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

Norepinephrine inhibits calcium-dependent potentials in rat sympathetic neurons

Norepinephrine reversibly antagonizes three calcium-dependent potentials recorded from rat postganglionic neurons. Norepinephrine inhibits the development of a shoulder on the aciton potential, the magnitude of the hyperpolarizing afterpotential, and the rate of rise and amplitude of the calcium spike. The action of norepinephrine is antagonized by the alpha-adrenergic antagonist phentolamine, but not by MJ 1999, a beta-adrenergic antagonist. These results suggest that activation of an alpha-adrenergic receptor may antagonize a voltage-sensitive calcium current.

Transmitter release: ruthenium red used to demonstrate a possible role of sialic acid containing substrates

1. The possible function of sialic acid-containing substrates (SACS) in synaptic terminals of Aplysia was studied by intracellular injection of ruthenium red and of neuraminidase. 2. Ruthenium red, a dye known to have sialic acid as a molecular target, blocked transmission irreversibly in both cholinergic (buccal ganglion) and non-cholinergic (cerebral ganglion) synapses. 3. An intracellular site of action is likely because much less ruthenium red was necessary to block transmission when it was injected intracellularly than when it was presented by bath perfusion. 4. Ca2+ spikes recorded in the presence of tetrodotoxin or in Na+-free solution were not modified by ruthenium red or neuraminidase injections or perfusions. It is therefore improbable that these substances blocked transmission by blocking voltage-dependent Ca2+ influx. 5. Strong electrotonic depolarization of a pre-synaptic interneurone in the presence of 10(-4) M-tetrodotoxin caused a sustained post-synaptic response, which was abolished by ruthenium red. This result eliminates axonal conduction block as the principal mechanism of ruthenium red action. 6. Post-synaptic responses to ionophoretically applied acetylcholine (ACh) were not modified by bath perfusion of 2 x 10(-2) M-ruthenium red. 7. Biochemical analysis of pools of [3H]ACh was performed after injection of a precursor, [3H]acetate, into an identified interneurone. Ruthenium red appeared to increase significantly the 'free' (cytoplasmic) ACh pool without any change of 'bound' (vesicular) [3H]ACh-pool. 8. A model is proposed in which SACS act as intracellular Ca2+ receptors involved in transmitter release.

Calcium-dependent potentials in the mammalian sympathetic neurone

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

Evidence for an intracellular calcium store releasable by surface stimuli ifibroblasts (L cells)

A spontaneously occurring or electrically elicited hyperpolarizing activation (HA) in L cells was previously shown to be due to a specific increase in the membrane K+ permeability (Nelson et at. 1972. J. Gen. Physiol. 60:58--71). Intracellular injection of Ca++ elicits an identical hyperpolarizing response which suggests that the increased K+ permeability associated with the HA is mediated by an increase in cytoplasmic Ca++. In zero-Ca, EGTA-containing saline the proportion of cells in which HA's can be evoked decreases, but the amplitude of those HA's that are produced is comparable to that of HA's in normal Ca saline. Co++ does block the HA but only after a period of 2 h or longer; D-600 does not affect the HA. The observations, with others, suggest that the primary source of the Ca mediating the HA response is intracellular. In L cells the endoplasmic reticulum forms morphologically specialized appositions with the surface membrane which resemble structures at the triads of muscle that are thought to mediate coupling between surface membrane electrical activity and contraction via Ca release from the sarcoplasmic reticulum. The similar structures in L cells may mediate coupling between surface membrane electrical, mechanical, or chemical stimuli and the HA response via release of Ca from the endoplasmic reticulum. Surface-coupled release of Ca from intracellular stores might also regulate a number of other intracellular functions in nonmuscle cells.

