Inhibition of GABA metabolism in rat brain slices by halothane

Naloxone and flumazenil fail to antagonize the isoflurane-induced suppression of dorsal horn neurons in cats

Effects of naloxone and flumazenil on isoflurane activities were examined on dorsal horn neurons in cats. Isoflurane suppressed bradykinin-induced nociceptive responses in transected feline spinal cords. The bradykinin-induced neuronal firing rates were significantly suppressed by 60.0%, 35.3% and 32.2% at 10, 20 and 30 min after isoflurane administration, respectively. The 32.3% suppression on bradykinin-induced neuronal responses at 30 min after isoflurane administration was not reversed 5 min after administration of naloxone (36.4% suppression). The suppressive effects of isoflurane were not reversed by naloxone (0.2 mg.kg(-1), i.v.). Similarly, the benzodiazepine antagonist, flumazenil (0.2 mg.kg(-1), i.v.), did not affect the suppressive effects of isoflurane. Failure of naloxone and flumazenil to reverse the suppressive effects of isoflurane suggests that isoflurane interacts with neither opioid nor benzodiazepine receptors in producing its suppressive action on nociceptive responses in dorsal horn neurons of the feline spinal cord.

Accessing the tonotopic organization of the ventral cochlear nucleus by intranuclear microstimulation

This study is part of a program to develop an auditory prosthesis for the profoundly deaf, based on multichannel microstimulation in the cochlear nucleus. The functionality of such a device is dependent on its ability to access the tonotopic axis of the human ventral cochlear nucleus in an orderly fashion. In these studies, we utilized the homologies between the human and feline ventral cochlear nuclei and the known tonotopic organization of the central nucleus of the inferior colliculus (IC). In anesthetized cats, stimuli were delivered to three or four locations along the dorsal-to-ventral axis of the posteroventral cochlear nucleus (PVCN), and for each stimulus location, we recorded the multiunit neuronal activity and the field potentials at 20 or more locations along the dorsolateral-ventromedial (tonotopic) axis of the IC. The current source-sink density (CSD), which delimits regions of neuronal activity, was computed from the sequence of field potentials recorded along this axis. The multiunit activity and the CSD analysis both showed that the tonotopic organization of the PVCN can be accessed in an orderly manner by intranuclear microstimulation in several regions of the PVCN, using the range of stimulus pulse amplitudes that have been shown in previous studies to be noninjurious during prolonged intranuclear microstimulation via chronically implanted microelectrodes. We discuss the applicability of these findings to the design of clinical auditory prostheses for implantation into the human cochlear nucleus.

Effects of thiopental, halothane and isoflurane on the calcium-dependent and -independent release of GABA from striatal synaptosomes in the rat

The effects of the anesthetic agents thiopental, halothane and isoflurane on the release of GABA induced by depolarization and/or reversal of the GABA carrier were investigated in a synaptosomal preparation obtained from the rat striatum. Veratridine (1 microM) and KCl (9 mM) elicited a significant, Ca(2+)-dependent release of [3H]GABA. The KCl-evoked release was not significantly modified in the presence of nipecotic acid (10(-5) M), a selective blocker of the neuronal GABA carrier. The [3H]GABA release was significantly decreased by omega-conotoxin (10(-7) M, a blocker of the N voltage-dependent Ca2+ channels, but was affected by neither nifedipine (10(-4) M) nor omega-Aga-IVA (10(-7) M which block the L and P Ca2+ channels, respectively. Thiopental application (10(-5) to 10(-3) M) was followed by a dose-related, significant, decrease in both the veratridine and KCl-induced releases, whether nipecotic acid was present or not. In contrast, halothane and isoflurane (1-3%) failed to alter [3H]GABA release. Altogether, these results suggest that reduction of the depolarization-evoked GABA release might contribute to thiopental anesthesia, but this seems unlikely for volatile anesthetics.

The effect of halothane dose on striatal dopamine: an in vivo microdialysis study

The relationship between a level of an anesthesia state and striatal dopamine concentration was investigated. The effect of halothane administration on dopamine level in rat striatal extracellular fluid was determined in tracheotomized rats, using an in vivo brain microdialysis method. The striatal dopamine was increased by halothane doses. The correlation coefficient between halothane and dopamine concentrations was 0.69.

