Chieko Okuda

Cerebral Extracellular Glucose and Lactate Concentrations during and after Moderate Hypoxia in Glucose-and Saline-infused Rats

To assess the effect of a glucose infusion on brain extracellular fluid (ECF) during systemic hypoxia, changes in glucose and lactate concentrations in cerebral ECF during and after moderate hypoxic hypoxia were measured in adult, conscious, unrestrained rats, with a microdialysis probe in the posterior hippocampus. The rats were given either saline (n = 6) or 50% glucose solution (n = 6) for 3 h, starting 60 min before the onset of hypoxia. Hypoxia was produced by circulating 7% O2 gas in a plastic chamber for 90 min. In saline-infused animals, brain ECF glucose concentrations decreased slightly during hypoxia, although blood glucose concentrations did not change. Blood lactate concentrations increased to 6.28 +/- 0.91 mM, at 60 min after the onset of hypoxia (P less than 0.05). Brain ECF lactate concentrations increased to 3.53 +/- 0.20 mM and remained constant during 60 min of steady-state hypoxia (P less than 0.05) and decreased to the basal level within 60 min after the end of hypoxia. When sodium lactate solution was infused intravenously for 90 min (n = 4), blood lactate concentrations increased to a level as high as those found during hypoxia. However, the brain ECF lactate concentration increased only to 1.86 +/- 0.09 mM. In glucose-infused animals, the blood glucose concentration reached 339.1 +/- 32.3 mg/dl at the end of the glucose infusion, and the brain ECF glucose concentration increased to 54.7 +/- 7.3 mg/dl.(ABSTRACT TRUNCATED AT 250 WORDS)

The involvement of central cholinergic mechanisms in cardiovascular responses to intracerebroventricular and intravenous administration of thyrotropin-releasing hormone.

Intracerebroventricular (i.c.v.) administration of thyrotropin-releasing hormone (TRH) in a range from 0.1 to 100 micrograms induced a dose-related increase in blood pressure in conscious rats, whereas TRH-free acid (TRH-OH) and histidyl-proline diketopiperazine (His-Pro-DKP), metabolites of TRH, did not. The blood pressure responses to intravenous (i.v.) injection of 5 mg/Kg TRH were similar to those induced by TRH (i.c.v.). Pretreatment with atropine (50 micrograms, i.c.v.) significantly reduced the pressor effect of TRH administered through either route. Hemicholinium-3 (50 micrograms, i.c.v.), an inhibitor of choline uptake, also prevented the increase in blood pressure induced by TRH (10 micrograms, i.c.v.). These results indicate that both centrally and peripherally administered TRH have pressor effects that are mediated by central cholinergic mechanisms, probably by activating cholinergic neurons.

Cardiovascular effect of intravenously administered thyrotropin-releasing hormone and its concentration in push-pull perfusion of the fourth ventricle in conscious and pentobarbital-anesthetized rats

Changes in the concentration of thyrotropin-releasing hormone (TRH) in cerebrospinal fluid (CSF) were examined by the push-pull perfusion method after intravenous (i.v.) administration of the peptide in conscious and pentobarbital-anesthetized rats. The concentration of endogenous TRH in the perfusate was not changed during the 160-min perfusion period and was similar to that in the CSF (0.92 +/- 0.26 ng/ml) collected before the perfusion in conscious as well as in anesthetized rats. After i.v. administration of TRH (5 mg/kg) to the conscious rats, the peptide concentration in the perfusate increased to 42.23 +/- 14.33 ng/ml during the first 20 min and gradually returned to the basal level 2 hr after administration. The total amount of TRH detected in the perfusate was 20.0 ng. It was reduced by 75% in the anesthetized animals. The increases in blood pressure and heart rate, seen after i.v. as well as intracerebroventricular administration of TRH in the conscious rats, was significantly inhibited in the anesthetized rats. These results indicate that systemically administered TRH exerts its cardiovascular effect at central site(s), and that the transportation and the effect of the peptide is suppressed by pentobarbital anesthesia.

Hypothalamic control of pituitary and adrenal hormones during hypothermia.

In order to investigate neuroendocrinological mechanisms of hypothermia, we determined the changes in plasma concentrations of corticosterone (CS), prolactin (PRL), and thyrotropin (TSH), and their correlations with alterations in hypothalamic dopamine (DA) and thyrotropin releasing hormone (TRH), in rats restrained and immersed in a water bath at various temperatures. A graded decrease of body temperature induced a progressive increase in the plasma level of CS, whereas that of PRL showed a drastic decrease. The plasma level of TSH also showed an increase during mild hypothermia (about 35 degrees C), but this increase was not evident during profound hypothermia (below 24 degrees C). The changes in these hormones were readily reversed by rewarming animals. Although DA content in the hypothalamus was not affected, its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), showed an increase following the decrease of body temperature. Pretreatment of the animals with sulpiride, a D2-antagonist, prevented the hypothermia-induced inhibition of PRL release. Hypothalamic TRH was significantly decreased during mild hypothermia, and it returned to control levels after rewarming. These results suggest that the decrease in plasma PRL induced by hypothermia may be associated with the activation of hypothalamic DA neurons, whereas the increase in plasma TSH during mild hypothermia seems to be caused by the increased release of TRH in the hypothalamus.

