Calcium-dependent potentials with different sensitivities to calcium agonists and antagonists in guinea-pig hippocampal neurons

Nimodipine Exerts Time-Dependent Neuroprotective Effect after Excitotoxical Damage in Organotypic Slice Cultures

During injuries in the central nervous system, intrinsic protective processes become activated. However, cellular reactions, especially those of glia cells, are frequently unsatisfactory, and further exogenous protective mechanisms are necessary. Nimodipine, a lipophilic L-type calcium channel blocking agent is clinically used in the treatment of aneurysmal subarachnoid haemorrhage with neuroprotective effects in different models. Direct effects of nimodipine on neurons amongst others were observed in the hippocampus as well as its influence on both microglia and astrocytes. Earlier studies proposed that nimodipine protective actions occur not only via calcium channel-mediated vasodilatation but also via further time-dependent mechanisms. In this study, the effect of nimodipine application was investigated in different time frames on neuronal damage in excitotoxically lesioned organotypic hippocampal slice cultures. Nimodipine, but not nifedipine if pre-incubated for 4 h or co-applied with NMDA, was protective, indicating time dependency. Since blood vessels play no significant role in our model, intrinsic brain cell-dependent mechanisms seems to strongly be involved. We also examined the effect of nimodipine and nifedipine on microglia survival. Nimodipine seem to be a promising agent to reduce secondary damage and reduce excitotoxic damage.

Hydroxysafflor Yellow A Protects Neurons From Excitotoxic Death through Inhibition of NMDARs

Excessive glutamate release causes overactivation of N-methyl d-aspartate receptors (NMDARs), leading to excitatory neuronal damage in cerebral ischemia. Hydroxysafflor yellow A (HSYA), a compound extracted from Carthamus tinctorius L., has been reported to exert a neuroprotective effect in many pathological conditions, including brain ischemia. However, the underlying mechanism of HSYA's effect on neurons remains elusive. In the present study, we conducted experiments using patch-clamp recording of mouse hippocampal slices. In addition, we performed Ca²⁺ imaging, Western blots, as well as mitochondrial-targeted circularly permuted yellow fluorescent protein transfection into cultured hippocampal neurons in order to decipher the physiological mechanism underlying HSYA's neuroprotective effect.

Through the electrophysiology experiments, we found that HSYA inhibited NMDAR-mediated excitatory postsynaptic currents without affecting α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor and γ-aminobutyric acid A-type receptor-mediated currents. This inhibitory effect of HSYA on NMDARs was concentration dependent. HSYA did not show any preferential inhibition of either N-methyl d-aspartate receptor subtype 2A- or N-methyl d-aspartate receptor subtype 2B- subunit-containing NMDARs. Additionally, HSYA exhibits a facilitatory effect on paired NMDAR-mediated excitatory postsynaptic currents. Furthermore, HSYA reduced the magnitude of NMDAR-mediated membrane depolarization currents evoked by oxygen-glucose deprivation, and suppressed oxygen-glucose deprivation–induced and NMDAR-dependent ischemic long-term potentiation, which is believed to cause severe reperfusion damage after ischemia. Through the molecular biology experiments, we found that HSYA inhibited the NMDA-induced and NMDAR-mediated intracellular Ca²⁺ concentration increase in hippocampal cultures, reduced apoptotic and necrotic cell deaths, and prevented mitochondrial damage. Together, our data demonstrate for the first time that HSYA protects hippocampal neurons from excitotoxic damage through the inhibition of NMDARs. This novel finding indicates that HSYA may be a promising pharmacological candidate for the treatment of brain ischemia.

pH modulation of currents that contribute to the medium and slow afterhyperpolarizations in rat CA1 pyramidal neurones

