Alpha-chloralose opens the chloride channel of frog isolated sensory neurons

Anesthetics and control of breathing

An important side effect of general anesthetics is respiratory depression. Anesthetics have multiple membrane targets of which ionotropic receptors such as gamma-aminobutyric acid-A (GABA(A)), glycine, N-methyl-D-aspartate and nicotinic acetylcholinergic (nACh) receptors are important members. GABA, glutamate and ACh are crucial neurotransmitters in the respiratory neuronal network, and the ability of anesthetics to modulate their release and interact with their receptors implies complex effects on respiration. Metabotropic receptors and intracellular proteins are other important targets for anesthetics suggesting complex effects on intracellular signaling pathways. Here we briefly overview the effects of general anesthetics on protein targets as far as these are relevant for respiratory control. Subsequently, we describe some methods with which the overall effect of anesthetics on the control of breathing can be measured, as well as some promising in vivo approaches to study their synaptic effects. Finally, we summarize the most important respiratory effects of volatile anesthetics in humans and animals and those of some intravenous anesthetics in animals.

Ethanol inhibits the sensory responses of cerebellar granule cells in anesthetized cats

BACKGROUND

Granule cells occupy a strategic position in the transmission of afferent information to the cerebellar cortex. They are also the most abundant type of neurons in the cerebellum. The functions of the cerebellum are thought to be sensitive to acute alcohol intoxication. The effects of acute alcohol intoxication on the in vivo physiology of cerebellar granule cells are, however, not completely known.

METHODS

We studied chloralose-anesthetized cats at ethanol doses relevant to human drinking (0.3-1.2 g/kg). We recorded the electrophysiological responses of granule cell clusters to auditory and visual stimulation, and simultaneously monitored the concentration of ethanol in the cerebrospinal fluid (CSF).

RESULTS

At an intravenous ethanol dose of 0.3 g/kg, CSF ethanol concentration peaked in 10 minutes at 17 mM, equivalent to a blood alcohol concentration (BAC) of about 0.08 g/dL. Ethanol quickly and almost completely abolished both auditory and visual responses from granule cells. Complete or near-complete inhibition lasted 15 to 20 minutes; approximately 50% recovery required an additional 15 minutes, and a full recovery yet another 15 minutes. A higher ethanol dose at 1.2 g/kg resulted in a more severe inhibition and required longer time for recovery. The relationship between ethanol dose, CSF ethanol concentration, and granule cell responses was dynamic and nonlinear, critically depending upon the elapsed time.

CONCLUSIONS

Cerebellar granule cell sensory responses are highly sensitive to ethanol inhibition. A rapid development of acute tolerance appears to be a major factor contributing to the dynamic and nonlinear relationship among ethanol dosage, CSF ethanol concentration, and granule cell responses. It is likely that a generalized de-afferentation of the cerebellum from its mossy fiber afferents, followed by the subsequent development of acute tolerance may play major roles by which alcohol intoxication affects cerebellar functions.

The actions of ether, alcohol and alkane general anaesthetics on GABAA and glycine receptors and the effects of TM2 and TM3 mutations

The actions of 13 general anaesthetics (diethyl ether, enflurane, isoflurane, methoxyflurane, sevoflurane, chloral hydrate, trifluoroethanol, tribromoethanol, tert-butanol, chloretone, brometone, trichloroethylene, and alpha-chloralose) were studied on agonist-activated Cl(-) currents at human GABA(A) alpha(2)beta(1), glycine alpha(1), and GABA(C) rho(1) receptors expressed in human embryonic kidney 293 cells. All 13 anaesthetics enhanced responses to submaximal (EC(20)) concentrations of agonist at GABA(A) and glycine receptors, except alpha-chloralose, which did not enhance responses at the glycine alpha(1) receptor. None of the anaesthetics studied potentiated GABA responses at the GABA(C) rho(1) receptor. Potentiation of submaximal agonist currents by the anaesthetics was studied at GABA(A) and glycine receptors harbouring mutations in putative transmembrane domains 2 and 3 within GABA(A) alpha(2), beta(1), or glycine alpha(1) receptor subunits: GABA(A) alpha(2)(S270I)beta(1), alpha(2)(A291W)beta(1), alpha(2)beta(1)(S265I), and alpha(2)beta(1)(M286W); glycine alpha(1)(S267I) and alpha(1)(A288W). For all anaesthetics studied except alpha-chloralose, at least one of the mutations above abolished drug potentiation of agonist responses at GABA(A) and glycine receptors. alpha-Chloralose produced efficacious direct activation of the GABA(A) alpha(2)beta(1) receptor (a 'GABA-mimetic' effect). The other 12 anaesthetics produced minimal or no direct activation of GABA(A) and glycine receptors. A non-anaesthetic isomer of alpha-chloralose, beta-chloralose, was inactive at GABA(A) and glycine receptors and did not antagonize the actions of alpha-chloralose at GABA(A) receptors. The implications of these findings for the molecular mechanisms of action of general anaesthetics at GABA(A) and glycine receptors are discussed.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

