Correlates of anesthetic properties in isolated spinal cord: cyclobutanes

Spinal Reflexes and Windup In Vitro: Effects of Analgesics and Anesthetics

The spinal cord is the first relay center for nociceptive information. Following peripheral injury, the spinal cord sensitizes. A sign of spinal sensitization is the hyper-reflexia which develops shortly after injury and can be detected in the isolated spinal cord as a "memory of pain." In this context, it is easy to understand that many analgesic compounds target spinally located sites of action to attain analgesia. In vitro isolated spinal cord preparations have been used for a number of years, and experience on the effects of compounds of diverse pharmacological families on spinal function has accumulated. Recently, we have proposed that the detailed study of spinal segmental reflexes in vitro may produce data relevant to the evaluation of the analgesic potential of novel compounds. In this review, we describe the main features of segmental reflexes obtained in vitro and discuss the effects of compounds of diverse chemical nature and pharmacological properties on such reflexes. Our aim was to compare the different profiles of action of the compounds on segmental reflexes in order to extract clues that may be helpful for pharmacological characterization of novel analgesics.

Hydrocarbon molar water solubility predicts NMDA vs. GABAA receptor modulation

Background

Many anesthetics modulate 3-transmembrane (such as NMDA) and 4-transmembrane (such as GABA_A) receptors. Clinical and experimental anesthetics exhibiting receptor family specificity often have low water solubility. We hypothesized that the molar water solubility of a hydrocarbon could be used to predict receptor modulation in vitro.

Methods

GABA_A (α₁β₂γ_2s) or NMDA (NR1/NR2A) receptors were expressed in oocytes and studied using standard two-electrode voltage clamp techniques. Hydrocarbons from 14 different organic functional groups were studied at saturated concentrations, and compounds within each group differed only by the carbon number at the ω-position or within a saturated ring. An effect on GABA_A or NMDA receptors was defined as a 10% or greater reversible current change from baseline that was statistically different from zero.

Results

Hydrocarbon moieties potentiated GABA_A and inhibited NMDA receptor currents with at least some members from each functional group modulating both receptor types. A water solubility cut-off for NMDA receptors occurred at 1.1 mM with a 95% CI = 0.45 to 2.8 mM. NMDA receptor cut-off effects were not well correlated with hydrocarbon chain length or molecular volume. No cut-off was observed for GABA_A receptors within the solubility range of hydrocarbons studied.

Conclusions

Hydrocarbon modulation of NMDA receptor function exhibits a molar water solubility cut-off. Differences between unrelated receptor cut-off values suggest that the number, affinity, or efficacy of protein-hydrocarbon interactions at these sites likely differ.

Structural basis for high‐affinity volatile anesthetic binding in a natural 4‐helix bundle protein

Physiologic sites for inhaled anesthetics are presumed to be cavities within transmembrane 4-alpha-helix bundles of neurotransmitter receptors, but confirmation of binding and structural detail of such sites remains elusive. To provide such detail, we screened soluble proteins containing this structural motif, and found only one that exhibited evidence of strong anesthetic binding. Ferritin is a 24-mer of 4-alpha-helix bundles; both halothane and isoflurane bind with K(A) values of approximately 10(5) M(-1), higher than any previously reported inhaled anesthetic-protein interaction. The crystal structures of the halothane/apoferritin and isoflurane/apoferritin complexes were determined at 1.75 A resolution, revealing a common anesthetic binding pocket within an interhelical dimerization interface. The high affinity is explained by several weak polar contacts and an optimal host/guest packing relationship. Neither the acidic protons nor ether oxygen of the anesthetics contribute to the binding interaction. Compared with unliganded apoferritin, the anesthetic produced no detectable alteration of structure or B factors. The remarkably high affinity of the anesthetic/apoferritin complex implies greater selectivity of protein sites than previously thought, and suggests that direct protein actions may underlie effects at lower than surgical levels of anesthetic, including loss of awareness.

Inhaled anesthetics and immobility: mechanisms, mysteries, and minimum alveolar anesthetic concentration

Studies using molecular modeling, genetic engineering, neurophysiology/pharmacology, and whole animals have advanced our understanding of where and how inhaled anesthetics act to produce immobility (minimum alveolar anesthetic concentration; MAC) by actions on the spinal cord. Numerous ligand- and voltage-gated channels might plausibly mediate MAC, and specific amino acid sites in certain receptors present likely candidates for mediation. However, in vivo studies to date suggest that several channels or receptors may not be mediators (e.g., gamma-aminobutyric acid A, acetylcholine, potassium, 5-hydroxytryptamine-3, opioids, and alpha(2)-adrenergic), whereas other receptors/channels (e.g., glycine, N-methyl-D-aspartate, and sodium) remain credible candidates.

