The effects of strontium and barium ions at synapses in sympathetic ganglia

Preferential affinity of calcium ions to charged phosphatidic acid surface from a mixed calcium/barium solution: X-ray reflectivity and fluorescence studies

X-ray reflectivity and fluorescence near total reflection experiments were performed to examine the affinities of divalent ions (Ca(2+) and Ba(2+)) from aqueous solution to a charged phosphatidic acid (PA) surface. A phospholipid (1,2-dimyristoyl-sn-glycero-3-phosphate, DMPA), spread as a monolayer at the air/water interface, was used to form and control the charge density at the interface. We find that, for solutions of the pure salts (i.e., CaCl(2) and BaCl(2)), the number of bound ions per DMPA at the interface is saturated at concentrations that exceed 10(-3) M. For 1:1 Ca(2+)/Ba(2+) mixed solutions, we find that the bound Ca(2+)/Ba(2+) ratio at the interface is 4:1. If the only property determining charge accumulation near PA were the ionic charges, the concentration of mixed Ca(2+)/Ba(2+) at the interface would equal that of the bulk. Our results show a clear specific affinity of PA for Ca compared to Ba. We provide some discussion on this issue as well as some implications for biological systems. Although our results indicate an excess of counterion charge with respect to the surface charge, that is, charge inversion, the analysis of both reflectivity and fluorescence do not reveal an excess of co-ions (namely, Cl(-) or I(-)).

Barium-stimulated chemosensory activity may not reflect inhibition of background voltage-insensitive K+ channels in the rat carotid body

To test the hypothesis that the voltage-insensitive background leak K+ channel is responsible for the oxygen-sensitive properties of glomus cells in the rat carotid body (CB) we used Ba2+, a non-specific inhibitor of K+ currents. In vitro changes in cytosolic calcium ([Ca2+]c) and chemosensory discharge were studied to measure the effect of Ba2+. In normal Tyrode buffer, Ba2+ (3 and 5 mM) significantly increased carotid sinus nerve (CSN) discharge over baseline firing rates under normoxia (PO2 approximately 120 Torr) from approximately 150 to approximately 600 imp/0.5 s. However, addition of 200 microM Cd2+ which completely blocked increase in CSN activity stimulated by hypoxia (PO2 approximately 30 Torr), hypercapnia (PCO2 approximately 60 Torr, PO2 approximately 120 Torr) and high CO (PCO approximately 550 Torr, PO2 approximately 120 Torr) did not significantly inhibit Ba2+-stimulated CSN discharge. The response to hypoxia is abolished with Ca2+-free tyrode buffer containing 10 mM EGTA. Yet, in the same buffer, Ba2+ increased CSN discharge from approximately 2 to approximately 180 imp/0.5 s. With 200 microM Cd2+ and 10 mM EGTA, Ba2+ still increased CSN discharge from approximately 2 to approximately 150 imp/0.5 s. Oligomycin (2 microg) abolished the hypoxic response. However, in the presence of oligomycin CSN response to Ba2+ was significant. Since Ba2+ increased neural discharge under conditions where hypoxia stimulated CSN discharge is completely abolished, we suggest that the effect of Ba2+ on CSN discharge may not have anything to do with the oxygen sensing mechanism in the CB.

Modulation of N-type calcium channels translocation in RINm5F insulinoma cells

An intracellular pool of N-type voltage-operated calcium channels has recently been described in both IMR32 human neuroblastoma and PC12 rat pheochromocytoma cells. These channels were found to be accumulated in subcellular fractions where the chromogranin B-containing secretory granules were also enriched. Upon exocytosis N-type calcium channels were reversibly inserted in the plasma membrane. We have now extended this study to RINm5F rat insulinoma cells, and characterized the parallelism between the 'regulated' secretion of serotonin and the recruitment of surface calcium channels. Exocytosis was stimulated by different means, such as depolarization with high KCl, high Ba(2+)alone or protein kinase C activation; on the other hand exocytosis was inhibited with the non-selective calcium channel antagonist Cd(2+)or with noradrenaline. Stimulated release was always accompanied, with parallel kinetics, by calcium channel recruitment, while inhibition of secretion blocked calcium channel recruitment too. During repetitive depolarizations we revealed a potentiation of [Ca(2+)]()i transients in single Fura-2 loaded RINm5F cells, that was accompanied by an increase in surface VOCCs, suggesting a physiological role for the newly recruited channels. 2000 Academic Press@p$hr

Antagonism of calcium currents and neurotransmitter release by barium ions at frog motor nerve endings

1. The effects of Ba(2+) (0.1 - 2 mM) on the component of the perineural voltage change associated with nerve terminal calcium currents (prejunctional Ca(2+) currents) were compared with the effects of this ion to antagonize calcium-dependent acetylcholine (ACh) release. These experiments were made on isolated neuromuscular junctions of the frog. 2. In the presence of sufficient concentrations of K(+) channel blockers to eliminate measurable prejunctional K(+) currents, low concentrations of Ba(2+) selectively antagonized prejunctional Ca(2+) currents in normal Ca(2+) solutions. Higher concentrations of Ba(2+) also substantially reduced the Na(+) component of the perineural waveform. 3. Ba(2+) inhibited the prolonged prejunctional Ca(2+) currents that developed in the presence of higher concentrations of K(+) channel blockers. 4. Simultaneous measurements of the prejunctional Ca(2+) currents and the electrophysiological correlates of ACh release (i.e. end-plate potentials, EPPs) were made under conditions of modest K(+) channel blockade. Under these conditions, Ba(2+) generally produced simultaneous decreases in both Ca(2+) currents and EPP amplitudes. In some instances, a prolongation of prejunctional Ca(2+) currents and a transient increase in EPP amplitudes preceded the decreases in both electrophysiological events. 5. These results suggest that Ba(2+) ions can antagonize the entry of calcium into motor nerve endings and this effect is likely to be responsible for the inhibitory effects of Ba(2+) on evoked ACh release.

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.

Two components of transmitter release from the chick ciliary presynaptic terminal and their regulation by protein kinase C

1. A study was made of the effects of phorbol ester (phorbol 12-myristate 13-acetate, PMA, 0.1 microM) on the two components of evoked transmitter release, namely the fast synchronous and the slow asynchronous components, from the giant presynaptic terminal of the chick ciliary ganglion. The excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp of the postsynaptic neuron. 2. The decay time constant of the slow component was prolonged by replacing Ca2+ with Sr2+. In 5 mM [Sr2+]o the fast component decayed with a time constant of 2.6 +/- 1.4 ms whereas the slow component decayed with a time constant of 19 +/- 7 ms. 3. When stimulated with twin pulses with a short interpulse interval, the fast component of the second EPSC was often depressed whereas the slow component was usually facilitated. Both components were positively dependent on [Sr2+]o in a saturable manner, but the fast component approached its maximum at a lower [Sr2+]o than the slow component. 4. PMA potentiated both the fast and slow components to a similar extent and with a similar time course. For each component, the effect of PMA was less potent at high [Sr2+]o than at low [Sr2+]o. For either the fast or the slow component the PMA-induced potentiation was accompanied by a reduction in the paired-pulse ratio (PPR). 5. Despite the different dissociation constant for dextran-conjugated fura-2, the fluorescent ratio for intraterminal [Sr2+] ([Sr2+]i) decayed to the baseline after the nerve-evoked increment with a time course similar to that for [Ca2+]i, suggesting that intraterminal Sr2+ is buffered less efficiently than Ca2+. PMA did not increase the [Sr2+]i transients produced by stimulation of the presynaptic oculomotor nerve. 6. It is suggested that protein kinase C (PKC) modulates both the fast and slow components through common molecular mechanisms that upregulate the Sr2+ sensitivity of the vesicle fusion probability.

