Characterization of a radula opener neuromuscular system in Aplysia

1. Several lines of evidence suggest that the I7-I10 muscle group contributes to the radula opening phase of behavior in Aplysia; 1) extracellular stimulation of these muscles in reduced preparations causes the halves of the radula to separate, 2) synaptic activity can be recorded from muscles I7-I10 in intact animals when the radula is opening, and 3) motor neurons innervating I7-I10 are activated out of phase with retractor/closer motor neurons during cycles of buccal activity driven by the cerebral-to-buccal interneuron 2 (CBI-2). 2. All of the opener muscles are innervated by the B48 neurons, a bilaterally symmetrical pair of cholinergic motor neurons. B48 neurons produce excitatory junction potentials (EJPs) in opener muscle fibers that summate to produce muscle contractions. Contraction size is determined by the size of depolarization in muscle fibers and/or by action potentials that are triggered by summation of B48-evoked EJPs. 3. In addition to input from B48 neurons, opener muscles also receive excitatory input from the cholinergic multiaction neurons B4/B5. EJPs evoked by stimulation of neurons B4/B5 are 1/10 the size of B48-evoked EJPs. Consequently, changes in muscle tension produced by B4/B5 activity are relatively small. In contrast to B48 neurons, neurons B4/B5 are likely to be active during the closing/retraction phase of behavior. During cycles of buccal activity driven by neuron CBI-2, neurons B4/B5 fire in phase with closer/retractor motor neurons. Thus opener muscles may develop a modest amount of tension during the closing/retraction phase of behavior as a result of synaptic input from neurons B4/B5. 4. Opener muscles may also develop tension during closing/retraction simply by virtue of the fact that they have been stretched. When isolated opener muscles are lengthened, depolarizations are recorded from individual muscle fibers, and muscle tension increases. With sufficient changes in fiber length, action potentials are elicited. These action potentials produce twitchlike muscle contractions that become rhythmic with maintained stretch. Stretch-activated depolarizations are generally first apparent when muscle length is increased by 1 mm. Length changes of 4-5 mm are generally necessary to elicit twitchlike muscle contractions. Changes of 1-2 mm in muscle length are observed when the opener muscle's antagonist, the accessory radula closer, is activated in reduced preparations. 5. Stretch may also modulate B48-induced contractions of the opener muscles. When muscle length is increased, B48-elicited contractions of the I7 muscle are larger. These increases in contraction amplitude are accompanied by decreases in contraction latency. 6. We conclude that muscles I7-I10 contract vigorously in response to strong excitatory input from neuron B48 and contribute to radula opening. Stretch may potentiate this activity. Thus, if radula closer muscles contract vigorously and pull on the opener muscles, the opener muscles will respond by contracting more vigorously themselves. This may be a mechanism for maintaining amplitude relationships between antagonistic muscles. Additionally, it is likely that the opener muscles will develop at least a modest amount of tension during closure/retraction of the radula. Part of this activation may derive from the weak excitatory input that the muscles receive from neurons B4/B5. Another part may derive from the stretch. One function of this co-contraction may be to act as a brake on closure, bringing this phase of feeding behavior to a smooth halt.

A pair of identified interneurons in Aplysia that are involved in multiple behaviors are necessary and sufficient for the arterial-shortening component of a local withdrawal reflex

A bilateral pair of cerebral interneurons, called CC5, contribute to the generation of a number of different behaviors involving head movements. Each cell sends its axon to the ipsilateral and contralateral pedal and pleural ganglia. A weak tactile stimulus to the head excites the ipsilateral CC5; a strong stimulus excites both the ipsilateral and contralateral cells. Firing of CC5 produces powerful shortening of the ipsilateral pedal artery (PA) by means of monosynaptic excitation of the pedal artery shortener (PAS) neuron, the single motor neuron for the artery. A weak touch to a tentacle excites the ipsilateral PAS and evokes a local withdrawal response accompanied by shortening of the ipsilateral PA. In vivo recording of the pedal artery nerve (PAn) showed that PAS was activated bilaterally during defensive head withdrawal elicited by a strong stimulus and was activated unilaterally by a weak stimulus. The responses were eliminated by cutting the ipsilateral cerebral-pleural connective (C-PLC). Electrical stimulation of the cerebral-pleural connective provided evidence that all of the excitatory input to PAS via this connective is provided by CC5. A variety of experimental results indicates that during a local withdrawal reflex of the tentacle, CC5 is necessary and sufficient for the unilateral PA-shortening component of the response and therefore functions as a command neuron for a component of the behavior. The data suggest that during defensive head withdrawal, the two CC5 neurons may act conjointly as a two-neuron command system that is necessary and sufficient for the bilateral arterial-shortening component of the behavior.