Short-term memory resists the depressant effect of the nonimmobilizer 1-2-dichlorohexafluorocyclobutane (2N) more than long-term memory

The nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N, also termed F6) does not suppress movement to noxious stimuli but does suppress learning of fear-potentiated startle. The mechanism whereby 2N suppresses this learning is unknown. Herein, we report the effect of 2N on suppression of two other forms of learning, fear conditioning to context and to tone. Because 2N does not cause sedation, we could study the effect of 2N on short-term memory (memory for fear conditioning measured during or immediately after training) as well as on long-term memory (measured 24 h after training). The EC(50) for suppression of long-term memory (the concentration decreasing memory by 50%) of fear conditioning to context was 2.00% plus/minus 0.01% (mean plus/minus SEM), and for fear conditioning to tone was 3.45% plus/minus 0.26%, (P < 0.05). The EC(50) for suppression of short-term memory of fear conditioning to context was 2.59% plus/minus 0.21% (P < 0.05, compared with long-term memory of context conditioning), whereas short-term memory of fear conditioning to tone was not suppressed by 3.5%, the largest concentration studied. Thus, short-term memory resists the depressant effect of 2N more than long-term memory, fear conditioning to tone is less vulnerable to the effect of 2N than fear conditioning to context, and 3.5% 2N does not preclude transmission of tone and shock signals to the site where tone-shock associations are formed.

IMPLICATIONS

The nonimmobilizer 1,2-dichlorohexafluorocyclobutane has a greater depressant effect on long-term memory than short-term memory, suggesting that it impairs the processes responsible for the retention of memory more than for the formation of memory itself.

Multiple specific binding targets for inhaled anesthetics in the mammalian brain

Previous work showed widespread saturable binding of halothane in rat brain. To determine whether this represents selective binding to a few widespread proteins or less selective binding to many different proteins, we used [(14)C]halothane photolabeling and quantitative electrophoresis/autoradiography in rat cerebellar homogenates. Many proteins incorporate label. Stoichiometry values ranged from 0 to 4 at 0.2 mM [(14)C]halothane in a group of 24 randomly selected protein bands. Apparent IC(50) values from unlabeled halothane competition experiments ranged from 0.2 to 2.0 mM, with soluble protein having significantly lower values (higher affinity) than membrane protein. Chloroform inhibited halothane labeling similar to unlabeled halothane but with higher apparent IC(50) values, whereas isoflurane and an anesthetic, cyclobutane (1-chloro-1,2,2-trifluorocyclobutane), inhibited halothane labeling to a smaller degree. A nonanesthetic, cyclobutane (1,2-dichlorohexafluorocyclobutane), inhibited halothane labeling the least. We conclude that halothane binding motifs are sufficiently degenerate to be found in many proteins, both soluble and membrane-bound.

Antinociceptive properties of neurosteroids IV: pilot study demonstrating the analgesic effects of alphadolone administered orally to humans

Fourteen patients scheduled for orthopaedic knee reconstruction surgery were enrolled in a prospective, double-blind, randomized study in which they received alphadolone (25-500 mg, n = 9) or placebo (lactose, n = 5) given orally 1 h after operation. All the subjects received a standardized general anaesthetic and the same type of surgery followed by physiotherapy using a continuous passive movement machine. Morphine was administered intravenously after operation by patient-controlled analgesia. Verbal rating and visual analogue scores assessed pain experiences for 6 h. Orally administered alphadolone up to 500 mg caused no increase in sedation, respiratory depression, nausea or vomiting. The experiences of these side-effects were all rated as none, mild or moderate. Orally administered alphadolone caused statistically significant reductions in morphine use and simultaneous highly significant reductions in pain scores. We conclude that alphadolone is a useful analgesic in humans when given by the oral route.

