Sodium- and calcium-dependent spike potentials in the secretory neuron soma of the X-organ of the crayfish

Histamine operates Cl–-gated channels in crayfish neurosecretory cells

We describe a histamine-activated Cl– conductance in the X-organ neurons from crayfish Cherax quadricarinatus, which has comparable properties to the homomultimeric histamine-gated ion channels described in Drosophila. Topical application of histamine inhibited spontaneous neuronal firing in the X-organ sinus gland tract, concomitant with an increase in the membrane conductance. In X-organ neurons in culture and under voltage-clamp conditions, histamine evoked outward currents at –40 mV that reversed at the Cl– equilibrium potential. Histamine sensitivity in these neurons had a half-maximal response(EC50)=3.3±1 μmol l–1, with a Hill number of 2.6±0.4. The histamine-evoked current was blocked by tiotidine, cimetidine, ranitidine and 256±11 and 483±11 μmol l–1, respectively) and d-tubocurarine(IC50=21±2 μmol l–1), but was insensitive to picrotoxin, bicuculline and strychnine. Neither GABA nor glutamate was capable of desensitizing the histamine response, indicating that histamine activates a particular Cl– conductance. The presence of immunoreactive neurons to histamine in the medulla terminalis with axonal projections to the neuropile suggests a possible histaminergic modulation of the X-organ sinus gland system.

Glutamate and GABA activate different receptors and Cl(-) conductances in crab peptide-secretory neurons

Responses to rapid application of glutamic acid (Glu) and gamma-aminobutyric acid (GABA), 0.01-3 mM, were recorded by whole-cell patch clamp of cultured crab (Cardisoma carnifex) X-organ neurons. Responses peaked within 200 ms. Both Glu and GABA currents had reversal potentials that followed the Nernst Cl(-) potential when [Cl(-)](i) was varied. A Boltzmann fit to the normalized, averaged dose-response curve for Glu indicated an EC(50) of 0.15 mM and a Hill coefficient of 1.05. Rapid (t(1/2) approximately 1 s) desensitization occurred during Glu but not GABA application that required >2 min for recovery. Desensitization was unaffected by concanavalin A or cyclothiazide. N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, quisqualate, and kainate (to 1 mM) were ineffective, nor were Glu responses influenced by glycine (1 microM) or Mg(2+) (0-26 mM). Glu effects were imitated by ibotenic acid (0.1 mM). The following support the conclusion that Glu and GABA act on different receptors: 1) responses sum; 2) desensitization to Glu or ibotenic acid did not diminish GABA responses; 3) the Cl(-)-channel blockers picrotoxin and niflumic acid (0.5 mM) inhibited Glu responses by approximately 90 and 80% but GABA responses by approximately 50 and 20%; and 4) polyvinylpyrrolydone-25 (2 mM in normal crab saline) eliminated Glu responses but left GABA responses unaltered. Thus crab secretory neurons have separate receptors responsive to Glu and to GABA, both probably ionotropic, and mediating Cl(-) conductance increases. In its responses and pharmacology, this crustacean Glu receptor resembles Cl(-)-permeable Glu receptors previously described in invertebrates and differs from cation-permeable Glu receptors of vertebrates and invertebrates.

