Effects of monoamine transmitters on neurons and astrocytes: correlation between energy metabolism and intracellular messengers

Fasting prevents medetomidine-induced hyperglycaemia and alterations of neurovascular coupling in the somatosensory cortex of the rat during noxious stimulation

Medetomidine and isoflurane are commonly used for general anaesthesia in fMRI studies, but they alter cerebral blood flow (CBF) regulation and neurovascular coupling (NVC). In addition, medetomidine induces hypoinsulinemia and hyperglycaemia, which also alter CBF regulation and NVC. Furthermore, sudden changes in arterial pressure induced by noxious stimulation may affect NVC differently under medetomidine and isoflurane anaesthesia, considering their different effects on vascular functions. The first objective of this study was to compare NVC under medetomidine and isoflurane anaesthesia during noxious stimulation. The second objective was to examine whether fasting may improve NVC by reducing medetomidine-induced hyperglycaemia. In male Wister rats, noxious electrical stimulation was applied to the sciatic nerve in fasted or non-fasted animals. CBF and local field potentials (LFP) were recorded in the somatosensory cortex to assess NVC (CBF/LFP ratio). The CBF/LFP ratio was increased by medetomidine compared with isoflurane (p = 0.004), but this effect was abolished by fasting (p = 0.8). Accordingly, medetomidine produced a threefold increase in blood glucose (p < 0.001), but this effect was also abolished by fasting (p = 0.3). This indicates that isoflurane and medetomidine anaesthesia alter NVC differently, but the undesirable glucose dependent effects of medetomidine on NVC can be prevented by fasting.

L-Tyrosine availability affects basal and stimulated catecholamine indices in prefrontal cortex and striatum of the rat

We previously found that L-tyrosine (L-TYR) but not D-TYR administered by reverse dialysis elevated catecholamine synthesis in vivo in medial prefrontal cortex (MPFC) and striatum of the rat (Brodnik et al., 2012). We now report L-TYR effects on extracellular levels of catecholamines and their metabolites. In MPFC, reverse dialysis of L-TYR elevated in vivo levels of dihydroxyphenylacetic acid (DOPAC) (L-TYR 250-1000 μM), homovanillic acid (HVA) (L-TYR 1000 μM) and 3-methoxy-4-hydroxyphenylglycol (MHPG) (L-TYR 500-1000 μM). In striatum L-TYR 250 μM elevated DOPAC. We also examined L-TYR effects on extracellular dopamine (DA) and norepinephrine (NE) levels during two 30 min pulses (P2 and P1) of K+ (37.5 mM) separated by t = 2.0 h. L-TYR significantly elevated the ratio P2/P1 for DA (L-TYR 125 μM) and NE (L-TYR 125-250 μM) in MPFC but lowered P2/P1 for DA (L-TYR 250 μM) in striatum. Finally, we measured DA levels in brain slices using ex-vivo voltammetry. Perfusion with L-TYR (12.5-50 μM) dose-dependently elevated stimulated DA levels in striatum. In all the above studies, D-TYR had no effect. We conclude that acute increases within the physiological range of L-TYR levels can increase catecholamine metabolism and efflux in MPFC and striatum. Chronically, such repeated increases in L-TYR availability could induce adaptive changes in catecholamine transmission while amplifying the metabolic cost of catecholamine synthesis and degradation. This has implications for neuropsychiatric conditions in which neurotoxicity and/or disordered L-TYR transport have been implicated.

A glycogen phosphorylase inhibitor selectively enhances local rates of glucose utilization in brain during sensory stimulation of conscious rats: implications for glycogen turnover

Glycogen is degraded during brain activation but its role and contribution to functional energetics in normal activated brain have not been established. In the present study, glycogen utilization in brain of normal conscious rats during sensory stimulation was assessed by three approaches, change in concentration, release of (14)C from pre-labeled glycogen and compensatory increase in utilization of blood glucose (CMR(glc)) evoked by treatment with a glycogen phosphorylase inhibitor. Glycogen level fell in cortex, (14)C release increased in three structures and inhibitor treatment caused regionally selective compensatory increases in CMR(glc) over and above the activation-induced rise in vehicle-treated rats. The compensatory rise in CMR(glc) was highest in sensory-parietal cortex where it corresponded to about half of the stimulus-induced rise in CMR(glcf) in vehicle-treated rats; this response did not correlate with metabolic rate, stimulus-induced rise in CMR(glc) or sequential station in sensory pathway. Thus, glycogen is an active fuel for specific structures in normal activated brain, not simply an emergency fuel depot and flux-generated pyruvate greatly exceeded net accumulation of lactate or net consumption of glycogen during activation. The metabolic fate of glycogen is unknown, but adding glycogen to the fuel consumed during activation would contribute to a fall in CMR(O2)/CMR(glc) ratio.