Antinociceptive properties of neurosteroids I. Spinally-mediated antinociceptive effects of water-soluble aminosteroids

Four water-soluble aminosteroid intravenous anaesthetic agents (ORG 20380, 20549, 21047 and 20599) were investigated for antinociceptive properties following intrathecal injection in rats. Two compounds, ORG 20380 and 20549, produced spinally-mediated antinociception assessed by tail flick and electrical current nociceptive tests. These effects were dose-related and suppressed by concurrent administration of the GABA(A) receptor antagonist, bicuculline. ORG 21047 and 20599 caused no antinociceptive effects when given intrathecally. Experiments in which nociceptive thresholds were measured after intravenous injections of ORG 20549 showed that subanaesthetic doses of this compound caused antinociceptive effects revealed by both nociceptive tests. This was equal in magnitude to that obtained with intrathecal administration of the same drug. We conclude that ORG 20380 and 20549 produce spinally-mediated antinociception by combination with spinal cord GABA(A) receptors. These spinal receptors are different from the GABA(A) receptors responsible for the anaesthetic effects of these drugs.

Halothane binding to a G protein coupled receptor in retinal membranes by photoaffinity labeling

General anesthetics have been reported to alter the functions of G protein coupled receptor (GPCR) signaling systems. To determine whether these effects might be mediated by direct binding interactions with the GPCR or its associated G protein, we studied the binding character of halothane on mammalian rhodopsin, structurally the best understood GPCR, by using direct photoaffinity labeling with [(14)C]halothane. In the bleached bovine rod disk membranes (RDM), opsin and membrane lipids were dominantly photolabeled with [(14)C]halothane, but none of the three G protein subunits were labeled. In opsin itself, halothane labeling was inhibited by unlabeled halothane with an IC(50) of 0.9 mM and a Hill coefficient of -0.8. The stoichiometry was 1.1:1.0 (halothane:opsin molar ratio). The IC(50) values of isoflurane and 1-chloro-1,2, 2-trifluorocyclobutane were 5.0 and 15 mM, respectively. Ethanol had no effect on opsin labeling by halothane. A nonimmobilizer, 1, 2-dichlorohexafluorocyclobutane, inhibited halothane labeling by 50% at 0.05 mM. The present results demonstrate that halothane binds specifically and selectively to GPCRs in the RDM. The absence of halothane binding to any of the G protein subunits strongly suggests that the functional effects of halothane on GPCR signaling systems are mediated by direct interactions with receptor proteins.

Distinctly different interactions of anesthetic and nonimmobilizer with transmembrane channel peptides

Although it plays no clinical role in general anesthesia, gramicidin A, a transmembrane channel peptide, provides an excellent model for studying the specific interaction between volatile anesthetics and membrane proteins at the molecular level. We show here that a pair of structurally similar volatile anesthetic and nonimmobilizer (nonanesthetic), 1-chloro-1,2,2-trifluorocyclobutane (F3) and 1, 2-dichlorohexafluorocyclobutane (F6), respectively, interacts differently with the transmembrane peptide. With 400 microM gramicidin A in a vesicle suspension of 60 mM phosphatidylcholine-phosphatidylglycerol (PC/PG), the intermolecular cross-relaxation rate constants between (19)F of F3 and (1)H in the chemical shift regions for the indole and backbone amide protons were 0.0106 +/- 0.0007 (n = 12) and 0.0105 +/- 0.0014 (n = 8) s(-1), respectively. No cross-relaxation was measurable between (19)F of F6 and protons in these regions. Sodium transport study showed that with 75 microM gramicidin A in a vesicle suspension of 66 mM PC/PG, F3 increased the (23)Na apparent efflux rate constant from 149.7 +/- 7.2 of control (n = 3) to 191.7 +/- 12.2 s(-1) (n = 3), and the apparent influx rate constant from 182.1 +/- 15.4 to 222.8 +/- 21.7 s(-1) (n = 3). In contrast, F6 had no effects on either influx or efflux rate. It is concluded that the ability of general anesthetics to interact with amphipathic residues near the peptide-lipid-water interface and the inability of nonimmobilizer to do the same may represent some characteristics of anesthetic-protein interaction that are of importance to general anesthesia.

