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P Lipton

Publications and source records attributed to P Lipton.

32 records · Page 2Linked to original sources
PubMed1986

Calcium and long-term transmission damage following anoxia in dentate gyrus and CA1 regions of the rat hippocampal slice.

The mechanism of long-term anoxic damage in brain tissue is investigated using the rat hippocampal slice as a model system. The effects of short durations of anoxia on subsequent transmission through two neural pathways are studied. 10 min of anoxia irreversibly abolishes transmission between the perforant path and the dentate granule cells while only 7 min of anoxia irreversibly abolishes transmission between the Schaeffer collaterals and the CA1 pyramidal cells. We examine the involvement of Ca2+ in this irreversible transmission damage and, also, the differential sensitivities of the dentate gyrus and CA1 regions. Substitution of a buffer containing 0 Ca2+ and 10 mM-Mg2+ during the anoxic period substantially improves the recovery of synaptic transmission in both regions of the slice. Dentate gyrus transmission recovers completely after 20 min of anoxia and CA1 transmission survives 10 min of anoxia. These results suggest that Ca2+ influx during anoxia may be an important cause of the long-term damage. The uptake of 45Ca2+ into the intracellular space of the slice is increased during anoxia. This effect is approximately twice as large in CA1 as in the dentate gyrus. Thus, in the dentate gyrus the calculated exchangeable pool of Ca2+ is increased 30% by anoxia and in the CA1 it is increased by 70%. Two incubating conditions which decrease the amount of 45Ca2+ uptake during anoxia protect transmission against long-term damage. (a) Pre-incubation of the slices with 25 mM-creatine elevates tissue phosphocreatine and attenuates the fall in adenosine 5'-triphosphate (ATP) during anoxia. This is associated with partial protection against transmission damage and an approximate 50% attenuation of the anoxic uptake of 45Ca2+. (b) Inclusion of 2 mM-cobalt in the buffer reduces the normoxic uptake of 45Ca2+ so that the uptake during anoxia is no greater than normoxic uptake in the absence of cobalt. This is associated with a complete protection against long-term transmission damage following 10 min of anoxia in the dentate gyrus. A kinetic analysis of the 45Ca2+ uptake shows that the anoxic uptake results primarily from inhibition of the unidirectional efflux of Ca2+ from the cells; there is no calculable increase in the undirectional influx. This suggests that anoxia increases Ca2+ uptake by inhibiting one or more Ca2+-extrusion processes and not by opening depolarization-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphate↗
PubMed1985

Regulation of serotonin release from the in vitro rat hippocampus: effects of alterations in levels of depolarization and in rates of serotonin metabolism.

We analyze the time course of 5-hydroxytryptamine (5-HT, serotonin) release from K+-depolarized hippocampal slices using a two-compartment kinetic model. The model is based on the assumptions that the rate of release is dependent on the amount of 5-HT in a releasable pool and that this pool may be resupplied during depolarization by newly synthesized 5-HT. Comparisons were made between predictions of the model and observed changes in 5-HT metabolism and in 5-HT release studied under a variety of experimental conditions. In accordance with predictions of the model, experimental manipulation of 5-HT synthesis and breakdown rates did not affect release immediately after depolarization but did affect the release rate during prolonged depolarization. Increasing bath tryptophan from 0 to 10 microM approximately doubled both 5-HT synthesis and the release rate after 40 min of K+-induced depolarization while having a smaller effect on release during the first 2 min. Inhibition of 5-HT breakdown did not significantly affect release during the first 2 min of depolarization but increased it over threefold after 40 min. In contrast, altering the concentrations of K+ or Ca2+ in the incubation medium affected mainly the early phase of 5-HT release and not the late phase. Reducing Ca2+ from 2.4 to 0.4 mM reduced 5-HT release by about 30% during the first 9 min of depolarization but did not affect release during the subsequent 30 min. Increasing the concentration of K+ from 18 to 60 mM stimulated release by sixfold during the first 2 min but only twofold after a subsequent 30 min. These results support our kinetic model and suggest that regulation of 5-HT metabolism at the site of the nerve terminal could be a mechanism for modulation of 5-HT release during prolonged discharge of serotonergic neurons.

