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K Krnjević

Publications and source records attributed to K Krnjević.

At least 37 records · Page 2Linked to original sources
PubMed1997

Endogenous adenosine on membrane properties of CA1 neurons in rat hippocampal slices during normoxia and hypoxia.

The effects of endogenous adenosine release on CA1 neurons in hippocampal slices were studied under normoxic and hypoxic conditions, by using extra-/intracellular and whole-cell recordings. During normoxia, the adenosine antagonist, 8-(p-sulphophenyl) theophylline (8-SPT) or adenosine deaminase (ADA) potentiated both evoked CA1 EPSPs and spontaneous synaptic activity, but not monosynaptic IPSPs; there was a minimal depolarization (by 1 mV), probably caused by the enhanced synaptic activity, but no increase in input conductance. Under voltage-clamp with KCl electrodes (with holding potential (VH) near -70 mV), hypoxia (4-5 min) elicited a rise in input conductance and an outward current that reversed near -90 mV, in keeping with the activation of K conductance. These effects of hypoxia were partly attenuated by 8-SPT (10 microM). The hypoxia-induced outward current and conductance increase were abolished by 1 mM Ba, being replaced by a small inward current and a conductance decrease. These data indicate that adenosine tonically inhibits excitatory, but not inhibitory, synaptic transmission, has no direct effect on input conductance, and contributes to the hyperpolarization and fall in input resistance induced by hypoxia.

Adenosine↗
PubMed1997

2-Deoxy-D-glucose-induced changes in membrane potential, input resistance, and excitatory postsynaptic potentials of CA1 hippocampal neurons.

Temporary block of glycolysis by 2-deoxy-D-glucose (2-DG) reversibly suppresses synaptic transmission in the CA1 region of hippocampal slices. Recovery of responses is followed by a sustained potentiation of field excitatory postsynaptic potentials (EPSPs) (2-DG-LTP). To investigate the mechanisms involved in this type of LTP, we studied the effects of 2-DG on membrane properties of CA1 neurons (in slices from Sprague-Dawley rats), recorded with sharp intracellular electrodes containing 3 M KCl, as well as patch electrodes, filled mainly with 150 mM KMeSO4 and Hepes. The predominant change produced by 15- to 20-min applications of 2-DG (10 mM, replacing glucose) was hyperpolarization (-5.6 +/- 1.1 mV for 18 intracellular recordings and -7.2 +/- 0.80 mV for 17 whole-cell recordings) accompanied by a fall in resistance (-33 +/- 2.5% for 14 intracellular recordings and -11.6 +/- 7.1% for 15 whole-cell recordings). Virtually identical hyperpolarizations were recorded in the presence of 20 microM glyburide (-5.5 +/- 1.5 mV, n = 6), but they were abolished by adenosine antagonists 8-(p-sulfophenyl)theophylline (8-SPT) and 8-cyclopentyl-3,7-dihydro-1,3-dipropyl-1H-purine-2,6-dione (DPCPX) (2.8 +/- 1.6 and 4.0 +/- 1.7 mV, respectively; n = 5 for both). It was concluded that the hyperpolarization is most likely caused by an increase in K+ conductance, activated by a 2-DG-induced rise in adenosine release. After such applications of 2-DG, a sustained potentiation of EPSPs (similar to the 2-DG-LTP of field EPSPs) was evident in five neurons recorded with intracellular electrodes but not in any of nine whole-cell recordings, where it was replaced by sustained, LTD-like depression. We conclude that a factor essential for 2-DG-LTP induction is lost during whole-cell recording.

Animals↗
PubMed1996

Tolbutamide blocks Ca(2+)- and voltage-dependent K+ currents of hippocampal Ca1 neurons.

