Effects of neuromuscular blocking agents on excitatory transmission and gamma-aminobutyric acidA-mediated inhibition in the rat hippocampal slice

Safety of general anaesthetics on the developing brain: are we there yet?

Thirty years ago, neurotoxicity induced by general anaesthetics in the developing brain of rodents was observed. In both laboratory-based and clinical studies, many conflicting results have been published over the years, with initial data confirming both histopathological and neurodevelopmental deleterious effects after exposure to general anaesthetics. In more recent years, animal studies using non-human primates and new human cohorts have identified some specific deleterious effects on neurocognition. A clearer pattern of neurotoxicity seems connected to exposure to repeated general anaesthesia. The biochemistry involved in this neurotoxicity has been explored, showing differential effects of anaesthetic drugs between the developing and developed brains. In this narrative review, we start with a comprehensive description of the initial concerning results that led to recommend that any non-essential surgery should be postponed after the age of 3 yr and that research into this subject should be stepped up. We then focus on the neurophysiology of the developing brain under general anaesthesia, explore the biochemistry of the observed neurotoxicity, before summarising the main scientific and clinical reports investigating this issue. We finally discuss the GAS trial, the importance of its results, and some potential limitations that should not undermine their clinical relevance. We finally suggest some key points that could be shared with parents, and a potential research path to investigate the biochemical effects of general anaesthesia, opening up perspectives to understand the neurocognitive effects of repetitive exposures, especially in at-risk children.

Intravenous infusion of rocuronium bromide prolongs emergence from propofol anesthesia in rats

BACKGROUND

Neuromuscular blocking agents induce muscle paralysis via the prevention of synaptic transmission at the neuromuscular junction and may have additional effects at other sites of action. With regard to potential effects of neuromuscular blocking agents on the central nervous system, a definitive view has not been established. We investigated whether intravenous infusion of rocuronium bromide affects the emergence from propofol anesthesia.

METHODS

Using an in vivo rat model, we performed propofol infusion for 60 minutes, along with rocuronium bromide at various infusion rates or normal saline. Sugammadex or normal saline was injected at the end of the infusion period, and we evaluated the time to emergence from propofol anesthesia. We also examined the neuromuscular blocking, circulatory, and respiratory properties of propofol infusion along with rocuronium bromide infusion to ascertain possible factors affecting emergence.

RESULTS

Intravenous infusion of rocuronium bromide dose-dependently increased the time to emergence from propofol anesthesia. Sugammadex administered after propofol infusion not containing rocuronium bromide did not affect the time to emergence. Mean arterial pressure, heart rate, partial pressures of oxygen and carbon dioxide, and pH were not affected by rocuronium bromide infusion. Neuromuscular blockade induced by rocuronium bromide, even at the greatest infusion rate in the emergence experiment, was rapidly antagonized by sugammadex.

CONCLUSIONS

These results suggest that intravenous infusion of rocuronium bromide dose-dependently delays the emergence from propofol anesthesia in rats. Future studies, such as detection of rocuronium in the cerebrospinal fluid or central nervous system, electrophysiologic studies, microinjection of sugammadex into the brain, etc., are necessary to determine the mechanism of this effect.

Neural Correlates of Consciousness at Near-Electrocerebral Silence in an Asphyxial Cardiac Arrest Model

Recent electrophysiological studies have suggested surges in electrical correlates of consciousness (i.e., elevated gamma power and connectivity) after cardiac arrest (CA). This study examines electrocorticogram (ECoG) activity and coherence of the dying brain during asphyxial CA. Male Wistar rats (n = 16) were induced with isoflurane anesthesia, which was washed out before asphyxial CA. Mean phase coherence and ECoG power were compared during different stages of the asphyxial period to assess potential neural correlates of consciousness. After asphyxia, the ECoG progressed through four distinct stages (asphyxial stages 1-4 [AS1-4]), including a transient period of near-electrocerebral silence lasting several seconds (AS3). Electrocerebral silence (AS4) occurred within 1 min of the start of asphyxia, and pulseless electrical activity followed the start of AS4 by 1-2 min. AS3 was linked to a significant increase in frontal coherence between the left and right motor cortices (p < 0.05), with no corresponding increase in ECoG power. AS3 was also associated with a significant posterior shift of ECoG power, favoring the visual cortices (p < 0.05). Although the ECoG during AS3 appears visually flat or silent when viewed with standard clinical settings, our study suggests that this period of transient near-electrocerebral silence contains distinctive neural activity. Specifically, the burst in frontal coherence and posterior shift of ECoG power that we find during this period immediately preceding CA may be a neural correlate of conscious processing.

Different F-Wave Recovery After Neuromuscular Blockade with Pancuronium and Mivacurium

We performed this study to assess the recovery period after neuromuscular blockade by electromyographic F-wave analysis, a method that supplies more information about more proximal parts of the motor system than conventionally used methods, e.g., mechanomyography (MMG). In 20 neurosurgical ASA physical status I or II patients anesthesia was induced and maintained with IV fentanyl and midazolam. Patients were randomly assigned to receive either 0.25 mg/kg mivacurium (MV group, n = 10) or 0.1 mg/kg pancuronium (PC group, n = 10) intraoperatively. MMG monitoring of the adductor pollicis muscle was performed continuously. F waves were recorded at the abductor pollicis muscle of the contralateral hand at train-of-four (TOF) ratios of 0.1, 0.25, 0.5, 0.7, 0.75, 0.8, 0.85, 0.9, and 0.95. Recovery of F-wave amplitudes after neuromuscular blockade with pancuronium was significantly slower compared with mivacurium (P = 0.004) during the clinically important recovery period defined by MMG TOF ratios from 0.7 to 0.95. This electrophysiologic finding suggests a differential recovery of the motor system after administration of pancuronium and mivacurium not detected by MMG.