P-type Ca2+ current in crayfish peptidergic neurones

Inward Ca2+ current through voltage-gated Ca2+ channels was recorded from freshly dissociated crayfish X-organ (XO) neurones using the whole-cell voltage-clamp technique. Changing the holding potential from −50 to −90 mV had little effect on the characteristics of the current-voltage relationship: neither the time course nor the amplitude of the Ca2+ current was affected. Inactivation of the Ca2+ current was observed over a small voltage range, between −35 and −10 mV, with half-inactivation at −20 mV. The activation of the Ca2+ current was modelled using Hodgkin-Huxley kinetics. The time constant of activation, τ m, was 568+/−66 micros at −20 mV and decreased gradually to 171+/−23 micros at 40 mV (means +/− s.e.m., N=5). The steady-state activation, m(infinity), was fitted with a Boltzmann function, with a half-activation voltage of −7.45 mV and an apparent threshold at −40 mV. The instantaneous current-voltage relationship was adjusted using the Goldman-Hodgkin-Katz constant-field equation, giving a permeation of 4.95×10(−5)cm s-1. The inactivation of the Ca2+ current in XO neurones was dependent on previous entry of Ca2+. Using a double-pulse protocol, the inactivation was fitted to a U-shaped curve with a maximal inactivation of 35 % at 30 mV. The time course of the recovery from inactivation was fitted with an exponential function. The time constants were 17+/−2.6 ms for a prepulse of 10 ms and 31+/−3.2 ms for a prepulse of 20 ms. The permeability sequence of the Ca2+ channels was as follows: Ba2+>Sr2+~Ca2+>>Mg2+. Other divalent cations blocked the Ca2+ current, and their effects were voltage-dependent; the potency of blockage was Cd2+~Zn2+>>Co2+~Ni2+. The peptide ω -agatoxin-IVA, a selective toxin for P-type Ca2+ channels, blocked 85 % of the Ca2+ current in XO neurones at 200 nmol l-1, but the current was insensitive to dihydropyridines, phenylalkylamines, ω -conotoxin-GVIA and ω -conotoxin-MVIIC, which are blockers of L-, N- and Q-type Ca2+ channels, respectively. From the voltage- and Ca2+-dependent kinetics, the higher permeability to Ba2+ than to Ca2+ and the higher sensitivity of the current to Cd2+ than to Ni2+, we conclude that the Ca2+ current in XO neurones is generated by high-voltage-activated (HVA) channels. Furthermore, its blockage by ω -agatoxin-IVA suggests that it is mainly generated through P-type Ca2+ channels.

Regulation of crustacean neurosecretory cell activity

1. The X organ-sinus gland system is a conglomerate of 150-200 neurosecretory cells in the eyestalk of crustaceans. It is the source of a host of peptide neurohormones which partake in the control of a wide range of physiological functions. Distinct families of X organ peptides have been chemically characterized: (a) two chromatophorotropic hormones of small sizes, one of 8 residues and the other of 15-20 residues; and (b) three metabotropic hormones of high molecular weight (70-80 residues), related to the control of blood sugar levels, molting, and gonad activity. Some of these hormones have been identified only in crustaceans; others are common to various arthropod groups. A number of peptides orginally described in other zoological groups are also present in the X organ-sinus gland system; such is the case for members of the FMRF-amide family, enkephalins, and other peptides. 2. Cells specifically containing each hormone have been located in the X organ and some information is available on the cellular and molecular substrate of the biosynthesis, transport, storage, and release of various hormones. The electrical activity of X organ neurons has been recorded at the cell soma, arborizations, axons, and neurosecretory terminals. Conspicuous regional differences have been defined for the various patterns of activity, as well as the distribution of their underlying ion currents. 3. The release of hormones and the electrical activity of X organ neurons are regulated by environmental and endogenous influences, such as light and darkness, stress, and circadian rhythms. These influences appear to be mediated by a host of neurotransmitters/modulators, most noticeably, gamma-aminobutyric acid, 5-hydroxytryptamine and other amines, and enkephalins. Each of these mediators acts upon a definite ionic substrate(s) and exerts specific regulatory effects on X organ cell activity. A given neuron may be under the control of more than one neurotransmitter, and a transmitter may mediate different and even opposite influences on different neurons.

Excitatory action of gamma-aminobutyric acid (GABA) on crustacean neurosecretory cells