In current-clamp recordings with KMeSO4 electrodes (either whole-cell or intracellular), though tolbutamide (0.5-1 mM) did not change the resting potential, it increased both input resistance (by 12 +/- 3.8%) and spontaneous firing, and spikes were evoked by smaller depolarizing pulses. Tolbutamide reduced in a dose-dependent manner both components of post-burst afterhyperpolarizations: IC50 was 0.15 mM for medium afterhyperpolarizations and 0.33 mM for slow afterhyperpolarizations. In whole-cell recordings under voltage-clamp, 0.5-1 mM tolbutamide depressed slow outward currents by 65 +/- 5.3%. The tolbutamide-sensitive current was Ca(2+)-dependent-tolbutamide being ineffective in Mn2+, low Ca(2+)-containing medium-though tolbutamide did not significantly depress high voltage-activated Ca2+ currents. Tolbutamide reduced C-type outward currents by 45 +/- 5.9% and M-type current inward relaxations by 41 +/- 12.9%, as well as Q-type current inward relaxations by 22 +/- 5.7%. Glyburide (10 microM) did not depress afterhyperpolarizations or outward currents, even in recordings with electrodes containing 1 mM guanosine diphosphate. We conclude that the most prominent effects of 0.5-1 mM tolbutamide on CA1 neurons are caused by suppression of Ca(2+)-and voltage-dependent outward currents, including IAHP, IC and IM.

Animals↗
PubMed1996

Calcium dependence of LTP induced by 2-deoxyglucose in CA1 neurons.

1. As previously reported, in hippocampal slices from Sprague-Dawley rats, 13-min applications of 2-deoxy-D-glucose (2-DG) (substituting 10 mM 2-DG for glucose)-which sharply depress field excitatory postsynaptic potentials (EPSPs)-are followed by a sustained potentiation of the initial slopes of EPSPs (2-DG-LTP). 2. Such 2-DG-LTP is not prevented by exposing slices to Ca(2+)-free medium for 25 min before the 13-min 2-DG applications (in Ca(2+)-free medium). Therefore 2-DG-LTP is not dependent on influx of external Ca2+ during the 2-DG applications. 3. When the Ca(2+)-free conditions begin 15 min before, and are maintained for 10 min after, the 13-min 2-DG applications (in Ca(2+)-free medium), 2-DG-LTP is either totally suppressed or much reduced. A delayed Ca2+ influx thus plays a crucial role in the induction of 2-DG-LTP. 4. Much longer Ca(2+)-free pretreatment (for 77 min) largely abolishes 2-DG-LTP. Therefore Ca2+ release from a compartment (presumably intracellular) that is not readily depleted is also important for the induction of 2-DG-LTP. 5. This intracellular Ca2+ store is sensitive to dantrolene sodium (10 microM)-which prevents 2-DG-LTP-but not 10 microM thapsigargin. 2-DG-LTP of isolated N-methyl-D-aspartate-receptor-mediated EPSPs is only partly reduced by dantrolene. 6. Dantrolene (10 microM) also reduces or abolishes posttetanic potentiation, but not paired-pulse facilitation. 7. Depotentiation by 1-Hz stimulation is abolished by 20 microM dantrolene. 8. In contrast to the above, long-term potentiation (LTP) elicited by tetanic stimulation is prevented by 10 microM thapsigargin but not by dantrolene (< or = 50 microM). 9. In conclusion, two mechanisms of intracellular Ca2+ concentration increase appear to be essential for LTP induction by 2-DG. One is Ca2+ influx after the 2-DG application; the other is Ca2+ release from a dantrolene-sensitive internal store. The opposite effects of thapsigargin and dantrolene on 2-DG-LTP and tetanic LTP suggest that distinct internal sources of Ca2+ may be needed for the induction of these two forms of LTP.

Animals↗
PubMed1996

Dantrolene depolarizes hippocampal neurons in slices from rats.

In intracellular recordings by current clamp, dantrolene (sodium salt, 20 microM) regularly depolarized CA1 neurons (by approximately 13 mV), even in the presence of tetrodotoxin. Dantrolene almost abolished slow (but not medium) afterhyperpolarizations (AHP) (down by approximately 80%). As in previous experiments, dantrolene strongly depressed the hyperpolarization and resistance drop caused by hypoxia. Under voltage clamp (at approximately -70 mV), dantrolene accordingly elicited an inward current of approximately -100 pA and suppressed a voltage-dependent slow outward current and tail current, as well as the outward current generated by hypoxia. In addition, dantrolene enhanced Q-current (IQ) inward relaxations; but it had no effect on high voltage activated Ca currents and did not prevent their suppression by hypoxia. We conclude that depression of slow AHP current and increase in IQ probably account for the depolarizing action of dantrolene.

