Reduction by sevoflurane of adenosine 5'-triphosphate-activated inward current of locus coeruleus neurons in pontine slices of rats

Brain areas modulation in consciousness during sevoflurane anesthesia

Sevoflurane is presently one of the most used inhaled anesthetics worldwide. However, the mechanisms through which sevoflurane acts and the areas of the brain associated with changes in consciousness during anesthesia remain important and complex research questions. Sevoflurane is generally regarded as a volatile anesthetic that blindly targets neuronal (and sometimes astrocyte) GABAA receptors. This review focuses on the brain areas of sevoflurane action and their relation to changes in consciousness during anesthesia. We cover 20 years of history, from the bench to the bedside, and include perspectives on functional magnetic resonance, electroencephalogram, and pharmacological experiments. We review the interactions and neurotransmitters involved in brain circuits during sevoflurane anesthesia, improving the effectiveness and accuracy of sevoflurane's future application and shedding light on the mechanisms behind human consciousness.

Effects of single-dose antipurinergic therapy on behavioral and molecular alterations in the valproic acid-induced animal model of autism

Autism spectrum disorder (ASD) is characterized by deficits in communication and social interaction, restricted interests, and stereotyped behavior. Environmental factors, such as prenatal exposure to valproic acid (VPA), may contribute to the increased risk of ASD. Since disturbed functioning of the purinergic signaling system has been associated with the onset of ASD and used as a potential therapeutic target for ASD in both clinical and preclinical studies, we analyzed the effects of suramin, a non-selective purinergic antagonist, on behavioral, molecular and immunological in an animal model of autism induced by prenatal exposure to VPA. Treatment with suramin (20 mg/kg, intraperitoneal) restored sociability in the three-chamber apparatus and decreased anxiety measured by elevated plus maze apparatus, but had no impact on decreased reciprocal social interactions or higher nociceptive threshold in VPA rats. Suramin treatment did not affect VPA-induced upregulation of P2X4 and P2Y2 receptor expression in the hippocampus, and P2X4 receptor expression in the medial prefrontal cortex, but normalized an increased level of interleukin 6 (IL-6). Our results suggest an important role of purinergic signaling modulation in behavioral, molecular, and immunological aberrations described in VPA model, and indicate that the purinergic signaling system might be a potential target for pharmacotherapy in preclinical studies of ASD.

Opportunities and Challenges for Single-Unit Recordings from Enteric Neurons in Awake Animals

Advanced electrode designs have made single-unit neural recordings commonplace in modern neuroscience research. However, single-unit resolution remains out of reach for the intrinsic neurons of the gastrointestinal system. Single-unit recordings of the enteric (gut) nervous system have been conducted in anesthetized animal models and excised tissue, but there is a large physiological gap between awake and anesthetized animals, particularly for the enteric nervous system. Here, we describe the opportunity for advancing enteric neuroscience offered by single-unit recording capabilities in awake animals. We highlight the primary challenges to microelectrodes in the gastrointestinal system including structural, physiological, and signal quality challenges, and we provide design criteria recommendations for enteric microelectrodes.

Anesthetic action of volatile anesthetics by using Paramecium as a model

Although empirically well understood in their clinical administration, volatile anesthetics are not yet well comprehended in their mechanism studies. A major conundrum emerging from these studies is that there is no validated model to assess the presumed candidate sites of the anesthetics. We undertook this study to test the hypothesis that the single-celled Paramecium could be anesthetized and served as a model organism in the study of anesthetics. We assessed the motion of Paramecium cells with Expert Vision system and the chemoresponse of Paramecium cells with T-maze assays in the presence of four different volatile anesthetics, including isoflurane, sevoflurane, enflurane and ether. Each of those volatiles was dissolved in buffers to give drug concentrations equal to 0.8, 1.0, and 1.2 EC50, respectively, in clinical practice. We could see that after application of volatile anesthetics, the swimming of the Paramecium cells was accelerated and then suppressed, or even stopped eventually, and the index of the chemoresponse of the Paramecium cells (denoted as I ( che )) was decreased. All of the above impacts were found in a concentration-dependent fashion. The biphasic effects of the clinical concentrations of volatile anesthetics on Paramecium simulated the situation of high species in anesthesia, and the inhibition of the chemoresponse also indicated anesthetized. In conclusion, the findings in our studies suggested that the single-celled Paramecium could be anesthetized with clinical concentrations of volatile anesthetics and therefore be utilized as a model organism to study the mechanisms of volatile anesthetics.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