1. Intracellular and voltage-clamp recordings were obtained from a selected population of neurosecretory (ns) cells in the X organ of the crayfish isolated eyestalk. Pulses of gamma-aminobutyric acid (GABA) elicited depolarizing responses and bursts of action potentials in a dose-dependent manner. These effects were blocked by picrotoxin (50 microM) but not by bicuculline. Picrotoxin also suppressed spontaneous synaptic activity. 2. The responses to GABA were abolished by severing the neurite of X organ cells, at about 150 microns from the cell body. Responses were larger when the application was made at the neuropil level. 3. Topical application of Cd2+ (2 mM), while suppressing synaptic activity, was incapable of affecting the responses to GABA. 4. Under whole-cell voltage-clamp, GABA elicited an inward current with a reversal potential dependent on the chloride equilibrium potential. The GABA effect was accompanied by an input resistance reduction up to 33% at a -50 mV holding potential. No effect of GABA was detected on potassium, calcium, and sodium currents present in X organ cells. 5. The effect of GABA on steady-state currents was dependent on the intracellular calcium concentration. At 10(-6) M [Ca2+]i, GABA (50 microM) increased the membrane conductance more than threefold and shifted the zero-current potential from -25 to -10 mV. At 10(-9) M [Ca2+]i, GABA induced only a 1.3-fold increase in membrane conductance, without shifting the zero-current potential. 6. These results support the notion that in the population of X organ cells sampled in this study, GABA acts as an excitatory neurotransmitter, opening chloride channels.

Electrical activity of the sinus gland of the terrestrial isopod, Oniscus asellus: characteristics of identified potentials recorded extracellularly from neurosecretory terminals

Spontaneously occurring neurosecretory action potentials recorded extracellularly from the sinus gland (SG) of the terrestrial isopod. Oniscus asellus, are of 5 types (A through E) identified by their amplitudes and patterns of discharge. Type A have the largest (200-450 microV) and type E the smallest (25-50 microV) amplitude. Types A, B and C originate from the bulb of the SG, and discharge at high frequencies (30-60 Hz) in coordinated bursts ranging from seconds to several minutes in duration. Coordination of their discharges suggests a mechanism for synchronizing bursting activity among different cell types. Types D and E originate from the lateral extension of the SG, and discharge at low frequencies (0.5-1.0 Hz) for prolonged periods (5-10 min). Their activity is not synchronized with discharges of other potentials. Following transection of the brain through the lateral part of the central protocerebral neuropile, A, B and C potentials are eliminated whereas D and E potentials remain active. This result suggests A, B and C potentials arise from neurosecretory cells (NSCs) whose cell bodies are located in the medial protocerebrum, and D and E potentials arise from NSCs identified in the optic lobe. Alterations in the appearance of action potentials following exposure to salines deficient in Na+ or Ca2+, or containing tetrodotoxin or cobalt, reveal that A and B potentials are primarily Ca2+ dependent whereas C potentials are both Ca2+ and Na+ dependent.

Heterogeneity of neurons in the crustacean X-organ as revealed by intracellular recording and injection of horseradish peroxidase

The intracellular injection technique of HRP with simultaneous recordings of intracellular potential was applied to the crab X-organ-sinus gland peptidergic neurosecretory neurons. At least two classes of neurons were discriminated from the usual type of neurosecretory neurons morphologically as well as electrophysiologically. Possible roles of those neurons were suggested as the modulation and coordination of activities of the neurosecretory system.

Electrophysiology of identified neurosecretory and non-neurosecretory cells in the cockroach pars intercerebralis

Two cell types can be distinguished with intracellular recording from the pars intercerebralis of the American cockroach (Periplaneta americana). The first type, which corresponds morphologically to the medial neurosecretory cell, always had spontaneously occurring, overshooting action potentials. These action potentials are probably endogenously produced. Tetrodotoxin experiments revealed that sodium is the dominant ion of the action potential. The action potentials are followed by a relatively long after-hyperpolarization. The input resistance of these cells ranged from 120 to 390 M omega. A mathematical model, based on cellular morphology and response to current pulses, revealed a membrane time constant of about 100 msec and an axonal:somatic conductance ratio of approximately 13. Area-specific membrane resistance was estimated at 33 k omega cm2. These cells also often had reversible and spontaneous inhibitory postsynaptic potentials. The second cell type, which is non-neurosecretory, never produced spontaneous action potentials and rarely had synaptic potentials. Action potentials could be evoked by current injection into the cell body or by extracellular stimulation of their axons in the posteroventral portion of the the protocerebrum. These action potentials also depend on sodium ions. Their input resistance ranged from 16 to 35 M omega. They had a membrane time constant of approximately 15 msec and an axonal:somatic conductance ratio of about 9. Their area specific membrane resistance was estimated at 14 k omega cm2.

