Effect of decreased oxygen on in vitro release of endogenous 3,4-dihydroxyphenylethylamine from mouse striatum

Relation between brain tissue pO2 and dopamine synthesis of basal ganglia--a 18FDOPA-PET study in newborn piglets

Perinatal hypoxic-ischemic cerebral injury is a major determinant of neurologic morbidity and mortality in the neonatal period and later in childhood. There is evidence that the dopaminergic system is sensitive to oxygen deprivation. However, the respective enzyme activities have yet not been measured in the living neonatal brain. In this study, we have used 18F-labelled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emission tomography (PET) to estimate the activity of the aromatic amino acid decarboxylase (AADC), the ultimate enzyme in the synthesis of dopamine, in the brain of newborn piglets under normoxic and moderate asphyxial conditions. The study was performed on 8 newborn piglets (2-5 days old). In each piglet PET studies were performed under control conditions and during 2-hour asphyxia. Simultaneously, brain tissue pO2 was recorded, cerebral blood flow (CBF) was measured with colored microspheres and cerebral metabolic rate of oxygen (CMRO2) was determined. Asphyxia was induced by lowering the inspired fraction of oxygen from 0.35 to 0.10 and adding about 6% CO2 to the inspired gas. Asphyxia elicited a more than 3-fold increase of the CBF (p < 0.01) so that CMRO2 remained unchanged throughout the asphyxial period. Despite this, brain tissue pO2 was reduced from 19 +/- 4 mm Hg to 6 +/- 3 mm Hg (p < 0.01). Blood-brain transfer of FDOPA as well as permeability-surface area product (PS) from striatum were unchanged. Striatal synthesis rate of fluoro-dopamine from FDOPA (k3) was, however, significantly increased (p < 0.01). This increase of the AADC activity is associated with reduced brain tissue pO2. Asphyxia-induced CBF increase impedes an alteration of brain oxidative metabolism.

Relationship of extracellular dopamine in striatum of newborn piglets to cortical oxygen pressure

The present studies describes the relationship between extracellular dopamine in striatum of newborn piglets and cortical oxygen pressure. The extracellular level of dopamine was measured by in vivo microdialysis and the oxygen pressure in the cortex was measured by phosphorescence lifetime of oxygen probe in the blood. Controlled, graded levels of hypoxic insult to the brain of animals were generated by decreasing of the oxygen fraction in the inspired gas (FiO2) from 21% to 14%, 11%, and 9%. This resulted in decrease in the cortical oxygen pressure from 31-35 Torr to about 24 Torr, 15 Torr and 4 Torr, respectively. The changes in extracellular level of dopamine, DOPAC and HVA were dependent on changes in cortical oxygen pressure. Stepwise decrease in the cortical oxygen pressure (see above) caused increases in extracellular dopamine of about 80%, 200% and 550%, respectively. The levels of DOPAC and HVA progressively decreased and when cortical oxygen decreased to 4-6 Torr were about 50% and 70% of control, respectively. After return of FiO2 to control (21%), the cortical oxygen pressure rapidly increased to above normal, then returned to control values. The extracellular levels of dopamine, DOPAC, and HVA recovered more slowly, attaining control values in about 30 minutes. The data show that extracellular levels of dopamine increase with even very small decreases in oxygen pressure. Thus, there is no "oxygen reserve" which protects dopamine release and metabolism from decrease in oxygen pressure.

Early postnatal hypoxia induces long-term changes in the dopaminergic system in rats

A rat model of a mild, chronic, early postnatal hypoxia, characterized by long-term consequences in the behavioural outcome, was used to study long-term consequences in the dopaminergic system. Exposure of newborn rats to an early postnatal hypoxia (hypobaric hypoxia, 11 kPa pO2 in the inspiratory air, 2nd-10th day of life, 10 hours daily) brings about the following lasting neurochemical changes: an increased stimulated dopamine release rate from striatum slices by about 30%, an increased low affinity, high capacity dopamine uptake into striatum synaptosomes by about 100%. The critical period to produce an increased release rate of dopamine was estimated as day 2-6 postnatally. There are no long-term changes in the concentration of dopamine and its metabolites and in the tyrosine hydroxylase activity in consequences of this early postnatal hypoxia. Treatment of newborn animals with L-DOPA (10-50 micrograms/g body weight) previous to hypoxia normalizes the DA release rate.

