Ultrastructural observations on transendothelial transport from extracellular neuronal spaces towards the vascular lumina of cerebral capillaries of hydrocephalic human biopsies.
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Biopsy↗
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Extracellular potassium changes in the spinal cord of the cat and their relation to slow potentials, active transport and impulse transmission.
1. By means of K-specific double-barrelled micro-electrodes the time course of changes in K+ concentration in the extracellular space of the lumbar spinal cord was examined after peripheral tetanic stimulation and after a single volley in a mixed peripheral nerve in non-anaesthetized, intercollicularly decerebrated and spinalized cats. 2. Tetanic stimulation (100 Hz) which increases the [K]e from 3 to 9 mM is followed by a phase of reduced [K]e during which [K]e decreases by 0.5 mM below resting level, lasting 1-2 minutes before returning to its original resting level. Evidence is presented that this subnormal phase of [K]e reflects active processes redistributing accumulated K+ from extracellular space. 3. The subnormal phase of [K]e can be registered only when the microelectrode is located in very close vicinity of discharging neurones and is not primarily dependent on the absolute level of increased [K]e. This can be considered as evidence that the neurones and not the glial cells are responsible for active reabsorption of K+ from the extracellular space. 4. Increased E1K]e is reflected in focally recorded potentials as a negativity and decreased [K]e as a positivity. The latency of focally recorded positivity is, however, shorter than the latency of reduced [K]e. This makes it likely that the positivity reflects not only passive hyperpolarization of glial elements, but also an active, electrogenic ion transport across neuronal membrane. 5. The shortest latency of increased [K]e induced by a single volley in a mixed peripheral nerve was found to be 9 msec; the peak, representing 0.5 mM, was attained after 40 msec and the total duration was 200 msec. A theoretical consideration is put forward that the time course of transient increase in [K]e is consistent with the suggestion that K+ which accumulates in the spinal cord after neuronal discharge is responsible for primary afferent depolarization. 6. Evidence is presented that increased [K]e, induced by a long lasting peripheral stimulation, is accompanied by decreased efficacy of impulse transmission.
Animals↗
Human epidermal energy metabolism is functionally anaerobic.
We have reported that epidermal Langerhans cells possess an H(+)-extruding mechanism signalling their existence in an anaerobic environment. This study highlights the energy metabolism of human epidermis. In their habitual state the keratinocytes contain more lactate than do most other cell types. Their lactate production in vitro is vigorous and independent of oxygen and most of it is released to the medium. Autoincubation of the epidermis under starved conditions resulted in a 30% increase of lactate, indicating ongoing glycogenolysis. Iodoacetate inhibited lactate production by > 90%. Energy charge values were low, approximately 0.82, and comparable with those previously reported for smooth muscle. Moreover, the overwhelming majority of the keratinocytic mitochondria had an appearance markedly deviating from those in the Langerhans cells, melanocytes and fibroblasts, and, above all, were characterized by an enormous reduction of the inner membrane. This structure is in all probability incompatible with an effective oxidative metabolism of glucose. We conclude that epidermal energy metabolism is predominantly anaerobic in spite of the formal presence of mitochondria. The high production of lactate obviously demands extracellular transport pathways for rapid elimination of this organic acid. An extracellular space complying with such a demand emerges on electron microscopy when an isotonic glutaraldehyde-based fixative is used. The prevailing view regarding the size of the extracellular space is based on the common use of hypotonic fixatives, such as Karnovski's fixative, which causes gross cellular swelling and concomitant near total elimination of the extracellular space, leaving interstices with a diameter significantly smaller than that allowing fluid flow.
Adult↗
Penetration of substances into tumor tissue: a methodological study with microelectrodes and cellular spheroids.
A new method was tested for studies of penetration of substances into tumorlike tissue. The penetration of the ions K+, Cl-, and Ca2+ through several layers of tumor cells was demonstrated by using double barrelled, ion sensitive microelectrodes with extra thin tip diameters. Spheroids consisting of human glioma, U-118 MG, and human thyroid cancer, HTh-7, cells were used as models of tumor tissue. A microelectrode was inserted into the center of a spheroid. Thereafter, the concentration of the test substance was increased in the surrounding medium. The change in concentration inside the spheroid was recorded and the penetration pattern evaluated. All three types of tested ions penetrated easily through the spheroids. The K+ ions penetrated most efficiently, and the Ca2+ ions showed the slowest penetration. The Ca2+ ions penetrated somewhat more slowly in the U-118 MG spheroids (which had rather small extracellular spaces) than in the HTh-7 spheroids (which had larger extracellular spaces). Ion sensitive electrodes, which are easily available, were used in this study only to demonstrate the principle. We hope that the method described can be used for penetration studies of various substances. For example, all substances that can be detected by enzyme microelectrodes could be studied. The main advantage of the method is that the complete penetration pattern can be studied as a function of time in individual spheroids. Previously described methods require histological procedures for each analyzed penetration time.