Coupling of electrical activity from contralateral sinus glands

Bursts of electrical activity recorded extracellularly from the sinus gland (SG) of the isopod, Oniscus asellus, occur synchronously in right and left SGs. Synchronization results from the electrical activity of two physiologically identifiable neurosecretory cell (NSC) types in one SG being coupled to the electrical activity of their respective contralateral counterparts. Furthermore, the coupling mechanism which serves to coordinate hormone release from contralateral SGs appears to differ for each of the two NSC types.

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.

Dye coupling and gap junctions between crustacean neurosecretory cells

Neurosecretory cells in the X organ-sinus gland system of the crayfish were impaled and Lucifer Yellow was intracellularly iontophoresed. In some neurons the injected dye was transferred to neighboring neurons. The interneuronal dye transfer was between adjacent somata. Coupling was also observed between neurons and smaller cells, possibly glia. Gap junctions were identified by freeze-fracture in neuron somata and glial cells in the X organ and also in neurosecretory axons in the sinus gland.

Retinal illumination produces synaptic inhibition of a neurosecretory organ in the crayfish, Pacifastacus leniusculus (Dana)

We have identified a cluster of neurosecretory cells in the crayfish eyestalk that possess dendrites in the second optic neuropil (Medulla) and project axons to the first optic neuropil (Lamina). Illumination of the ipsilateral retina produces a synaptic inhibition of these cells that is mimicked by iontophoresis of gamma-aminobutyric acid within the medullary neuropil. The neurosecretory nature of the cells, the efferent projection of their axons, and the strong inhibition of their spiking activity upon retinal illumination suggest that they may be involved in the feedback control of dark adaptation and/or circadian changes in visual sensitivity.

Light input to crustacean neurosecretory cells

Electrical activity was recorded intracellularly from neurosecretory cells in the crayfish eyestalk identified by lucifer yellow injection. The activity is most commonly enhanced by illumination of retinal fields. Increments in spontaneous activity as well as bursts in otherwise silent cells were the most common type of response. Occasionally light-induced inhibitory responses were recorded. At neuropil level, light pulses result in EPSPs with amplitudes dependent on intensity of light and the previous adaptation to darkness.

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.

The ionic dependence of black widow spider venom action at the stretch receptor neuron and neuromuscular junction of crustaceans

The effects of black widow spider venom (BWSV) on the crayfish stretch receptor and the lobster neuromuscular junction were examined. In crayfish stretch receptor neurons, BWSV caused a slight hyperpolarization followed by a large depolarization. The venom-induced depolarization of the stretch receptor was caused by an increase in membrane conductance to Na+ and Ca2+. Black widow spider venom also caused an increase in the frequency of miniature inhibitory postsynaptic potentials recorded in the stretch receptor. The ability of BWSV to increase the frequency of miniature excitatory postsynaptic potentials (MEPSPs) at the lobster neuromuscular junction was dependent on the divalent cation composition of the bathing medium. Ringer solutions containing Ca2+ supported the greatest venom-induced increase in MEPSP frequency, Mg2+ and Mn2+ supported a moderate increase in MEPSP frequency, while Co2+ and Zn2+ blocked this venom effect entirely. Black widow spider venom did not block axonal conduction in lobster walking leg axons or in the axon of the crayfish stretch receptor. The results suggest that in crustaceans, BWSV interacts specifically with membrane of the soma-dendritic region of the stretch receptor and with nerve terminal membrane, causing an increase in Na+ and Ca2+ conductance.

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.

