Release of ethanolamine, but not of serine or choline, in rat pontine nuclei on stimulation of afferents from the cortex, in vivo

Ethanolamine: A Potential Promoiety with Additional Effects in the Brain

Ethanolamine is a bioactive molecule found in several cells, including those in the central nervous system (CNS). In the brain, ethanolamine and ethanolamine-related molecules have emerged as prodrug moieties that can promote drug movement across the blood-brain barrier. This improvement in the ability to target drugs to the brain may also mean that in the process ethanolamine concentrations in the brain are increased enough for ethanolamine to exert its own neurological ac-tions. Ethanolamine and its associated products have various positive functions ranging from cell signaling to molecular storage, and alterations in their levels have been linked to neurodegenerative conditions such as Alzheimer's disease. This mini-review focuses on the effects of ethanolamine in the CNS and highlights the possible implications of these effects for drug design.

Ethanolamine Produced from Oleoylethanolamide Degradation Contributes to Acetylcholine/Dopamine Balance Modulating Eating Behavior

Oleoylethanolamide is a well-recognized anorectic compound which also has noteworthy effects on food-reward, influencing the acetylcholine (ACh)/dopamine (DA) balance in the cholinergic system. After its administration, oleoylethanolamide is quickly degraded into oleic acid and ethanolamine. The effect of oleic acid on the gut-brain axis has been extensively investigated, whereas ethanolamine has received scarce attention. However, there is scattered evidence from old and recent research that has underlined the influence of ethanolamine on the cholinergic system. In the present article, we propose a model by which the released ethanolamine contributes to the overall balance between DA and ACh after oleoylethanolamide administration.

Ethanolamine and related amino alcohols increase basal and evoked release of [3H]-D-aspartic acid from synaptosomes by enhancing the filling of synaptic vesicles

This research examines the effects of ethanolamine and other amino alcohols on the dynamics of acridine orange (AO), oxonol V, and [3H]-D-aspartic acid in synaptic preparations isolated from mammalian brain. Ethanolamine concentration-dependently enhanced AO release from synaptosomes. Similar effects were observed with methylethanolamine and dimethylethanolamine, but not choline. The enhancement of AO efflux by ethanolamine was independent of extrasynaptosomal calcium (in contrast to KCl-induced AO efflux), was unaffected by tetrodotoxin and did not involve depolarization of the synaptosomal plasma membrane. KCl was unable to release AO from synaptosomes following exposure to ethanolamine, however ethanolamine and other amino alcohols were found to enhance both basal and KCl-evoked release of [3H]-D-aspartic acid from synaptosomes. Using isolated synaptic vesicles we demonstrate that amino alcohols are able to 1) abolish the ATP-dependent intravesicular proton concentration (i.e. stimulate efflux of AO) in a similar way to carbonyl cyanide m-chlorophenylhydrazone (CCCP), 2) increase the ATP-supported transvesicular membrane potential (i.e. quench oxonol V fluorescence) in contrast to CCCP and 3) enhance intravesicular uptake of [3H]-D-aspartic acid. These results suggest that positively charged, membrane impermeant amino alcohol species are generated within synaptic vesicles as they sequester protons. Cationic forms of these amino alcohols boost the transvesicular electrical potential which increases transmitter uptake into synaptic vesicles and facilitates enhancement of basal and evoked release of transmitter. Our data suggest a potential role for ethanolamine and related amino alcohols in the regulation of synaptic vesicle filling. These findings may also have relevance to neuropathophysiological states involving altered production of ethanolamine.

Depolarization-induced release of ethanolamine from brain synaptic preparations in vitro

The release of ethanolamine from mouse brain synaptosomes and synaptoneurosomes has been investigated. The depolarizing agents veratridine (50 microM), KCl (35 mM) and 4-aminopyridine (2 mM) enhanced the release of [3H]ethanolamine from preloaded synaptosomes under superfusion conditions. Tetrodotoxin (2 microM) strongly inhibited veratridine- and 4-aminopyridine-stimulated release of [3H]ethanolamine but had no effect on KCl-evoked or resting release. In the absence of calcium, a reduction in the resting release of [3H]ethanolamine occurred and release evoked by veratridine, and KCl was markedly reduced. Exposure of preloaded synaptosomes to 5 mM ethanolamine (but not 5 mM serine or 5 mM choline) calcium-dependently increased the efflux of [3H]ethanolamine, however, this was not accompanied by membrane depolarization. When these experiments were performed using synaptoneurosomes, qualitatively similar results were obtained. The resting and evoked release of [3H]ethanolamine was however approximately 2.5-fold higher compared to synaptosomes on a brain equivalent basis, suggesting that uptake and release occur at sites in addition to the nerve ending. Our data are consistent with the idea that a significant amount of ethanolamine accumulates presynaptically and undergoes calcium-dependent release upon depolarization possibly via classical exocytosis. In contrast, ethanolamine-induced release of [3H]ethanolamine likely involves mostly diffusional exchange across the neuronal membrane rather than base exchange. The present results add support to the concept that ethanolamine may play a role as a synaptic signaling molecule in mammalian brain.

