The membrane components of crustacean neuromuscular systems. II. Analysis of interactions among the electrogenic components

Novel inward rectifier blocked by Cd2+ in crayfish muscle

The characteristics of a voltage- and time-dependent inward rectifying current were examined with voltage clamp techniques in crayfish muscle. The inward current, carried by K+, was activated by hyperpolarization. Although this inward current increased with the extracellular K+ concentration [( K+]o), the voltage-dependence of the underlying conductance was independent of [K+]o. The current was unaffected by Cs+ and Ba2+, but was blocked by low concentrations of Cd2+. Therefore, this inward rectifier is different than previously described ones.

Block and activation of the hyperpolarization-activated inward current by Ba and Cs in frog sinus venosus

Voltage clamp experiments were performed on isolated frog sinus venosus trabeculae using the double mannitol gap voltage clamp technique. On hyperpolarization from the holding potential (-30, -50 mV) to various potential levels slow activation of inward current was recorded. Several basic features of this current system resemble those of the current if in mammalian pace-maker tissues. The current activates from a threshold ranging between -50, -70 mV and increases in the inward direction with the negative pulse amplitude. Conductance measurements during current development show a conductance increase. The current is strongly reduced during perfusion with Na-free medium. However, there were several important differences in its properties from those of the if current in other preparations. Ba in concentrations of 0.3-5 mM reduces the amplitude of the inward current in a concentration-dependent manner. Cs in low concentration range (1-10 mM) fails to have any effect on the time dependent current. Cs concentrations higher than 10 mM increase the current amplitude in a dose-dependent manner. The current increase induced by Cs still remains in Na-free solution and is not affected by Cl replacement. These results suggest that Cs may carry inward current. The identity of the ionic mechanism responsible for the observed current is discussed.

Effect of changes in intra- and extracellular sodium on the inward (anomalous) rectification in salamander photoreceptors

Solitary rod inner segments were obtained by enzymic dissociation of the tiger salamander retina. Ih, an inward current activated by membrane hyperpolarization, was studied using the single-pipette voltage-clamp technique with patch pipettes. In order to investigate Ih in isolation from voltage-dependent potassium and calcium currents, it was necessary to superfuse with a solution containing TEA and cobalt. When the solution in the patch pipette contained 45 mM-KCl and 50 mM-NaCl, the characteristics of Ih were indistinguishable from those previously described with fine-tip micro-electrodes: the reversal potential was near-30 mV and Ih was blocked by extracellular caesium and enhanced by an increase in the extracellular potassium concentration. The increase in Ih observed when the extracellular potassium concentration is raised is due to an increase in conductance and in driving force. Replacement of sodium in the patch pipette with choline caused a 15 mV displacement of the reversal potential for Ih in the depolarized direction. When using sodium-free patch pipettes, replacement of extracellular sodium displaced the reversal potential for Ih to -74 mV, a value in the range of the potassium equilibrium potential in solitary inner segments. Intracellular or intra- and extracellular sodium substitution affected neither the activation range of Ih nor the maximum conductance. From points 3-6 it can be concluded that Ih is carried mainly, if not exclusively, by sodium and potassium and that the channel responsible for Ih is insensitive to modifications of the intra- or extracellular sodium concentration. The results of long-term hyperpolarization, of partial block with caesium and of total sodium substitution are consistent with sodium and potassium permeating the same type of channel.

Further structure activity studies on the excitatory amino acid receptors of the crustacean neuromuscular junction

Intracellular recordings of the membrane potential and evoked excitatory junction potentials were made from the opener muscle in the walking leg of the Hermit crab ( Eupagurus bernhardus ). A variety of amino acid analogues including cis and trans 1-amino-1,3- dicarboxycyclopentane were tested for agonist activity and their potencies were compared with L-glutamate. Quisqualic acid was the most powerful excitant whereas N-methyl-DL-aspartic acid and ibotenic acid were the least active. Threshold concentrations of L-glutamate and quisqualic acid potentiated the ionophoretic L-glutamate potential and the excitatory junction potentials without affecting membrane input resistance. The results are discussed in terms of the structure-activity relationship of the crustacean excitatory receptor compared to other vertebrate and invertebrate nervous systems.

