Effects of temperature on the decline in miniature end-plate potential frequency following a tetanus

When frog nerve-muscle preparations are stimulated tetanically in saline solutions containing Ca²⁺, Mn²⁺, Co²⁺, or Ni²⁺ there is a substantial rise in miniature end-plate potential (MEPP) frequency. After stimulation is ended, there is a decline back toward pre-stimulus MEPP frequencies. If we assume that MEPP frequencies are an index of the concentration of the divalent cations at some critical site in the nerve terminal, then the fall in MEPP frequency reflects the clearing of the divalent metal cation from the critical region. As a first step in investigating the clearing mechanism, we have studied the effects of temperature on the decline in MEPP frequencies following a tetanus. The Q₁₀s for the fall in MEPP frequencies following the tetanus in Ca²⁺ or Mn²⁺ range between 1.2 and 1.6. The results can be interpreted in terms of a two-stage model for the fall-off in release probability following stimulation. In Co²⁺ or Ni²⁺ containing solutions at 7°C or lower the MEPP frequency is sustained at an elevated level following the tetanus; at higher temperatures the decline does not reach the initial control level.     

Aging increases calcium influx at motor nerve terminal

To determine whether increased transmitter release from soleus nerve terminals of old C57BL/6J mice is caused by an altered Ca²⁺ regulation, the time course of post-tetanic potentiation of miniature endplate potential (MEPP) frequency was used as an indicator of the kinetics of Ca²⁺ metabolism in young (10 months) and old (24 months) mice. Post-tetanic potentiation properties were studied in either (1) 0.2 mM Ca²⁺, 5.0 mM Mg²⁺ Krebs; or (2) Ca²⁺-free/EGTA Krebs to eliminate Ca²⁺ influx, and thereby isolated Ca²⁺ buffering. In the 0.2 mM Ca²⁺ Krebs, the time constants of decay of augmentation (T_A) and potentiation (T_P) were longer in old (T_A = 10.3 ± 1.0 sec, T_P = 195.3 ± 5.4 sec) than in young (T_A = 7.0 ± 0.7 sec, T_P = 78.8 ± 6.6 sec) nerve terminals. Evoked transmitter release was measured in 0.4 mM Ca²⁺, 2.75 mM Mg²⁺ Krebs. Quantal content of the endplate potential was positively correlated with T_A (r = 0.95) and with T_P (r = 0.98). In the Ca²⁺-free/EGTA Krebs, there was no difference in post-tetanic potentiation properties between young and old terminals. These results suggest that Ca²⁺ influx into the soleus nerve terminal increases with aging. This may explain, at least in part, the increased quantal content observed at old terminals.     

4-Aminopyridine does not increase m.e.p.p. frequencies at junctions depolarized by potassium

4-aminopyridine (4-AP) is known to produce large increase in quantal acetylcholine release from stimulated motor nerve terminals. It has been suggested that the drug might act directly on Ca²⁺ channels to increase Ca²⁺ influx. This possibility was tested at frog neuromuscular junctions depolarized in elevated [K⁺]_out. The 4-AP did not increase miniature end-plate potential frequencies. Also, 4-AP did not alter the increase in frequency that follows a rise in [Ca²⁺]_out at a depolarized junction. Therefore, under these conditions, 4-AP does not appear to change Ca²⁺ entry into or elimination from the nerve terminal. The results support the hypothesis that 4-AP acts by lengthening the nerve terminal action potential.     

Influence of cations on the electrical activity of neuroblastoma × glioma hybrid cells

Electrical excitability is one of the various neuronal properties of neuroblastoma × glioma hybrid cells. At a Ca²⁺ concentration of 1.8mM the action potential is inhibited by tetrodotoxin, suggesting that the inward current is carried by Na⁺ ions. In contrast, at a Ca²⁺ concentration of 20–36mM and even in the absence of Na⁺, spikes (sometimes repetitive) with strong hyperpolarizing afterpotential occur, which are no longer affected by tetrodotoxin. They are, however, blocked by antagonists of Ca²⁺ like La³⁺, Co²⁺, Mn²⁺, and the synthetic compounds D-600 and BAY a-1040. This seems to indicate that at high concentrations of Ca²⁺, the inward current of the action potential is essentially carried by Ca²⁺. Sr²⁺, but not Mg²⁺ can effectively substitute for Ca²⁺. It shows down the time course of the action potential. Ba²⁺ depolarizes the membrane gradually. If Ca²⁺ is also present, Ba²⁺ causes a reduced depolarization and spontaneous action potentials with no hyperpolarizing after-potential are observed.     

