurons that would otherwise silence the glutamatergic neurons. This effect of morphine is dependent on the drug acting on a specific receptor type on the interneurons.

Chen et al. show that injecting a drug that blocks these receptors straight into the VTA of rats prevents morphine from increasing the animals' activity levels. It also prevents the animals from developing a preference for being in locations where they have previously received morphine. This suggests that morphine could primarily exert its pleasurable effects by preventing the glutamatergic neurons from being inhibited, and thus allowing them to activate the VTA neurons.

DOI:http://dx.doi.org/10.7554/eLife.09275.002

Modulation of inhibitory and excitatory fast neurotransmission in the rat CNS by heavy water (D2O)

The effects of heavy water (deuterium oxide, D2O) on GABAergic and glutamatergic spontaneous and evoked synaptic transmission were investigated in acute brain slice and isolated "synaptic bouton" preparations of rat hippocampal CA3 neurons. The substitution of D2O for H2O reduced the frequency and amplitude of GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) in a concentration-dependent manner but had no effect on glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs). In contrast, for evoked synaptic responses in isolated neurons, the amplitude of both inhibitory and excitatory postsynaptic currents (eIPSCs and eEPSCs) was decreased in a concentration-dependent manner. This was associated with increases of synaptic failure rate (Rf) and paired-pulse ratio (PPR). The effect was larger for eIPSCs compared with eEPSCs. These results clearly indicate that D2O acts differently on inhibitory and excitatory neurotransmitter release machinery. Furthermore, D2O significantly suppressed GABAA receptor-mediated whole cell current (IGABA) but did not affect glutamate receptor-mediated whole cell current (IGlu). The combined effects of D2O at both the pre- and postsynaptic sites may explain the greater inhibition of eIPSCs compared with eEPSCs. Finally, D2O did not enhance or otherwise affect the actions of the general anesthetics nitrous oxide and propofol on spontaneous or evoked GABAergic and glutamatergic neurotransmissions, or on IGABA and IGlu. Our results suggest that previously reported effects of D2O to mimic and/or modulate anesthesia potency result from mechanisms other than modulation of GABAergic and glutamatergic neurotransmission.

Tetrodotoxin abruptly blocks excitatory neurotransmission in mammalian CNS

The present study utilised a 'synaptic bouton' preparation of mechanically isolated rat hippocampal CA3 pyramidal neurons, which permits direct physiological and pharmacological quantitative analyses at the excitatory and inhibitory single synapse level. Evoked excitatory and inhibitory postsynaptic currents (eEPSCs and eIPSCs) were generated by focal paired-pulse electrical stimulation of single boutons. The sensitivity of eEPSC to tetrodotoxin (TTX) was higher than that of the voltage-dependent Na(+) channel whole-cell current (INa) in the postsynaptic CA3 soma membrane. The synaptic transmission was strongly inhibited by 3 nM TTX, at which concentration the INa was hardly suppressed. The IC50 values of eEPSC and INa for TTX were 2.8 and 37.9 nM, respectively, and complete inhibition was 3-10 nM for eEPSC and 1000 nM for INa. On the other hand, both eEPSC and eIPSC were equally and gradually inhibited by decreasing the external Na(+) concentration ([Na]o), which decreases the Na(+)gradient across the cell membrane. The results indicate that TTX at 3-10 nM could block most of voltage-dependent Na(+) channels on presynaptic nerve terminal, resulting in abruptly inhibition of action potential dependent excitatory neurotransmission.

