Anesthetic actions in the hippocampal formation

Network and neuronal membrane properties in hybrid networks reciprocally regulate selectivity to rapid thalamocortical inputs

Rapidly changing environments require rapid processing from sensory inputs. Varying deflection velocities of a rodent's primary facial vibrissa cause varying temporal neuronal activity profiles within the ventral posteromedial thalamic nucleus. Local neuron populations in a single somatosensory layer 4 barrel transform sparsely coded input into a spike count based on the input's temporal profile. We investigate this transformation by creating a barrel-like hybrid network with whole cell recordings of in vitro neurons from a cortical slice preparation, embedding the biological neuron in the simulated network by presenting virtual synaptic conductances via a conductance clamp. Utilizing the hybrid network, we examine the reciprocal network properties (local excitatory and inhibitory synaptic convergence) and neuronal membrane properties (input resistance) by altering the barrel population response to diverse thalamic input. In the presence of local network input, neurons are more selective to thalamic input timing; this arises from strong feedforward inhibition. Strongly inhibitory (damping) network regimes are more selective to timing and less selective to the magnitude of input but require stronger initial input. Input selectivity relies heavily on the different membrane properties of excitatory and inhibitory neurons. When inhibitory and excitatory neurons had identical membrane properties, the sensitivity of in vitro neurons to temporal vs. magnitude features of input was substantially reduced. Increasing the mean leak conductance of the inhibitory cells decreased the network's temporal sensitivity, whereas increasing excitatory leak conductance enhanced magnitude sensitivity. Local network synapses are essential in shaping thalamic input, and differing membrane properties of functional classes reciprocally modulate this effect.

Duplicate preconditioning with sevoflurane in vitro improves neuroprotection in rat brain via activating the extracellular signal-regulated protein kinase

OBJECTIVE

Sevoflurane preconditioning has been demonstrated to reduce cerebral ischemia-reperfusion (IR) injury, but the underlying mechanisms have not been fully elucidated. Besides, different protocols would usually lead to different results. The objective of this study was to determine whether dual exposure to sevoflurane improves the effect of anesthetic preconditioning against oxygen and glucose deprivation (OGD) injury in vitro.

METHODS

Rat hippocampal slices under normoxic conditions (95% O2/5% CO2) were pre-exposed to sevoflurane 1, 2 and 3 minimum alveolar concentration (MAC) for 30 min, once or twice, with 15-min washout after each exposure. The slices were then subjected to 13-min OGD treatment (95% N2/5% CO2, glucose-free), followed by 30-min reoxygenation. The population spikes (PSs) were recorded in the CA1 region of rat hippocampus. The percentage of PS amplitude at the end of 30-min reoxygenation to that before OGD treatment was calculated, since it could indicate the recovery degree of neuronal function. In addition, to assess the role of mitogen-activated protein kinases (MAPKs) in preconditioning, U0126, an inhibitor of extracellular signal-regulated protein kinase (MEK-ERK1/2, ERK1/2 MAPK), and SB203580, an inhibitor of p38 MAPK, were separately added 10 min before sevoflurane exposure.

RESULTS

Preconditioning once with sevoflurane 1, 2, and 3 MAC increased the percentage of PS amplitude at the end of 30-min reoxygenation to that before OGD treatment, from (15.13+/-3.79)% (control) to (31.88+/-5.36)%, (44.00+/-5.01)%, and (49.50+/-6.25)%, respectively, and twice preconditioning with sevoflurane 1, 2, and 3 MAC increased the percentage to (38.53+/-4.36)%, (50.74+/-7.05)% and (55.86+/-6.23)%, respectively. The effect of duplicate preconditioning with sevoflurane 3 MAC was blocked by U0126 [(16.23+/-4.62)%].

CONCLUSION

Sevoflurane preconditioning can induce neuroprotection against OGD injury in vitro, and preconditioning twice enhances this effect. Besides, the activation of extracellular signal-regulated protein kinase (MEK-ERK1/2, ERK1/2 MAPK) may be involved in this process.