Ca2+/S100 regulation of giant protein kinases

Protein phosphorylation by protein kinases plays a central regulatory role in cellular processes and these kinases are themselves tightly regulated. One common mechanism of regulation involves Ca2+-binding proteins (CaBP) such as calmodulin (CaM). Here we report a Ca2+-effector mechanism for protein kinase activation by demonstrating the specific and >1,000-fold activation of the myosin-associated giant protein kinase twitchin by Ca2+/S100A1(2). S100A1(2) is a member of a large CaBP family that is implicated in various cellular processes, including cell growth, differentiation and motility, but whose molecular actions are largely unknown. The S100A1(2)-binding site is a part of the autoregulatory sequence positioned in the active site that is responsible for intrasteric autoinhibition of twitchin kinase; the mechanism of autoinhibition based on the crystal structures of two twitchin kinase fragments is described elsewhere. Ca2+/S100 represents a likely physiological activator for the entire family of giant protein kinases involved in muscle contractions and cytoskeletal structure.

Two ion currents activated by acetylcholine in the ARC muscle of Aplysia

1. This work continues our examination of the electrophysiology and contractions of single fibers dissociated from a widely studied molluscan muscle, the accessory radula closer (ARC) muscle of Aplysia californica, aimed at understanding its excitation-contraction mechanisms and their modulation. 2. Extensive previous work has characterized a number of basal ion currents present in the fibers and effects of transmitters and peptide cotransmitters that modulate ARC-muscle contractions in vivo. Here we use current clamp, voltage clamp, and contraction measurements to examine the actions of acetylcholine (ACh), the transmitter that induces the contractions. 3. As in the whole ARC muscle, ACh depolarizes unclamped fibers maximally to about -25 mV where, no matter how much ACh is applied, the depolarization saturates. 4. The underlying ACh-activated current is in fact the sum of two quite distinct components, IACh,cat and IACh,Cl. 5. IACh,cat is itself a mixed current carried by cations (physiologically mainly by Na+, but to a significant degree also by Ca2+), reverses near +20 mV, rectifies inwardly, exhibits prominent voltage-dependent kinetics of activation with hyperpolarization, and is selectively blocked by hexamethonium. 6. In contrast, IACh,Cl is carried by Cl-, reverses near -60 mV, exhibits little rectification or voltage-dependent kinetics, is activated selectively by suberyldicholine, and is blocked by alpha-bungarotoxin. 7. Both currents activate fast when ACh is applied, desensitize relatively slowly in its presence, then deactivate fast. Both currents are activated at similar ACh concentrations, half-maximally at approximately 10 microM. Both currents also are activated by carbachol and propionylcholine and blocked by d-tubocurarine, bicuculline and paraoxon. Picrotoxin and atropine block IACh,cat better, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and anthracene 9-carboxylic acid IACh,Cl better. 8. The two currents are virtually identical to ACh-activated cationic (Na) and Cl currents that are ubiquitous in molluscan neurons. As has been proposed for the neuronal currents, IACh,cat resembles vertebrate neuronal nicotinic ACh-receptor (nAChR) currents, whereas IACh,Cl resembles vertebrate skeletal muscle nAChR currents. 9. Functionally, we believe that IACh,cat serves primarily to depolarize the ARC muscle to open voltage-activated L-type Ca channels, allow Ca2+ influx, and initiate contraction. Physiologically significant Ca2+ may also enter through the ACh,cat channels themselves. 10. By superimposing on IACh,cat, IACh,Cl brings the reversal potential of the combined current to around -25 mV and thereby sets a relatively negative upper limit to the ACh-induced depolarization. We propose that this is its physiological role. By limiting the depolarization, IACh,Cl limits the degree of activation of the Ca current and Ca2+ influx, and so prevents excessive contraction. More importantly, it moderates the voltage during contraction to a range where small voltage changes can finely grade contraction amplitude in this nonspiking muscle. 11. Indeed, in contraction experiments on the single fibers, there is an inverse correlation between the IACh,Cl/IACh,cat ratio and the magnitude of the ACh-induced depolarization and contraction. Furthermore, increased pharmacological activation of IACh,Cl depresses, and block of IACh,Cl enhances, both the depolarization and contraction. 12. Obligatory simultaneous coactivation of IACh,cat and IACh,Cl in the ARC muscle may be part of a peripheral control mechanism that automatically keeps the size of its contractions within behaviorally optimal limits.

Studies of the calmodulin-binding site of twitchin with synthetic peptides using fluorescence and CD spectroscopy

The calcium-dependent interaction of two synthetic peptides derived from the putative calmodulin-binding site in the protein kinase autoinhibitory region of twitchin was studied by fluorescence and CD spectroscopy. The peptides interacted with dansylcalmodulin in the presence of Ca2+ as shown by a change in the fluorescence emission spectra. Fluorescence titration of dansylcalmodulin with the peptides was used to quantify this interaction. The peptides appeared to assume a helical conformation in a non-polar environment as seen by CD spectroscopy. The ellipticity of Ca2+ calmodulin was enhanced in the presence of peptides compared with that of Ca2+ calmodulin and peptides alone, indicating that the peptides had formed a complex with calmodulin. These results support the assignment of the twitchin calmodulin-binding site.