Proton-induced currents in substantia gelatinosa neurons of the rat trigeminal subnucleus caudalis

Acid-sensing ion channels (ASICs) are widely expressed in both the peripheral and central nervous system, and contribute to the modulation of central nociceptive transmission under both physiological and pathophysiological conditions. In this study, we characterized the proton-induced membrane currents in acutely isolated rat substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis using the whole cell patch-clamp technique. Exposure to acidic conditions (pH<6.5) induced the inward currents in a pH-dependent manner. Amiloride, a general ASIC antagonist, significantly blocked the proton-induced currents in a non-competitive manner. The pH 6.0-induced membrane current (IpH6.0) was greatly attenuated in the Na(+)-free external solution, and the reversal potential of the proton-induced currents was similar to the theoretical Na(+) equilibrium potential. The IpH6.0 was reciprocally potentiated by a lower extracellular Ca(2+) concentration. The modulation of IpH6.0 by divalent cations and other modulators suggests that the proton-induced currents are mediated by multiple types of ASIC subunits, including ASIC1a and ASIC2a. Multi-cell RT-PCR analysis revealed that SG neurons express these subunits. Exposure to a pH 6.0 solution directly depolarized the membrane potential, and generated a burst of action potentials in a current-clamp mode. This acidic pH-induced depolarization was significantly blocked by amiloride. The present results suggest that ASICs expressed on SG neurons play important roles in the regulation of nociceptive transmission from the orofacial tissues.

Transmembrane AMPAR regulatory protein γ-2 is required for the modulation of GABA release by presynaptic AMPARs

Presynaptic ionotropic glutamate receptors (iGluRs) play important roles in the control of synaptogenesis and neurotransmitter release, yet their regulation is poorly understood. In particular, the contribution of transmembrane auxiliary proteins, which profoundly shape the trafficking and gating of somatodendritic iGluRs, is unknown. Here we examined the influence of transmembrane AMPAR regulatory proteins (TARPs) on presynaptic AMPARs in cerebellar molecular layer interneurons (MLIs). 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a partial agonist at TARP-associated AMPARs, enhanced spontaneous GABA release in wild-type mice but not in stargazer mice that lack the prototypical TARP stargazin (γ-2). These findings were replicated in mechanically dissociated Purkinje cells with functional adherent synaptic boutons, demonstrating the presynaptic locus of modulation. In dissociated Purkinje cells from stargazer mice, AMPA was able to enhance mIPSC frequency, but only in the presence of the positive allosteric modulator cyclothiazide. Thus, ordinarily, presynaptic AMPARs are unable to enhance spontaneous release without γ-2, which is required predominantly for its effects on channel gating. Presynaptic AMPARs are known to reduce action potential-driven GABA release from MLIs. Although a G-protein-dependent non-ionotropic mechanism has been suggested to underlie this inhibition, paradoxically we found that γ-2, and thus AMPAR gating, was required. Following glutamate spillover from climbing fibers or application of CNQX, evoked GABA release was reduced; in stargazer mice such effects were markedly attenuated in acute slices and abolished in the dissociated Purkinje cell-nerve bouton preparation. We suggest that γ-2 association, by increasing charge transfer, allows presynaptic AMPARs to depolarize the bouton membrane sufficiently to modulate both phasic and spontaneous release.

Modulation of excitatory synaptic transmission in rat hippocampal CA3 neurons by triphenyltin, an environmental pollutant

Triphenyltin (TPT) is an organometallic compound that poses a known environmental hazard to some fish and mollusks, as well as mammals. However, its neurotoxic mechanisms in the mammalian brain are still unclear. Thus, we have investigated mechanisms through which TPT modulates glutamatergic synaptic transmission, including spontaneous, miniature, and evoked excitatory postsynaptic currents (sEPSCs, mEPSCs, and eEPSCs respectively), in a rat hippocampal CA3 'synaptic-bouton' preparation. TPT, at environmentally relevant concentrations (30 nM to 1 μM), significantly increased the frequency of sEPSCs and mEPSCs in a concentration-dependent manner, without affecting the currents' amplitudes. The facilitatory effects of TPT on mEPSC frequency were seen even in a Ca(2+)-free external solution containing tetrodotoxin. These effects were further prolonged by adding caffeine, which releases Ca(2+) from intracellular Ca(2+) storage sites. In glutamatergic eEPSCs evoked by paired-pulse stimuli, TPT at concentrations greater than or equal to 100 nM markedly increased the current amplitude by the first pulse and decreased failure rate and pair-pulse ratio. On the other hand, both voltage-dependent Na(+) and Ca(2+) channels were unaffected by submicromolar concentrations of TPT. Overall, these results suggest that TPT, at environmentally relevant concentrations, affects presynaptic transmitter release machinery by directly modulating Ca(2+) storage. Further, findings of this study imply that excitotoxic mechanisms may underlie TPT-induced neuronal damage.