Effects of hypothermia on thyrotropin-releasing hormone content in the rat brain

The thyrotropin-releasing hormone (TRH) content in the brain was determined in normothermic and hypothermic rats subjected to immobilization stress. TRH contents in the hypothalamus, midbrain and cerebral cortex significantly decreased during mild hypothermia (body temperature about 34 degrees C), but not during profound hypothermia (about 24 degrees C). The decreases in the TRH content during mild hypothermia were readily reversed by rewarming the animal. These results indicate that cerebral TRH is involved in the response to a mild body temperature drop when the animal is exposed to a cold environment.

Involvement of endogenous thyrotropin-releasing hormone in central regulation of the cardiovascular system after bleeding in conscious rats

The 4th ventricle of a conscious rat was perfused using a push-pull cannula. The concentration of thyrotropin-releasing hormone (TRH) in the perfusate was significantly increased after withdrawal of 30% of the total blood. Administration of antiserum of TRH into the ventricle potentiated and prolonged the hypotension induced by the bleeding. These results suggest that endogenous brain TRH is involved in the central regulation of the cardiovascular system after bleeding in conscious rats.

Suppression of the pressor effect of centrally administered thyrotropin-releasing hormone under halothane, pentobarbital and flunitrazepam anaesthesia

Intracerebroventricular (i.c.v.) administration of thyrotropin‐releasing hormone (TRH) caused an increase in blood pressure (BP) and heart rate (HR) in conscious rats. The pressor effect was greatly diminished by adrenalectomy as well as after pretreatment with phentolamine, an α‐receptor antagonist or with mecamylamine, a ganglion blocker, suggesting that centrally administered TRH increases BP mainly by stimulating sympathetic activity. Under halothane (0.8%), pentobarbital (33 mg/kg, i.p.) and flunitrazepam (0.8 mg/kg, i.v.) anaesthesia, the pressor effect of TRH was almost completely blocked. The increase in BP induced by peripheral α‐receptor stimulation with phenylephrine was not affected by the anaesthetics at these doses. Pretreatment with atropine (50 μg, i.c.v.) significantly reduced the pressor effect of TRH. Intracerebroventricularly administered haloperidol and bicuculline also partially diminished the increase in BP produced by TRH, while other neurotransmitter blockers such as phentolamine, propranolol and naloxone did not. These results indicate that the anaesthetics at the doses employed interfere with the central neuronal pathway(s), probably cholinergic pathways, through which TRH exerts its pressor effect.

Hemorrhage-induced regional brain thyrotropin-releasing hormone release in conscious rats measured by microdialysis

The septum, nucleus accumbens and preoptic area in the brains of conscious, freely moving rats were perfused using microdialysis probes. The TRH concentration significantly increased in the septum after withdrawal of 30% of the total blood volume but remained at constant levels in the other brain areas. Also, high potassium dose-dependently stimulated TRH release in vivo. These results suggest that blood loss stimulates septal TRH release, probably by membrane depolarization of TRH-containing nerve terminals.

The interaction of pancuronium and vecuronium with cardiac muscarinic receptors

The interaction of vecuronium, a monoquaternary analogue of pancuronium, with the cardiac muscarinic receptors in canine hearts was investigated in vitro by [3H] QNB binding assay and compared with that of pancuronium. Both pancuronium and vecuronium, which are nicotinic antagonists of competitive types, inhibited the binding of [3H] QNB to cardiac muscarinic receptors with values of IC50 of 5.41 × 10‐7 mol/l and 3.97 × 10‐6 mol/l respectively. According to the Kd and Bmax values on the Scatchard plot, pancuronium, while not influencing the number of receptors, had a 3‐fold greater inhibitory effect than vecuronium on the affinity of the muscarinic receptors. The K1 values of vecuronium and pancuronium showed that pancuronium had a 7.3‐fold greater affinity with the receptors than vecuronium. We concluded that vecuronium had a direct inhibitory action on the binding of [3H] QNR to the canine heart muscarinic receptors, but this action was much weaker than pancuronium.

Alterations in cerebral β-adrenergic receptor-adenylate cyclase system induced by halothane, ketamine and ethanol

The effect of halothane, ketamine and ethanol on ?-adrenergic receptor adenylate cyclase system was studied in the brain of rats. An anesthetic concentration of halothane and ketamine added in vitro decreased the stimulatory effect of norepinephrine on cyclic AMP formation in slices from the cerebral cortex. On the other hand, ethanol increased the basal activity of cerebral adenylate cyclase without affecting on the norepinephrine-stimulated activity. The increase of the basal activity induced by ethanol was not antagonized by propranolol, a ?-adrenergic antagonist. In the crude synaptosomal (P(2)) fraction, these drugs had no significant effect on the basal adenylate cyclase activity, binding of [(3)H]dihydroalprenolol to ?-receptor, and binding of [(3)H]guanylylimido diphosphate ([(3)H]Gpp(NH)p) to guanyl nucleotide binding site. In contrast, the adenylate cyclase activity stimulated by Gpp(NH)p or NaF was significantly inhibited by an anesthetic concentration of these drugs. An anesthetic concentration of these drugs increased the membrane fluidity of P(2) fraction monitored by the fluorescence polarization technique. The addition of linoleic acid (more than 500 ?M) also induced not only the increase of fluidity, but also the decrease of Gpp(NH)p- or NaF-stimulated adenylate cyclase activity in the cerebral P(2) fraction. The present results suggest that general anesthetics may interfere with the guanyl nucleotide binding regulatory protein-mediated activation of cerebral adenylate cyclase by disturbing the lipid region of synaptic membrane.