Reversal potentials of L-glutamate and the excitatory transmitter at the neuromuscular junction of the crayfish

Operant avoidance learning in crayfish, Orconectes rusticus: Computational ethology and the development of an automated learning paradigm

Research in crustaceans offers a valuable perspective for studying the neural implementation of conserved behavioral phenomena, including motivation, escape, aggression, and drug-sensitive reward. The present work adds to this literature by demonstrating that crayfish successfully learn to respond to spatially contingent cues. An integrated video-tracking system automatically delivered a mild electric shock when a test animal entered or remained on a substrate paired with punishment. Following a few instances of shock delivery, crayfish quickly learned to avoid these areas. Comparable changes in substrate preference were not exhibited by yoked controls, but locomotion differed significantly from both pre-conditioning levels and from those of their masters receiving shock in a contingent fashion. The results of this work provide valuable insights into the principles governing avoidance learning in an invertebrate system and provide a behavioral template for exploring the neural changes during associative learning. Serving as a case study, this project introduces a new computer framework for the automated control of learning paradigms. Based on routines contained within the JavaGrinders library (free download at iEthology.com), it integrates real-time video tracking with robotic interfaces, and provides a suitable framework for implementing automated learning paradigms.

Amine neurochemistry and aggression in crayfish

A primary goal of our research is to explore proximate mechanisms important in recruiting adaptive social behaviors. For instance, if one of three different behaviors may be expressed in a particular set of circumstances, how do neurochemical mechanisms bias behavior towards the expression of one act in lieu of the other possibilities? In this article, we review recent results suggesting that serotonin may play such a role in the control of aggression in crayfish. First, we summarize techniques that have been optimized for sensitive characterization of neurochemical profiles in crayfish. Then, borrowing concepts from behavioral ecology, we review a framework for quantitative investigation, which regards behavior as a set of individual decisions, each with a particular probability for occurrence, a motivational context, and controlled by its own distinct neurochemical mechanisms.

Regulation of synaptic vesicle recycling by calcium and serotonin

Serotonin, a neuromodulator at the crayfish neuromuscular junction, regulates neurotransmission without changing intracellular calcium levels. However, the mechanism of this regulation remains unclear. By analysis of synaptic depression using a depletion model and measurement of vesicle recycling using the styryl dye FM1-43, we show that serotonin increases the number of vesicles available for transmitter release (total synaptic vesicle pool size). This regulation is due either to an increase in the number of vesicles at each release site or to an activation of previously nonsecreting or silent synapses. We also observed that low calcium medium rendered part of the vesicle pool unavailable for release. These results suggest a new mechanism for regulating synaptic transmission.

Mitochondrial involvement in post-tetanic potentiation of synaptic transmission

Posttetanic potentiation (PTP) is an essential aspect of synaptic transmission that arises from a persistent presynaptic [Ca2+]i following tetanic stimulation. At crayfish neuromuscular junctions, several inhibitors of mitochondrial Ca2+ uptake and release (tetraphenylphosphonium or TPP+, carbonyl cyanide m-chlorophenylhydrazone or CCCP, and ruthenium red) blocked PTP and the persistence of presynaptic residual [Ca2+]i, while endoplasmic reticulum (ER) Ca2+ pump inhibitors and release channel activators (thapsigargin, 2,5-di-(tert-butyl)-1,4-benzohydroquinone or BHQ, and caffeine) had no effects. PTP apparently results from the slow efflux of tetanically accumulated mitochondrial Ca2+.

Calcium released by photolysis of DM-nitrophen triggers transmitter release at the crayfish neuromuscular junction