Ionic currents in response to membrane depolarization in an Aplysia neurone

1. Action potentials recorded in the soma of R15 neurones in the abdominal ganglia of Aplysia juliana were not suppressed by selective inhibition of either Na or Ca conductance alone. It was necessary to block both conductances to suppress action potentials. 2. Membrane currents generated by step depolarizations of the soma consisted of early transient and delayed steady-state currents. The early transient current could have one or two components depending on the activating depolarization. 3. The early more rapid component had a reversal potential at +54 mV and the reversal potential changed with extracellular Na concentration in accord with the Nernst equation. It was blocked by substitution of impermeant cations for Na, by TTX and by internal injections of Zn. It was concluded that this component was normally a Na current. 4. The later slower component of the transient current had a reversal potential at about +65 mV and the reversal potential changed with extracellular Ca concentration is accord with the Nernst equation. It was blocked by substitution of Mg for Ca or addition of Mn, Co, Ni or verapamil to the extracellular solution. It was concluded that this component was normally a Ca current. 5. Na and Ca currents were generated at different threshold potentials, Na currents first appearing at about -20 mV and Ca currents at -5 to 0 mV. 6. The time-to-peak of both Na and Ca currents was affected by the holding potential, by the amplitude of the activating depolarization, by temperature and by divalent ion concentration. 7. The peak Na and Ca conductances both increased sigmoidally with increasing depolarization, the maximum Na conductance of 10--15 microS being approximately twice the maximum Ca conductance. Peak conductances for Na and Ca reached half-maximum at -8 and +3 mV, respectively. 8. The amplitude of the delayed steady-state current could be varied by changing the extracellular K+ ion concentration or by adding tetraethylammonium to the extracellular solution. The reversal potential for 'tail currents' was -67 mV and shifted 18 mV when the extracellular K concentration was doubled. It was concluded that the delayed steady-state current was K current. 9. With prolonged depolarizations, K current decayed with a time constant of the order of 1 sec. Peak K conductance increased with increasing depolarization with the half-maximum occurring at a potential more positive than +20 mV. The maximum rate of fractional activation of K conductance was independent of the amplitude of the clamp step.

Current-voltage relationships of repetitively firing neurons

The current-voltage curves of repetitively firing neurons show non-linearities in the subthreshold region. Microsurgically isolated molluscan neuron somata were studied under voltage clamp using ramp voltage command signals. During the depolarizing 1/2 cycle a region of negative slope conductance was observed. Ion substitution experiments suggest that this results from non-inactivating or slowly inactivating Na+ and Ca2+ currents. The hyperpolarizing 1/2 cycle reveals a hysteresis effect which results at least in part from a Ca2+ activated 5+ current. Similar characteristics have been described in bursting neurons. Their occurrence in the non-bursting neurons studied here shows that they are not unique to this class of neurons and suggests that their primary contribution is to create electrical instability necessary for repetitive firing.

Tetrodotoxin sensitivity and Ca component of action potentials of mouse dorsal root ganglion cells cultured in vitro

In the mouse dorsal root ganglia cultured in vitro, neurons were classified into 3 groups according to the responses of their action potentials to tetrodotoxin (TTX) and removal of Na ions from bathing medium: (1) the neurons whose action potentials were not affected by TTX by TTX (10(-6) - 10 (-5)g/ml) and which generated Ca-dependent regenerative responses under Na-free condition, (2) the neurons whose spike potentials were resistant to TTX but failed to survive in Na-free saline and (3) the neurons whose action potentials were suppressed by TTX(10(-8)g/ml) as well as Na removal. The mean duration of spike and after-hyperpolarization was longest in the first group of the neurons and shortest in the third, probably reflecting the difference in the contribution of Ca currents to action potentials. The unresponsiveness of the neurons to TTX was shown to be due to the insensitivity of Na as well as Ca components of action potentials to the toxin. It was discussed that the occurrence of TTX-resistant action potentials to the toxin. It was discussed that the occurrence of TTX-resistant action might be related to the neuronal development.

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.