Calcium homeostasis in rat septal neurons in tissue culture

Septal neutons from embryonic rats were grown in tissue culture. Microfluorimetric and electrophysiological techniques were used to study Ca²⁺ homeostasis in these neurons. The estimated basal intracellular free ionized calcium concentration ([Ca²⁺]_i) in the neurons was low (50–100 nM). Depolarization of the neurons with 50 mM K⁺ resulted in rapid elevation of [Ca²⁺]_i to 500–1,000 nM showing recovery to baseline [Ca²⁺]_i over several minutes. The increases in [Ca²⁺]_i caused by K⁺ depolarization were completely abolished by the removal of extracellular [Ca²⁺], and were reduced by 80% by the ‘L-type’ Ca²⁺ channel blocker, nimodipine (1 μM). [Ca²⁺]_i was also increased by the excitatory amino andl-glutamate, quisqualate, AMPA and kainate. Responses to AMPA and kainate were blocked by CNOX and DNOX. In the absence of extracellular Mg²⁺, large fluctuations in [Ca²⁺]_i were observed that were blocked by removal of extracellular Ca²⁺, by tetrodotoxin (TTX), or by antagonists ofN-methyld-aspartate (NMDA) such as 2-amino 5-phosphonovalerate (APV). In zero Mg²⁺ and TTX, NMDA caused dose-dependent increases in [Ca²⁺]_i that were blocked by APV. Caffeine (10 mM) caused transient increases in [Ca²⁺]_i in the absence of extracellular Ca²⁺, which were prevented by thapsigargin, suggesting the existence of caffeine-sensitive ATP-dependent intracellular Ca²⁺ stores. Thapsigargin (2 μM) had little effect on [Ca²⁺]_i, or on the recovery from K⁺ depolarization. Removal of extracellular Na⁺ had little effect on basal [Ca²⁺]_i or on responses to high K⁺, suggesting that Na⁺/Ca²⁺ exchange mechanisms do not play a significant role in the short-term control of [Ca²⁺]_i in septal neurons. The mitochondrial uncoupler, CCCP, caused a slowly developing increase in basal [Ca²⁺]_i; however, [Ca²⁺]_i recovered as normal from high K⁺ stimulation in the presence of CCCP, which suggests that the mitochondria are not involved in the rapid buffering of moderate increases in [Ca²⁺]_i. In simultaneous electrophysiological and microfluorimetric recordings, the increase in [Ca²⁺]_i associated with action potential activity was measured. The amplitude of the [Ca²⁺]_i increase induced by a train of action potentials increased with the duration of the train, and with the frequency of firing, over a range of frequencies between 5 and 200 Hz. Recovery of [Ca²⁺]_i from the modest Ca²⁺ loads imposed on the neuron by action potential trains follows a simple exponential decay (τ = 3–5s).     

Spontaneous and evoked transmitter releases after concanavalin A treatment are affected differently by hypertonic low calcium solutions at frog neuromuscular junction

Adding sucrose to the low calcium bathing solution made with no added calcium but containing Mg2+ EGTA to increase the tonicity elevated the basal frequency of miniature end-plate potentials (MEPPs) in the frog. The hypertonic low calcium solution also increased the rate at which the MEPP frequency rose in response to tetanic stimulation and elevated the level of the maximum frequency, which approached an asymptote. Pretreatment with concanavalin A (con A) greatly reduced the elevation in the basal frequency produced by hypertonic solutions. However, tetanic stimulation gave the same results as in untreated preparations when the tonicity was increased. Pretreatment with colchicine before the con A treatment eliminated the blocking action of con A on the MEPP frequency when the preparation was exposed to hypertonic solutions. Tetanic stimulation produced increases in MEPP frequency similar to those observed in normal junctions immersed in hypertonic solutions. Caffeine elevated the basal level of the MEPP frequency. Tetanic stimulation in the caffeine solution caused the increase in the MEPP frequency; the higher the basal level rose, the higher the maximum level became. However, the rate at which the MEPP frequency rose remained unchanged. The present results indicate that hypertonicity increases not only the basal frequency of MEPPs but also the slope at which the MEPP frequency is elevated by tetanic stimulation, both mechanisms being different and that the rate and magnitude of the tetanic potentiation of MEPP frequency are not simply determined by the pre-tetanus frequency.     