Animals↗
PubMed1995

Nitric oxide tonically depresses a voltage- and Ca-dependent outward current in hippocampal slices.

In whole-cell recordings from CA1 neurons, net outward currents (at ca. -20 mV, from VH ca. -50 mV) were 40-50% depressed by sodium nitroprusside (100-500 microM) or L-arginine (L-ARG; 50-200 microM), but not by D-arginine (100 microM). The NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME; 200 microM) restored the L-ARG-depressed current to ca. 80% of control. In naive cells, L-NAME increased outward currents by 45 +/- 12.6%; the enhanced currents were then reduced by adding L-ARG (200-400 microM). The NO-sensitive current is Ca-dependent, because L-NAME and L-ARG were ineffective in Mn/low Ca medium or when electrodes contained 2.2 mM EGTA. Since high voltage-activated Ca-currents were unaltered by L-NAME, we conclude that NO tonically enhances excitability in slices by depressing a voltage- and calcium-dependent (IK(Ca)-type) outward current.

Animals↗
PubMed1995

Econazole, a blocker of Ca2+ influx, selectively suppresses LTP of EPSPs in hippocampal slices.

Econazole--an agent that suppresses Ca2+ influx triggered by depletion of internal Ca-stores in non-excitable cells--was bath-applied to submerged slices from Sprague-Dawley rats. Econazole (15-20 microM) had no consistent effect on afferent volleys, EPSPs or population spikes; but in seven out of nine slices, it prevented the induction of tetanic long-term potentiation (LTP) of field EPSPS. By contrast, all slices showed a marked LTP of population spikes. Ca2+ influx induced by tetanic depletion of Ca2+ stores may be essential for LTP of excitatory synaptic transmission.

Animals↗
PubMed1995

Internal Ca2+ stores involved in anoxic responses of rat hippocampal neurons.

1. During whole-cell recordings from CA1 neurons of rat brain slices with electrodes containing only KMeSO4 and Hepes, brief anoxia (2-3 min) consistently evoked a hyperpolarization (delta V approximately 14 mV) and reduction in input resistance (delta R approximately -20%). 2. As in previous intracellular recordings, Dantrolene sodium (10 microM) suppressed the anoxic delta V and delta R, confirming the release of internal Ca2+ is a major component of the anoxic response. 3. To identify the relevant intracellular Ca2+ store, other blockers of Ca2+ release were applied either externally (in the bath) or internally, by addition to the contents of the recording electrode. 4. The anoxic hyperpolarization was abolished or much reduced by heparin (10-20 micrograms ml-1, internal), thapsigargin (10 microM, external), Ruthenium Red (50 microM, internal) and external procaine (0.5-2 mM), but not by internal procaine (0.5-1 mM) or ryanodine (10 microM, external). 5. The anoxic fall in resistance was also abolished or reduced by heparin, thapsigargin and external procaine, but not by ryanodine, internal procaine or Ruthenium Red. 6. In addition, external procaine (0.5-2 mM) eliminated the early (transient) depolarization and reduced the post-anoxic hyperpolarization by 60 +/- 22%. 7. None of these agents consistently changed the resting potential, but the input resistance was significantly increased by Dantrolene and external procaine. 8. In view of the marked effects of heparin and thapsigargin, but not ryanodine and internal procaine, we conclude that the anoxic response seen in such whole-cell recordings is initiated predominantly by Ca2+ release from an internal store that is InsP3 sensitive rather than Ca2+ sensitive. 9. Comparable but less pronounced effects of external procaine were seen during intracellular recordings with 3 M KCl-containing electrodes. The dose-dependent suppression of various features of the anoxic response by external procaine (EC50 approximately 0.2 mM) is presumed to be mediated by a superficial membrane trigger or modulating site.

Animals↗
PubMed1995

Adenosine antagonists have differential effects on induction of long-term potentiation in hippocampal slices.