Biological Transport↗
Extracellular matrix formation by chondrocytes in monolayer culture.
In previous studies were have reported on the secretion and extracellular deposition of type II collagen and fibronectin (Dessau et al., 1978, J. Cell Biol., 79:342-355) and chondroitin sulfate proteoglycan (CSPG) (Vertel and Dorfman, 1979, Proc. Natl. Acad. Sci. U. S. A. 76:1261-1264) in chondrocyte cultures. This study describes a combined effort to compare sequence and pattern of secretion and deposition of all three macromolecules in the same chondrocyte culture experiment. By immunofluorescence labeling experiments, we demonstrate that type II collagen, fibronectin, and CSPG reappear on the cell surface after enzymatic release of chondrocytes from embryonic chick cartilage but develop different patterns in the pericellular matrix. When chondrocytes spread on the culture dish, CSPG is deposited in the extracellular space as an amorphous mass and fibronectin forms fine, intercellular strands, whereas type II collagen disappears from the chondrocyte surface and remains absent from the extracellular space in early cultures. Only after cells in the center of chondrocyte colonies shape reassume spherical shape does the immunofluorescence reveal type II collagen in the refractile matrix characteristic of differentiated cartilage. By immunofluorescence double staining of the newly formed cartilage matrix, we demonstrate that CSPG spreads farther out into the extracellular space that type II collagen. Fibronectin finally disappears from the cartilage matrix.
Animals↗
Alpha adrenergic-mediated accumulation of calcium in reperfused myocardium.
Reperfusion of ischemic myocardium is associated with increases in total myocardial calcium (Ca+2), which may influence the ultimate extent of ischemic damage as well as the development of arrhythmias. Since reperfusion is also associated with enhanced alpha-adrenergic responsivity, this study was performed to determine the potential interactions between alpha-adrenergic receptors and myocardial calcium during reperfusion. Cats were subjected to 35 min of left anterior descending coronary artery occlusion and 10 min of reperfusion. Total myocardial calcium was measured by atomic absorption spectrometry. Intracellular calcium was calculated from measurements of extracellular space [( 3H]inulin). In control animals with reperfusion, total calcium increased from 0.32 +/- 0.03 to 0.65 +/- 0.05 mmol/100 g dry tissue (P less than 0.0001), while intracellular calcium increased from 0.15 +/- 0.03 to 0.40 +/- 0.05 mmol/100 g dry tissue (P less than 0.001). Pretreatment with the alpha-adrenergic blocking agents phentolamine or prazosin prevented the increase in total and intracellular calcium. Phentolamine and the aqueous soluble alpha 1-adrenergic antagonist BE-2254 administered as late as 2 min before reperfusion similarly attenuated the increase in tissue calcium. Although administration of BE-2254 2 min before reperfusion failed to block the reperfusion-induced increase in extracellular space, the increase in calculated intracellular calcium was prevented. beta-Adrenergic blockade with propranolol partially attenuated but did not prevent an increase in total tissue calcium. Labetalol, a combined alpha- and beta-adrenergic blocking agent completely blocked the increase in tissue calcium during reperfusion. Additional experiments performed after 70 min of ischemia with reperfusion demonstrated a 49% attenuation of the increase in tissue calcium with alpha-adrenergic blockade. Electron microscopy with pyroantimonate and x-ray microprobe analysis demonstrated a large increase in calcium precipitate in mitochondria after reperfusion in untreated animals. Though alpha-adrenergic blockade prevented the calcium deposition in mitochondria, other criteria of ischemia persisted. Thus, alpha-adrenergic blockade specifically prevents the increase in intracellular calcium during reperfusion in reversibly injured tissue, independent of alterations in extracellular space and tissue water.
Adrenergic alpha-Antagonists↗
In vivo metabolism and clearance of substance P and co-expressed tachykinins in rat striatum.