1. Spontaneous and evoked transmitter release at the crayfish neuromuscular junction were potentiated in response to photolytic release of calcium from the 'caged' calcium compound DM-nitrophen, which had previously been injected into presynaptic terminals. 2. The amount of calcium released from DM-nitrophen photolysis depends on the concentration of DM-nitrophen, its photoproducts, Ca2+, Mg2+, H+, ATP and the cell's native buffer. Since none of these are known in the crayfish terminal, the study was conducted in a qualitative fashion. 3. Photolytic release of calcium from DM-nitrophen increased excitatory junctional potentials (EJPs) by a range of 2-31 times over control values and the miniature excitatory junctional potential (MEJP) frequency increased from resting values of 1-10 quanta/s to 3000-11,000 quanta/s. 4. Extracellular calcium was not required for the light-evoked asynchronous release of transmitter. Calcium-bound DM-nitrophen previously pressure injected into crayfish presynaptic terminals increased the MEJP frequency from resting values of 1-8 quanta/s to 800-10,000 quanta/s during photolysis in a calcium-free cobalt Ringer solution. 5. Iontophoresis of calcium-free DM-nitrophen into presynaptic terminals released transmitter upon photolysis, but only in a calcium-containing Ringer solution. This suggests that DM-nitrophen is capable of binding calcium once injected into terminals, but this is dependent on the presence of external calcium. 6. Photolysis of DM-nitrophen at lower light intensities produced a slower rate of transmitter release. 7. Brief light exposures, i.e. those which photolysed 5-20% of the DM-nitrophen, resulted in a rapid decay of postsynaptic responses on extinguishing the light, due to rebinding of photolytically released calcium to unphotolysed DM-nitrophen. Longer light exposures which completely photolysed DM-nitrophen, leaving only the low affinity photoproducts, produced a slow decay of transmitter release after the light pulse, presumably due to the active extrusion of calcium from the presynaptic terminals. 8. During photolysis of DM-nitrophen, the time courses of changes in EJP amplitude and MEJP frequency were different, indicating that the two measures of transmitter release were not linearly related. 9. MEJP frequency and EJP amplitudes during DM-nitrophen photolysis were fitted to a 'non-linear summation model' in which photolytically released calcium sums with calcium entering during an action potential to evoke transmitter release with a calcium co-operativity of five.

Effects of stimulus timing on transmitter release and postsynaptic membrane potential at crayfish neuromuscular junctions

Different synaptic terminals of the single excitor axon to the opener muscle of crayfish (Procambarus clarkii) often release transmitter in a very different manner when stimulated with the same equal-interval, doublet, or triplet patterns. Compared to synapses that show little facilitation (low Fe synapses), highly facilitating (high Fe) synapses show greater percentage increases in several measures of synaptic efficacy when stimulated with any of these patterns. Low Fe synapses usually show the greater absolute changes in these measures of synaptic efficacy. Changes in the span and pattern of doublets and triplets can independently affect both pre- and postsynaptic measures of synaptic efficacy at either low Fe or high Fe synapses.

Permeability changes induced by L-glutamate at the crayfish neuromuscular junction

Two different methods are described which allow the reversal potential (Er) for the channels opened by L-glutamate at the voltage-clamped, crayfish neuromuscular junction to be measured accurately. In both cases the value of Er was found to be about +6 mV. Reversal potentials were also measured in solutions where Na+ was replaced by K+, Ca2+, or Mg2+; or in which Cl- was replaced by isethionate. In solutions where Na+ was partially replaced by K+, the measured reversal potentials were compared to theoretical values predicted by both the constant-field and equivalent-circuit equations. The experimental values were more accurately described by the constant-field equation. Permeability ratios (PX/PNa) for K+, Ca2+, Mg2+, and Cl- were calculated using the constant-field equation. K+ and Na+ were equally permeant while Ca2+ and Mg2+ were about half as permeant as the monovalent cations. Cl- was impermeant. The results of these experiments indicate that the L-glutamate activated channel is non-selective for cations. Furthermore, the value of the permeability ratios for the physiological cations tested are very similar to those obtained for the acetylcholine activated channel in vertebrate skeletal muscle.