Pyrethroids of the most potent class antagonize GABA action at the crayfish neuromuscular junction

Deltamethrin and three insecticidal cyano analogs causing the Type II pyrethroid syndrome increased the input resistance of crayfish claw opener muscle fibers bathed in gamma-aminobutyric acid (GABA). In contrast, two non-toxic stereoisomers and three insecticidal pyrethroids causing the Type I syndrome were inactive. Known GABA antagonists including picrotoxinin (PTX) induced an effect similar to, although quicker than, that caused by the active pyrethroids. Two benzodiazepines reduced the potency of PTX and deltamethrin. Cyanophenoxybenzyl pyrethroids therefore appear to act on the GABA receptor-ionophore complex.

Characterization of a chloride conductance activated by hyperpolarization in Aplysia neurones

A voltage-clamp study was made of some properties of the non-synaptic hyperpolarization-activated Cl- conductance recently described in Aplysia neurones loaded with Cl- ions (Chesnoy-Marchais, 1982). The experiments were performed on an identified family of neurones, which present cholinergic responses allowing an easy measurement of the equilibrium potentials of Cl- (ECl) and K+ ions (EK). The Cl- selectivity of the hyperpolarization-activated conductance was deduced from four observations: (1) the extrapolated reversal potential of the hyperpolarization-activated current, Er, was close to the reversal potential of the cholinergic Cl- response, which is the equilibrium potential for Cl- ions, ECl. (2) Modifications of the intracellular or extracellular Cl- concentration induced changes of the reversal potential Er. (3) A prolonged and intense activation of the current lowered the intracellular Cl- concentration. (4) The current persisted after complete substitution of intracellular and extracellular cations by CS+ ions, as well as after replacement of extracellular Na+ ions by Tris. The steady-state Cl- conductance (gss) increases steeply with hyperpolarization. The kinetics of activation and deactivation are exponential and are characterized by the same voltage-dependent time constant (tau), of the order of a few seconds or fractions of seconds. The curves gss(V) and tau (V) can both be fitted by a two-state model in which the rate constants are exponential functions of the membrane potential (e-fold change for 12-16 mV). The Cl- current is much more affected by changes of the intracellular Cl- concentration than predicted simply from the change in Cl- driving force. Both the conductance and the time constant of activation are strongly modified. Modifications of the extracellular Cl- concentration do not always alter the amplitude of the hyperpolarization-activated Cl- current, but systematically affect its kinetics. The hyperpolarization-activated current is abolished after prolonged exposure of the cell to an artificial sea water where NO3- ions replace Cl- ions, as well as after intracellular injections of NO3- ions. Increasing the external pH shifts the gss(V) and tau (V) curves to the left. Lowering the external pH has reverse but less pronounced effects. In cells which were not loaded with Cl- ions and did not present the hyperpolarization-activated Cl- current, this current could be detected if the hyperpolarizing jump was preceded by short depolarizing pulses. In cells which were loaded with Cl- ions, the Cl- current became larger after a short depolarizing pulse. In the presence of extracellular Co2+ ions, depolarizing pulses no longer increased the Cl- current.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-dependent drug blockade of L-glutamate activated channels of the crayfish

The actions of d-tubocurarine (d-TC) and local anaesthetics on the L-glutamate activated channel at the voltage-clamped crayfish neuromuscular junction were studied. The effect of d-TC and local anaesthetics on the dose-response relationship between ionophoretically applied L-glutamate and synaptic current suggested that both acted as non-competitive inhibitors. The amount of inhibition was voltage dependent, and increased as the membrane potential was hyperpolarized. This voltage-dependent block was also manifest in a flattening of the I-V relationship between L-glutamate induced current and membrane potential in the presence of d-TC. However, the reversal potential for the L-glutamate activated channel was not affected; it was about +7 mV in both the presence and absence of d-TC. The neurally evoked excitatory post-synaptic current (e.p.s.c.) was depressed in the presence of these drugs and this effect was also voltage dependent. The time course of the e.p.s.c. was affected, but less so than expected if the L-glutamate activated channel were identical to the channel opened by acetylcholine at the vertebrate neuromuscular junction. Possible reasons for this discrepancy are discussed.

