Calcium dependent action potentials produced in leech Retzius cells by tetraethylammonium chloride

Ketamine Produces a Long-Lasting Enhancement of CA1 Neuron Excitability

Ketamine is a clinical anesthetic and antidepressant. Although ketamine is a known NMDA receptor antagonist, the mechanisms contributing to antidepression are unclear. This present study examined the loci and duration of ketamine’s actions, and the involvement of NMDA receptors. Local field potentials were recorded from the CA1 region of mouse hippocampal slices. Ketamine was tested at antidepressant and anesthetic concentrations. Effects of NMDA receptor antagonists APV and MK-801, GABA receptor antagonist bicuculline, and a potassium channel blocker TEA were also studied. Ketamine decreased population spike amplitudes during application, but a long-lasting increase in amplitudes was seen during washout. Bicuculline reversed the acute effects of ketamine, but the washout increase was not altered. This long-term increase was statistically significant, sustained for >2 h, and involved postsynaptic mechanisms. A similar effect was produced by MK-801, but was only partially evident with APV, demonstrating the importance of the NMDA receptor ion channel block. TEA also produced a lasting excitability increase, indicating a possible involvement of potassium channel block. This is this first report of a long-lasting increase in excitability following ketamine exposure. These results support a growing literature that increased GABA inhibition contributes to ketamine anesthesia, while increased excitatory transmission contributes to its antidepressant effects.

Endothelium- and smooth muscle-dependent vasodilator effects of Citrus aurantium L. var. amara: Focus on Ca(2+) modulation

Neroli, the essential oil of Citrus aurantium L. var. amara, is a well-characterized alleviative agent used to treat cardiovascular symptoms. However, because it has been found to have multiple effects, its mechanism of action requires further exploration. We sought to clarify the mechanism underlying the actions of neroli in mouse aorta. In aortic rings from mice precontracted with prostaglandin F2 alpha, neroli induced vasodilation. However, relaxation effect of neroli was decreased in endothelium-denuded ring or pre-incubation with the nitric oxide synthase inhibitor NG-Nitro-l-arginine-methyl ester (L-NAME). And also, neroli-induced relaxation was also partially reversed by 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ), a soluble guanylyl cyclase (sGC) inhibitor. In addition, neroli inhibited extracellular Ca(2+)-dependent, depolarization-induced contraction, an effect that was concentration dependent. Pretreatment with the non-selective cation channel blocker, Ni(2+), attenuated neroli-induced relaxation, whereas the K(+) channel blocker, tetraethylammonium chloride, had no effect. In the presence of verapamil, added to prevent Ca(2+) influx via smooth muscle voltage-gated Ca(2+) channels, neroli-induced relaxation was reduced by the ryanodine receptor (RyR) inhibitor ruthenium red. Our findings further indicate that the endothelial component of neroli-induced vasodilation is partly mediated by the NO-sGC pathway, whereas the smooth muscle component involves modulation of intracellular Ca(2+) concentration through inhibition of cation channel-mediated extracellular Ca(2+) influx and store-operated Ca(2+) release mediated by the RyR signaling pathway.

Calcium transients in subcompartments of the leech Retzius neuron as induced by single action potentials

Regional Ca(2+) influx into neurons plays an essential role for fast signal processing, yet it is little understood. We have investigated intracellular Ca(2+) transients induced by a single action potential (AP) in Retzius neurons in situ of isolated ganglia of the leech Hirudo medicinalis using confocal laser scanning microscopy in the cell body, in different axonal branches, and in dendrites. In the cell body, a single AP induced a Ca(2+) transient in submembrane regions, while in central regions no fluorescence change was detected. Burst activity evoked a much larger Ca(2+) influx, which elicited Ca(2+) signals in central somatic regions, including the cell nucleus. A single AP induced a Ca(2+) transient in distal branches of the axon and in dendrites that was significantly larger than in the proximal axon and in the cell body (p <.05), and the recovery of the Ca(2+) transient was significantly faster in axonal branches than in dendrites (p <.01). The AP-induced Ca(2+) transient was inhibited by Co(2+) (2 mM). The P/Q-type Ca(2+) channel blocker omega-agatoxin TK (500 nM) and the L-type Ca(2+) channel blocker nifedipine (20 microM) had no effect on the Ca(2+) transient, whereas the L-type Ca(2+) channel blocker methoxyverapamil (D600, 0.5-1 mM) irreversibly reduced the Ca(2+) transient by 37% in axons and by 42% in dendrites. Depletion of intracellular Ca(2+) stores following inhibition of endoplasmic Ca(2+)-ATPases by cyclopiazonic acid (10 microM) decreased the AP-induced Ca(2+) transient in the dendrites by 21% (p <.01), but not in axons, and increased the Ca(2+) recovery time constant (tau) in the axonal branches by 129% (p <.01), but not in dendrites. The results indicate that an AP evokes a voltage-gated Ca(2+) influx into all subcompartments of the Retzius neuron, where it produces a Ca(2+) signal of different size and/or kinetics. This may contribute to the modulation of electrical excitation and propagation of APs, and to different modes of synaptic and nonsynaptic processes.