Characterization of proton-induced currents in rat trigeminal mesencephalic nucleus neurons

Acid-sensing ion channels (ASICs) are widely expressed in central as well as peripheral neurons. Here we have characterized the proton-induced currents in acutely isolated rat trigeminal mesencephalic nucleus (Vmes) neurons using a whole cell patch-clamp technique. In a voltage-clamp condition, the application of acid extracellular solution (≤ pH 6.5) induced the inward currents in a pH-dependent manner. The proton-induced currents disappeared in the Na(+)-free external solution, and were concentration-dependently blocked by amiloride, a general ASIC blocker. The reversal potential of proton-induced currents was similar to the theoretical Na(+) equilibrium potential, suggesting that the proton-induced currents are mainly mediated by the activation of ASICs, which are highly selective to Na(+). The modulation of proton-induced currents by divalent cations and the expression patterns of ASIC transcripts using by the multi-cell RT-PCR assay suggest that Vmes neurons express functional ASIC2a and ASIC1b subunits. In a current-clamp condition, acidic pH directly depolarized the membrane potential and generated a burst of action potentials at Vmes neurons, which innervate the masseter muscle spindles. Considering that cell bodies of Vmes neurons are located within the central nervous system, ASICs expressed on Vmes neurons, by sensing peripheral and/or central acidosis, might play pivotal roles in the transduction of proprioceptive information from the masseter muscles and periodontal ligaments.

Inhibition of excitatory synaptic transmission in hippocampal neurons by levetiracetam involves Zn²⁺-dependent GABA type A receptor-mediated presynaptic modulation

Levetiracetam (LEV) is an antiepileptic drug with a unique but as yet not fully resolved mechanism of action. Therefore, by use of a simplified rat-isolated nerve-bouton preparation, we have investigated how LEV modulates glutamatergic transmission from mossy fiber terminals to hippocampal CA3 neurons. Action potential-evoked excitatory postsynaptic currents (eEPSCs) were recorded using a conventional whole-cell patch-clamp recording configuration in voltage-clamp mode. The antiepileptic drug phenytoin decreased glutamatergic eEPSCs in a concentration-dependent fashion by inhibiting voltage-dependent Na⁺ and Ca²⁺ channel currents. In contrast, LEV had no effect on eEPSCs or voltage-dependent Na⁺ or Ca²⁺ channel currents. Activation of presynaptic GABA type A (GABA(A)) receptors by muscimol induced presynaptic inhibition of eEPSCs, resulting from depolarization block. Low concentrations of Zn²⁺, which had no effect on eEPSCs, voltage-dependent Na⁺ or Ca²⁺ channel currents, or glutamate receptor-mediated whole cell currents, reduced the muscimol-induced presynaptic inhibition. LEV applied in the continuous presence of 1 µM muscimol and 1 µM Zn²⁺ reversed this Zn²⁺ modulation on eEPSCs. The antagonizing effect of LEV on Zn²⁺-induced presynaptic GABA(A) receptor inhibition was also observed with the Zn²⁺ chelators Ca-EDTA and RhodZin-3. Our results clearly show that LEV removes the Zn²⁺-induced suppression of GABA(A)-mediated presynaptic inhibition, resulting in a presynaptic decrease in glutamate-mediated excitatory transmission. Our results provide a novel mechanism by which LEV may inhibit neuronal activity.

