The gamma-subunit governs the susceptibility of recombinant gamma-aminobutyric acid type A receptors to block by the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (F6, 2N)

To identify anesthetic effects that produce the different components of the complex anesthetic state, the so-called nonanesthetics/nonimmobilizer classes of compounds have been introduced. Because ionotropic gamma-aminobutyric acid type A (GABA(A)) receptors play an important role in the mediation of the central nervous system (CNS) effects of general anesthetics, and their susceptibility to modulation by various drugs depends on subunit composition, we have compared the effect of the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (F6) on GABA(A) receptors expressed in human embryonic kidney 293 cells transfected with alpha1beta2 versus alpha1beta2gamma2s subunits. Using rapid perfusion and whole-cell recording techniques, we found that, like isoflurane, F6 blocked GABA-induced currents through alpha1beta2 receptors but, unlike isoflurane, the presence of the gamma2s subunit conferred complete resistance to block by F6. Also, in contrast to isoflurane, F6 had no effect on deactivation kinetics of GABA-induced currents in either type of receptor. We conclude that modulation of alphabetagamma receptors plays little or no role in the actions of F6, but the block of alphabeta receptors may contribute to its effects on the CNS. IMPLICATIONS: Gamma-aminobutyric acidA receptors are the target of numerous drugs affecting the central nervous system. The subunit composition of the GABAA receptors governs their interaction with many drugs. We investigated whether the gamma-subunit influences the interaction with the nonimmobilizer F6.     

The differential effects of the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (F6, 2N) and isoflurane on extrasynaptic gamma-aminobutyric acid A receptors

The nonimmobilizer 1,2-dichlorohexafluorocyclobutane (F6; also known as 2N) causes amnesia and seizures at concentrations less than and more than, respectively, than that predicted to cause immobility (MACpred). The molecular and cellular basis of these effects is not known. We reported previously that F6 has no effect on synaptic gamma aminobutyric acid (GABA)A receptors located on the somata of hippocampal pyramidal cells. However, in hippocampal neurons, GABAA receptors that are located subsynaptically have different pharmacologic properties from those at extrasynaptic sites, and these classes of receptors may serve different physiologic functions. Therefore, we investigated the effects of F6 and isoflurane on currents mediated predominantly by extrasynaptic GABAA receptors harvested from hippocampal neurons by exposing nucleated excised patches to brief, high-concentration pulses of GABA. We found that extrasynaptic GABAA receptors in the majority of neurons located in the pyramidal cell layer are insensitive to F6 at concentrations up to 110 microM, although receptors harvested from one putative interneuron were potently inhibited by 43 microM of F6. By contrast, isoflurane consistently reduced the peak amplitude and slowed deactivation of currents mediated by extrasynaptic receptors, similar to its effect on synaptic receptors. These results demonstrate the selective sensitivity of extrasynaptic GABAA receptors on pyramidal neurons to isoflurane but not F6.     

Glial cells of the oligodendrocyte lineage express proton-activated Na+ channels

Neurons and oligodendrocytes, but not type I astrocytes and Schwann cells, generate large Na+ currents in response to a step increase of [H+]. Proton-activated Na+ channels are the first cationic channels expressed in neuronal precursor cells from the mammalian brain. Glial precursor cells cultured from mouse brain are also capable of generating Na+ currents in response to step acidification (INa(H]. With further development along the oligodendrocyte lineage, this property is retained, whereas voltage-activated Na+ and K+ currents disappear. Comparing INa(H) of oligodendrocytes with INa(H) of their precursor cells did not reveal a difference in current amplitude, suggesting a higher density of INa(H) channels on the (smaller) precursor cells. The properties of INa(H) in glial precursor cells and oligodendrocytes are similar to those of neurons, with respect to activation conditions, time course, and the effect of extracellular Ca2+ concentrations. The results are consistent with previous observations which showed that oligodendrocytes partially preserve their chemically activated, but completely lose their voltage-activated, ion channels.     

Halothane Blocks Synaptic Excitation of Inhibitory Interneurons

Background: Activation of principal hippocampal neurons is controlled by feedforward and feedback inhibition mediated by gamma-aminobutyric acidergic interneurons. The effects of halothane on glutamate receptor-mediated synaptic excitation of inhibitory interneurons have not been reported yet.

Methods: The effects of halothane on glutamatergic excitatory postsynaptic currents and on spike threshold in visually identified interneurons were studied with tight-seal, whole-cell voltage- and current-clamp recordings in thin slices from adult mouse hippocampus. The excitatory postsynaptic currents were pharmacologically isolated into their N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated components using selective antagonists.