Modulatory effects of myomodulin on the excitability and membrane currents in Retzius cells of the leech

Ion channel modulation by the peptide myomodulin (MM) has been demonstrated in a wide variety of organisms including Aplysia, Lymnaea, and Pleurobranchaea. This neural and muscular modulation has been shown to be important for shaping and modifying behavior. In this paper, we report that MM modulates several distinct ionic channels in another species, the medicinal leech Hirudo medicinalis. Experiments have focused on the Retzius cell (R) because the R cell is a multifunction neuron that has been implicated in a number of behaviors including feeding, swimming, secretion, thermal sensing, and the touch elicited shortening reflex and its plasticity. Previous work had identified a MM-like peptide in the leech and demonstrated that this peptide modulated the excitability of the R cell. Using combined current- and voltage-clamp techniques to examine the effects of MM on the R cell, we found that in response to a step pulse, MM increased the excitability of the R cell such that the cell fires more action potentials with a shorter latency to the first action potential. We found that this effect was mediated by the activation of a Na+-mediated inward current near the cell resting membrane potential. Second, we found that MM differentially modulated the potassium currents IA and IK. No effect of MM was found on IA, whereas MM significantly reduced both the peak and steady-state amplitudes of IK by 49 +/- 2.9% and 43 +/- 7.2%, respectively (means +/- SE). Finally we found that MM reduced the amplitude of the Ca2+ current by approximately 20%. The ionic currents modulated by MM are consistent with the overall effect of MM on the cellular activity of the R cell. An understanding of the cellular mechanisms by which MM modulates the activity of the R cell should help us to better understand the roles of both MM and the R cell in a variety of behaviors in the leech.

Effect of extracellular K+ on the intracellular free Ca2+ concentration in leech glial cells and Retzius neurones

The effects of extracellular K+ on the intracellular free Ca2+ concentration ([Ca2+]i) of neuropile glial cells and Retzius neurones in intact segmental ganglia of the leech Hirudo medicinalis were investigated by using iontophoretically injected fura-2. In both cell types, an elevation of the extracellular K+ concentration ([K+]o) caused an increase in [Ca2+]i, which was blocked by Co2+, Ni2+ and menthol, whereas nicardipine, flunarizine, omega-conotoxin GVIA and omega-agatoxin IVA were ineffective. In Ca(2+)-free solution, the K(+)-induced [Ca2+]i increase was largely suppressed in neuropile glial cells and completely abolished in Retzius neurones. The results indicate that the K(+)-induced [Ca2+]i increase was mainly due to Ca2+ influx through voltage-dependent Ca2+ channels. The Ca2+ channels of the two cell types were activated at different membrane potentials but at the same [K+]o. In both cell types, the recovery from a K(+)-induced [Ca2+]i increase was unaltered in Na(+)-free solution, indicating that active Ca2+ transport across the plasma membrane is mediated by Na(+)-independent mechanisms.

Diversity and modulation of ionic conductances in leech neurons

A complete understanding of animal behavior at the cellular level requires detailed information on the intrinsic biophysical properties of neurons, muscles, and the synaptic connections they make. In the past 10 to 15 years, electrophysiological studies of leech neurons have revealed a diverse array of voltage-gated ionic conductances distinguished by their pharmacological sensitivity to classic ion channel blockers. Voltage-clamp studies have provided new information about the kinetics and voltage-dependence of Na+ conductances, several K+ currents, including IA, IK and IK(Ca.), and high- and low-voltage-gated Ca2+ conductances. These studies showed that the action potentials of most leech neurons result from the usual sequence of permeability changes to Na+, K+, and Ca2+ ions. They also added insight as to the role played by particular combinations of conductances in providing individual neurons with electrical properties appropriate for the particular information they encode. Evidence is accumulating on the modulatory actions fo endogenous neurotransmitters such as FMRFamide, serotonin, and octopamine on motor behaviors in the animal. Parallel studies suggest that changes in behavior can be explained, at least in part, by the alteration of firing patterns of selected neurons and muscles resulting from modulation of multiple ion conductances. This makes the leech exceptionally attractive for neuroethological studies because it is one of the simplest organisms in which the methods of psychology and neurobiology can be combined. Information gathered from this animal will therefore increase our understanding regarding general principles underlying the cellular basis of behavior.

Neurotoxic action of some antiarrhythmic agents: comparative effects of propafenone, lidocaine and amiodarone on leech Retzius nerve cell

A series of antiarrhythmic drugs was studied on spontaneous spike activity and depolarizing outward potassium current in leech Retzius nerve cells. Propafenone (0.7 microM/ml) produced a cardiac-like action potential with a rapid depolarization followed by a sustained depolarization or plateau, which is terminated after 250 msec by a rapid repolarization. The effect of lidocaine (0.7 microM/ml) on spontaneous spike activity was less pronounced, and early afterdepolarization has been recorded. Amiodarone at the same and much higher concentrations (3 microM/ml) did not generate either a cardiac-like action potential or an early afterdepolarization. In the voltage clamp experiments, fast and slow calcium-activated outward potassium currents were suppressed with propafenone and lidocaine but not with amiodarone. These results suggest that the antiarrhythmic drugs, propafenone and lidocaine modulate calcium-activated potassium channels in leech Retzius nerve cells.

Protein phosphatase inhibitors prolong Ca(2+)-transients and divalent cation-dependent action potentials in leech Retzius cells

Elevation of [K+]o for 30 s from 4 to 120 mM produced a fast and reversible depolarization and transient increase in [Ca2+]i in fura-2 loaded Retzius cells of the leech. The protein phosphatase inhibitor, okadaic acid, significantly slowed the return of [Ca2+]i toward baseline without affecting the amplitude of depolarization or rate of repolarization. Furthermore, okadaic acid and another phosphatase inhibitor, calyculin A, prolonged Ba(2+)-dependent action potentials. These results suggest that the kinetics of Ca2+ influx may be regulated by the activity of phosphatases PP-1 and/or PP-2A.