Calcium uptake related to K-depolarization and Na/Ca exchange in sheep brain synaptosomes

The uptake of Ca²⁺ by synaptosomes induced by K⁺-depolarization andby Na⁺/Ca²⁺ exchange was studied in synaptosomes in which the internal Na⁺ and K⁺ contents were varied by prolonged incubation at 30 °C or by inhibiting the Na⁺, K⁺-ATPase with 1 mM ouabain. Increased Na⁺ content of the synaptosomes is associated with an increase in Ca²⁺ uptake when the synaptosomes are placed in depolarizing K⁺ media. Furthermore, reduction in the [Na⁺]_o, when the [K⁺]_o is increased, in substitution for [Na⁺]_o, to depolarize the membrane, further increases the Ca²⁺ uptake. Under these conditions, Ca²⁺ entry probably occurs through voltage-sensitive channels and through the Na⁺/Ca²⁺ exchanger. Destruction of the Na⁺ gradient by monensin, or preloading the synaptosomes with K⁺, completely inhibits the Ca²⁺ uptake in a K⁺-depolarizing medium. It is shown that if the Na⁺ gradient is maintained constant during K⁺-depolarization, the Ca²⁺ uptake is very low and that most of the Ca²⁺ uptake is correlated with the Na⁺ gradient. Evidence is presented that K⁺ may stimulate the Na⁺/Ca²⁺ exchange mechanism. Furthermore, divalent cations, Mg²⁺, Mn²⁺ and Zn²⁺, known to block Ca²⁺ channels, also inhibit Na⁺/Ca²⁺ exchange.     

The action of valproate on spontaneous epileptiform activity in the absence of synaptic transmission and on evoked changes in [Ca2+]o and [K+]o in the hippocampal slice

The effects of valproate (VPA) on neuronal excitability and on changes in extracellular potassium ([K⁺]₀) and calcium ([Ca²⁺]₀) were investigated with ion selective-reference electrode pairs in area CA₁ of rat hippocampal slices. Field potential responses to single ortho- and antidromic stimuli were unaltered by VPA (1–5 mM). The afferent volley evoked in the Schaffer-commissural fibers was also unaffected. In contrast, VPA (1 mM) depressed frequency potentiation and paired pulse facilitation markedly. Decreases in [Ca²⁺]₀ induced either by repetitive stimulation or by application of the excitatory amino acids N-methyl-d-aspartate and quisqualate were reduced, and the latter results suggest that VPA interferes with postsynaptic Ca²⁺ entry. When synaptic transmission was blocked by lowering [Ca²⁺]₀ (0.2 mM) and elevating [Mg²⁺]₀ (7 mM), prolonged afterdischarges elicited by antidromic stimulation were blocked by VPA. VPA also suppressed the spontaneous epileptiform activity seen when [Ca²⁺]₀ was lowered to 0.2 mM, without elevating [Mg²⁺]₀. The amplitudes of the rises in [K⁺]₀ induced by repetitive orthodromic stimulation were only slightly depressed and those elicited by antidromic stimulation were generally unaltered by VPA, as were laminar profiles of stimulus-evoked [K⁺]₀ signals. These results indicate that VPA has membrane actions in addition to known effects on excitatory and inhibitory transmitter pools.     

Prejunctional modulation of nitroxidergic nerve function in canine cerebral arteries

Modulation by acetylcholine, VIP, clonidine, ω-conotoxin and Mg²⁺ of the relaxant response to electrical and chemical stimulations of nitroxidergic nerves, in which nitric oxide (NO) acts as a neurotransmitter, was investigated in isolated canine cerebral arteries. Acetylcholine attenuated the response, the inhibition being reversed by atropine; however, physostigmine failed to reduce the response. VIP in submaximal doses did not alter the neurally induced relaxation. The same was true with clonidine, morphine and naloxane. Treatment with ω-conotoxin depressed the response to electrical nerve stimulation but did not influence the nicotine-induced relaxation. Mg²⁺ inhibited the relaxation caused by nerve stimulation and Ca²⁺ reversed the response. It is concluded that activation of prejunctional muscarinic receptors seems to inhibit the synthesis or release of NO from nerve terminals but endogenous acetylcholine from cholinergic nerve does not play a role in inhibiting the nitroxidergic nerve function. Prejunctional VIP, ₂ adrenergic and opioid receptors are not likely to participate in the regulation of nerve function. Ca²⁺ responsible for NO synthase activation would be introduced into nerve terminals via N-type Ca²⁺ channels when electrically stimulated and via non-N-, non-L-type channels when stimulated by nicotine. Mg²⁺ and Ca²⁺ counteract in the neurally induced relaxation, although the underlying mechanism was not determined.     