The role of GABAergic inputs for coincidence detection in the neurones of nucleus laminaris of the chick

1. Synaptic inputs to nucleus laminaris (NL) neurones were studied in a brainstem slice preparation of chick embryos (E15-20) using the whole-cell patch clamp technique. NL neurones are third order auditory neurones and are proposed to behave as coincidence detectors concerned with interaural timing discrimination. 2. Under voltage clamp conditions, electrical stimuli applied to either ventral or dorsal dendritic layers evoked EPSCs. These fast currents decayed with a time constant of 1.1 ms near the resting potential, reversed close to 0 mV, and were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20 microM) or 6, 7-dinitro-quinoxaline-2,3-dione (DNQX, 20 microM). Coincident or near coincident stimulation of the ventral and the dorsal dendritic layers increased the probability of action potential generation (response probability). 3. In the presence of CNQX (40 microM) other postsynaptic currents (PSCs) were observed, which reversed close to the equilibrium potential for chloride (ECl), and were reversibly blocked by bicuculline (20 microM) and, therefore, were mediated by GABAA receptors. Spontaneous GABAergic PSCs were inward going near the resting membrane potential immediately after starting whole-cell recording with a low Cl- (5 mM, ECl = -90 mV) pipette medium, but became outward-going with time. This indicates that GABAergic inputs may generate depolarizing potentials in intact NL neurones. 4. Local GABA (10 microM) application reduced both the EPSP and EPSC amplitude and shortened the EPSP decay time constant (from 5.3 to 2. 1 ms), while the EPSC decay time constant was not affected (from 1.3 to 1.2 ms). These GABA effects were mostly due to the shunting conductance of the postsynaptic GABAA receptors. 5. Depolarizing current injections combined with electrical stimuli to a unilateral axon bundle simulated bilateral synaptic inputs. Response probability increased with decreased interstimulus intervals, while local GABA (10 microM) application to the soma narrowed the time dependence of the response probability. 6. These results suggest that GABAergic inputs to NL neurones may serve to improve coincidence detection of the bilateral excitatory inputs through an increase in membrane conductance.

Calcium- and barium-dependent exocytosis from the rat insulinoma cell line RINm5F assayed using membrane capacitance measurements and serotonin release

Electrophysiological measurements of cell capacitance (Cm) and biochemical assays of [3H] serotonin ([3H]5-hydroxytryptamine or [3H]5-HT) release were combined to study the control of secretion in rat insulinoma RINm5F cells. Depolarizing pulses produced Cm changes (DeltaCm), indicative of exocytosis, with the same voltage and Ca2+ dependency as the inward Ca2+ currents (ICa). Ba2+ was able to substitute for Ca2+ in stimulating exocytosis, but not endocytosis. However, both the relative potency and kinetics of Ca2+-versus Ba2+-triggered exocytosis differed significantly. 5-HT synthesis and uptake were demonstrated in RINm5F cells. This allowed the use of [3H]5-HT to study hormone release from cell populations. [3H]5-HT was released in a depolarization-, Ca2+- and time-dependent manner. Ba2+ also substituted for Ca2+ in depolarization-induced [3H]5-HT release. Thapsigargin, used to deplete Ca2+ stores, had no effects on Ca2+-triggered Cm increases, but Ca2+-triggered [3H]5-HT release was abolished. Ba2+-triggered [3H]5-HT release, however, was only slightly affected by Ca2+ store depletion. Ba2+ was found to act directly as a secretagogue of [3H]5-HT in intact cells, but not in Cm measurements of voltage-clamped cells, suggesting that cell depolarization is a prerequisite for this action.

A comparison between potencies of external calcium, strontium and barium to support GABAergic synaptic transmission in rat cultured hippocampal neurons

Relative potencies of external Ca2+, Sr2+ and Ba2+ to trigger GABAergic synaptic transmission were evaluated by applying the patch-clamp technique to both presynaptic and postsynaptic hippocampal neurons prepared from neonatal rats. Action potentials were evoked by application of voltage pulses to presynaptic neurons, and GABAergic synaptic currents were recorded in voltage-clamped postsynaptic neurons. No stimuli were delivered during replacement with test solutions and only five pulses were applied to the presynaptic neuron in each test solution. During the five-pulse application, the amplitude of synaptic currents was constant in Ca(2+)-containing solutions, but decreased successively in Ba(2+)- and Sr(2+)-containing solutions without Ca2+. Thus, the amplitude of synaptic currents induced by the first pulse in each ionic condition was used to evaluate the potency of divalent cations. The lowest external concentration required to trigger the transmission was 0.3 mM for Ca2+, 1 mM for Sr2+ and 2 mM for Ba2+, and the concentration required to achieve the same effect as with 2 mM Ca2+ was 6 mM for Sr2+ and 10 mM for Ba2+. These results strongly suggest that Ba2+ as well as Sr2+ can be substituted for Ca2+ in GABAergic synaptic transmission and the order of potency is Ca2+ > Sr2+ > Ba2+.

Bidirectional modulation of GABA-gated chloride channels by divalent cations: inhibition by Ca2+ and enhancement by Mg2+

The effects of the divalent cations Ca2+, Sr2+, Ba2+, Mg2+, Mn2+, and Cd2+ were studied on gamma-aminobutyric acidA (GABAA) responses in rat cerebral cortical synaptoneurosomes. The divalent cations produced bidirectional modulation of muscimol-induced 36Cl- uptake consistent with their ability to permeate and block Ca2+ channels. The order of potency for inhibition of muscimol responses was Ca2+ > Sr2+ > Ba2+, similar to the order for permeation of Ca2+ channels in neurons. The order of potency for enhancement of muscimol responses was Cd2+ > Mn2+ > Mg2+, similar to the order for blockade of Ca2+ channels in neurons. Neither Ca2+ nor Mg2+ caused accumulation of GABA in the extravesicular space due to increased GABA release or decreased reuptake of GABA by the synaptoneurosomes. The inhibition of muscimol responses by Ca2+ was most likely via an intracellular site of action because additional inhibition could be obtained in the presence of the Ca2+ ionophore, A23187. This confirms electrophysiologic findings in cultured neurons from several species. In contrast, the effects of Cd2+, Mn2+, and Mg2+ may be mediated via blockade of Ca2+ channels or by intracellular sites, although the results of these studies do not distinguish between the two loci. The effects of Zn2+ were also studied, because this divalent cation is reported to have widely divergent effects on GABAA responses. In contrast to other studies, we demonstrate that Zn2+ inhibits GABAA responses in an adult neuronal preparation. Zn2+ produced a concentration-dependent inhibition (limited to 40%) of muscimol responses with an EC50 of 60 microM. The inhibition of muscimol-induced 36Cl- uptake by Zn2+ was noncompetitive.(ABSTRACT TRUNCATED AT 250 WORDS)

Barium evokes glutamate release from rat brain synaptosomes by membrane depolarization: involvement of K+, Na+, and Ca2+ channels

