Sodium and calcium components of action potentials in the Aplysia giant neurone

Sense and Insensibility - An Appraisal of the Effects of Clinical Anesthetics on Gastropod and Cephalopod Molluscs as a Step to Improved Welfare of Cephalopods

Recent progress in animal welfare legislation stresses the need to treat cephalopod molluscs, such as Octopus vulgaris, humanely, to have regard for their wellbeing and to reduce their pain and suffering resulting from experimental procedures. Thus, appropriate measures for their sedation and analgesia are being introduced. Clinical anesthetics are renowned for their ability to produce unconsciousness in vertebrate species, but their exact mechanisms of action still elude investigators. In vertebrates it can prove difficult to specify the differences of response of particular neuron types given the multiplicity of neurons in the CNS. However, gastropod molluscs such as Aplysia, Lymnaea, or Helix, with their large uniquely identifiable nerve cells, make studies on the cellular, subcellular, network and behavioral actions of anesthetics much more feasible, particularly as identified cells may also be studied in culture, isolated from the rest of the nervous system. To date, the sorts of study outlined above have never been performed on cephalopods in the same way as on gastropods. However, criteria previously applied to gastropods and vertebrates have proved successful in developing a method for humanely anesthetizing Octopus with clinical doses of isoflurane, i.e., changes in respiratory rate, color pattern and withdrawal responses. However, in the long term, further refinements will be needed, including recordings from the CNS of intact animals in the presence of a variety of different anesthetic agents and their adjuvants. Clues as to their likely responsiveness to other appropriate anesthetic agents and muscle relaxants can be gained from background studies on gastropods such as Lymnaea, given their evolutionary history.

Conditional rhythmicity and synchrony in a bilateral pair of bursting motor neurons in Aplysia

This investigation examined the activity of a bilateral pair of motor neurons (B67) in the feeding system of Aplysia californica. In isolated ganglia, B67 firing exhibited a highly stereotyped bursting pattern that could be attributed to an underlying TTX-resistant driver potential (DP). Under control conditions, this bursting in the two B67 neurons was infrequent, irregular, and asynchronous. However, bath application of the neuromodulator dopamine (DA) increased the duration, frequency, rhythmicity, and synchrony of B67 bursts. In the absence of DA, depolarization of B67 with injected current produced rhythmic bursting. Such depolarization-induced rhythmic burst activity in one B67, however, did not entrain its contralateral counterpart. Moreover, when both B67s were depolarized to potentials that produced rhythmic bursting, their synchrony was significantly lower than that produced by DA. In TTX, dopamine increased the DP duration, enhanced the amplitude of slow signaling between the two B67s, and increased DP synchrony. A potential source of dopaminergic signaling to B67 was identified as B65, an influential interneuron with bilateral buccal projections. Firing B65 produced bursts in the ipsilateral and contralateral B67s. Under conditions that attenuated polysynaptic activity, firing B65 evoked rapid excitatory postsynaptic potentials in B67 that were blocked by sulpiride, an antagonist of synaptic DA receptors in this system. Finally, firing a single B65 was capable of producing a prolonged period of rhythmic synchronous bursting of the paired B67s. It is proposed that modulatory dopaminergic signaling originating from B65 during consummatory behaviors can promote rhythmicity and bilateral synchrony in the paired B67 motor neurons.

Calcium and voltage dependent inactivation of sodium and calcium currents limits calcium influx in Helisoma neurons

The control of free intracellular calcium concentration ([Ca2+]i) is necessary for cell survival because of the ubiquitous and essential role this second messenger plays in regulating numerous intracellular processes. Calcium regulation in neurons is especially vigorous because of the large calcium influx that occurs through voltage-gated channels during membrane depolarization. In this study we examined changes in ionic currents that can limit calcium influx into neurons during electrical activity. We found that the [Ca2+]i in electrically stimulated Helisoma B4 neurons initially increased to a peak and then relaxed to lower concentrations in tandem with a decline in the action potential peak voltage. The decline in [Ca2+]i and the peak action potential voltage in this sodium and calcium driven neuron was found to be a dual manifestation of I(Na) and I(Ca) inactivation. I(Na) and I(Ca) both displayed voltage dependent inactivation. Additionally, I(Na) and I(Ca) progressively inactivated at [Ca2+]i above 200 nM, concentrations readily attained in electrically stimulated B4 neurons. Calcium and voltage dependent I(Na) and I(Ca) inactivation were found to reduce calcium influx during continuous electrical stimulation by decreasing both the magnitude of I(Ca) that could be activated and the percent of the available I(Ca) that would be activated due to the diminished peak action potential voltage. Calculations based on data herein suggest that the voltage and calcium dependent I(Na) and I(Ca) inactivation that occurs during continuous electrical stimulation dramatically reduces calcium influx in this sodium and calcium driven neuron and thus limits the increase in [Ca2+]i.

