Inhibition by halothane of release of norepinephrine, but not of dopamine-beta-hydroxylase, from guinea-pig vas deferens

The effects of general anesthetics on norepinephrine release from isolated rat cortical nerve terminals

Intravenous and volatile general anesthetics inhibit norepinephrine (NE) release from sympathetic neurons and other neurosecretory cells. However, the actions of general anesthetics on NE release from central nervous system (CNS) neurons are unclear. We investigated the effects of representative IV and volatile anesthetics on [(3)H]NE release from isolated rat cortical nerve terminals (synaptosomes). Purified synaptosomes prepared from rat cerebral cortex were preloaded with [(3)H]NE and superfused with buffer containing pargyline (a monoamine oxidase inhibitor) and ascorbic acid (an antioxidant). Basal (spontaneous) and stimulus-evoked [(3)H]NE release was evaluated in the superfusate in the absence or presence of various anesthetics. Depolarization with increased concentrations of KCl (15-20 mM) or 4-aminopyridine (0.5-1.0 mM) evoked concentration- and Ca(2+)-dependent increases in [(3)H]NE release from rat cortical synaptosomes. The IV anesthetics etomidate (5-40 microM), ketamine (5-30 microM), or pentobarbital (25-100 microM) did not affect basal or stimulus-evoked [(3)H]NE release. Propofol (5-40 microM) increased basal [(3)H]NE release and, at larger concentrations, reduced stimulus-evoked release. The volatile anesthetic halothane (0.15-0.70 mM) increased basal [(3)H]NE release, but did not affect stimulus-evoked release. These findings demonstrate drug-specific stimulation of basal NE release. Noradrenergic transmission may represent a presynaptic target for selected general anesthetics in the CNS. Given the contrasting effects of general anesthetics on the release of other CNS transmitters, the presynaptic actions of general anesthetics are both drug- and transmitter-specific.

IMPLICATIONS

General anesthetics affect synaptic transmission both by altering neurotransmitter release and by modulating postsynaptic responses to transmitter. Anesthetics exert both drug-specific and transmitter-specific effects on transmitter release: therapeutic concentrations of some anesthetics stimulate basal, but not evoked, norepinephrine release, in contrast to evoked glutamate release, which is inhibited.

Pentobarbitone inhibits the stress response to transport in male goats

Pentobarbitone (20 mg/kg i.v.) blocked plasma cortisol release when administered either before a 20 min journey or during a 2 h journey. This confirms that pentobarbitone can block stimulated, as well as resting, cortisol secretion. In general, blood glucose concentrations were not increased above 90 mg/100 ml until at least 30 min after the start of transport; however, this increase was also blocked by pentobarbitone administered 30 min into the 2 h journey. Significant increases in respiratory and heart rates occurred within 15 min of the start of transport; pentobarbitone caused an immediate decrease in these parameters. In conclusion, pentobarbitone was shown to reverse many metabolic changes induced by transport.

Anaesthetic concentrations of enflurane and methoxyflurane in rat brain in-vivo and in-vitro

We measured concentrations of enflurane and methoxyflurane in brains of anaesthetized rats and established conditions for reproducing these concentrations in brain tissue in-vitro. Despite a 12-fold difference in inspired potency, brain concentrations resulting in anaesthesia were similar for both compounds. However, substantially lower concentrations in the equilibrating gas were necessary to achieve similar tissue concentrations in-vitro, probably because anaesthetic-induced respiratory depression or changes in cardiac output causes incomplete equilibration in-vivo. These studies provide direct evidence that brain concentrations associated with anaesthesia are similar for anaesthetics with different inspired potencies. They also suggest that lower concentrations in the equilibrating gas should be used in-vitro to reproduce clinically relevant tissue concentrations that are necessary to cause anaesthesia in-vivo.

The effects of anesthetics on the concentrations of cholecystokinin octapeptide sulfate-like immunoreactivity in rat brain regions

Cholecystokinin octapeptide sulfate-like immunoreactivity (CCK-8S-LI) was determined by radioimmunoassay in rat brain areas following injections of pentobarbital, halothane and chloral hydrate. Time-dependent changes in the concentrations of CCK-8S-LI were different between pentobarbital and chloral hydrate in all brain regions studied. After pentobarbital injection, CCK-8S-Li peaked at 30-60 min in the frontal cortex, nucleus accumbens, striatum and substantia nigra; after chloral hydrate injection, CCK-8S-LI peaked at 120 min in the hypothalamus, nucleus accumbens and substantia nigra. Both anesthetics induced almost the same sleeping times. Halothane inhalation caused increases in the concentrations of CCK-8S-LI in the amygdala and hippocampus. In addition, following intracardial perfusion of saline for 30 min after pentobarbital anesthesia, the concentrations of CCK-8S-LI increased in the nucleus accumbens, and decreased in the frontal cortex. These results suggest that since different anesthetics cause different changes in CCK levels, anesthetics affect studies of these neurons.

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.

