Monoamine neurotransmitter metabolism and locomotor activity during chemical hypoxia

Neurological effects of respiratory dysfunction

The respiratory and the nervous systems are closely interconnected and are maintained in a fine balance. Central mechanisms maintain strict control of ventilation due to the high metabolic demands of brain which depends on a continuous supply of oxygenated blood along with glucose. Moreover, brain perfusion is highly sensitive to changes in the partial pressures of carbon dioxide and oxygen in blood, which in turn depend on respiratory function. Ventilatory control is strictly monitored and regulated by the central nervous system through central and peripheral chemoreceptors, baroreceptors, the cardiovascular system, and the autonomic nervous system. Disruption in this delicate control of respiratory function can have subtle to devastating neurological effects as a result of ensuing hypoxia or hypercapnia. In addition, pulmonary circulation receives entire cardiac output and this may act as a conduit to transmit infections and also for metastasis of malignancies to brain resulting in neurological dysfunction. Furthermore, many neurological paraneoplastic syndromes can have underlying lung malignancies resulting in respiratory dysfunction. It is essential to understand the underlying mechanisms and the resulting manifestations in order to prevent and effectively manage the many neurological effects of respiratory dysfunction. This chapter explores the various neurological effects of respiratory dysfunction with focus on their pathophysiology, etiologies, clinical features and long-term neurological sequelae.

Acetyl-l-Carnitine ameliorates hypobaric hypoxic impairment and spatial memory deficits in rats

Inadequate oxygen availability at high altitude causes oxidative stress and generation of reactive oxygen species, which may lead to memory impairment. Hippocampus, which plays a key role in the learning and memory processes, is especially vulnerable to hypoxic damage. The present study was aimed at investigating the effect of acetyl-L-carnitine on spatial working and reference memory deficits along with oxidative and apoptotic damage, caused by hypobaric hypoxia in male Sprague Dawley rats. Rats were trained in Morris Water Maze for eight days after which they were submitted to chronic hypobaric hypoxia exposure at a simulated altitude of 6100 m for three days. Rats received daily acetyl-L-carnitine at a dosage of 75 mg/kg body weight orally during exposure. Subsequent to exposure, performance of the animals was tested in Morris Water Maze, which revealed working memory impairment that was significantly improved by acetyl-L-carnitine. However, there was no change in the reference memory after hypobaric hypoxia exposure. Following behavioral study animals were sacrificed and biomarkers of oxidative damage like free radical production, lactate dehydrogenase activity, lipid peroxidation, antioxidant status and expression of apoptotic [viz. caspase-3, Apoptosis activating factor (Apaf-1), bax, cytochrome c] and anti-apoptotic protein-Bcl-2 were studied in the hippocampus. There was a significant increase in oxidative stress along with increased expression of apoptotic proteins and NR1 subunit of glutamate receptor indicating occurrence of excitotoxicity in hypoxia exposed rats. These results suggested that supplementation with acetyl-L-carnitine improves spatial working memory deficits reduces oxidative stress and inhibits apoptotic cascade induced by hypoxia.

Socially-mediated differences in brain monoamines in rainbow trout: effects of trace metal contaminants

Monoaminergic systems play a crucial role in linking behaviour and physiology. Here the physiological and behavioural effects of metal exposure in relation to monoaminergic systems were considered by exposing rainbow trout dyads, demonstrating stable dominance relationships, to cadmium or lead. Fish exposed to 4 microg l(-1) cadmium accumulated more cadmium at the gill than fish held in control water. Fish exposed to 7 microg l(-1) cadmium had higher gill, liver and kidney cadmium concentrations. No significant lead accumulation was seen after exposure to 46 microg l(-1) for 48 h but exposure to 325 microg l(-1) lead caused an increase in gill, liver and kidney lead concentrations. Brain accumulation of both cadmium and lead was only seen after exposure to the highest concentrations. Exposure to 4 or 7 microg l(-1) cadmium, or 46 or 325 microg l(-1) lead for 48 h did not disrupt established dominance hierarchies. As expected with this stable behavioural situation, in control pairs, animals of different social status displayed different physiological profiles. Subordinate fish had higher concentrations of circulating plasma cortisol and telencephalic 5-hydroxyindoleacetic acid/5-hydroxytryptamine (serotonin) (5-HIAA/5-HT) ratios. However, these physiological profiles were affected by metal exposure, with a trend towards higher serotonergic activity in dominant fish. Dominants exposed to 325 microg l(-1) lead had significantly higher hypothalamic 5-HIAA/5-HT ratios when compared with subordinates. The results demonstrate that if stable social hierarchies are established in control water they may not be affected by exposure to cadmium and lead although physiological changes may be evident.