How adenosine leakage and tetanic release might affect long-term potentiation (LTP) was investigated by applying adenosine antagonists 8(p-sulfophenyl)theophylline (8SPT) or 8-cyclopentyl-3,7-dihydro-1,3-dipropyl-1H-purine-2,6-dione (DPCPX) to slices, while recording CA1 field EPSPs and population spikes. In the first series of experiments, we applied weak double tetani (at 100 Hz, for 1 s) that were subliminal for evoking LTP in initial control runs. In the presence of 8SPT--at concentrations (10-50 microM) which block both A1 and A2 receptors--the same tetani consistently evoked LTP of population spikes but not of excitatory postsynaptic potentials (EPSPs), whereas DPCPX (50 nM), which blocks only A1 receptors, facilitated LTP of both EPSPs and population spikes. These results are consistent with previous evidence that tetanic adenosine release on the one hand depresses LTP via A1 receptors but on the other facilitates LTP via A2 receptors. In a second set of experiments, 8SPT (50-100 microM) did not prevent the induction of LTP of both EPSPs and population spikes by stronger tetanic stimulation. Therefore A2 receptor activation is not essential for the induction of LTP when stronger tetani are applied. Overall, the main effect of endogenous adenosine release is to oppose LTP induction.

Adenosine↗
PubMed1995

Actions of diazoxide on CA1 neurons in hippocampal slices from rats.

Membrane effects of diazoxide (DZX) were examined in CA1 pyramidal neurons, mainly by whole-cell recording in slices kept at 33 degrees C (from Sprague-Dawley rats). Bath applications of DZX (0.65 mM) did not significantly change the resting input conductance; but instantaneous inward rectification was reduced by 47 +/- 14% (near -110 mV). There was a similar depression of a large, sustained voltage-dependent outward current (by 44 +/- 11% near 0 mV). A nearly identical reduction of the outward current recorded in a Ca current suppressing medium (but not in 30 mM tetraethylammonium) indicated that the DZX-sensitive current includes the delayed rectifier. In Mn, low-Ca medium containing tetraethylammonium and carbachol, DZX potentiated (by 43 +/- 12%) the D-type slowly decaying outward current seen after hyperpolarizing pulses at a holding potential of approximately -50 mV. DZX abolished or depressed slow inward currents, such as the tetrodotoxin-sensitive persistent Na current, high voltage activated Ca currents (IC50 = 0.47 mM), and the Q current. In 6 of 13 cells recorded with electrodes containing either guanosine or adenosine diphosphate, DZX potentiated the voltage-dependent outward current, but input conductances were reduced. In conclusion, although there was little indication that it activates classical KATP channels in CA1 neurons, DZX strongly depresses several voltage-dependent, slowly inactivating outward and inward currents, which are important modulators of cell excitability.

Animals↗
PubMed1995

Long-term potentiation in hippocampal slices induced by temporary suppression of glycolysis.

1. Temporary suppression of glycolysis by 2-deoxy-D-glucose (2-DG)-long enough to abolish CA1 population spikes (PSs) and reduce field excitatory postsynaptic potentials (EPSPs) by two-thirds-is followed by a sustained rebound of EPSPs and PSs (both up by 70-150%). 2. Post 2-DG long-term potentiation (2-DG-LTP) is prevented by block of N-methyl-D-aspartate (NMDA) receptors (NMDARs). Though 2-DG-LTP is normally expressed by other receptors, in presence of picrotoxin 2-DG causes similar LTP of NMDAR-mediated EPSPs. 3. Stimulation at 1 s-1 fully depotentiates 2-DG-LTP. 4. Unlike tetanic LTP, 2-DG-LTP is not pathway-specific, is not occluded by a preceding tetanic LTP (or vice versa) and is insensitive to block of NO synthesis. 5. Hypoglycemic states may have long-lasting after-effects on cerebral synaptic function.

Animals↗
PubMed1994

Tolbutamide suppresses slow and medium afterhyperpolarization in hippocampal slices.