Propagating calcium spikes in an axon of Aplysia

1. Intracellular recordings were obtained from the axon of the giant neurone R2 of Aplysia in order to study the ionic dependency of action potentials. 2. The overshoot potential of the axon spike increases with Na concentration in the manner predicted for a Na electrode. The maximum rate of rise (Vm) is linearly related to Na concentration. The overshoot potential is insensitive to Ca concentration at Na concentrations as low as 250 mM. 3. Tetrodotoxin (TTX) or replacement of Na with Tris abolishes action potentials in the axon but not soma of R2. Addition of 4-aminopyridine to a Na-free solution permits axon spikes to be generated. These action potentials are blocked by 30 mM-Co2+, but not by TTX. The overshoot potential and Vm of these action potentials increase monotonically with Ca concentration. 4. Axonal action potentials can be generated when an equimolar concentration of Sr is substituted for all of the Ca and Mg in Na-free medium. These action potentials are abolished by 30 mM-Ca2+ or mM-Co2+, and increase with Sr concentration. 5. TTX-resistant Ba spikes can similarly be elicited in R2 axon. These action potentials are reduced by Ca, Co, or Cd, and enhanced by raising the Ba concentration. 6. The Vm of Na spikes in the absence of Ca is greater in the axon than in the soma of R2, whereas the Vm of divalent spikes is greater in the soma. 7. During repetitive stimulation the axon spikes incrase in duration. This broadening is inhibited by replacing Ca in the bath with Mn or by the addition of 30 mM-CoCl2, and is enhanced by raising the Ca concentration by 30 mM. 8. The action potention of R2 axon has a mixed Na/Ca dependency. The density of Ca current may be greater in the some than in the axon of this cell.

Presynaptic modulation of voltage-dependent Ca2+ current: mechanism for behavioral sensitization in Aplysia californica

Behavioral sensitization of the gill-withdrawal reflex of Aplysia is the result of a prolonged increase in transmitter release from the presynaptic terminals of sensory neurons. Earlier work suggested that this presynaptic facilitation might be mediated by a serotonin-sensitive adenylate cyclase in the sensory neuron terminals. Here we present evidence that presynaptic facilitation results from a cyclic AMP-dependent increase in the calcium current that underlies action potentials in the sensory neurons. The action potentials of sensory neuron cell bodies have, in addition to a sodium current, a calcium current that is enhanced by blocking the opposing potassium current with tetraethylammonium. Under these conditions, the action potentials show a slowly repolarizing plateau that follows the Nernst potential for a calcium electrode and serves as a sensitive assay for changes in calcium current. Stimulation of the pathway that mediates sensitization, incubation with serotonin or phosphodiesterase inhibitors, or intracellular injection of cyclic AMP produces an increase in the calcium plateau in the presence of tetraethylammonium. In addition, both before and after sensitizing stimulation, the duration of the plateau potential parallels transmitter release as measured by the amplitude of monosynaptic excitatory postsynaptic potentials evoked in the motor neurons by intracellular stimulation of single sensory neurons. These results are consistent with the idea that presynaptic facilitation is caused by a cyclic AMP-mediated increase in a voltage-sensitive calcium current in sensory neuron presynaptic terminals. This synaptic action is novel in that it can produce little or no change in the resting potential, is of long duration, and exerts its influence directly on a conductance triggered by the action potential, rather than on non-voltage-sensitive conductances, as is typical of conventional synaptic actions.