Using EEG to monitor anesthesia drug effects during surgery

The use of processed electroencephalography (EEG) using a simple frontal lead system has been made available for assessing the impact of anesthetic medications during surgery. This review discusses the basic principles behind these devices. The foundations of anesthesia monitoring rest on the observations of Guedel with ether that the depth of anesthesia relates to the cortical, brainstem and spinal effects of the anesthetic agents. Anesthesiologists strive to have a patient who is immobile, is unconscious, is hemodynamically stable and who has no intraoperative awareness or recall. These anesthetic management principles apply today, despite the absence of ether from the available anesthetic medications. The use of the EEG as a supplement to the usual monitoring techniques rests on the observation that anesthetic medications all alter the synaptic function which produces the EEG. Frontal EEG can be viewed as a surrogate for the drug effects on the entire central nervous system (CNS). Using mathematical processing techniques, commercial EEG devices create an index usually between 0 and 100 to characterize this drug effect. Critical aspects of memory formation occur in the frontal lobes making EEG monitoring in this area a possible method to assess risk of recall. Integration of processed EEG monitoring into anesthetic management is evolving and its ability to characterize all of the anesthetic effects on the CNS (in particular awareness and recall) and improve decision making is under study.

Intravenous anesthetics are more effective than volatile anesthetics on inhibitory pathways in rat hippocampal CA1

In this study, we have examined the effects of both volatile and IV general anesthetics on excitatory synaptic transmission, with and without recurrent inhibition, to clarify whether excitatory or inhibitory synapses are the major targets of action. Field population spike amplitudes (fPSs) of CA1 pyramidal neurons were recorded in rat hippocampal slices. Schaffer-collateral-commissural fibers (Sch) were stimulated orthodromically, and the evoked fPSs (PS[Sch]) in CA1 area were measured. In addition, the fPSs (PS[Alv+Sch]) elicited by stimulation of the Sch after antidromic stimulation of the alveus hippocampi (Alv) to produce recurrent inhibition were determined. It was observed that sevoflurane (0.5%-5%) and isoflurane (0.5%-5%) primarily inhibited PS[Sch] and also produced additive inhibition on the PS[Alv+Sch] in a concentration-dependent manner. The calculated 50% effective concentration (EC50) values for PS[Sch] and PS[Alv+Sch] were 5.3 vol% and 3.9 vol% (sevoflurane) and 1.7 vol% and 1.1 vol% (isoflurane), respectively. In comparison, thiopental (2.0 x 10(-5)-5.0 x 10(-4) mol/L) reduced both the PS[Sch] and PS[Alv+Sch] in a concentration-dependent manner. The calculated EC50 values for thiopental on PS[Sch] and PS[Alv+Sch] were 3.4 x 10(-4) and 5.7 x 10(-5) mol/L, respectively. Propofol (2.0 x 10(-5)-3.5 x 10(-4) mol/L) had little effect on the PS[Sch] but reduced PS[Alv+Sch] with a calculated EC(50) value of 5.1 x 10(-4) mol/L. The effects of the IV anesthetics with recurrent inhibition were antagonized in the presence of the gamma-aminobutyric acid-A-receptor antagonist bicuculline methiodide. In addition, all anesthetics prolonged recurrent inhibition from 100 ms (sevoflurane and isoflurane) to 400 ms (propofol). The results suggest that sevoflurane and isoflurane inhibit mainly on glutamate-mediated orthodromic pathways, whereas thiopental and propofol enhance gamma-aminobutyric acid-A-mediated recurrent inhibitory pathways in CA1 neurons, thus providing further evidence that the mechanisms of general anesthetics are drug- and pathway-specific.

