A hypothetical model on the mechanism of anesthesia

Correlating in vivo anaesthetic effects with ex vivo receptor density data supports a GABAergic mechanism of action for propofol, but not for isoflurane

If the in vivo effects of anaesthesia are mediated through a specific receptor system, then a relationship could exist between the regional changes in brain metabolism caused by a particular agent and the underlying regional distribution of the specific receptors affected by that agent. Positron emission tomography data from volunteers studied while unconscious during propofol (n=8) or isoflurane (n=5) anaesthesia were used retrospectively to explore for evidence of relationships between regional anaesthetic effects on brain glucose metabolism and known (ex vivo) regional distribution patterns of human receptor binding sites. The regional metabolic reductions caused by propofol differed significantly from those of isoflurane. Propofol's reductions negatively correlated most significantly with the regional distribution of [3H]diazepam and [3H]flunitrazepam (benzodiazepine) binding site densities (r=-0.86, P<0.0005; r=-0.79, P<0.005, respectively) and less strongly with [3H]naloxone (opioid) binding density (r=-0.69, P<0.05). Isoflurane's reductions positively correlated only with muscarinic (acetylcholine) binding density (r=0.85, P<0.05). These findings are consistent with the hypothesis that some of propofol's in vivo anaesthetic effects may be mediated through a GABAergic mechanism and suggest some of isoflurane's in vivo effects might involve antagonism of central acetylcholine functioning.

[Acute neurotoxic organophosphate poisoning: insecticides and chemical weapons]

OBJECTIVE

To review clinical and therapeutic bases of an organophosphate poisoning, either with insecticide or nerve agent.

DATA SOURCES

References were obtained from computerized bibliographic research (Medline), from personal data (academic memoir, documents under approbation of the National Defense Office), from Internet's data.

DATA SYNTHESIS

Generally, organophosphate poisoning occurs during accidental exposure with agricultural insecticide or suicide. The effects of organophosphate compounds are due to the inhibition of the enzyme acetylcholinesterase. The intoxication symptoms can be divided into muscarine-like, nicotine-like effects, effects on the central nervous system and symptoms related to the dysfunction of the neuromuscular junction. The interest of biological acetylcholinesterase's measuring is minimal because it is weakly specific or sensitive. The immediate severity is due to hypoxia. Respiratory failure results from the lack of central drive inflated with excessive bronchial secretions, bronchospasm and respiratory muscles paralysis. The secondary complications are early myopathies whose gravity is correlated with the decrease of acetylcholinesterases, or later neuropathies induced by a different mechanism. Beside the symptomatic measures, atropine is the specific anticholinergic treatment. When promptly used, oximes can regenerate cholinesterases. The attempted effects of the treatment are mouth dryness, pupilar dilatation and flushing of the skin. Nerve agents are lethal toxics which have a short onset time and produce severe neurological pathology. In a terrorist incident, it is as important to identify rapidly the toxic agent and provide emergency decontamination as to manage medical care. An effective response must be multidisciplinary, involving clinicians, toxicologists, Emergency Medical Service and public's health personnel.

Toward a unified theory of narcosis: brain imaging evidence for a thalamocortical switch as the neurophysiologic basis of anesthetic-induced unconsciousness

A unifying theory of general anesthetic-induced unconsciousness must explain the common mechanism through which various anesthetic agents produce unconsciousness. Functional-brain-imaging data obtained from 11 volunteers during general anesthesia showed specific suppression of regional thalamic and midbrain reticular formation activity across two different commonly used volatile agents. These findings are discussed in relation to findings from sleep neurophysiology and the implications of this work for consciousness research. It is hypothesized that the essential common neurophysiologic mechanism underlying anesthetic-induced unconsciousness is, as with sleep-induced unconsciousness, a hyperpolarization block of thalamocortical neurons. A model of anesthetic-induced unconsciousness is introduced to explain how the plethora of effects anesthetics have on cellular functioning ultimately all converge on a single neuroanatomic/neurophysiologic system, thus providing for a unitary physiologic theory of narcosis related to consciousness.

Diaspirin crosslinked hemoglobin (DCLHb) does not affect the anesthetic potency of isoflurane in rats

Hemoglobin solutions are being developed as oxygen carrying fluids for multiple clinical indications. Despite an early report of accentuation of ether anesthesia, the effect of hemoglobin on anesthetic potency has not been assessed. We assessed the effect of alpha-alpha diaspirin crosslinked hemoglobin (DCLHb) on the anesthetic requirement of isoflurane necessary to keep rats unresponsive to noxious stimuli (1.0 MAC [minimum alveolar concentration]). During isoflurane administration, each rat received one of the following fluid regimens: 44Hct/N-normal hematocrit and volume; 44Hct/H-8.0 ml of donor blood given as a hypervolemic bolus; 30Hct/H-5.0 ml of DCLHb given as an exchange transfusion and 8.0 ml as a hypervolemic bolus; or 16Hct/H-15.0 ml of DCLHb given as an exchange transfusion and 8.0 ml as a hypervolemic bolus. MAC was determined using a standard tail clamp technique. The isoflurane requirement to achieve 1.0 MAC was not different between the four groups. These results are consistent with a hypothesis that DCLHb does not change the anesthetic state.