During K(+)-induced depolarization of isolated rat brain nerve terminals (synaptosomes), 1 mM Ba2+ could substitute for 1 mM Ca2+ in evoking the release of endogenous glutamate. In addition, Ba2+ was found to evoke glutamate release in the absence of K(+)-induced depolarization. Ba2+ (1-10 mM) depolarized synaptosomes, as measured by voltage-sensitive dye fluorescence and [3H]-tetraphenylphosphonium cation distribution. Ba2+ partially inhibited the increase in synaptosomal K+ efflux produced by depolarization, as reflected by the redistribution of radiolabeled 86Rb+. The release evoked by Ba2+ was inhibited by tetrodotoxin (TTX). Using the divalent cation indicator fura-2, cytosolic [Ca2+] increased during stimulation by approximately 200 nM, but cytosolic [Ba2+] increased by more than 1 microM. Taken together, our results indicate that Ba2+ initially depolarizes synaptosomes most likely by blocking a K+ channel, which then activates TTX-sensitive Na+ channels, causing further depolarization, and finally enters synaptosomes through voltage-sensitive Ca2+ channels to evoke neurotransmitter release directly. Though Ba(2+)-evoked glutamate release was comparable in level to that obtained with K(+)-induced depolarization in the presence of Ca2+, the apparent intrasynaptosomal level of Ba2+ required for a given amount of glutamate release was found to be several-fold higher than that required of Ca2+.

Strontium and barium induce exocytosis in electropermeabilized neutrophils

Calcium, strontium and barium induced an exocytotic response in electropermeabilized rabbit neutrophils while magnesium was without any effect. The extent of enzyme release was found to depend upon the concentration of these cations. For all cations, an optimum concentration was found with the same maximum enzyme release. At concentrations higher than optimum a decrease in lysozyme release was observed. Efficiency to induce enzyme release was in the order: Ca2+ > Sr2+ > Ba2+. Enzyme release was significantly enhanced by guanosine-5'-[gamma-thio]triphosphate (GTP gamma S) resulting in a shift to the left of the dose/response curve. The enhancement by GTP gamma S was strongest with Ca2+, was less with Sr2+, and was very little with Ba2+. The time course of lysozyme release was the same for Ca2+, Sr2+, and Ba2+ in the presence and absence of GTP gamma S when suboptimal cation concentrations were used. A decrease in responsiveness to the effectors after electropermeabilization was observed with Ca2+, Sr2+ and Ba2+ in the presence and absence of GTP gamma S. The lysozyme release induced by the different cations was not inhibited by the protein kinase C inhibitor staurosporine and was slightly affected by pertussis toxin. Ca2+ and Sr2+, but not Ba2+, potentiated formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) induced enzyme release in intact neutrophils. The divalent cation ionophore A23187 induced enzyme release in the presence of Ca2+ and Sr2+ but not in the presence of Ba2+. The results obtained with electropermeabilized neutrophils indicate that Sr2+ and Ba2+ can act as substitutes for Ca2+ in activating exocytosis, and that permeabilized neutrophils provide the best tool to investigate the effects of alkaline earth ions in exocytosis.

Barium-induced exocytosis is due to internal calcium release and block of calcium efflux

The concentration of cytosolic free Ca2+ ([Ca2+]i) and the release of tritiated norepinephrine ([3H]NE) were monitored during Ba2+ stimulation of sympathetic neurons cultured from chick embryos. Ba2+ (2.5 mM in Ca(2+)-free medium) caused a rise in [Ca2+]i in all regions (cell bodies, neurites, and growth cones) of sympathetic neurons and evoked [3H]NE release in the absence of other stimuli. The increase in [Ca2+]i and release of [3H]NE were sustained for up to 30 min in the presence of Ba2+. When Ba(2+)-stimulated cells were immediately washed in Ca(2+)-free Ba(2+)-free EGTA solution, both the elevated [Ca2+]i and [3H]NE release returned to basal levels, with similar, fast, time courses. Ba2+ also blocked Ca2+ efflux from neurons loaded with 45Ca. We conclude from the parallel effects of Ba2+ on [Ca2+]i and [3H]NE release that Ba2+ stimulates exocytosis by a Ca(2+)-dependent mechanism. The Ba(2+)-induced rise in [Ca2+]i is a result of two separate actions: (i) the release of Ca2+ from intracellular sites and (ii) an effective block of Ca2+ extrusion. The ability of Ba2+ to release Ca2+ in growth cones that are insensitive to caffeine suggests that Ba2+ may displace Ca2+ from binding sites other than endoplasmic reticulum.

Effects of inorganic potassium channel blockers on calcium requirement of transmission in a sympathetic ganglion

The effects of the alkali metal ions cesium (Cs+) and rubidium (Rb+) and alkaline earth metal ions barium (Ba2+) and strontium (Sr2+) on ganglionic transmission in various calcium (Ca2+) concentrations were investigated in rat isolated superior cervical ganglia. Cesium (1 and 2 mM) moderately supported transmission in low Ca2+ and potentiated the compound action potential (CAP) at high Ca2+ concentration. Higher concentrations of Cs+ caused depression of CAP especially at lower Ca2+ concentration. Cesium (1-4 mM) induced large spontaneous rhythmic spikes or burst of spikes. At 4 mM, Rb+ potentiated CAP at all Ca2+ concentrations. As with Cs+, higher concentrations of Rb+ inhibited CAP except at large Ca2+ concentrations. Barium (1-6 mM) effectively enhanced transmission at all Ca2+ concentrations. Strontium enhanced transmission only at very low Ca2+ concentrations. No enhancement was seen with Sr2+ in the presence of normal or higher Ca2+. The results indicate that Cs+ and Rb+ may antagonize Ca2+ effects at release sites while allowing more Ca2+ influx into the nerve terminal as a result of K+ channel blockade. Barium and to a lesser extent Sr2+ may substitute for Ca2+ in the process of transmitter release in the superior cervical ganglia of rats.

Effects of vagal stimulation and applied acetylcholine on the arrested sinus venosus of the toad

1. The effects of vagal stimulation and applied acetylcholine were compared on sinus venosus preparations of the toad, Bufo marinus, in which beating had been inhibited by adding the organic calcium antagonist nifedipine. 2. Bath-applied acetylcholine and vagal stimulation each caused membrane hyperpolarizations which were abolished by hyoscine. 3. Whereas the hyperpolarization that accompanied vagal stimulation was largely unaffected by barium ions, that produced by bath-applied acetylcholine was almost abolished. 4. Caesium ions also prevented the hyperpolarization produced by bath-applied acetylcholine but potentiated the responses to vagal stimulation. 5. The membrane resistance of arrested sinus venosus cells was found to be higher during vagal stimulation than in the absence of stimulation. In contrast when a similar hyperpolarization was produced by adding acetylcholine, the membrane resistance was found to be lower than in control solution. 6. The results are discussed in relation to the idea that neuronally released acetylcholine causes membrane hyperpolarization by suppressing inward current flow and applied acetylcholine acts to increase outward current flow.

Heat generation associated with synaptic transmission in the mammalian superior cervical ganglion

By use of a thermal detector constructed with a thin polyvinylidene fluoride film (PVDF), heat production in the superior cervical ganglion (SCG) of the guinea pig was examined. A single electric shock applied to the preganglionic nerve evokes a temperature rise of approximately 1.5 x 10(-6) deg. The thermal responses summate when the preganglionic nerve is stimulated repetitively. The amplitude of the thermal response is increased when the preparation is treated with a high Ca2+ medium. Treatment with agents that block ganglionic transmission (high Mg2+, d-tubocurarine, hexamethonium, TTX) reversibly suppresses thermal response. It is thus concluded that the thermal responses described in this paper are generated by the physico-chemical events underlying postsynaptic electrogenesis in the SCG cells.