Phosphorylation of myosin regulatory light chains by the molluscan twitchin kinase

The unusually large (approximately 600 to > 3000 kDa) myosin-associated proteins of the titin/twitchin superfamily are considered to be important cytoskeletal rulers for thick filament assembly in muscle. This function is maintained by approximately 60-240 modular fibronectin-type-III and immunoglobulin-C2 repeats in these proteins which further contain a protein serine/threonine kinase domain of unknown function. In this study, the bacterially expressed kinase domain of Aplysia twitchin was used in order to identify a potential physiological substrate. Addition of the recombinant kinase to Aplysia actomyosin preparations resulted in the specific phosphorylation of the 19-kDa myosin regulatory light chains. The twitchin kinase phosphorylated purified light chains on Thr15 in a region which shared a high degree of similarity with the phosphorylation site for vertebrate smooth muscle myosin light chain kinase. Peptide analogs of the twitchin substrate sequence and the similar sequence in vertebrate smooth muscle myosin light chains were phosphorylated with good kinetic properties. These data reveal the first potential substrate for any of the giant protein kinases and support a dual role of twitchin in molluscan muscle as a cytoskeletal protein as well as a myosin light chain kinase.

Distribution in the central nervous system of Aplysia of afferent fibers arising from cell bodies located in the periphery

The present study using autoradiography to determine the location of the projections of presumptive peripheral afferent neurons into the central nervous system of Aplysia. Selected peripheral tissues (with an emphasis on structures involved in feeding behavior) were exposed to radioactive amino acids, and the distribution of macromolecules transported into the nervous system via afferent fibers was determined by autoradiography. Different regions of the body exhibited different patterns of projections, and, within the neuropil of the cerebral ganglion, there was a loose topographical organization of projections from the head. For some regions of the body, the projections was largely limited to the ganglion from which the nerve enters; for other regions, the projection was very widespread. In some cases (e.g., rhinophore to eye), there was evidence of projections from one peripheral structure to another. Experiments with all peripheral tissues that were studied resulted in extensive labeling of central ganglia, indicating that afferents with peripheral cell bodies may provide a major source of sensory input to the central nervous system and suggesting that many or all of the numerous ultrafine axons visualized via electron microscopy in the nerves of Aplysia may originate from first- or second-order sensory afferents whose cell bodies are located in the periphery.

Nine members of the myomodulin family of peptide cotransmitters at the B16-ARC neuromuscular junction of Aplysia

1. Neuromodulation by multiple related peptides with different spectra of physiological effects appears an effective way to integrate complex physiological functions. A good opportunity to examine this issue occurs in the accessory radula closer (ARC) neuromuscular circuit of Aplysia, where, extensive previous work has shown, acetylcholine-induced contractions of the muscle are variously modulated by several families of peptide cotransmitters released under appropriate behavioral circumstances from the muscle's own two motor neurons. 2. In this work we focused on the myomodulins (MMs) released from motor neuron B16. Previous work has characterized MMA (PMSMLRLamide) and MMB (GSYRMMRLamide). We now similarly purified from ARC neuromuscular material and sequenced MMC (GWSMLRLamide), MMD (GLSMLRLamide), MME (GLQMLRLamide), and MMF (SLNMLRLamide). Three additional MMs, MMG (TLSMLRLamide), MMH (GLHMLRLamide), and MMI (SLSMLRLamide), are encoded by a known MM gene. B16 probably synthesizes, and coreleases, all nine MMs. Further MMs have been found in other mollusks. All evidence indicates that the MMs are a major, widely distributed family of molluscan neuropeptides important as neuromuscular modulators and probably also central transmitters or modulators. 3. MM effects on motor neuron B16-elicited ARC muscle contractions were best analyzed as the sum of three distinct actions: potentiation, depression of the amplitude of the contractions, and acceleration of their relaxation rate. We compared the effectiveness of all nine MMs in these respects. We correlated this with their effectiveness in enhancing the L-type Ca current and activating a specific K current in voltage-clamped dissociated ARC muscle fibers, effects we previously proposed to underlie, respectively, the potentiation and the depression of contractions. 4. All nine MMs were similarly effective in enhancing the Ca current and, as far as it was possible to determine, potentiating the amplitude as well as accelerating the relaxation rate of the contractions. 5. In contrast, the MMs' ability to activate the K current and depress the contractions varied greatly. MMB and MMC, in particular, were weak, whereas the other seven MMs were considerably more effective in both respects. 6. Altogether, we were able to explain the potentiating and depressing strengths of the MMs by the magnitude of their modulation of the Ca and K currents, providing further support for our hypothesis that the effects on contraction amplitude are mediated by the effects on the two currents. 7. The net effect on contraction amplitude was determined by the balance between the potentiation and depression. Although most MM concentrations had both potentiating and depressing actions, potentiated contractions predominated at low and depressed contractions (but with accelerated relaxation rate) at high concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca(2+)-activated K current in the ARC muscle of Aplysia