Characterization of age-related changes in synaptic transmission onto F344 rat basal forebrain cholinergic neurons using a reduced synaptic preparation

Basal forebrain (BF) cholinergic neurons participate in a number of cognitive processes that become impaired during aging. We previously found that age-related enhancement of Ca(2+) buffering in rat cholinergic BF neurons was associated with impaired performance in the water maze spatial learning task (Murchison D, McDermott AN, Lasarge CL, Peebles KA, Bizon JL, and Griffith WH. J Neurophysiol 102: 2194-2207, 2009). One way that altered Ca(2+) buffering could contribute to cognitive impairment involves synaptic function. In this report we show that synaptic transmission in the BF is altered with age and cognitive status. We have examined the properties of spontaneous postsynaptic currents (sPSCs) in cholinergic BF neurons that have been mechanically dissociated without enzymes from behaviorally characterized F344 rats. These isolated neurons retain functional presynaptic terminals on their somata and proximal dendrites. Using whole cell patch-clamp recording, we show that sPSCs and miniature PSCs are predominately GABAergic (bicuculline sensitive) and in all ways closely resemble PSCs recorded in a BF in vitro slice preparation. Adult (4-7 mo) and aged (22-24 mo) male rats were cognitively assessed using the water maze. Neuronal phenotype was identified post hoc using single-cell RT-PCR. The frequency of sPSCs was reduced during aging, and this was most pronounced in cognitively impaired subjects. This is the same population that demonstrated increased intracellular Ca(2+) buffering. We also show that increasing Ca(2+) buffering in the synaptic terminals of young BF neurons can mimic the reduced frequency of sPSCs observed in aged BF neurons.

Potent and direct presynaptic modulation of glycinergic transmission in rat spinal neurons by atrial natriuretic peptide

Atrial and brain natriuretic peptides (ANP and BNP) exist in the central nervous system and modulate neuronal function, although the locus of actions and physiological mechanisms are still unclear. In the present study we used rat spinal sacral dorsal commissural nucleus (SDCN) and hippocampal 'synaptic bouton' preparations, to record both spontaneous and evoked glycinergic inhibitory postsynaptic currents (sIPSCs and eIPSCs) in SDCN neurons, and the evoked excitatory postsynaptic currents (eEPSCs) in hippocampal CA3 neurons. ANP potently and significantly reduced the sIPSC frequency without affecting the amplitude. ANP also potently reduced the eIPSCs amplitude concurrently increasing the failure rate and the paired pulse ratio response. These ANP actions were blocked by anantin, a specific type A natriuretic peptide receptor (NPR-A) antagonist. The results clearly indicate that ANP acts directly on glycinergic presynaptic nerve terminals to inhibit glycine release via presynaptic NPR-A. The ANP effects were not blocked by the membrane permeable cGMP analog (8Br-cGMP) suggesting a transduction mechanisms not simply related to increasing cGMP levels in nerve terminals. BNP did not affect on glycinergic sIPSCs and eIPSCs. Moreover, both ANP and BNP had no effect on glutamatergic EPSCs in hippocampal CA3 neurons. The results indicate a potent and selective presynaptic inhibitory action of ANP on glycinergic transmission in spinal cord sensory circuits.

Presynaptic glycine receptors increase GABAergic neurotransmission in rat periaqueductal gray neurons