Afterhyperpolarizations (AHPs) were recorded (in whole-cell mode, with KMeSO4-containing electrodes) after multiple spikes evoked with 200 ms current pulses. Bath applications of tolbutamide (0.5-1 mM) to 12 CA1 neurones nearly abolished the medium and slow component of AHPs. Thus the AHPs generated by 7-8 spikes (mostly) were reduced by 82.6 +/- 5.2% (s.e.m.) at the initial peak (mAHP) and by 85.0 +/- 5.8% 1 s later (sAHP). Glibenclamide (10 microM) had no comparable blocking effect. The previous finding that tolbutamide (but not glibenclamide) selectively suppresses the anoxic hyperpolarization of CA1 neurones is therefore consistent with the idea that the anoxic hyperpolarization is mediated by Ca-dependent (especially AHP-type) K channels.

Animals↗
PubMed1994

Guanosine diphosphate is required for activation of a glyburide, ATP and cromakalim-sensitive outward current in rat hippocampal neurones.

Voltage-dependent outward currents were examined in CA1 neurones by whole-cell recording in slices. Nine cells were recorded with the 'standard' internal solution (KMeSO4, HEPES, EGTA, CaCl2, and MgCl2) and held a potential (-54 +/- 3 mV) at which there was no significant outward current. Cromakalim (CROM, 100 microM) reduced both input conductance (GN) (by 14 +/- 4%) and outward currents, evoked over a wide range of potentials by brief depolarizing pulses: at -4.0 +/- 3.0 mV, currents diminished by 30 +/- 10%. When 1 mM GDP was added to the standard internal solution, there was a significant outward current at approximately -54 mV; and CROM greatly increased outward currents near -4.0 mV (by 99 +/- 26.4%, n = 10). The enhanced outward currents were reduced by CROM washout (in two cells) and by 10 microM glyburide (GLYB, in four cells). When six other cells were recorded with electrodes containing both ATP (5 mM) and GDP (1 mM), there was no net outward current at approximately -54 mV and CROM reduced outward currents (at approximately 0 mV, by 37.5 +/- 10.9%). We conclude that GDP in hippocampal neurones appears to activate an ATP- and GLYB-sensitive outward current, which is much potentiated by CROM.

Action Potentials↗
PubMed1994

Anoxia selectively depresses excitatory synaptic transmission in hippocampal slices.

EPSPs/IPSPs were recorded with intracellular electrodes from CA1 neurons close to site of stimulation. Brief anoxia (3 min) abolished EPSPs but reduced IPSPs by 64.8 +/- 4.0% (n = 10); the remaining IPSP was presumed to be monosynaptic. The effects of anoxia on purely monosynaptic IPSPs were examined after pharmacological blockade of excitatory synaptic transmission with 2 mM kynurenate or 20 microM CNQX + 20 microM APV. In these tests, after 3 min of anoxia the slopes of IPSPs vs. membrane potential were reduced by only 38.2 +/- 4.3% (n = 12). The present study demonstrates that, contrary to previous reports, inhibitory synaptic transmission is quite resistant to anoxia.

2-Amino-5-phosphonovalerate↗
PubMed1994

Endogenous adenosine deaminase does not modulate synaptic transmission in rat hippocampal slices under normoxic or hypoxic conditions.