Serotonin depletion inhibits feeding in carnivorous leeches (Haemopis)

The effect of reserpine on the serotonin content of the central nervous system and the feeding behavior of the carnivorous leech Haemopis marmorata was investigated. Leeches were hand-fed to satiation by presentation of pieces of chicken liver held in forceps for three meals at 4-day intervals prior to and 1 week following three consecutive injections of 100 micrograms reserpine in the crop. A group matched by weight and preinjection food intake was injected with the vehicle in a blind experimental design. Histochemical and chemical determinations showed that reserpine effectively depleted the serotonin content of the animal's central nervous system. Furthermore, the food intake of reserpine-treated animals was significantly less than that of the sham-injected group. However, the depleted animals continued to ingest approximately 40% of the amount consumed during the preinjection period. These results show that reserpine successfully depleted the serotonin in the carnivorous leech H. marmorata and decreased the food intake of this leech species. They suggest that feeding behavior in Haemopis is partially but not exclusively dependent on serotonin and that, in contrast to the sanguivorous leeches, additional factors may be necessary for its regulation.

FMRF-amide modulates the electrical activity of the leech Retzius cell

The effect of the peptide FMRF-amide on the electrical activity of the leech Retzius (R) cell was investigated using electrophysiological techniques. FMRF-amide and six structurally related analogs increased the excitability of the R cell in several distinct ways that could act in concert to modulate transmitter release. 'Puffs' of FMRF-amide transiently depolarized the cell leading to a barrage of action potentials. This depolarization was followed by a phase of rhythmical bursting that appeared intrinsic to the neuron. FMRF-amide also broadened the plateau of the Ca(2+)-dependent action potential. The results suggest that the terminal Phe and Arg as well as the C-terminal amide are critical for the activity of these peptides.

Effects of acid Ca2+ Ringer on passive electrical properties and intracellular ion activities in leech Retzius neuron

1. A significant drop in effective input resistance of the free membrane and an increase in effective coupling resistance in acid Ca2+ Ringer (complete replacement of Na+ with Ca2+, pH 4) compared to control medium has been obtained in leech Retzius neurons. 2. In neutral Ca2+ Ringer (pH 7.2), effective input resistance increased while effective coupling resistance did not change. In acid sodium, leech Ringer (pH 4) effective input resistance increased while coupling resistance decreased. 3. Ten millimolar manganese and 10 mmol tetraethylammonium did not block conductance changes obtained in acid Ca2+ Ringer. 4. Intracellular activity of Na+ decreased, cellular activity of Cl- increased and intracellular K+ activity was unchanged in both acid and neutral Ca2+ Ringer. 5. The main difference was intracellular acidification in acid Ca2+ Ringer while intracellular pH was unchanged in neutral Ca2+ Ringer. 6. We discuss the possibility that in acid Ca2+ Ringer, intracellular acidification in leech neurons may be responsible for accompanying conductive changes.

Mechanism of action of triethyltin on identified leech neurons

The effects of triethyltin (TET) have been examined using intracellular electrophysiological recording techniques from identified neurons of the leech (Hirudo medicinalis) CNS and from salivary glands of the giant Amazon leech (Haementeria ghilianii). TET, at concentrations as low as 10(-5) M, caused a reversible neuronal membrane depolarisation accompanied by an increase in firing frequency of action potentials (which could lead to conduction block at 10(-4) M) and a concomitant decrease in membrane resistance. TET-induced membrane depolarisation still occurred in saline where Na+, K+ and Ca2+ had been replaced by choline. TET decreased the rate of the depolarising and repolarising phases of the action potential. This also occurred in Haementeria salivary gland cells, in which the only inward cation channel is a calcium channel. The calcium channel blocker, manganese, did not block the effects of TET. TET counteracted the effects on the action potential of the potassium channel blocker, tetraethylammonium chloride (TEA). TET-induced neurotoxicity occurred independently of any resultant toxic effects on the myelin sheath. The action of TET is consistent with our view that it causes an increase of intracellular free Ca2+ probably via release from intracellular stores and inhibition of Ca2+ reuptake. A resulting inhibition of the Na+/K+ and Ca2+ pumps may also occur.

Pharmacological control of the pattern of activity in leech Retzius neurones

1. Changes in the activity pattern of leech Retzius (R) cells were investigated using intracellular recording. 2. The presence of an after-hyperpolarisation (AHP) is closely related to activity pattern; regular firing being associated with an AHP, bursting with its absence. 3. Increasing external calcium (Cao), cyclic AMP levels or activity of kinase A enhanced the AHP. 4. Bursting was induced by low Cao, EGTA, barium, cobalt or injection of phorbol ester. 5. Reduction of Cao to zero caused long paroxysmal depolarising shifts of potential which could be reversed to bursting by cobalt, IBMX or injection of kinase A catalytic subunit. 6. The possible roles of a calcium-activated potassium channel and protein phosphorylation in regulating the activity of the cell are discussed.

In situ patch-clamp recording of calcium-activated potassium channels from an identified leech neuron

The paired anterior lateral giant cells of the leech Haementeria have only two active voltage gated ionic currents. We took advantage of this simple complement of ionic currents to investigate the single channel properties of this cells' calcium-activated K+ current (I(K,Ca) in situ. Cell-attached patch recordings showed large, bursting events with a conductance of approximately 90-100 pS which had extrapolated reversal potential consistent with K+ events. The channel open time distribution was well described by a single exponential process while the channel closed times were bi-exponentially distributed. The results show that the single channel properties of I(K,Ca) in annelids closely resemble those of similar currents described in vertebrates.

