Paul S. García

General Anesthetic Actions on GABAA Receptors

General anesthetic drugs interact with many receptors in the nervous system, but only a handful of these interactions are critical for producing anesthesia. Over the last 20 years, neuropharmacologists have revealed that one of the most important target sites for general anesthetics is the GABAA receptor. In this review we will discuss what is known about anesthetic – GABAA receptor interactions.

Modulation of Vigilance in the Primary Hypersomnias by Endogenous Enhancement of GABAA Receptors

A component of cerebrospinal fluid in excessively sleepy people activates an inhibitory signaling pathway and mimics the actions of sedative-hypnotics. Awake and Refreshed A spindle prick on the finger, and Princess Aurora couldn’t keep her eyes open; one hundred years later, Sleeping Beauty was awakened with a kiss. But persistent daytime sleepiness—hypersomnolence—is no fairy tale, and neither the cause nor a cure is apparent. Now, Rye et al. begin to illuminate, in patients with primary hypersomnias, the neurobiology that underlies sleepiness of unknown etiology. A disabling condition, primary hypersomnia is characterized by lethargy, trance-like states, and “sleep drunkenness” even after prolonged, deep, nonrestorative sleep. The authors showed that cerebrospinal fluid (CSF) from these hypersomnolent subjects contains a small (500 to 3000 daltons) trypsin-sensitive substance that stimulates the in vitro function of selected γ-aminobutyric acid (GABA) receptors only in the presence of GABA—an inhibitory neurotransmitter that stimulates GABA receptors, quells consciousness, and induces sleep. GABA receptors are known to bind a class of psychoactive sedating drugs called benzodiazepines (BZDs). Hypersomnolent CSF samples mimicked the effects of BZD on GABA receptors but did not compete with BZD binding in human brain tissue, suggesting that the newly identified substance functions by a distinct mechanism. Furthermore, the BZD receptor antagonist flumazenil reversed hypersomnolent-CSF activation of GABA signaling, even though the drug is known to be a competitive antagonist of BZD and blocks BZD action by binding to the classical BZD-binding domain of GABA receptors. Most importantly, flumazenil restored vigilance in some hypersomnolent subjects. Together, these mechanistic studies pinpoint a potential new neuropharmacological pathway for a 25-year-old drug. The current study suggests that one of the “spindle pricks” that puts hypersomnolent subjects to sleep is a substance in CSF that augments inhibitory GABA signaling. A deeper understanding of the neurobiology of primary hypersomnia should help scientists discover new “kisses” that restore wakefulness—in fewer than 100 years. The biology underlying excessive daytime sleepiness (hypersomnolence) is incompletely understood. After excluding known causes of sleepiness in 32 hypersomnolent patients, we showed that, in the presence of 10 μM γ-aminobutyric acid (GABA), cerebrospinal fluid (CSF) from these subjects stimulated GABAA receptor function in vitro by 84.0 ± 40.7% (SD) relative to the 35.8 ± 7.5% (SD) stimulation obtained with CSF from control subjects (Student’s t test, t = 6.47, P < 0.0001); CSF alone had no effect on GABAA signaling. The bioactive CSF component had a mass of 500 to 3000 daltons and was neutralized by trypsin. Enhancement was greater for α2 subunit– versus α1 subunit–containing GABAA receptors and negligible for α4 subunit–containing ones. CSF samples from hypersomnolent patients also modestly enhanced benzodiazepine (BZD)–insensitive GABAA receptors and did not competitively displace BZDs from human brain tissue. Flumazenil—a drug that is generally believed to antagonize the sedative-hypnotic actions of BZDs only at the classical BZD-binding domain in GABAA receptors and to lack intrinsic activity—nevertheless reversed enhancement of GABAA signaling by hypersomnolent CSF in vitro. Furthermore, flumazenil normalized vigilance in seven hypersomnolent patients. We conclude that a naturally occurring substance in CSF augments inhibitory GABA signaling, thus revealing a new pathophysiology associated with excessive daytime sleepiness.

Brief-reports: elevated myostatin levels in patients with liver disease: a potential contributor to skeletal muscle wasting.

Loss of skeletal muscle mass is a poorly understood complication of end-stage liver disease (ESLD). Based on recent stem cell literature, we hypothesized that the potent negative regulator of muscle mass, myostatin, could play a role in the muscle loss associated with ESLD. In this preliminary investigation, we measured myostatin levels in patients undergoing liver transplant evaluation, using a novel enzyme-linked immunosensitivity assay. Myostatin levels were significantly elevated in patients with ESLD compared with healthy controls. These data suggest that myostatin deserves further investigation as a target for therapies designed to preserve muscle mass in patients with ESLD.