We examined the effects of changes in pH(o) and pH(i) on currents contributing to the medium and slow afterhyperpolarizations (mI(AHP) and sI(AHP), respectively) in rat CA1 neurones. Reducing pH(o) from 7.4 to 6.7 inhibited mI(AHP) and sI(AHP) whereas increasing pH(o) to 7.7 augmented mI(AHP) and, to a greater extent, sI(AHP). The ability of changes in pH(o) to modulate mI(AHP) reflected changes in the Ca(2+)-activated K(+) current, I(AHP), and a Co(2+)- and XE991-resistant component of mI(AHP), but not the muscarine-sensitive current, I(M). In the presence of 1 microM TTX and 5 mM TEA, low pH(o)-evoked reductions in sI(AHP) were associated with reductions in Ca(2+)-dependent depolarizing potentials; because neither effect was attenuated when internal H(+) buffering power was raised by including 100 mm tricine in the patch pipette, the actions of reductions in pH(o) to inhibit sI(AHP) and, possibly, I(AHP) in large part appear to reflect a low pH(o)-dependent decrease in Ca(2+) influx. In contrast, the effects of high pH(o) to augment mI(AHP) and sI(AHP) were associated with relatively small increases in Ca(2+) potentials but were significantly attenuated by 100 mM internal tricine, indicating that a rise in pH(i) consequent upon the rise in pH(o) was largely responsible. The possibility that changes in pH(i) could act to modulate mI(AHP) and sI(AHP), independently of changes in Ca(2+) influx, was also suggested by experiments in which pH(i) was lowered at a constant pH(o) by the external application of propionate or by the withdrawal of HCO(-)(3) from the perfusing medium. Lowering pH(i) at a constant pH(o) had little effect on Ca(2+) potentials but inhibited mI(AHP) and, to a greater extent, sI(AHP), effects that were attenuated by 100 mM internal tricine. Together, the results indicate that changes in pH(o) and pH(i) modulate mI(AHP) and sI(AHP) in rat CA1 neurones and suggest that, depending on the direction of the pH(o) change, the sensitivities of the underlying currents to changes in Ca(2+) influx and/or pH(i) may contribute to the effects of changes in pH(o) to modulate mI(AHP) and sI(AHP).

Mechanisms underlying the depression of evoked fast EPSCs following in vitro ischemia in rat hippocampal CA1 neurons

The mechanisms underlying the depression of evoked fast excitatory postsynaptic currents (EPSCs) following superfusion with medium deprived of oxygen and glucose (in vitro ischemia) for a 4-min period in hippocampal CA1 neurons were investigated in rat brain slices. The amplitude of evoked fast EPSCs decreased by 85 +/- 7% of the control 4 min after the onset of in vitro ischemia. In contrast, the exogenous glutamate-induced inward currents were augmented, while the spontaneous miniature EPSCs obtained in the presence of tetrodotoxin (TTX, 1 microM) did not change in amplitude during in vitro ischemia. In a normoxic medium, a pair of fast EPSCs was elicited by paired-pulse stimulation (40-ms interval), and the amplitude of the second fast EPSC increased to 156 +/- 24% of the first EPSC amplitude. The ratio of paired-pulse facilitation (PPF ratio) increased during in vitro ischemia. Pretreatment of the slices with adenosine 1 (A1) receptor antagonist, 8-cyclopenthyltheophiline (8-CPT) antagonized the depression of the fast EPSCs, in a concentration-dependent manner: in the presence of 8-CPT (1-10 microM), the amplitude of the fast EPSCs decreased by only 20% of the control during in vitro ischemia. In addition, 8-CPT antagonized the enhancement of the PPF ratio during in vitro ischemia. A pair of presynaptic volleys and excitatory postsynaptic field potentials (fEPSPs) were extracellularly recorded in a proximal part of the stratum radiatum in the CA1 region. The PPF ratio for the fEPSPs also increased during in vitro ischemia. On the other hand, the amplitudes of the first and second presynaptic volley, which were abolished by TTX (0.5 microM), did not change during in vitro ischemia. The maximal slope of the Ca(2+)-dependent action potential of the CA3 neurons, which were evoked in the presence of 8-CPT (1 microM), nifedipine (20 microM), TTX (0.5 microM), and tetraethyl ammonium chloride (20 mM), decreased by 12 +/- 6% of the control 4 min after the onset of in vitro ischemia. These results suggest that in vitro ischemia depresses the evoked fast EPSCs mainly via the presynaptic A1 receptors, and the remaining 8-CPT-resistant depression of the fast EPSCs is probably due to a direct inhibition of the Ca(2+) influx to the axon terminals.