Results: Halothane (0.37-2.78 mM) reversibly blocked non-N-methyl-D-aspartate and N-methyl-D-aspartate excitatory postsynaptic currents in hippocampal oriens-alveus interneurons. Half-maximal inhibition was observed at similar concentrations (0.59 mM and 0.50 mM, respectively). Halothane inhibited synaptically generated action potentials at concentrations that did not elevate the spike threshold.     

Between scylla and charybdis: a bleomycin-exposed patient with Cohen syndrome

Chemotherapy with bleomycin may cause a syndrome of pulmonary sensitivity to supranormal inspired oxygen concentrations that persists for an unknown period of time after exposure. We present a mentally retarded adolescent patient in whom supranormal inspired oxygen was temporarily necessary to manage her difficult airway. Subsequently her pulmonary function deteriorated acutely and, after intermittent stabilization, irreversibly. In this case, bleomycin exposure may have played a pivotal role in modulating minor insults to trigger fatal acute respiratory distress syndrome (ARDS).     

Differential uptake of volatile agents into brain tissue in vitro. Measurement and application of a diffusion model to determine concentration profiles in brain slices

BACKGROUND: The rate of onset of drug actions in experiments with brain slices in vitro can vary widely. One factor that influences the rate is access to tissue sites of action. To study the effects of the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (F6, also termed 2N in the literature) on physiologic processes under defined tissue concentrations, the authors performed electrophysiologic measurements of the effects of F6 and halothane, measured the uptake of these agents into brain tissue, and performed computational modeling to determine concentration-depth profiles during drug application. METHODS: Hippocampal brain slices 500 microm thick were prepared from adult rats. Evoked population responses in the CA1 region were obtained using extracellular recordings and electrical stimulation of the Schaffer collateral pathway. F6 (24 microm) and halothane (270 microm) were applied via superfusion for 40 min. Uptake of drug into tissue slices was measured using gas chromatography. Computational modeling was used to obtain estimates of drug diffusion coefficients in brain tissue and to calculate tissue concentration as a function of time and depth during drug application. RESULTS: Halothane reduced the amplitude of the evoked population spike and reduced the population excitatory postsynaptic potential slope. F6 had no effect on either measure. Uptake experiments yielded a diffusion coefficient of 0.1 x 10-6 cm2/s for F6 and 0.8 x 10-6 cm2/s for halothane. After 40 min of drug application, the concentration reached at tissue depths from which physiologic signals were obtained, approximately the top 200 microm of the slice, was estimated to be 58% of the final equilibrium value for F6 and 93% for halothane. CONCLUSIONS: Diffusion into tissue is substantially slower for F6 than for halothane, and its impact is great enough that this must be considered when designing or interpreting in vitro experiments. However, impaired access does not account for the lack of effect of F6 on electrophysiologic responses in rat hippocampal slices.     

Is anesthesia caused by potentiation of synaptic or intrinsic inhibition? Recent insights into the mechanisms of volatile anesthetics

Volatile anesthetics modulate synaptic (GABAA receptor-mediated) and intrinsic (K+ channel-controlled) neuronal inhibition. GABAA receptor activity is enhanced, leading to increased charge transfer and prolonged synaptic inhibition, and members of the two pore domain family of potassium channels are activated, leading to neuronal hyperpolarization and reduced excitability. These effects may underlie different components of the complex anesthetic state.     

Sea urchin puncture resulting in PIP joint synovial arthritis: case report and MRI study

Of the 600 species of sea urchins, approximately 80 may be venomous to humans. The long spined or black sea urchin, Diadema setosum may cause damage by the breaking off of its brittle spines after they penetrate the skin. Synovitis followed by arthritis may be an unusual but apparently not a rare sequel to such injury, when implantation occurs near a joint. In this case report, osseous changes were not seen by plain x-rays. Magnetic resonance imaging (MRI) was used to expose the more salient features of both soft tissue and bone changes of black sea urchin puncture injury 30 months after penetration. In all likelihood, this type of injury may be more common than the existing literature at present suggests. It is believed to be the first reported case in this part of the world as well as the first MRI study describing this type of joint pathology. Local and systemic reactions to puncture injuries from sea urchin spines have been described previously. These may range from mild, local irritation lasting a few days to granuloma formation, infection and on occasions systemic illness. The sea urchin spines are composed of calcium carbonate with proteinaceous covering. The covering tends to cause immune reactions of variable presentation. There are only a handful of reported cases with sea urchin stings on record, none of them from the Red Sea. However, this condition is probably more common than is thought and can present difficulty in diagnosis. In this case report, the inflammation responded well to heat treatment, mobilization and manipulation of the joint in its post acute and chronic stages. As some subtle changes in soft tissues and the changes in bone were not seen either on plain x-rays or ultrasound scan, gadolinium-enhanced MRI was used to unveil the marked changes in the joint.