Astrocytic contributions to bioenergetics of cerebral ischemia

Astrocytes are multifunctional cells that interact with neurons and other astrocytes in signaling and metabolic functions, and their resistance to pathophysiological conditions can help restrict loss of tissue after an ischemic event provided adequate nutrients are supplied to support their requirements. Astrocytes have substantial oxidative capacity and mechanisms to upregulate glycolytic capability when respiration is impaired. An astrocytic enzyme that synthesizes a powerful activator of glycolysis is not present in neurons, endowing astrocytes with the ability to sustain ATP production under restrictive conditions. The monocarboxylic acid transporter (MCT) isoforms predominating in astrocytes are optimized to facilitate very large increases in lactate flux as lactate concentration increases within (1-3 mM) and above (>3 mM) the normal range. In sharp contrast, the major neuronal MCT serves as a barrier to increased transmembrane transport as lactate rises above 1 mM, restricting both entry and efflux. Lactate can serve as fuel during recovery from ischemia but direct evidence that lactate is oxidized by neurons (vs. astrocytes) to maintain synaptic function is lacking. Astrocytes have critical roles in regulation of ionic homeostasis and control of extracellular glutamate levels, and spreading depression associated with ischemia places high demands on energy supplies in astrocytes and contributes to metabolic exhaustion and demise. Disruption of Ca2+ homeostasis, generation of oxygen free radicals and nitric oxide, and mitochondrial depolarization contribute to astrocyte death during and after a metabolic insult. Novel pharmaceutical agents targeted to astrocytes and hyperoxic therapy that restores penumbral oxygen level during energy failure might improve postischemic outcome.

Receptor subtype and dose dependence of dexmedetomidine-induced accumulation of [14C]glutamine in astrocytes suggests glial involvement in its hypnotic-sedative and anesthetic-sparing effects

Dexmedetomidine, a selective alpha(2)-adrenergic agonist, increases accumulation of [14C]glutamine and its labeled metabolites in primary cultures of mouse astrocytes. The concentration dependence is biphasic and identical to that previously described for dexmedetomidine's effect on free cytosolic calcium concentration ([Ca(2+)](i)) in astrocytes, and both effects are exerted on the alpha(2A) subtype of the alpha(2) receptor, suggesting a Ca(2+)-mediated effect. The concentration corresponding to the most potent effect is similar to that with which dexmedetomidine exerts its anesthetic-sparing activity in vivo, and the second peak corresponds to its hypnotic-sedative effect. It is suggested that both effects may be caused by decreased glutamatergic neurotransmission, secondary to reduced availability of glutamine as a glutamate precursor in glutamatergic neurons.

Nordidemnin potently inhibits inositol uptake in cultured astrocytes and dose-dependently augments lithium's proconvulsant effect in vivo

It has been suggested that inositol uptake across the cell membrane is of importance for maintenance of the inositol pool involved in lithium's therapeutic effect in bipolar disease and in the lithium-pilocarpine seizure test in freely moving rats (measuring the latency of a normally subconvulsive concentration of pilocarpine to seizure induction in the additional presence of lithium). We have tested this hypothesis by: 1) demonstrating an extremely high potency of nordidemnin as an inhibitor of myo-inositol uptake in primary cultures of mouse astrocytes; and 2) determining the dose-response correlation of a nordidemnin-induced decrease in the latency before appearance of seizures in the lithium-pilocarpine test after intracerebroventricular injection of minute samples (10 microl) of virtually isotonic saline solution.

The role of astrocytes and noradrenaline in neuronal glucose metabolism

In the classical model the energy requirements during neuronal activation are provided by the delivery of additional glucose directly into the extracellular compartment that results from the increase in local cerebral blood flow (rCBF). The present review proposes that astrocytes play a key role in the response to neuronal activation. Arginine for the synthesis of NO, which has a major role in the increase in rCBF, is released from astrocytes in response to stimulation of astrocytic glutamate receptors. The increased delivery of glucose by the blood stream enters astrocytes, where some of it is converted to glycogen. During neuronal activation there is a decrease in extracellular glucose owing to increased utilization followed by a delayed increase; this results from stimulation of astrocytic beta-adrenergic receptors, which leads to a breakdown of glycogen and the export of glucose.

Noradrenaline effects on pyruvate decarboxylation: correlation with calcium signaling

Noradrenaline effects on the rate of metabolism of pyruvate to acetyl coenzyme A, catalyzed by the pyruvate dehydrogenase complex, was measured in primary cultures of mouse astrocytes as rate of production of labeled CO(2) from 1-[(14) C]pyruvate in the absence of competing glucose in the medium. The subtype specificity of a noradrenaline-stimulated increase in rate of CO(2) formation was identical to that for noradrenaline-induced increase in free intracellular calcium ([Ca(2+)](i)), suggesting a causal relationship between these two phenomena. The noradrenaline-induced stimulation of pyruvate decarboxylation was abolished in the presence of 10 mM magnesium chloride in the medium, combined with the omission of calcium, a procedure known to prevent an increased [Ca(2+)] in the cytosol from raising intramitochondrial [Ca(2+)]. Thus, the stimulation of metabolic flux through the reaction catalyzed by the pyruvate dehydrogenase complex appears to result from an increase in intramitochondrial [Ca(2+)] ions in astrocytes. Such a mechanism for stimulation of the same enzyme has been convincingly demonstrated in other cell types, primarily heart muscle and hepatic cells, but it has not previously been demonstrated in any cell type from the central nervous system.