Changes in the intracellular concentration of free calcium ions in a pace-maker neurone, measured with the metallochromic indicator dye arsenazo III

1. The bursting pace-maker R-15 cell of Aplysia was injected with the Ca2+ sensitive dye arsenazo III. Changes in absorbance were measured with a differential spectrophotometer to monitor changes in free intracellular Ca2+, [Ca-a], during activity. 2. Dye absorbance increased during each pace-maker-induced burst of action potentials and decreased during the hyperpolarizing phase of the pace-maker cycle. 3. The increase in dye absorbance was, at least in part, dependent upon action potential discharge and was greater when action potential duration was prolonged by treatment with tetraethylammonium chloride. 4. Changes in dye absorbance occurred under voltage clamp conditions when the membrance was depolarized 5-15 mV from a holding potential near the resting potential and were larger with greater step depolarizations. 5. These changes were completely blocked by the addition of 3mM-La3+ to normal ASW. 6. The ratio of the absorbance change between two pairs of wave-lengths during the pace-maker cycle was compared with the ratio observed following injection of Ca2+, Mg2+ and H+ ions. The ratio for the pace-maker cycle was well matched by that for Ca2+ injection, but not by that for injection of Mg2+ or H+. 7. Intracellular Ca2+ injections which increased [Ca]1 to the same amount as occurred during the pace-maker cycle also produced an outward current of sufficient magnitude to account for the post-burst hyperpolarization. 8. Depolarization of the cell membrane by extrinsic current during the burst increased and prolonged the change in dye absorbance as well as the post-burst hyperpolarization. 9 It is suggested that Ca2+ enters during the pace-maker cycle, thereby increasing [Ca]i, and that this increase is sufficient to activate an outward current carried by K+ ions which causes or contributes to the post-burst hyperpolarization.

Developmental changes in the inward current of the action potential of Rohon-Beard neurones

1. Rohon-Beard cells in the spinal cord of Xenopus tadpoles have been studied in animals from early neural tube to free-swimming larval stages. The onset and further development of electrical excitability of these neurones has been investigated in different ionic environments, to determine the ionic species carrying the inward current of the action potential.2. The cells appear inexcitable at early stages (Nieuwkoop & Faber stages 18-20) and do not give action potentials to depolarizing current pulses.3. The action potential is first recorded at stage 20. (A) The inward current is carried by Ca(2+) at stages 20-25, since it is blocked by mm quantitites of La(3+), Co(2+) or Mn(2+) and is unaffected by removal of Na(+) or the addition of tetrodotoxin (TTX). (B) The action potential is an elevated plateau of long duration (mean 190 msec at stages 20-22). The duration decreases exponentially with repetitive stimulation. (C) The specific Ca(2+) conductance (g(Ca)) at the onset of the plateau of the action potential is 2.6 x 10(-4) mho/cm(2). Calculations show that a single action potential raises [Ca(2+)](1) by more than 100-fold.4. At later times (stages 25-40), the inward current of the action potential is carried by both Na(+) and Ca(2+): the action potential has two components, an initial spike which is blocked by removal of Na(+) or addition of TTX, followed by a plateau which is blocked by La(3+), Co(2+) or Mn(2+).5. Finally (stages 40-51), the inward current is primarily carried by Na(+), since the action potential is blocked only by removal of Na(+) or addition of TTX, and the overshoot agrees with the prediction of the Nernst equation for a Na-selective membrane. When the outward current channel is blocked and cells exposed to Na-free solutions, 67% of cells at the latest stages studied were incapable of producing action potentials in which the inward current is carried by divalent cations.6. The duration of the action potential decreases from a maximum of about 1000 msec to about 1 msec during development. The maximum input resistance (R(in)) decreases from ca. 1000 to 100 MOmega.7. The calcium action potential may play a role in the development of excitability and the growth of the neurones.

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.