Characteristics of protective effects of NMDA antagonist and calcium channel antagonist on ischemic calcium accumulation in rat hippocampal CA1 region

Effects of excitatory amino acid receptor antagonists and voltage-dependent Ca(2+) channel antagonists on ischemia-induced intracellular free Ca(2+) accumulation in rat hippocampal slices were examined. Ischemia caused a large Ca(2+) accumulation in CA1 region but a small Ca(2+) accumulation in CA3 and dentate gyrus regions. When applied during ischemia, the NMDA receptor antagonist MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine maleate) inhibited the ischemic Ca(2+) accumulation only in the CA1, but the non-NMDA receptor antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) inhibited it in all the three regions. The L-type Ca(2+) channel antagonists nifedipine and verapamil inhibited the ischemic Ca(2+) accumulation only in the CA1 region, but omega-conotoxin, a N- and L-type Ca(2+) channel antagonist inhibited the Ca(2+) accumulation in all the three regions of the hippocampus. When applied after 5-min ischemia, nifedipine but not MK-801, inhibited sustained postiscehmic Ca(2+) elevation in the CA1 region but not in the CA3 and dentate gyrus regions. These findings suggest that the enhanced ischemia-induced Ca(2+) accumulation in the CA1 region is mediated via activation of both NMDA receptors and L-type-like Ca(2+) channels. It appears that sustained postischemic Ca(2+) elevation in the CA1 region is mediated via activation of L-type-like Ca(2+) channels, but not of NMDA receptors.

Effects of nifedipine on rhythmic synchronous activity of human neocortical slices

The antiepileptic effect of the dihydropyridine calcium channel blocker nifedipine was tested in neocortical slice preparations (n=27) from patients ranging in age from four to 46 years (mean=25) who underwent surgery for the treatment of intractable epilepsy. Epileptiform events consisted of spontaneously occurring rhythmic sharp waves as well as of untriggered epileptiform field potentials induced by omission of Mg(2+) from the superfusate, or epileptiform field potentials elicited by application of bicuculline and triggered by single electrical stimuli. (1) Spontaneous rhythmic sharp waves (n=6): with nifedipine (40micromol/l), the repetition rate was decreased down to 30% of initial value, whereas the area under the field potential remained nearly unchanged. (2) Untriggered low Mg(2+) epileptiform field potentials (n=6): with nifedipine (40micromol/l) the area under the field potentials was reduced while the action on the repetition rate was ambiguous. (3) Triggered bicuculline epileptiform field potentials (n=15): with nifedipine (40micromol/l; n=4), no antiepileptic effect was found. There was, however, a marked increase in the area under the epileptiform field potentials. The area under the field potentials was reduced only at a dosage of 60micromol/l (n=11). This effect was stronger when nifedipine was applied with a K(+) concentration raised from 4 to 8mmol/l. The results show that the calcium channel blocker nifedipine is able to reduce differential epileptiform discharges in human neocortical tissue. These observations are in line with previous findings, suggesting that calcium flux into neurons is involved in epileptogenesis. The present results therefore support the idea that some organic calcium antagonists may be useful in human epilepsy therapy, although the etiology of epileptic seizures seems to be a critical factor for the efficacy of the drug.

Calcium-activated potassium currents in mammalian neurons

1. Influx of calcium via voltage-dependent calcium channels during the action potential leads to increases in cytosolic calcium that can initiate a number of physiological processes. One of these is the activation of potassium currents on the plasmalemma. These calcium-activated potassium currents contribute to action potential repolarization and are largely responsible for the phenomenon of spike frequency adaptation. This refers to the progressive slowing of the frequency of discharge of action potentials during sustained injection of depolarizing current. In some cell types, this adaptation is so marked that despite the presence of depolarizing current, only a single spike (or a few spikes) is initiated. Following cessation of current injection, slow deactivation of calcium-activated potassium currents is also responsible for the prolonged hyperpolarization that often follows. 2. A number of macroscopic calcium-activated potassium currents that can be separated on the basis of kinetic and pharmacological criteria have been described in mammalian neurons. At the single channel level, several types of calcium-activated potassium channels also have been characterized. While for some macroscopic currents the underlying single channels have been unambiguously defined, for other currents the identity of the underlying channels is not clear. 3. In the present review we describe the properties of the known types of calcium-activated potassium currents in mammalian neurons and indicate the relationship between macroscopic currents and particular single channels.