A Cl- conductance activated by hyperpolarization in Aplysia neurones

Although many voltage-gated cation channels have been described and extensively studied in biological membranes, there are very few examples of voltage-gated anion channels. Chloride conductances activated by depolarization have been observed in skate electroplaque and in frog and chick skeletal muscle. A Cl- conductance activated by hyperpolarization has been suggested both for frog muscle treated with acid (pH 5) solutions, and for crayfish muscle where it could account for the fact that the pronounced inward-going rectification of the I-V curve disappears if the fibres have been soaked in a Cl(-)-free solution. More recently, voltage-dependent anion channels extracted from biological membranes have been incorporated into artificial membranes. I now report that in Aplysia neurones, and in particular those in which the internal Cl- concentration has been increased, a Cl- conductance can be observed which is slowly activated by hyperpolarization and shows a vary steep voltage dependence. This time- and voltage-dependent Cl- conductance probably exists also in many other cells. Its presence might explain why it is difficult when using KCl-filled microelectrodes to maintain prolonged hyperpolarizations. This Cl- conductance constitutes a new type of inward-going rectification distinct both from the classical "anomalous rectification' which involves selective K+ channels and from the current termed if in heart muscle that is presently attributed to a cationic conductance.

The role of intracellular chloride in hyperpolarizing post-synaptic inhibition of crayfish stretch receptor neurones

1. The intracellular Cl(-) activity (a(Cl) (i)) of isolated crayfish stretch receptor neurones was measured using liquid ion exchanger Cl(-)-selective micro-electrodes. The potential developed due to the difference between the normal extracellular Cl(-) activity (a(Cl) (o)) and a(Cl) (i) (V(Cl)) was compared with the simultaneously measured reversal potential of the inhibitory post-synaptic potential (E(i.p.s.p.)) to further clarify the ionic basis of the i.p.s.p..2. In normal Ringer solution, V(Cl) (63.3 +/- 2.3 mV) was found to be close to the resting membrane potential (E(m), 62.6 +/- 3.9 mV) while E(i.p.s.p.) (74.5 +/- 1.9 mV) was more negative than either. The V(Cl) value corresponds to an apparent a(Cl) (i) of 12.7 +/- 1.3 mM, which is about 4 mM more than required for a Cl(-) governed E(i.p.s.p.) of 74.5 mV.3. Reducing a(Cl) (o) caused smaller changes in V(Cl) than predicted for passive Cl(-) re-distributions. On complete removal of extracellular Cl(-) (Cl(o) (-)), V(Cl) increased to 84.6 +/- 2.7 mV, equivalent to an apparent a(Cl) (i) of about 5 mM-Cl(-). This value can be used as an estimate of the level of intracellular interference on the Cl(-)-selective micro-electrode.4. Increasing extracellular K(+) (K(0) (+)) decreased both V(Cl) and E(i.p.s.p.). Decreasing K(o) (+) had the converse effect. The time course of the changes in V(Cl) and E(i.p.s.p.) was much the same. The difference between V(Cl) and E(i.p.s.p.) decreased to about 3 mV in high K(o) (+), and increased to about 30 mV in low K(o) (+). This variation in the difference between E(i.p.s.p.) and V(Cl) is consistent with the assumption that anions other than Cl(-) contribute to the recorded V(Cl) rather than another ion contributes to the inhibitory current.5. Application of 5 mM-NH(4) (+) or of frusemide (6 x 10(-4) M) decreased V(Cl) and E(i.p.s.p.). The difference between V(Cl) and E(i.p.s.p.) was also decreased.6. We conclude that a(Cl) (i) is lower than predicted from a passive distribution and thus the chloride equilibrium potential (E(Cl)) is more negative than E(m). If a constant intracellular interference equivalent to about 4 mM-Cl(-) is assumed to contribute to the recorded V(Cl), E(Cl) was approximately equal to E(i.p.s.p.) in all the experimental conditions. Therefore we suggest that the i.p.s.p. is solely generated by Cl(-) ions.