Separation of sodium and calcium currents in the somatic membrane of mollusc neurones

1. Characteristics of the transmembrane ionic currents under controlled changes in ionic composition of extra- and intracellular medium were studied in isolated neurones from the ganglia of molluscs, Helix pomatia, Limnea stagnalis and Planorbis corneus. The neurones were investigated by a new technique which allows for dialysis of their interior and for clamping of the potential at the surface membrane without using micro-electrodes.2. Replacement of K ions by Tris inside the neurones eliminated the outward K current so that the actual time course of the inward current could be measured. The latter was separated into two additive components, one of which was carried by Na ions and the other one by Ca ions.3. Both inward currents were unaltered by tetrodotoxin (TTX); however, Ca current could be separately blocked by externally applied Cd ions (K(d) = 7.2 x 10(-5)M) and by the use of fluoride as an intracellular anion.4. No reversal of Na inward current could be achieved in neurones dialysed with Na-free solution, indicating the absence of outward current carrying ions through the corresponding channels. With 5 mM-Na inside the cell, the equilibrium potential was close to the value predicted by the Nernst equilibrium.5. A non-specific outward current could be detected in K-free cells at membrane potentials exceeding 20-40 mV. Its time course was proportional to 1 - exp (-t/tau(ns)). Cd ions depressed this current. The presence of the non-specific outward current made an exact measurement of the equilibrium potential for the Ca inward current impossible.6. The kinetics of Na inward currents could be described by m(3)h and those of the Ca current by m(2)h law. The corresponding values for V(m) = 0 are: tau(m)(Na) = 1.1 +/- 0.5 msec, tau(m)(Ca) = 2.4 +/- 1.0 msec, tau(h)(Na) = 7.9 +/- 2.0 msec. The inactivation of Ca current included two first-order kinetic processes with tau(h1) = 50 +/- 10 msec and tau(h) = 320 +/- 30 msec.7. The data presented are considered to be a proof of the existence of separate systems of Na and Ca ion-conducting channels in the nerve cell membrane.

Calcium-dependent depression of a late outward current in snail neurons

Neuron cell bodies of Helix pomatia were voltage-clamped with a 300-millisecond depolarizing test pulse (pulse II) delivered I second after a depolarizing conditioning pulse (pulse I). The outward current, measured 200 milliseconds after the onset of pulse. II, exhibited a strong depression that was dependent on the presence of pulse. I. The maximum depression of the pulse II outward current occurred when pulse I voltages lay in the range over which calcium influx is inferred to be greatest; depression of the pulse II current subsided as pulse I potentials approached the putative calcium equilibrium potential. In the presence of extracellular [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) or D600, the intensity of the pulse II current became largely independent of pulse I, approaching the values of maximal depression seen in normal Ringer solution. On the other hand, lowering the intracellular pH with extracellular carbon dioxide-carbonate buffer had no measurable effect on the outward currents. Other experiments showed that it is primarily the calcium-dependent, outward-current hump of the N-shaped late current-voltage curve that is depressed by presentation of the conditioning pulse. It was concluded that distinct from an early potassium-activating role, calcium entering during a depolarization leads, during a subsequent depolarization, to a depression of the calcium-activated potassium system that persists for many seconds.

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

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

The action potential of chick dorsal root ganglion neurones maintained in cell culture

1. The directly evoked action potential of dissociated, embryonic, chick, dorsal root ganglion (DRG) neurones maintained in cell culture is prolonged compared to spinal cord cell spikes and the re-polarization phase is marked by a plateau. 2. Evidence was obtained that both Ca2+ and Na+ carry inward current across the active soma membrane. Ca2+ because: overshooting spikes persist in tetrodotoxin (TTX) or Na+-free media; in the presence of TTX (or absence of Na+) spike size varies directly with extracellular Ca2+ and spikes are eliminated by Co2+. Na+ because: spikes persist in the presence of Co2+ or Ca2+-free media; in the presence of Co2+ (or absence of Ca2+) spike varies directly with extracellular Na+ and spikes are blocked by TTX. 3. On the other hand, Ca2+ plays less if any role in action potentials conducted along sensory nerve cell processes. Conducted spikes could not be evoked in TTX containing or Na+-free media. 4. A long-lasting depolarization follows the action potential in some neurones. This depolarization is associated with an increase in membrane conductance and appears to drive the membrane potential to ca. -30mV. It persists when conducted impulses are blocked so it is probably not a recurrent synaptic potential. 5. It is suggested that combined Ca2+-Na+ spikes observed in isolated sensory neurones in vitro reflect the action potential of adult sensory cells but the possibility that they represent an early stage in development is also discussed.