The Effects of General Anesthetics on P2X7 and P2Y Receptors in a Rat Microglial Cell Line

BACKGROUND

Microglial cells play important roles in coordinating the inflammatory brain responses to hypoxia and trauma. Ionotropic P2X receptors and metabotropic P2Y receptors (P2YRs) expressed in microglia can be activated by extracellular adenosine triphosphate (ATP) derived from damaged cells or astrocytes, and participate in the signaling pathways evoked in brain insult. Although several inhaled and IV anesthetics produce neuroprotective effects through neuronal mechanisms, little is known about how general anesthetics modulate microglial responses in the pathological state. We examined the effects of various general anesthetics on purinergic responses in a rat microglial cell line.

METHODS

Currents were consistently activated by applications of ATP via a U-tube system under the whole-cell configuration. ATP-induced nondesensitizing currents observed after several applications of ATP exhibited characteristics of P2X7 receptors. The P2YRs-mediated mobilization of intracellular Ca2+ was measured using a Ca2+-sensitive fluorescent dye (fura-2).

RESULTS

Inhaled anesthetics (sevoflurane, isoflurane, and halothane) at doses three times as high as minimum alveolar concentrations had no effect on the P2X7Rs-mediated currents. IV anesthetics (ketamine, propofol, and thiopental) enhanced the P2X7Rs-mediated currents reversibly. The potencies for activation of P2X7Rs were not correlated with the octanol/buffer partition coefficients. Thiopental, at low concentrations, slightly inhibited the P2X7Rs-mediated currents, suggesting its dual actions on P2X7Rs. The P2YRs-mediated mobilization of intracellular Ca2+ was not affected by any of the general anesthetics tested.

CONCLUSIONS

Our results suggest that IV anesthetics, particularly thiopental and propofol, may modulate microglial functions through P2X7Rs in pathological conditions.

ATP- and adenosine-mediated signaling in the central nervous system: synaptic purinoceptors: the stage for ATP to play its "dual-role"

The studies aiming to understand the function of purinoceptors in the central nervous system (CNS), which has been explored mostly in isolated and cultured cell systems, are now at the stage of identifying their physiological and pathophysiological significance in the native organs, tissues, and whole animals. The results of our recent studies made in brain slice preparations are not in full accordance with what have been demonstrated in isolated cells, mostly due to strong interplay between ATP receptors, adenosine receptors, and ecto-nucleotidases. This suggests that these proteins form coordinated regulation systems in the native tissue, controlling the local network behaviors through regulating the balance between the effects of ATP and adenosine on synaptic transmissions. We propose that this tripartite regulation system by extracellular purines may be an important target of CNS drugs.

Effects of pentobarbital on purinergic P2X receptors of rat dorsal root ganglion neurons

Purinergic P2X receptors are ligand-gated ion channels that are activated by extracellular adenosine triphosphate (ATP) and are widely expressed not only in the central and peripheral nervous system but also in tissues throughout the body, playing an important role in the transfer of nociceptive information. Since the influence of barbiturates on P2X receptor subtypes is not known, we studied the effects of pentobarbital sodium (PB) on ATP responses in dorsal root ganglion (DRG) neurons. DRG neurons were dissected from 10- to 14-day-old rats and dissociated after enzyme treatment. Electrical measurements were performed using the nystatin-perforated patch recording mode under voltage-clamp conditions. Drugs were applied using the Y-tube method. ATP evoked three types of inward current at -60 mV: fast desensitizing, slow desensitizing, and mixed. The fast-type current was attributed to activation of P2X3 subtype and the slow type to the P2X2 subtype. PB suppressed the fast-type current in a concentration-dependent manner, while the slow type was slightly reduced. A noncompetitive inhibition was suggested by a downward shift of the ATP concentration-response curves. The current-voltage relationships showed inward rectification, and the extent of suppression was not affected by the holding potential. The reduction was greater in external solutions of higher pH. PB had subtype-specific effects on P2X receptors. The ionized form is likely to be responsible for the suppression of the P2X3 receptor current, which may result in a reduction of the excitability of central and peripheral neurons and may contribute to the anesthetic and analgesic actions of the agent.