An analysis of the inhibitory post-synaptic current in the voltage-clamped crayfish muscle

1. Inhibitory post-synaptic currents (i.p.c.s) were recorded from the feed-back current through a wire electrode inserted longitudinally into the opener muscle fibre of the claw in the crayfish (Cambarus clarkii). 2. I.p.s.c. rose to its peak in about 3-4 msec and decayed approximately exponentially. The decay time constant at -100 mV was 9.4 msec. 3. The decay time constant decreased as the membrane was hyperpolarized and increased during depolarization. The time constant (tau) depends on voltage (V) according to the relation tau = a exp (AV), with a = 18.6 msec and A = 0.0065 mV-1. Voltage dependence was opposite in direction to that seen at frog end-plates, but in the same direction as that of e.p.s.c. in crayfish muscle. 4. At lower temperatures, the rise and fall times of i.p.s.c.s were prolonged. Q10 for the decay time constant was 2.4 between 22.6 and 12.5 degrees C. 5. When pH was decreased from 7.2 to 5.5, the decay time constant increased by about 50%, with little change in the voltage dependence of the time course. 6. When chloride in the solution was changed to iodide, the decay time constant was increased by a factor of 3, while voltage dependence of the time course was not changed. In bromide solution the decay time constant increased by about 50%. 7. Peak amplitudes of i.p.s.c.s were approximately linear as the membrane was depolarized, but they levelled off as the membrane was hyperpolarized beyond reversal potential. The non-linear I-V relation did not result from inadequate voltage clamping, nor from a change in the inside concentration of chloride. After equilibration with iodide solution the I-V relation was approximately linear. 8. The decay time constant was increased after repetitive nerve stimulation. This prolongation became more pronounced at lower temperatures. 9. The kinetic process of the transmitter action is discussed. It is suggested that the rate limiting process for i.p.s.c. is binding and unbinding of the transmitter to the receptor.

Effects of membrane potential and temperature on the excitatory post-synaptic current in the crayfish muscle

1. Effects of membrane potential and temperature on the excitatory post-synaptic current (e.p.s.c.) were studied in the voltage-clamped crayfish muscle. E.p.c. was recorded either by measuring the feedback current through an intracellular wire electrode or by focal recording with an extracellular micro-electrode. 2. The amplitude of the e.p.s.c. obtained by the voltage clamp method varied almost linearly with membrane potential between -100 mV and +70 mV, whilst the reversal potential was +23.8 +/- 3.9 mV (S.E. of mean). 3. The declining phase of the extracellular e.p.s.c. was slightly prolonged by depolarization and shortened by hyperpolarization. Potential dependence of the decay time constant was expressed by tau = a exp (AV), with a = 2.78 msec and A = 0.0037 mV-1. 4. The decay time constant had a Q10 of 2.3 and the growth time had a Q10 of 1.5. 5. The voltage dependence of the decay phase of the e.p.s. was the reverse of that found in frog end-plate. It is concluded that the voltage dependence of the time course is not related either to the charge of ions which carry the synaptic current or to the charge of the transmitter.

Antagonism by some antihistamines of the amino acid-evoked responses recorded from the lobster muscle fibre and the frog spinal cord

1 The effects of some antihistamines on the lobster muscle fibre and the frog spinal cord were investigated using intracellular and extracellular recordings, respectively. 2. On lobster muscle, histamine H1-blockers reversibly antagonized responses to bath-applied glutamate, aspartate and quisqualate but not responses to gamma-aminobutyric acid (GABA). Iontophoretic glutamate potentials were also reduced. Histamine (up to 1 mM) had no effect on this preparation. 3 The H1-antagonists produced a small increase in muscle membrane conductance and a slight hyperpolarization. These effects were largely unchanged in a low C1- bathing solution. Procaine (1 mM) decreased membrane conductance and did not affect responses to GABA or glutamate. 4 The H2-antagonist burimamide blocked both glutamate and GABA-evoked responses on the lobster muscle without affecting resting potential or conductance. 5 In the frog cord, bath-applied histamine produced ventral root depolarizations and dorsal root hyperpolarizations (sometimes biphasic responses). These effects were reduced by tetrodotoxin (TTX) but not by antazoline (H1-blocker) or burimamide; the latter reversibly antagonized responses to both glutamate and GABA on TTX-treated cords while antazoline was ineffective. 6 It is suggested that antihistamines can act as non-specific amino acid antagonists by interacting at the level of the receptor-coupled ionophores.