A molecular approach to the development of anticonvulsants

Anticonvulsants are neuronal stabilizing compounds that exhibit multiple clinical effects, including anticonvulsant, anxiolytic, sedative, and muscle-relaxant properties. This complex therapeutic picture complicates the treatment of seizure disorders in individuals with mental and developmental disorders, and frequently impairs the routine integration into society for these individuals. In order to improve the therapeutic effectiveness of these compounds, it is necessary to identify their precise molecular actions on the neuronal membrane and their effects on neuronal function. We have identified two major classes of low-affinity BZ binding sites that seem to function as generalized anticonvulsant receptors and that may mediate the anticonvulsant and sedative effects produced by these compounds. The identification of these binding sites and their anticonvulsant binding profile may clarify the complex picture of anticonvulsant mechanisms and elucidate the site(s) at which anticonvulsants produce their inhibition of MES-induced seizures and sedative effects. We will continue to examine the physiological changes induced by anticonvulsant binding at these BZ binding sites that may be a foundation for understanding the molecular basis of sedation and MES-induced seizure inhibition. Specifically, we will investigate the specific membrane components associated with the inhibition of Ca2+ channels, Na+ channel rectification, and CaM kinase II. If these goals can be achieved, then model systems could be developed to screen potential anticonvulsant or sedative compounds in the search for more effective therapeutic drugs.

Barbiturates block divalent cation action potentials in leech nociceptive cells

Phenobarbital (PNB), pentobarbital (PTB) and methohexital (MTX) decreased the maximum rate of depolarization Vmax and duration of divalent cation action potentials elicited in leech nociceptive neurons in Na+-free solutions containing the K+-channel blocker TEA, without significantly affecting resting membrane potential or conductance. The block of the divalent cation action potentials was reversible and dose-dependent, ED50 for inhibition of Vmax being 560 microM for MTX, 800 microM for PTB and 3000 microM for PNB. This order of potency correlated well with the ratio of unchanged/charged form of the drugs at physiological pH suggesting that in leech, as in other preparations, the non-ionized form was the active one. In Na+-containing Ringer, the 3 barbiturates depolarized and decreased membrane resistance in the lateral nociceptive cells, but not the medial nociceptive cells. These results provide additional information regarding the newly described pharmacological differences among closely related neurons. These membrane actions may be related to some of the excitatory properties described for other barbiturates in invertebrate and mammalian preparations.

Properties of action potentials carried by divalent cations in identified leech neurons

Properties of divalent cation potentials carried by either Sr2+ or Ca2+ ions in Na+-free, TEA-Ringer solution were characterized in identified neurons of two species of leeches (Macrobdella and Haementeria). In Macrobdella, the overshoot of the potentials varied logarithmically with [Sr2+]0 (28.5 mV per 10-fold change). The overshoot, Vmax, and duration of the potentials increased with increasing divalent cation concentration and saturated at about 20 to 30 mM [Sr2+]0. The Vmax, amplitude, and duration of the potentials were reversibly blocked by Co2+ and Mn2+. The block by Mn2+ could be well-fitted by a reverse Langmuir-curve with an apparent KI of 100 micromolar. The local anesthetic procaine also reversibly inhibited the Vmax and duration of the potentials. The inhibition was greater at alkaline pH suggesting that procaine blocks the calcium channel from inside the membrane. The identified leech neurons examined in Macrobdella varied considerably in their ability to sustain somatic divalent cation potentials. Stimulation of T cells and most motoneurons produced no or only weak potentials, whereas stimulation of Retzius, N, Nut, and AP cells evoked overshooting potentials of several seconds' duration. Stimulation of the ALG cell of Haementeria in normal Ringer solution evoked a slowly-rising, purely Ca2+-dependent potential of approximately 100 ms duration. This response was TTX-resistant, unaffected by complete removal of Na+ from the Ringer solution, and abolished by 1 mM Mn2+. The overshoot varied logarithmically with a slope of 28 mV/decade change in [Ca2+]0.

Inhibition of Ca2+ conductance in identified leech neurons by benzodiazepines

Benzodiazepines (BZs) in micromolar concentrations inhibit Mn2+- and Co2+-sensitive regenerative divalent cation potentials, which are revealed in the presence of tetraethylammonium ion, in leech nociceptive neurons (N cells). This BZ effect is reversible and dose-dependent. The BZs, like Mn2+ and Co2+, inhibit the maximum rate of depolarization (Vmax) and duration of divalent cation potentials at concentrations that do not significantly affect resting membrane potential or Vmax of the Na+-dependent action potential. Ultraviolet-induced BZ binding to micromolar-affinity sites in ganglia and isolated cells irreversibly blocks Ca2+ conductance in neurons without significantly affecting resting membrane potentials. BZ binding studies with leech neuronal membrane show saturable, specific binding in the micromolar concentration range that was similar to BZ binding to synaptosomal membrane fractions. The apparent Kd obtained from the micromolar-affinity BZ binding curve for leech ganglionic membrane preparations agrees well with the apparent Ki estimated from the dose-response curve measuring BZ inhibition of Vmax of the divalent cation potentials. These findings indicate that BZs act like Ca2+-channel antagonists in intact neuronal preparations and are consistent with the hypothesis that BZ binding to micromolar-affinity receptors modulates voltage-gated Ca2+ channels.