Activity-dependent accumulation of Ca2+ in axon and dendrites of the leech Leydig neuron

We have investigated Ca2+ changes evoked by single action potentials (APs) in axon and dendrites of leech Leydig neurons. Dendritic Ca2+ transients induced by an AP were twice as large as in the axon, and Ca2+ recovery was significantly faster in the dendrites as compared to the axon. The AP-induced Ca2+ transients were blocked by Co2+ and suppressed in Ca2+-free saline, indicating Ca2+ influx through voltage-activated channels. During a train of APs, Ca2+ accumulated significantly more in the axon than in the dendrites. Suppression of the Ca2+ influx changed the shape of the action potential and increased the firing frequency. The results suggest a functional role of Ca2+ influx and Ca2+ accumulation during electrical activity in different neuronal subcompartments.

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.

Initiation and propagation of calcium-dependent action potentials in a coupled network of olfactory interneurons

Coherent oscillatory electrical activity and apical-basal wave propagation have been described previously in the procerebral (PC) lobe, an olfactory center of the terrestrial slug Limax maximus. In this study, we investigate the physiological basis of oscillatory activity and wave propagation in the PC lobe. Calcium green dextran was locally deposited in the PC lobe; this led to cellular uptake and transport of dye by bursting and nonbursting neurons of the PC lobe. The change of intracellular calcium concentration was measured at several different positions in neurites of individual bursting neurons in the PC lobe with a two-photon laser-scanning microscope. Fluorescence measurements were also made from neurons intracellularly injected with calcium green-1. Two different morphological classes of bursting neurons were found, varicose (VB) and smooth (SB). Our results from concurrent optical and intracellular recordings suggest that Ca2+ is the major carrier for the inward current during action potentials of bursting neurons. Intracellular recordings from bursting neurons with nystatin perforated-patch electrodes made while simultaneously recording the local field potential (LFP) with extracellular electrodes indicate that the burster spikes are precisely phase-locked to the periodic LFP events. By referencing successive calcium measurements to the common LFP signal, we could therefore accurately determine the relative timing of calcium transients at different points along a neurite. Measuring the relation of temporal to spatial differences allowed us to estimate the velocity of action potential propagation, which was 4.3 +/- 0.2 (SE) mm/s in VBs, and 1.3 +/- 0.2 mm/s in SB.

Cloning and tissue distribution of the Aplysia Na+ channel alpha-subunit cDNA

Voltage-gated Na+ channels generate the depolarizing inward current that is critical for the initiation and conduction of action potentials. To study the roles of Na+ channels in neuronal signaling, we have begun the molecular analysis of Na+ channels in Aplysia californica. We have isolated cDNAs that encode a neuronal Na+ channel alpha-subunit, which we have named SCAP1. DNA sequence analysis of the SCAP1 cDNA revealed an open reading frame that predicts a protein of 1,993 amino acids, which is highly similar to other members of the Na+ channel alpha-subunit gene family. RNase protection assays carried out on various Aplysia tissues indicated that SCAP1 is expressed predominantly in the nervous system. All of the nonneuronal tissues tested were negative with the exceptions that low levels of expression were observed in ovotestis and parapodium, probably due to the presence of small numbers of neurons within these tissue preparations. Southern blot hybridization at reduced stringency indicated that the genome of Aplysia contains more than one Na+ channel alpha-subunit gene.

A new family of conotoxins that blocks voltage-gated sodium channels

Conus peptides, including omega-conotoxins and alpha-conotoxins (targeting calcium channels and nicotinic acetylcholine receptors, respectively) have been useful ligands in neuroscience. In this report, we describe a new family of sodium channel ligands, the mu-O-conotoxins. The two peptides characterized, mu-O-conotoxins MrVIA and MrVIB from Conus marmoreus potently block the sodium conductance in Aplysia neurons. This is in marked contrast to standard sodium channel blockers that are relatively ineffective in this system. The sequences of the peptides are as follows. mu-O-conotoxin MrVIA: ACRKKWEYCIVPIIGFIYCCPGLICGPFVCV mu-O-conotoxin MrVIB: ACSKKWEYCIVPILGFVYCCPGLICGPFVCV mu-O-conotoxin MrVIA was chemically synthesized and proved indistinguishable from the natural product. Surprisingly, the mu-O-conotoxins show no sequence similarity to the mu-O-conotoxins. However, ananalysis of cDNA clones encoding the mu-O-conotoxin MrVIB demonstrated striking sequence similarity to omega- and delta-conotoxin precursors. Together, the omega-, delta-, and mu-O-conotoxins define the O-superfamily of Conus peptides. The probable biological role and evolutionary affinities of these peptides are discussed.

Electrophysiological characterization of a novel conotoxin that blocks molluscan sodium channels

A novel peptide toxin, PnIVB, isolated from the venom of Conus pennaceus blocks voltage-gated sodium current in Aplysia neurons. Complete blockade is obtained at a PnIVB concentration of 80 +/- 2.2 nM and 50% blockade at 16 +/- 0.86 nM. The potency of PnIVB in blocking Aplysia sodium current is four orders of magnitude larger than that of tetrodotoxin. The toxin has no paralytic activity when injected into fish. The rapid blockade of sodium current by PnIVB is not associated with a change in the activation or inactivation kinetics of the current, or with the reversal potential. Sodium current blockade is reversible after a 30 min wash with 50 times the bath volume. The novel conotoxin PnIVB can be used as a powerful tool for mollusc neurobiological research and as a molecular probe to explore the structure-function relations of voltage-gated sodium channel subtypes.

