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

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.

Mediatory role of the dopaminergic system through D1 receptor on glycine-induced hypophagia in neonatal broiler-type chickens

The present study aimed to examine the mediatory role of the dopaminergic system in the food intake induced by intracerebroventricular (ICV) injection of glycine in neonatal 3-h feed-deprived (FD3) meat-type chickens. In the first and second experiments, birds were ICV injected using low and high doses of glycine (50, 100 and 200 nmol) and strychnine (50, 100 and 200 nmol), respectively. In experiments 3-9, the behaviorally subeffective doses of dopamine (10 nmol), 6-OHDA (2.5 nmol), SCH 23,390 (D1 antagonist; 5 nmol), AMI-193 (D2 antagonist; 5 nmol), NGB2904 (D3 antagonist; 6.4 nmol) and L-741,742 (D4 antagonist; 6 nmol) were, respectively, co-administrated with glycine (200 nmol) in FD3 5-day-old chicks to investigate possible interplay of dopamine receptors in glycine-induced feeding behavior. Then, cumulative food intake based on body weight percentage (%BW) was determined at 30, 60 and 120 min after the injection. According to the results, dopamine significantly boosted the hypophagia induced by glycine at all-time intervals (p ≤ 0.001). These results combined with the previous findings suggest an interplay between dopamine and glycine in chicken's brain in which D1 receptor-mediated food intake induced by glycine.

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.

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.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

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.

Inhibition by general anesthetic propofol of compound action potentials in the frog sciatic nerve and its chemical structure

Although the intravenous general anesthetic propofol (2,6-diisopropylphenol) has an ability to inhibit nerve conduction, this has not been fully examined. Various agents inhibit compound action potentials (CAPs) in a manner dependent on their chemical structures. To determine propofol's chemical structure that is important in nerve conduction inhibition, we examined the effects of propofol and its related compounds on fast-conducting CAPs recorded from the frog sciatic nerve by using the air-gap method. Propofol concentration-dependently reduced the peak amplitude of the CAP with a half-maximal inhibitory concentration (IC₅₀) value of 0.14 mM. A similar inhibition was produced by other phenols, 4-sec-butylphenol and 4-amylphenol (IC₅₀ values: 0.33 and 0.20 mM, respectively). IC₅₀ values for these and more phenols (4-isopropylphenol, 4-tert-butylphenol, and 4-ter-amylphenol; data published previously) were correlated with the logarithm of their octanol-water partition coefficients. A phenol having ketone group (raspberry ketone) and alcohols (3-phenyl-1-propanol and 2-phenylethylalcohol) inhibited CAPs less effectively than the above-mentioned phenols. The local anesthetic (LA) benzocaine reduced CAP peak amplitudes with an IC₅₀ of 0.80 mM, a value larger than that of propofol. When compared with other LAs, propofol activity was close to those of ropivacaine, levobupivacaine, and pramoxine, while benzocaine activity was similar to those of cocaine and lidocaine. It is concluded that propofol inhibits nerve conduction, possibly owing to isopropyl and hydroxyl groups bound to the benzene ring of propofol and to its lipophilicity; propofol's efficacy is comparable to those of some LAs. These results could serve to develop propofol-related agents exhibiting analgesia when applied topically.

Propofol Inhibits Cerebellar Parallel Fiber-Purkinje Cell Synaptic Transmission via Activation of Presynaptic GABAB Receptors in vitro in Mice

Propofol is a widely used intravenous sedative-hypnotic agent, which causes rapid and reliable loss of consciousness via activation of γ -aminobutyric acid A (GABA_A) receptors. We previously found that propofol inhibited cerebellar Purkinje cells (PC) activity via both GABA_A and glycine receptors in vivo in mice. We here examined the effect of propofol on the cerebellar parallel fiber (PF)-PC synaptic transmission in mouse cerebellar slices by whole-cell recording technique and pharmacological methods. We found that following blockade of GABA_A and glycine receptors activity, propofol reversely decreased the amplitude of PF-PC excitatory postsynaptic currents (PF-PC EPSCs), and significantly increased paired-pulse ratio (PPR). The propofol-induced decrease in amplitude of PF-PC EPSCs was concentration-dependent. The half-inhibitory concentration (IC₅₀) of propofol for inhibiting PF-PC EPSCs was 4.7 μM. Notably, the propofol-induced changes in amplitude and PPR of PF-PC EPSCs were abolished by GABA_B receptor antagonist, saclofen (10 μM), but not blocked by N-methyl-D-aspartate receptor (NMDA) receptor antagonist, D-APV (50 μM). Application of the GABA_B receptor agonist baclofen induced a decrease in amplitude and an increase in PPR of PF-PC EPSCs, as well masked the propofol-induced changes in PF-PC EPSCs. Moreover, the propofol-induced changes in amplitude and PPR of PF-PC EPSCs were abolished by a specific protein kinase A (PKA) inhibitor, KT5720. These results indicate that application of propofol facilitates presynaptic GABA_B receptors, resulting in a depression of PF-PC synaptic transmission via PKA signaling pathway in mouse cerebellar cortex. The results suggest that the interaction with GABA_B receptors may contribute to the general anesthetic action of propofol.