Effects of tetraethylammonium-chloride and divalent cations on the afterhyperpolarization following repetitive firing in leech neurons

In leech Retzius cells, repetitive activity evoked a prolonged Ca2+-dependent after-hyperpolarization (PAH) (30-60 s) accompanied by an increase in input conductance. PAH persisted in Retzius cells, as well as in nociceptive (N) cells, when Sr2+ but not Mg2+ was substituted for Ca2+. In the presence of tetraethylammonium-chloride (TEA) or Ba2+, PAH was replaced by a Ca2+-dependent, Mg2+-blockable depolarization which was present in the order N greater than R. Careful study of the differences in such phenomena in identified cells may improve our understanding of the differential susceptibility of various neurons to hyperexcitability.

Procaine actions on tetrodotoxin sensitive and insensitive leech neurons

Procaine (0.1-10 mM) was applied to two kinds of identified neurons in segmental leech ganglia. Both Retzius (R) cells and nociceptive (N) cells responded by dose-dependent reduction of maximum rates of de- and repolarization during action potentials. However, the N cells, which are more sensitive to tetrodotoxin than R cells, were also 3 times more sensitive to procaine. The prolongation of action potentials produced by procaine in R cells was enhanced by low Ca but antagonized by high Ca. This implies that the drug interfered with repolarization by affecting a Ca-dependent mechanism. In alkaline solution (pH 8.5), sensitivity of the R cell to procaine approached that of the N cell at pH 7.4 suggesting that the drug acted at an intracellular site after passage through membrane lipids in its uncharged form. The combined effects of TTX and procaine, in concentrations which produced about 50% inhibition of dV/dTdep in N cells when given separately, were found to be intermediate between those predicted by two models which assume identical and independent sites of action, respectively. These data extend our earlier observations regarding the existence of two types of Na channels in mature leech neurons. They imply that the differential sensitivity to procaine among these cells may be a consequence of variable access to an otherwise identical 'receptor' and that TTX and procaine act on separate sites which may interact with each other.

Action potential prolongation: an effect of physostigmine (eserine) upon Retzius cells in the leech C.N.S

Physostigmine (PHY; eserine) prolongs the action potentials in the Retzius cells within leech ganglia to about 800 ms. The effect was reversible and occurred at concentrations of 1-10 mM which are several orders of magnitude greater than those required to inhibit cholinesterase. The prolonged action potentials showed an early, spike-like depolarization followed by a plateau. The initial depolarization exhibited a strong dependence on external Na+ while the amplitude of the plateau had somewhat less Na+ dependence: 52 and 24 mV/decade, respectively. The duration of the plateau was increased by elevating Na+ and decreased by elevating Ca2+. Increasing the action potential frequency, by intracellular stimulation, decreased both the duration and amplitude of the plateau. Neostigmine, di-isopropylphosphofluoridate, and acetylcholine did not prolong RZ action potentials. Thus, the membrane effects of physostigmine appear to be independent of any inhibition of cholinesterase or accumulation of acetylcholine.

Differential action of tetrodotoxin on identified leech neurons

1. In leech segmental ganglia, the maximum rate of depolarization of action potentials was found to depend largely on Na in the Retzius (R) cell, the mechanosensory P, N and T cells and an identifiable neuron of unknown function, the X cell. 2. Tetrodotoxin (TTX) 15 100 mumol/l had little or no effect on R and X cells. In contrast, membrane excitation in N, P and T cells was depressed in dose- and use-dependent fashion. 3. The data imply the existence of two kinds of Na channels in normal, fully differentiated leech neurons. Correlation of such differences should lead to a better understanding of how particular neurons perform different functions.

Chemical transmission between individual Retzius and sensory neurones of the leech in culture

1. Chemical synaptic transmission develops between individual identified neurones dissected from leech ganglia and maintained in culture. Impulses in Retzius cells give rise to hyperpolarizing synaptic potentials in pressure (P) sensory cells. In suitable medium the potentials develop by 3 days and can be observed for more than 3 weeks. 2. The synaptic potentials occur after a synaptic delay, exhibit facilitation and depression and are reversed by hyperpolarization. The blocking effects of reduced calcium and raised magnesium concentrations in the bathing fluid provide additional evidence for the chemical nature of transmission. 3. An increase in chloride conductance is involved in the generation of the synaptic potential in the P cell. With high intracellular Cl in the post-synaptic cell, the synaptic potentials become reversed and amplified. The amplitudes of these reversed responses range from 1 to 20 mV with a falling phase lasting for seconds. 4. Changes in the membrane potential of the presynaptic cell that modify the amplitude and duration of the action potential influence the efficacy of transmission. In addition, impulses in Retzius cells initiated from hyperpolarized values of membrane potential evoke smaller synaptic potentials in the P cells than impulses arising from a depolarized level. 5. With neurones placed directly next to one another in the dish, maintained depolarization of the presynaptic Retzius cell in the absence of conducted action potentials gives rise to slow synaptic potentials in the P cells. In some pairs, the response in the P cell consists of a marked increase in 'noise'. 6. Injection of horseradish peroxidase into the Retzius cell reveals neurites with distinctive varicosities growing over the P cell.