Neurons expressing the preprotachykinin A gene, which encodes the sequences of substance P, neurokinin A, neuropeptide gamma and neuropeptide K, exemplify peptide co-existence. Furthermore, there is also evidence that substance P fragments have biological activity. However, the relative contribution of each of these peptides to tachykinin signalling is still poorly understood. An important factor which will determine the characteristics of the signal mediated by co-localised peptides is their clearance from the extracellular space. The striatum, in which tachykinins are present and exert neuromodulatory roles, can be used as a model to investigate this aspect. Therefore, in this study we characterised in vivo in the striatum the metabolism and clearance of substance P and of the other three co-expressed peptides. After intrastriatal administration of 1 pmol, tritiated substance P disappeared too rapidly for metabolites to be detected. However, when 10 nmol substance P and 1 pmol tritiated substance P were co-injected, substance P(1-4) and substance P(1-7), which are biologically active, were detected as major metabolites. Under these conditions, the rate of decay of tritiated substance P was 0.2 nmol/min. The effects of the peptidase inhibitors thiorphan, bestatin and captopril suggested that neutral endopeptidase 24.11 and aminopeptidases were involved in primary substance P cleavages, whereas angiotensin-converting enzyme was involved in secondary cleavages. The monitoring of the decay of unlabelled substance P by high-performance liquid chromatography gave a rate of 0.16 nmol/min. Using high-performance liquid chromatography with capillary electrophoresis, the rates of decay of 10 nmol neurokinin A or neuropeptide gamma were five and seven times faster than that of substance P. In contrast, over the time course of the experiment, no significant decay of neuropeptide K was detected. These results show that substance P disappears rapidly from the extracellular space, and supports the formation in vivo of major N-terminal active substance P metabolites. Our study also highlights significant differences in the clearance of co-expressed tachykinins and suggests that certain species may disappear relatively slowly from the extracellular space, and thus may make a significant temporal and spatial contribution to signalling.
Animals↗
Myelin membrane structure and composition correlated: a phylogenetic study.
We have correlated myelin membrane structure with biochemical composition in the CNS and PNS of a phylogenetic series of animals, including elasmobranchs, teleosts, amphibians, and mammals. X-ray diffraction patterns were recorded from freshly dissected, unfixed tissue and used to determine the thicknesses of the liquid bilayer and the widths of the spaces between membranes at their cytoplasmic and extracellular appositions. The lipid and protein compositions of myelinated tissue from selected animals were determined by TLC and sodium dodecyl sulfate-polyacrylamide gel electrophoresis/immunoblotting, respectively. We found that (1) there were considerable differences in lipid (particularly glycolipid) composition, but no apparent phylogenetic trends; (2) the lipid composition did not seem to affect either the bilayer thickness, which was relatively constant, or the membrane separation; (3) the CNS of elasmobranch and teleost and the PNS of all four classes contained polypeptides that were recognized by antibodies against myelin P0 glycoprotein; (4) antibodies against proteolipid protein (PLP) were recognized only by amphibian and mammalian CNS; (5) wide extracellular spaces (ranging from 36 to 48 A) always correlated with the presence of P0-immunoreactive protein; (6) the narrowest extracellular spaces (approximately 31 A) were observed only in PLP-containing myelin; (7) the cytoplasmic space in PLP-containing myelin (approximately 31 A) averaged approximately 5 A less than that in P0-containing myelin; (8) even narrower cytoplasmic spaces (approximately 24 A) were measured when both P0 and 11-13-kilodalton basic protein were detected; (9) proteins immunoreactive to antibodies against myelin P2 basic protein were present in elasmobranch and teleost CNS and/or PNS, and in mammalian PNS, but not in amphibian tissues; and (10) among mammalian PNS myelins, the major difference in structure was a variation in membrane separation at the cytoplasmic apposition. These findings demonstrate which features of myelin structure have remained constant and which have become specifically altered as myelin composition changed during evolutionary development.
Animals↗
Electron microscopic and immunocytochemical study of rapidly frozen, freeze-substituted neural lobes of rats.