Structural consequences of anesthetic and nonimmobilizer interaction with gramicidin A channels

Although interactions of general anesthetics with soluble proteins have been studied, the specific interactions with membrane bound-proteins that characterize general anesthesia are largely unknown. The structural modulations of anesthetic interactions with synaptic ion channels have not been elucidated. Using gramicidin A as a simplified model for transmembrane ion channels, we have recently demonstrated that a pair of structurally similar volatile anesthetic and nonimmobilizer, 1-chloro-1,2,2-trifluorocyclobutane (F3) and 1,2-dichlorohexafluorocyclobutane (F6), respectively, have distinctly different effects on the channel function. Using high-resolution NMR structural analysis, we show here that neither F3 nor F6 at pharmacologically relevant concentrations can significantly affect the secondary structure of the gramicidin A channel. Although both the anesthetic F3 and the nonimmobilizer F6 can perturb residues at the middle section of the channel deep inside the hydrophobic region in the sodium dodecyl sulfate micelles, only F3, but not F6, can significantly alter the chemical shifts of the tryptophan indole N-H protons near the channel entrances. The results are consistent with the notion that anesthetics cause functional change of the channel by interacting with the amphipathic domains at the peptide-lipid-water interface.

Nonimmobilizers and transitional compounds may produce convulsions by two mechanisms

Some inhaled compounds cause convulsions. To better appreciate the physical basis for this property, we correlated the partial pressures that produced convulsions in rats with the lipophilicity (nonpolarity) and hydrophilicity (polarity) of 45 compounds: 3 n-alkanes, 18 n-haloalkanes, 3 halogenated aromatic compounds, 3 cycloalkanes and 3 halocycloalkanes, 13 halogenated ethers, and 2 noble gases (He and Ne). In most cases, convulsions were quantified by averaging the alveolar partial pressures just below the pressures that caused and slightly higher pressures that did cause clonic convulsions (ED50). The ED50 did not correlate with hydrophilicity (the saline/gas partition coefficient), nor was there an obvious correlation with molecular structure. For 80% of compounds (36 of 45), the ED50 correlated closely (r2 = 0.99) with lipophilicity (the olive oil/gas partition coefficient). Perhaps because they block the effect of GABA on GABA(A) receptors, five compounds were more potent than would be predicted from their lipophilicity. Conversely, four compounds may have been less potent than would be predicted because they (like conventional inhaled anesthetics) enhance the effect of GABA on GABA(A) receptors.

IMPLICATIONS

Nonimmobilizers and transitional compounds may produce convulsions by two mechanisms. One correlates with lipophilicity (nonpolarity), and the other correlates with an action on GABA(A) receptors.

Desflurane and the nonimmobilizer 1,2-dichlorohexafluorocyclobutane suppress learning by a mechanism independent of the level of unconditioned stimulation

We previously demonstrated that anesthetics and non-immobilizers suppress learning and memory in rats. In the training portion of the test, rats received a light plus a footshock and learned to associate the two, as evidenced by subsequent potentiation of the response (jumping) to light plus a noise (fear-potentiated startle). However, anesthetics and nonimmobilizers also decreased the response of animals receiving footshocks during training, which suggests that the reduction in fear-potentiated startle might reflect analgesia, rather than an impairment of learning and memory. Furthermore, although we previously demonstrated that the nonimmobilizer 2,3-dichlorohexafluorocyclobutane (2N) could completely abolish learning, we did not demonstrate the minimal dose required. In the present study, we eliminated analgesia as a confounding factor by training rats breathing desflurane and 2N with footshock intensities that produced responses at least equal to those produced in control animals. Both desflurane and 2N suppressed learning at 0.2 times the minimum alveolar anesthetic concentration (MAC) or the MAC predicted from lipid solubility, despite the increased footshock intensity. This partial pressure of desflurane equals that previously shown to suppress learning at lower footshock intensities. We conclude that suppression of learning and memory by desflurane and 2N does not result from decreased sensitivity to the unconditioned stimulus (the footshock) and that the potency of 2N is consistent with its lipophilicity.

IMPLICATIONS

General anesthesia eliminates recall of intraoperative events, including pain. Using an animal model, we refuted the hypothesis that lack of recall results from the analgesia (i.e., the reduced response to painful stimuli produced by inhaled drugs) rather than from a direct effect on learning.

Desflurane and nitrous oxide, but not nonimmobilizers, affect nociceptive responses

Nonimmobilizers (previously called nonanesthetics) do not prevent movement in response to a noxious stimulus, even at doses predicted to produce anesthesia. We hypothesized they would also lack antinociceptive effects. We tested this prediction using the tail-flick latency (TFL) test. As predicted, the two nonimmobilizers tested (1,2-dichlorohexafluorocyclobutane and perfluoropentane) did not alter TFL, whereas desflurane and nitrous oxide both lengthened TFL (nitrous oxide at a lower minimum alveolar anesthetic concentration [MAC]-multiple than desflurane). In addition, we found that 0.1 MAC desflurane had a hyperalgesic effect (shortened TFL).