Halothane-induced hyperpolarization and depression of postsynaptic potentials of guinea pig thalamic neurons in vitro

Intralaminar thalamic nuclei have been considered to be a component of the non-specific sensory system which is involved in physiological functions related to consciousness and pain sensation. The effect of halothane on membrane potentials and synaptic properties of neurons of the parafascicular (Pf) nucleus in guinea pig brain slices was investigated using intracellular recording methods. Halothane at concentrations of 0.4-1.0 mM, which are in the range of clinical concentrations, produced hyperpolarizations of 2-8 mV in approximately 50% of the cells. The halothane-induced hyperpolarization was nullified at a membrane potential close to the K+ equilibrium potential. The amplitude of the hyperpolarization was dependent on the external K+ concentration, and was decreased by either Ba2+, or 4-aminopyridine, or intracellular injection of Cs+. All these results indicate that the hyperpolarization was due to an increase in K+ conductance. Halothane at clinical concentrations depressed both excitatory and inhibitory postsynaptic potentials in a concentration-dependent manner. On the other hand hyperpolarizing responses to exogenous gamma-aminobutyric acid (GABA) in the presence of bicuculline were suppressed by halothane, but depolarizing responses to L-glutamate were not altered. The results indicate that the depressant action of the anesthetic on the excitatory postsynaptic potential (EPSP) may occur presynaptically, whereas the blocking action on the inhibitory postsynaptic potential (IPSP) may occur postsynaptically.

The effects of halothane on the contents of putative transmitter amino acids in whole rat brain

The effects of halothane on brain amino acid contents were investigated in rat by using a gas chromatography-mass spectrometry system. The contents of glycine and glutamate did not show any consistent changes in 1% or 2% halothane, but increased in 3% or 4% halothane. The levels of gamma-aminobutyric acid were not altered in any concentrations of halothane. The levels of aspartate increased in a dose-dependent manner up to 3% halothane, but returned to the control level in 4% halothane. These findings suggest that halothane brings the alteration of brain amino acid contents by affecting their synthesis and catabolism.

Effects of halothane anaesthesia on extracellular levels of dopamine, dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindolacetic acid in rat striatum: a microdialysis study

The effects of halothane anaesthesia on striatal extracellular levels of dopamine, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5HIAA) were investigated in microdialysis experiments. Induction of anaesthesia was accompanied by a rapid increase in dopamine levels and a slower increase in DOPAC and HVA. 5HIAA was not affected. The reduction of dopamine levels induced by apomorphine 0.05 mg/kg appeared with a shorter latency in conscious rats than in anaesthetised rats but the maximum decrease was unaffected by anaesthesia. The decreases in dopamine and DOPAC induced by alpha-methyl-p-tyrosine 50 mg/kg were affected in opposite directions by halothane: the dopamine reduction was more pronounced while the DOPAC reduction was less pronounced in anaesthetized than in conscious animals. In no case was a qualitative shift in the response observed. It is concluded that halothane may influence the levels of dopamine as well as the response to dopaminergic drugs.

Effects of halothane on the efflux of [3H]D-aspartate from rat brain slices

The in vitro effects of halothane on the potassium-stimulation-induced efflux of [3H]D-aspartate in rat cerebral cortex slices were studied. The slices were initially incubated with Krebs-Ringer's solution containing [3H]D-aspartate, a putative excitatory transmitter. The slices were then stimulated with high concentrations of K+ in the presence and absence of halothane, and the efflux was measured using a scintillation counter. Halothane, 1% and 2%, had little effect on the potassium-stimulation-induced efflux, but that of 4 and 8% increased the efflux significantly. The spontaneous efflux was unaffected by all concentrations of halothane studied. The control study of pentobarbital, in the concentration of 0.05 to 1.00 mmol/l, reduced the efflux in a dose-related manner. These findings indicate that the release of an excitatory transmitter, aspartate, may not be involved in the mechanism of halothane anaesthesia.

