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M Chesler

Publications and source records attributed to M Chesler.

At least 37 records · Page 2Linked to original sources
PubMed1996

Temporal resolution of activity-dependent pH shifts in rat hippocampal slices.

1. The rise time of activity-dependent extracellular pH shifts was measured in the CA1 stratum radiatum of rat hippocampal slices by recording pH-sensitive fluorescence of a fluorescein-conjugated dextran. Optical data were compared with simultaneous pH microelectrode recordings. 2. The pH shifts generated by CO2 or by stimulation of the Schaffer collaterals were paralleled by shifts in fluorescence emissions at 535 nm when the probe was excited with 490-nm light (delta F490). Emissions at 535 nm induced by 440-nm light were unchanged in these paradigms. 3. A train of three stimuli at 100 Hz was repeated at 30-s intervals and the stimulus-triggered delta F490 was averaged. The mean rise time of the delta F490 was 69 +/- 24 (SE) ms (range 20-200 ms, n = 6). The mean increase in emission was 0.75 +/- 0.22% of baseline, associated with a pH microelectrode response of +0.06 +/- 0.02 unit pH. 4. These data demonstrate that synaptically evoked alkaline transients develop within tens of milliseconds. The occurrence of the alkalinization in the same time frame as excitatory postsynaptic currents indicates that these pH shifts arise with sufficient speed to modulate synaptic transmission.

Alkalies↗
PubMed1995

Addition of carbonic anhydrase augments extracellular pH buffering in rat cerebral cortex.

1. The ability of the extracellular space to buffer rapid alkaline shifts was studied in rat cortex in vitro and in vivo. Alkaline shifts were generated by iontophoresis of OH- or were evoked by pressure ejection of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). 2. In cortical slices, alkaline shifts induced by OH- were enhanced by the carbonic anhydrase (CA) inhibitor benzolamide (83 +/- 16%, mean +/- SE), and were decreased by superfusion of 10 mg/l CA (-59 +/- 4%). CA had no effect at 1 mg/l, and no additional effect at 100 mg/l. 3. In slices, and in vivo, alkaline shifts induced by AMPA were similarly enhanced by benzolamide and decreased by superfusion of CA. 4. These data indicate that extracellular CA activity is less than that required for maximum buffering. This suggests that equilibrium between CO2 and bicarbonate may not be attained during rapid extracellular pH shifts.

Aging↗
PubMed1995

Benzolamide inhibits low-threshold calcium currents in hippocampal pyramidal neurons.

1. Benzolamide is a poorly permeant sulfonamide inhibitor of the enzyme carbonic anhydrase. We studied the effect of benzolamide on low-threshold (LT) Ca currents in neonatal hippocampal CAl neurons. 2. In hippocampal slices, benzolamide (2-10 microM) inhibited the LT current 30-75% in voltage-clamped CAl pyramidal cells (n = 6). In slices bathed in N-2-hydroxypiperazine-N'-2-ethane-sulfonic acid (HEPES)-buffered Ringer, benzolamide also reduced the LT current, indicating that the action of the drug was not bicarbonate dependent. 3. Benzolamide inhibited LT Ca currents 20-75% in acutely dissociated CAl neurons in HEPES (n = 18): inhibition was 36 +/- 8% (mean +/- SE; n = 7) and 50 +/- 8% (n = 7) at 10 and 50 microM benzolamide, respectively. By contrast, high-threshold calcium currents recorded in CAl pyramidal cells (n = 18) and dorsal root ganglion neurons (n = 4) were virtually unaffected by benzolamide. 4. These results indicate that benzolamide inhibits LT Ca channels in central neurons and suggest caution in the use of this agent to inhibit extracellular carbonic anhydrase in excitable tissues.

Animals↗
PubMed1994

Calcium dependence of glutamate receptor-evoked alkaline shifts in hippocampus.

Glutamate receptor activation induces an extracellular alkalinization in rodent hippocampus. We studied its Ca2+ dependence and pharmacology in hippocampal slices. Glutamate-evoked alkaline shifts were blocked by 0 Ca2+ saline. Alkalinizations induced by AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) and NMDA (N-methyl-D-aspartate) were abolished by 20 microM CNQX (6-cyano-7-nitro-nitroquinoxaline-2,3-dione) and 50 microM APV (DL-2-amino-5-phosphonovalerate), respectively. The AMPA- and NMDA-evoked alkaline shifts were blocked by 0 Ca2+, however, AMPA-induced [K+]o elevation was unaffected. These data suggest that the glutamate receptor-channel does not mediate H+ influx, and support a role for Ca(2+)-H+ exchange.