Cholinergic motor neurones in the stomatogastric system of the lobster

1. A study of the neurotransmitters used by each of the eleven types of excitatory motor neurones (identified according to the muscle innervated) of the lobster stomatogastric ganglion was undertaken. 2. The dorsal dilator muscle is innervated by the two motor neurones designated 'PD'. Bath and iontophoretic applications of acetylcholine (ACh) produce contractures and depolarizations respectively in the dorsal dilator muscle. 3. Pharmacological experiments support the cholinergic nature of the excitatory junctional potentials (e.j.p.s) recorded in the dorsal dilator muscle when the PD motor nerve is stimulated. 4. The apparent reversal potentials for the e.j.p.s and the iontophoretic ACh response in the dorsal dilator muscle are the same. 5. On the basis of choline acetyltransferase assays on identified stomatogastric ganglion motor neurone somata and tension measurements on the muscles innervated by each type of stomatogastric ganglion motor neurone, a transmitter candidate was established for each type of motor neurone. Motor neurones named VD, LPG, GM, MG, LG, and DG are putatively cholinergic. L-Glutamate is a transmitter candidate for the motor neurones called LP, PY, IC, and AM. 6. Potential correlations between the distribution of putatively cholinergic and glutaminergic motor neurones and the electrical coupling among the stomatogastric ganglion motor neurones are discussed.

Penicillin decreases chloride conductance in crustacean muscle: a model for the epileptic neuron

The effects of penicillin were studied on the neuromuscular preparation of the ghost crab, Ocypoda cursor. Penicillin in doses lower than 2 mM reduced both the amplitude of inhibitory junction potentials and conductance increases induced by external application of GABA. The nature of the latter effect appears to be 2-fold, a weaker competitive inhibition and a more powerful non-competitive effech which may be ionophore blockade. Penicillin in concentrations above 2 mM diminished resting conductance, especially that of chloride. The action of penicillin is, in general, to decrease chloride conductance in this preparation. The crustacean neuromuscular preparation may provide a useful analogue for understanding penicillin evoked epilepsy. The reduced chloride conductance could explain decreased inhibition, increased excitation and depolarization shifts in cortical neurons.

Permeability changes produced by L-glutamate at the excitatory post-synaptic membrane of the crayfish muscle

1. Permeability changes produced by L-glutamate at the neuromuscular junction of the crayfish (Cambarus clarkii) were investigated by application of the drug iontophoretically to the voltage-clamped junction and measuring the resulting 'glutamate current'. 2. Reversal potentials were determined by measuring the glutamate current at different membrane potentials. They were +39-1 +/- 3-6 mV (mean +/- S.E. of mean) in normal solution and +16-5 +/- 2-0 mV in solutions made twice as hypertonic by the addition of sucrose. 3. Decreasing external Na+ concentration shifted the reversal potential in the negative direction; increased Na+ in the positive direction. 4. The relation between the amplitude of the glutamate current and extracellular Na+ concentration was approximately linear. 5. Alteration of the external K+ or Cl- concentration did not affect the amplitude or reversal potential of glutamate current. 6. In Na+-free solution the application of L-glutamate produced a small inward current at the resting potential and its amplitude was augmented by increasing the external Ca2+ concentration. 7. Increasing the Ca2+ concentration in the normal Na+ media produced no appreciable effect on the reversal potential but decreased the amplitude of glutamate current. 8. The results indicate that L-glutamate increases the membrane permeability mainly to Na+ and slightly to Ca2+. 9. The time course of glutamate current was shorter than that of the concentration calculated from the diffusion equation and it was simulated more closely by the square of the concentration.

Agonistic and antagonistic activity of glutamate analogs on neuromuscular excitation in the walking limbs of lobsters

Forty-six analogs of L-glutamate were tested for activity on muscle fibers in the walking limbs of lobsters. Effects on the membrane potential, input resistance, and amplitude of neurally evoked EPSPs and IPSPs were studied as well as effects on applied L-glutamate. Seventeen of the compounds studied depolarized the muscle fibers in a manner indicative of an agonistic action on receptors in the neuromuscular excitatory membrane. Six analogs selectively reduced the amplitude of evoked EPSPs, and at least three of these (kainic acid, D-glutamate, and D-aspartate) antagonized the excitatory action of applied L-glutamate. Kainic acid was the most potent of the blockers of neuromuscular excitation, but even it was relatively weak since a concentration of 1 mM was required for an apparent effect. Generally those analogs in the L-configuration which possessed activity, had agonistic actions, whereas those in the D-configuration were usually antagonistic. These observations provide pharmacological evidence for the concept that L-glutamate is the transmitter agent which mediates neuromuscular excitation in the walking limbs of lobsters. In addition, our results are consistent with recent studies which indicate that L-aspartate may also function in this neuromuscular excitatory process.