Animals↗
PubMed1983

Glycolysis and brain function: [K+]o stimulation of protein synthesis and K+ uptake require glycolysis.

Glucose is essentially the sole energy substrate in the normally functioning brain. There are, however, situations in which other substrates can partially substitute for glucose and maintain an apparently normal brain function. However, in no case has it been possible to completely substitute other substrates for glucose and maintain normal brain function. Studies on insulin-induced hypoglycemia suggest that this glucose dependence does not result from its involvement in ATP generation. Two explantation that have been offered are that toxic catabolites arise if nonglucose substrates are oxidized or that glycolysis is necessary to maintain neurotransmitter metabolism. We consider a third basis for the glucose requirement: our past studies have shown that hippocampal slice protein synthesis is activated by small increases in extracellular [K+] ([K+]o), and that this results from activation of K+ uptake into brain cells. We find that this process specifically requires aerobic glycolysis. The basis for the requirement appears to be that [K+]o activation of the Na+-K+ pump is specifically dependent on glycolytically generated energy. Thus, it is possible that glucose is required to maintain normal K+ clearance from the extracellular space during neural activity. This could partially account for the dependence of brain function on glycolysis.

Adenosine Triphosphate↗
PubMed1982

Reduced ATP concentration as a basis for synaptic transmission failure during hypoxia in the in vitro guinea-pig hippocampus.

1. Experiments were performed to determine whether a decrease in tissue ATP contributes to the rapid failure of cerebral synaptic transmission during hypoxia. Transmission between the perforant path and the dentate granule cells in the in vitro hippocampus was studied.2. Hippocampal slice ATP is decreased by approximately 15% at the time that the evoked response begins to diminish in standard Krebs bicarbonate buffer. This is about 2 min after the onset of hypoxia.3. When transmission failure is accelerated by increasing extracellular K(+) from 4.4 to 13.4 mM, the evoked response begins to decay about 30 sec after exposure to hypoxia. There is no decrease in hippocampal slice ATP at this time.4. However, ATP in the molecular layer (the synaptic region of the tissue) is decreased by approximately 15% at the time the evoked response begins to decay in the slices exposed to elevated K(+) concentration.5. Exposing the hippocampal slice to 25 mM-creatine for 3 hr elevates molecular layer phosphocreatine fourfold. Synaptic transmission during hypoxia survives three times as long as it does in the absence of creatine.6. In the creatine fortified medium, molecular layer ATP no longer declines within 30 sec of hypoxia. However the molecular layer ATP does decline within 90 sec of hypoxia, the time at which the evoked response begins to decay in this creatine-fortified buffer.7. The results establish that ATP in the region of the active synapses is lowered when the first signs of electrophysiological failure appear during hypoxia. They also show that maintaining ATP for longer than normal during hypoxia is associated with a prolonged maintenance of the evoked response. They thus suggest that a decline in ATP is one factor causing hypoxic block of synaptic transmission.8. It is further suggested that the very rapid failure of the electroencephalogram during anoxia may also result from a decline in ATP.

Adenosine Triphosphate↗
PubMed1982

Mechanisms involved in irreversible anoxic damage to the in vitro rat hippocampal slice.

1. We have studied the effects of anoxia on the recovery of neural transmission between the perforant path and the dentate granule cells in the in vitro rat hippocampal slice. There is almost no recovery of the evoked population spike following 10 min of anoxia in slices from adult rats.2. A 2 h exposure of slices to creatine markedly improves the recovery of the population spike (80% vs. 5%). The creatine pre-incubation builds up phosphocreatine levels in the slice and prevents the large fall in ATP during anoxia; ATP falls to 7.9 rather than 3.6 nM/mg protein. The intracellular pH of both groups falls to the same level during anoxia.3. If calcium concentration in the medium is reduced to 0 while magnesium concentration is raised to 10 mM during anoxia the evoked response recovers to about 65%.4. The data suggest that an attenuation of the fall in ATP or entry of calcium during anoxia protects the tissue against irreversible transmission damage. Thus both of these factors participate in the generation of this damage. It is not yet clear if they act independently or if one acts by altering the other.5. In the post-anoxic recovery period the intracellular concentration of potassium is reduced by about 25%. However, it is still much higher than in slices that show only partial block of the evoked response when treated with ouabain. Therefore a fall in intracellular potassium 1 h after anoxia cannot explain the lack of recovery of the evoked response in adult tissue.6. ATP levels in the post-anoxic recovery period are reduced from their pre-anoxic levels (9.7 vs. 13.9 nM/mg protein). However when azide or antimycin A are used to directly reduce ATP to the level found 1 h after anoxia the evoked response is reduced by only about 45%. Thus the reduced post-anoxic ATP levels are not sufficient to explain the loss of the evoked response in adult tissue.7. The data show that the irreversible loss of transmission is not due to decreased cell ATP or to decreased cell K/Na levels 1 h after the anoxic period.8. Since creatine pre-incubation protects against irreversible transmission loss this compound or one closely related to it may prove useful in attenuating irreversible brain damage in situ.