Soma spike of neuroendocrine bag cells of Aplysia californica

Soma action potentials of the neuroendocrine bag cells of Aplysia californica were studied with intracellular recording and current injection. Spikes in artificial sea water (ASW) were either graded with increasing depolarizing current pulses, or had a well-defined threshold. The latter spikes typically had faster rise times with larger overshoots and hyperpolarizing afterpotentials. Repetitive stimulation led to spike potentiation (SP), manifested as an increase in overshoot amplitude and duration of successive spikes in a train. SP was usually detectable at 0.5 Hz, and maximal between 0.8 and 4 Hz. Concomitant accommodation occurred rapidly at greater than or equal to 5 Hz. The increase in spike duration during SP resulted from a progressive enhancement of an inflection on the repolarizing phase. The inflection was dependent on membrane potential; small depolarizations (5-10 mV) enhanced it; hyperpolarization (less than 35 mV) reduced it. Solutions with O--Na+ (Tris-substituted) or O--Ca2+ (1 mM EGTA) revealed mixed Na+/Ca2+ spikes with variable degrees of Na+ versus Ca2+ dominance. Cd2+, Co2+, and Mn2+ reversibly abolished the inflection on the repolarizing phase, indicating that it is Ca2+ mediated; the spike was reduced irreversibly at higher concentrations. SP was generally reduced only if the spike was severely attenuated. It is proposed that SP results primarily from a voltage- and time-dependent potassium inactivation which then unmasks a calcium current. SP may play a role in augmenting the release of egg-laying hormone.

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.

Optical recording of calcium action potentials from growth cones of cultured neurons with a laser microbeam

Simultaneous recordings of calcium action potentials directly from growth cones and from somata of neuroblastoma cells indicated that they could be generated in the neurites at or near growth cones. Growth cone responses were measured with a fluorescent voltage-sensitive dye and a 5-milliwatt helium-neon laser microbeam as a monitoring light source.

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+.

Two reciprocating current components underlying slow oscillations in Aplysia bursting neurons

The mechanisms of the slow oscillatory potential in burst firing neurons in the abdominal ganglion of Aplysia californica (L3-L6 and R15) were studied using voltage clamp methods, including a novel tract and hold technique. The steady-state negative resistance characteristic (NRC) of these neurons is attributed to the activation of a moderately fast, persistent, inward current over a range of membrane potential below spike threshold. This inward current is quite sensitive to changes in external sodium concentration (Na)0 and insensitive to potassium (K)0. By contrast, the portion of the I-V curve below the NRC range is insensitive to (Na)0, but highly sensitive to (K)0. The results of 'track and store' voltage clamping show that there are actually two reciprocating currents whose combined action produces the slow oscillation. In addition to the inward current, there is a slow outward current which develops during the depolarized (burst) phase. The slow outward current can also be evoked, and more completely examined, with prolonged depolarizing voltage commands. The extremely slow decay of this current (tau approximately 45 sec) appears to be the factor underlying the slow, ramplike depolarization of Vm during the interburst interval. This slow outward current is insensitive to changes of (Na)0, but changes with (K)0 in a manner consistent with the Nerst equation. We conclude that the burst-inducing slow oscillations are generated as follows: a moderately fast inward sodium dependent current (INa) produces a regenerative depolarization, and this in turn, produces a much slower outward potassium current (IS) which hyperpolarizes the cell. The cycle is completed when IS has decayed sufficiently to allow Vm to depolarize enough to reactivate INa. We have used a quantitative version of this model to determine the time courses of gNa and gK throughout the oscillation, and to explain why different portions of the oscillatory cycle display 'graded' or 'all-or-none' behavior.

Electrical excitability of outgrowing neurites of embryonic neurones in cultures of dissociated neural plate of Xenopus laevis