Dopamine metabolism in the rabbit carotid body in vitro: effect of hypoxia and hypercapnia

Dopamine (DA) and noradrenaline (NA) were measured in the rabbit carotid body (CB) in vitro bv HPLC-ED under the following experimental conditions: 1h superfusion in normoxic, hypoxic (10% O2 in N2) or hypercapnic (8% CO2, 20% O2, 72% N2) medium, 5h superfusion in normoxia or hypoxia. The contents of DA and NA were decreased by hypoxia and hypercapnia after 1 h and 5h indicating a possible DA and NA secretion. Under the same experimental conditions synthesis of DA and NA and catabolism of DA were studied with enzymatic inhibition of tyrosine hydroxylase and monoamine oxidase (MAO) respectively. In hypoxia (1 h and 5h) the rate constant of DA synthesis was the same as in normoxia; however NA synthesis was decreased after 1 h hypoxia. On the contrary, hypercapnia, appeared to be a very effective stimulus of DA and NA synthesis.

Hypoxia induces different responses of striatal high- and low- affinity dopamine uptake sites

The dopamine (DA) uptake over a concentration range from 0.03 to 100 microM was studied in S1 fractions of the rat striatum prepared from control rats and those exposed for 14 h to hypobaric hypoxia. The uptake exhibited non-Michaelis-Menten kinetics, which were evaluated by applying an equation assuming two transport sites. The high-affinity uptake site was characterized by an apparent Michaelis-Menten constant of 0.47 microM and an apparent maximal transport rate of 113 pmol/mg protein/30 s. The respective constants of the low-affinity uptake site were 52.8 microM and 1490 pmol/mg protein/30 s. One hour after hypoxia kinetic constants of the high-affinity uptake were unchanged but the maximal transport rate of the low-affinity uptake was increased by 50%. The elevated low-affinity uptake capacity may represent a means of adaptation to hypoxia allowing a faster removal of high extracellular concentrations of DA.

Simultaneous measurement of oxygen and dopamine: coupling of oxygen consumption and neurotransmission

Fast-scan cyclic voltammetry was used to simultaneously measure increases in dopamine concentration and decreases in O2 concentration evoked by brief electrical stimulation (two pulses at 10 Hz) in slices of rat caudate nucleus. Dopamine concentration began increasing immediately after the first pulse and reached a maximum within 200 ms of stimulation. The O2 concentration began to decrease 300-700 ms after onset of stimulus. Responses for both dopamine and O2 were dependent on external Ca2+ and were Cd2+ and tetrodotoxin sensitive. Only the O2 response was sensitive to CN- (0.15 mM). At short times after exposure to 50 microM ouabain, electrically stimulated dopamine overflow was increased by 150% and electrically stimulated changes in O2 concentration were unaffected. Maximum dopamine concentration was increased 28% by sulpiride (2 microM), 78% by L-DOPA (60 microM), 105% by nomifensine (10 microM) and unaffected by nialamide (10 microM). Maximum decrease in O2 concentration was increased by 25% by sulpiride and unaffected by nialamide, L-DOPA, or nomifensine. The decreases in O2 concentration are indicative of increased O2 consumption and are a measure of oxidative energy production evoked by electrical stimulation. The increase in dopamine is due to the release of dopamine balanced by uptake and serves as an indication of neurotransmitter activity. The results indicate that increases in oxidative energy production following electrical stimulation are dependent on external Ca2+ entry through Cd(2+)-sensitive channels. Possible mechanisms for this coupling are discussed.