Adenosine Triphosphate↗
PubMed1982

Vasopressin augments depolarization-induced release and synthesis of serotonin in hippocampal slices.

Vasopressin may be a neurotransmitter and in vivo experiments suggest that it acts on monoamine metabolism. The rat hippocampal slice contains serotonergic nerve terminals but not cell bodies; we studied the effect of vasopressin on the synthesis and release of serotonin from these nerve terminals during depolarization. Incubation of slices in a buffer containing 60 mM K+ (high K buffer) for 10 min stimulated the release of serotonin into bathing medium and resulted in a Ca2+-dependent depletion of tissue serotonin from about 4.2 to about 2.8 ng/mg of protein. Vasopressin (10(-7) M) inhibited this depletion by about 70% so that serotonin levels fell only to 3.8 ng/mg of protein. The peptide also augmented the high K+-induced release of serotonin into the bathing medium by about 60%. The synthesis of serotonin was measured by determining its accumulation during a period when its catabolism was inhibited by pargyline. Vasopressin augmented the synthesis of serotonin in slices incubated in high K buffer by about 60%. There are serotonergic nerve endings in both the dentate gyrus and CA1 regions of the hippocampus. The effect of vasopressin on tissue serotonin was confined to the dentate gyrus regions. The data show that vasopressin acts on a specific group of hippocampal nerve endings to increase serotonin synthesis. The resulting increase in tissue serotonin may be the factor leading to the observed increase in serotonin release.

Animals↗
PubMed1981

Cerebral synaptic transmission during anoxia is protected by creatine.

Synaptic transmission in cerebral tissue fails very rapidly in the absence of oxygen; the metabolic basis for this is not known. We report here that the transmission failure in the guinea pig hippocampal slice can be delayed threefold by exposing the tissue to extracellular creatine (Cr) for 3 h. The improved survival is associated with an increase of tissue phosphocreatine (PCr) concentration. These data argue that the metabolic basis for synaptic transmission failure is a fall in tissue ATP concentrations. They also indicate a way to protect brain tissue against anoxic damage.

Anaerobiosis↗
PubMed1979

The effect of hypoxia on evoked potentials in the in vitro hippocampus.

1. We have studied the effect of hypoxia on transmission of electrical activity between the perforant path and the dentate granule cells in the in vitro guinea-pig hippocampus. 2. Hypoxia abolishes the evoked field potential within about 3 min, a time similar to that occurring in vivo (Andersen, 1960). 3. The evoked potential is very rapidly abolished by extracellular K+ concentrations greater than 13.4 mM; it is abolished by ouabain concentrations greater than 10(-5) M. The rate at which it is abolished increases with increasing ouabain concentrations: concentrations of about 8 x 10(-5) M abolish the evoked potential at the same rate as does hypoxia. 4. The time required to abolish the evoked potential during hypoxia decreases markedly as the extracellular K+ concentration is elevated from 4.4 to 13.4 mM. The time to abolish the potential during hypoxia is also decreased by partial replacement of the Cl- in the bathing medium by less permeant anions and by the presence of a low (10(-7) M) concentration of ouabain. All these are conditions which are expected to depolarize neuronal cell membranes. None of these alterations in the perfusing medium affect the concentrations of ATP or creatine phosphate in the hippocampal slice. Increasing extracellular Mg2+/Ca2+ to levels which reduce the evoked response by about 50% has no effect upon the time required to abolish the evoked potential during hypoxia at any concentration of extracellular [K+]. 5. These results provide evidence that the basis for the hypoxic block of the evoked potential is a depolarization of neuronal processes. They are consistent with the hypothesis that this depolarization is a result of inhibition of the Na+/K+ pump.