Contribution of L-type calcium channels to epileptiform activity in hippocampal and neocortical slices of guinea-pigs

The aim of the present investigation was to compare the antiepileptic efficacy of the specific L-type calcium channel blocker nifedipine in hippocampal and neocortical slice preparations in the Mg2+-free model of epilepsy. The main findings were as follows. (1) In hippocampal slices, in general, nifedipine (20-80 micromol/l) exerted a suppressive effect both on repetition rate and on area under epileptiform field potentials. This effect was clearly dose dependent. In the majority of cases, this suppression was preceded by an increase, which was transient in nature. Only in the lowest concentration (20 micromol/l) used, in normal K+, instead of a depression, a persistent increase occurred. (2) In neocortical slices, in the majority of experiments, nifedipine (20-80 micromol/l) showed a depressive action only on the area under the epileptiform field potentials. The depressive effect of nifedipine on the area was dose dependent, although to a lesser extent than in the hippocampus. In nearly half of the slices this suppression was preceded by a transient increase. By contrast, the repetition rate of epileptiform field potentials increased transiently in about 20% of the slices followed by a decrease. In the remaining 80% of the slices the repetition rate increased persistently. (3) An elevation of the K+ concentration accentuated the depressive actions of nifedipine only in the hippocampus. In contrast to elevated K+, in both the hippocampus and the neocortex, epileptiform field potentials were not suppressed in all experiments in normal K+. (4) The reversibility of the depressive effects of nifedipine was differential in the two tissue types. In the hippocampus, after suppression of epileptiform field potentials they reappeared in the overwhelming majority of slices. In the neocortex, this was the case in only one experiment. These findings may indicate the existence of L-type calcium channels with a differential functional significance for epileptogenesis and/or the existence of different forms of L-type channels in hippocampal and neocortical tissue. As a whole, the differential effects of L-type calcium channel blockade in the hippocampus and neocortex point to differences in the network properties of the two tissue types.

Electrophysiological roles of L-type channels in different classes of guinea pig sympathetic neuron

The electrophysiological consequences of blocking Ca(2+) entry through L-type Ca(2+) channels have been examined in phasic (Ph), tonic (T), and long-afterhyperpolarizing (LAH) neurons of intact guinea pig sympathetic ganglia isolated in vitro. Block of Ca(2+) entry with Co(2+) or Cd(2+) depolarized T and LAH neurons, reduced action potential (AP) amplitude in Ph and LAH neurons, and increased AP half-width in Ph neurons. The afterhyperpolarization (AHP) and underlying Ca(2+)-dependent K(+) conductances (gKCa1 and gKCa2) were reduced markedly in all classes. Addition of 10 microM nifedipine increased input resistance in LAH neurons, raised AP threshold in Ph and LAH neurons, and caused a small increase in AP half-width in Ph neurons. AHP amplitude and the amplitude and decay time constant of gKCa1 were reduced by nifedipine in all classes; the slower conductance, gKCa2, which underlies the prolonged AHP in LAH neurons, was reduced by 40%. Surprisingly, AHP half-width was lengthened by nifedipine in a proportion of neurons in all classes; despite this, neuron excitability was increased during a maintained depolarization. Nifedipine's effects on AHP half-width were not mimicked by 2 mM Cs(+) or 2 mM anthracene-9-carboxylic acid, a blocker of Cl(-) channels, and it did not modify transient outward currents of the A or D types. The effects of 100 microM Ni(2+) differed from those of nifedipine. Thus in Ph neurons, Ca(2+) entry through L-type channels during a single action potential contributes to activation of K(+) conductances involved in both the AP and AHP, whereas in T and LAH neurons, it acts only on gKCa1 and gKCa2. These results differ from the results in rat superior cervical ganglion neurons, in which L-type channels are selectively coupled to BK channels, and in hippocampal neurons, in which L-type channels are selectively coupled to SK channels. We conclude that the sources of Ca(2+) for activating the various Ca(2+)-activated K(+) conductances are distinct in different types of neuron.