Cellular communication in the circadian clock, the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus functions as the circadian clock in the mammalian brain. Communication between the cells of the suprachiasmatic nucleus is likely to be responsible for the generation and accuracy of this biological clock. Communication between many cells of the brain is mediated by action potentials that pass down the axon and cause release of neurotransmitters at the neuronal synaptic junction. Additional mechanisms of cellular communication appear to operate in the suprachiasmatic nucleus. Several lines of evidence point to multiple modes of cellular communication: these include the continuing operation of the clock after Na(+)-mediated action potentials have been blocked, the orchestrated metabolic rhythms of suprachiasmatic nucleus cells prior to synaptogenesis, the entrainment of fetal to maternal rhythms, and the rapid recovery of function after suprachiasmatic nucleus transplants into arrhythmic rodents. Possible alternative means of intercellular communication in the suprachiasmatic nucleus are examined, including calcium spikes in presynaptic dendrites, ephaptic interaction, paracrine communication, glial mediation, and gap junctions. This paper identifies and examines some of the unanswered questions related to intercellular communication of suprachiasmatic nucleus cells.

The Desheathed Periphery of Aplysia Giant Neuron. Fine Structure and Measurement of [Ca2+]o Fluctuations with Calcium-selective Microelectrodes

The visceral ganglion of Aplysia was mechanically desheathed after protease softening of the connective tissue to permit the positioning of ion-selective electrodes in the vicinity of the neuronal membrane. The effects of this treatment on satellite glia and neuronal cytology were observed by electron microscopy. The intracellular alterations were not suggestive of serious membrane damage but the cohesion between glial and neuronal membranes was affected-the glial processes appeared to retract from the trophospongium and in some cases the neuronal membrane was completely naked. The external calcium activity [Ca2+]o at the surface of identified giant neuron, R2, was measured using double-barrelled calcium-selective microelectrodes. A decrease of approximately 1 mM in [Ca2+]o could be recorded only during trains of action potentials induced by intracellular depolarizing current injection, and when the electrode was pushed firmly against the neuron surface. A recovery from this decrease in [Ca2+]o could sometimes, but not always, be observed during the phase of induced neuronal activity.

Parallel processing of short-term memory for sensitization in Aplysia

How is the short-term memory for a single form of learning distributed among the various elements of a neuronal circuit? To answer this question, we examined the short-term memory for sensitization, using the siphon component of the defensive gill- and siphon-withdrawal reflex. We found that the memory for short-term sensitization is represented by at least four sites of circuit modification, each involving a different type of plasticity. These include (1) presynaptic facilitation of the sensory neuron connections onto both interneurons and motorneurons; (2) presynaptic inhibition at the connections of the L30 inhibitory neurons onto the excitatory interneuron L29; (3) posttetanic potentiation of the excitatory connections made by L29 onto a specific subclass of siphon motorneurons, the LFS cells; and (4) an increase in the tonic firing rate of the LFS siphon motor neurons, resulting in neuromuscular facilitation. Each of the heterosynaptic changes seems to involve a common modulatory transmitter and to utilize a common second messenger system. Moreover, each of these sites seems capable of encoding a different component of the short-term memory. Facilitation of the connections of sensory neurons should contribute to the increase in amplitude of the response; the disinhibition of the L29 interneurons and the posttetanic potentiation at L29 synapses should contribute to an increase in the duration of the response; and the increase in tonic firing of the LFS subclass of siphon motor neurons seems capable of contributing both to an increase in response amplitude and to changes in response topography.

Action potentials, macroscopic and single channel currents recorded from growth cones of Aplysia neurones in culture

Action potentials, macroscopic ionic currents and single channel currents were recorded from growth cones of Aplysia right upper quadrant (r.u.q.) cells in culture, using the patch-clamp technique. Recordings were obtained from both intact growth cones and from growth cones that had been mechanically isolated from the rest of the neurone. In current-clamp mode, greater than half of the isolated growth cones display an all-or-none action potential when depolarized above 0 mV with outward current pulses. The remaining growth cones display only a graded depolarization that is unaffected by tetrodotoxin (TTX). In whole-cell voltage clamp almost all isolated growth cones display a rapidly activating and inactivating inward current followed by a delayed outward current in response to depolarizations positive to -20 mV. The rapid inward current reverses direction at around +70 to +80 mV and is completely suppressed by 100 microM-TTX, which suggests that this current is carried by the fast Hodgkin-Huxley sodium current channels. The delayed outward current appears to result from the activation of both the delayed rectifier potassium current, IK, and the calcium-activated potassium current, IC. The growth cones do not display any prominent early transient outward current, IA. The sodium current, INA, was studied in isolation by substituting caesium for potassium ions in the pipette solution. INa is half-inactivated at a holding potential of -36 mV, reaches half-maximal activation with a depolarization to 0 mV, and has a mean peak current density of 13 microA/cm2. The time course of inactivation is well described by a single exponential (tau = 3 ms at 0 mV). In cell-attached patches, a rapidly activating and inactivating inward current channel was recorded with an average unit conductance of 6.9 pS. The activation and inactivation parameters of the ensemble averaged current closely match the measured values from the macroscopic sodium current. At very positive potentials we recorded a voltage-dependent outward current channel with a conductance of around 35 pS. No significant inward calcium current was observed in whole-cell measurements and few single calcium channel currents were measured in cell-attached patches, suggesting a sparse distribution of calcium channels in the r.u.q. growth cones.