The periaqueductal gray (PAG) is involved in the central regulation of nociceptive transmission by affecting the descending inhibitory pathway. In the present study, we have addressed the functional role of presynaptic glycine receptors in spontaneous glutamatergic transmission. Spontaneous EPSCs (sEPSCs) were recorded in mechanically dissociated rat PAG neurons using a conventional whole-cell patch recording technique under voltage-clamp conditions. The application of glycine (100 µM) significantly increased the frequency of sEPSCs, without affecting the amplitude of sEPSCs. The glycine-induced increase in sEPSC frequency was blocked by 1 µM strychnine, a specific glycine receptor antagonist. The results suggest that glycine acts on presynaptic glycine receptors to increase the probability of glutamate release from excitatory nerve terminals. The glycine-induced increase in sEPSC frequency completely disappeared either in the presence of tetrodotoxin or Cd²⁺, voltage-gated Na⁺, or Ca²⁺ channel blockers, suggesting that the activation of presynaptic glycine receptors might depolarize excitatory nerve terminals. The present results suggest that presynaptic glycine receptors can regulate the excitability of PAG neurons by enhancing glutamatergic transmission and therefore play an important role in the regulation of various physiological functions mediated by the PAG.

Pregnenolone sulfate modulates glycinergic transmission in rat medullary dorsal horn neurons

The neurosteroid pregnenolone sulfate (PS), a representative excitatory neuromodulator, has a variety of neuropharmacological actions, such as memory enhancement and convulsant effects. In this study, the effects of PS on glycinergic transmission, such as glycinergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs), were investigated in acutely isolated medullary dorsal horn neurons by use of a conventional whole-cell patch-clamp technique. PS significantly increased the frequency but decreased the amplitude of glycinergic mIPSCs in a concentration-dependent manner. PS also accelerated the decay time constant of glycinergic mIPSCs. The PS-induced decrease in mIPSC amplitude was due to the direct postsynaptic inhibition of glycine receptors because PS inhibited the glycine-induced Cl(-) currents in a noncompetitive manner. The PS-induced increase in mIPSC frequency was not due to the activation of α7 nicotinic acetylcholine, NMDA, σ1 receptors and voltage-dependent Ca(2+) channels, which are known to be molecular targets of PS. On the other hand, the PS-induced increase in mIPSC frequency was completely attenuated either in the Ca(2+)-free external solution or in the presence of transient receptor potential (TRP) channel blockers, suggesting that PS elicits an increase in Ca(2+) concentration within glycinergic nerve terminals via the activation of putative TRP channels. The PS-mediated modulation of glycinergic synaptic transmission, such as the enhancement of presynaptic glycine release and direct inhibition of postsynaptic glycine receptors, might have a broad impact on the excitability of medullary dorsal horn neurons and therefore affect the processing of nociceptive transmission from orofacial tissues.

Transsynaptic inhibition of spinal transmission by A2 botulinum toxin

Type A botulinum toxin blocks not only ACh release from motor nerve terminals but also central synaptic transmission, including glutamate, noradrenaline, dopamine, ATP, GABA and glycine. Neurotoxins (NTXs) are transported by both antero- and retrogradely along either motor or sensory axons for bidirectional delivery between peripheral tissues or the CNS. A newly developed type A2 NTX (A2NTX) injected into one rat foreleg muscle was transported to the contralateral muscle. This finding was consistent with the NTX traveling retrogradely via spinal neurons and then transsynaptically through motor neurons to the contralateral motor neurons within the spinal cord and on to the soleus muscle. In the present study we found that toxin injection into the rat left soleus muscle clearly induced bilateral muscle relaxation in a dose-dependent fashion, although the contralateral muscle relaxation followed the complete inhibition of toxin-injected ipsilateral muscles. The toxin-injected ipsilateral muscle relaxation was faster and stronger in A2NTX-treated rats than A1LL (BOTOX). A1LL was transported almost equally to the contralateral muscle via neural pathways and the bloodstream. In contrast, A2NTX was mainly transported to contralateral muscles via the blood. A1LL was more successfully transported to contralateral spinal neurons than A2NTX. We also demonstrated that A1LL and A2NTX were carried from peripheral to CNS and vice versa by dual antero- and retrograde axonal transport through either motor or sensory neurons.