Ionic mechanism of the excitatory synaptic membrane of the crayfish neuromuscular junction

1. The reversal potential for the excitatory neuromuscular junction of the crayfish (Cambarus clarkii) was measured using the voltage clamp method. The potential change was recorded with an intracellular microcapillary and the negative phase of the output of the feed-back amplifier was connected to the stainless-steel wire which was inserted longitudinally into the muscle fibre. 2. When the excitatory nerve was stimulated, a transient feed-back current flowed inwardly through the membrane. This current was called the excitatory junctional current (e.j.c.). 3. Reversal potentials were determined by extrapolating the e.j.c.s measured at different membrane potentials. They were about 10-20 mV positive with respect to the bath solution (11-5 +/- 1-2 mV, mean +/- S.E.). 4. The reversal potential for the iontophoretically applied glutamate was identical with that for the e.j.c. 5. In hypertonic solutions, the reversal potentials for e.j.c. and glutamate became more negative. 6. When the sodium concentration of the bath solution was decreased, the reversal potential became more negative. 7. When the chloride and potassium concentration were altered, little, if any, change was observed in the reversal potential. 8. It was concluded that the e.j.c. was carried mainly by sodium ions. Contribution of other ions, possibly calcium ions, was discussed.

Dual effect of L-glutamate on excitatory postjunctional membranes of crayfish muscle

Iontophoretically applied glutamate produces different excitatory postjunctional permeability changes on separate muscle fibers in a single crayfish muslce. At junctions on some fibers glutamate appears to increase the conductance to both sodium and potassium whereas at others its effect is primarily on the sodium conductance. These results obtained by studying the reversal potential for the extracellularly recorded glutamate potential under conditions of varied extracellular sodium and potassium concentrations.

Cooperative interaction of glutamate and aspartate with receptors in the neuromuscular excitatory membrane in walking limbs of the lobster

When applied to lobster muscle fibers, L-glutamate, L-aspartate, and combinations of the two amino acids can induce membrane depolarization. Under normal conditions, a quantitative analysis of the depolarization response or change in membrane conductance was precluded by nonlinearities in the voltage-current relationship of the membrane. By including gamma-aminobutyrate (GABA) in the bathing medium, the voltage-current relationship was made linear in the depolarizing direction over a range of 15-20 mV from the resting potential. However, a meaningful examination of the increase in membrane conductance caused by glutamate and aspartate was still not possible. Therefore, the depolarization responses caused by the excitatory amino acids were taken as a quantitative reflection of receptor activation in the excitatory postsynaptic membrane. In the presence of GABA, aspartate by itself, at concentrations up to 10 mM, had little excitatory activity, whereas glutamate effected an appreciable membrane depolarization at concentrations of 0.1 to 0.2 mM. Aspartate, at concentrations which exhibited no activity alone, markedly enhanced the excitatory action of glutamate. Aspartate shifted the glutamate dose-response curve to the left, but did not appear to affect the maximum depolarization response elicited by glutamate. These observations are consistent with the concept that aspartate increases the affinity between glutamate and the glutamate binding sites. Limiting slopes of log-dose versus log-response curves for the excitatory action of glutamate suggest that the interaction of glutamate with excitatory receptors is a cooperative process. The possibility exists that individual receptors contain multiple and distinct glutamate and aspartate binding sites. These results support the view that neuromuscular excitation in the lobster is mediated by glutamate and aspartate functioning synergistically.

Inhibitory and excitatory effects of dopamine on Aplysia neurones

1. Electrophoretic application of dopamine (DA) on Aplysia neurones elicits both excitatory and inhibitory effects, which in many cases are observed in the same neurone, and often result in a biphasic response.2. The DA receptors are localized predominantly on the axons. Desensitization, which occurs after repeated injections or with bath application of DA, is more marked for excitatory responses.3. Tubocurarine and strychnine block the DA excitatory responses without affecting the inhibitory ones, which can be selectively blocked by ergot derivatives. It is concluded that the excitatory and inhibitory effects are mediated by two distinct receptors.4. The two DA receptors can be pharmacologically separated from the three ACh receptors described in the same nervous system.5. In some neurones the dopamine inhibitory responses can be inverted by artificial hyperpolarization of the membrane at the potassium equilibrium potential, E(K), indicating that dopamine causes a selective increase in potassium permeability.6. In other neurones the reversal potential of dopamine inhibitory responses is at a more depolarized level than E(K), but can be brought to E(K) by pharmacological agents known to block the receptors mediating the excitatory effects of DA.7. In still other neurones, the hyperpolarization induced by DA cannot be inverted in normal conditions, but a reversal can be induced by ouabain or by the substitution of external sodium by lithium. These results are discussed in terms of an hypothesis in which dopamine increases the potassium permeability of a limited region of the axonal membrane.8. It is concluded that a selective increase in potassium permeability probably accounts for all dopamine inhibitory effects in the neurones studied.