Ca-K bi-ionic action potential in squid giant axons

Under intracellular perfusion with a solution containing K+ as the sole cation species, squid giant axons were found to be capable of developing all-or-none action potentials when immersed in a medium in which CaCl2 was the only electrolyte. The adequate range of ion concentration for demonstrating this capability was mentioned. The reversal potential level measured by the voltage-clamp technique varied directly with the logarithm of the concentration of extracellular Ca-ion; the proportionality constant was close to RT/2F. The action potential observed under this Ca-K bi-ionic condition could not be suppressed by addition of tetrodotoxin or saxitoxin to the external medium. The external Ca-ion could be replaced with Co- or Mn-ion without eliminating the capability of the axons to develop action potentials. D-600 could not suppress the inward current observed under the voltage-clamp condition, but 4-aminopyridine could suppress it. The experimental findings were interpreted based on the current channel hypothesis and on the macromolecular theory.

ACh-evoked complex membrane potential changes in mouse submaxillary gland acini. A study employing channel blockers and atropine

The responses in membrane potential and resistance of acinar cells to iontophoretically applied acetylcholine (ACh) were investigated using intracellular micro-electrode recording in superfused segments of mouse submaxillary gland. For measurements of membrane resistance and acetylcholine equilibrium potential (EACh), two micro-electrodes were inserted into neighbouring communicating cells. Current could be injected through one of the electrodes. The pattern of membrane potential change induced by ACh depended on the resting potential. Simple hyperpolarizations were induced at low resting potentials, while biphasic potential changes (depolarization followed by hyperpolarization) or simple depolarizations were observed at relatively high resting potentials. A similar dependence of the ACh induced potential change on the resting potential was obtained in experiments in which the resting membrane potential was set at different levels by injecting direct current and stimulating the same cell with equal doses of ACh. The ACh equilibrium potential ranged widely between -45 and -75 mV. Under special conditions the conversion in response to ACh from a hyperpolarization to depolarization could be obtained without change in resting potential. Small doses of ACh evoked simple depolarization, while medium doses induced biphasic responses and large doses of ACh caused hyperpolarization. The effect of a low concentration of atropine on the response was an initial block of hyperpolarization followed by a secondary block of depolarization. Intracellular injection of TEA ions converted the ACh induced potential response from hyperpolarization to depolarization. Both the depolarizing and hyperpolarizing ACh responses were accompanied by a marked reduction in membrane resistance. The depolarization was abolished by a severe reduction in external Na concentration, while the hyperpolarization was sensitive to alternations in external K concentration. These results indicate that some of the complex responses in submaxillary gland acinar cells to ACh may be explained by the interaction between two different kinds of potential change (Na dependent depolarization and K dependent hyperpolarization).

Effects of tetraethylammonium chloride on contractile, membrane and cable properties of rabbit artery muscle

1. Two types of effects of tetraethylammonium chloride (TEA) have been found in the smooth muscle cells of the rabbit main pulmonary artery. (a) With rapid onset of action TEA depolarizes the cell membrane, increases the membrane resistance, causes anomalous rectification and occasionally spike potentials in response to externally applied depolarizing current pulses and produces tonic contractions. (b) During prolonged (greater than 30 min) incubation in TEA phasic contractions develop progressively and the vascular strips respond to electrical stimulation with synchronized and powerful contractions. 2. There is a linear relationship between log concentration TEA and depolarization over the range of 10-100 mM-TEA. TEA (10 and 30 mM) raises the membrane resistance and decreases the core resistance. The latter effect appeared to develop more slowly than the former. 3. During short exposure to TEA part of the smooth muscle cells respond to depolarizing current pulses with spike potentials of variable amplitude and duration. These spikes are very sensitive to inhibition by verapamil or nickel chloride but are not affected by tetrodotoxin. The amplitude of electrotonic potentials, increased by TEA, is slightly further elevated by verapamil or nickle chloride. 4. TEA (10 mM) increases the mechanical response to low and intermediate potassium concentrations but has no effect on maximal contractions to high potassium. The slope of the line relating log potassium concentration to membrane potential is decreased by TEA. 5. TEA (10 mM) shifts the concentration response curve for the contractile effect of noradrenaline to the left and increases the maximum of noradrenaline-induced contractions. In the presence of TEA, noradrenaline reduces the membrane potential to markedly lower values than under control conditions. 6. It is concluded that the rapidly occurring effects of TEA on the vascular smooth muscle cells of the rabbit main pulmonary artery are a decrease in potassium and an increase in calcium conductance. The latter effect may be related to a blockade of potassium channels; however, we cannot rule out the possibility that TEA affects calcium conductance independent of its presumed action on potassium channels. The slowly developing effects of TEA may be ascribed to the formation of gap junctions and/or (less likely) to an intracellular accumulation of TEA.

Segregation of leech neurones by the effect of sparteine on action potential duration

1. Sparteine (SPT) and 3- or 4-aminopyridine, were applied to leech segmental ganglia and the electrophysiological responses of the Retzius (R) and sensory neurones responding to pressure (P), touch (T), and noxious (N) stimuli analysed. 2. SPT 0.05-0.5 mM when presented via the bath to the whole ganglion prolonged the action potentials of these neurones to characteristically different degrees; the cells were clearly segregated in the order R greater than N greater than P greater than T at 0.5 mM, regardless of exposure time. 3. The plateau of the prolonged action potentials in the R and N cells was sustained by either Ca or Sr and was blocked by Mn, in normal or Na-free Ringer. These responses were similar to those seen in the same cell types with TEA under the same condition. 4. The SPT prolongation of action potentials was favoured by alkalinization of the Ringer solution. This implies that the drug acted in its uncharged form. SPT was ineffective when applied by pressure into the somata of these four neurones. This may be because SPT was charged at the intracellular pH or because it acted at some external membrane site. 5. The aminopyridines when applied in the bath had no effect on the repolarization of these four neurones. 6. These results suggest that TEA and SPT probably act on repolarization by similar mechanisms. The parameter of membrane function principally affected is probably a K current which contributes to repolarization to different degrees in the four cells and which may be activated by Ca.