Two calcium-activated potassium conductances in a subpopulation of coeliac neurones of guinea-pig and rabbit

1. Some of the sympathetic neurones in coeliac ganglia isolated from young guinea-pigs and rabbits were found to generate action potentials followed by after-hyperpolarizations with durations of 3-8 s, much longer than those (congruent to 300-500 ms) observed in the majority of other mammalian sympathetic neurones. 2. This type of ganglion cell discharged only once at the onset of a depolarizing step unless a very high intensity current was applied. Passive and voltage-dependent membrane conductances studied in detail in guinea-pig ganglia differed from those in the two other classes of sympathetic ganglion cell described previously (Cassell, Clark & McLachlan, 1986). 3. By using a single microelectrode to voltage clamp the soma, it was possible to demonstrate that both fast and slow components of the tail current following initiation of an uncontrolled 'action current' in neurones with long after-hyperpolarizations (l.a.h.) were carried by K+ ions, as was the fast tail current (time constant, tau congruent to 130 ms) present in other coeliac neurones. 4. The amplitude of both components of the tail current in l.a.h. neurones was markedly reduced by the replacement of Ca2+ by Mn2+, Co2+ or Ba2+ ions. These manoeuvres had similar effects on the fast tail current in other coeliac neurones. 5. Both time course and amplitude of the fast tail current were increased when Ca2+ concentration was raised, or when several 'action currents' were initiated, whereas only the amplitude of the slow tail current was affected. 6. The time course of the slow tail current could be described by the sum of two exponentials with tau on = 285 ms and tau off = 1.3 s at 35 degrees C occurring after a delay of 60 ms. This current had a Q10 of about 4 between 35 and 25 degrees C. In contrast, the Q10 of the fast component was about 2. 7. Morphine (10(-6) M) and vasoactive intestinal polypeptide (10(-6) M) had no effect on the outward tail current in l.a.h. neurones, but 5-hydroxytryptamine (10(-6) M) was found to abolish the slow component without affecting the fast component. 8. The slow tail current was activated in the subthreshold range of membrane potentials, and its properties could account for the firing characteristics of this subpopulation of sympathetic neurones. 9. The two calcium-activated potassium conductances that are responsible for the prolonged after-hyperpolarization resemble those in a subpopulation of nodose ganglion cells with unmyelinated axons (Fowler, Greene & Weinreich, 1985).(ABSTRACT TRUNCATED AT 400 WORDS)

Platelet aggregation and endogenous 5-HT secretion in presence of Ca2+, Sr2+ and Ba2+. Effects of calcium antagonists

Washed rat platelets aggregation and endogenous serotonin release were studied after thrombin stimulation in the presence of different concentrations of Ca2+, Sr2+ or Ba2+. The extent of platelet aggregation and release was found to depend upon the external concentration of these cations. For all of them, an optimum concentration could be defined. Higher concentrations were shown to inhibit both aggregation and release. Efficiency to support thrombin-induced aggregation was in the order Ca2+ greater than Sr2+ greater than Ba2+. Complete inhibition of aggregation and release induced by thrombin was obtained after a 30 second preincubation with 38 uM nitrendipine, 1 mM Cd2+ or 1 mM Mn2+. Inhibition was obtained in the presence of Ca2+, Sr2+ or Ba2+. These results are consistent with the hypothesis that Sr2+ and Ba2+ are able to support platelet activation acting as Ca2+ substitutes. Following thrombin stimulation, they could penetrate the platelets and mimick a rise in cytoplasmic Ca2+.

Strontium, barium, and manganese metabolism in isolated presynaptic nerve terminals

To gain insight into the mechanisms by which the divalent cations Sr, Ba, and Mn affect neurotransmitter release from presynaptic nerve terminals, we examined the sequestration of these cations, in comparison to Ca, by mitochondrial and nonmitochondrial [presumably smooth endoplasmic reticulum (SER)] organelles and the extrusion of these cations from isolated nerve terminals. Sequestration was studied in synaptosomes made leaky to small ions by treatment with saponin; efflux was examined in intact synaptosomes that were preloaded with the divalent cations by incubation in depolarizing (K rich) media. The selectivity sequence for ATP-dependent mitochondrial uptake that we observed was Mn much greater than Ca greater than Sr much greater than Ba, whereas that for the SER was Ca greater than or equal to Mn greater than Sr much greater than Ba. When synaptosomes that were preloaded with divalent cations were incubated in Na- and Ca-free media, there was little efflux of Ca, Ba, Sr, or Mn. When the incubation was carried out in media containing Na without Ca, there was substantial stimulation of Ca and Sr efflux, but only slight stimulation of Ba or Mn efflux. In Na-free media, the addition of 1 mM Ca promoted the efflux of all four divalent cations, probably via Ca-divalent cation exchange. In summary, the sequestration and extrusion data suggest that, with equal loads, Mn will be buffered to the greatest extent, whereas Ba will be least well buffered. These results may help to explain why Mn has a very long-lasting effect on transmitter release, while the effect of Sr is much briefer.

The effect of calcium channel blockers on blood platelet function, especially calcium uptake

The effects of organic and inorganic calcium antagonists on washed platelets from rat and human have been studied. Platelet aggregation was assessed by turbidimetry. Endogenous serotonin release was measured on the same sample by means of electrochemically treated carbon fiber electrodes. The organic calcium antagonist, nitrendipine, and the inorganic calcium channel blockers (Co2+, Mn2+, Cd2+, La3+) drastically inhibited rat and human platelet aggregation induced by thrombin, ADP or adrenaline in the presence of 0.32 mM Ca2+. In our conditions, the thrombin-induced release of endogenous serotonin was found to be external Ca2+-dependent and completely inhibited by 20 microM nitrendipine or 1 mM Cd2+. In addition, Ba2+ or Sr2+ ions can be substituted for Ca2+ to bring about platelet aggregation as well as endogenous serotonin secretion. In Ba2+ or Sr2+-containing media, rat platelet aggregation and/or serotonin secretion can be inhibited by either nitrendipine or Cd2+. Finally, we have also studied the thrombin- and external Ca2+-dependence of radiolabeled calcium uptake by rat platelets. We found that the thrombin-induced 45Ca uptake was inhibited by either 18 microM nitrendipine or 1 mM Cd2+. These results provide strong evidence for the existence of an influx of divalent cations (Ca2+, Sr2+, Ba2+) triggering platelet function. They also suggest, although they do not prove, that the translocation of these cations occurs through an agonist-operated channel as proposed by Hallam and Rink (FEBS Lett. 186 (1986) 175-179).