Alkalies↗
PubMed1994

Endogenous H+ modulation of NMDA receptor-mediated EPSCs revealed by carbonic anhydrase inhibition in rat hippocampus.

1. The occurrence of extracellular alkaline transients during excitatory synaptic transmission suggests that the NMDA receptor H(+)-modulatory site may have a physiological role. Here we amplify these pH shifts using benzolamide (a carbonic anhydrase inhibitor) and describe concomitant effects on EPSCs in whole-cell clamped CA1 neurones in rat hippocampal slices. 2. In CO2-HCO3(-)-buffered media, benzolamide increased the time to 50% decay (t50) of the EPSCs by 78 +/- 14% (P < 0.01, n = 10). This occurred simultaneously with amplification of the extracellular alkaline shift (154 +/- 14%). 3. In CO2-HCO3(-)-buffered media containing DL-2-amino-5-phosphonovalerate (APV), the EPSC t50 was unaltered by benzolamide, while the extracellular alkaline shifts were increased (111 +/- 23%, n = 8). 4. In Hepes-buffered media, neither the EPSC t50 nor the extracellular alkaline shift was altered by benzolamide (n = 9). 5. These data demonstrate that NMDA receptor activity is dependent on the buffering kinetics of the brain extracellular space. The results suggest that endogenous pH shifts can modulate NMDA receptor function in a physiologically relevant time frame.

2-Amino-5-phosphonovalerate↗
PubMed1994

Elevation and clearance of extracellular K+ following graded contusion of the rat spinal cord.

The elevation and clearance of extracellular potassium concentration ([K+]e) were studied following graded contusion injury of the rat thoracic spinal cord. Animals were anesthetized, paralyzed, laminectomized at T9-T10, and then artificially ventilated. A 10-g rod was dropped 1.25, 2.5, or 5 cm onto the dorsal thoracic cord with the dura intact. After impact, and incision of the dura-arachnoid and pial membranes, double-barreled, potassium-selective microelectrodes were inserted midway between the midline and lateral edge of the cord. For all three injury levels, the elevation of [K+]e was greatest within the first 1000 microns from the dorsal surface. In 50 g-cm injuries, increasing [K+]e was sometimes observed between 250 and 1000 microns; however such gradients were not typically observed in 25 and 12.5 g-cm injuries. Measured at 3-7 min after injury, the mean peak elevations of [K+]e were significantly different, measuring 13 +/- 2.4, 27 +/- 5.5, and 44 +/- 4.2 mM following 12.5, 25, and 50 g-cm contusions, respectively. The exponential half-times of [K+]e clearance averaged 5.8 +/- 1.0, 9.2 +/- 1.8, and 17 +/- 5.7 min for the same respective injury levels. These results indicate that elevation of [K+]e following traumatic injury to the spinal cord is a graded phenomenon, dependent on the energy of impact. This finding is consistent with a mechanism in which simple mechanical injury of cell membranes is the proximate cause of potassium release.

Animals↗
PubMed1994

Effects of GABA on axonal conduction and extracellular potassium activity in the neonatal rat optic nerve.

GABA depolarizes rat optic nerve axons and modulates axonal conduction through the activation of GABA-A receptors. To address whether an increase of [K+]e plays a major role in GABA actions on the rat optic nerve, we studied the effects of GABA on axonal conduction and [K+]e in the neonatal rat optic nerve in vitro. Double-barrelled K(+)-sensitive microelectrodes were used to record [K+]e. GABA (10(-4)-10(-3) M) increased [K+]e in the neonatal optic nerve. During prolonged application, the [K+]e slowly recovered. The increase in [K+]e induced by GABA was markedly reduced by the GABA-A receptor blocker bicuculline (10(-4) M). Isoguvacine (10(-4) M), a GABA-A agonist, mimicked the effect of GABA but produced larger responses at the same concentration. In contrast, baclofen (10(-4) M), a GABA-B agonist, had no effect on [K+]e. The changes in the compound action potential induced by GABA correlated only partially with the [K+]e changes. Furthermore, the changes in the compound action potential induced by elevation of K+ were far less than those induced by GABA. These results demonstrate that the GABA-evoked accumulation of [K+]e plays a secondary role in GABA actions on the neonatal rat optic nerve.

Action Potentials↗
PubMed1994

Determination of extracellular bicarbonate and carbon dioxide concentrations in brain slices using carbonate and pH-selective microelectrodes.