1. This work continues our examination of the electrophysiology and contractions of single, functionally intact fibers dissociated from a widely studied molluscan muscle, the accessory radula closer (ARC) muscle of Aplysia californica, aimed at understanding its excitation-contraction mechanisms and their modulation. Extensive previous work has characterized a number of ion currents in the fibers. 2. Here we describe an additional major current that could not be studied earlier because, unlike all of the other currents in the ARC muscle fibers, it becomes prominent only during contraction of the fiber. It is a Ca(2+)-activated K current, associated with contraction most likely because both are activated by the same elevation in intracellular free Ca2+. 3. We used several manipulations to elicit the Ca(2+)-activated K current and contraction: depolarizing voltage steps in the presence of extracellular Ca2+, application of caffeine in the presence or absence of extracellular Ca2+ (and thus presumably working by releasing Ca2+ from intracellular stores), application of the Ca(2+)-ionophore A23187, and direct iontophoretic injection of Ca2+ into the fiber. 4. The Ca(2+)-activated K current reversed around -70 mV, not far from EK, and the reversal potential shifted substantially with elevated extracellular K+. Activation of the current was not only Ca2+ dependent, but also quite strongly voltage dependent, promoted by depolarization. The current was well blocked by tetraethylammonium (KD approximately 2 mM), but not blocked by even 10 mM 4-aminopyridine or low concentrations of the K-current blocking toxins charybdotoxin and apamin. 5. After a depolarizing voltage step in Ca(2+)-containing solution, the Ca(2+)-activated K current appeared, often with some delay, as a large peak of current that soon disintegrated into a prolonged period of individual oscillatory transients of Ca(2+)-activated K current, sometimes correlated with transient contractions. Similar transients could be elicited by caffeine or iontophoretic Ca2+ injection. More extensive study of the underlying Ca2+ dynamics will be presented elsewhere, but we interpret these phenomena in terms of our hypothesis that the ARC muscle generates both contraction and the Ca(2+)-activated K current by Ca(2+)-induced Ca2+ release (CICR), in which a small depolarization-induced influx of extracellular Ca2+ releases more Ca2+ from intracellular stores. 6. The Ca(2+)-activated K current is significant in the physiological operating voltage range of the ARC muscle, and its predicted hyperpolarizing action and consequent negative-feedback depression of contractions is likely to be an important part of the integrated set of mechanisms that regulate the muscle's contractility.

A population of SCP-containing neurons in the buccal ganglion of Aplysia are radula mechanoafferents and receive excitation of central origin

The rostral cluster of SCP-immunoreactive cells, originally identified in each buccal hemiganglion of juvenile Aplysia, was examined in mature specimens. Immunohistochemical and dye-fill experiments showed that each rostral cluster consists of approximately 40 cells. Although these neurons exhibit heterogeneity of size and shape, all cells project an axon into the radula nerve. Tracing of dye-filled cells showed that they project to the layer of tissue that lines the inner surface of the food-grasping portion of the chitinous radula. This tissue contains SCP-immunoreactive nerve fibers and varicosities in regions corresponding to the projections of dye-filled neurons. Several observations indicate that rostral cluster neurons transduce tactile stimuli applied to the radula surface: (1) each cell responds to touch of a circumscribed receptive field with a rapidly adapting burst of action potentials, (2) the evoked spikes arise abruptly from the resting potential without prepotentials, and (3) the responses persist when central and peripheral synaptic transmission is blocked in high Mg2+, low Ca2+ artificial seawater solutions. These cells, designated radula mechanoafferent (RM) neurons, do not respond to chemical stimuli including NaCl, glutamate, and seaweed extract. The highest density of receptive fields is found on the posterodorsal edges of the radula halves, areas most directly involved in grasping food. The RM neurons are electrically coupled cells, with coupling coefficients ranging from 0.006 to 0.22. They fire phasically during buccal motor programs, even in the absence of peripheral feedback from the radula or other portions of the buccal mass. In radiolabeling studies the RM cells were found to synthesize authentic SCPA and SCPB. Sensorin-A, a peptide that is localized to other Aplysia mechanoafferent neurons, was not detected immunohistochemically in these cells.

FRF peptides in the ARC neuromuscular system of Aplysia: purification and physiological actions

1. One preparation that has proven to be advantageous for the study of neuromuscular modulation is the accessory radula closer (ARC) muscle of Aplysia californica and its motor neurons B15 and B16. In this study three members of a new peptide family have been purified from this well-characterized preparation. Because these peptides terminate in Phe-Arg-Phe-amide, we have named them FRFA, FRFB, and FRFC. The FRFs are thus RFamide peptides and are related to the widely studied neuropeptide FMRFamide. 2. The FRFs are present in the ARC motor neuron B15 in small quantities. 3. When they are exogenously applied, the FRFs decrease the size of ARC muscle contractions elicited by stimulation of either motor neuron B15 or B16. They appear to do this by a combination of presynaptic and postsynaptic actions. 4. Presynaptically, the FRFs appear to act like the buccalins, another family of inhibitory ARC neuropeptides. Both families of peptides reduce the size of motor neuron-elicited excitatory junction potentials (EJPs) presumably by decreasing presynaptic acetylcholine (ACh) release. 5. Postsynaptically, the FRFs appear to depress contractions because they activate a characteristic voltage-dependent, 4-amino-pyridine-sensitive K current in the ARC muscle. The same current is activated by a second class of ARC modulators: those that exert potentiating actions at low doses and inhibitory actions at high doses, i.e., serotonin, the small cardioactive peptides (SCPs), and particularly the myomodulins. Receptors mediating activation of the K current by the FRFs and the other modulators do, however, appear to be different. 6. We hypothesize that the inhibitory actions of the FRFs prevent excessively large muscle contractions. If contraction size is limited, then contraction duration is also limited. This may allow faster and more energetically favorable switching between contractions of antagonistic muscles.