General anesthetics attenuate gap junction coupling in P19 cell line

Gap junction communication is widespread throughout the mammalian nervous system among neurons as well as glia. We addressed the hypothesis that general anesthetics attenuate gap junction mediated coupling in P19 cell line that can differentiate into neuronal-like cells and astrocytes and oligodendrocytes. We characterized the extent of dye coupling over time in the P19 cell line using colocalization of chlormethylbenzamido-1,1 dioctadecyl-3,3,3',3'-tetramethylindocarbocyamine (CM-DiI) and calcein-AM in donor and recipient cells in cocultures. After seeding, the gap junction permeant dye calcein spreads from donor to recipient cells. CM-DiI and calcein fluorescence identified donor and recipient cells, respectively. The extent of intercellular connections was evaluated using cell counting and flow cytometry up to 2 hr after treatment. Clinically relevant concentrations of the intravenous anesthetics propofol (15 microM) and thiopental (10 microM) attenuated gap junction permeability in P19 cell cultures. In contrast, halothane, a volatile anesthetic in a concentration (0.64 mM) relevant to its free aqueous EC50 had no effect on gap junction coupling; however, very high halothane concentrations (2.8 mM) blocked dye transfer by approximately 90%. The results indicate that halothane concentrations pertinent to clinical anesthesia were unable to attenuate gap junction communication in a cell line that can express neuronal and glial gap junction proteins; however, clinically relevant concentrations of propofol and thiopental depressed gap junction coupling.

Increased sensitivity to halothane but decreased sensitivity to propofol in mice lacking the N-type Ca2+ channel

Volatile anesthetics are known to depress excitatory synaptic transmission. Inhibition of voltage-dependent Ca2+ channels is speculated to underlie this mechanism, which remains to be clarified in vivo. We examined the sensitivity to halothane in mice lacking the N-type Ca2+ channel, a major contributor of presynaptic neurotransmitter release. Sensitivity to halothane was significantly increased in the knockout mice compared with the wild-type littermates. Halothane also depressed field excitatory postsynaptic potentials recorded from the Schaffer collateral-CA1 hippocampal synapses more greatly in the knockout mice. We further examined sleep time induced by injection of propofol, an intravenous anesthetic that mainly affects inhibitory synaptic transmission. In contrast, sensitivity to propofol was significantly decreased in the knockout mice. We suggest that inhibition of the N-type Ca2+ channel underlies mechanisms of halothane anesthesia but counteracts propofol anesthesia.

Anesthetic sensitivities to propofol and halothane in mice lacking the R-type (Cav2.3) Ca2+ channel

Because inhibition of voltage-dependent Ca(2+) channels can be a mechanism underlying general anesthesia, we examined sensitivities to propofol and halothane in mice lacking the R-type (Ca(v)2.3) channel widely expressed in neurons. Sleep time after propofol injection (26 mg/kg IV) and halothane MAC(RR) and MAC (50% effective concentrations for the loss of the righting reflex and for the tail pinch/withdrawal response, respectively) were determined. Significantly shorter propofol-induced sleep time (291.6 +/- 16.8 s versus 344.4 +/- 12.1 s) and larger halothane MAC(RR) (1.11% +/- 0.04% versus 0.98% +/- 0.03%) were observed in Ca(v)2.3 channel knockouts (Ca(v)2.3(-/-)) than in wild-type (Ca(v)2.3(+/+)) litter mates. To investigate the basis of the decreased anesthetic sensitivities in vivo, field excitatory postsynaptic potentials and population spikes (PSs) were recorded from Schaffer collateral CA1 synapses in hippocampal slices. Propofol (10-30 micro M) inhibited PSs by potentiating gamma-aminobutyric acid-ergic inhibition, and this potentiation was markedly smaller at 30 micro M in Ca(v)2.3(-/-) mice, possibly accounting for the decreased propofol sensitivity in vivo. Halothane (1.4%-2.2%) inhibited field excitatory postsynaptic potentials similarly in both genotypes, whereas 1%-2% halothane depressed PSs more in Ca(v)2.3(-/-) mice, suggesting the postsynaptic role of the R-type channel in the propagation of excitation and other mechanisms underlying the increased halothane MAC(RR) in Ca(v)2.3(-/-) mice.