Removal of toxin (tetrodotoxin) from puffer ovary by traditional fermentation

The amounts of puffer toxin (tetrodotoxin, TTX) extracted from the fresh and the traditional Japanese salted and fermented "Nukazuke" and "Kasuzuke" ovaries of Takifugu stictonotus (T. stictonotus) were quantitatively analyzed in the voltage-dependent sodium current (I(Na)) recorded from mechanically dissociated single rat hippocampal CA1 neurons. The amount of TTX contained in "Nukazuke" and "Kasuzuke" ovaries decreased to 1/50-1/90 times of that of fresh ovary during a salted and successive fermented period over a few years. The final toxin concentration after fermentation was almost close to the TTX level extracted from T. Rubripes" fresh muscle that is normally eaten. It was concluded that the fermented "Nukazuke" and "Kasuzuke" ovaries of puffer fish T. Stictonotus are safe and harmless as food.

Ionotropic receptors at hippocampal mossy fibers: roles in axonal excitability, synaptic transmission, and plasticity

Dentate granule cells process information from the enthorinal cortex en route to the hippocampus proper. These neurons have a very negative resting membrane potential and are relatively silent in the slice preparation. They are also subject to strong feed-forward inhibition. Their unmyelinated axon or mossy fiber ramifies extensively in the hilus and projects to stratum lucidum where it makes giant en-passant boutons with CA3 pyramidal neurons. There is compelling evidence that mossy fiber boutons express presynaptic GABA_A receptors, which are commonly found in granule cell dendrites. There is also suggestive evidence for the presence of other ionotropic receptors, including glycine, NMDA, and kainate receptors, in mossy fiber boutons. These presynaptic receptors have been proposed to lead to mossy fiber membrane depolarization. How this phenomenon alters the excitability of synaptic boutons, the shape of presynaptic action potentials, Ca²⁺ influx and neurotransmitter release has remained elusive, but high-resolution live imaging of individual varicosities and direct patch-clamp recordings have begun to shed light on these phenomena. Presynaptic GABA_A and kainate receptors have also been reported to facilitate the induction of long-term potentiation at mossy fiber—CA3 synapses. Although mossy fibers are highly specialized, some of the principles emerging at this connection may apply elsewhere in the CNS.

Comparative effects of pentobarbital on spontaneous and evoked transmitter release from inhibitory and excitatory nerve terminals in rat CA3 neurons

Pentobarbital (PB) modulates GABA(A) receptor-mediated postsynaptic responses through various mechanisms, and can directly activate the channel at higher doses. These channels exist both pre- and postsynaptically, and on the soma outside the synapse. PB also inhibits voltage-dependent Na⁺ and Ca²⁺ channels to decrease excitatory synaptic transmission. Just how these different sites of action combine to contribute to the overall effects of PB on inhibitory and excitatory synaptic transmission is less clear. To compare these pre- and postsynaptic actions of PB, we used a 'synaptic bouton' preparation of isolated rat hippocampal CA3 pyramidal neurons where we could measure in single neurons the effects of PB on spontaneous and single bouton evoked GABAergic inhibitory and glutamatergic excitatory postsynaptic currents (sIPSCs, sEPSCs, eIPSCs and eEPSCs), respectively. Low (sedative) concentrations (3-10 μM) of PB increased the frequency and amplitude of sIPSCs and sEPSCs, and also presynaptically increased the amplitude of both eIPSCs and eEPSCs. There was no change in current kinetics at this low concentration. At higher concentrations (30-300 μM), PB decreased the frequency, and increased the amplitude of sIPSCs, and presynaptically decreased the amplitude of eIPSCs. The current decay phase of sIPSCs and eIPSCs was increased. An increase in both frequency and amplitude was seen for sEPSCs, while the eIPSCs was also decreased by a bicuculline-sensitive presynaptic effect. The results confirm the multiple sites of action of PB on inhibitory and excitatory transmission and demonstrate that the most sensitive site of action is on transmitter release, via effects on presynaptic GABA(A) receptors. At low concentrations, however, both glutamate and GABA release is similarly enhanced, making the final effects on neuronal excitability difficult to predict and dependent on the particular systems involved and/or on subtle differences in susceptibility amongst individuals. At higher concentrations, release of both transmitters is decreased, while the postsynaptic effects to increase IPSPs and decrease EPSCs would be expected to both results in reduced neuronal excitability.