Voltage sensitive calcium entry in frog motoneurones

1. The electrical properties of motoneurone membrane were investigated in the isolated and hemisected spinal cord of frogs, using intracellular recording techniques. 2. TTX (1 x 10(-6) g/ml.) blocked action potentials produced either by intracellular depolarizing current pulses or ventral root stimuli. Voltage--current relations from these cells showed a diminishing slope for depolarizing current pulses of increasing intensity. 3. If TEA (5--10 mM) was added to the media containing TTX, intracellular depolarizing pulses elicited prolonged regenerative depolarizations characterized by a peak of variable amplitude and a repolarizing phase preceded by a prolonged plateau of variable duration. 4. During the plateau of the response, the membrane conductance was increased above its resting value. 5. The response was shortened during repetitive stimulation and could be curtailed by applying a hyperpolarizing pulse during the plateau. 6. The response depended on the presence of external Ca2+ and increased in size and duration with increasing Ca2+ concentration. Sr2+ substituted effectively for Ca2+. Sr2+-dependent responses were considerably longer than the Ca2+-dependent ones. Ca2+ or Sr2+ dependent responses persisted in Na+-free media containing isotonic TEA, and were abolished by addition of Co2+. 7. Ca2+ or Sr2+-dependent regenerative responses were followed by a hyperpolarization which could last several seconds. The current responsible for this after-hyperpolarization was TTX and TEA resistant. 8. It is concluded that the TTX-resistant regenerative response is probably generated in the soma-dendritic membrane, and is due to influx of Ca2+ or Sr2+ through voltage sensitive channels different to those through which Na+ permeates during generation of 'normal' action potentials. In addition it is shown that the hyperpolarization following 'Ca spikes', and which might be due to an increase in K+ conductance can also be triggered by Sr2+.

9-Aminoacridine- and tetraethylammonium-induced reduction of the potassium permeability in pancreatic B-cells. Effects on insulin release and electrical properties

The effects of 9-aminoacridine and tetraethylammonium on insulin release and rubidium efflux from perifused rat islets were investigated and correlated with their effects on the electrical properties of mouse B cells studied with microelectrode techniques. 9-Aminoacridine (0.05--1 mmol/l) and tetraethylammonium (2--40 mmol/l) produced a dose-dependent, reversible potentiation of glucose-stimulated insulin release. This effect was rapid, affected both phases of secretion and was maximum in the presence of 6 mmol/l glucose, but no longer significant at 20 mmol/l glucose. It was unaltered by atropine or propanolol, and abolished by mannoheptulose or omission of extracellular calcium. 9-Aminoacridine, but not tetraethylammonium, also induced insulin release in the absence of glucose stimulation. Neither drug modified glucose metabolism in islet cells and only 9-aminoacridine increased 45Ca2+ uptake. In the presence of 0, 3 or 6 mmol/l glucose, but no longer at 20 mmol/l glucose, 9-aminoacridine and tetraethylammonium reduced the rate of 86Rb+ efflux from the islets. Both drugs also slightly reduced 86Rb+ uptake by islet cells. In the presence of 11 mmol/l glucose, 9-aminoacridine reduced the amplitude and the duration of the polarization phases between the bursts of electrical activity; concomitantly these periods of spike activity were markedly prolonged. At lower glucose concentrations (3 or 7 mmol/l), 9-aminoacridine progressively depolarized B cells and induced electrical activity in otherwise silent cells. Tetraethylammonium also suppressed the repolarization phases between the bursts of spikes in the presence of a stimulating concentration of glucose. At low glucose, tetraethylammonium produced only a limited and not maintained depolarization. These results show that a reduction of the potassium permeability in pancreatic B cells potentiates the insulin-releasing effect of glucose and may even stimulate secretion. They also suggest that the initial depolarizing effect of glucose is due to a reduction of the potassium permeability, whereas the repolarization at the end of each burst of electrical activity is mediated, at least in part, by an increase in the potassium permeability of B cells.

Intracellular citrate or externaly applied tetraethylammonium ions produce calcium-dependent action potentials in an insect motoneurone cell body

1. Electrophysiological observations have been made upon the cell body of an identified motoneurone of the cockroach, Periplaneta americana. Normal responses were compared with those observed after intracellular injection of citrate anions or when the preparation was bathed in solutions containing tetraethylammonium ions (TEA+).2. Normally when depolarized, the motoneurone soma gave a series of damped oscillations; the amplitude of these responses increased with increase in the applied current.3. After citrate ions had been injected into the neurone soma, all-or-none action potentials were evoked by depolarization; such responses appeared about 5-10 min after the onset of citrate injection. Injection of EGTA produced similar effects. Citrate and EGTA probably produce their effect through a reduction in the intracellular free calcium concentration.4. When preparations were bathed in saline solution containing 50 mM-TEA+, soma depolarization produced prolonged all-or-none action potentials (up to approximately 100 msec duration).5. The action potentials produced by citrate injection or externally applied TEA+ appeared to have a similar ionic mechanism; they were not depressed by sodium-free solutions or by tetrodotoxin (4 x 10(-6)M) but were reversibly blocked in saline solution containing 40 mM-manganous chloride.6. The overshoot amplitude of action potentials recorded after injection of citrate anions or in solutions containing TEA+ showed a 22.5 mV change for a ten-fold change in the external calcium concentration.7. Both intracellular citrate and external TEA+ caused a significant increase in the input resistance and membrane time constant of the motoneurone.8. It is concluded that action potentials generated under various experimental conditions in the soma of this insect motoneurone map have differing ionic mechanisms.