Animals↗
PubMed1973

Effects of membrane depolarization on nicotinamide nucleotide fluorescence in brain slices.

1. Simultaneous measurement of tissue NAD(P)H fluorescence and respiration was used to elucidate some of the early chemical changes after depolarization of the membranes of cerebral-cortical cells. Depolarization was effected by the application of a train of short-duration voltage pulses across a slice of guinea-pig cerebral cortex. The pulses cause a biphasic change in fluorescence corresponding to an early (within 1s) oxidation and later (about 10s) reduction of nicotinamide nucleotide. The major portions of both these redox changes are unrelated to the accompanying increase in slice respiration of about 75%. 2. The early oxidation requires Ca(2+) and phosphate in the bathing medium and appears to be largely due to an early mitochondrial uptake of these ions. 3. The ensuing reduction occurs in the cytosol, as judged by its almost complete elimination in the presence of exogenous pyruvate. It becomes markedly attenuated with increasing time of incubation. This and the ability of exogenous 6-N-2'-O-dibutyryl cyclic AMP to cause a reduction in the absence of electrical pulses suggest that it results from a cyclic AMP-mediated activation of glycogenolysis. 4. Further results, obtained in the presence of exogenous pyruvate, indicate that in addition to the above effects a net transfer of NADH from the mitochondrial to cytosolic space (presumably via a shuttle mechanism) is activated by the electrical pulses. 5. The lack of any sizeable (more than 2% of basal fluorescence) fluorescence change associated with the respiratory increase caused by the pulses, and the fact that any change which may be associated with it corresponds to a small reduction of mitochondrial nicotinamide nucleotide, show that a simple state 4 --> state 3 mitochondrial transition cannot account for the increased respiration. The results suggest, rather, a co-ordinated control of respiration, at more than one site.

Animals↗
PubMed1973

Effects of membrane depolarization on light scattering by cerebral cortical slices.

1. A system is described for simultaneously measuring the respiration and the reflectance of a tissue slice and is applied to a study of guinea-pig cerebral cortical slices.2. Reducing bathing medium osmolarity led to a reversible decrease in reflectance of these slices (as well as slices from liver and kidney cortex). In half isotonic solutions reflectance was reduced by 31%.3. Anoxia led to a decreased reflectance which was eliminated if all the Cl was substituted by the larger glucuronate anion.4. It is concluded that slice reflectance is lowered when cellular volumes are increased by water or isotonic solution influx.5. Membrane depolarization effected by ouabain, high (60 mM) K bathing medium, veratridine or repeated electrical pulses led to rapid decreases in reflectance of 25, 27, 31 and 7.5% respectively. Turning off the electrical pulses caused reflectance to return to control values. Reversibility of the chemical effectors was not tested.6. Substitution of Cl by glucuronate abolished the reflectance changes, although it did not inhibit the increased respiration induced by the depolarizing stimuli.7. Tetrodotoxin abolished both the respiratory and reflectance effects of veratridine and electrical pulses but had no effect upon those of high K or ouabain.8. The decrease in reflectance began about 1 sec after initiation of the pulses and was half maximal by 8 sec.9. Titrating reflectance against [K] showed that an increase of 5 mM-K led to a 4% decrease in reflectance and that reflectance became minimal between 60 and 80 mM-K(+).10. It is concluded that membrane depolarization in excitable cells of the cerebral cortex (and also, possibly, in the glia) causes rapid increases in cell volume due to influx of isotonic solution.11. The results indicate, more specifically, that changes in intercellular K concentrations of size and duration thought to occur following nervous activity in the C.N.S. cause cell volume changes large enough to drastically reduce the intercellular volumes and so, transiently, increase extracellular molecular and ionic concentrations. Increases of extracellular [K] and [Ca] have significant effects upon synaptic transmission and upon spontaneous nervous activity. It is suggested that nervous activity in one cell (or portion of it) might, in this way, strongly influence function in neighbouring elements.

Animals↗