Effects of pH changes on calcium-mediated potentials in rat hippocampal neurons in vitro

The effects of changes in extra- and intracellular pH (pHo and pHi, respectively) on potentials mediated by the influx of Ca2+ ions were investigated in intracellular "current-clamp" recordings from CA1 pyramidal neurons in rat hippocampal slices. In neurons which exhibited a "regular-spiking" discharge in response to depolarizing current injection at pH 7.3, perfusion with pH 7.7 medium led to the development of burst firing. Conversely, neurons which were "burst-firing" at pH 7.3 became regular spiking upon exposure to pH 6.9 medium. In addition, the rebound depolarization following a current-evoked hyperpolarization to >- 60 mV, which in part reflects activation of a low-voltage-activated Ca2+ conductance, was reduced at pHo 6.9 and enhanced at pHo 7.7. Neither the burst firing pattern of discharge nor the augmented rebound depolarization observed during perfusion with pH 7.7 medium was due to the reduction in [Cl-]o consequent upon the increase in [HCO3-]o at a constant PCO2. The magnitudes of the fast afterhyperpolarization which follows a single depolarizing current-evoked action potential and the slow afterhyperpolarization which follows a train of action potentials were attenuated and enhanced, respectively, during perfusion with pH 6.9 and pH 7.7 media, compared to responses obtained at pH 7.3. Reducing pHi at a constant pHo (by exposure to pH 7.3 HCO3-/CO2-free medium buffered with 30 mM HEPES) also attenuated fast and slow afterhyperpolarizations. In tetrodotoxin- and tetraethylammonium-poisoned slices, perfusion with pH 6.9 and pH 7.7 media reduced and increased, respectively, the magnitude of current-evoked Ca2+-dependent depolarizing potentials and their associated slow afterhyperpolarizations, compared with responses obtained at pH 7.3. In contrast, reducing pHi at a constant pHo elicited only a small reduction in the magnitude of Ca2+ spikes but markedly attenuated the subsequent slow afterhyperpolarization. The results suggest that, in rat CA1 hippocampal pyramidal neurons, Ca2+-dependent depolarizing potentials mediated by the influx of Ca2+ ions through voltage-activated Ca2+ channels are sensitive to changes in pHo. These effects of changes in pHo are not dependent upon changes in pHi consequent upon the changes in pHo. Changes in pHo also affect the magnitudes of fast and slow afterhyperpolarizations mediated by Ca2+-dependent K+ conductances. In these cases, however, the effects of changes in pHo are mimicked by changes in pHi at a constant pHo, suggesting in turn that the effects of changes in pHo on fast and slow afterhyperpolarizations may be mediated both by changes in Ca2+ influx (reflecting mainly changes in pHo) and by direct effects of changes in pHi (consequent upon changes in pHo) on Ca2+-dependent K+ conductances.

Specificity in the interaction of HVA Ca2+ channel types with Ca2+-dependent AHPs and firing behavior in neocortical pyramidal neurons

Intracellular recordings and organic and inorganic Ca2+ channel blockers were used in a neocortical brain slice preparation to test whether high-voltage-activated (HVA) Ca2+ channels are differentially coupled to Ca2+-dependent afterhyperpolarizations (AHPs) in sensorimotor neocortical pyramidal neurons. For the most part, spike repolarization was not Ca2+ dependent in these cells, although the final phase of repolarization (after the fast AHP) was sensitive to block of N-type current. Between 30 and 60% of the medium afterhyperpolarization (mAHP) and between approximately 80 and 90% of the slow AHP (sAHP) were Ca2+ dependent. Based on the effects of specific organic Ca2+ channel blockers (dihydropyridines, omega-conotoxin GVIA, omega-agatoxin IVA, and omega-conotoxin MVIIC), the sAHP is coupled to N-, P-, and Q-type currents. P-type currents were coupled to the mAHP. L-type current was not involved in the generation of either AHP but (with other HVA currents) contributes to the inward currents that regulate interspike intervals during repetitive firing. These data suggest different functional consequences for modulation of Ca2+ current subtypes.