Rapid freezing of freshly dissected or incubated neural lobes was explored as a means of obtaining ultrastructural preservation of the more natural state of this tissue. A quantitative assessment of the region of good fixation was made in order to determine the relative fractions occupied by axons, pituicytes and the extracellular space. The immunocytochemical distributions of neurophysins and the glycopeptide portion of the vasopressin precursor were evaluated using the immunogold technique in order to determine the relative numbers of oxytocin and vasopressin fibre types in the fixed region, and the subcellular localization of these antigens. The uncut surface of rat neural lobes was rapidly frozen against a highly polished copper plug and freeze-substituted in osmium-acetone either immediately after dissection (approximately 2 min), or after a 15 min incubation period in vitro in an oxygenated, balanced salt solution. Substituted neural lobes were prepared for either conventional electron microscopy, or for immunogold labelling of neurophysins and the glycopeptide precursor to vasopressin. Membranes, subcellular organelles and extracellular matrix were well preserved 10 microns deep to the contacted surface. The extracellular space accounted for approximately 30% of the cross-sectional area of the neuropil and could be divided into two domains: an extended perivascular space (28-29% of total area); and a narrow (approximately 24 nm; approximately 1% of total) space between closely apposed neurosecretory processes or between these processes and pituicytes. Pituicytes accounted for about 30% of the area and axons 20-25%. Pituicytes occupied close to 60% of the basal lamina at the neurohaemal contact zone, while axons occupied approximately 20%. There were no differences between neural lobes frozen immediately after dissection and those incubated for 15 min in any of these measures, suggesting minimal fluid redistribution. Gold particles were specifically localized over large (100-200 nm) dense core vesicles, and less frequently over multivesicular bodies and lysosomes. No etching of the plastic or reduction of osmium was necessary to achieve labelling. Specific labelling of one set of terminals and axons (about 80%) was observed with the monoclonal antibody previously shown to be specific for oxytocin-neurophysin, while in neighbouring sections the remaining 20% of the processes were labelled with the antiserum to the vasopressin precursor, or with non-specific antibodies to neurophysins. In conclusion, ultrarapid freezing preserves a large extracellular space in the neural lobe and provides for high resolution morphological and immunocytochemical studies of neurohypophysial structure.
Animals↗
The effect of hemorrhagic shock on the extracellular fluid space in the nephrectomized baboon.
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Animals↗
Localisation of the protein and glycoprotein components of bovine nasal epithelial desmosomes by immunoelectron microscopy.
Desmosomal proteins (dp1-4) and glycoproteins (dg1-3) have been localised within desmosomes of bovine nasal epithelium by immunogold labelling of ultrathin frozen sections. Beginning in the extracellular space and proceeding through the plaque to the tonofilaments, the following localisations were found. Labelling for the 130,000 and 115,000 Mr glycoproteins (dg2 and dg3) was predominantly in the extracellular space, a location consistent with their proposed adhesive function. The glycoproteins of 175,000-164,000 Mr (dg1) were also found in the extracellular space and in addition had cytoplasmic domains extending throughout the cytoplasmic plaque. The 83,000 Mr protein (dp3) was located along the cytoplasmic face of the membrane and extended into the plaque, whereas an antibody which recognises both the 83,000 Mr protein (dp3) and the 75,000 Mr protein (dp4) gave labelling both in and beyond the plaque. Labelling for the high mol. wt proteins of Mr 250,000 and 215,000 (dp1 and dp2) was largely excluded from the plaque, and was located distally, adjacent to the tonofilaments. Hemidesmosomes could not be labelled with antibodies to dg1-3 or dp3 and 4, but some labelling was obtained with antibody to dp1 and 2.
Animals↗
Sodium and water contents of sarcoplasm and sarcoplasmic reticulum in rat skeletal muscle: effects of anisotonic media, ouabain and external sodium.