Clinical concentrations of volatile anesthetics reduce depolarization-evoked release of [3H]norepinephrine, but not [3H]acetylcholine, from rat cerebral cortex

We compared the potencies of halothane, enflurane, and methoxyflurane in producing unconsciousness in vivo and in inhibiting the release of [3H]norepinephrine and [3H]acetylcholine in vitro. Rats were anesthetized with various concentrations of each anesthetic, and responsiveness was determined by a hemostat tail pinch. Slices of cerebral cortex were equilibrated with similar concentrations of each agent in vitro, and potassium-evoked release of [3H]norepinephrine and [3H]acetylcholine was determined. For both studies, brain concentrations of the anesthetics were determined by heptane extraction and gas chromatography. Using this method, we found that brain concentrations of all three agents which caused unconsciousness in vivo also reduced depolarization-evoked release of [3H]norepinephrine by approximately 30% in vitro. The release of [3H]acetylcholine was unaffected by similar concentrations of these anesthetics. Such selective interference with stimulus-secretion coupling in central noradrenergic, and possibly other, neurons might contribute to the depressant actions of volatile anesthetics. The differential effects on norepinephrine and acetylcholine release also suggest differences in the mechanisms by which these two transmitters are released.

Partial reversal of the cerebral effects of isoflurane in the dog by the benzodiazepine antagonist flumazenil

Six dogs initially anaesthetized with isoflurane-N2O-O2 for surgery with cannulation of the sinus sagittalis for direct measurement of cerebral blood flow (CBF) and maintained with 1% isoflurane-N2-O2 anaesthesia, were given two 2-mg doses of the benzodiazepine antagonist flumazenil with an interval of 10 min. This was accompanied by a significant increase in the cerebral metabolic rate for oxygen (CMRO2) 10 min after the first 2-mg dose, and 5, 10 and 15 min after the second 2-mg dose, the increase varying in mean value from 9% to 14%. EEG was recorded in five of the six dogs and administration of flumazenil converted the EEG from a sleep pattern to an awake pattern in two of them. There were no significant changes in the CBF or mean arterial blood pressure (MABP) values. In the brain biopsies taken at the end of the study there were no significant changes in the adenylate energy charge (EC) and the lactate/pyruvate ratio (L/P) when compared to five other dogs not given flumazenil. It is concluded that flumazenil in this study demonstrated a partial antagonism of the cerebral effects of isoflurane.

A hypothetical model on the mechanism of anesthesia

An hypothesis on the mechanism of action of general anesthetic agents is proposed. It is based on a potentiation of chloride influx due to the action of anesthetic agents on the GABA-receptor complex at the lipid-protein interface. This hypothesis accommodates various observations such as lipid solubility of anesthetic molecules, their lack of stringent structural requirement, pressure reversal of anesthetic action, and other neurochemical and neuropharmacological data.

Electroretinogram as a possible monitor of anesthetic depth

The effects of volatile anesthetics, i.e., methoxyflurane, halothane and enflurane, on the electroretinogram (ERG) were studied in 15 albino rabbits. The ERG was analyzed in terms of the a-wave, and the first oscillatory component (01) in the b-wave. The 01 peak latency showed a significant dose-related prolongation when anesthetic end-tidal concentrations were in excess of 0.8 minimum alveolar concentration (MAC). One MAC, a measure of anesthetic potency, is the end-tidal concentration of an anesthetic at 1 atmosphere that induces immobility in 50% of animals against a noxious stimulus. The amplitudes of the a-wave and the 01 decreased in dose-dependent manners, but their changes were less striking than those of the 01 latency. The peak latency of the a-wave remained unchanged. We conclude that the 01 peak latency is a useful monitor of the depth of inhalational anesthesia.

Induced enzyme release from synaptosomes by halothane

GABA-transaminase has been found to be released from rat brain synaptosomes by halothane in a dose-related manner. The releases of both GABA-transaminase and succinic semialdehyde dehydrogenase were increased with time. The release of other enzymes (creatine kinase, glutamate decarboxylase, aspartate transaminase, lactate dehydrogenase, and malate dehydrogenase) was less in magnitude and not related to the duration of incubation. Such observations suggested a specific event in the halothane-induced release of GABA-catabolizing enzymes. A suggestion linking mode of anesthetic action to a mitochondrial effect of volatile anesthetics was made.