IMPLICATIONS

Because inhibition of neuronal Ca(2+) currents can be a mechanism underlying general anesthesia, we examined anesthetic sensitivities in mice lacking the R-type (Ca(v)2.3) Ca(2+) channels both in vivo and in hippocampal slices. Decreased sensitivities in mutant mice imply a possibility that agents blocking this channel may increase the requirements of anesthetics/hypnotics.

The effects of general anesthetics on excitatory and inhibitory synaptic transmission in area CA1 of the rat hippocampus in vitro

It is unclear whether general anesthetics induce enhancement of neural inhibition and/or attenuation of neural excitation. We studied the effects of pentobarbital (5 x 10(-4) mol/L), propofol (5 x 10(-4) mol/L), ketamine (10(-3) mol/L), halothane (1.5 vol%), and isoflurane (2.0 vol%) on both excitatory and inhibitory synaptic transmission in rat hippocampal slices. Excitatory or inhibitory synaptic pathways were isolated using pharmacological antagonists. Extracellular microelectrodes were used to record electrically evoked CA1 neural population spikes (PSs). In the presence of the gamma-aminobutyric acid type A (GABA(A)) receptor antagonist (bicuculline), the inhibitory actions of pentobarbital and propofol were completely antagonized, whereas those of ketamine, halothane, and isoflurane were only partially blocked. To induce the N-methyl-D-aspartate (NMDA) receptor-mediated PS (NMDA PS), the non-NMDA and GABA(A) receptors were blocked in the absence of Mg2+. Ketamine, halothane, and isoflurane decreased the NMDA PS, and pentobarbital and propofol had no effect on the NMDA PS. The non-NMDA receptor-mediated PS (non-NMDA PS) was examined using the antagonists for the NMDA and GABA(A) receptors. Volatile, but not i.v., anesthetics reduced the non-NMDA PS. These findings indicate that pentobarbital and propofol produce inhibitory actions due to enhancement in the GABA(A) receptor; that ketamine reduces NMDA receptor-mediated responses and enhances GABA(A) receptor-mediated responses; and that halothane and isoflurane modulate GABA(A), NMDA, and non-NMDA receptor-mediated synaptic transmission.

IMPLICATIONS

Volatile anesthetics modulate both excitatory and inhibitory synaptic transmission of in vitro rat hippocampal pathways, whereas i.v. anesthetics produce more specific actions on inhibitory synaptic events. These results provide further support the idea that general anesthetics produce drug-specific and distinctive effects on different pathways in the central nervous system.

The short-acting anesthetic propofol produces biphasic effects-depression and withdrawal rebound overshoot-on some (but not all) limbic evoked potentials in the behaving rat

Propofol, the relatively new, short-acting general anesthetic, markedly enhances the action of GABA at the GABAA receptor. To evaluate its effects on field potentials evoked in the dentate gyrus (DG) during the anesthetic and recovery periods, propofol was administered intraperitoneally to behaving rats bearing stimulating electrodes in the dorsal perforant path (DPP), where medial perforant path fibers predominate, and in the anterior piriform cortex (PC; i.e., olfactory cortex), and recording electrodes in the DG. Input from the PC reaches the DG via the lateral perforant path. Population slow waves (SWs) were evoked by paired-pulse stimulation of the PC at a 32 ms interstimulus interval (ISI) to produce paired-pulse facilitation in the awake animal. We had previously demonstrated that amplitude of SW2 (produced by the second stimulus) was greatly decreased by GABAergic drugs and increased by antiGABAergic convulsant agents. After administration of propofol, mean amplitude of SW2 decreased immediately and remained low for 30-60 min during propofol-induced sleep (as expected), then unexpectedly increased to about 1.5- to 2-fold above pretreatment levels at 2-4 h before gradually returning to pretreatment levels. In addition, the DPP was stimulated to produce either paired-pulse inhibition (20 ms ISI) or facilitation (32 ms ISI) of DG population spikes (PSs) in the awake animal. PS2 was much more inhibited during propofol-induced sleep, than during the pretreatment period, consistent with an expected marked increase in recurrent inhibition. An overshoot in PS2 amplitude was observed only occasionally during recovery, suggesting that withdrawal overshoot in amplitudes is more characteristic of PC-evoked DG SW2 potentials. The overshoot in SW2 amplitude during recovery may have been related to propofol's 'rapid on-rapid off' actions on the GABAA receptor, perhaps resulting in a phenomenon like the 'GABA withdrawal syndrome'. Such an effect, if true, may help explain the rare occurrence of seizures, especially during recovery, associated with its use clinically.