Decreases of action potential amplitudes, in sodium-free and calcium-free conditions, of identifiable giant neurons of an African giant snail (Achatina fulica Férussac)--I. The right parietal ganglion

Decreases of the action potential amplitude in sodium- and calcium-free states were observed with respect to the four giant neurons, PON (periodically oscillating neuron), Tan (tonically autoactive neuron), RAPN (right anterior pallial neuron) and d-RPLN (dorsal-right parietal large neuron), identified in the right parietal ganglion of the suboesophageal ganglia of an African giant snail (Achatina fulica Férussac). The decrease of the PON action potential amplitude, caused in the sodium-free state, was observed to be 25.4 +/- 2.1% (23.0 +/- 2.0 mV), expressed by M +/- SE, while that of the calcium-free state was 35.0 +/- 2.1% (30.9 +/- 1.7 mV). Then, the two ionic dependencies of the PON action potential were estimated to be about 40-50% on sodium and 50-60% on calcium. The decrease of the TAN action potential in the sodium-free state, was observed to be 20.7 +/- 1.2% (18.8 +/- 1.3 mV), whereas that of the calcium-free state was 42.2 +/- 2.7% (39.0 +/- 2.2 mV), indicating that the two ionic dependencies were 30-40% on sodium and 60-70% on calcium. The decrease of the RAPN action potential in the sodium-free state, was 45.8 +/- 3.7% (40.3 +/- 3.1 mV), whereas that of the calcium-free state was 21.7 +/- 2.5% (17.8 +/- 2.0 mV), indicating that the two ionic dependencies were about 70% on sodium and about 30% on calcium. The decrease of the d-RPLN action potential in the sodium-free state was found to be 17.6 +/- 2.4% (15.2 +/- 1.8 mV), whereas that of the calcium-free state was 23.1 +/- 1.4% (20.8 +/- 1.4 mV), indicating that the two ionic dependencies were 40-50% on sodium and 50-60% on calcium. The action potential amplitudes of all the neurons tested were decreased in both sodium-free and calcium-free states. However, their ionic dependencies were estimated to vary from 70% on sodium (30% on calcium) to 30% on the sodium (70% on the calcium), according to the neurons tested.

Differentiation and the cell membrane excitability of teratocarcinoma OTT6050 in culture

The electrophysiological properties of differentiated cells which were derived from teratocarcinoma OTT6050 in culture showed two different types of spike generation: (1) In the differentiated cells treated by retinoic acid, the action potential of cells involved Ca2+; (2) in the differentiated cells treated by both retinoic acid and nerve growth factor (NGF), the action potential of cells involved Na+ and CA2+. The results indicate that the effects of drugs appear to be important factors in the induction as well as in the regulation of cellular functions in the differentiated cells.

Comparative electrical properties of identified neurons in Elysia chlorotica before and after low salinity acclimation

Identified neurons in the abdominal ganglion of Elysia chlorotica adapted to 50% seawater (SW) had significantly different electrical properties from the same cells in animals adapted to 100% SW. Resting potential, action potential (AP) overshoot, (AP) duration, threshold and after potential were all different following salinity acclimation. The resting potential of these cells behaves as an ideal potassium electrode above 10 mM [K+]. The action potential has both sodium and calcium components to the rising phase.

A novel membrane sodium current induced by injection of cyclic nucleotides into gastropod neurones

Injection of cyclic AMP (cAMP) or cyclic GMP into identifiable neurones from several different gastropod species immediately depolarized the cell membranes in a dose-dependent manner. Doses were monitored photometrically and evidence is presented for depolarizing effects following nucleotide injections of as little as 30-35 mumol. The depolarizing effect was reversible and was demonstrated under voltage clamp to be primarily the result of a nucleotide-induced, transient increase in a membrane Na current, INa (cAMP). The charge-carrying species was identified by using ion-substituted salines, reversal potential in low-Na saline, and intracellular ion-sensitive electrode measurements. The current was resistant to tetrodotoxin, ouabain and amiloride. Substituting Trisma, tetramethylammonium or bis-tris propane for Na prevented the induced current, whereas Li substitution did not. Duration of the induced current was greatly prolonged in neurones bathed in the phosphodiesterase inhibitor isobutylmethylxanthine, or following injection of any of several cAMP analogues, indicating that the reversible nature of the current stems primarily from in situ hydrolysis of the injected dose and not current inactivation. Amplitude of the induced current either remained constant or decreased over the voltage range where it could be easily measured, i.e. -30 greater than Vm greater than -100 mV, reflecting a voltage as well as a chemical sensitivity of INa (cAMP).