The effects of volatile anesthetics on synaptic and extrasynaptic GABA-induced neurotransmission

Examination of volatile anesthetic actions at single synapses provides more direct information by reducing interference by surrounding tissue and extrasynaptic modulation. We examined how volatile anesthetics modulate GABA release by measuring spontaneous or miniature GABA-induced inhibitory postsynaptic currents (mIPSCs, sIPSCs) or by measuring action potential-evoked IPSCs (eIPSCs) at individual synapses. Halothane increased both the amplitude and frequency of sIPSCs. Isoflurane and enflurane increased mIPSC frequency while sevoflurane had no effect. These anesthetics did not alter mIPSC amplitudes. Halothane increased the amplitude of eIPSCs, with a decrease in failure rate (Rf) and paired-pulse ratio. In contrast, isoflurane and enflurane decreased the eIPSC amplitude and increased Rf, while sevoflurane decreased the eIPSC amplitude without affecting Rf. Volatile anesthetics did not change kinetics except for sevoflurane, suggesting that presynaptic mechanisms dominate changes in neurotransmission. Each anesthetic showed somewhat different GABA-induced response and these results suggest that GABA-induced synaptic transmission cannot have a uniformly common site of action as suggested for volatile anesthetics. In contrast, all volatile anesthetics concentration-dependently enhanced the GABA-induced extrasynaptic currents. Extrasynaptic receptors containing α4 and α5 subunits are reported to have high sensitivities to volatile anesthetics. Also, inhibition of GABA uptake by volatile anesthetics results in higher extracellular GABA concentration, which may lead to prolonged activation of extrasynaptic GABAA receptors. The extrasynaptic GABA-induced receptors may be major site of volatile anesthetic-induced neurotransmission. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

GABAA receptors facilitate spontaneous glutamate release in rat periaqueductal gray neurons

The functional role of presynaptic γ-aminobutyric acid (GABA)(A) receptors in excitatory glutamatergic transmission was examined in rat periaqueductal gray neurons recorded using a conventional whole-cell patch clamp technique. Muscimol, a GABA(A) receptor agonist, significantly increased the frequency of spontaneous excitatory postsynaptic currents without affecting their amplitude, and this effect was completely blocked by the selective GABA(A) receptor antagonist. The muscimol-induced facilitation of spontaneous excitatory postsynaptic current frequency disappeared either in the presence of tetrodotoxin or Cd. The results suggest that the activation of presynaptic GABA(A) receptors directly depolarizes glutamatergic terminals resulting in the facilitation of spontaneous glutamate release, and that presynaptic GABA(A) receptors play an important role in the regulation of various physiological functions mediated by the periaqueductal gray.

Muscarinic M4 receptors regulate GABAergic transmission in rat tuberomammillary nucleus neurons

Histaminergic neurons within the tuberomammillary nucleus (TMN) play an important role in sleep-wakefulness regulation. Here, we report the muscarinic modulation of GABAergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) in mechanically dissociated rat histaminergic neurons using a conventional whole-cell patch clamp technique. Muscarine, a nonselective muscarinic acetylcholine (mACh) receptor agonist, reversibly decreased mIPSC frequency without affecting the current amplitude, indicating that muscarine acts presynaptically to decrease the probability of spontaneous GABA release. The muscarine action on GABAergic mIPSC frequency was completely blocked by atropine, a nonselective mACh receptor antagonist, and tropicamide, an M(4) receptor antagonist. The muscarine-induced decrease in mIPSC frequency was completely occluded in the presence of Cd(2+), a general voltage-dependent Ca(2+) channel blocker, or in a Ca(2+)-free external solution. However, pharmacological agents affecting adenylyl cyclase or G-protein coupled inwardly rectifying K(+) channel activity did not prevent the inhibitory action of muscarine on GABAergic mIPSCs. These results suggest that muscarine acts on M(4) receptors on GABAergic nerve terminals projecting to histaminergic neurons to inhibit spontaneous GABA release via the inhibition of Ca(2+) influx from the extracellular space. Muscarine also inhibited action potential-dependent GABA release by activating presynaptic M(4) receptors in more physiological conditions. The M(4) receptor-mediated modulation of GABAergic transmission onto TMN neurons may contribute to the regulation of sleep-wakefulness.