1. I have studied the electrical excitability of outgrowing processes of individual neurones in cultures made from dissociated neural plates of embryos of Xenopus laevis prior to the time of neurite outgrowth in vivo. 2. The electrical excitability of neurites was tested by stimulating them extracellularly and recording responses with an intracellular electrode in their cell bodies; neurites were excitable at all times examined. 3. The ionic basis of the excitability of neurites was tested by recording from cells while changing the composition of the salines perfusing the cultures. 4. In cultures less than 10 hr old, all neurites tested made responses which depended on Ca2+. The action potentials of the cell bodies were also Ca2+-dependent at these times. 5. Between 10 and 12 hr in culture, a time at which the cell bodies still made Ca2+-dependent action potentials, neurites acquired the ability to make Na+-dependent responses. At these times, two-thirds of neurites tested retained the ability to produce divalent cation-dependent action potentials when perfused with solutions of isotonic Ba2+. 6. After 12 hr in culture, no neurites were observed to make Ca2+-or Ba2+-dependent responses; only Na+-dependent responses were observed. Cells continued to initiate and elongate new neurites until about 24 hr in culture. Thus neurites sent out at different times in culture differed in their development of excitability. 7. Cell bodies making exclusively Ca2+-dependent action potentials could be found until about 15 hr in culture, after which time a Na+-dependent component appeared. Cell bodies could then be observed to make action potentials which depended on both Ca2+ and Na+ until about 3 days in culture. After 3 days, most cell bodies made predominately Na+-dependent action potentials. Unlike the neurites, cell bodies retained the ability to make action potentials in isotonic Ba2+ for as long as the cultures were maintained (up to 5 days). 8. The possibility that changes in the ionic basis of action potentials reflected the death of one population of cells and the simultaneous appearance of another population with different properties was eliminated by observing the fate of single cells while changes in the physiological properties were occurring. Such observations showed that the majority of cells in each culture were surviving throughout the period of study. 9. Thus the membranes of the neurites and cell bodies of neurones in these cultures appeared to undergo independently timed changes in the ionic basis of their action potentials.

Functional characterization of the neurodepressing hormone in the crayfish

The effect of the neurodepressing hormone (NDH) was studied on different identified motoneurons in the abdominal ganglia of the crayfish Procambarus bouvieri (Ortmann). Although differences in sensitivity were apparent, all the neurons tested responded to NDH with a reduction in spontaneous firing rate, which lasted as long as NDH was present, and, depending on the concentration and time of action of the hormone, for even longer periods. NDH activity was determined in the various parts of the central nervous system of the crayfish, being highest in the eyestalk, gradually diminishing away from the eyestalk, with a cephalo-caudal gradient, being lowest in the abdominal ganglia. High levels of NDH activity were detected in the blood. After eyestalk ablation, NDH concentration steadily diminishes in the blood and central nervous system, until virtually disappearing after 4 days; from day 5 onwards, the activity is recovered up to its original levels. NDH synthesis takes place with a time constant of approximately 3 hr in cultured isolated segments of central nervous system, being highest in the eyestalk.

Effects of divalent cation ionophores on the neuron membrane of the crayfish

The effects of divalent cation ionophores, A23187 and X-537A, on the electrical membrane properties were investigated by using the soma membrane of the X-organ of the crayfish. They reduced the amplitude and maximum rate of rise of Ca-action potential in lower concentration. As the concentration increased, a reduction of membrane resistance and hyperpolarization occurred simultaneously. Further increase resulted in membrane depolarization with a further decrease in resistance. The threshold concentration of X537A was 100 times higher than that of A23187. These effects were reversible only when the application period was relatively short, while a longer application resulted in an incomplete reversibility or in no reversibility at all. The ionophore effect was facilitated in high Ca medium and diminished in low Ca medium. In Sr medium, the same effects on the resistance and the membrane potential were barely observable. TEA reduced the effects of A23187 but did not completely inhibit the effects. The Na-cation potential was also reduced by the higher concentration of the ionophore. From these results it is concluded that the divalent cation ionophores. A23187 and X537A, carry divalent cations, Ca ions in a physiological medium, into the neuron soma through the membrane and the consequent increase of the intracellular divalent cations induces K conductance increase and that higher concentration of the ionophore induces the increase in the conductance of the other ion species, such as Na.

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.

Synaptic junctions in the sinus gland of the freshwater prawn Palaemon paucidens

Two types of neurosecretory fibers, designated as Type 5 and Type 6 axons, in the sinus gland of the freshwater prawn, Palaemon, establish contact with other neurosecretory axons by means of synaptic junctions. This finding strongly supports the view that release of some neurohormones from the eyestalk may be regulated by neurosecretory neurons through synaptic transmission.

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.