Mild hypothermia protects rat hippocampal CA1 neurons from irreversible membrane dysfunction induced by experimental ischemia

In order to examine the effects of hypothermia on the changes in membrane potential induced by experimental ischemia (deprivation of oxygen and glucose), intracellular recordings were made from single CA1 pyramidal neurons in slice preparations of rat hippocampus. Application of ischemic medium caused irreversible changes in membrane potential consisting of an initial hyperpolarization, then a slow depolarization and a rapid depolarization. At temperatures of 35 degrees C and 37 degrees C, once the rapid depolarization occurred, readministration of oxygen and glucose failed to restore the membrane potential, a state referred to as irreversible membrane dysfunction. When the temperature was lowered to between 27 degrees C and 33 degrees C, the membrane potential returned to the control resting membrane potential in 75% of the neurons. The temperature coefficients (Q10) of the latency, the amplitude, and the maximal slope of the rapid depolarization were 2.5, 1.4 and 2.9, respectively. It is concluded that the critical neuroprotective temperature in ischemia-induced membrane dysfunction is found to be 33 degrees C in single CA1 neurons in vitro.

Nitric oxide contributes to irreversible membrane dysfunction caused by experimental ischemia in rat hippocampal CA1 neurons

The effects of agents which affect the action of nitric oxide (NO) were studied intracellularly on the ischemia-induced changes in membrane potential of single CA1 pyramidal neurons of the rat hippocampal slice preparations. The N-methyl-D-aspartate (NMDA) receptor antagonists, (+/-)-2-amino-5-phosphonopentanoic acid (AP5, 250 microM) or Co2 (2 mM) restored the membrane potential in more than 80% of the neurons. In about 60% of the neurons, the membrane potential was partially recovered as a result of exposure to the NO synthase inhibitor, NG-nitro-L-arginine (100 microM). The NO scavengers, carboxy-2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO, 300 microM) and hemoglobin (10 microM) restored the membrane potential in all neurons examined. Superoxide dismutase (50 U/ml) protected about 75% of the neurons from irreversible membrane dysfunction. It is concluded that the release of NO induced by experimental ischemia may result in the irreversible membrane dysfunction, and that a NO scavenger, carboxy-PTIO, prevents the ischemic changes in membrane potential. With respect to ischemic brain damage, the neuroprotection provided by carboxy-PTIO may have clinical relevance in the management of a variety of neurological conditions.

Factors that reverse the persistent depolarization produced by deprivation of oxygen and glucose in rat hippocampal CA1 neurons in vitro

In CA1 pyramidal neurons in rat hippocampal tissue slices, superfusion with ischemia-simulating medium produced a rapid depolarization after 6 min of exposure. The membrane potential eventually reached 0 after 5 min (a persistent depolarization), even when oxygen and glucose were reintroduced. The role of various ions in the reversal of this persistent depolarization after reintroduction of oxygen and glucose was investigated. The peak of the persistent depolarization was decreased in solutions containing reduced Na+ or Ca2+ and in solutions containing Co2+ or Ni2+. In contrast, the depolarization was not affected by reduction of external K+ or Cl- or by addition of tetrodotoxin (TTX), flunarizine, or nifedipine. These results suggest that sustained Na+ and Ca2+ influxes produce the persistent depolarization. The membrane potential recovered after reintroduction of oxygen and glucose in low Ca2+, low Cl-, or K+-rich medium and in TTX- or tetraethylammonium-containing medium, but not in low Na+ or low K+ medium and in flunarizine- or nifedipine-containing medium. Either reduction in extracellular Ca2+ or addition of Co2+ was the most effective in promoting recovery from the persistent depolarization, suggesting that Ca2+ influx has a key role in causing the membrane dysfunction. The peak of the persistent depolarization was reduced by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), DL-2-amino-5-phosphonopentanoic acid (AP5), DL-amino-3-phosphonopropionic acid (AP3), or DL-amino-4-phosphonobutyric acid, suggesting that activation of non-N-methyl-D-aspartate (non-NMDA), NMDA, and metabotropic glutamate (Glu) receptors is involved in the generation and maintenance of the persistent depolarization. Among these Glu receptor antagonists, only CNQX or AP5 was able to reduce dose dependently the level of depolarization, suggesting that Ca2+ influx via both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate type II receptors and NMDA receptors contributes to the membrane dysfunction. trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) did not affect the peak potential of the persistent depolarization, but it dose-dependently restored the membrane potential. AP3 antagonized the protective action of t-ACPD. The membrane potential also recovered after reintroduction when the slice was pretreated by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester, ryanodol 3-(1H-pyrrole-2-carboxylate), 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride, and procaine, suggesting that raised [Ca2+]i from Ca2+-induced Ca2+ release pool contributes to the membrane dysfunction. It, therefore, is concluded that raised [Ca2+]i has a dominant role in causing irreversible changes. The increase in [Ca2+]i during the persistent depolarization may be the result of Ca2+ entry via both a leaky membrane and Glu-activated receptor channels as well as Ca2+ released from internal stores.