Myomodulin application increases cAMP and activates cAMP-dependent protein kinase in the accessory radula closer muscle of Aplysia

Myomodulin A (MMA) application or stimulation of neuron B16, which releases MMA, increases cAMP levels in the accessory radula closer (ARC) muscle of Aplysia. MMA application also increases cAMP-dependent protein kinase (cAPK) activity in one subcellular compartment of the muscle. These results suggest that at least part of MMA's effects in this system are mediated via the cAPK signal transduction pathway. Since the effects of the small cardioactive peptides (SCPs) on ARC muscle contraction are similar to those of MMA, our results suggest that the convergent physiological effects of MMA and SCPB in this system may be due, in part, to the two peptide neuromodulators utilizing the same signal transduction pathway.

SCP application or B15 stimulation activates cAPK in the ARC muscle of Aplysia

Application of small cardioactive peptide (SCP) or stimulation of motorneuron B15 increases the level of activated cAMP-dependent protein kinase (cAPK) in the ARC muscle. SCP application also appears to induce a translocation of cAPK between different subcellular compartments of the ARC muscle and this translocation is also induced by cAMP addition to muscle homogenates. These results suggest that the actions of SCP in the Aplysia ARC neuromuscular system are mediated via the cAPK signal transduction pathway.

cAMP-dependent phosphorylation of Aplysia twitchin may mediate modulation of muscle contractions by neuropeptide cotransmitters

Acting through a cAMP-cAMP-dependent protein kinase (cAPK) cascade, members of two neuropeptide families, the small cardioactive peptides and myomodulins, modulate contraction amplitude and relaxation rate in the accessory radula closer (ARC) muscle of the marine mollusc Aplysia californica. An approximately 750-kDa phosphoprotein was identified in the ARC muscle as the major substrate for cAPK activated either by application of neuropeptides or by peptides released by motorneuron stimulation at physiological frequencies. Immunoblot and immunoelectron microscopy experiments revealed the widespread presence of this protein in Aplysia muscles and its colocalization with contractile filaments in the ARC muscle. Sequence analysis of proteolytic peptide fragments derived from the protein indicated that it is structurally related to the muscle protein twitchin. Finally, the level of neuropeptide-induced phosphorylation of the protein correlated well with peptidergic modulation of the relaxation rate of the muscle. We propose that twitchin in Aplysia, and perhaps in other species, may mediate the modulation of the relaxation rate of muscle contractions.

Autophosphorylation of molluscan twitchin and interaction of its kinase domain with calcium/calmodulin

An approximately 750-kDa member of the family of giant titin/twitchin-like myosin-associated proteins was highly purified from muscle of the marine mollusc Aplysia californica. Purified twitchin was able to autophosphorylate on threonine, which demonstrates its protein serine/threonine kinase activity. cDNA sequence analysis of the cloned kinase domain of molluscan twitchin revealed that it is most closely related with the kinase domains of Caenorhabditis elegans twitchin (62% identity) and vertebrate myosin light chain kinases (45% average identity). Analysis of the cDNA sequence further suggested the presence of a potential calmodulin-binding site in a putative autoinhibitory region. The functional activity of this site was demonstrated by the calcium-dependent binding of purified twitchin to immobilized calmodulin and the fact that this interaction could be competed with synthetic peptides deduced from the cDNA sequence. Furthermore, biotinylated calmodulin bound to immobilized twitchin in gel-overlay assays with nanomolar affinity (EC50 approximately equal to 70 nM). The potential regulation of twitchin by calcium/calmodulin indicates that titin-like molecules may serve dynamic functions during contraction-relaxation cycles in muscle in addition to their functions as cytoskeletal proteins.

Activation of K current in the accessory radula closer muscle of Aplysia californica by neuromodulators that depress its contractions

The neural and cellular mechanisms of plasticity apparent in the feeding behavior of the mollusk Aplysia californica have been extensively studied in a simple neuromuscular circuit consisting of the accessory radula closer (ARC) muscle and its innervating motor and modulatory neurons. In this circuit, the plasticity is largely due to modulation of the amplitude and duration of the contractions of the muscle by a variety of modulatory neurotransmitters and peptide cotransmitters, among them the small cardioactive peptides (SCPs), myomodulins (MMs), and serotonin (5-HT). We have studied dissociated but functionally intact ARC muscle fibers to determine whether modulation of membrane ion currents in the muscle might underlie these effects. Using voltage-clamp techniques, we found that two currents were indeed modulated. In the preceding article, we proposed that enhancement of "L"-type Ca current is the mechanism by which the modulators potentiate the amplitude of ARC-muscle contractions. Here, we report that the modulators also activate a distinctive K current. Large K currents were activated, in particular, by MMA, while MMB, the SCPs, and 5-HT activated much smaller currents most likely of the same kind. Buccalins, modulators that do not act directly on the ARC muscle, were ineffective. The modulator-induced K current was strongly enhanced by depolarization, but relatively slowly so that its amplitude continued to increase for several hundred milliseconds following a depolarizing voltage step. The current was Ca2+ independent, not readily blocked by extracellular Cs+ or Ba2+ and only by high concentrations of tetraethylammonium. However, it was almost completely blocked by as little as 10 microM 4-aminopyridine. In contrast to the modulator-induced enhancement of Ca current, activation of the K current was not significantly mimicked by elevation of cAMP. In the intact as well as the dissociated ARC muscle, although low levels of all of the modulators potentiate contractions, even moderate levels of MMA strongly depress them, whereas the other modulators depress them weakly only at high concentrations. The modulator-induced K current appears well suited to counteract depolarization of the muscle and thus limit activation of the "L"-type Ca current that provides Ca2+ essential for contraction. We therefore propose that the modulators depress ARC-muscle contractions in large part by activating the K current. This occurs simultaneously with the enhancement of the Ca current; net potentiation or depression then depends on the balance between the relative strengths of the modulation of the two ion currents.