Postganglionic sympathetic nerve activity in halothane-anaesthetized rats during controlled and spontaneous ventilation

The aim of the study was to compare the effect of halothane anaesthesia on sympathetic nerve discharge in mechanically normoventilated and spontaneously breathing rats. Renal sympathetic nerve activity (rSNA), mean arterial pressure (MAP) and heart rate (HR) were measured in the conscious state and at the inspiratory halothane concentrations of 0.6%, 1.2% and 2.4% in one mechanically normoventilated and one spontaneously breathing group, while a third group was subjected to controlled hypoventilation at 1.2% halothane concentration. Halothane in blood was determined in two separate groups at 1.2%. In an additional group of spontaneously breathing rats, PaCO2 was analysed during consciousness and the halothane concentrations of 1.2% and 2.4%. There was a pronounced decrease in rSNA, MAP and HR at all levels of anaesthesia in the mechanically ventilated rats. However, rSNA, HR and MAP were significantly higher in the spontaneously breathing rats at increasing levels of halothane anaesthesia. Controlled hypoventilation at 1.2% halothane increased the variables significantly. In spontaneously breathing animals, PaCO2 increased significantly during the halothane exposure. The concentration of halothane in blood was significantly higher in the spontaneously breathing rats. Thus, the halothane-induced respiratory depression in the spontaneously breathing rats preserved rSNA during halothane anaesthesia, possibly via CO2-mediated chemoreceptor stimulation.

Mechanisms of halothane action on synaptic transmission in motoneurons of the newborn rat spinal cord in vitro

Action of halothane on synaptic transmission was studied on the isolated newborn rat spinal cord. Clinical doses of halothane (less than or equal to 3%) suppressed mono- and polysynaptic reflexes, dorsal root reflexes, excitatory and inhibitory postsynaptic potentials as well as the spontaneous synaptic potentials caused by impulse bombardment. However, the spontaneous miniature inhibitory postsynaptic potentials observed after blocking impulse activities by tetrodotoxin were not all suppressed by halothane. During halothane administration, the membrane potential of motoneurons was hyperpolarized by several millivolts, associated with an increase in input conductance. However, the threshold potential level for spike generation was virtually unaffected. Depression of synaptic transmission in spinal motoneurons by halothane is suggested to be due to two factors: a reduction in the amount of transmitter release secondary to interference with Ca2+ entry into nerve terminals, either by partial blockade of impulse invasion or voltage-dependent Ca2+ channels; and an increase in the depolarizing current necessary for excitation of motoneurons owing to hyperpolarization and decreased input resistance.

Cardiac arrhythmias induced by guanethidine in cats anesthetized with halothane

Rapid i.v. administration of guanethidine provokes severe and sustained ventricular arrhythmias in cats anesthetized with halothane. The arrhythmias include premature ventricular excitations, multifocal ventricular rhythm, bigeminy, trigeminy and ventricular tachycardia. They begin in about 20 sec and last from 4 to 100 min. By comparison, a standard dose of noradrenaline (10 micrograms/kg) induces ventricular arrhythmias which develop in about 12 sec and continue for 1-2.5 min. Pretreatment with beta-adrenoceptor blocking drugs prevents arrhythmias from both drugs, and pretreatment with imipramine or reserpine prevents arrhythmias for guanethidine but not noradrenaline. The adrenergic neuron blocking action of guanethidine does not alter the arrhythmogenic action of guanethidine since arrhythmias can still be produced after adrenergic neuron blockade. These results indicate that guanethidine causes arrhythmias by releasing noradrenaline from cardiac adrenergic neuron storage sites and, therefore, point out the vulnerability of the heart to arrhythmias when noradrenaline-releasing drugs interact with halogenated hydrocarbon anesthetics.

The effect of two anesthetic agents on norepinephrine and dopamine in discrete brain nuclei, fiber tracts, and terminal regions of the rat

Catecholaminergic neurons have been implicated in the mechanism of general anesthesia, but previous attempts at measuring changes in adrenergic neuron function during anesthesia have been limited by techniques to whole brain. Microdissection techniques and sensitive radioisotopic-enzymatic assays were used to measure levels of catecholamines in 20 different nuclei, fiber tracts or nerve terminal regions in brains of rats anesthetized for 90-105 min with 1% halothane or 18% cyclopropane. These two anesthetics were chosen because of their diverse effects on the electroencephalogram and on the cardiovascular and respiratory systems. Of the areas examined, significant increases in norepinephrine content with both anesthetic agents were found only in the nucleus accumbens, locus coeruleus and central gray catecholamine areas. Only in the nucleus accumbens was the dopamine level increased by both anesthetics; cyclopropane, but not halothane anesthesia, also increased the dopamine content of the caudate nucleus, while halothane, but not cyclopropane anesthesia, significantly decreased the dopamine level of the ventral nucleus of the thalamus. Changes in levels of transmitters do not distinguish cause from effect of anesthesia, and further experiments are needed to delineate what role, if any, the specific areas play in muscle relaxation, analgesia, sleep or anesthesia. This study shows that a drug can affect one nucleus or region without significantly affecting other regions that contain the same transmitter substance, and that changes in transmitter levels can occur selectively in different regions of brain even if the nerve endings are derived from contiguous cell bodies.