Effects of ethanolamine (Etn) administration on Etn and choline (Ch) levels in plasma, brain extracellular fluid (ECF) and brain tissue, and on brain phospholipid levels in rats: an in vivo study

The sources and fates of brain ethanolamine (Etn) are poorly known and the effects of its administration have not been investigated, even though cortical levels are known to be reduced in certain neurodegenerative diseases. We studied the effect of different Etn doses (10(-3), 5 x 10(-3) and 10(-2) mol/kg, i. p.) on its and choline's (Ch) levels in arterial plasma and brain extracellular fluid (ECF) of awake rats. We also studied its effects on brain levels of Etn, Ch, and their respective major phospholipids. Etn administration caused dose dependent increases in Etn levels within both plasma and brain ECF. For the 10(-2) mol/kg dose, Etn levels were significantly (p<0.01) greater than pre-injection values in both the plasma and ECF. Whole brain Etn and phosphatidylethanolamine were also significantly (p<0.05) increased by 10(-2) mol/kg Etn. Exogenous Etn significantly (p<0.05) increased Ch levels in plasma and whole brain; Etn also increased brain ECF Ch levels. Our data show for the first time that circulating Etn can act as a source of brain Ch. Metabolic pathways that might mediate the increases in Etn and Ch are discussed, as are possible mechanisms of the decreases in brain Eth seen in Alzheimer's disease.

Push-pull cannula for localized application of drugs and sampling of medium, combined with electrophysiological recordings in an interface slice chamber

These experiments combined electrophysiological recordings from hippocampal slices with application of drugs to and sampling of extracellular fluid from a restricted region of the slice using a push-pull cannula placed under the slice in an interface chamber. Stable and apparently normal extracellular and intracellular recordings could be obtained directly over the tip of the cannula and solutions changed without disturbing the recording. Relatively rapid effects (1-5 min) were observed when TTX, CNQX, or medium containing 50 mM K+ were applied via the cannula and recovery from these effects was achieved. In addition, effects were restricted to the immediate vicinity of the cannula; neurones recorded several hundred micrometers away were apparently unaffected. Samples of extracellular fluid obtained as minute fractions during the application of high K+ containing medium contained higher concentrations of GABA, aspartate and glutamate than control fractions but the same levels of other amino acids, e.g., isoleucine and leucine. With appropriate design of push-pull cannula and recording chamber, therefore, stable electrophysiological recordings can be combined with localized extracellular fluid sampling and rapid and localized application of test solutions in an interface slice chamber.

In vivo release of endogenous glutamate and aspartate in the rat striatum during stimulation of the cortex

The release of endogenous glutamate and aspartate from corticostriate neurons in the anaesthetized rat was investigated. Efflux of glutamate, aspartate and leucine in push-pull cannula perfusates of the striatum was measured in 1 min fractions collected before, during and after a 4 min stimulation period of ipsilateral frontal cortex. Efflux of the putative excitatory amino acid transmitters, glutamate and aspartate, determined during the first minute of stimulation, was significantly elevated above prestimulation resting level (by 167% and 316%, respectively), while efflux of leucine, a non-transmitter amino acid, remained unchanged. Efflux of glutamate and aspartate during the last 3 min of stimulation dropped rapidly, indicating the activation of a regulatory mechanism, presumably re-uptake. The data further support the hypothesis that glutamate and/or aspartate act as transmitters or are metabolites of transmitters in the corticostriate pathway.

Phosphoethanolamine and ethanolamine are decreased in Alzheimer's disease and Huntington's disease

Measurements of both phosphoethanolamine (PEA) and ethanolamine (EA) were made in postmortem brain samples from patients with Alzheimer's disease (AD) and Huntington's disease (HD) using high-performance liquid chromatography with electrochemical detection. In AD levels of PEA were significantly reduced by 64% in temporal cortex, 48% in frontal cortex and 40% in hippocampus. In HD concentrations of PEA were significantly reduced by 76% in the caudate, 53% in putamen and 48% in the nucleus accumbens. EA concentrations showed similar but smaller reductions in both diseases. Both PEA and EA are involved in phospholipid metabolism and can be released with depolarizing stimuli in some circumstances. Since two diverse neuropathologic processes can result in depleted levels of both amines in their respective regions of pathologic predilection, it is likely that the depletions accompany neuronal death.

Potassium-evoked release of endogenous primary amine-containing compounds from the trout saccular macula and saccular nerve in vitro

An in vitro preparation of the trout saccular macula, containing a large number of hair cells, served as a potential source of neurotransmitter(s) released at the acousticolateralis hair cell-afferent nerve synapse. An in vitro preparation of the saccular nerve, maintained in parallel, served to indicate the potential neural contribution to overall release from the macula. Efflux of 27 primary amine-containing compounds from the macula and nerve fractions was monitored by cation-exchange HPLC with fluorescence detection, and release by 53.5 mM potassium was determined at 1.45 mM calcium, 0.35 mM magnesium or 0 mM calcium, 10.1 mM magnesium. Taurine was released from the saccular macula in the greatest amount, accounting for 72% of the total evoked release of primary amine-containing compounds. Its release was calcium dependent and its time course prolonged. The contribution by myelinated nerve and associated Schwann cells within the macula to overall release of taurine from the macula in the presence of calcium, as determined from the saccular nerve preparation, was only 2%. Other components specifically released from the macula included ethanolamine, phosphoserine, beta-alanine, and glycine. Glutamate and aspartate were released from both the macula and saccular nerve fractions by potassium in the presence of calcium and in a ratio of 6:1 (glutamate:aspartate) for the macula and 7.5:1 for the nerve. The release of aspartate, but not that of glutamate, was lowered in saline containing 0 mM calcium, 10.1 mM magnesium. The calculated contribution from neural elements to overall release from the macula was 10% for aspartate and 18% for glutamate. These studies demonstrate that both the macula and saccular nerve fractions release the 'excitatory neurotransmitter' candidates aspartate and glutamate. Calcium-dependent, potassium-evoked release of taurine appears to be specific to the hair cell-supporting cell population of the saccular macula, and taurine may, therefore, be involved directly or indirectly in hair cell neurotransmission in labyrinthine organs. This study represents the first detailed biochemical characterization of efflux and release for an in vitro hair cell system of relatively high purity with respect to hair cells.