General Anesthetic, Respiratory Function and Gamma Amino Butyric ACID (GABA) Levels in Rat Brain

Abstract

One area of neuropharmacology that has received very little attention is the effect of the clinically used general anesthetics on GABA levels in the brain. The clinical importance of this topic can be emphasized by the fact that no satisfactory, empirically verifiable theory explaining the mechanism of actions of the general anesthetics has yet been documented, despite the fact that these agents have been employed in the operating room for more than a century.

A plethora of hypotheses concerning the mechanism of actions of the general anesthetics has emerged over the past one hundred years. Many of these hypothesis have been physically rather than biochemically oriented and have been hard to test in laboratory. An advantage of monitoring the effects on GABA metabolism by various anesthetics agents is that theories that may emerge concerning the dynamics of these drugs will be relatively easy to test using biochemical techniques already established in laboratory, such as high pressure liquid chromatography.

Respiratory depression is a highly undesirable effect of drug overdoses or unusual sensitivity to a given agent. Thus, quick management of respiratory depression is essential in order to minimize morbidity and mortality resulting from lack or oxygen. Previously, several drugs were used to stimulate depressed respiratory activity. Such drugs are called analeptics and include such agents as picrotoxin (a GABA antagonist), strychnine, and pentylenetetrazol. However, the main drawback of these agents is that they all produce convulsions at doses just above those needed to stimulate depressed respiratory function.

Earlier work in our laboratory has indicated that naloxone, at doses greater than those needed to antagonize narcotics can reverse the sedative, anxiolytic, and respiratory depressant effects of the benzodiazepine chlordiazepopxide (1 preliminary experiment have indicated that pretreatment with naloxone will reverse the respiratory depressant effects but not the bradycardia and hypotensions induced by chlordiazepoxide, but also increased the maximum tolerated i.v. dose of chlordiazepoxide from 160 mg/kg to 220 mg/kg. Further study is needed to define the mechanism of this unusual antagonism.

Conflicting studies have emerged from various laboratories concerning the effects of GABA metabolism by three broad classes of anesthetics: the inhalation anesthetics, dissociative anesthetics and barbiturates.

Cheng and Brunner (2) have reported that the inhalation anesthetic halothane inhibits the oxidation of reduced nicotinamide adenine dinucleotide (NADH) , thereby shutting down the tricarboxylic acid cycle (TCA cycle) (2). This leads to an intracellular accumulation of glutamate, which is presumably converted to GABA. The suggested results of TCA cycle inhibition are three fold: 1) reduced respiration and oxygen consumption could occur and less energy would be required to restore ionic equilibrium across neuronal membrane should neural transmission occur; 2) accumulation of NADH would favor an increase in the concentration of reduced substances such as GABA; and 3) accumulated GABA, probably in the presynaptic nerve terminals, could exert a physiological response by pre or postsynaptic inhibition of neural transmission. Whether the reported increase in GABA concentration is a cause or an effect of mitochondrial inhibition has yet to be determined.

Dye and Taberner (3) have found that injection of barbiturates, such as thiopental has no apparent effect on the enzymatic activity of glutamic acid decarboxylase (the enzymes which degrades GABA). Thus no effects were seen on GABA levels (3). On the other hand, dissociative anesthetics such as ketamine and gamma-hydroxybutyric acid (which is also a metabolite of GABA), have been reported to competitively inhibit glutamate decarboxylase, thereby reducing the levels of GABA in the brain.

Several interesting lines of evidence implicate GABA in the control of respiratory function. It is thought that the medullary control of respiration involves two systems; one neural circuit controlling inspiratory impulses and the other controlling expiratory impulses for respiration, mutually inhibit the other. This type of dual inhibition is thought to provide the neural mechanism which allows for rhythmic breathing. It is interesting to note that GABA has been localized in inhibitory interneurons in the spinal cord of many mammalians species (4). It is tempting to speculate that GABA is the inhibitory neurotransmitter between the inspiratory and expiratory centers of the medulla. Conceivably, the medulla would have neural pathways similar to those in the spinal cord, since the medulla is the first element of higher CNS organizations above the spinal cord.

Naloxone, which is usually thought of a specific narcotic antagonist, has recently been demonstrated by Breuker and coworkers to act as a reversible GABA antagonist when iontophoretically applied to the nucleus Accumbens in the rat brain (5). The evidence that naloxone can act as a GABA antagonist could explain several unexpected findings.

In summary it would appear as if GABA-ergic inhibition might play an important role in the neural control of respiration and that selective manipulation of medullary GABA would lead to either increased respiratory function (using GABA antagonists) or decreased respiratory function (using GABA agonists). Little research has been done in this area; however, the clinical significance of such research might be manifested in reduced mortality due to severe drug-induced respiratory depression.

Specific Objectives of the proposed work were:

1. To determine if GABA plays a regulatory role in the neural control of respiration in rodents.

2. To determine the effects of various GABA agonists (Benzodiazepines and muscimol) and putative GABA antagonists (Picrotoxin, Bicuculline, and Naloxone) on general respiratory depression.

3. To predict, based on animal studies, whether GABA antagonists could be used as therapeutic agents in management of respiratory depression.

4. To determine if a model linking GABA levels and general anesthetic can explain various anesthetic states produced by selected clinical agents.