Release of a neurodepressing hormone from the crustacean sinus gland

The nervous system of the crayfish contains a peptide of low molecular weight which depresses the spontaneous electrical activity of motoneurons in the abdominal ganglia. Most of this substance is contained in the sinus gland, a neurohemal organ in the eyestalk. Electrical stimulation of the isolated sinus gland, or its incubation in potassium-rich solutions (20--80 mM) results in the release of the neurodepressing peptide. The release is calcium-dependent and appears to involve a process of exocytosis. The released product shows electrophoretic properties undistinguishable from the substance present in the sinus gland.

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.

Mutants with reduced Ca activation in Paramecium aurelia

Two heat-sensitive "pawn" mutants of Paramecium aurelia are capable of avoiding reactions when grown at 23 degrees C but not at 35 degrees C. Electrophysiological analyses show that Ca activation is reduces in the mutants even when they are grown at 23 degrees C. The maximal rate of rise and the peak of the evoked action potential (Ca-spike) in the mutants are smaller than those of wild type in a K-solution. After suppression of K conductance by either TEA+ or Ba++, the action potentials of the mutants peak at the same level as that of wild type. However, the maximal rate of rise of the mutants remains only about half that of wild type. Thus, the mutations affect Ca activation but not K activation. Incubation at a high temperature (35 degrees C) further reduces Ca activation to almost zero in the mutants but has little or no effect on wild type. This almost complete loss of Ca activation explains the lack of avoiding reactions when the mutants are grown at high temperatures. A double mutant containing two heat-sensitive mutations shows extremely reduced Ca activation even when grown at 23 degrees C.

Potassium activation in Helix aspersa neurones under voltage clamp: a component mediated by calcium influx

1. Helix aspersa neurones under voltage clamp generate prolonged outward currents (potassium currents) in response to depolarizing command pulses. 2. The potassium currents recorded from cell A were reversibly reduced 25-50% by 10 mM cobalt ions in the bathing medium; 1 mM lanthanum, 10(-6) g/ml. D-600 and 10(-6) g/ml. iproveratril had similar effects but were only partially reversible. 3. The relationship between the potassium currents and the membrane potential had an "n" shape in normal saline. In calcium-free saline (containing 25 mM magnesium) the potassium currents were reduced and the "n" shape was abolished. The effect of calcium-free saline was readily reversible. 4. The voltage-dependence of the calcium-sensitive potassium currents was similar to that of the "late" calcium channel in squid axons (Baker, Hodgkin & Ridgway, 1971). 5. When cell A was depolarents were made up of two exponentially declining components. The slower of the two components was reduced in calcium-free saline. 6. When cell A was depolarized by 150 mV for 10 msec and then repolarized the "tail" currents were made up of a single rapidly declining component. The reversal potential of this component changed by 58 mV for a tenfold change in the external potassium concentration as predicted by the Nernst equation. 7. The reversal potential of "tail" currents having both components was less sensitive to changes in the external potassium concentration. 8. Tetraethylammonium (TEA) ions blocked both calcium dependent and voltage sensitive potassium currents. Each receptor was found to bind a single molecule of TEA. The dissociaton constant was about 10 mM in each case. 9. The intracellular concentration of ionized calcium was estimated from the potential at which there was no apparent calcium influx (the null point). It was between 3 x 10(-8) M and 8 x 10(-8) M with 10(-2) M calcium in the bathing medium. 10. The null point changed 30 mV for a tenfold change in the external calcium concentration as predicted by the Nernst equation. 11. It is concluded that depolarization of Helix neurones activates two typesof potassium channel. One channel is voltage dependent and highly selective for potassium. Activation of the other channel is dependent on the influx (or injection, see Meech, 1972, 1974a) of calcium. This calcium mediated potassium activation system saturates at high external calcium concentrations and is inhibited by external magnesium ions.