Ligand-dependent effects of ethanol and diethylether at brain benzodiazepine receptors

The GABAA receptor chloride channel complex interacts with various categories of sedatives, including the benzodiazepines, and possibly ethanol and volatile general anesthetics. Thus, specific binding of tritiated derivatives of a benzodiazepine antagonist, flumazenil, and an agonist, flunitrazepam, to rat brain membrane fragments was monitored at equilibrium in the presence and absence of anesthetizing concentrations of ethanol and diethylether. Ethanol produced a concentration-dependent inhibition of [3H]flumazenil binding, which was not reversed by the GABAA receptor competitive antagonist bicuculline, but had no effect on [3H]flunitrazepam binding. Both ethanol and diethylether decreased the affinity of the benzodiazepine site for [3H]flumazenil. These data indicate that ethanol and diethylether have GABA-independent effects at the benzodiazepine sites of the GABAA receptor. These findings are inconsistent with a two-state functional model of the benzodiazepine site and, instead, support a model containing a specific, antagonist-favored conformation.

Halothane enhances the binding of diazepam to synaptic membranes from rat cerebral cortex

The effects of halothane on diazepam binding to the rat cerebral synaptic membranes were investigated. Halothane at the concentrations of 0.63, 1.3 and 4.1 mmol/l increased the binding by 12, 23 and 33%, respectively, compared to the control in the absence of halothane. The increments were dependent on the presence of Cl-. The findings indicate that halothane and diazepam have an interaction at receptor level.

Mechanisms of general anesthesia: brain regional responses to baclofen

The GABAB agonist baclofen is reported to produce general anesthesia when administered either centrally into the lateral ventricles of rats or peripherally to mice. Previously we demonstrated that beta-endorphin given intracerebrally produces anesthesia in rats, a response localized to sites in or adjacent to the inferior third and fourth ventricles. In order to compare the anatomical localization of these two anesthetic responses, we administered baclofen into the inferior or superior lateral or third ventricles, the aqueduct, or fourth ventricle in rats. Although 10 micrograms baclofen infusions into several regions caused loss of the righting reflex, in no case did animals exhibit an unconscious state which satisfied strict criteria of anesthesia. Infusions of 20 micrograms into the inferior third and fourth ventricles elicited seizures followed by a postictal depression. Although unresponsive to some stimuli, these animals showed no impairment in the corneal reflex. Since this dose was often lethal, higher doses not tested. Baclofen, given to mice intraperitoneally at doses of 25, 50, or 75 mg/kg, failed to elicit strictly defined anesthesia, although, to varying degrees, animals exhibited analgesia, loss of the righting reflex, and loss of behavioral responses to loud sounds. Animals continued to show motor responses when handled and retained corneal reflexes. Baclofen does not evoke an unconscious anesthetic state when administered centrally or systemically, emphasizing the need for strict criteria to define general anesthesia and to categorize drugs that promote this state.

Modulation of the benzodiazepine/gamma-aminobutyric acid receptor chloride channel complex by inhalation anesthetics

Inhalation anesthetics, such as diethyl ether, halothane, and enflurane, increase 36Cl- uptake into rat cerebral cortical synaptoneurosomes in a concentration-dependent, picrotoxin-sensitive fashion. At concentrations consistent with those that stimulate 36Cl- uptake, inhalation anesthetics also inhibit the binding of t-[35S]butylbicyclophosphorothionate ([35S]TBPS) to well-washed cortical membranes. Scatchard analysis of [35S]TBPS binding indicates that these agents reduce the apparent affinity of this radioligand and have little effect on the Bmax. The ability of inhalation anesthetics to directly stimulate 36Cl- uptake and inhibit [35S]TBPS binding is a property shared by nonvolatile anesthetics. Nonetheless, there are differences between nonvolatile agents (such as barbiturates and alcohols) and inhalation anesthetics, because the former compounds augment muscimol (a GABAmimetic) stimulated 36Cl- uptake, whereas the latter group (such as ether and enflurane) inhibit this effect. These findings demonstrate that therapeutically relevant concentrations of inhalation anesthetics perturb the benzodiazepine/gamma-aminobutyric acid receptor chloride channel complex, and suggest this oligomeric protein may be a common mediator of some aspects of anesthetic action.