Effects of barium on isolated frog spinal cord

The effects of Ba2+ were studied in vitro on the isolated frog spinal cord. Ba2+ (25 microM-5 mM) caused a concentration-dependent depolarization of ventral (VR) and dorsal (DR) roots. TTX and Mg2+ substantially reduced the depolarization suggesting that interneuronal effects were involved. Ba2+ (25-500 microM) markedly increased the frequency and duration of spontaneous VR and DR potentials and substantially enhanced the duration (and frequently the amplitude) of VR and DR potentials evoked by DR stimulation. Higher concentrations of Ba2+ (1-5 mM) reduced both spontaneous and evoked potentials. Ba2+ (25-500 microM) enhanced the amount of K+ released by a DR volley and by application of L-glutamate and L-aspartate. The cation reduced VR and DR root depolarizations produced by elevated [K+]0. VR potentials induced by L-glutamate, L-aspartate, GABA and glycine and DR depolarizations caused by GABA were reduced by Ba2+. These results show that Ba2+ has complex actions on reflex transmission, interneuronal activity, the postsynaptic actions of excitatory and inhibitory amino acids and the evoked release of K+.

An intracellular study of synaptic transmission and dendritic morphology in sympathetic neurons of the chick embryo

The characteristics of synaptic transmission in whole embryonic avian sympathetic ganglia have been examined by intracellular recording. Neurons in lumbar paravertebral ganglia of chick embryos exhibit both fast nicotinic excitatory postsynaptic potentials (EPSPs) and non-cholinergic slow EPSPs. Fast nicotinic transmission is mediated by at least 3-5 convergent preganglionic inputs and can be detected at the earliest embryonic stage examined (Stage 38; 12 days of incubation). Two types of non-cholinergic slow EPSPs have been observed and distinguished by their time course and the resulting changes in input resistance. One of these slow synaptic potentials is mimicked by exogenously applied substance P, but not by exogenous luteinizing hormone-releasing hormone (LH-RH). Muscarinic agonists also evoke slow depolarizations in the ganglia, mediated at least in part by inhibition of the M-current. Intracellular labeling with horseradish peroxidase reveals cells with 5-10 primary dendrites at Stage 42 (16 days of incubation), the earliest stage to exhibit slow EPSPs. The active and passive membrane properties of avian sympathetic neurons, including the presence of the M-current, generally resemble those of adult mammalian and amphibian sympathetic neurons. Functional activity in chick sympathetic neurons is present at a developmental stage where both biochemical and morphological indices of synapse maturation are at low levels. Since this progression has also been observed in the avian ciliary ganglion, it may be of general relevance to neuronal development.

Membrane currents underlying the cholinergic slow excitatory post-synaptic potential in the rat sympathetic ganglion

Non-nicotinic slow synaptic currents were recorded from voltage-clamped neurones in isolated rat superior cervical ganglia bathed in a solution containing d-tubocurarine and (usually) 1 microM-neostigmine. Three components of slow synaptic current could be detected following repetitive preganglionic stimulation: a net inward current resulting from inhibition of the voltage-dependent outward K+ current IM; a net outward current associated with a fall in membrane conductance when IM was deactivated by membrane hyperpolarization or inhibited with external Ba2+ or internal Cs+; and an occasional late inward current associated with an increased membrane conductance. As a result, synaptic current amplitudes showed complex changes with changes in membrane potential. Both the inward current associated with IM inhibition and the outward current were enhanced by neostigmine and blocked by atropine or pirenzepine, and therefore resulted from activation of muscarinic receptors. In unclamped neurones, equivalent stimulation produced a membrane depolarization and induced or facilitated repetitive spike discharges. It is concluded that the principal synaptic response to muscarinic receptor activation is IM inhibition, leading to a net inward current and increased excitability, but that this response may be modified under certain circumstances by other synaptic currents.

Alteration of the fast excitatory postsynaptic current by barium in voltage-clamped amphibian sympathetic ganglion cells

Barium-induced alterations in fast excitatory postsynaptic currents (e.p.s.cs) have been studied in voltage-clamped bullfrog sympathetic ganglion B cells. In the presence of 2-8 mM barium, e.p.s.c. decay was prolonged and in many cells the e.p.s.c. decay phase deviated from a single exponential function. The decay phase in these cases was more accurately described as the sum of two exponential functions. The frequency of occurrence of a complex decay increased both with increasing barium concentration and with hyperpolarization. Miniature e.p.s.c. decay also was prolonged in barium-treated cells. E.p.s.c. amplitude was not markedly affected by barium (2-8 mM) in cells voltage-clamped to -50 mV whereas at -90 mV there was a progressive increase in peak size with increasing barium concentration. In control cells the e.p.s.c.-voltage relationship was linear between -20 and -100 mV; however, this relationship became progressively non-linear with membrane hyperpolarization in barium-treated cells. The e.p.s.c. reversal potential was shifted to a more negative value in the presence of barium. There was a voltage-dependent increase in charge movement during the e.p.s.c. in barium-treated cells which was not present in control cells. We conclude that the voltage-dependent alteration in e.p.s.c. decay time course, peak amplitude and charge movement in barium-treated cells is due to a direct postsynaptic action of barium on the kinetics of receptor-channel gating in postganglionic sympathetic neurones.

Divalent cations differentially support transmitter release at the squid giant synapse

The ability of Ca, Sr and Ba ions to support transmitter release was studied at the squid giant synapse by examining their respective actions on presynaptic current and post-synaptic responses. Transmitter-induced post-synaptic currents were smaller in Sr- than in Ca- containing solutions, and much smaller in Ba-containing solutions. The time course and amplitude of spontaneous miniature post-synaptic potentials were similar in the presence of all three divalent ions. Sr or Ba substitution has little effect on the resting potential of presynaptic terminals. In Sr-containing solutions, action potentials were similar in amplitude and time course to those recorded in Ca. Ba slightly prolonged action potential duration but had no effect on amplitude. Voltage-clamped presynaptic terminals exhibited inward Ca, Sr or Ba currents which were apparently carried through Ca channels. These currents were similar in amplitude and time course in all three ions, being somewhat larger in Ba. Although presynaptic currents were similar in these ions, transmitter release induced by these currents depended upon the divalent species entering the presynaptic terminal. Release was greatest in response to presynaptic current carried by Ca and smallest in response to current carried by Ba. Transfer curves relating presynaptic current to post-synaptic potential were sigmoidal in all three ions, and exhibited limiting slopes of approximately 2. Divalent cations differentially support transmitter release at the squid giant synapse in the sequence Ca greater than Sr much greater than Ba. The differential efficacy of the divalent cations is not due to post-synaptic alterations, presynaptic potential changes or differences in presynaptic divalent cation conductances. This sequence may reflect the cation selectivity of the exocytotic process responsible for transmitter release.

Effects of calcium, strontium, and barium ions on phosphorylation of hippocampal proteins in vitro

Calcium ion alone or in the presence of added calmodulin stimulated in vitro transfer of 32P from [gamma 32P]ATP into several proteins of mitochondrial and synaptosomal particulate fractions from rat brain. Strontium ion was capable of substituting for calcium ion in this stimulation, but barium ion lacked this capacity. These results bring into question the hypothesis that calcium-dependent protein phosphorylation of synaptic proteins is intrinsic to neurotransmitter release during neurotransmission, but they do not rule out that possibility.