Blood oxygen partial pressure affects plasma prolactin concentration in humans

Responses of plasma prolactin (PRL) concentration to acute and repeated changes in blood oxygen partial pressure (PO2a) at rest were investigated in two studies (A; B), with special reference to possible effects mediated via serotonin (5-HT) synthesis. In A, nine male subjects inhaled for 105 min gas containing different oxygen fractions for 6 days. Gas concentrations consisted of 14% (A14), 21 % (A21), 40% (A40), 60% (A60) and 80% (A80) O2 mixed with N2 as well as 100% O2 (A100). Venous and capillary blood samples were drawn before and every 15 min during gas inhalation for analysis of plasma PRL and PO2a. In B, two groups of subjects (B I; B II) were exposed to 30 min day(-1) of gas inhalation over 14 consecutive days. Gas concentration consisted for B I of 14% O2/86% N2 and for B II of 100% O2. During pre- and post-examination a baseline blood sample was drawn, followed by a neuroendocrine test of serotonergic function using a partial 5-HT1A receptor agonist (60 mg of buspirone hydrochloride). In A, each increase of inhaled oxygen fraction also resulted in higher blood POb2a. In A14, A21 and A40, plasma PRL concentrations did not change from basal level. Increases in plasma PRL concentration were found in A60 after 30 min as well as in A80 and A100 after 15 min. A higher blood PO2a induced a higher plasma PRL secretion but also an earlier decline from peak plasma PRL value despite continued inhalation of the respective oxygen concentration. During post-examination in B, basal plasma PRL concentrations were increased in B I and decreased in B II. Plasma PRL response to stimulation challenge was not affected by treatments. Thus, chronic adaptations of basal plasma PRL concentrations to decreased/increased blood PO2a were not related to up/down-regulation, respectively, of central serotonergic receptor function.

Effects of carbon monoxide exposure on serotonergic neuronal systems in rat brain

It is well known that some psychiatric sequelae exist after CO poisoning, but few animal studies on serotonergic neuronal function after CO exposure have been carried out. We investigated the effects of successive carbon monoxide (CO) exposure (6000 ppm, 10 min, 3 repetitions) on serotonergic neuronal systems in rat brain. Serotonin (5-HT) concentrations were significantly decreased only in the frontal cortex from 1 hr to 7 days after CO exposure. 5-Hydroxyindoleacetic acid (5-HIAA) concentrations were significantly increased at 1 hr in all six brain regions measured (frontal cortex, striatum, hypothalamus, hippocampus, midbrain, and pons). 5-HT synthesis, measured by the accumulation of 5-hydroxytryptophan (5-HTP) after the administration of m-hydroxybenzylhydrazine (NSD-1015), was significantly decreased in all regions from 1 hr to 7 days after CO exposure. [3H]Ketanserin (5-HT2A) binding sites in the frontal cortex were not affected by CO exposure. DOI-induced head shakes, a 5-HT2A receptor mediated behavior, were not changed after CO exposure. These findings indicated that CO exposure caused presynaptic serotonergic neuronal dysfunctions that consisted mainly of decreased concentration of 5-HT in the frontal cortex or a decrease of 5-HT synthesis in all six regions, without compensatory hyperfunction of 5-HT2A receptors.

Effect of O2 availability on neuroendocrine variables at rest and during exercise: O2 breathing increases plasma prolactin