Unc-1: a stomatin homologue controls sensitivity to volatile anesthetics in Caenorhabditis elegans

To identify sites of action of volatile anesthetics, we are studying genes in a functional pathway that controls sensitivity to volatile anesthetics in the nematode Caenorhabditis elegans. The unc-1 gene occupies a central position in this pathway. Different alleles of unc-1 have unique effects on sensitivity to the different volatile anesthetics. UNC-1 shows extensive homology to human stomatin, an integral membrane protein thought to regulate an associated ion channel. We postulate that UNC-1 has a direct effect on anesthetic sensitivity in C. elegans and may represent a molecular target for volatile anesthetics.

Enhanced isoflurane suppression of excitatory synaptic transmission in the aged rat hippocampus

1. The effects of the volatile anaesthetic, isoflurane, were investigated on evoked dendritic field excitatory postsynaptic potentials (f.e.p.s.p.) and antidromic and orthodromic population spikes recorded extracellularly in the CA1 cell layer region in the in vitro hippocampal slice taken from young mature (2-3 months) and old (24-27 months) Fisher 344 rats. 2. Isoflurane depressed the f.e.p.s.ps and the orthodromically-evoked population spikes in both old and young hippocampi. However, the magnitude of the anaesthetic-induced depression was greater in slices taken from old rats compared to those taken from young rats during the application of different isoflurane concentrations (0.5-5%). 3. In the presence of the GABA(A) antagonist, bicuculline methiodide (15 microM), isoflurane suppressed the f.e.p.s.ps to the same extent as was observed in the absence of the GABA(A) antagonist. 4. Orthodromically evoked population spikes were suppressed by isoflurane in a manner quantitatively similar to the suppression of the f.e.p.s.ps. However, antidromic population spikes and presynaptic volleys evoked in young and old slices were resistant to anaesthetic action. In addition, paired pulse facilitation ratio of the evoked dendritic f.e.p.s.ps was not affected in both young and old slices during the application of isoflurane. 5. When slices were exposed to low Ca2+/high Mg2+ solution, isoflurane (1 and 3%) depressed the f.e.p.s.ps in aged slices to the same extent as in young slices. 6. The augmented anaesthetic depression of f.e.p.s.ps in old compared to young hippocampi in the absence and presence of bicuculline, and the lack of anaesthetic effects on antidromic population spikes and presynaptic volleys in old and young slices, suggest that the increased sensitivity of anaesthetic actions in old hippocampi is due to age-induced attenuation of synaptic excitation rather than potentiation of synaptic inhibition. Furthermore, elimination of the increased sensitivity of old slices to anaesthetic actions when the slices were perfused with low Ca2+/high Mg2+ medium, which presumably would decrease intracellular [Ca2+], suggests that the enhanced anaesthetic effects in aged neurones might be related to increased intraneuronal [Ca2+] in the synaptic terminal.