The ionic mechanisms associated with the excitatory response of kainate, L-glutamate, quisqualate, ibotenate, AMPA and methyltetrahydrofolate on leech Retzius cells

Intracellular recordings were made from Retzius cells from segmental ganglia of the leech, Hirudo medicinalis. The ionic mechanisms of the following compounds were examined: L-glutamate, ibotenate, quisqualate, AMPA, kainate, methyltetrahydrofolate and carbachol. All these compounds depolarise and excite Retzius cells. In sodium-free Ringer, the responses to L-glutamate, kainate, ibotenate and AMPA were greatly reduced, the response to quisqualate was reduced, the response to methyltetrahydrofolate was normal while the response to carbachol was abolished. In sodium-free high calcium Ringer the responses to L-glutamate, ibotenate and carbachol were absent, the responses to quisqualate and AMPA greatly reduced, the responses to methyltetrahydrofolate and kainate were normal. The methyltetrahydrofolate and kainate responses in sodium-free high calcium Ringer were greatly reduced on addition of cobalt. All the responses are associated with an increase in conductance, the increase being the largest in the case of kainate. It is concluded that the response to L-glutamate, ibotenate and carbachol are dependent on sodium, the responses to quisqualate and AMPA are mainly sodium dependent, possibly with a small calcium component. The kainate response in normal Ringer is largely sodium dependent but in sodium-free Ringer calcium can completely substitute for sodium. The methyltetrahydrofolate response appears to be sodium independent but at least partly calcium dependent. These studies provide further evidence that L-glutamate and ibotenate act on a common receptor on leech Retzius cells while kainate acts on a separate receptor which can activate a calcium ionophore. It is probable that methyltetrahydrofolate acts on a different ionophore system to kainate. N-Methyl-D-aspartate has no agonist activity on any of these receptors.

Combined spikes induced by Ca and Na currents in cultured cerebellar neurons from the chick embryo

Evoked spikes in explanted cerebellar neurons cultured for 17-25 days, presumably including Purkinje cells, were not completely blocked by 10(-5) g/ml TTX. The TTX-resistant components of the spike were suppressed by Co2+ or Mn2+. It is suggested that combined Ca and Na components are involved in spike generation mechanisms in cultured neurons from the chick cerebellum and that they may be related to the maturation process of excitable membranes of cell soma.

Side effects of phosphorylated acetylcholinesterase reactivators on neuronal membrane and synaptic transmission

The side effects of four phosphorylated cholinesterase reactivators (oximes): contrathion, TMB4, toxogonine and 1574 SEBC on membrane properties and synaptic transmission of Aplysia central neurons were investigated. Applied in the bath at 10(-3) mol X 1(-1) to 10(-2) mol X 1(-1) concentrations, all these oximes had a depressive action on cholinergic transmission exerting a curare-like effect on the postsynaptic receptors. In addition, Toxogonin and TMB4 affected the presynaptic voltage dependent sodium conductance. None of these oximes interfered with the voltage dependent potassium or calcium conductances. The oximes had a transient facilitatory action on amplitude of the response to ionophoretically applied acetylcholine (ACh) on H-type ACh receptors, but not on cells with D-type ACh receptors. The K+ dependent response to ACh injection on pleural ganglion cells was selectively blocked by 5 X 10(-6) mol X 1(-1) contrathion. All oximes at 10(-2) mol X 1(-1) to 10(-3) mol X 1(-1) similarly depressed serotonin receptors in buccal ganglion cells. All the effects of oximes were reversible by washing. It was concluded that oximes can act as 1) inhibitors of Na+ conductance, 2) antagonists for various synaptic receptors, 3) reversible inhibitors of acetylcholinesterase.

Effects of anions on calcium component in sensory nerve terminal of frog muscle spindles

An increase in the Ca2+ component following individual sodium spike in spontaneous discharges was observed in the isolated sensory terminal of the frog muscle spindle perfused with isotonic sodium solution with a group of anions, of which the size in the aqueous solution ranged from 0.74 to 1.32 times that of hydrated sodium ion. The effects of these anions was counteracted with divalent cations. It is hypothesized that anions similar in size to hydrated sodium ions may form an anion-cation complex at the interaction site of the Na+ carrier, whereby the Na+ may enter the Ca2+ channel. This may be inactivated by the divalent cations.