1. During the first 2 hr washout of (24)Na from rat extensor digitorum longus muscle fits a sum of two exponentials, neither of which represents loss of extracellular tracer. This implies a model with two intracellular components.2. Results of suitably designed experiments indicate that the two components are bidirectionally connected to each other as well as to extracellular space. These results are incompatible with a model in which every fibre is homogeneous with respect to Na concentration and flux, but in which there is a distribution of these properties among fibres.3. Results are consistent with identification of the more slowly exchanging component as sarcoplasm and the more rapidly exchanging component as sarcoplasmic reticulum (SR).4. Parameters of the general model include six transport coefficients, two volumes, and contents of two Na pools. The number of equations is inadequate to yield unique solutions by which the values of these parameters can be calculated. However, we derive inequalities that place upper and lower limits on the parameters.5. If the model is correct, the rate constant for Na efflux from SR to extracellular space is at least five times greater than that across sarcolemma. Under standard conditions flux (per muscle weight) from SR is at least 100 times greater than that from sarcoplasm.6. Under standard conditions, only 2-4% of intracellular Na, or 0.5-0.9 m-equiv/kg wet wt., is in sarcoplasm, and the rest is in SR.7. Bounds on fluid volumes of sarcoplasm and SR under standard conditions are calculated with the assumption that Na concentration in SR is the same as in extracellular space. According to the calculations, fluid volume of sarcoplasm is 0.54 ml./g wet wt. Fluid volume of SR is about 0.124 ml./g wet wt., or 14.3% of fibre volume, in agreement with Peachey's estimate (1965) of volume of SR in frog muscle.8. Three tests are applied to the model, with the following results: (a) volume of sarcoplasm increases in hypotonic solution and decreases in hypertonic solutions, as predicted for an osmometer. Volume of SR tends to change in the opposite direction, in agreement with results of Birks & Davey (1969) from electron microscopy on frog muscle; (b) the major effect of partial substitution of external Na by Li is a reduction in Na content of SR, with no significant change in that of sarcoplasm or in volume of either component; (c) the major effect of 10(-5)M ouabain is an increase in Na content of sarcoplasm, with no demonstrable change in that of SR or in volume of either component.9. These results support the model, particularly our identification of the slowly exchanging component as sarcoplasm, identification of the rapidly exchanging component as SR, and the assumption that Na concentration in SR is close to that in extracellular fluid.
Animals↗
New ultrastructure of rat RPE cells: basal intracytoplasmic tubules.
Retinal pigment epithelial cells have prominent basal folds facing Bruch's membrane. In addition to folds I have observed intracytoplasmic tubules 60-90 nm in diameter in the basal cytoplasm of rat pigment epithelial cells. The tubules arise from the basal plasma membrane and open to the extracellular space. The tubules are most evident when intravenous horseradish peroxidase is used as a tracer. The tracer leaks out of the fenestrated choriocapillaris, into the extracellular space of the pigment epithelium and into the tubules. Electron microscopy at 1000 KV (High Voltage Electron Microscopy) confirms the tubular nature of these structures and their continuity with folds or the plasma membrane facing Bruch's membrane. The tubules are also observed in tissue not infiltrated with peroxidase. Morphometry shows that the tubules occupy about 21% of the surface area of the basal plasma membrane. Tubules appear plentiful where folds are reduced, and reduced where folds are plentiful; the tubules may be a different conformation of the normally slit-like fold extracellular space. The tubules are observed in all quadrants of the retina; centrally and peripherally; in young and adult rats and in pigmented and albino rats. The tubule's function may be linked to that of the folds, from which many of them arise.
Animals↗
Evidence for Sarcocystis as the etiologic agent of equine protozoal myeloencephalitis.
Equine protozoal myeloencephalitis (EPM) was diagnosed in 10 horses. By electron microscopy, schizonts were found in intact host cells of the spinal cords or, more frequently, free in the extracellular spaces. Developmental stages of schizonts differed morphologically, and the late stage of schizogony was characterized by endopolygeny. These findings permitted tentative identification of the protozoon as a Sarcocystis sp. Free merozoites were present in the extracellular spaces or in cells of the spinal cord. Pericytes of capillaries were most frequently parasitized by merozoites were present in the extracellular spaces or in cells of te spinal cord. Pericytes of capillaries were most frequently parasitized by merozoites, but the cytoplasm of neurons, macrophages, intravascular and tissue neutrophils, and axons of myelinated nerve fibers also contained these organisms. The presence of parasites in the cytoplasm of tissue and circulating neutrophils suggest that this putative Sarcocystis sp. may have a hematogenous phase of infection.
Animals↗
Extracellular matrix production by cat retinal pigment epithelium in vitro: characterization of type IV collagen synthesis.