Neuroendrocrine and substrate responses were investigated in eight male athletes during inhalation of either 100% O2 (HE), 14% O2 (HO) or normoxio gas (NO) before, during and after 60 min of cycle ergometry at the same absolute work rate. Concentrations of prolactin (PRL), growth hormone (GH), testosterone (T), adrenocorticotropic hormone (ACTH), cortisol (COR), adrenalin (A), noradrenalin (NA), insulin (INS), ammonia (NH3), free fatty acids, serotonin (5-HT), total protein, branched-chain amino acids (BCAA) and free tryptophan (free TRP) were determined in venous blood and lactate concentration [LA-], partial pressure of oxygen (PO2), oxygen saturation (SO2), partial pressure of carbon dioxide and pH in capillary blood. The PO2 and SO2 were augmented in HE and decreased in HO (P < or = 0.01). In HO and NO no significant changes were found for any other parameter during 30 min of rest prior to exercise. In HE, PRL increased by about 400% during this time, while NA declined (P < or = 0.01). Heart rate (HR) and [LA-] were higher during exercise in HO (P < or = 0.01). In all trials, NH3, NA, A, T, GH and ACTH increased during exercise (P < or = 0.01), while BCAA and INS declined. In comparison to NO and HE, increases of NA, A, GH, COR and ACTH were higher in HO (P < or = 0.01). The PRL in NO and COR in NO and HE did not change significantly. In HE, after the initial increase at rest, PRL declined during exercise but remained higher than in HO. Higher values for NA, A, GH, COR and ACTH in HO were likely to have reflected an augmented relative exercise intensity. Our results showed that PRL but no other hormone increased during acute exposure to hyperoxia. This PRL release was independent of exercise stress and greater than PRL augmentation during hypoxia, which was related to a higher relative exercise intensity as indicated by [LA-] and HR. Responses of plasma NH3, BCAA, free TRP and 5-HT could not explain PRL augmentation induced by the increment in blood SO2 during hyperoxia.

Estimation of the convulsive effect of cyanide in rats

Convulsions are frequently observed in severe, acute cyanide intoxication. The mechanisms involved are not well known. In the present study, the convulsive effect of cyanide was examined in the rat by means of a dose threshold determination after infusion of cyanide. An optimal dose rate of 1.80 mg/min./kg was determined. By infusing this optimal dose rate it was possible to divide another cyanide-treated group of rats into two groups: one without and another with convulsions. After the experiment all animals were killed by exposure to high intensity microwave irradiation. Regional dopamine, noradrenaline and main dopamine metabolites were isolated and analyzed using high pressure liquid chromatography and electrochemical detection. Striatal noradrenaline was decreased but dopamine and its metabolites were increased in rats infused with cyanide at the optimal dose rate until the convulsions started. Animals infused with the threshold dose at the optimal dose rate did not show any significant changes in noradrenaline or in dopamine metabolites, but all animals showing convulsions also had increased striatal dopamine levels. Thus changes in dopamine appear to be dependent on convulsions.

Nimodipine prevents the in vivo decrease in hippocampal extracellular acetylcholine produced by hypobaric hypoxia

Hypoxia decreases acetylcholine (ACh) synthesis and release in vitro, and ACh synthesis in vivo; however, its effect on extracellular concentration of ACh in vivo is not known. The calcium channel blocker nimodipine is a cerebrovascular dilator which also increases extracellular ACh in vivo. Therefore, it may provide protection from the effects of hypobaric hypoxia on the cholinergic system either via its effects on vascular function or by direct action on the nervous system. This study examined the effect of hypobaric hypoxia on extracellular ACh and choline levels, as measured by microdialysis, as well as the effects of nimodipine under hypoxia. Microdialysis guide cannulae were implanted into the hippocampal region of male Fischer rats so that probes would sample from the CA1 and DG regions. Animals were then exposed for eight hours to a simulated altitude of 5,500 m (18,000 ft) or tested at sea level for an equivalent duration. HPLC with electrochemical detection was used for analysis of the dialysates. At 5,500 m extracellular ACh levels in the placebo-treated group were significantly lower than the sea level group values. This decrement was reversed by nimodipine administered i.p. immediately preceding altitude ascent (10 mg/kg) and 250 min post-altitude ascent (10 mg/kg). These data suggest that nimodipine may provide protection from the detrimental effects of hypoxia on hippocampal cholinergic function.