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

1. Retzius cells of leech segmental ganglia were exposed to tetraethylammonium chloride (TEA) presented both extracellularly, dissolved in the perfusing fluid, and intracellulary, by iontophoresis from a microelectrode. 2. Extracellular TEA, 10 and 25 mM, greatly prolonged the cells' action potentials, and the higher concentration increased their amplitude as well. At 10 mM the characteristic changes developed gradually over a period of about half an hour, while at 25 mM they appeared much more rapidly. However, at both concentrations the changes were reversible within minutes, even after long soaks in drug-containing solution. It is therefore probable that the drug acted at the outer surface of the membrane. 3. Intracellular TEA also prolonged the action potentials but there were several differences from the response produced by extracellular application. The changes developed gradually, and for a time, each firing of the cell was a complex event consisting of several early, brief depolarizations followed by a single much larger and more prolonged one. The large, late depolarization eventually obliterated the early ones; its gradual development suggested that it was produced only after TEA diffused to some extrasomatic portion of the cell. Intracellular TEA always caused progressive depolarization; this and the changes in the action potential were both irreversible, suggesting that the site of action was on the inner surface of the membrane. 4. Manipulations of external Na and Ca provided evidence that (a) in the absence of TEA, Retzius cell action potentials were exclusively Na-dependent, (b) that the early depolarizations in the complex action potentials produced by intracellular TEA were Na-dependent, while the later, large depolarization was Ca-dependent and (c) that the prolonged action potentials produced by extracellular TEA contained a large Ca-dependent component. 5. We conclude that TEA, acting from either side of the membrane, caused a voltage-sensitive, slowly activated Ca current to become a major contributor to the inward current of the action potential, probably by blocking the outward K current which ordinarily counteracts it. However, we cannot rule out the possibility that TEA enabled a Ca current by some means independent of its presumed action on K conductance. 6. Data resembling ours in some respects have been obtained from studies of the action of TEA on frog dorsal root ganglion cells, frog neuromuscular junction, and squid stellate ganglion. No clear counterpart of our findings has been reported form experiments on squid and amphibian axons, molluscan neurones, or frog skeletal muscle fibres.

Action potential and non-linear current-voltage relation in starfish oocytes

1. The electrical properties of the oocyte membrane of the starfish, Asterina pectinifera, were investigated using an intracellular microelectrode. 2. The resting potential in artificial sea-water ranged from -70 to -80 mV. 3. The starfish oocyte membrane was capable of generating an action potential as a result of permeability increases to both Ca and Na ions.4. The Ca component of the action potential was reversibly suppressed by Co or Mg ions, while the Na component was not affected by tetrodotoxin 5 times 10-minus 6 g/ml. 5. The steady-state relation of voltage vs. current was not linear but S-shaped. The curve wascomposed of inward-going rectification at the membrane potential more negative than -65 mV, outward-going rectification at the potential more positive than OmV and the transitional region between them. These findings are compared with those obtained in the mature egg of the tunicate.

Electrical excitability in the egg cell membrane of the tunicate

1. With the purpose of studying the differentiation of an excitable membrane, the electrical properties of the tunicate egg, a mosaic egg, was examined by intracellular recording techniques. The species used were Halocynthia aurantium Pallas and H. roretzi Drashe.2. The membrane in the matured but unfertilized egg, the fertilized but uncleaved egg, and the cleaved egg showed the resting potential of +5 to -15 mV.3. Hyperpolarization beyond -60 mV elicited a regenerative response in the form of an ;off response' with a critical membrane potential of about -40 mV and with an over-shoot of 10-20 mV above the original resting potential.4. The removal of Na from standard artificial sea water and the enhancement of Ca in Na-free ASW revealed both Na and Ca components in the ;off response'.5. In both std ASW and Na-free ASW the I-V relation of the egg cell membrane showed marked non-linearity, forming an S-shaped curve. In the range of positive membrane potential, the embryonic membrane showed a moderate outward-going rectification; in the potential range below -60 mV there was a considerable inward-going rectification. So far as examined, the shape of the I-V relation was affected only by changes of K concentration in the ASW, but not by those of other cations.6. Specific capacity of the egg cell membrane was 1.0 muF/cm(2). The specific slope resistance below -70 mV was 130 kOmega cm(2) for the unicellular egg and 50 kOmega cm(2) for the two cell embryo.