The extracellular pH of the brain is subject to shifts during neural activity. To understand these pH changes, it is necessary to measure [H+], [HCO3-], [CO3(2-)] and [CO2]. In principle, this can be accomplished using CO3(2-) and pH-sensitive microelectrodes; however, interference from HCO3- and Cl-, and physiological changes in [HCO3-], complicate measurements with CO3(2-) electrodes. Calibration requires knowledge of slope response, interference constants and corrections for [HCO3-] shifts. We show that when [HCO3-] is altered at constant [CO2] in the absence of Cl-, the HCO3- interference cancels and the Nikolsky equation reduces to the Nernst equation for CO3(2-). Measurement of CO3(2-) slope response by this method yielded a value of 28.5 +/- 0.72 mV per decade change in [CO3(2-)]. In Cl(-)-containing solutions, interference coefficient for HCO3- and Cl- were determined by altering [HCO3-] at constant [CO2], changing [CO2] at constant [HCO3-], then solving the simultaneous Nikolsky equations for each transition. The mean interference constants corresponded to selectivity ratios of 245:1 and 1150:1 for CO3(2-) over HCO3- and Cl- respectively. To correct for possible changes in [HCO3-], the equilibrium relation between CO3(2-) and HCO3- was substituted into the Nikolsky equation to yield an equation in [CO3(2-)] and [H+]. By simultaneously measuring shifts in [H+] with a pH microelectrode, this equation is readily solved for [CO3(2-)]. These methods were tested by measuring [HCO3-] and [CO2] in experimental solutions, and in the extracellular fluid of rat hippocampal slices.

Animals↗
PubMed1994

Depolarization-induced acid secretion in gliotic hippocampal slices.

Gliotic hippocampal slices were used to study glial acid secretion in a tissue largely devoid of neural elements. Rat hippocampal slices were prepared 10-28 days after sterotaxic injection of kainate. Cresyl Violet staining and immunohistochemistry for glial fibrillary acidic protein demonstrated a loss of neurons and a proliferation of reactive astrocytes in area CA3. Extracellular pH and K+ shifts were recorded in CA3 in response to K+ iontophoresis. Elevation of K+ evoked an extracellular acid shift that was two- to three-fold larger in gliotic versus unlesioned tissue. Ba2+ caused a slow extracellular acidification, and blocked both the depolarizing responses of the glial cells and the acid shifts evoked by K+. The K(+)-evoked acid shifts were abolished in Na(+)-free media, and diminished in HEPES-buffered solutions. Inhibition of extracellular carbonic anhydrase caused a reversible enhancement of the K(+)-evoked acid shifts, an effect that could be mimicked during H+ iontophoresis in agarose gels. Gliotic acid shifts were unaffected by amiloride or its analogs, stilbenes, zero Cl- media, zero or elevated glucose, lactate transport inhibitors, zero Ca2+ or Cd2+. Smaller acid shifts could be evoked in normal slices which were also enhanced by benzolamide, and blocked by Ba2+ and zero Na+ media. It is concluded that acid secretion by reactive astrocytes is Na+ and HCO3(-)-dependent and is triggered by depolarization. The similar pharmacological and ionic sensitivity of the acid shifts in non-gliotic tissue suggest that these properties are shared by normal astrocytes. These characteristics are consistent with the operation of an electrogenic Na(+)-HCO3- co-transporter. However, the enhancement of the acid shifts by inhibitors of extracellular carbonic anhydrase suggests that CO3(2-), rather than HCO3-, is the transported acid equivalent.

Acids↗
PubMed1994

Depolarization-induced alkalinization of astrocytes in gliotic hippocampal slices.

Depolarization-induced, intracellular alkaline shifts were studied in reactive astrocytes within slices of gliotic hippocampus. Slices were prepared 10-28 days after sterotaxic injection of kainic acid into the hippocampus of anesthetized rats. Astrocytes in gliotic CA3 were impaled with double-barreled pH sensitive microelectrodes and depolarized by iontophoresis of K+ from an adjacent micropipette. Elevation of extracellular K+ produced an intracellular alkalinization that grew with increasing membrane depolarization, ranging from approximately 0.10 to 0.30 pH units. Exposure to Ba2+ depolarized the cells and produced a similar alkalinization. In the presence of Ba2+, the K(+)-induced depolarization and the associated alkaline shift were abolished. The depolarization-induced alkaline shifts were partially inhibited (40 +/- 8.9%) in Na(+)-free media and were enhanced in bicarbonate versus HEPES-buffered saline. The alkalinizations were unaffected by incubation in chloride-free media, or by the stilbene 4,4'-dinitrostilbene-2,2'-disulfonic acid. It is concluded that the depolarization-induced alkaline shift of reactive astrocytes is mediated in part by a Na+ and HCO3(-)-dependent mechanism that is insensitive to stilbenes. These characteristics correspond well with the properties of depolarization-induced acid secretion in the gliotic tissue. In addition, a separate, Na(+)-independent mechanism contributes to the depolarization-induced alkalinization. In view of the absolute Na+ dependence of acid secretion in the gliotic slices, we propose that the latter mechanism does not extrude acid across the plasma membrane.