Effect of pentobarbital on Na and Ca action potentials in an invertebrate neuron

It has been suggested that Ca currents may be more sensitive to barbiturate blockade than Na currents. This hypothesis has been tested by comparing the effect of pentobarbital (PB) on the maximum rate of rise (V max) of Ca-depended and Na-dependent action potentials in cell R2 of the Aplysia abdominal ganglion. In Ca-free medium ([Na]o = 494 mM), V max of Na-spikes ranged from 50 to 100 V/s, while in Na-free medium ([Ca]o = 30 mM), V max of Ca-spikes ranged from 7 to 20 V/s. Under these conditions, Ca-spikes were 3-4 times more sensitive to barbiturate blockade than were Na-spikes. However, it was found that the sensitivity of Na- and Ca-spikes to PB dependent on V max of the spike prior to drug addition. V max was manipulated by altering the driving force on the current carrying cation; this was accomplished by changing the concentration of the cation in the bathing medium. Thus, Ca-spikes, elicited in media containing 10 mM Ca, were more sensitive to PB than were Ca-spikes elicited in 30 mM Ca. Likewise, the sensitivity of Na-spikes to PB could be altered by changing the external Na concentration, and consequently, V max. When the external Na and Ca concentrations were adjusted so that V max of Na- and Ca-spikes were similar, prior to drug addition, the PB dose-response curves for Na- and Ca-spikes overlapped. The mechanism accounting for the dependence of PB sensitivity on V max prior to drug addition remains unclear. However, the observation that PB dose-response curves for Na- and Ca-spikes are similar when V max of the spikes is similar, suggests that Na and Ca currents may be equally sensitive to PB in this particular cell.

Na+- and Ca2+-dependent components in action potentials of the ovulation hormone producing caudo-dorsal cells in Lymnaea stagnalis (Gastropoda)

Action potentials in the afterdischarge of the ovulation hormone producing caudo-dorsal cells (CDC) of Lymnaea stagnalis are strikingly different from electrically evoked spikes in the silent resting and inhibited states of these cells. Spikes evoked in the silent states consist of one fast peak (80-100 mV; 10-15 ms). The overshoot in Na+- and Ca2+-dependent. Spikes are blocked in Na+-free saline and by TTX. Repolarization is retarded by TEA. Co2+ increases the overshoot. Active state action potentials (60-80 mV) last up to 125 ms, due to activation of a slow component following the TTX-sensitive spike. The slow component is Na+- and Ca2+-dependent. In normal saline it is blocked by Co2+ and La3+. In Ca2+-free saline the remaining part of the slow component is blocked by La3+ only. The slow component is voltage-dependent in a graded fashion. Activation is bound to the active state in which the CDC are depolarized by 20 mV. TEA and Ca2+-free saline greatly increase spike duration in the active state. This suggests that, in addition to the classical TEA-sensitive channel, a Ca2+-dependent K+ channel is involved in repolarization of active state action potentials. The underlying membrane properties and the functional significance are discussed in relation to the pacemaking mechanism of the CDC.

Age-diminished motor neuronal function of central neuron L7 in Aplysia

The efficacy of central neuron L7 to elicit gill pinnule contractions was tested in mature and old Aplysia. The difference in age between groups was no less than 70 days and as much as 150 days. Spike in L7 were necessary to elicit pinnule contractions in both age groups. Spike rates of 8 spikes per second and higher elicited pinnule contractions that were significantly smaller in old than in mature animals. Synaptic activity in the pinnule muscles innervated by L7 was recorded extracellularly during contractions, and it was significantly less facilitated by spike trains in old compared to that in mature Aplysia. This suggests that the reduction of facilitated synaptic transmission between L7 and pinnule muscles results in diminished motor neuron function.

Calcium-independent release of GABA from isolated horizontal cells of the toad retina

1. When toad retinae were incubated first with veratrine, then with antibodies that reacted with the outer segments of photoreceptors, and finally with complement, horizontal cells survived and most other neurones died. This preparation of 'isolated' horizontal cells accumulated radioactive GABA from the incubation medium. The subsequent release of radioactive GABA could then be measured. 2. The efflux of GABA was increased by exposure to an elevated potassium concentration or added glutamate. Both procedures are known to depolarize horizontal cells. 3. GABA in the external medium also increased the efflux of GABA. 4. The increase in GABA efflux produced by an elevated potassium concentration was unaffected with calcium in the external medium was replaced with cobalt and when sodium was replaced with choline or lithium. 5. The increase in GABA efflux produced by glutamate was unaffected when calcium was replaced with cobalt and when sodium was replaced with lithium, but was inhibited when sodium was replaced with choline. 6. The increase in GABA efflux produced by external GABA was unaffected when calcium was replaced with cobalt but required sodium. Neither choline nor lithium would substitute for sodium. 7. An increase in GABA efflux was accompanied by an increase in sodium efflux. 8. After a high concentration of GABA (2-20 mM) had produced a maximal increase in GABA efflux, the addition of glutamate produced no further effect. Conversely, after a high concentration of glutamate (2-20 mM) had produced a maximal increase in efflux, the addition of external GABA produced only a small further increase. These and the preceding results could occur if GABA release were mediated by a carrier system which could be activated by either depolarization or homoexchange.