Development of spike potentials in skeletal muscle cells differentiated in vitro from chick embryo

The development of spike potential mechanisms during cell differentiation was studied in chick myotubes formed in vitro from trypsin-dissociated myoblasts. The spike potential and its rate of rise were measured in myotubes from 4-14 day old cultures. A depolarizing current pulse was delivered to evoke the spike potential after the steady membrane potential had been adjusted to a standard level of -80 mV in all cases. This gives the greatest maximum rate of rise of the spike potential and eliminates variation due to differences in the resting membrane potential of the myotubes. The size and maximum rate of rise of the spike potential increased significantly during the period examined. The spike potential was blocked by tetrodotoxin in almost all myotubes. These results suggest that during differentiation myotubes develop the ability to generate a spike potential due to an inward current carried by sodium ions.

The abdominal ganglion of Aplysia brasiliana: a comparative morphological and electrophysiological study, with notes on A. dactylomela

The ultrastructure and electrophysiological properties of neurons in the abdominal (visceral) ganglion of the marine opisthobranch gastropod Aplysia brasiliana have been investigated to determine whether this preparation compares favorably with the well studied A. californica for neurobiological research. In general, the topography, morphology and physiological characteristics, including synaptic connections, of neurons in this ganglion are quite similar to those of A. californica. There is close correspondence between the two animals in terms of each of the identified cells or neuronal clusters in the ganglion, including the presence of the cell L10 (interneuron I) in A. brasiliana which makes synaptic connections comparable with those in A. californica. New follower cells of this interneuron have been found in A. brasiliana. This species offers some advantages in that the connective tissue surrounding the ganglion is thinner and more transparent, making cell identification and penetration easier. A. brasiliana appears to exhibit the behaviors of A. californica that have been used in previous functional analyses of neural circuits. In addition, this species swims and exhibits a "burrowing" activity less commonly seen in A. californica. The rich repertoire of behaviors and accessibility of large identifiable and functionally interconnected neurons makes this species of Aplysia an excellent model preparation for future neurobiological studies. Similar, less thorough, investigations of the abdominal ganglion of A. dactylomela indicate that this species is also very similar to A. californica in terms of the identified cells in the abdominal ganglion.

Spike initiation by transmembrane current: a white-noise analysis

1. Those features of a transmembrane current correlated with spike initiation were examined in Aplysia neurones using a Gaussian white-noise stimulus. This stimulus has the advantages that it presents numerous wave forms in random order without prejudgement as to their efficacies, and that it allows straightforward statistical calculations. 2. Stimulation with a repeating segment of Gaussian white-noise current revealed remarkable invariance in the firing times of the tested neurones and indicated a high degree of reliability of their response. 3. Frequencies (less than 5 Hz) involved in spike triggering propagated faithfully for up to several millimetres, justifying intrasomatic current injection to examine spike initiation at the trigger locus. 4. Examination of current wave forms preceding spikes indicated that a wide variety could be effective. Hence, a statistical analysis was performed, including computation of probability densities, averages, standard deviations and correlation coefficients of pairs of current values. Each statistic was displayed as a function of time before the spike. 5. The average current trajectory preceding a spike was multiphasic and depended on the presence and polarity of a d.c. bias. An early relatively small inward- or outward-going phase was followed by a large outward phase before the spike. The early phase tended to oppose the polarity of the d.c. bias. 6. The late outward phase of the average current trajectory reached a maximum 40--75 msec before triggering the action potential (AP) and returned to near zero values at the moment of triggering. The fact that the current peak occurs in advance of the AP may be partially explained by a phase delay between the transmembrane current and potential. The failure of the average current trajectory to return to control values immediately following the peak argues for a positive role of the declining phase in spike triggering. 7. Probability densities preceding spikes were Gaussian, indicating that the average was also the most probable value. Although the densities were broad, confirming that spikes were preceded by a wide variety of current wave forms, their standard deviations were reduced significantly with respect to controls, suggesting preferred status of the average current trajectory in spike triggering. 8. The matrix of correlation coefficients between current pairs suggested that spikes tended to be preceded by wave forms that in part kept close to the average current trajectory and in part preserved its shape. 9. The average first and second derivatives of spike-evoking epochs revealed that current slope and acceleration, respectively, were most crucial in the last 200 msec before spike triggering, and that these dynamic stimulus components were more important for a cell maintained under a depolarizing, rather than a hyperpolarizing bias. 10...