Muscle relaxants in neurosurgical anaesthesia: a critical appraisal

The use of muscle relaxants, considered until recently as common practice in current neurosurgical anaesthesia protocols, becomes increasingly more questionable today. The reasons rely on the evolution of neurosurgery including the advent of new surgical techniques, the evolution of anaesthesia having the benefit of new drugs and devices, and the rationale for using muscle relaxants balanced against their potential side-effects and possible pharmacodynamic alterations in neurosurgical patients.

Cerebrospinal fluid concentrations of atracurium, laudanosine and vecuronium following clinical subarachnoid hemorrhage

BACKGROUND

Neuromuscular blocking agents may exert central nervous system effects when they reach the brain. This study assessed the concentrations and the time course of passage of vecuronium, atracurium, and its metabolite laudanosine in the cerebrospinal fluid (CSF) of patients undergoing intracranial aneurysm clipping.

METHODS

Twenty-five patients with subarachnoid hemorrhage were randomly allocated to receive an intravenous infusion of vecuronium (n=13) or atracurium (n=12). Arterial blood and lumbar CSF were sampled before and 1, 2, 3, 4 and 8 h after the start of the relaxant infusion. The samples were analyzed by liquid chromatography-electrospray ionization mass spectrometry (vecuronium) and high-pressure liquid chromatography (atracurium and laudanosine).

RESULTS

The data of 20 patients (10 in both groups) were analyzed. In 11 CSF samples from five patients atracurium was detected in concentrations from 10 to 50 ng/ml. Laudanosine was retrieved in all CSF samples at 1, 2, 3, 4 and 8 h; the highest CSF concentration of laudanosine occurred at 3 h [38 (18-63) ng/ml: median (range)]. Vecuronium was not found in any CSF sample.

CONCLUSION

Significant concentrations of atracurium and laudanosine but not of vecuronium were detected in the CSF of patients during and immediately after intracranial aneurysm surgery.

The role of nicotinic acetylcholine receptors in the mechanisms of anesthesia

Nicotinic acetylcholine receptors are members of the ligand-gated ion channel superfamily, that includes also gamma-amino-butiric-acid(A), glycine, and 5-hydroxytryptamine(3) receptors. Functional nicotinic acetylcholine receptors result from the association of five subunits each contributing to the pore lining. The major neuronal nicotinic acetylcholine receptors are heterologous pentamers of alpha4beta2 subunits (brain), or alpha3beta4 subunits (autonomic ganglia). Another class of neuronal receptors that are found both in the central and peripheral nervous system is the homomeric alpha7 receptor. The muscle receptor subtypes comprise of alphabetadeltagamma (embryonal) or alphabetadeltaepsilon (adult) subunits. Although nicotinic acetylcholine receptors are not directly involved in the hypnotic component of anesthesia, it is possible that modulation of central nicotinic transmission by volatile agents contributes to analgesia. The main effect of anesthetic agents on nicotinic acetylcholine receptors is inhibitory. Volatile anesthetics and ketamine are the most potent inhibitors both at alpha4beta2 and alpha3beta4 receptors with clinically relevant IC(50) values. Neuronal nicotinic acetylcholine receptors are more sensitive to anesthetics than their muscle counterparts, with the exception of the alpha7 receptor. Several intravenous anesthetics such as barbiturates, etomidate, and propofol exert also an inhibitory effect on the nicotinic acetylcholine receptors, but only at concentrations higher than those necessary for anesthesia. Usual clinical concentrations of curare cause competitive inhibition of muscle nicotinic acetylcholine receptors while higher concentrations may induce open channel blockade. Neuronal nAChRs like alpha4beta2 and alpha3beta4 are inhibited by atracurium, a curare derivative, but at low concentrations the alpha4beta2 receptor is activated. Inhibition of sympathetic transmission by clinically relevant concentrations of some anesthetic agents is probably one of the factors involved in arterial hypotension during anesthesia.

Modulation of synaptic transmission by nicotine and nicotinic antagonists in hippocampus

Using rat hippocampal slices, we studied the effects of nicotine and three antagonists of neuronal nicotinic receptors on excitatory and inhibitory transmission. We report that nicotine at concentrations between 0.5 and 100 microM enhanced excitatory synaptic responses and increased the size of the presynaptic fiber volley. This effect was reproduced by three neuronal nicotinic receptor antagonists: dihydro-beta-erythroidine, methyllycaconitine and mecamylamine. In contrast, nicotine, but not nicotinic antagonists, produced a dual effect on inhibition: nicotine enhanced gamma-aminobutyric-acid A (GABA(A)) receptor-mediated synaptic responses at low concentration (0.5 microM) and blocked them at high concentration (100 microM). We conclude that the excitatory effects of nicotine are reproduced by nicotinic receptor antagonists, thereby suggesting that these effects might be mediated through receptor desensitization. These results also indicate that nicotine differentially affects GABAergic inhibition at low and high concentrations-effects that are not reproduced by antagonists.