Potassium inactivation and impedance changes during spike electrogenesis in eel electroplaques

Various degrees of pharmacological K inactivation were induced by Cs or Ba in isolated single electroplaques of the electric eel. The resulting changes in K conductance give rise to very different steady-state current-voltage characteristics. They also induce differences in ion dynamics during spike electrogenesis. The dynamic changes were studied by AC bridge methods, registering the changes in impedance in synchrony with the neurally or directly evoked spikes. While spike electrogenesis was virtually unaffected by addition of Cs or Ba, the patterns of impedance changes were very different. The various patterns are accounted for by the changes in the respective current-voltage characteristics. The data constitute new evidence for regarding the electrically excitable component of the reactive membrane as a heterogeneous electrochemical system with separate and independently reactive channels that in the electroplaques are permselective for Na and K, respectively.

Crayfish muscle fiber: spike electrogenesis in fibers with long sarcomeres

Most of the muscle fibers in the walking legs of the crayfish Procambarus clarkii generate only graded electrical responses. However, some fibers in extensor muscles of the carpopodite have long sarcomeres, about 10 micrometers in length, and generate overshooting spikes that have conduction velocities of 0.3 meter per second. The spikes induce twitch contractions.

Inhibitory synapses on pacemaker neurons in the heart ganglion of a stomatopod, Squilla oratoria

The pacemaker neurons of the Squilla heart ganglion are innervated from the CNS through three pairs of extrinsic nerves. One of them, the alpha-nerve, is inhibitory to the heart beat. The effect of alpha-nerve stimulation on the pacemaker potential was examined with intracellular electrodes. Without extrinsic nerve stimulation the membrane potential of the pacemaker cell fluctuated spontaneously. On application of a tetanic train of stimuli to the alpha-nerve the membrane potential was shifted and fixed to a steady level, which with K(2)SO(4)-filled electrodes was near the peak of hyperpolarization after a spontaneous burst, but was less negative with KCl-filled electrodes. The shift of the membrane potential was due to the summated IPSP's. By changing the level of the membrane potential with injection of the polarizing current the IPSP could be reversed in sign, and the size of the IPSP was linearly correlated with the membrane potential level. During inhibition the membrane conductance increased. The increase depended on divalent cation concentrations in the outside medium. In Ca-rich saline the IPSP was greatly enhanced. In Mg-rich saline it was suppressed. The amplitude of antidromic spikes was reduced during inhibition especially when the spike frequency was high.

Permeability of alkali metal cations in lobster muscle. A comparison of electrophysiological and osmometric analyses

Single muscle fibers from lobster walking legs are effectively impermeable to Na, but are permeable to K. They shrink in hyperosmotic NaCl; they swell in low NaCl media which are hyposmotic or which are made isosmotic with the addition of KCl. In conformity, the membrane potential is relatively insensitive to changes in external Na, while it responds according to the Nernst relation for changes in external K. When the medium is made isosmotic or hyperosmotic with RbCl the volume and membrane potential changes are of essentially the same magnitudes as those in media enriched with KCl. The time courses for attaining equilibrium are slower, indicating that Rb is less permeant than K. Substitution of CsCl for NaCl (isosmotic condition) produces no change in volume of the muscle fiber. Addition of CsCl (hyperosmotic condition) causes a shrinkage which attains a steady state, as is the case in hyperosmotic NaCl. Osmotically, therefore, Cs appears to be no more permeant than is Na. However, the membrane depolarizes slowly in Cs-enriched media and eventually comes to behave as an ideal Cs electrode. Thus, the electrode properties of the lobster muscle fiber membrane may not depend upon the diffusional relations of the membrane and ions, and the osmotic permeability of the membrane for a given cation may not correspond with the electrophysiologically deduced permeability. Comparative data on the effects of NH(4) and Li are also included and indicate several other degrees of complexity in the cell membrane.