A Conserved Tyrosine in the 2 Subunit M4 Segment Is a Determinant of -Aminobutyric Acid Type A Receptor Sensitivity to Propofol

Background:The γ-aminobutyric acid type A receptor (GABAA-R) β subunits are critical targets for the actions for several intravenous general anesthetics, but the precise nature of the anesthetic binding sites are unknown. In addition, little is known about the role the fourth transmembrane (M4) segment of the receptor plays in receptor function. The aim of this study was to better define the propofol binding site on the GABAA-R by conducting a tryptophan scan in the M4 segment of the β2 subunit. Methods:Seven tryptophan mutations were introduced into the C-terminal end of the M4 segment of the GABAA-R β2 subunit. GABAA-R subunit complementary DNAs were transfected into human embryonic kidney 293 cells grown on glass coverslips. After transfection (36–72 h), coverslips were transferred to a perfusion chamber to assay receptor function. Cells were whole cell patch clamped and exposed to GABA, propofol, etomidate, and pregnenolone. Chemicals were delivered to the cells using two 10-channel infusion pumps and a rapid solution exchanger. Results:All tryptophan mutations were well tolerated, and with one exception, all resulted in minimal changes in receptor activation by GABA. One mutation, β2(Y444W), selectively suppressed the ability of propofol to enhance receptor function while retaining normal sensitivity to etomidate and pregnenolone. Conclusions:This is the first report of a mutation that selectively reduces propofol sensitivity without altering the action of etomidate. The reduction in propofol sensitivity is consistent with the loss of a hydrogen bond within the propofol binding site. These results also suggest a possible orientation of the propofol molecule within its binding site.

Postoperative cognitive dysfunction, Alzheimer's disease, and anesthesia

Although aging itself is not a disease, there are many comorbidities that become more common with aging. Heart disease, cancer, and other chronic illnesses are either more common or more severe in aging patients. Approximately 5.5 million people in the United States have Alzheimers disease (AD), with the principal risk factor being age. It is estimated that the incidence of AD diagnosis doubles every 5 years after the age of 65 [1]. Therefore, as the population ages, the impact of AD on the healthcare landscape will increase. Understanding how to manage patients with AD is critical as we begin to care for more elderly patients in the perioperative period [2]. In addition to their other health considerations, aging surgical patients are increasingly more likely to have pre-existing AD or be at risk for developing AD. There is growing interest to determine how anesthesia affects the development or progression of AD. Similarly, a best practice for the anesthetic management of patients with AD is not yet defined. Finally, the relationship between AD and susceptibility to or exacerbation of postoperative cognitive dysfunction (POCD) is not well understood. In this review, we will discuss both the clinical and the preclinical data related to anesthesia and AD, describe the overlapping pathophysiology of neurodegeneration and provide some insight into the anesthetic care of patients with AD.

Neuro-Modulating Effects of Honokiol: A Review

Honokiol is a poly-phenolic compound that exerts neuroprotective properties through a variety of mechanisms. It has therapeutic potential in anxiety, pain, cerebrovascular injury, epilepsy, and cognitive disorders including Alzheimer’s disease. It has been traditionally used in medical practices throughout much of Southeast Asia, but has now become more widely studied due to its pleiotropic effects. Most current research regarding this compound has focused on its chemotherapeutic properties. However, it has the potential to be an effective neuroprotective agent as well. This review summarizes what is currently known regarding the mechanisms involved in the neuroprotective and anesthetic effects of this compound and identifies potential areas for further research.

Electroencephalographic Variation during End Maintenance and Emergence from Surgical Anesthesia

The re-establishment of conscious awareness after discontinuing general anesthesia has often been assumed to be the inverse of loss of consciousness. This is despite the obvious asymmetry in the initiation and termination of natural sleep. In order to characterize the restoration of consciousness after surgery, we recorded frontal electroencephalograph (EEG) from 100 patients in the operating room during maintenance and emergence from general anesthesia. We have defined, for the first time, 4 steady-state patterns of anesthetic maintenance based on the relative EEG power in the slow-wave (<14 Hz) frequency bands that dominate sleep and anesthesia. Unlike single-drug experiments performed in healthy volunteers, we found that surgical patients exhibited greater electroencephalographic heterogeneity while re-establishing conscious awareness after drug discontinuation. Moreover, these emergence patterns could be broadly grouped according to the duration and rapidity of transitions amongst these slow-wave dominated brain states that precede awakening. Most patients progressed gradually from a pattern characterized by strong peaks of delta (0.5–4 Hz) and alpha/spindle (8–14 Hz) power (‘Slow-Wave Anesthesia’) to a state marked by low delta-spindle power (‘Non Slow-Wave Anesthesia’) before awakening. However, 31% of patients transitioned abruptly from Slow-Wave Anesthesia to waking; they were also more likely to express pain in the post-operative period. Our results, based on sleep-staging classification, provide the first systematized nomenclature for tracking brain states under general anesthesia from maintenance to emergence, and suggest that these transitions may correlate with post-operative outcomes such as pain.