Comparison of excitatory currents activated by different transmitters on crustacean muscle. II. Glutamate-activated currents and comparison with acetylcholine currents present on the same muscle

The properties of glutamate-activated excitatory currents on the gm6 muscle from the foregut of the spiny lobsters Panulirus argus and interruptus and the crab Cancer borealis were examined using either noise analysis, analysis of synaptic current decays, or slow iontophoretic currents. The properties of acetylcholine currents activated in nonjunctional regions of the gm6 muscle were also examined. At 12 degrees C and -80 mV, the predominant time constant of power spectra from glutamate-activated current noise was approximately 7 ms and the elementary conductance was approximately 34 pS. At 12 degrees C and -80 mV, the predominant time constant of acetylcholine-activated channels was approximately 11 ms with a conductance of approximately 12 pS. Focally recorded glutamatergic extracellular synaptic currents on the gm6 muscle decayed with time constants of approximately 7-8 ms at 12 degrees C and -80 mV. The decay time constant was prolonged e-fold about every 225-mV hyperpolarization in membrane potential. The Q10 of the time constant of the synaptic current decay was approximately 2.6. The voltage dependence of the steady-state conductance increase activated by iontophoretic application of glutamate has the opposite direction of the steady-state conductance activated by cholinergic agonists when compared on the gm6 muscles. The glutamate-activated conductance increase is diminished with hyperpolarization. The properties of the marine crustacean glutamate channels are discussed in relation to glutamate channels in other organisms and to the acetylcholine channels found on the gm6 muscle and the gm1 muscle of the decapod foregut (Lingle and Auerbach, 1983).

L-Glutamate effects on electrical potentials of synaptic plasma membrane vesicles

The electrogenic nature of the L-glutamate-stimulated Na+ flux was examined by measuring the distribution of the lipophilic anion [35S]thiocyanate (SCN-) into synaptic membrane vesicles that were incubated in a NaCl medium. Concentrations of L-glutamate from 10(-7) to 10(-4) M added to the incubation medium caused an enhanced intravesicular accumulation of SCN-. Based on the SCN- distribution in synaptic membrane vesicles it was calculated that 10 microM L-glutamate induced an average change in the membrane potential of + 13 mV. L-Glutamate enhanced both the Na+ and K+ conductance of these membranes as determined by increases in SCN- influx. Other neuroexcitatory amino acids and amino acid analogs (D-glutamate, L-aspartate, L-cysteine sulfinate, kainate, ibotenate, quisqualate, N-methyl-D-aspartate, and DL-homocysteate) also increased SCN- accumulation in synaptic membrane vesicles. These observations are indicative of the activation by L-glutamate and some of its analogs of excitatory amino acid receptor ion channel complexes in synaptic membranes.

Serotonin-induced depolarization of rat facial motoneurons in vivo: comparison with amino acid transmitters

Intracellular recordings were obtained from facial motoneurons in anesthetized rats. The effects of iontophoretically applied serotonin were compared to those of the excitatory amino acids glutamate and DL-homocysteic acid (DLH), and the inhibitory amino acids, glycine, GABA and muscimol, under various conditions of membrane polarization and intracellular chloride concentration. Iontophortically applied serotonin caused a depolarization of facial motoneurons which was accompanied by increased input resistance and increased neuronal excitability. Experiments comparing the response to serotonin with those of glycine, GABA, and muscimol demonstrated that the serotonin effect does not involve changes in membrane conductance to chloride. Comparisons of serotonin with glutamate and DLH at varying levels of membrane hyperpolarization indicated that the serotonin-induced depolarization is not caused by increased conductance to sodium or calcium, and differs in its underlying ionic mechanism from depolarizations induced by glutamate and DLH. Results were consistent with the hypothesis that serotonin causes depolarization, increased input resistance, and increased excitability in rat facial motoneurons by decreasing resting membrane conductance to potassium ions. Such changes in motoneurons in the brain stem and spinal cord probably account for some of the physiological and behavioral effects observed during pharmacological activation of serotonin receptors.

L-proline as a glutamate antagonist at a crustacean neuromuscular junction

The fast as well as the slow contractions of the adductor muscle in the claw of Procambarus clarkii are inhibited by L-proline. This inhibition is dose dependent and decreases with increasing frequency of stimulation of the "slow" fiber. Contractions caused by perfusing the adductor muscle with L-glutamate solutions are also inhibited by L-proline. The inhibiting potency of L-proline is small; the effective concentration of this amino acid is 50--100 times that of the L-glutamate applied. It was postulated that the inhibitory effect of L-proline is based on competition for excitatory receptor sites of L-glutamate, which causes depolarization and contraction, and L-proline, which lacks these actions. Theoretical considerations suggested a linear relationship between the stimulating L-glutamate and the just-inhibiting L-proline concentrations. Experimental evidence supported this model.