Restoration of cholinomimetic activity by clonidine in cholinergic plus noradrenergic lesioned rats

Lactate production (J_lac), oxygen consumption rate (Q_O2), plasma membrane potentials (E_m) and cytosolic free calcium levels [Ca²⁺]_i were studied on symaptosomes isolated from rat brains, incubated in presence of high doses of nicardipine (90 μM), diltiazem (0.5 mM) and verapamil (0.25 mM), and submitted to depolarizing stimulation or inhibition of mitochondrial respiration. Nicardipine was able to completely prevent the veratridine-induced stimulation ofJ_lac, Q_O2andE_m depolarization, whereas diltiazem and verapamil were less effective, although the concentrations used were 5 and 3 times higher, respectively, than nicardipine. Diltiazem, verapamil and nicardipine (9 μM) also prevented the veratridine-induced increase in [Ca²⁺]_i, this effect being much less pronounced if the drugs were added after veratridine. Monensin (20 μM) was also able to increase [Ca²⁺]_i but this effect was not affected by verapamil. Synaptosomes were also submitted to an inhibition of respiration of intrasynaptic mitochondria by incubation with rotenone (5 μM); in this condition of mimicked hypoxiaE_m was more positive of about 11 mV; none of the drugs utilized modified this situation. The rotenone-induced 3-fold increase inJ_lac was barely modified by diltiazem and verapamil but it was completely abolished by nicardipine. The possible mechanism of the counteracting action of the drugs towards veratridine stimulation and rotenone inhibition and the involvement of Na⁺/Ca²⁺ exchanger in affecting [Ca²⁺]_i are discussed.     

Blocking of the receptor-stimulated calcium entry into human platelets by verapamil and nicardipine

Verapamil (ED₅₀=3×10⁻⁶ M) and nicardipine (ED₅₀=10⁻⁶ M) inhibited the platelet activating factor (PAF)-induced increase of free cytosolic calcium concentration ([Ca²⁺]_i) in quin2-loaded human platelets. In a Ca-free medium containing 5 mM BaCl₂, PAF stimulated the inflow of Ba²⁺ ions which is completely abolished by verapamil and nicardipine. Simultaneous determination of quin2 fluorescence and ⁴⁵Ca absorption showed that the action of verapamil is accounted for by blocking of the Ca²⁺ entry. Nicardipine suppresses also Ca²⁺ mobilization from intracellular stores. The effects of verapamil and nicardipine are not competitive with respect to PAF.The blockers reduce the [Ca²⁺]_i increase induced by ADP, vasopressin, and PGH₂ analogue U46619.     

Potassium and sodium channels in human malignant glioma cells

Human malignant glioma cells from 5 different cell lines were voltage clamped and examined for the presence of depolarization-activated ion channels. Outward K-currents were elicited at membrane potentials > 40 mV, which had two main components, one which was delayed and blocked by externally applied tetraethylammonium (TEA, 10 mM), and another which was instantaneous and insensitive to TEA in the outside solution. The proportion of the two K-current components varied between cell lines. An increase in [Ca²⁺] in the range 0–4 mM, decreased the leak conductance and shifted the activation of the instantaneous outward K-current towards more positive potenttials. Mg²⁺, Zn²⁺ and Co²⁺ had qualitatively similar effects. Patch recordings with 150–160 mM K⁺-solution on both sides of the membrane revealed that the delayed outward K-current was carried through large conductance (250–300 pS) channels. Changes in free [Ca²⁺]_i from 0 to 2 × 10⁻⁸ M increased the activation of the large conductance K-channel. Small Na-currents were identified in cells from one cell line (Tp-378MG). The Na-conductance rangedfrom 0.5 to 7.5 nS in 25% of the cells, and was less than 0.5 nS in 75%. The Na-channels were activated and inactivated at 30–40 mV more positive potentials than in the mammalian peripheral nerve. Tetrodotoxin (100 mM) blocked g_Na almost completely.     

Relation between the effect of carbacholine on spontaneous mediator release from motor nerve endings and extracellular potassium concentration in the rat

The action of carbachol (10 microM/l) on miniature end-plate potentials (MEPP) frequency was studied in soleus muscle of rats during changes of extracellular potassium concentration from 2 to 15 mM. The cholinomimetic did not influence the spontaneous transmitter release within potassium concentration range from 2 to 7.5 mM and increased MEPP frequency with concentrations exceeding the 7.5 mM rate. This facilitating effect grew with potassium concentration and at 15 mM the MEPP frequency increased up to 160%. The data obtained supported the hypothesis according to which action of cholinomimetics on the spontaneous transmitter release realized by means of presynaptic cholinereceptors depends on the initial level of the membrane potential of nerve terminals.     