The physiological effects of serotonin on spontaneous and amino acid-induced activation of cerebellar nuclear cells: an in vivo study in the cat

It is well established that cerebellar efferents originate from neurons located within the cerebellar nuclei. Neurons within these nuclei receive excitatory inputs derived from the axons that arise from cells in several different regions of the brainstem and spinal cord, some of which continue on to terminate as mossy fibers and climbing fibers in the cerebellar cortex. GABA-induced inhibition in the nuclei is derived primarily from Purkinje cells located in the overlying cortex and possibly from axonal collaterals of a population of small, GABAergic nuclear neurons. In addition, a third chemically defined system of afferents that contain the monoamine serotonin forms a dense plexus of fibers throughout the cat's cerebellar nuclei. The intent of this study is to determine the physiological effects of serotonin on the spontaneous activity of cerebellar nuclear cells as well as that induced by application of the excitatory amino acids glutamate and aspartate in an adult in vivo preparation. Iontophoretic application of serotonin in anesthetized preparations suppresses both spontaneous and excitatory amino acid induced activity. In addition, interactions between serotonin and the amino acid analogs quisqualate and NMDA were analyzed; 5HT suppresses the excitatory responses of neurons to both analogs. However, there is a stronger suppressive effect on quisqualate-induced excitation as compared to that elicited by NMDA. In addition to modulating the effects of the excitatory amino acids, serotonin also potentiates the inhibitory effects of GABA. However, the effect was greatest if the neuron was initially preconditioned with GABA. In summary, serotonin acts to suppress amino acid induced activity in cerebellar nuclear neurons and to enhance gABA-mediated inhibition. The net effect is a decrease in nuclear cell activity and consequently in cerebellar output.

Trichloroethanol potentiation of gamma-aminobutyric acid-activated chloride current in mouse hippocampal neurones

1. The action of 2,2,2-trichloroethanol on gamma-aminobutyric acid (GABA)-activated Cl- current was studied in mouse hippocampal neurones in tissue culture by use of whole-cell patch-clamp recording. 2. Trichloroethanol increased the amplitude of currents activated by 1 microM GABA or 0.1 microM muscimol. Trichloroethanol, 1-25 mM, potentiated current activated by 1 microM GABA in a concentration-dependent manner with an EC50 of 3.0 +/- 1.4 mM and a maximal response (Emax) of 576 +/- 72% of control. 3. Trichloroethanol potentiated currents activated by GABA concentrations < 10 microM, but did not increase the amplitude of currents activated by concentrations of GABA > or = 10 microM. Despite marked potentiation of currents activated by low concentrations of GABA, trichloroethanol did not significantly alter the EC50, slope, or Emax of the GABA concentration-response curve. 4. Trichloroethanol, 5 mM, potentiated GABA-activated current in neurones in which ethanol, 10-500 mM, did not. The effect of trichloroethanol was not altered by the putative ethanol antagonist, Ro 15-4513. Trichloroethanol did not potentiate currents activated by pentobarbitone. 5. In the absence of exogenous GABA, trichloroethanol at concentrations > or = 2.5 mM activated a current that appeared to be carried by Cl- as its reversal potential changed with changes in the Cl- gradient and as it was inhibited by the GABAA antagonists, bicuculline methiodide and picrotoxin. 6. Since trichloroethanol is thought to be the active metabolite of chloral hydrate and other chloral derivative anaesthetics, potentiation of the GABA-activated current in central nervous system neurones by trichloroethanol may contribute to the sedative/hypnotic effects of these agents.