Improvement in daytime sleepiness with clarithromycin in patients with GABA-related hypersomnia: Clinical experience

The macrolide antibiotic clarithromycin can enhance central nervous system excitability, possibly by antagonism of GABA-A receptors. Enhancement of GABA signaling has recently been demonstrated in a significant proportion of patients with central nervous system hypersomnias, so we sought to determine whether clarithromycin might provide symptomatic benefit in these patients. We performed a retrospective review of all patients treated with clarithromycin for hypersomnia, in whom cerebrospinal fluid enhanced GABA-A receptor activity in vitro in excess of controls, excluding those with hypocretin deficiency or definite cataplexy. Subjective reports of benefit and objective measures of psychomotor vigilance were collected to assess clarithromycin’s effects. Clinical and demographic characteristics were compared in responders and non-responders. In total, 53 patients (38 women, mean age 35.2 (SD 12.8 years)) were prescribed clarithromycin. Of these, 34 (64%) reported improvement in daytime sleepiness, while 10 (19%) did not tolerate its side effects, and nine (17%) found it tolerable but without symptomatic benefit. In those who reported subjective benefit, objective corroboration of improved vigilance was evident on the psychomotor vigilance task. Twenty patients (38%) elected to continue clarithromycin therapy. Clarithromycin responders were significantly younger than non-responders. Clarithromycin may be useful in the treatment of hypersomnia associated with enhancement of GABA-A receptor function. Further evaluation of this novel therapy is needed.

The role of thrombin and protease-activated receptors in pain mechanisms

As our knowledge of the mechanisms underlying the sensation of pain continues to expand, researchers are constantly searching for novel therapeutic targets. One such novel pain pathway involves thrombin and its associated protease-activated receptor (PAR). Besides its traditional role in haemostasis, thrombin has multiple roles in both the central and peripheral nervous system including activation of microglia, regulation of neuronal death and neurite outgrowth, and influencing the transmission of pain signals in the nociceptive circuitry. Eventually therapeutic modalities directed at these targets could provide novel therapeutic approaches for treating chronic pain. The thrombin-associated PARs also have roles in inflammation, neurodevelopment, and conducting pain, both in conjunction with thrombin and independently. Recent laboratory evidence suggests that the PARs can attenuate pain mediated by the enteric nervous system in animal models (for example in pancreatitis and colitis). This review highlights several pathways in the mediation of pain sensation that can be influenced by thrombin.

Emergence from general anesthesia and the sleep-manifold

The electroencephalogram (EEG) during the re-establishment of consciousness after general anesthesia and surgery varies starkly between patients. Can the EEG during this emergence period provide a means of estimating the underlying biological processes underpinning the return of consciousness? Can we use a model to infer these biological processes from the EEG patterns? A frontal EEG was recorded from 84 patients. Ten patients were chosen for state-space analysis. Five showed archetypal emergences; which consisted of a progressive decrease in alpha power and increase peak alpha frequency before return of responsiveness. The five non-archetypal emergences showed almost no spectral EEG changes (even as the volatile general anesthetic decreased) and then an abrupt return of responsiveness. We used Bayesian methods to estimate the likelihood of an EEG pattern corresponding to the position of the patient on a 2-dimensional manifold in a state space of excitatory connection strength vs. change in intrinsic resting neuronal membrane conductivity. We could thus visualize the trajectory of each patient in the state-space during their emergence period. The patients who followed an archetypal emergence displayed a very consistent pattern; consisting of progressive increase in conductivity, and a temporary period of increased connection strength before return of responsiveness. The non-archetypal emergence trajectories remained fixed in a region of phase space characterized by a relatively high conductivity and low connection strength throughout emergence. This unexpected progressive increase in conductivity during archetypal emergence may be due to an abating of the surgical stimulus during this period. Periods of high connection strength could represent forays into dissociated consciousness, but the model suggests all patients reposition near the fold in the state space to take advantage of bi-stable cortical dynamics before transitioning to consciousness.