Enhancement of Ca current in the accessory radula closer muscle of Aplysia californica by neuromodulators that potentiate its contractions

A major goal of neuroscience is to identify the neural and cellular mechanisms of behavior and its plasticity. Progress toward this goal has come particularly from work with a small number of tractable model preparations. One of these is the simple neuromuscular circuit consisting of the accessory radula closer (ARC) muscle of the mollusk Aplysia californica and its innervating motor and modulatory neurons. Contraction of the ARC muscle underlies a component of Aplysia feeding behavior, and plasticity of the behavior is in large part due to modulation of the amplitude and duration of the contractions of the muscle by a variety of modulatory neurotransmitters and peptide cotransmitters, among them the small cardioactive peptides (SCPs), myomodulins (MMs), and serotonin (5-HT). We have studied single dissociated ARC muscle fibers in order to determine whether modulation of membrane ion currents in the muscle might underlie these effects. First, we confirmed that the dissociated fibers were functionally intact: just as with the whole ARC muscle, their contractions were potentiated by 5-HT and SCPB and potentiated as well as depressed by MMA, and their cAMP content was greatly elevated by 5-HT, SCPA and SCPB, and to a lesser extent by MMA and MMB. Next, using voltage-clamp techniques, we found that two ion currents present in the fibers were indeed modulated. The fibers possess a dihydropyridine-sensitive, high-threshold "L"-type Ca current. This current was enhanced by the modulators that potentiate ARC-muscle contractions--5-HT, SCPA and SCPB, and MMA and MMB--but not by buccalinA, a modulator that does not act directly on the ARC muscle. All of the potentiating modulators, as well as elevation of cAMP in the fibers by forskolin or a cAMP analog, maximally enhanced the current about twofold and mutually occluded each other's effects. Since the Ca current supplies Ca2+ necessary for contraction of the muscle, the enhancement of the current is a good candidate to be a major mechanism of the potentiation of the contractions. In the following article we report that the modulators also, to different degrees, activate a distinctive K current and thereby depress the contractions. Net potentiation or depression then depends on the balance between the relative strengths of the modulation of the two ion currents.

Body postural muscles active during food arousal in Aplysia are modulated by diverse neurons that receive monosynaptic excitation from the neuron C-PR

1. We previously found that identified neuron C-PR may mediate the appetitive feeding posture of Aplysia by actions on appropriate motor neurons and perhaps on modulatory neurons innervating the foot and neck. In the present experiments, we attempted to further investigate this hypothesis by characterizing the modulatory neurons that are excited by CP-R. 2. We identified three types of modulatory neurons all of which are excited, at least in part, by monosynaptic excitatory connections from C-PR. 3. The cell bodies of these neurons are located in the posterior region of the pedal ganglion. 4. The neurons send axons to muscles, but rather than producing contractions, they enhance, depress, or alter the relaxation rate of contractions produced by motor neurons. Each of these types of modulatory neurons produces a highly specific effect in terms of the region of the body affected and the nature of the modulation. 5. The primary effect of P1R-E neurons was to enhance longitudinal contractions of the anterior foot. 6. P1R-D neurons depressed longitudinal and transverse contractions of the anterior foot. 7. P8R neurons enhanced longitudinal and transverse contractions of the neck. 8. The results obtained from extracellular recordings of muscle junction potentials suggest that the firing of the modulatory neurons may enhance or depress muscle contractions, at least in part, by increasing or decreasing the size of the excitatory input the motor neurons produce on the appropriate muscles. These changes in excitatory drive to the muscle are likely to underlie, at least in part, the alterations in contraction size produced by the modulatory neurons, but changes in relaxation rate are likely related to other actions of the modulatory neurons. 9. We have evidence for at least nine neurons that modulate the foot or neck and are excited by C-PR, and it is very likely that there are more, perhaps considerably more, of these types of neurons. In fact, it appears as if a significant proportion of the efferent output to the muscles that mediate the appetitive phase of feeding consists of modulatory output rather than conventional motor neuron output that produces discreet contractions.