Two types of spike generation of human Auerbach's plexus cells in culture

Cultured cells from Auerbach's plexus derived from human small intestine were used in this study. The cultured cells were classified into two types according to their morphological characteristics. Type I cells have small soma, and few processes. Type 2 cells have large soma, many processes, and form complex networks. Intracellularly applied currents cause action potentials, followed by hyperpolarizing afterpotentials. Delayed rectification was seen. The electrical properties of these cells in culture showed different types of spike generation. Action potentials of type I cells involve Ca ions and Na ions, and those of type 2 cells, Ca ions.

Modulation of synaptic output by the transient outward potassium current in aplysia

The mechanisms involved in synaptic output modulation by presynaptic membrane potential was studied in identified Aplysia synapses, where a presynaptic hyperpolarization reduces the postsynaptic potential amplitude. The experiment reported here reveals that a presynaptic conditioning hyperpolarization induces a decreased presynaptic spike amplitude and that the reduction is due to a superimposition of a transient outward potassium current on the inward current. This is demonstrated by the external application of 4-aminopyridine which suppresses the early outward current and thus suppresses the modulation both of the presynaptic spike amplitude and the synaptic output.

Effects of furosemide on neural mechanisms in Aplysia

The effects of furosemide on action potentials and responses to several neurotransmitters have been studied in the neurons of Aplysia. Furosemide (10(-7) and 10(-3) M) does not visibly affect the normal action potential in R15 neurons. However, when TTX (30 microM) is used to block the sodium component in R15, the remaining spike (presumably the calcium component) is increased in amplitude in the presence of furosemide. Furosemide also alters transmitter-induced conductances. Furosemide greatly reduces the amplitude and shifts, in a depolarizing direction, the reversal potential of chloride-dependent responses to gamma-aminobutyric acid (GABA) and acetylcholine (ACh). This suggests that furosemide both blocks the chloride channel and inhibits a chloride pump. ACh-induced sodium responses were also reduced by furosemide but to a lesser extent than chloride responses. The potassium response to ACh and a voltage-dependent calcium response to serotonin were not altered. These results indicate that furosemide could alter synaptic responses both presynaptically by enhancement of calcium flux during the action potential and postsynaptically by blockade of chloride and sodium conductances.

Batrachotoxin blocks saltatory organelle movement in electrically excitable neuroblastoma cells

Batrachotoxin has been reported to inhibit fast axonal transport. We have examined the effect of batrachotoxin on saltatory organelle movements in N115 neuroblastoma cells (a model of fast axonal transport) using time-lapse video intensification microscopy. Batrachotoxin (0.1-1.0 microM) inhibits saltatory organelle movement. Contrary to previously published hypotheses, this inhibition of intra-axonal movement depends upon the action of batrachotoxin on action potential Na+ channels. Evidence for this conclusion is: (1) the range of concentrations of batrachotoxin which inhibit saltatory organelle movement is consistent with the dose-response curve for the activation of action potential Na+ channels by batrachotoxin in N18 neuroblastoma cells; (2) tetrodotoxin, which blocks action potential Na+ channels, prevents the inhibition of organelle movements by batrachotoxin; (3) batrachotoxin has no effect on saltatory movement in cells, including some neuroblastoma cell lines, which lack action potential Na+ channels; and (4) in Na+-free or low Na+ media, batrachotoxin does not block organelle movement. We suggest that changes in internal ion concentrations which follow the influx of Na+ are responsible for the inhibition of fast axonal transport by batrachotoxin.

Action of colchicine on membrane currents and synaptic transmission in Aplysia ganglion cells

The action of colchicine, a drug known to disrupt microtubules, on synaptic transmission and voltage-dependent phenomena was studied. Colchicine depressed transmission in both cholinergic and noncholinergic Aplysia ganglionic synapses. In some synapses, this effect was partly due to the curare like properties of the alkaloid. CA2+ currents, analyzed by voltage clamp techniques, were rapidly depressed by intracellular injection of colchicine and more slowly depressed by external application. Injected colchicine acted at much lower concentrations than required extracellularly. The implication of the reduced calcium influx in synaptic transmission is discussed. Colchicine caused a shift in the reversal potential of acetylcholine-activated chloride channels in a direction consistent with an increased intracellular chloride activity. It was concluded that the wide range of actions of colchicine on membrane properties should be taken into account when this drug is used in biological research.

Calcium action potentials in single freshly isolated smooth muscle cells

The ionic basis of the action potential was investigated using intracellular microelectrodes in single smooth muscle cells freshly isolated from the stomach of the toad Bufo marinus. When [Ca2+]0 was elevated (> 8mM), action potentials were readily elicited, which had similar characteristics to those found in many tissue preparations of visceral smooth muscle. There was a decrease in membrane resistance at the peak of the action potential and during the undershoot. The following evidence indicated that the inward current is carried by Ca2+: 1) Raising [Ca2+]0 from 15 to 49.6 mM in the presence of 18.2 mM tetraethylammonium chloride (TEA) increased the maximum rate of rise and the overshoot amplitude, the latter by 15 mV, i.e., 29.5 mV/10-fold change in [Ca2+]0. Changing [Na2+]0 from 11.8 to 81.8 mM had no significant effect on the maximum rate of rise or the overshoot. 2) The action potentials were blocked by 8 mM Mn2+ ([Ca2+]0 = 14.6 mM) but not by 14.3 microM tetrodotoxin (TTX) ([Na2+]0 = 100 mM). 3) Action potentials could be elicited when [Ba2+]0 or [Sr2+]0 were present in high concentrations ([Ca2+]0 less than or equal to 31 microM,[Na2+]0 = 11.8 mM). Both the maximum rate of rise and overshoot amplitude of the action potential increased as the membrane potential became more negative, suggesting increased activation of the inward current. Both TEA and Ba2+ prolonged the action potential, suggesting that a K+ current is responsible for repolarization. Action potentials could also be elicited on anode break at elevated [K+]0 (91 mM).