Atrophic effects of proximal tendon transection with and without denervation on mouse soleus muscles

The response of mouse soleus muscles to transection of the proximal tendon was examined at both gross and microscopic levels. Changes in length and mass of the muscle, and in the dimensions, number, and ATPase staining characteristics of the muscle fibers, were determined at various times to 4 to 6 weeks after tenotomy. Muscles shortened by more than 50% and lost about 60% of their mass. Fiber cross-sectional area increased, and then decreased and stabilized below control after 7 days, and degenerative lesions appeared in some fibers. Fiber numbers decreased by nearly 20% and then remained constant, apparently by regeneration of new fibers replacing atrophied ones. The proportion of muscle fibers of the slow-twitch type was reduced. No structural or functional changes in motor nerve terminals could be detected, except for detachment from severely atrophied fibers. Concurrent denervation decreased both shortening and the loss of mass after tenotomy. Fiber diameter decreased but no significant degenerative lesions or loss of fibers could be detected. Fiber-type composition was unchanged. These experiments provide quantitative information about the degenerative changes in tenotomized muscles without the variable complications arising from tendon reattachment. Comparison of the responses with those that follow transection of the Achilles tendon shows that the degree of atrophy is independent of the extent of active muscle shortening after tenotomy.

Antinociceptive and lethal effects of intraventricularly administered barium and strontium: antagonism by atropine sulfate or naloxone hydrochlorine

The intracerebroventricular injection of Ba++ and Sr++ produced naloxone and atropine reversible antinociception as measured by the mouse tail-flick test. Naloxone antagonized the antinociception produced by Ba++ more effectively than atropine (pA2 5.9 vs. 7.0, respectively). Naloxone was less efficient than atropine in blocking lethality. Together these results suggest different mechanisms involved in the production of antinociception and lethality by these ions.

Pharmacological inhibition of the M-current

1. The effects of muscarinic agonists, luteinizing hormone-releasing hormone (LHRH) analogues, uridine triphosphate (UTP) and divalent cations on K(+)-currents in voltage-clamped bullfrog sympathetic neurones have been studied.2. Muscarine (1-10 muM), D-ala(6) LHRH (1-5 muM), UTP (50-100 muM) and Ba(2+) (1-4 mM) selectively depressed the M-current (I(M)), without appreciable effect on the delayed rectifier, Ca(2+)-activated or transient outward currents (I(K), I(C) or I(A)).3. I(M)-inhibition was characterized by: (a) elimination of slow current relaxations accompanying voltage jumps in the membrane potential range -30 to -60 mV; (b) reduced voltage-dependent chord conductance over this range with no change in the voltage-independent chord conductance at more negative membrane potentials; (c) suppression of outward rectification in the steady-state current-voltage curve between -70 and -25 mV; and (d) development of an inward current which increased in amplitude between -70 and -20 mV in proportion to the decrease in steady-state I(M). The kinetics and voltage sensitivity of residual I(M) were unchanged.4. The magnitude of the inward current produced by muscarine or LHRH could be accounted for quantitatively by the reduction in steady-state I(M). No increase in leak current could be detected in the range -60 to -30 mV. In two cells muscarine (10 muM) increased the leak current and conductance at -70 to -100 mV, but not at more depolarized levels.5. I(M) was not modified by removing extracellular Ca(2+), adding a selective Ca(2+)-channel blocker (Cd(2+)), adding 1 mM-dibutyryl cyclic AMP or 8'Br cyclic GMP, or by intracellular ionophoresis of Ca(2+), 8'Br cyclic GMP, dibutyryl cyclic AMP, GTP-gamma-S or S-adenosylmethionine.6. It is concluded that the principal effects of these agents in unclamped neurones - depolarization, increased input resistance, reduced outward rectification and increased excitability - are due entirely to a selective inhibition of I(M). The intracellular transduction mechanism for I(M) inhibition is unknown.

Ionic mechanisms of cholinergic excitation in mammalian hippocampal pyramidal cells

Intracellular recordings from CA1 hippocampal pyramidal neurons were obtained using the in vitro hippocampal slice preparation. Responses to ACh were monitored in the presence of blockers of voltage-dependent conductances including Mn2+, TTX and Ba2+. When Mn2+ was used to block voltage-dependent Ca conductance and possible indirect presynaptic cholinergic actions, ACh still induced a significant voltage-sensitive increase in apparent input resistance (Ra) (29%), but only an insignificant depolarization of membrane potential (Vm). When both voltage-dependent Ca and Na conductances were blocked by application of Mn2+ and TTX, respectively, ACh produced voltage-dependent increases in Ra (31%) without significant depolarization. In solutions containing TTX alone, ACh produced voltage-sensitive increases in Ra (32%) as well as a significant depolarization (6.2 +/- 3.1 mV (S.D.)). ACh transiently blocked the conductance increase which followed presumed Ca spikes, suggesting an action on the Ca-activated K-dependent conductance. The effects of Ba2+ application (100-200 microM) on Ra mimicked those of ACh. When ACh was applied to neurons in the presence of Ba2+, Ra remained unchanged, although Vm depolarization of 5-15 mV was still seen. The data indicate that ACh decreases both a voltage-dependent K conductance (distinct from that of the delayed rectifier) and a Ca-activated K conductance. Muscarinic cholinergic depolarization occurs as a result of blockade of K conductance, and is mediated by voltage-dependent Ca and Na conductances, and perhaps by presynaptic actions.

Influx of calcium, strontium, and barium in presynaptic nerve endings

Depolarization-induced (potassium-stimulated) influx of 45Ca, 85Sr, and 133Ba was measured in synaptosomes prepared from rat brain. There are two phases of divalent cation entry, "fast" and "slow;" each phase is mediated by channels with distinctive characteristics. The fast channels inactivate (within 1 s) and are blocked by low concentrations (less than 1 micro M) of La. The slow channels do not inactivate (within 10 s), and are blocked by high concentrations (greater than 50 micro M) of La. Divalent cation influx through both channels saturates with increasing concentrations of permeant divalent cation; in addition, each permeant divalent cation species competitively blocks the influx of other permeant species. These results are consistent with the presence of "binding sites" for divalent cations in the fast and slow channels. The Ca:Sr:Ba permeability ratio, determined by measuring the influx of all three species in triple-label experiments, was 6:3:2 for the fast channel and 6:3:1 for the slow channel. A simple model for ion selectivity, based on the presence of a binding site in the channel, could account well for slow and, to some extent, for fast, channel selectivity data.

An intracellular analysis of dendrodendritic inhibition in the turtle in vitro olfactory bulb