Feline retinal pigment epithelial cells (RPE) produced an extracellular matrix (ECM) in vitro which was located between the basal surface of the RPE and the culture plate. This ECM had three morphological components: bundle, granular and fibrillar. After 14 days in culture the basal extracellular space contained small amounts of bundle material; granular and fibrillar material were infrequently observed at this time. The amount of ECM material increased with increasing time in culture. The accumulation of the granular component extracellularly was greatest between 60 and 108 days. Fibrillar material, although occasionally observed in the ECM, appeared to be an infrequent component. By 145 days, the ECM filled the extracellular space between the RPE and the culture plate. The time-dependent increase of the ECM indicated continued synthesis and secretion of ECM into the basal extracellular space by the RPE. Confluent RPE cultures, or choroidal/scleral fibroblasts, were incubated for 24 hr with [14C]-proline. Newly synthesized collagen, either in the culture medium or the cell layer, was co-precipitated with added carrier collagen by (NH4)2 SO4. The samples, with or without reduction and alkylation, were digested with pepsin and fractioned by selective salt precipitation and carboxymethyl(CM)-cellulose chromatography. The resulting fractions were further analyzed, or purified for thin layer chromatography (TLC) amino acid analysis, by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Cultured RPE cells, but not choroidal/scleral fibroblasts, produced labelled peptides which were characterized as alpha 1 (IV), and alpha 2 (IV) collagen chains by CM-cellulose chromatography, SDS-PAGE, proline: hydroxyproline ratios and sensitivity to bacterial collagenase. In contrast, choroidal/scleral fibroblasts produced labelled alpha 1 (I), beta 12 (I) and alpha 2 (I) collagen chains. The synthesis of type IV collagen by RPE cells may reflect the production of ECM observed by electron microscopy in cultured feline RPE cells.
Animals↗
Influence of manganese on the release of neurotransmitters in rat striatum.
On the basis of the evidence that manganese may be released along with glutamate into the extracellular space in the hippocampus and amygdala, the release of manganese and its influence in the striatum was examined by using the in vivo microdialysis method in the present study. The release of 54Mn previously taken up by the striatum into the extracellular space was enhanced during stimulation with 100 mM KCl. This enhancement of 54Mn release into the striatal extracellular space was inhibited by addition of 1 micro M tetrodotoxin. When the rat striatum was perfused with artificial CSF containing 200 nM MnCl(2), the levels of GABA in the perfusate were remarkably decreased, while the levels of glutamate, aspartate, and glycine in the perfusate were not appreciably decreased. These results suggest that manganese released into the synaptic cleft in a calcium- and impulse-dependent manner may influence GABA release in the striatum.
Animals↗
[Desobstructive sodium and water diuresis: pathophysiologic and clinical aspects of bilateral obstructive nephropathy (author's transl)].
The measurement of sodium and water loss after relieve of the obstruction shows that the obstructive nephropathy originates in three pathogenetic mechanisms. (1) Acute complete bilateral ureteric obstruction causes tubular atrophy, prevents both resorption and glomerular filtration, reduces the renal blood flow and increases the extracellular space by retention of water and products subject to urinary excretion. (2) Relieve of obstruction results in excess polyuria as blood flow and glomerular filtration recover rapidly, the extracellular space gets rid of its osmotic load, and the tubular dysfunction of resorption continues for several days until the epithelium has recovered from its pressure atrophy. All this will result in a high loss of sodium and water which requires adequate substitution; otherwise, natriuretic shock will result. (3) The chronic (bilateral) obstruction behaves in a similar way, yet is less reversible. The tubular damage is the same. Moderate polyuria occurs already during the stage of obstruction. Hereby the extracellular space decreases. After relieve of the obstruction the polyuria increases significantly, yet less rapidly than after acute obstruction as the glomerular function does not recover completely. The renal blood flow remains diminished, the vascular calibers stay narrowed, and the kidney remains shrunken. Loss of sodium and water will endanger the patient with chronic obstruction. Furthermore, the patient will be at risk due to dehydration, acidosis, anemia and uremia. The infusion therapy of the desobstructive nephropathy syndrome is based upon the venous pressure and the serum electrolytes which are measured twice daily.
Adult↗
Glycine release in the substantia nigra: Interaction with glutamate and GABA.
Previous studies have reported a high number of glycine (GLY) receptors in the substantia nigra (SN) but a low number of GLY-neurons, suggesting that taurine, a partial agonist of GLY-receptors, is the natural substrate for SN GLY-receptors. By using microdialysis to quantify amino acids in the extracellular space of the SN, we observed an extracellular pool of GLY in the rat that increased after depolarizing with high-K+ in a Ca2+-dependent manner and that diffuses through the extracellular space. GLY markedly increased after blocking either the tricarboxylic cycle with fluorocitrate or the glutamine synthetase activity with MSO. Because these products act selectively on glial cells, their effects show glia as a key cell in maintaining the extracellular pool of GLY in the SN. Extracellular GLY was modified by glutamate and glutamate receptor agonists. The local administration of GLY modified the extracellular concentration of GABA. Taken together, the complex regulation of the extracellular level of GLY, its possible glial origin and interaction with glutamate and GABA suggest a volume transmitter role for GLY in the SN, a possibility which also agrees with the recent finding of GLY-transporters in this centre.
Analysis of Variance↗