Calcium mediation of cyanide-induced catecholamine release: implications for neurotoxicity

Exposure of rat pheochromocytoma (PC12) cells to KCN (1.0-10 mM) over a 30-min period stimulated secretion of dopamine (DA) and decreased intracellular DA content. Addition of KCN (10 mM) to rat frontal cortex slices preloaded with 1-[7-3H]norepinephrine ([3H]NE) increased secretion of NE over a 10- to 30-min incubation period. In PC12 cells release of DA by KCN was nearly abolished in calcium-free media or by prior addition of diltiazem, a calcium channel antagonist. Release of [3H]NE from rat cortical slices by cyanide was only partly inhibited by diltiazem suggesting that intracellular calcium may be involved in this response. In PC12 cells KCN also produced a dose-related release of the DA precursor dihydroxyphenylalanine, without altering intracellular stores. Levels of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were enhanced at lower concentrations of KCN. These observations indicate cyanide elicits exocytotic release of neurotransmitters in a calcium-dependent manner and also show that cyanide alters catecholamine metabolism. These actions of cyanide may be important in CNS symptoms of intoxication.

Effects of hypoxia on the activity of the dopaminergic neuron system in the rat striatum as studied by in vivo brain microdialysis

The purpose of the present study is to clarify the effects of hypoxia on the activity of the dopaminergic neurons in the brain and its mechanism of action. For this purpose, the effects of hypoxia on the extracellular levels of 3,4-dihydroxyphenylethylamine (dopamine) were examined in the rat striatum using in vivo brain microdialysis in the presence or absence of pretreatment with either tetrodotoxin (a blocker of voltage-dependent sodium channels) or nomifensine (a blocker of dopamine reuptake). Exposure to various degrees of hypoxia (15, 10, and 8% O2 in N2) increased dopamine levels in striatal dialysates to 200, 400, and 1,100%, respectively, of the control value. On reoxygenation, dopamine levels in the dialysates rapidly returned to the control level. Reexposure to hypoxia increased the dopamine levels to the same extent as during the first exposure. After addition of tetrodotoxin (40 microM) to the perfusion fluid or pretreatment with nomifensine (100 mg/kg, i.p.), exposure to hypoxia no longer increased the dopamine levels. These results suggest that although hypoxia induces an increase in the extracellular dopamine levels (hence, an apparent increase in the activity of the dopaminergic neurons), this increase is not the result of an increase in dopamine release itself, but rather the result of inhibition of the dopamine reuptake mechanism.

Effects of pentobarbital on proopiomelanocortin opioid products of neonatal piglets during normoxia and hypoxia

Abstract Endogenous opioids have been shown to suppress physiological functions in the neonate. It has been suggested that anesthesia with barbiturates might enhance this suppression by influencing opioid systems directly. To explore this possibility, naive piglets, 2.2+/-0.8 (X+/-SD) days old, underwent one of five protocols: 1) normoxia (control); 2) 10% 0(2)/90% N(2) (hypoxia); 3) saline injection ip during normoxia (sham anesthesia); 4) pentobarbital sodium, 25 mg/kg ip, during normoxia (barbiturate anesthesia); and 5) pentobarbital sodium, 25 mg/kg ip, during hypoxia (combined hypoxia and barbiturate anesthesia). Following the inhalation of either gas mixture for at least 30 min, and precisely 30 min after an injection, blood, cerebrospinal fluid and a dorsal medullary slice containing the nucleus tractus solitarii were collected and processed for measurement by radioimmunoassay of opioid proopiomelanocortin products. These comprised beta-lipotropin (the precursor), beta-endorphin-like immunoreactivity (containing the active peptide beta-endorphin) and N-acetyl beta-endorphin (a deactivated peptide). The most striking result was seen in the cerebrospinal fluid: As compared to barbiturate anesthesia, peptide levels with all other treatments, including combined hypoxia and barbiturate anesthesia, were consistently higher. In the plasma, peptide levels after either combined hypoxia and barbiturate anesthesia or hypoxia alone were generally higher than those of their respective controls (sham anesthesia, control). Plasma levels of beta-endorphin-like immunoreactivity and estimated beta-endorphin with combined hypoxia and barbiturate anesthesia were also higher than those with barbiturate anesthesia. The latter pattern was reversed in the nucleus tractus solitarii, in which beta-endorphin-like immunoreactivity and estimated beta-endorphin levels were lower with combined hypoxia and barbiturate anesthesia than with barbiturate anesthesia alone, although no significant differences were achieved. These results suggest that pentobarbital may decrease the central neuronal release of active endorphins, and thus decrease the quantity of these ligands available for interaction with opioid receptors. Hypoxia, on the other hand, appears to increase such release even in the presence of pentobarbital. Thus, during a hypoxic insult, the suppressive influence of opioids on physiological functions would be enhanced regardless of the presence of barbiturate anesthesia.