Neuromuscular transmission without sodium activation of the presynaptic nerve terminal in the lobster

1. We studed Na-independent synaptic transmission in the inhibitory synapse of the walking leg of the spiny lobster (Palinurus japonicus). 2. After loading the preparation with tetrodotoxin (TTX), brief depolarizing current injected in the inhibitory axon produced a small action potential, which propagated to the nerve terminal and gave rise to inhibitory post-synaptic potentials (i.p.s.p.) 3. The presynaptic action potential, in the presence of TTX, failed to propagate after removing Na+ in the solution. The TTX-resistant action potential was decreased, but not blocked by 30 mM-CoCl2. 4. When 4-aminopyridine (4-AP) was added to low Na+ or Na-free solution containing TTX synaptic transmission was restored. When the duration of the current pulse was increased, graded i.p.s.p. were evoked. 5. In high Ca2+ solutions containing K blockers, action potentials with prolonged duration were evoked. 6. The action potential of the presynaptic axon of the lobster neuromuscular junction depends on both Na+ and Ca2+.

Divalent ion currents and the delayed potassium conductance in an Aplysia neurone

1. In Na- and Ca-free external solutions, Sr or Ba (but not Mg) could act as carriers of inward current during action potentials in the neurone, R15 of the Aplysia abdominal ganglion. These action potentials exhibited a prolonged plateau phase, the duration of which was dependent on the concentration and species of divalent cation and activity of the neurone. 2. Depolarization of the soma membrane in Na-free Ba solution generated a prolonged, 'late' inward current the amplitude of which was dependent on the external Ba concentration. The Ba current was insensitive to tetrodotoxin but could be blocked by Mn2+ and Co2+ ions. 3. The peak current-voltage relation and threshold for activation of the late inward current was shifted to more negative potentials on replacement of Ca with Ba. The zero-current (reversal) potentials for both Sr and Ba were more negative than for Ca, indicating that the 'Ca' channel is less permeable to Sr2+ or Ba2+ ions than to Ca2+ ions. 4. Inactivation of the 'Ca' channel is slower in Ba than in Ca solution. The time course of Ba currents during a maintained depolarization of 2 sec could be reasonably described by the expression, I'Ba(t) = I'Ba (infinity) [1-exp(-t/tau M)]2exp(-t/tau H). 5. Time constants for activation (tau M) and inactivation (tau H) were voltage-dependent. In the range -10 to +30 mV, tau M varied from 15 to 5 msec and tau H from 2.0 to 0.5 sec (12 degrees C). Steady-state Ba conductance (corrected for inactivation) was voltage-dependent, increasing sigmoidally with depolarization to a maximum of approximately 12 microS at potentials beyond +15 mV. 6. Steady-state inactivation of Ba conductance (hBa(infinity)) varied with holding potential (VH). Conditioning holding potentials more negative than the resting potential (-40 to -50 mV) produced depression of Ba currents. Complete inactivation of Ba currents occurred at holding potentials more positive than 0 mV or with repetitive activation at frequencies greater than 1 Hz. 7. The divalent ions, Ba2+ and Sr2+, reversible depressed the total delayed K+ current at a rate dependent on the frequency of activation. Ba and Sr shifted the delayed K+ current-voltage curve to more positive voltages and depressed the delayed outward current at all membrane potentials. 8. Comparison of the effect of Ba on delayed K+ currents with those obtained in the presence of Mn2+ ions indicated that Ba2+ ions depress both the voltage-dependent and Ca-dependent components of the delayed K+ current. However, the mechanism by which Ba acts to inhibit the two components of the delayed K+ current appears to be different.

Quantal release of acetylcholine examined by current fluctuation analysis at an identified neuro-neuronal synapse of Aplysia

Quantal events have been analyzed at a neuro-neuronal synapse of Aplysia where the nature of the transmitter is established and both presynaptic and postsynaptic neurons are identified and can be voltage-clamped. Prolonged depolarizations were applied to the presynaptic neuron, which gave rise in the postsynaptic cell to a current response characterized by current fluctuations or noise. Acetylcholine was also applied ionophoretically on the same postsynaptic cell. Amplitude and time course of miniature currents and acetylcholine-activated chloride channels of the same cell were examined by using a current fluctuation analysis. It was estimated that one presynaptic spike releases about 180 quanta, each opening 500 chloride channels.