Halothane depresses action potential conduction in hippocampal axons

General anesthetics are thought to depress the central nervous system (CNS) by acting at synapses; however, only a few studies have compared effects on axonal conduction with effects on synaptic responses using mammalian CNS preparations. The present study used glutamate receptor antagonists (CNQX/APV) or low calcium to block synaptic transmission, allowing Schaffer-collateral axon fiber volleys to be recorded from rat hippocampal brain slices. Since fiber volleys are compound action potentials, they provide a measure of axonal conduction in Schaffer-collateral fibers. Clinical concentrations of the inhalational anesthetic, halothane (1 rat MAC, 1.2 vol.%), produced an 18 +/- 2.3% depression of fiber volley amplitudes (mean +/- S.D.; p < 0.001 ANOVA, n = 10). Depression of action potential conduction accounted for approximately 30% of the overall depression of synaptic transmission produced by halothane at this concentration. Halothane-induced fiber volley depression occurred with little change in conduction velocity, similar to the effect seen with decreased stimulus intensity, but significantly different from the decreased velocity produced by tetrodotoxin (100 nM, p < 0.005). The results indicate that halothane can depress axonal conduction at clinically relevant concentrations and that this depression could contribute to the CNS depression that is associated with anesthesia.

Intrathecally administered NMDA receptor antagonists reduce the MAC of isoflurane in rats

PURPOSE

We studied the effects of intrathecal administration of an N-methyl-D-aspartate (NMDA) receptor antagonist and an antagonist of the glycine site of the NMDA receptor on the minimum alveolar anaesthetic concentration (MAC) of isoflurane in rats, and on locomotor activity in conscious rats.

METHODS

In Wistar rats fitted with indwelling intrathecal catheters, we determined the MAC of isoflurane after the administration of saline (control group); the competitive NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosponic acid(CPP) at 0.01, 0.1, and 1.0 nM; and the selective antagonist of the glycine site on the NMDA receptor complex 7-chlorokynurenic acid (7CKA) at 0.1, 1.0, and 10 nM. After measurement of MAC following administration of the antagonist, the equipotent reversal dose of NMDA or D-serine was administered. The rats were examined for the presence of locomotor dysfunction by intrathecal administration of NMDA receptor antagonists, NMDA and D-serine in conscious rats. All of the experiments were performed using randomization and masking of drugs.

RESULTS

CPP at 0.1 and 1.0 nM decreased the MAC of isoflurane by 9.9-17.6% (P < 0.05). 7CKA at 1.0 and 10 nM reduced MAC from 10.5-15.5% (P < 0.05). Intrathecal administration of NMDA or D-serine reversed the decreases in MAC to control values. Locomotor activity was not changed.

CONCLUSIONS

We believe that NMDA receptor plays an important role in determining the MAC of isoflurane in the spinal cord.

The antinociceptive effect of S-(+)-ibuprofen in rabbits: epidural versus intravenous administration

This study was designed to determine whether systemic absorption plays any role in the antinociceptive effect of epidural (EP) sodium S(+)-ibuprofen (IB). One week after surgical implantation of EP catheters, six rabbits were given EP injections with either normal saline (NS) 0.4 mL or IB 10 mg in 0.4 mL NS (Group 1) on separate days. Each animal was injected with IB 10 mg intravenously (i.v.) on another day. Six control rabbits (Group 2) had neither surgery nor any injection. Analgesic testing was performed using electric stimulation through two electrocardiogram (ECG) skin electrodes with built-in adhesive, attached to shaved hip areas using 50 V, 1 Hz, 3 ms, before and 0.5,1,2 and 3 h after injection in Group 1, and in similar times in controls. The 95% confidence intervals (CI) of the mean difference between baseline and maximal nociceptive response latency of all groups were compared using analysis of covariance (ANCOVA) adjusted for baseline measurements. This comparison covered all possible pairs among all groups. Significant antinociceptive effects were seen after EP IB but not after control or i.v. IB. Neither motor dysfunction nor evidence of systemic toxicity or neurotoxicity was observed in any animal.