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.

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.

Characteristics of crayfish neuromuscular facilitation and their calcium dependence

1. A quantitative description of facilitation in the crayfish claw opener muscle is presented. The facilitation of a test response following one or more conditioning stimuli, and the growth of facilitation during a tetanus, are measured.2. In superficial central fibres facilitation following one or more impulses can be described as the sum of two components which are both maximum at the end of the conditioning train and decline simultaneously and exponentially with different time constants thereafter.3. During a tetanus, facilitation to successive stimuli grows more rapidly than is predicted by assuming that each impulse adds a constant facilitative effect to an accumulating total state of facilitation.4. Sufficiently large values of tetanic facilitation are predicted by a model which assumes that transmitter release is proportional to the nth power of a substance or factor accumulating in nerve terminals. But no single value of n predicts the correct rise of facilitation in a tetanus and the time course of its subsequent decline from the facilitation following a single spike.5. A model which assumes that the facilitative effects of successive spikes multiply in a tetanus predicts responses that are larger than those observed.6. The effects of varying the calcium concentration ([Ca(2+)]) on transmitter release and facilitation were studied. When a magnesium-EDTA buffering system is used to vary [Ca(2+)], transmitter release is found to be nearly linearly related to [Ca(2+)] in the range 0.1-13.5 mM.7. The magnitude and time course of facilitation during and following a tetanus are unaffected by varying [Ca(2+)] between 1.0 and 40 mM.8. The relation between ;steady-state' facilitation and stimulus frequency is also unaffected by changing [Ca(2+)], except that in high [Ca(2+)] transmitter release appears to saturate at high frequencies (above 30 Hz).9. The results are discussed in terms of the ;calcium accumulation' hypothesis of facilitation. The findings in crayfish appear to be qualitatively consistent with this hypothesis if certain modifications are made in the hypothesis.

The accumulative properties of facilitation at crayfish neuromuscular synapses

1. Synaptic facilitation was measured with intracellular recording at two classes of neuromuscular synapses in the opener muscle of the crayfish dactyl by placing a test stimulus at various intervals after either a single conditioning stimulus or a short conditioning train.2. The facilitative effect of one stimulus reaches approximately the same level with both the superficial central and superficial distal synapses. The facilitation decreases smoothly in two phases after the conditioning stimulus at superficial central synapses and in a more complex fashion at superficial distal synapses.3. The two synaptic types differ in the manner in which they add up the facilitative effects produced by each of the stimuli in a short train. With superficial distal synapses the facilitative effects of conditioning stimuli add linearly, while with superficial central synapses the facilitative effects accumulate exponentially.4. The linear addition of facilitation at superficial distal synapses is not altered when the quantal content is lowered by decreasing the external Ca concentration from 13.5 to 3 mM.5. The rate of decay of facilitation is the same following both one and three conditioning stimuli, even though the facilitation is nearly six times larger in the latter case.6. The results are discussed in terms of mechanisms for synaptic facilitation.

Calcium and facilitation at two classes of crustacean neuromuscular synapses

The closer muscle of the crab, Chionoecetes, has at least two classes of excitatory neuromuscular synapses. In one class of synapses an action potential depolarizing the synaptic region releases much more transmitter if it has been preceded recently by another action potential. The other class of synapses shows this property, called facilitation, to a far lesser extent. Immediately after one conditioning stimulus the level of facilitation is similar in both classes. The rate of the ensuing decay of the facilitation is the critical factor differentiating the two classes of synapses. The relationship between external Ca(++) concentration and transmitter release is similar for both classes of synapses. The slope of a double logarithmic plot of this relationship varies from 3.1 between 5 and 10 mM Ca(++) to 0.9 between 30 and 40 mM Ca(++). Facilitation does not significantly change when tested in external Ca(++) concentrations ranging from 7 to 30 mM. The extracellularly recorded nerve terminal action potential does not increase in amplitude during facilitation. The results suggest that the mechanism of synaptic facilitation is similar for both classes of synapses and occurs after the stage in transmitter release involving Ca(++).