1. Intracellular recordings from an in vitro preparation of turtle olfactory bulb were used to determine the pathway responsible for producing synaptic inhibition of mitral cells.2. Inhibitory post-synaptic potentials (i.p.s.p.s) could be elicited in mitral cells by both olfactory nerve (orthodromic) and mitral cell axon (antidromic) stimulation and by suprathreshold depolarizing current pulses injected intracellularly through the recording micro-electrode. Reversing the chloride gradient by either intracellular injection of chloride or lowering the external chloride concentration reversed the i.p.s.p.s into depolarizing potentials. The GABA antagonists, bicuculline and picrotoxin, blocked the i.p.s.p.s.3. A large increment in the size of the orthodromic and antidromic i.p.s.p. was associated with an action potential. Grading the stimulus intensity on either side of threshold resulted in graded changes in the size of the i.p.s.p. The increment associated with an action potential and the ability to evoke an i.p.s.p. by direct stimulation of a mitral cell suggested that these phenomena were due to activation of the dendrodendritic reciprocal synapses between mitral and granule cells.4. Orthodromic, antidromic and directly produced action potentials could be fractionated such that regenerative activation of the soma-dendritic membrane could be blocked. Only when this membrane was allowed to reach threshold was a large i.p.s.p. recorded. This indicated that the increment in the i.p.s.p. was due to activation of a synaptic pathway involving the soma-dendritic membrane.5. When spike propagation in the mitral cell axons was blocked by tetrodotoxin (TTX), an i.p.s.p. could still be produced by direct stimulation, indicating that the mitral cell soma-dendritic membrane is functionally both pre- and post-synaptic. TTX blocked the fast, high amplitude somatic spikes and revealed higher threshold, broader spikes of lower amplitude that were blocked by cobalt and calcium-free Ringer.6. Tetraethylammonium (TEA) increased the duration and the amplitude of the calcium spike. The amplitude was also increased by barium which prolonged the spike only if TEA was present. In the presence of TEA, bicuculline also prolonged the calcium spike. This suggests that three ionic conductances limit the duration of the calcium action potential: a voltage-dependent potassium conductance, a calcium-dependent potassium conductance, and the chloride conductance associated with the i.p.s.p.7. Spontaneous, bicuculline-sensitive, depolarizing potentials were recorded in mitral cells impaled with KCl-filled electrodes. Orthodromic or antidromic stimulation increased the frequency of these small potentials for the duration of the i.p.s.p., indicating prolonged GABA release.8. Stimulation of the olfactory nerves, the mitral cell axons, and direct stimulation could elicit action potentials in granule layer cells. Orthodromic and antidromic activation was followed by a hyperpolarization of about the same duration as the mitral cell i.p.s.p. and was probably the result of dysfacilitation.9. Paired stimulation of the mitral cell axons resulted in the diminution of the granule cell e.p.s.p. evoked by the second shock, indicating that the predominant excitatory input to the granule cells is through the mitral cell dendrites.10. It is concluded that both synaptic inhibition of mitral cells and excitation of granule cells is mediated primarily by the dendrodendritic reciprocal pathway.

Changes in miniature endplate potential frequency during repetitive nerve stimulation in the presence of Ca2+, Ba2+, and Sr2+ at the frog neuromuscular junction

Miniature endplate potentials (MEPPs) were recorded from frog sartorious neuromuscular junctions under conditions of reduced quantal contents to study the effect of repetitive nerve stimulation on asynchronous (tonic) quantal transmitter release. MEPP frequency increased during repetitive stimulation and then decayed back to the control level after the conditioning trains. The decay of the increased MEPP frequency after 100-to 200-impulse conditioning trains can be described by four components that decayed exponentially with time constants of about 50 ms, 500 ms, 7 s, and 80 s. These time constants are similar to those for the decay of stimulation-induced changes in synchronous (phasic) transmitter release, as measured by endplate potential (EPP) amplitudes, corresponding, respectively, to the first and second components of facilitation, augmentation, and potentiation. The addition of small amounts of Ca2+ or Ba2+ to the Ca2+-containing bathing solution, or the replacement of Ca2+ with Sr2+, led to a greater increase in the stimulation-induced increases in MEPP frequency. The Sr-induced increase in MEPP frequency was associated with an increase in the second component of facilitation of MEPP frequency; the Ba-induced increase with an increase in augmentation. These effects of Sr2+ and Ba2+ on stimulation-induced changes in MEPP frequency are similar to the effects of these ions on stimulation-induced changes in EPP amplitude. These ionic similarities and the similar kinetics of decay suggest that stimulation induced changes in MEPP frequency and EPP amplitude have some similar underlying mechanisms. Calculations are presented which show that a fourth power residual calcium model for stimulation-induced changes in transmitter release cannot readily account for the observation that stimulation-induced changes in MEPP frequency and EPP amplitude have similar time-courses.

Who do barium ions imitate acetylcholine?

The action of 1-4 mM barium on bullfrog sympathetic ganglion cells was studied using voltage clamp. Barium imitated the action of muscarine, causing depolarization, increased input resistance, tendency to repetitive firing, and a specific inhibition of a slow small outward current termed the M current. Barium did not produce significant short-term inhibition of three other outward currents; the delayed rectifier, the calcium-dependent K current, and the A current.

Epileptiform burst afterhyperolarization: calcium-dependent potassium potential in hippocampal CA1 pyramidal cells

Synaptic excitation of hippocampal cells during blockade of synaptic inhibition results in an epileptiform "burst" potential followed by a prolonged afterhyperpolarization. This afterhyperpolarization resembles the one that is seen after the epileptic interictal spike and that is considered of critical importance in preventing seizure development. The afterhyperpolarization produced in the presence of y-aminobutyric acid antagonists is associated with a conductance increase and is inhibitory. It can occur in an all-or-none fashion after a burst, is independent of chloride, and is depressed by barium. The afterhyperpolarization has a reversal potential of (-86) millivolts, and the reversal potential is strongly dependent on the extracellular concentration of potassium. The afterhyperpolarization appears to be an intrinsic, inhibitory potassium potential mediated by calcium. This finding has implications for understanding the cellular mechanisms of epilepsy.

Barium and strontium can substitute for calcium in noradrenaline output induced by excess potassium in the guinea-pig

1. The ability of Ba2+ and Sr2+ to substitute for Ca2+ in the noradrenaline output induced by excess K+ was examined using isolated guinea-pig vas deferens. 2. When the vas deferens was repeatedly exposed to excess K+ (60 mM) at 40 min intervals, the noradrenaline output increased at least three-fold in incubation medium which contained either Ca2+, Ba2+ or Sr2+. The response decreased on repetition. The order of effectiveness was roughly Ba2+ > Ca2+ > Sr2+. 3. In the absence of excess K+, these cations had no significant stimulating effect on the noradrenaline output even when added after exposure to Ca2+-free solution. 4. As the concentration of divalent cation was increased from 0.2 to 2.5 mM the noradrenaline output induced by excess K+ increased. The maximum noradrenaline output was achieved at a divalent cation concentration of 2.5 mM and was 29.56 +/- 3.52, 15.02 +/- 1.12 and 7.45 +/- 0.84 (mean +/- S.E. of mean) n-mole/g per hr in the presence of either Ba2+, Ca2+ or Sr2+, respectively. Further increase in the concentration of the cations reduced the response. 5. The addition of either Sr2+ (2 mM) or Ca2+ (1 mM) to a solution containing various concentrations of Ba2+ facilitated the K+-induced increase in the noradrenaline output when the Ba2+ concentration was low, but inhibited release of noradrenaline when higher concentrations of Ba2+ were used. The addition of Sr2+ (1 mM) to Ca2+-containing solutions had a similar effect. 6. Mg2+ competitively inhibited the K+-induced increase in the noradrenaline output in the presence of either Ba2+ or Sr2+ and blocked that in the presence of CA2+. 7. The results indicate that both Ba2+ and Sr2+ can substitute for Ca2+ in the cations act though the same site at some stage in the process of K+-induced transmitter release.

Strontium supports synaptic transmission and long-lasting potentiation in the hippocampus

(1) Synaptic transmission was studied in isolated transverse hippocampal slices from guinea pigs. Extracellular evoked potentials were recorded in the region CA1. (2) Changing the normal perfusion solution (containing 2 mM Ca2+) to calcium-free Ringer abolished synaptic transmission which was again restored by adding strontium. A synaptic efficacy of 25--50% ofn normal was obtained for 10 mM Sr2+. (3) Two different synaptic inputs to CA1 pyramidal cells were tested with respect to their ability to produce long-lasting synaptic potentiation after tetanization in strontium Ringer. Following a brief tetanus the field EPSP and, especially, the population spike were greatly enhanced. (4) The potentiation so produced was similar to the long-lasting potentiation seen in the normal slice, because it (i) had a very long duration (hours), (ii) was specific for the tetanized pathway, (iii) showed potentiation of both 'volley-EPSP' and 'EPSP-spike' relations, and (iv) was accompanied by short-lasting (less than 5 min) generalized depression.