Calcium-dependent increase in spike duration during repetitive firing of Aplysia axon in the presence of TEA

Repetitive stimulation was studied in the axon of the giant neuron, R2, of Aplysia in the presence of TEA. In 25 or 50 mM extracellular TEA, a plateau develops on the axon spike during repetitive stimulation at frequencies greater than 3/sec. The plateau in extracellular TEA is inhibited by 30 mM CoCl2 or 1 mM CdCl2, and is enhanced by raising the Ca concentration. Intracellular TEA induces a plateau on the axon spike at frequencies less than 1/30sec. This plateau increases in duration with repetitive stimulation at higher frequencies and is inhibited by 30 mM CoCl2 or 1 mM CdCl2. The increase in spike duration during repetitive firing in the presence of TEA is indicative of an increased entry of Ca during the spike train.

Ionic currents in cultured mouse neuroblastoma cells under voltage-clamp conditions

1. Ionic currents in differentiated cells of mouse neuroblastoma clone N1E-115 have been studied under voltage-clamp conditions. 2. Depolarizing voltage steps from a holding potential of -85 mV to levels more positive than -40 mV produced fast transient inward currents followed by delayed outward currents. 3. The fast inward current is carried by Na+: it is blocked by tetrodotoxin and is absent in Na+-free solutions. Its kinetic behaviour resembles that of the Na+ current in squid giant axon. A mean value of 85 mmho/cm2 was found for the maximum Na+ conductance (GNa).4. The delayed outward current is carried primarily by K+: it is blocked by externally applied tetraethylammonium (TEA, 15 mM) and has a reversal potential (mean -71 mV) close to the theoretical K+ equilibrium potential. Its instantaneous I--V curve is linear. By analogy with the formulation of Hodgkin & Huxley (1952c), the outward current can be described by IK = -GKn2(V--EK) where GK = 12 mmho/mc2. 5. During prolonged depolarizations the delayed outward current declines. This decline, which occurs in two phases, represents a partial inactivation of the K+ conductance. 6. A weak inward current with slow activation and inactivation kinetics appears in Na+-free solution containing 10 mM-Ca2+. It is activated at a membrane potential of -55 mV and reaches its maximum at -20 mV with a time to peak of about 10 msec. This current is tetrodotoxin-resistant, reversibly blocked by Co2+ (5mM) and is suggested to be carried by Ca2+. 7. An increase in the external divalent cation concentration results in a parallel shift of the steady-state I--V curve along the voltage axis in positive direction. The activation of delayed outward currents is suggested not to depend on Ca2+ influx. 8. It is concluded that separate voltage-dependent Na+, K+ and Ca2+ channels exist in the differentiated neuroblastoma membrane with kinetic and pharmacological properties similar to those observed in non-mammalian preparations.

A prolonged, voltage-dependent calcium permeability revealed by tetraethylammonium in the soma and axon of Aplysia giant neuron

The soma but not the axon of the giant neuron, R2, of Aplysia can generate an all-or none Ca spike in Na-free or TTX-containing medium (Junge and Miller, 1974). Extracellular axonal recordings made at several distances from the soma provide evidence that the transition in ability to fire a spike in Na-free medium occurs within the first 250 micrometer of the axon. Application of 25 mM TEA-Br to the bathing medium causes a more than tenfold increase in the duration of the somatic action potential. The duration of the axonal action potential in TEA decreases with distance from the soma. At distances greater than 3 mm from the soma this concentration of TEA causes little or no increase in the duration of the axon spike. The effect of 25 mM TEA on both the soma and proximal axon is blocked reversibly by 30 mM CoCl2 or 1 mM CdCl2. The duration of the somatic action potential in TEA increases with an increase in Ca concentration of the bath. At a constant concentration of Na, the voltage level of the somatic plateau increases with Ca concentration in the manner predicted for a Ca electrode. In the presence of 11 mM Ca2+ the potential of the plateau is relatively insensitive to Na concentration. The TEA plateau in R2 reveals a prolonged voltage-dependent permeability to Ca. The duration of the plateau may indicate the degree of Ca activation during a spike.

Close relation between TEA responses and Ca-dependent membrane phenomena of four identified leech neurones

1. Tetraethylammonium chloride (TEA) was applied to four kinds of identified neurones in leech segmental ganglia, namely, the sensory cells responding to touch (T), pressure (P) and noxious (N) stimuli and the Retzius cell (R).2. TEA prolonged the action potentials of these cells to characteristically different degrees, in the order R > N > P > T, regardless of exposure time. This result was the same whether TEA was presented to the whole ganglion via the bathing fluid or injected iontophoretically into the soma of the cell under study.3. TEA in Na-free solution caused the behaviour of the N cell membrane to be dominated by a Ca-dependent, Mn-blockable event identical in every respect except smaller size to the previously described behaviour of the R cell under the same conditions. The P cell displayed a still smaller event of the same kind, but none was detectable in the T cell.4. In the absence of both TEA and Na, when Ca was the only extra-cellular cation available to carry current, active membrane responses to depolarization were present in the R cell (previous study) and the N cell; such responses were minimal in the P cell and absent from the T cell.5. Differences among the four cells in density of a divalent cation conductance mechanism are the simplest explanation for these observations, though a more complex explanation based on multiple, pharmacologically distinct K conductances is not excluded by our data.