Alkalies↗
PubMed1994

Dynamics of extracellular calcium activity following contusion of the rat spinal cord.

The role of Ca2+ in cellular injury has received particular attention in studies of acute spinal cord trauma. In this context, the spatial and temporal distribution of extracellular Ca2+ ([Ca2+]e) may have an important bearing on the development of secondary tissue injury. We therefore studied the spatial-temporal distribution of [Ca2+]e following moderate (25 g-cm) contusive injury to the rat thoracic (T9-T11) spinal cord. Double-barreled, Ca(2+)-selective microelectrodes were used to measure the magnitude and time course of [Ca2+]e at increasing depths from the dorsal spinal cord surface. After 2 h, the tissue was frozen and later analyzed for total Ca concentration using atomic absorption spectroscopy. [Ca2+]e fell at all depths, but the decrease was maximal at 250 and 500 microns from the dorsal surface, where, at 0-10 min after injury, [Ca2+]e averaged 0.09 +/- 0.03 and 0.06 +/- 0.03 mM respectively. By 2 h postinjury, [Ca2+]e recovered to nearly 1 mM across all depths. Over this time, total tissue calcium concentration ([Ca]t) was 4.54 +/- 0.16 mumol/g in injured cords vs 2.75 +/- 0.1 mumol/g in sham-operated controls. These data place emphasis on the dorsal gray matter as a principal site of ionic derangement in acute spinal cord injury. The implications of these findings are discussed with reference to secondary injury processes.

Animals↗
PubMed1993

Non-synaptic modulation of dorsal column conduction by endogenous GABA in neonatal rat spinal cord.

GABAA receptor activation can modulate axonal conduction in the isolated dorsal column of the neonatal rat spinal cord in vitro. However, it is not known whether axonal conduction in the dorsal column can be modulated by endogenous GABA in the developing spinal cord. We consequently compared the effects of GABA, a GABAA agonist, and a GABA uptake inhibitor on axonal conduction in the dorsal column of hemisected neonatal (0- to 9-day-old) rat spinal cords in vitro. Extracellular compound action potentials evoked by supramaximal stimuli were recorded at two points with glass microelectrodes. GABA (10(-4) to 10(-3) M) reversibly decreased the compound action potential amplitude and the population conduction velocity. At 10(-4) M, compound action potential amplitudes fell by 45.0 +/- 6.5% of control while the conduction velocity slowed by 11.8 +/- 4.3% (n = 5). The GABAA receptor agonist, isoguvacine, mimicked the effects of GABA on the dorsal column compound action potential. In contrast, while GABA at 10(-5) M decreased the amplitude by 7.7 +/- 3.1%, it increased conduction velocity by 9.7 +/- 1.3% (n = 5). The GABA uptake inhibitor, nipecotic acid (10(-3) M), consistently decreased the compound action potential amplitude by 17.7 +/- 6.5% (n = 6) but the conduction velocity slowed in four out of six preparations. In two instances, nipecotic acid decreased the amplitude and increased the conduction velocity. The effects of nipecotic acid on the dorsal column compound action potential were blocked in the presence of the GABAA antagonist bicuculline.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1992

pH transients evoked by excitatory synaptic transmission are increased by inhibition of extracellular carbonic anhydrase.

Excitatory synaptic transmission has been associated with a rapid alkalinization of the brain extracellular space. These pH shifts are markedly increased by acetazolamide, an inhibitor of carbonic anhydrase. Although this effect can be readily explained by inhibition of extracellular carbonic anhydrase, this enzyme has been considered strictly intracellular in the central nervous system. To determine whether these alkaline shifts are regulated by extracellular carbonic anhydrase, we studied the effects of a membrane impermeant, dextran-bound inhibitor of this enzyme. Extracellular alkaline transients, measured with pH-sensitive microelectrodes, were generated in the CA1 region of rat hippocampal slices by repetitive electrical stimulation of Schaeffer collateral fibers or by local ejection of glutamate. More direct alkalinizations were elicited by focal ejection of NaOH in the vicinity of a pH microelectrode. These pH transients were reversibly enhanced by addition of the dextran-bound inhibitor. We conclude that there is significant carbonic anhydrase activity in the extracellular space of the brain. We postulate that this enzyme functions in the regulation and modulation of extracellular pH transients associated with neuronal activity.

Acetazolamide↗
PubMed1992

Modulation of pH by neuronal activity.