Attenuation of Gap-Junction-Mediated Signaling Facilitated Anesthetic Effect of Sevoflurane in the Central Nervous System of Rats

Accumulating evidence suggests that reduction of intrinsic excitability or synaptic excitation and/or an enhancement of synaptic inhibition underlie the general anesthetic condition. Besides chemical synapse, neurons could communicate with each other by electrical coupling via gap-junctions. We hypothesized that inhibition of cell-to-cell signaling through gap-junction in the central nervous system (CNS) is involved in the anesthetic mechanism of volatile anesthetics. The minimum alveolar concentration (MAC) of sevoflurane was measured after the intracerebroventricular (ICV) or intrathecal (IT) administration of carbenoxolone (CBX), a gap-junction inhibitor, in vivo. The spontaneous oscillation in membrane currents of locus coeruleus neurons that results from electrical coupling between neurons was also recorded from young rat pontine slices by the patch clamp method, and the effect of sevoflurane on this oscillation was examined in vitro. The ICV administration of CBX (125 and 250 micro g/rat) significantly reduced the MAC of sevoflurane dose-dependently, whereas IT injection failed to inhibit the MAC. Sevoflurane at clinically relevant concentrations (0.1-0.5 mM) suppressed the spontaneous oscillation in membrane current concentration-dependently. These suppressions were significant at 0.5 mM with both amplitude and frequency. We suggest that suppression of gap-junction-mediated signaling in the CNS is involved in the anesthetic-induced immobilization by sevoflurane.

IMPLICATIONS

The intracerebroventricular administration of the gap-junction inhibitor, carbenoxolone, reduced the MAC of sevoflurane, and sevoflurane suppressed the signaling through gap-junctions in the central nervous system. The inhibition of gap-junctions may be one of the mechanisms and the site of action of sevoflurane.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

The effects of general anaesthetics on ligand-gated ion channels

The experimental effort that has been expended in investigating the effects of general anaesthetics on LGICs has been enormous over the past decade. Members of all three LGIC superfamilies have been examined using electrophysiological techniques. Anaesthetics that have been examined include volatile anaesthetics, gaseous anaesthetics, alcohols, i.v. anaesthetics and non-immobilizers. Obsolete anaesthetics (ether, cyclopropane, butane) have been used in order to increase the variability of the structure and polarity of experimental compounds. The tools of molecular biology have been used to make chimeric receptors and to make single-site mutations. Interestingly, this work has been taking place in parallel with efforts to understand the structure of these proteins. Anaesthetic research often stimulates structural research as well as vice versa. There are some common themes in the interactions between anaesthetics and the three superfamilies of LGICs. In many cases, anaesthetics have both inhibitory and potentiating effects on the channels. It is likely that the number of examples of this will increase when experiments are designed to look specifically for one or the other type of effect. So we must conclude that there are multiple binding sites for anaesthetics on LGICs. The degree of inhibition or potentiation is not easily predictable. In retrospect, this is not surprising when we consider that the sensitivity of a channel to anaesthetics can be altered by a single amino-acid mutation. The large structural differences between the cys-loop, glutamate-activated and P2X superfamilies do not lead to large differences in anaesthetic sensitivity. It is the smaller, almost insignificant, changes that do this. This observation that small changes may lead to large effects reinforces the idea that at least some of the interactions between anaesthetics and LGICs are direct drug-protein interactions that are not mediated by the lipids. This review has not addressed the question of whether the effects of anaesthetics seen on LGICs are relevant to anaesthesia. This question cannot really be answered at present. Although potent effects can be observed on the channels themselves, we have only begun to try to understand whether these effects are important for a synapse, a neuronal circuit or the function of an animal's nervous system. We have studied the trees; now we must go on to study the forest and the ecosystem.