Hypoxia induced metabolism dysfunction of rat astrocytes in primary cell cultures

In order to study the astroglial contribution to hypoxic injury on brain tissue metabolism, modifications of glutamine synthetase (GS) lactate dehydrogenase (LDH) enolase and malate dehydrogenase activity produced by reduced oxygen supply have been determined in primary cultures of astrocytes prepared from newborn rat cerebral cortex. Enzymatic activities were measured immediately after the hypoxic treatment (9 h) and during post injury recovery. GS level is significantly decreased in response to low oxygen pressure and increased above control value during the post hypoxic recovery period. The magnitude of GS reduction by hypoxia depends on the age of the cells in culture. Lactate dehydrogenase and enolase levels were significantly enhanced during the two periods considered. No modification of the MDH level was observed. The synthesis of LDH isoenzymes containing mainly M subunits is specifically induced by hypoxia. Our results suggest that astroglial cells may represent a particularly sensitive target toward hypoxia injury in brain tissue. Low oxygen pressure available may modify some fundamental metabolical functions of these cells such as glutamate turnover and lactic acid accumulation.

Hypoxia-induced neurotransmitter deficits in neonatal rats are partially corrected by exogenous GM1 ganglioside

Exposure of 7-day-old rats to 7% oxygen/balance nitrogen for 2 h results in selective changes of cholinergic, serotonergic, and dopaminergic neuronal markers in the frontal cortex, hippocampus, and striatum when evaluated 3 weeks after the insult. There is also about a 15% deficiency in brain weight. Treatment with GM1 ganglioside, 50 mg/kg i.p., for 2 days before and for 3 weeks after the hypoxic insult partially corrects the neurodevelopmental abnormalities including the deficiency in brain weight. We conclude that GM1 ganglioside might have therapeutic potential for treating suspected neonatal hypoxia.

Dopamine and serotonin in rat striatum during in vivo hypoxic-hypoxia

Dopamine and serotonin were determined in extracellular fluid of rat striatum by semiderivative in vivo voltammetry during normoxia and a single or repeated exposure to 15% O2 (i.e., mild hypoxia) or 12.5% O2 (i.e., moderate hypoxia). A single exposure to 15% oxygen increased extracellular dopamine 76%. With reintroduction of air to the animals, dopamine values returned to baseline. During a second episode of 15% oxygen, dopamine increased 63% and remained elevated even during a final exposure to air. On the other hand, serotonin was unaffected by 15% oxygen. Moderate hypoxia (12.5% oxygen) increased dopamine (79%) and serotonin (26%) and both remained elevated even after the initial reintroduction of air. These studies demonstrate that in vivo hypoxia increases rat striatal extracellular dopamine and, to a lesser extent, extracellular serotonin. Furthermore, after repeated, mild hypoxic episodes or moderate hypoxia, the increases in rat striatal extracellular dopamine and serotonin continue even during normoxia. These studies further support a role for dopamine and serotonin in hypoxic-induced changes in brain function. The hypoxic-induced elevation of these two neurotransmitters during normoxia may be important in the production of hypoxic/ischemic-induced cell damage.