Gating currents associated with sodium and calcium currents in an Aplysia neuron

In a voltage-clamped aplysia neuron (R15), depolarization beyond -30 millivolts produces an inward sodium current. Depolarization beyond -10 millivolts produces an additional inward calcium current with slower kinetics than the sodium current. When these ionic currents have been suppressed and capacitive currents subtracted out, a small outward displacement current can be seen with depolarizations beyond -30 millivolts. An additional, slower displacement current is seen with depolarizations beyond -10 millivolts. The currents have an exponential decay with an increase in rate per 10 degrees C increase in temperature of about 3 and are thought to be sodium and calcium gating currents.

Photoreceptor-bipolar cell transmission in the perfused retina eyecup of the mudpuppy

The hypothesis that a synaptic transmitter is released by photoreceptors in the dark is supported by experiments in which cobalt was used as a synaptic blocking agent, while intracellular recordings of receptors and neurons that are directly postsynaptic to receptors were maintained. In the dark the depolarizing bipolars are hyperpolarized, whereas the hyperpolarizing bipolars are depolarized.

A voltage-sensitive persistent calcium conductance in neuronal somata of Helix

1. An intracellular voltage clamp in conjunction with a patch pipette utilizing feed-back to monitor local current from the soma membrane were used to analyse transient and stationary currents in bursting pacemaker neurones in Helix pomatia and H. levantina. 2. A weak, net inward current flows during small (less than or equal 20 mV) depolarizations. This current exhibits slow activation kinetics, persistence during prolonged depolarization, and slow turning off at end of depolarization. Consequently, the steady-state current-voltage curve exhibits a region of negative resistance from about -55 to -35 mV. 3. The slow inward current and the negative resistance characteristic are rapidly and completely abolished by substitution of Co2+ or La3+ for Ca2+ and are partially blocked by the Ca-blocking drug D-600. Substitution of Tris or glucose for Na+ significantly reduces the inward current only after 15-20 min exposure, recovery being equally slow. 4. The inward current and the negative resistance characteristic of the I-V curve are greatly enhanced by Ba2+ substitution for Ca2+. This is ascribed in part to Ba2+ carrying current through the slow inward current channels and in part to a suppression of the late K+ current by Ba2+. 5. The inward current is also present in many non-bursting neurones but fails to appear as a net inward current due to short circuiting by a leakage current or by the delayed potassium current. In these cells the slow inward current contributes to inward going rectification. Replacement of Ca2+ with Ba2+ enhances the current so as to produce a net inward current during small depolarizations in these neurones. 6. It is concluded that the slow inward current is carried primarily by Ca2+ in the soma membrane of bursting pace-maker neurones and a number of non-bursting cells examined in the parietal ganglion of Helix. 7. The sensitivity to small depolarizations and persistence during prolonged depolarization suggests two roles for the Ca system in the generation of slow pace-maker oscillations. In this model the Ca system contributes to the slow depolarization which constitutes the onset of the pace-maker wave, and also contributes to the increment in [Ca] in which activates the Ca-sensitive K+ conductance responsible for repolarization. The inhibition of spontaneous bursting by Ca-blocking agents supports this model.