Dependence of the acid-sensitive ion channel, ASIC1a, on extracellular Ca(2+) ions

The early H⁺-induced current in the embryonic spinal cord neurone depends on extracellular Ca²⁺ for its function. We have studied the Ca²⁺-dependence of homo- and heteromultimeric acid-sensing ion channels (ASICs) expressed in Cos cells. It was found that single-channel conductance of both the ASIC2a and the ASIC1a channel is reduced at membrane potentials more negative than −40 mV by elevated extracellular Ca²⁺. Due to this effect on unitary currents, the macroscopic ASIC2a peak current at −60 mV decreases gradually with rising extracellular Ca²⁺ concentration. In addition, the macroscopic ASIC1a current is very small at low extracellular Ca²⁺ and increases with rising Ca²⁺ up to 5 mM before decreasing again at still higher concentrations of extracellular Ca²⁺.     

Mechanism of action of lead on neuromuscular junctions

Effects of bath-applied lead acetate on nerve-evoked and spontaneous neurotransmitter release were studied at the neuromuscular junction of the rat using conventional microelectrode recording techniques. Lead (20, 100 microM) depressed end-plate potential (EPP) amplitude within 5 min of application without affecting miniature end-plate potential (MEPP) amplitude. Increasing bath Ca2+ concentration from 2 mM to 4 or 8 mM caused a concentration-dependent reversal of lead-induced block of nerve-evoked EPPs. When lead was washed out of the bath, EPP amplitude either returned to control values, or was potentiated. Mean quantal content (m) was reduced significantly by lead treatment, an effect due primarily to a decrease in the immediately available store of transmitter (n). The probability of transmitter release (p) was either unchanged or slightly increased by lead. In contrast to its depressant effect on evoked transmitter release, spontaneous transmitter release, as measured by MEPP frequency, was increased by lead. MEPP frequency was increased from control levels of 1.2-3.2/sec to 12-16/sec by 100 microM lead. When lead was removed from the bath solution, MEPP frequency returned to control levels. Lead-induced increases in MEPP frequency still occurred when Ca2+ was removed from the external bath solution, or when 1 mM Mn2+ was added to block nerve terminal Ca2+ channels, suggesting that extracellular Ca2+ is not required for lead to increase spontaneous release. It is suggested that lead exerts actions at multiple sites at the presynaptic nerve terminal. An extracellular action of lead on transmitter release mechanisms is likely to be due to a competitive antagonism with Ca2+ for entry through Ca2+ channels. An intracellular action of lead is indicated by the depression of n, and the ability of lead to stimulate spontaneous acetylcholine release in the absence of external Ca2+ entry.     

Selective inhibition of the slow K current at motor nerve ending by plasma from a myasthenia gravis patient

The effect of plasma from a myasthenia gravis (MG) patient, containing anti-presynaptic membrane receptor (PsmR) antibody on the membrane currents of motor nerve ending was investigated in mouse intercostal nerve triangularis sterni preparations by perineurial recording. After inhibition of both the fast K⁺ current and Ca²⁺-dependent K⁺ current by 30 mM Tetraethyl-ammonium (TEA) unmasked the voltage dependent fast Ca²⁺ current and the “Ca plateau”, which was contributed by the voltage-dependent slow Ca²⁺ current and slow K⁺ current. Application of the MG plasma caused further prolongation and increase of the Ca plateau, due to blockage of the slow K⁺ current. This effect was observed immediately after the application and could be partially reversed by washing, whereas no change was found by addition of the plasma from healthy persons. When K⁺ current was completely blocked by 30 mM TEA and 300 μM 3,4-diaminopyridine (3,4-DAP), the fast Ca²⁺ current and the slow Ca²⁺ current were revealed. Neither the fast nor the slow Ca²⁺ current could be affected by the MG plasma; It was also shown that the MG plasma was devoid of noticeable effect on the voltage dependent Na⁺ current, fast K⁺ current as well as the Ca²⁺-dependent K⁺ current. So the effect of the MG plasma with antibody to PsmR was concluded to inhibit the slow K⁺ current selectively. As we knew, the β-bungarotoxin binding protein was a kind of K⁺ channel, these results further confirmed that the β-bungarotoxin binding protein should be the target of the antibody to PsmR found in the plasma of some patients suffering from MG.     