Characterization of the membrane ion currents of a model molluscan muscle, the accessory radula closer muscle of Aplysia californica. III. Depolarization-activated Ca current

1. The accessory radula closer (ARC) muscle of Aplysia californica and its innervation is a model preparation for the study of the neural and cellular mechanisms of behavioral plasticity. Much of the plasticity is due to modulation of contractions of the muscle by a variety of neurotransmitters and peptide cotransmitters. Preliminary to investigating the cellular mechanisms of this modulation, we have characterized the major membrane ion currents present in the unmodulated ARC muscle and their likely roles in normal contraction. We have studied single dissociated but functionally intact ARC muscle fibers under voltage clamp. This is the last of three papers describing this work. In the first paper we characterized two currents prominent at hyperpolarized voltages, a classical inwardly rectifying K current and a Cl current induced by elevated intracellular Cl-. In the second paper we examined two large outward K currents activated at more depolarized voltages, an "A" current and a delayed rectifier. 2. In this paper, we describe an inward depolarization-activated Ca current that underlies and is normally completely masked by the K currents and is revealed when they are blocked. 3. The Ca current begins to activate above -40 or -30 mV. It is fully available for activation at voltages more negative than -60 mV. It activates in milliseconds, then inactivates relatively slowly with maintained depolarization. The current is larger and inactivates slower when it is carried by Ba2+ rather than Ca2+. The inactivation is current rather than voltage dependent. The current is blocked by Co2+, Cd2+, and with a characteristic time dependence, by the dihydropyridine Ca-channel antagonist nifedipine. 4. These properties of the current characterize it as a high-threshold, L-type Ca current. 5. This current most likely provides Ca2+ necessary for contraction of the ARC muscle.

Characterization of the membrane ion currents of a model molluscan muscle, the accessory radula closer muscle of Aplysia california. I. Hyperpolarization-activated currents

1. The simple neuromuscular circuit consisting of the accessory radula closer (ARC) muscle of the mollus Aplysia californica together with its innervating motor and modulatory neurons has been extensively studied as a model preparation in which it might be possible to reach an integrated understanding of the neural and cellular mechanisms of behavioral plasticity, in this case of a component of Aplysia feeding behavior. Previous work has suggested that much of the plasticity of this behavior is implemented by appropriate release of modulatory neurotransmitters and peptide cotransmitters that modulate several parameters of the contractions of the ARC muscle. However, little is as yet known about the underlying cellular mechanisms. 2. We have begun to study single, functionally intact fibers dissociated from the ARC muscle to assess to what extent the modulation of its contraction might be mediated by one candidate mechanism, modulation of its membrane ion currents. First, however, it was necessary to gain a thorough understanding of the unmodulated currents and their likely roles in normal contraction. Using voltage-clamp techniques, we have therefore identified and characterized the major currents present in the ARC muscle fibers. We describe these currents in this and the following two papers. These results constitute the first detailed description of ion currents in a molluscan muscle and lay the foundation for further study, to be presented in subsequent papers, of the roles of two currents that we have indeed found to be modulated in ways likely to contribute to the modulation of contraction. 3. In this paper we first describe the general electrophysiological characteristics of the dissociated fibers and present evidence that the fibers can be adequately space clamped. 4. The physiological operating voltage range of the nonspiking ARC muscle most likely extends from about -80 to about -25 mV. The steady-state current-voltage (I-V) relation of total membrane current rectifies inwardly in the negative and outwardly in the positive portion of this voltage range, with a plateau region of high or even negative slope resistance separating the two regions of rectification. 5. The current responsible for the inward rectification at negative voltages is a classical inwardly rectifying K current. It is activated by hyperpolarization with quasi-instantaneous kinetics; its whole I-V relation shifts along the voltage axis in a Nernstian manner with altered extracellular K+ concentration; it is blocked by low extracellular Ba2+ and Cs+, and the block is promoted by hyperpolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the membrane ion currents of a model molluscan muscle, the accessory radula closer muscle of Aplysia californica. II. Depolarization-activated K currents

1. The accessory radula closer (ARC) muscle of Aplysia californica and its innervation is a model preparation for the study of the neural and cellular mechanisms of behavioral plasticity. Much of the plasticity is mediated by release of neurotransmitters and peptide cotransmitters that modulate contractions of the muscle. Preliminary to investigating the cellular mechanisms of action of these modulators, we have characterized the major membrane ion currents present in the unmodulated ARC muscle and their likely roles in normal contraction. We have studied single dissociated but functionally intact ARC muscle fibers under voltage clamp. This is the second of three papers describing this work. In the preceding paper we described the electrophysiological properties of the fibers at hyperpolarized voltages, and characterized the two major hyperpolarized-activated currents present, a classical inwardly rectifying K current and a Cl current induced by elevated intracellular Cl-. 2. In this paper we dissect the large outward current that becomes activated when the fibers are depolarized above -50 or -40 mV. We find that this current consists of two major depolarization-activated K currents, a fast transient "A"-type current and a slower maintained delayed rectifier, with perhaps a small component of Ca(2+)-activated K current. 3. The A current begins to activate with voltage steps above -50 or -40 mV. It activates in milliseconds, then inactivates virtually completely within 100-200 ms. It is fully available for activation below -80 mV, and almost completely inactivated above -40 mV. It is Ca2+ independent, half-maximally blocked by approximately 3 mM 4-aminopyridine (4-AP) but only 460 mM tetraethylammonium (TEA). 4. The delayed rectifier both activates and inactivates more slowly and more positive than the A current. Thus it begins to activate only above -30 or -20 mV; it activates in tens of milliseconds, then inactivates incompletely over several seconds; it is fully available below -70 mV and inactivated above 0 mV. It is Ca2+ independent, half-maximally blocked by 10 mM TEA and 3-10 mM 4-AP. 5. In the following paper we describe a depolarization-activated Ca current that underlies the K currents and most likely provides Ca2+ necessary for contraction of the muscle. By activating simultaneously with the Ca current, the K currents serve to prevent spikes, so that the depolarization is confined to a range where small voltage changes provide fine control over a wide range of contraction strengths.