Crayfish muscle: permeability to sodium induced by calcium depletion

Membrane of crayfish muscle fibers becomes selectively permeable to sodium when the calcium concentration of the bathing medium is reduced. Removal of calcium or its reduction below 1 or 2 millimole per liter causes large transient depolarizations up to 70 millivolts in amplitude. They resemble pro longed action potentials and occur only in the presence of sodium. The responses are abolished when tris(hydroxymethyl)aminomethane or lithium is substituted for sodium, and are blocked by tetrodotoxin even in the presence of sodium.

Permeability changes associated with the action potential in procaine-treated crayfish abdominal muscle fibers

Permeability changes associated with prolonged action potentials have been analyzed in procaine-treated crayfish abdominal muscle fibers. The effect of external Ca indicates that the increase in membrane conductance observed during the rising phase of the action potential is primarily due to a permeability increase for Ca. A remnant of the permeability increase may cause the succeeding plateau as shown by its high conductance and by the effect of low Mn. A delayed increase in conductance precedes the termination of the plateau phase. This is due to a delayed increase in permeability, probably for K, that is observed when depolarizing electrogenesis is eliminated. High external Ca reduces the action potential duration, the falling phase starting at a higher depolarization. These changes may be related to an earlier onset of the delayed increase in permeability, induced by a larger inside positivity in the presence of higher Ca. No "anomalous rectification" is seen in early or late I-V curves for small depolarizations. Ba may replace Ca in its role in depolarizing electrogenesis, and the first action potential induced in Ba saline has a large overshoot and a long duration. In higher Ba salines, action potentials are greatly prolonged. Long term soaking in Rb-containing or K-free saline also augments and prolongs the action potential. These changes are assumed to be related to depression of the K permeability of the membrane.

Crayfish muscle fiber: ionic requirements for depolarizing synaptic electrogenesis

Presence of sodium in the bathing medium is not essential for the electrically excitable depolarizing electrogenesis of crayfish muscle fibers, production of action potentials being dependent on calcium. The depolarizing electrogenesis of the excitatory synaptic membrane component does require sodium, however, and this ion cannot be replaced by lithium as it can in spike electrogenesis of many cells. Ionophoretic applications of glutamate, which in the presence of sodium depolarize the cell by activating the excitatory synaptic membrane, are without effect in the absence of sodium. Not only is there no depolarization, but the membrane conductance also remains unchanged. Thus, in the absence of inward movement of sodium across the synaptic membrane there is also no outward movement of potassium. Accordingly, it seems that increased conductance for potassium is not an independent process in the synaptic membrane, whereas it is independent of sodium activation in spike electrogenesis. Chloride activation is independent, however; increase in conductance and the electrogenesis of the inhibitory synaptic component are not affected by the absence of sodium. Implications of these findings regarding the structure of differently excitable membrane components are discussed.

Analysis of depolarizing and hyperpolarizing inactivation responses in gymnotid electroplaques

In electroplaques of several gymnotid fishes hyperpolarizing or depolarizing currents can evoke all-or-none responses that are due to increase in membrane resistance as much as 10- to 12-fold. During a response the emf of the membrane shifts little, if at all, when the cell either is at its normal resting potential, or is depolarized by increasing external K, and in the case of depolarizing responses when either Cl or an impermeant anion is present. Thus, the increase in resistance is due mainly, or perhaps entirely, to decrease in K permeability, termed depolarizing or hyperpolarizing K inactivation, respectively. In voltage clamp measurements the current-voltage relation shows a negative resistance region. This characteristic accounts for the all-or-none initiation and termination of the responses demonstrable in current clamp experiments. Depolarizing inactivation is initiated and reversed too rapidly to measure with present techniques in cells in high K. Both time courses are slowed in cells studied in normal Ringer's. Once established, the high resistance state is maintained as long as an outward current is applied. Hyperpolarizing inactivation occurs in normal Ringer's or with moderate excess K. Its onset is more rapid with stronger stimuli. During prolonged currents it is not maintained; i.e., there is a secondary increase in conductance. Hyperpolarizing inactivation responses exhibit a long refractory period, presumably because of persistence of this secondary increase in conductance.