IMPLICATIONS

We studied the response of animals inhaling anesthetics or nonimmobilizers (compounds predicted to be anesthetics from the Meyer-Overton relation) to painful stimuli. Nonimmobilizers had no effect on these responses; at a low partial pressure, desflurane was hyperalgesic; nitrous oxide and, at higher partial pressures, desflurane were antinociceptive.

Comparison of anaesthetic and non-anaesthetic effects on depolarization-evoked glutamate and GABA release from mouse cerebrocortical slices

1. Investigation with substances that are similar in structure, but different in anaesthetic properties, may lead to further understanding of the mechanisms of general anaesthesia. 2. We have studied the effects of two cyclobutane derivatives, the anaesthetic, 1-chloro-1,2,2-trifluorocyclobutane (F3), and the non-anaesthetic, 1,2-dichlorohexafluorocyclobutane (F6), on K+-evoked glutamate and gamma-aminobutyric acid (GABA) release from isolated, superfused, cerebrocortical slices from mice, by use of h.p.l.c. with fluorescence detection for quantitative analysis. 3. At clinically relevant concentrations, the anaesthetic, F3, inhibited 40 mM K+-evoked glutamate and GABA release by 72% and 47%, respectively, whereas the structurally similar non-anaesthetic, F6, suppressed evoked glutamate release by 70% but had no significant effects on evoked GABA release. A second exposure to 40 mM KCl after a approximately 30 min washout of F3 or F6 showed recovery of K+-evoked release, suggesting that F3 and F6 did not cause any non-specific or irreversible changes in the brain slices. 4. Our findings suggest that suppression of excitatory neurotransmitter release may not be directly relevant to the primary action of general anaesthetics. A mechanism involving inhibitory postsynaptic action is implicated, in which a moderate suppression of depolarization-evoked GABA release by the anaesthetic may be consistent with the enhancement of postsynaptic GABAergic activities.

Ventilatory effects of the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N) in swine

Nonimmobilizers (inhaled compounds that do not suppress movement in response to a noxious stimulus) resemble anesthetics in their capacity to suppress memory, but unlike anesthetics, they can cause convulsions. Higher concentrations of nonimmobilizers may cause death, even with apparent suppression of convulsions by the concurrent administration of conventional inhaled anesthetics. We hypothesized that nonimmobilizers can depress ventilation and can cause death by adding to the depression of ventilation produced by conventional anesthetics. To test these hypotheses, we administered 1,2-dichlorohexafluorocyclobutane (2N) to four pigs anesthetized with desflurane. The addition of 2N decreased PaCO2 and tended to increase the slope of the ventilatory response to imposed increases in PETCO2. Limited results from study of two other nonimmobilizers (2,3-dichlorooctafluorobutane and perfluoropentane), in two pigs each, were consistent with the findings for 2N. However, experimental limitations (e.g., toxicity of 2,3-dichlorooctafluorobutane, and hypoxia from perfluoropentane) confound interpretation of these latter results. Our findings do not support our hypotheses--2N (and presumably all nonimmobilizers) seems to be a respiratory stimulant, not a depressant.

IMPLICATIONS

A new class of inhaled compounds, nonimmobilizers, allow tests of how inhaled anesthetics act. Nonimmobilizers may act like anesthetics (e.g., impair learning) or may not (e.g., do not prevent movement in response to a noxious stimulus). The present work shows that, unlike anesthetics,nonimmobilizers do not depress breathing.

Anesthetic actions within the spinal cord: contributions to the state of general anesthesia

The behavioral state known as general anesthesia is the result of actions of general anesthetic agents at multiple sites within the neuraxis. The most common end point used to measure the presence of anesthesia is absence of movement following the presentation of a noxious stimulus. The actions of general anesthetics within the spinal cord have been shown to contribute significantly to the suppression of pain-evoked movements, an important component of clinical anesthesia. Studies in the spinal cord are likely to increase our understanding of the pharmacology by which general anesthetics alter the transmission of somatomotor information. It now appears that the pharmacology responsible for the production of anesthesia is agent- and site-selective, and not the result of a unitary mechanism of action.