Effects of acute lethal cyanide intoxication on central dopaminergic pathways

In rats treated with sodium cyanide (NaCN), 20 mg/kg intraperitoneally, the striatal dopamine (DA) level was decreased within 60 sec. compared to controls injected with NaCl 0.9%. Treatment with NaCN also increased the naturally occurring L-DOPA in the striatum, but not in the other brain regions studied. Decreased DA levels but increased L-DOPA accumulation were also seen in cyanide-treated animals after inhibition of neuronal L-aromatic amino decarboxylase. In rats given a non-lethal dose of NaCN, 2.5 mg/kg intraperitoneally, 30 min. before sacrifice and L-DOPA, 100 mg/kg intraperitoneally, 25 min. before sacrifice, regional L-DOPA levels were not significantly changed, but the striatal DA levels were diminished compared to controls. Decreased DA levels might indicate that cyanide inhibits the synthesis of brain DA. However, both increased L-DOPA and increased accumulation of L-DOPA after neuronal decarboxylase were observed after cyanide. Furthermore, we have earlier reported that lethal doses of NaCN decreased the DA metabolite HVA in the striatum but did not significantly change the oxidatively deaminated metabolite of DA, DOPAC. Inhibition of L-aromatic amino acid decarboxylase appears to play a minor role in causing decreased striatal DA levels. However, our findings might be compatible with cyanide-produced inhibition of the energy-demanding granular uptake and/or release of DA.

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.

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.

Locomotor activity as a predictor of times and dosages for studies of nicotine's neurochemical actions

Nicotine's action on the central nervous system is complex and likely involves an interaction of neurotransmitters. To determine the time after administration of nicotine and dosage for neurochemical studies, locomotor activity of CD-1 mice was determined at 5 min intervals between 0-60 min. A low nicotine dosage (0.05 mg/kg) did not alter activity 5-15 min after drug injection, but increased activity 28% at 15-25 min post-injection. A high dosage (0.8 mg/kg) reduced total distance 62% and rearing 87% at 5-15 min; at 15-25 minutes total distance declined 56% and rearing 69%; all measures returned to control values after 30 minutes; rearing then increased at 40 min after nicotine. Pretreatment (15 min before nicotine) with mecamylamine (1.0 mg/kg), but not hexamethonium (1.0 mg/kg), prevented the depressant effect of nicotine. Dopamine (DA) and its metabolites as well as acetylcholine (ACh) synthesis were measured at the point of nicotine's maximal depressant action. Striatal levels of dihydroxyphenylacetic acid (DOPAC) were increased and ACh utilization was reduced in striatum (-25%) and cortex (-24%) 10 min after nicotine (0.8 mg/kg). Mecamylamine, while preventing the depressant effect of nicotine on locomotor activity, did not alter its effects on DA metabolism. These results demonstrate that the behavioral outcome of acute nicotine treatment is time and dose-dependent. Nicotine's depressant action appears not to be due to altered DA but may be related to changes in carbohydrate and acetylcholine metabolism.

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

The effects of hypoxia on release of endogenous 3,4-dihydroxyphenylethylamine (DA, dopamine) were investigated in mouse striatal slices. Following a 60-min preincubation, potassium increased DA release 12 times between zero and 1 min. By 10 min, uptake processes exceeded release and DA levels in the media decreased. Hypoxia (low oxygen) and anoxia (no oxygen) increased DA in the media by 120 and 205%, respectively, but did not alter dihydroxyphenylacetic acid concentrations. Under similar conditions, anoxia increased [3H]DA uptake eight-fold. For the uptake studies, the amount of DA added to the media was critical; in the presence of high concentrations of DA, anoxia reduced reuptake. Regardless of exogenous DA, hypoxia and anoxia increased extracellular DA, which may play a role in ischemic cell damage.

Behavioral and neurochemical correlates of morphine and hypoxia interactions

Decreased oxygen availability (hypoxia) impairs the synthesis of dopamine and serotonin in parallel with a decline in open-field behavior. If hypoxic-induced deficits in dopamine and serotonin metabolism are physiologically important, then stimulation of their synthesis may help reverse hypoxic-induced neurochemical and behavioral deficits. Acute morphine sulfate (50 mg/kg) increased dihydroxyphenylacetic acid/dopamine ratios (DOPAC/DA) (+20%), the conversion of [3H]tyrosine to [3H]dopamine (+73%) and open-field activity (+130%) in CD-1 male mice. However, morphine failed to significantly alter the incorporation of [3H]tryptophan to [3H]serotonin. Morphine antagonized the hypoxic-induced impairment of dopamine metabolism and locomotor activity. DOPAC/DA ratios of hypoxic animals that were treated with morphine were identical to controls, and conversion rates of [3H]tyrosine to [3H]dopamine were increased. Total distance in an automated activity monitor following the combination of morphine and hypoxia increased 79% compared to a 48% decrease with hypoxia alone. These results suggest that both hypoxia and morphine alter the dopaminergic system, but in opposite directions. These interactions may help to explain why morphine is able to ameliorate hypoxic-induced changes in behavior.