Effects of dihydropyridine calcium antagonists on rat midbrain dopaminergic neurones

1. The effects of the dihydropyridine calcium channel antagonists, nifedepine and nimodipine (300 nM-30 microM) were tested in vitro on intracellularly recorded dopaminergic neurones in the rat ventral mesencephalon. 2. Bath applied nifedipine and nimodipine inhibited in a concentration-dependent manner the spontaneous firing discharge of the action potentials, whereas, the dihydropyridine calcium channel agonist, Bay K 8644 increased the firing rate. 3. Pacemaker oscillations and bursts of action potentials were produced by loading the cells with caesium. Nifedipine and nimodipine reduced the rate and the duration of the caesium-induced membrane oscillations and decreased the number of action potentials in a burst. During the blockade of potassium currents the dopaminergic neurones often developed a prolonged (100-800 ms) afterdepolarization that was also inhibited by dihydropyridines. 4. The spontaneous discharge of calcium spikes was also inhibited by both dihydropyridine calcium antagonists. The apparent input resistance and the level of membrane potential were not affected by the dihydropyridine calcium antagonists. 5. If the action potential duration was less than 150 ms the shape of the spike was not clearly influenced by both calcium antagonists. However, when the duration of the action potential was longer than 150-200 ms due to the intracellular injection of caesium ions plus the extracellular application of tetraethylammonium (10-50 mM), both nifedipine and nimodipine reversibly shortened the plateau potential. 6. It is suggested that nifedipine and nimodipine depress the rhythmic and bursting activity of the dopaminergic cells and shorten the calcium action potential by blocking dihydropyridine-sensitive high-threshold calcium currents.

Effects of diltiazem in convulsive states differ from those previously reported for dihydropyridine calcium channel antagonists

Unlike the dihydropyridine calcium channel antagonists studied previously, the benzothiazepine calcium channel antagonist, diltiazem, increased the incidence of convulsions caused by bicuculline, N-methyl-DL-aspartate or 4-aminopyridine. However, the latencies to convulsions were also increased. Diltiazem increased the ratings of convulsive behaviour on handling after intraperitoneal administration of bicuculline, or pentylenetetrazol and after the calcium channel activator, Bay K 8644, administered ICV. When the binding of the dihydropyridine, [3H]-nitrendipine in the CNS was measured in vivo, this was increased by diltiazem. This compound therefore showed a different pattern of interaction with convulsant drugs then that previously demonstrated for other calcium channel antagonists, appearing to possess both pro- and anticonvulsant actions, and a different pattern of interaction with the dihydropyridine receptor complex.