Cytosolic-free calcium and neurotransmitter release with decreased availability of glucose or oxygen

Exposing brain slices to reduced oxygen tensions or impairing their ability to utilize oxygen with KCN decreases acetylcholine (ACh) but increases dopamine (DA) and glutamate in the medium at the end of a release incubation. To determine if these changes are due to alterations in the presynaptic terminals, release from isolated nerve endings (i.e. synaptosomes) was determined during histotoxic hypoxia (KCN). KCN reduced potassium-stimulated synaptosomal ACh release and increased dopamine and glutamate release. Since several lines of evidence suggest that altered calcium homeostasis underlies these changes in release, the effects of reducing medium calcium concentrations from 2.3 to 0.1-mM were determined. In low calcium medium, KCN still increased dopamine and glutamate release, but had no effect on ACh release. Hypoxia increased cytosolic-free calcium in both the normal and low calcium medium, although the elevation was less in the low calcium medium. Thus, the effects of histotoxic hypoxia on cytosolic free calcium concentration paralleled those on glutamate and dopamine release. Reducing the glucose concentration of the medium also increased cytosolic-free calcium. The data are consistent with the hypothesis that hypoxia and hypoglycemia increase cytosolic-free calcium, which stimulates the release of dopamine and glutamate, whose excessive release may lead to subsequent cellular damage postsynaptically.

Dopamine, acetylcholine, and glutamate interactions in aging. Behavioral and neurochemical correlates

Aging, hypoxia, and thiamin deficiency diminish motor performance. Similar alterations of ACh, DA, and glutamate metabolism accompany hypoxia, thiamin deficiency, and aging. Both aging and hypoxia reduce ACh release and stimulate DA and glutamate release. Presynaptic enhancement of DA and glutamate release may be important in the production of cell damage that may contribute, in part, to age-related deficits in motor as well as cognitive function. The decline in ACh release may be important in the production of the cognitive deficits. An understanding of the interactions of neurotransmitters in hypoxia and thiamin deficiency aids our understanding of normal aging and increases the possibility of developing better treatments for the multiple neurotransmitter deficiencies that accompany many metabolic, age-related, and chronic degenerative disorders.

Differential alteration of dopamine, acetylcholine, and glutamate release during anoxia and/or 3,4-diaminopyridine treatment

The potassium-stimulated release of acetylcholine (ACh), glutamate (GLU) and dopamine (DA) from mouse striatal slices was studied during anoxia and/or 3,4-diaminopyridine (DAP) treatment. Anoxia, in the presence of calcium, increased DA and GLU release, but depressed ACh release. Omission of calcium from an anoxic incubation further stimulated GLU and DA release and impaired ACh release. Under normoxic conditions, DAP (100 microM) increased the release of all three neurotransmitters; the sensitivity of the slices to DAP changed with the presence or absence of an acetylcholinesterase inhibitor in the preincubation media. During an anoxic incubation, DAP did not ameliorate the anoxic-induced, K+-stimulated impairment of ACh release, but significantly reduced the K+-stimulated release of GLU and DA. These results are consistent with the hypothesis that hypoxia induces a presynaptic deficit that may underlie postsynaptic ischemic-induced changes. Amelioration of these presynaptic alterations in neurotransmitter release may be an effective approach to preventing hypoxic-induced damage.

Selective alteration of mouse brain neurotransmitter release with age

The release of acetylcholine (ACh), glutamate (GLU) and dopamine (DA) from various brain regions was investigated in young (3 month) and old (30 month) Balb/c mice. Aging increased the basal release of GLU (77%) and DA (29%) in striatum and GLU in hippocampus (94%); the concentrations of these neurotransmitters in the media after K+ stimulation were unaltered by aging. Although the basal release of ACh was not altered by age, K+-stimulated ACh release was reduced in striatum. The age-related increases in basal GLU and DA release may be important in the pathophysiology of cell death during aging.