TRPA1-like channels enhance glycinergic transmission in medullary dorsal horn neurons

The effect of icilin, a potent agonist of transient receptor potential ankyrin 1 (TRPA1) and TRPM8, on glycinergic transmission was examined in mechanically isolated rat medullary dorsal horn neurons by use of the conventional whole-cell patch-clamp technique. Icilin increased the frequency of glycinergic spontaneous miniature inhibitory post-synaptic currents (mIPSCs) in a dose-dependent manner. Either allyl isothiocyanate(AITC) or cinnamaldehyde, other TRPA1 agonists, also increased mIPSC frequency, but the extent of facilitation induced by AITC or cinnamaldehyde was less than that induced by icilin. However, menthol, a TRPM8 agonist, had no facilitatory effect on glycinergic mIPSCs. The icilin-induced increase in mIPSC frequency was significantly inhibited by either HC030031, a selective TRPA1 antagonist, or ruthenium red, a non-selective transient receptor potential channel blocker. Icilin failed to increase glycinergic mIPSC frequency in the absence of extracellular Ca(2+), suggesting that the icilin-induced increase in mIPSC frequency is mediated by the Ca(2+) influx from the extracellular space. In contrast, icilin still increased mIPSC frequency either in the Na(+) -free external solution or in the presence of Cd(2+), a general voltage-dependent Ca(2+) channel blocker. The present results suggest that icilin acts on pre-synaptic TRPA1-like ion channels, which are permeable to Ca(2+), to enhance glycinergic transmission onto medullary dorsal horn neurons. The TRPA1-like channel-mediated enhancement of glycinergic transmission in medullary dorsal horn neurons would contribute to the regulation of pain information from the peripheral tissues.

Co-release of glutamate and GABA from single, identified mossy fibre giant boutons

Several laboratories have provided immunohistochemical, molecular biological and electrophysiological evidence that the glutamatergic granule cells of the dentate gyrus can transiently express a GABAergic phenotype during development. Electrophysiological recordings on hippocampal slices obtained during this period have shown that stimulation of the mossy fibres (MFs) provokes simultaneous monosynaptic GABA(A) and glutamate receptor-mediated responses in their target cells,which have the pharmacological and physiological characteristics of MF neurotransmission. This evidence, although strongly supporting the hypothesis that MFs co-release glutamate and GABA, is indirect, as the extracellular stimulation used in slice experiments could activate fibres other than MFs. In this study, we show that selective stimulation of single, identified MF boutons (MFBs) attached to the apical dendrites of dissociated pyramidal cells of developing rats produced synaptic currents mediated by either glutamate receptors only or by both glutamate and GABA(A) receptors. By contrast, stimulation of MFBs of adult rats produced exclusively glutamate receptor-mediated responses. All responses evoked by stimulation of MFBs underwent strong frequency-dependent potentiation and were depressed by the activation of presynaptic metabotropic glutamate receptors. On the other hand, synaptic responses evoked by stimulation of interneuronal boutons located on the soma or on the basal dendrites of the same pyramidal cells were exclusively mediated by GABA(A) receptors, underwent frequency-dependent depression and were unaffected by mGluR agonists.We here demonstrate that the simultaneous glutamatergic and GABAergic responses evoked by MF stimulation in pyramidal cells of CA3 during development have a common origin in the giant MFBs.