Interactions between diltiazem and ethanol: differences from those seen with dihydropyridine calcium channel antagonists

It has previously been shown that dihydropyridine calcium channel antagonists prevent the ethanol withdrawal syndrome and potentiate the acute effects of ethanol and other central depressant drugs. We now report that, in contrast, the benzothiazepine calcium channel antagonist, diltiazem, gave no protection against the behavioural hyperexcitability seen during ethanol withdrawal, when given either acutely, on withdrawal, or chronically, during the ethanol treatment. A significant increase in convulsive behaviour on handling was seen during the withdrawal period when diltiazem was given on cessation of a mild chronic ethanol treatment schedule. Diltiazem decreased the acute general anaesthetic effects of ethanol, and did not appear to potentiate the ataxic action of ethanol. Centrally administered diltiazem did not enhance the hypothermic action of ethanol, but this effect was significantly increased by diltiazem when the calcium channel antagonist was given peripherally. When given alone by the intraperitoneal route, diltiazem decreased spontaneous locomotor activity and lowered body temperature. When the intracerebral route was used for administration of diltiazem, a significantly decrease in body temperature was seen when this compound was given alone, accompanied by a brief hyperexcitability. The interactions between ethanol and diltiazem therefore appear to differ from those seen with other calcium channel antagonists.

A change from HCO3(-)-CO2- to hepes-buffered medium modifies membrane properties of rat CA1 pyramidal neurones in vitro

1. Intracellular recordings were obtained from CA1 pyramidal neurones in rat hippocampal slices. Perfusion with a HCO3(-)-CO2-free, HEPES-buffered medium at pH 7.4 produced a wide variety of reversible effects on neuronal excitability, compared to responses obtained under standard (21 mM-HCO3-, 5% CO2, pH 7.4) conditions. 2. Introduction of HCO3(-)-CO2-free medium most commonly elicited, within 5-20 min, a fall in resting membrane potential (Vm), a rise in threshold for Na(+)-dependent action potential generation, and a reduction in input resistance. Anomalous inward rectification in the hyperpolarizing direction and subthreshold inward rectification were commonly reduced in HEPES-buffered medium. More prolonged exposure (> or = 25 min) to HCO3(-)-CO2-free medium produced, on occasion, Na+ spike inactivation. 3. The amplitudes of the fast and medium after-hyperpolarizations (AHPs) following a single depolarizing current-evoked action potential were attenuated during perfusion with HEPES-buffered medium at pH 7.4, as was the composite AHP following a train of action potentials. 4. Perfusion with HEPES-buffered medium at pH 7.4 reduced the degree of spike frequency adaptation and abolished depolarizing current-evoked burst-firing behaviour when this was present under standard conditions. 5. In tetrodotoxin (TTX)- and tetraethylammonium (TEA)-poisoned neurones, perfusion with HCO3(-)-CO2-free medium at pH 7.4 slightly raised the threshold for activation of Ca(2+)-dependent potentials and slightly reduced their duration, compared to responses obtained in HCO3(-)-CO2-buffered medium at the same pH. The AHP following the Ca2+ spike was, however, markedly attenuated. 6. Perfusion with a low-pH HCO3(-)-CO2-buffered medium (7 mM-HCO3-, 5% CO2, pH 6.9) produced changes qualitatively similar to those observed during perfusion with HEPES-buffered medium at pH 7.4. Raising the pH of the HEPES-buffered medium to 7.8 or 7.9 reversed inconsistently and then only in part the changes noted on the transition from a HCO3(-)-CO2- to a HEPES-buffered medium at the same pH (7.4). 7. The effects noted are unlikely to be due to a direct action of HEPES itself on neuronal membrane conductances. Rather, I suggest that they are likely to be caused by intracellular acidosis consequent upon the omission of HCO3- and CO2 from the extracellular medium.

Novel form of long-term potentiation produced by a K+ channel blocker in the hippocampus

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is a widely studied model of memory processes. In the CA1 region, LTP is triggered by the entry of Ca2+ through N-methyl-D-aspartate (NMDA) receptor channels and maintained by the activation of Ca2(+)-sensitive intracellular messengers. We now report that in CA1, a transient block by tetraethylammonium of IC, IM and the delayed rectifier (IK) produces a Ca2(+)-dependent NMDA-independent form of LTP. Our results suggest that this new form of LTP (referred as to LTPK) is induced by a transient enhanced release of glutamate which generates a depolarization by way of the non-NMDA receptors and the consequent activation of voltage-dependent Ca2+ channels.