Functional studies in cultured astrocytes

Studies using primary cultures of astrocytes have made essential contributions to the understanding of astrocytic functions and neuronal-astrocytic interactions. The purposes of this article are to (i) outline principles and methodologies used in the preparation of such cultures and caveats for the interpretation of the observations made; (ii) summarize astrocytic functions in turnover of the amino acid transmitters glutamate and gamma-aminobutyric acid (GABA), in energy metabolism and in Na+,K+-ATPase-catalyzed processes and emphasize the degree to which the observations have been confirmed in intact tissue; (iii) describe regulations of astrocytic functions by transmitters and by calcium channel activity; and (iv) indicate suggestions for future functional studies using astrocytes in primary cultures and emphasize that some of the conclusions about neuronal-astrocytic interactions reached on the basis of studies in cultured cells and confirmed in intact tissue may not yet have been completely integrated into general neuroscience knowledge.

Astrocyte-neuron interaction during one-trial aversive learning in the neonate chick

During two specific stages of the Gibbs-Ng model of one-trial aversive learning in the neonate chick, we have recently found unequivocal evidence for a crucial involvement of astrocytes. This evidence is metabolic (utilization of the astrocyte-specific energy store, glycogen, during normal learning and inhibition of memory formation by the astrocyte specific metabolic inhibitors, fluoroacetate and methionine sulfoximine) as well as physiological (abolition of memory formation in the presence of ethacrynic acid, an astrocyte-specific inhibitor of cellular reaccumulation of potassium ions). These findings are discussed in the present review in the framework of a more comprehensive description of metabolic and physiological neuronal-astrocytic interactions across an interstitial (extracellular) space bounded by minute processes from either cell type.

What do retinal müller (glial) cells do for their neuronal 'small siblings'?

Müller (radial glial) cells are the predominant glia of the vertebrate retina. They arise, together with rod photoreceptor cells, bipolar cells, and a subset of amacrine cells, from common precursor cells during a late proliferative phase. One Müller cell and a species-specific number of such neurons seem to form a columnar unit within the retinal tissue. In contrast, 'extracolumnar neurons' (ganglion cells, cone photoreceptor cells, horizontal cells, and another subset of amacrine cells) are born and start differentiation before most Müller cells are generated. It may be essential for such neurons to develop metabolic capacities sufficient to support their own survival, whereas late-born ('columnar') neurons seem to depend on a nursing function of their 'sisterly' Müller cell. Thus, out of the cell types within a retinal column it is exclusively the Müller cell that possesses the enzymes for glycogen metabolism. We present evidence that Müller cells express functional insulin receptors. Furthermore, isolated Müller cells rapidly hydrolyse glycogen when they are exposed to an elevated extracellular K+ ion concentration, a signal that is involved in the regulation of neuronal-glial metabolic cooperation in the brain. Müller cells are also thought to be essential for rapid and effective retinal K+ homeostasis. We present patch-clamp measurements on Müller cells of various vertebrate species that all demonstrate inwardly rectifying K+ channels; this type of channel is well-suited to mediate spatial buffering currents. A mathematical model is presented that allows estimation of Müller cell-mediated K+ currents. A simulation analysis shows that these currents greatly limit lateral spread of excitation beyond the borders of light-stimulated retinal columns, and thus help to maintain visual acuity.

Long-lasting abolishment of noradrenaline induced stimulation of oxidative metabolism after chronic exposure of developing mouse astrocytes to cocaine

Rate of 14CO2 production from [l-14C]glutamate was determined as a measurement of oxidative metabolism in developing primary cultures of astrocytes, obtained from the neonatal mouse brain and grown in the absence (control) or presence of cocaine. From the age of 3 days, the drug-exposed cultures were grown in a tissue culture medium containing either 1 or 3 microM cocaine. After 2 months of chronic exposure to cocaine the metabolic rate showed an increase of approximately 50%, but there was a long lag period (several weeks) before this response occurred. In contrast to a marked stimulation of CO2 production when noradrenaline was added to untreated cultures of the same age, there was no similar effect of noradrenaline on cultures treated with cocaine. After exposure to cocaine for 21 days (24-day-old cultures), both the enhanced CO2 production and the abolishment of the normal response to noradrenaline persisted during 'withdrawal' (cessation of drug exposure) throughout the total period investigated, i.e. to an age of 60 days (corresponding to a withdrawal period of 36 days). The correlation of these findings with in vivo data is discussed.