Although the requirement for a strict regulation of pH in the brain is frequently emphasized, recent studies indicate that neuronal activity gives rise to significant changes in intracellular and extracellular pH. Given the sensitivity of many ion channels to hydrogen ions, this modulation of local pH might influence brain function, particularly where pH shifts are sufficiently large and rapid. Studies using pH-sensitive microelectrodes have demonstrated marked cellular and regional variability of activity-dependent pH shifts, and have begun to uncover several of their underlying mechanisms. Accumulating evidence suggests that regional and subcellular pH dynamics are governed by the respective localization of glial cells, ligand-gated ion channels, and extracellular and intracellular carbonic anhydrase.

Animals↗
PubMed1992

Alkaline extracellular pH shifts generated by two transmitter-dependent mechanisms.

Recent studies of the effect of gamma-aminobutyric acid (GABA) on brain extracellular pH are reviewed. Experiments were performed on isolated turtle cerebellum, using double-barrelled pH-sensitive microelectrodes. Superfusion of GABA (1 mM) caused a rapid extracellular alkaline shift accompanied by a rise in extracellular K+. Washout of GABA was often associated with an acid rebound, concomitant with an undershoot of extracellular K+. The GABA-evoked alkaline shift was blocked by picrotoxin and mimicked by the GABA-A agonists isoguvacine and muscimol. The response persisted in the nominal absence of extracellular calcium, but it was reversibly abolished in nominally bicarbonate free media. In contrast, extracellular alkaline shifts evoked by repetitive stimulation of the parallel fibers were amplified in bicarbonate-free media and were insensitive to picrotoxin. These results indicate the existence of separate, transmitter-dependent mechanisms of extracellular alkalinization: (i) a GABA-A receptor mediated process, most likely associated with efflux of bicarbonate ions across GABA-A anion channels and (ii) a bicarbonate-independent process associated with excitatory synaptic transmission.

Animals↗
PubMed1992

Modulation of extracellular pH by glutamate and GABA in rat hippocampal slices.

1. Alkaline extracellular pH transients evoked by afferent stimulation, and local pressure ejection of glutamate and gamma-aminobutyric acid (GABA), were studied in the CA1 region of rat hippocampal slices. Amino acid-evoked responses were obtained by use of a dual micromanipulator, with the tip of a double-barreled pH-sensitive microelectrode positioned 50 microns from a pressure ejection pipette. 2. At 31 degrees C, in Ringer solutions buffered with 26 mM HCO3- and 5% CO2, mean extracellular pH in submerged 300-microns slices was 7.15 +/- 0.12 (n = 27 slices), at a tissue depth of approximately 150 microns. In Ringer buffered with 35 mM HCO3- and 5% CO2, extracellular pH was 7.29 +/- 0.10 (n = 19 slices). 3. Repetitive stimulation of the Schaffer collaterals caused an extracellular alkaline shift in stratum oriens, pyramidale, and radiatum, averaging 0.05 +/- 0.03 pH units among all regions (n = 138), with a maximum response of 0.16 pH units. Alkaline transients of similar appearance were obtained by local ejection of glutamate (0.01-0.12 pH units, n = 110) and GABA (0.01-0.18 pH units, n = 137). Control ejection of these amino acids into dilute agar caused only small acid shifts. 4. Superfusion of 100 microM picrotoxin abolished the GABA-evoked alkaline shift but failed to inhibit the Schaffer collateral- and glutamate-evoked alkalinizations. 5. Superfusion of 10(-5)-10(-3) M acetazolamide acidified the baseline by 0.05-0.10 pH units and amplified the Schaffer collateral- and glutamate-evoked alkaline shifts.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetazolamide↗
PubMed1992

Extracellular alkaline shifts in rat hippocampal slice are mediated by NMDA and non-NMDA receptors.

1. The pharmacology of synaptically evoked extracellular alkaline shifts was studied in the CA1 area of rat hippocampal slices. 2. Stimulus-evoked alkalinizations were unaffected by 2-amino-5-phosphonovalerate (APV) (20 microM). 3. 6-Cyano-7-nitro-nitroquinoxaline-2,3-dione (CNQX) (10 microM) inhibited the alkalinizations. In the continued presence of CNQX, an APV-sensitive, picrotoxin-insensitive, alkaline shift was elicited in low Mg2+ media. 4. Antidromic stimulation produced small alkaline shifts in comparison with orthodromic activation. 5. Our results demonstrate that in the hippocampal CA1 region, synaptically evoked alkalinizations can arise through both N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors. These responses cannot be explained by cell firing per se.

2-Amino-5-phosphonovalerate↗