Effects of some divalent cations on synaptic transmission in frog spinal neurones

1. Synaptic transmission between dorsal root afferents and motoneurones was studied in the isolated and hemisected spinal cord of frogs, using intracellular and extracellular recording techniques, and ionic substitutions of divalent cations in the bathing fluid. 2. Delayed components of excitatory post-synaptic potentials (e.p.s.p.s) evoked in motoneurones by dorsal root supramaximal stimuli, as well as the Ca2+-dependent slow after-hyperpolarization which follows antidromic spikes, were reversibly blocked by superfusing the cords with 'Ca2+-free' media containing Co2+ (4 mM) or Mg2+ (6-10 mM). However, short latency e.p.s.p.s persisted in these media for more than 8 hr. 3. The minimum synaptic delay of the Co2+ and Mg2+, resistant e.p.s.p.s, measured from the peak negativity of the extracellularly recorded presynaptic spike to the onset of the e.p.s.p., was 0.3 msec at 10 +/- 1 degrees C. 4. The Co2+, Mg2+-resistant e.p.s.p.s were graded, and could be elicited by stimulation of segmental or adjacent roots. Those evoked by each of two adjacent roots showed linear summation when the roots were stimulated simultaneously. 5. The Co2+, Mg2+-resistant e.p.s.p.s decreased in amplitude at stimulating frequencies between 10 and 100 Hz, and with paired stimuli at intervals shorter than 20-40 msec. These reductions in amplitude were paralleled by decreases in amplitude of the presynaptic population spike. 6. Solutions free of divalent ions, containing EGTA (2 mM) abolished the Co2+, Mg2+-resistant e.p.s.p.s. They remained blocked for a variable time after returning to Ca2+-free Ringer containing Mg2+ (8 mM). Their continued abolition at this stage is probably not due to changes in electrical properties of motoneuronal membranes. Eventually, the Mg2+-resistant e.p.s.p.s started recovering in the Ca2+-free Ringer containing Mg2+. The time of onset of this recovery depended on the duration of exposure to EGTA. 7. Sr2+ (2-11 mM), although less effective than Ca2+, restored the composite e.p.s.p.s evoked by dorsal root supramaximal stimuli, as well as the Ca2+-dependent slow after-hyperpolarization of the motoneurone. The composite e.p.s.p.s could not be restored with Ba2+ (2-10 mM). 8. The results suggest that the Co2+, Mg2+-resistant e.p.s.p is generated by electrical coupling between some afferent fibres (probably primary afferents) and motoneurones. The after-effects of EGTA treatments probably reflect uncoupling of electrotonic junctions. In contrast, the delayed components of the composite e.p.s.p.s are generated through chemical synapses whose divalent cation requirement is similar to that of the neuromuscular junction.

Calcium-dependent potentials in the mammalian sympathetic neurone

1. Intracellular recordings from post-ganglionic neurones of the rat superior cervical ganglion revealed two non-synaptic potentials dependent upon Ca2+, a hyperpolarizing afterpotential (h.a.p.) and a tetrodotoxin (TTX)-insensitive spike. 2. The h.a.p. followed regeneration discharge of the membrane potential in normal and TTX-containing Locke solution. 3. The h.a.p. appeared to arise from an increased K+ conductance because it was associated with a decrease in input resistance, reversed at -90 mV, and was proportional in magnitude to the extracellular K+ concentration. 4. Tetraethylammonium (TEA) and 4-aminopyridine (4-AP) apparently antagonized a voltage-sensitive K+ conductance because they broadened the action potential. However, these substances reduced only slightly the peak amplitude and earliest phases of the h.a.p. 5. The TTX-insensitive spike was most apparent when TEA was present and was invariably followed by an h.a.p. with a magnitude proportional to that of the spike. 6. The magnitude of the h.a.p. and the TTX-insensitive spike was directly proportional to the external Ca2+ concentration and was antagonized by Co2+ and Mn2+ in a dose-dependent fashion. 7. In normal Locke solution, Ba2+ antagonized the h.a.p. and allowed the neurone to sustain discharge during prolonged depolarization. In Locke solution containing TTX and TEA, Ba2+ reduced the magnitude of the h.a.p. but greatly increased the duration of the TTX-insensitive spike. 8. The h.a.p. was not significantly affected by altering external Cl- concentration and the TTX-insensitive spike was not reduced by altering external Na+ concentration. 9. It is concluded that the post-ganglionic neurone supports a regenerative Ca2+ conductance mechanism which in turn triggers an increased K+ conductance. The h.a.p. appears to result from outward K+ current in both a Ca2+ and voltage-dependent fashion.

Enhancement by an antagonist of transmitter release from frog motor nerve terminals

1 The effect of Ba2+ on the synchronous release of acetylcholine from frog motor nerve terminals was studied by conventional electrophysiological techniques. 2 When Ca2+ and Ba2+ were the only divalent cations in the bathing fluid, Ba2+ caused a presynaptic reduction in the amplitude of the endplate potential (e.p.p.). This effect was surmountable by increasing the Ca2+ concentration. 3 The affinity constant (KA) for Ba2+, calculated on the assumption that Ba2+ is a competitive inhibitor of the agonist, Ca2+, was 1.1 +/- 0.4 mM-1 (mean +/- s.e. mean, n = 8). 4 When e.p.ps were depressed by the addition of 1 mM Mg2+, addition of Ba2+ (1 to 3 mM) caused either a further presynaptic depression of moderate magnitude or had no additional effect. 5 When e.p.p.s were depressed with [Mg2+] greater than or equal to 2 mM, addition of Ba2+ greater than or equal to 0.9 mM enhanced the e.p.p. amplitude by a presynaptic mechanism. 6 The interaction of the divalent cation antagonists Mg2+ and Ba2+ with the agonist, Ca2+ is discussed. It is demonstrated that a model which considers the nonequilibrium, kinetic properties of binding can be used to describe interactions between divalent cations at the external surface of the motor nerve ending.

On the conductance pathway traversed by strontium in mediating the asynchronous release of acetylcholine by motor nerve impulses

A study was made to determine whether the Sr2+ dependent asynchronous release of acetylcholine by nerve impulses is mediated by the conventional Ca2+ conductance channel or, as has been suggested recently, through an alternative ion pathway. Experiments were performed on the frog neuromuscular junction by the use of standard electrophysiological techniques. Repetitive nerve stimulation in Sr2+-Ringer solutions caused a marked increase in miniature end-plate potential (m.e.p.p.) frequency which was dependent on Sr2+ concentration and inhibited in a competitive fashion by the known Ca2+ antagonists, Co2+ and Mg2+. The equilibrium dissociation constants (KdS) determined for both Co2+ (0.09 +/- 0.01 mM, mean +/- s.e. mean, n = 5) and Mg2+ (3.7 +/- 0.3 mM, mean +/- s.e. mean, n = 4) were essentially the same as the reported values for these antagonists in blocking Ca2+ -mediated transmitter release by nerve impulses. These results suggest that Sr2+ mediates asynchronous evoked transmitter release through the conventional calcium conductance channel.