Presence of Inhibitory Glycinergic Transmission in Medium Spiny Neurons in the Nucleus Accumbens

It is believed that the rewarding actions of drugs are mediated by dysregulation of the mesolimbic dopaminergic system leading to increased levels of dopamine in the nucleus accumbens (nAc). It is widely recognized that GABAergic transmission is critical for neuronal inhibition within nAc. However, it is currently unknown if medium spiny neurons (MSNs) also receive inhibition by means of glycinergic synaptic inputs. We used a combination of proteomic and electrophysiology studies to characterize the presence of glycinergic input into MSNs from nAc demonstrating the presence of glycine transmission into nAc. In D1 MSNs, we found low frequency glycinergic miniature inhibitory postsynaptic currents (mIPSCs) which were blocked by 1 μM strychnine (STN), insensitive to low (10, 50 mM) and high (100 mM) ethanol (EtOH) concentrations, but sensitive to 30 μM propofol. Optogenetic experiments confirmed the existence of STN-sensitive glycinergic IPSCs and suggest a contribution of GABA and glycine neurotransmitters to the IPSCs in nAc. The study reveals the presence of glycinergic transmission in a non-spinal region and opens the possibility of a novel mechanism for the regulation of the reward pathway.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

Propofol Protects Rat Hypoglossal Motoneurons in an In Vitro Model of Excitotoxicity by Boosting GABAergic Inhibition and Reducing Oxidative Stress

In brainstem motor networks, hypoglossal motoneurons (HMs) play the physiological role of driving tongue contraction, an activity critical for inspiration, phonation, chewing and swallowing. HMs are an early target of neurodegenerative diseases like amyotrophic lateral sclerosis that, in its bulbar form, is manifested with initial dysphagia and dysarthria. One important pathogenetic component of this disease is the high level of extracellular glutamate due to uptake block that generates excitotoxicity. To understand the earliest phases of this condition we devised a model, the rat brainstem slice, in which block of glutamate uptake is associated with intense bursting of HMs, dysmetabolism and death. Since blocking bursting becomes a goal to prevent cell damage, the present report enquired whether boosting GABAergic inhibition could fulfill this aim and confer beneficial outcome. Propofol (0.5 µM) and midazolam (0.01 µM), two allosteric modulators of GABA_A receptors, were used at concentrations yielding analogous potentiation of GABA-mediated currents. Propofol also partly depressed NMDA receptor currents. Both drugs significantly shortened bursting episodes without changing single burst properties, their synchronicity, or their occurrence. Two hours later, propofol prevented the rise in reactive oxygen species (ROS) and, at 4 hours, it inhibited intracellular release of apoptosis-inducing factor (AIF) and prevented concomitant cell loss. Midazolam did not contrast ROS and AIF release. The present work provides experimental evidence for the neuroprotective action of a general anesthetic like propofol, which, in this case, may be achieved through a combination of boosted GABAergic inhibition and reduced ROS production.

Neurotoxicity of propofol on rat hypoglossal motoneurons in vitro

Although propofol is a widely used intravenous general anaesthetic, many studies report its toxic potential, particularly on the developing central nervous system. We investigated its action on hypoglossal motoneurons (HMs) that control two critical functions in neonates, namely tongue muscle activity and airway patency. Thus, clinically relevant concentrations of propofol (1 and 5μM) were applied (4h) to neonatal rat brainstem slices to evaluate the expression of apoptosis-inducing factor (AIF) as biomarker of toxicity. This anaesthetic strongly increased AIF in the cytoplasm and the nucleus, without early loss of HMs. Electrophysiological recordings from HMs showed that propofol (5μM) enhanced GABA- and glycine-evoked current amplitude and lengthened GABAergic current decay time. Propofol also depressed NMDA receptor-mediated responses without affecting AMPA receptors. Since GABA and glycine depolarize neonatal HMs, we propose that the damaging action by propofol on these motoneurons might arise from the facilitated action of these transmitters with subsequent cytoplasmic Ca²⁺ overload. This phenomenon, in turn, may trigger cell death mechanisms manifested as increased expression of AIF and its translocation into the nucleus. Since propofol is also employed for induction and maintenance of paediatric surgery, caution is needed because its potential neurotoxicity might negatively impact neurodevelopment.

Delayed application of the anesthetic propofol contrasts the neurotoxic effects of kainate on rat organotypic spinal slice cultures

Excitotoxicity due to hyperactivation of glutamate receptors is thought to underlie acute spinal injury with subsequent strong deficit in spinal network function. Devising an efficacious protocol of neuroprotection to arrest excitotoxicity might, therefore, spare a substantial number of neurons and allow later recovery. In vitro preparations of the spinal cord enable detailed measurement of spinal damage evoked by the potent glutamate analogue kainate. Any clinically-relevant neuroprotective treatment should start after the initial lesion and spare networks for at least 24h when cell damage plateaus. Using this strategy, we have observed that the gas anesthetic methoxyflurane provided strong, delayed neuroprotection. It is unclear if this beneficial effect was due to the mechanism of action by methoxyflurane, or it was the consequence of anesthetic depression. To test this hypothesis, we investigated the effect by propofol (commonly injected i.v. for general anesthesia) after kainate excitotoxicity induced on organotypic spinal slices. At 5μM concentration, propofol significantly attenuated cell death, including neuronal losses and, especially, damage to the highly vulnerable motoneurons. The action by propofol was fully prevented when co-applied with the GABAA antagonist bicuculline, indicating that neuroprotection required intact GABAA receptor function. Although bicuculline per se was not neurotoxic, it largely enhanced the lesional effects of kainate, suggesting that GABAA receptor activity could limit excitotoxicity. Our data might offer an explanation for the beneficial clinical outcome of neurosurgery performed as soon as possible after spinal lesion: we posit that general anesthesia contributes to this outcome, regardless of the type of anesthetic used.