Temperature dependence of carbachol-induced modulation of miniature end-plate potential frequency in rats

In the rat soleus, the frequency of miniature end-plate potentials (MEPP) did not change after application of 10⁻⁵ M of the cholinomimetic drug carbachol between 18 °C and 34 °C but decreased by 40% at physiological temperatures of 37–38 °C. The carbachol-induced decrease in MEPP frequency was not eliminated by 10⁻⁷ to 10⁻⁸ M atropine or 3 × 10⁻⁷ (+)-tubocurarine similarly as had been previously found at frog neuromuscular junction.     

Platelet aggregation and endogenous 5-ht secretion in presence of ca, sr and ba. 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⁺>Sr²⁺>Ba²⁺. Complete inhibition of aggregation and release induced by thrombin was obtained after a 30 second preincubation with 38 uM nitrendipine, 1 nM Cd^2- 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⁺.     

Effects of micromolar and nanomolar calcium concentrations on non-synaptic bursting in the hippocampal slice

To determine how [Ca²⁺]₀ affects non-synaptic epileptogenesis in the CA1 area of hippocampal slices, we compared the extracellularly recorded hyperactivity induced by ACSF containing either micromolar (‘low’-Ca²⁺, LC-ACSF) or nanomolar concentrations of Ca²⁺ (‘zero’-Ca²⁺, ZC-ACSF). Both solutions effectively blocked chemical synaptic transmission but spontaneous bursts developed more quickly and consistently in ZC-ACSF and were longer in duration and more frequent than those recorded in LC-ACSF. Antidromically evoked bursts were less epileptiform, i.e., they exhibited fewer population spikes (PSs), in ZC-ACSF. Increasing [Mg²⁺]₀ or decreasing [K⁺]₀ suppressed spontaneous LC-ACSF bursting but only decreased the intensity and frequency of bursting in ZC-ACSF. Either manipulation increased the epileptiform nature of the antidromically evoked field potential, thereby mimicking the effect of increasing [Ca²⁺]₀ from nanomolar to micromolar levels. Bath application of 250–500 μM GABA commonly arrested spontaneous bursting in LC-ACSF. In ZC-ACSF, GABA decreased the burst frequency but paradoxically superimposed high amplitude PSs on each burst. These effects were reversed by the GABA_A receptor antagonists bicuculline methiodide or picrotoxin (50–100 μM). These results indicate that simply lowering [Ca²⁺]₀ from micromolar to nanomolar concentrations increases the burst propensity and intensity of the CA1 population and can dramatically alter responses to pharmacological agents.     

Tetanic stimulation increases the frequency of miniature end-plate potentials at the frog neuromuscular junction in Mn-, CO-, and Ni-saline solutions

The effects of Mn²⁺, Co²⁺, and of Ni²⁺ on quantal acetylcholine (ACh) release have been studied with conventional microelectrode techniques. Increasing the [Co²⁺]₀ or [Ni²⁺]₀ (in the absence of extracellular Ca²⁺) caused an increase in miniature end-plate potential (MEPP) frequency. [Mn²⁺]₀ caused some increase in frequency at low levels, but then there was no rise as the concentration was increased further. In preparations depolarized with 20 mM K⁺, the MEPP frequency was a monotonically increasing function of [Co²⁺]₀ or of [Ni²⁺]₀. In increasing concentrations of [Mn²⁺]₀ there was an increase followed by a levelling off or a depression at higher concentrations. Tetanic stimulation of the motor nerve in solutions containing no added divalent cations or containing MgEGTA produced slight or no increases in MEPP frequency. In Mn²⁺-, Co²⁺- or Ni²⁺- saline solution stimulation of the motor nerve led to substantial increases in MEPP frequencies. The maximum frequency attained in Mn²⁺, Co²⁺, or Ni²⁺ was a power function of : (a) the duration of the tetanus; (b) the frequency of stimulation during the tetanus; or (c) the extracellular concentration of the divalent cation. During stimulation in Mn²⁺-saline solution the MEPP frequency reached a maximum; further stimulation led to a fall in frequency. We conclude that Mn²⁺, Co²⁺, and Ni²⁺ can enter the nerve terminal through a voltage-gated channel. Once within the terminal, they can stimulate quantal release by releasing Ca²⁺ or by causing the liberation of an activator, like H⁺, within the terminal.