Structure, localization, and action of buccalin B: a bioactive peptide from Aplysia

The cholinergic motor neurons for the accessory radula closer (ARC) contain several neuropeptides that affect muscle contractions. In the present study, we have purified and sequenced a sixth ARC neuropeptide, using a combination of high pressure liquid chromatography and bioassays. This neuropeptide, Gly-Leu-Asp-Arg-Tyr-Gly-Phe-Val-Gly-Gly-Leu-amide, has been named buccalin B (BUCb) because it is significantly homologous to the previously characterized neuropeptide buccalin A. BUCb was found to be two-three times more potent than buccalin A in depressing motor neuron induced contractions.

Dopaminergic neuron B20 generates rhythmic neuronal activity in the feeding motor circuitry of Aplysia

We have identified a buccal neuron (B20) that exhibits dopamine-like histofluorescence and that can drive a rhythmic motor program of the feeding motor circuitry of Aplysia. The cell fires vigorously during episodes of patterned buccal activity that occur spontaneously, or during buccal programs elicited by stimulation of identified cerebral command-like neurons for feeding motor programs. Preventing B20 from firing, or firing B20 at inappropriate times, can modify the program driven by the cerebral feeding command-like neuron CBI-2. When B20 is activated by means of constant depolarizing current it discharges in phasic bursts, and evokes a sustained coordinated rhythmic buccal motor program. The program incorporates numerous buccal and cerebral neurons associated with aspects of feeding responses. The B20-driven program can be reversibly blocked by the dopamine-antagonist ergonovine, suggesting that dopamine may be causally involved in the generation of the program. Although firing of B20 evokes phasic activity in cerebral command-like neurons, the presence of the cerebral ganglion is not necessary for B20 to drive the program. The data are consistent with the notion that dopaminergic neuron B20 is an element within the central pattern generator for motor programs associated with feeding.

Effects of cerebral neuron C-PR on body postural muscles associated with a food-induced arousal state in Aplysia

1. Firing of cerebral neuron, C-PR, produced complex bilateral movements of various regions of the body of the marine mollusc Aplysia californica. The movements were similar to those seen when the animal assumes the head-up feeding posture during food-induced arousal. Muscles of the neck largely contracted in transverse and longitudinal directions, and large transverse movements were also induced in the middle part of the foot. On the other hand, firing of C-PR appeared to relax the anterior part of the foot in transverse and longitudinal directions. 2. We identified pedal-ganglion motor neurons that innervate various regions of the animal, and explored the synaptic connections of C-PR with these neurons. Firing of C-PR produced synaptic potentials bilaterally in most of the identified motor neurons. 3. Motor neurons for the neck were largely excited by C-PR firing. C-PR firing also excited the motor neurons that produce transverse movements of the middle part of the foot. On the other hand, C-PR inhibited the spontaneous spike activity of the motor neurons for the anterior part of the foot. 4. One neck motor neuron was found to receive a monosynaptic excitatory postsynaptic potential (EPSP) from C-PR, but the postsynaptic potentials (PSPs) induced by C-PR in the other identified motor neurons were mediated polysynaptically. 5. We also found that the C-PR can modulate movements evoked by firing of the motor neurons for the ipsilateral neck and anterior foot. C-PR enhanced both transverse and longitudinal contractions of the neck. 6. For the anterior foot region, C-PR had different modulatory effects on the longitudinal and the transverse contractions. C-PR largely enhanced or initially depressed and then enhanced longitudinal contractions, whereas C-PR depressed transverse contractions. 7. The overall results support the hypothesis that C-PR is involved in controlling the head-up posture when the animal is aroused by food.

The myomodulin-related neuropeptides: characterization of a gene encoding a family of peptide cotransmitters in Aplysia

The myomodulin-related peptides comprise a family of cotransmitters that modulate neuromuscular signaling in the feeding system of Aplysia. In this study, cDNA clones encoding a myomodulin precursor polypeptide were isolated and characterized. This precursor contains seven different myomodulin-related peptides, one of which, myomodulin A, is present in 10 contiguous copies. The sequence of a myomodulin genomic clone indicates that all of these myomodulin-related peptides are encoded on a single exon. The myomodulin gene is expressed in a tissue-specific manner and myomodulin mRNA is localized to specific neurons in the Aplysia CNS. The presence of multiple related neuropeptides can greatly increase the range and precision of signaling at synapses where they act as modulator cotransmitters.