Field and intracellular potentials were recorded from CA1 pyramidal stratum in submerged slices (at 33 degrees). During "normal" oxygenation (95% O2 + 5% CO2), tonic depression of population spikes and field excitatory postsynaptic potentials by endogenous adenosine was demonstrated by (i) the marked enhancement by the adenosine antagonists 8-(p-sulfophenyl)theophylline (10 microM) and caffeine (0.2 mM), (ii) depression by the transport blocker dipyridamole (5 microM), and (iii) enhancement by exogenous adenosine deaminase (all tested by bath application). Thus, adenosine deaminase (0.5 units/ml) reduced by 10.7 +/- 3.0% (S.E.) the half-maximal stimulus intensity (for population spikes). The effects of adenosine deaminase were prevented by the specific inhibitor, deoxycoformycin (30 microM). In intracellular recordings, excitatory postsynaptic potentials were enhanced in a comparable manner by adenosine deaminase. By contrast, neither deoxycoformycin (5 and 30 microM) nor erythro-9-(2-hydroxy-3-nonyl)adenine (another adenosine deaminase inhibitor; 10 and 50 microM) had significant effects on population spikes. Superfusion with anoxic medium (saturated with 95% N2 + 5% CO2) for 2-3 min suppressed population spikes reversibly, by a mechanism involving adenosine, because 8-(p-sulfophenyl)theophylline (10 microM) and caffeine (0.2 mM) delayed the onset of anoxic block and accelerated the subsequent recovery, and the recovery was much slower or incomplete in the presence of dipyramidole (0.5 microM). However, the anoxic suppression of population spikes was not affected by deoxycoformycin (30 microM) or erythro-9-(2-hydroxy-3-nonyl)adenine (10 microM); the corresponding 50% postanoxic recovery times were also unchanged (e.g. 4.0 +/- 0.2 min for controls and 4.1 +/- 0.3 min in deoxycoformycin).(ABSTRACT TRUNCATED AT 250 WORDS)

Adenine↗
PubMed1994

Actions of cromakalim on outward currents of CA1 neurones in hippocampal slices.

1. Membrane effects of cromakalim (Crom; 50-300 microM) were examined in CA1 neurones recorded mainly by intracellular, single-electrode voltage-clamping in slices (from Sprague-Dawley rats) kept in an interface chamber at 33 degrees C. 2. In 14 cells held at -63 +/- 3.5 mV, in the presence of tetrodotoxin, kynurenic acid and (in most cases) bicuculline, bath applied Crom produced no consistent change in holding current (-59 +/- 66 pA) or input conductance (GN) (-3.9 +/- 5.2%). 3. Overall there were no significant changes in instantaneous inward rectification or in Q-current inward relaxations. 4. In 18 out of 22 cells, outward currents, evoked by 0.5 s pulses to voltages > -50 and < -20 mV, were depressed by Crom (by 42 +/- 11%, for n = 22). Because this effect was consistently seen in Ca current-blocking media, containing either Mn and low Ca, or Cd (and also carbachol), the K channels depressed by Crom were probably of the delayed rectifier (IDR) type. 5. The Crom-control difference current (ICrom), obtained with slow depolarizing ramps, had a biphasic character, inward in the voltage (V) range > -50 < -20 mV (where outward currents are depressed by Crom) and tending outward for V > or = -20 mV. 6. In 10 out of 11 cells, Crom potentiated a D-like, slowly-inactivating outward current (by 88 +/- 31%, for n = 11). 7 The effects of Crom and of 2 min periods of anoxia were compared in 12 cells: unlike anoxia, Cromproduced no consistent increases in GN; the currents evoked in the same cells by anoxia differed significantly from those evoked by Crom (by 150 +/- 60 pA); the directions of current changes induced byCrom and anoxia respectively were not significantly correlated. Crom strongly depressed anoxic outward currents (by 80 +/- 12%, n = 4).8 Some Crom-induced effects (increases in D-like current and the outward current elicited at V>- 20 mV) were always reversed by tolbutamide (1 mM), but much less consistently by glibenclamide(10-30 microM).9 In conclusion, the effects of Crom, recorded with intracellular electrodes in CA1 neurones in slices,show little resemblance to the effects of activation of ATP-sensitive K channels.

Animals↗
PubMed1993

Diazoxide suppresses slowly-inactivating outward and inward currents in CA1 hippocampal neurones.

Diazoxide (DZX) opens ATP-sensitive K (KATP) channels in muscle and other cells. In whole-cell voltage-clamp recordings in slices, in the presence of kynurenate and bicuculline to minimize indirect effects, DZX (0.65 mM) did not increase input conductance; but it sharply reduced persistent inward and outward currents. An inward current (peak near -20 mV) was especially clear in the presence of K channel blockers; was fully evident in Ca-channel blocking medium; but was abolished by tetrodotoxin. The main direct effects of DZX on these neurones are thus mediated not by activation of KATP channels, but rather by modulation of voltage-dependent channels, including a TTX-sensitive persistent NA current and possibly a Ca current.

Animals↗