Acetylcholine-evoked afterdischarge in Aplysia bag cell neurons

Hydrogen peroxide and phosphoinositide metabolites synergistically regulate a cation current to influence neuroendocrine cell bursting

In various neurons, including neuroendocrine cells, non-selective cation channels elicit plateau potentials and persistent firing. Reproduction in the marine snail Aplysia californica is initiated when the neuroendocrine bag cell neurons undergo an afterdischarge, that is, a prolonged period of enhanced excitability and spiking during which egg-laying hormone is released into the blood. The afterdischarge is associated with both the production of hydrogen peroxide (H₂ O₂ ) and activation of phospholipase C (PLC), which hydrolyses phosphatidylinositol-4,5-bisphosphate into diacylglycerol (DAG) and inositol trisphosphate (IP₃ ). We previously demonstrated that H₂ O₂ gates a voltage-dependent cation current and evokes spiking in bag cell neurons. The present study tests if DAG and IP₃ impact the H₂ O₂ -induced current and excitability. In whole-cell voltage-clamped cultured bag cell neurons, bath-application of 1-oleoyl-2-acetyl-sn-glycerol (OAG), a DAG analogue, enhanced the H₂ O₂ -induced current, which was amplified by the inclusion of IP₃ in the pipette. A similar outcome was produced by the PLC activator, N-(3-trifluoromethylphenyl)-2,4,6-trimethylbenzenesulfonamide. In current-clamp, OAG or OAG plus IP₃ , elevated the frequency of H₂ O₂ -induced bursting. PKC is also triggered during the afterdischarge; when PKC was stimulated with phorbol 12-myristate 13-acetate, it caused a voltage-dependent inward current with a reversal potential similar to the H₂ O₂ -induced current. Furthermore, PKC activation followed by H₂ O₂ reduced the onset latency and increased the duration of action potential firing. Finally, inhibiting nicotinamide adenine dinucleotide phosphate oxidase with 3-benzyl-7-(2-benzoxazolyl)thio-1,2,3-triazolo[4,5-d]pyrimidine diminished evoked bursting in isolated bag cell neuron clusters. These results suggest that reactive oxygen species and phosphoinostide metabolites may synergize and contribute to reproductive behaviour by promoting neuroendocrine cell firing. KEY POINTS: Aplysia bag cell neurons secrete reproductive hormone during a lengthy burst of action potentials, known as the afterdischarge. During the afterdischarge, phospholipase C (PLC) hydrolyses phosphatidylinositol-4,5-bisphosphate into diacylglycerol (DAG) and inositol trisphosphate (IP₃ ). Subsequent activation of protein kinase C (PKC) leads to H₂ O₂ production. H₂ O₂ evokes a voltage-dependent inward current and action potential firing. Both a DAG analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG), and IP₃ enhance the H₂ O₂ -induced current, which is mimicked by the PLC activator, N-(3-trifluoromethylphenyl)-2,4,6-trimethylbenzenesulfonamide. The frequency of H₂ O₂ -evoked afterdischarge-like bursting is augmented by OAG or OAG plus IP₃ . Stimulating PKC with phorbol 12-myristate 13-acetate shortens the latency and increases the duration of H₂ O₂ -induced bursts. The nicotinamide adenine dinucleotide phosphate oxidase inhibitor, 3-benzyl-7-(2-benzoxazolyl)thio-1,2,3-triazolo[4,5-d]pyrimidine, attenuates burst firing in bag cell neuron clusters.

Hydrogen Peroxide Gates a Voltage-Dependent Cation Current in Aplysia Neuroendocrine Cells

Nonselective cation channels promote persistent spiking in many neurons from a diversity of animals. In the hermaphroditic marine-snail, Aplysia californica, synaptic input to the neuroendocrine bag cell neurons triggers various cation channels, causing an ∼30 min afterdischarge of action potentials and the secretion of egg-laying hormone. During the afterdischarge, protein kinase C is also activated, which in turn elevates hydrogen peroxide (H₂O₂), likely by stimulating nicotinamide adenine dinucleotide phosphate oxidase. The present study investigated whether H₂O₂ regulates cation channels to drive the afterdischarge. In single, cultured bag cell neurons, H₂O₂ elicited a prolonged, concentration- and voltage-dependent inward current, associated with an increase in membrane conductance and a reversal potential of ∼+30 mV. Compared with normal saline, the presence of Ca²⁺-free, Na⁺-free, or Na⁺/Ca²⁺-free extracellular saline, lowered the current amplitude and left-shifted the reversal potential, consistent with a nonselective cationic conductance. Preventing H₂O₂ reduction with the glutathione peroxidase inhibitor, mercaptosuccinate, enhanced the H₂O₂-induced current, while boosting glutathione production with its precursor, N-acetylcysteine, or adding the reducing agent, dithiothreitol, lessened the response. Moreover, the current generated by the alkylating agent, N-ethylmaleimide, occluded the effect of H₂O₂ The H₂O₂-induced current was inhibited by tetrodotoxin as well as the cation channel blockers, 9-phenanthrol and clotrimazole. In current-clamp, H₂O₂ stimulated burst firing, but this was attenuated or prevented altogether by the channel blockers. Finally, H₂O₂ evoked an afterdischarge from whole bag cell neuron clusters recorded ex vivo by sharp-electrode. H₂O₂ may regulate a cation channel to influence long-term changes in activity and ultimately reproduction.SIGNIFICANCE STATEMENT Hydrogen peroxide (H₂O₂) is often studied in a pathological context, such as ischemia or inflammation. However, H₂O₂ also physiologically modulates synaptic transmission and gates certain transient receptor potential channels. That stated, the effect of H₂O₂ on neuronal excitability remains less well defined. Here, we examine how H₂O₂ influences Aplysia bag cell neurons, which elicit ovulation by releasing hormones during an afterdischarge. These neuroendocrine cells are uniquely identifiable and amenable to recording as individual cultured neurons or a cluster from the nervous system. In both culture and the cluster, H₂O₂ evokes prolonged, afterdischarge-like bursting by gating a nonselective voltage-dependent cationic current. Thus, H₂O₂, which is generated in response to afterdischarge-associated second messengers, may prompt the firing necessary for hormone secretion and procreation.

A Closely Associated Phospholipase C Regulates Cation Channel Function through Phosphoinositide Hydrolysis

In the hemaphroditic sea snail, Aplysia californica, reproduction is initiated when the bag cell neurons secrete egg-laying hormone during a protracted afterdischarge. A source of depolarization for the afterdischarge is a voltage-gated, nonselective cation channel, similar to transient receptor potential (TRP) channels. Once the afterdischarge is triggered, phospholipase C (PLC) is activated to hydrolyze phosphatidylinositol-4,5-bisphosphate (PIP₂) into diacylglycerol (DAG) and inositol trisphosphate (IP₃). We previously reported that a DAG analog, 1-oleoyl-2-acetyl-sn-glycerol (OAG), activates a prominent, inward whole-cell cationic current that is enhanced by IP₃ To examine the underlying mechanism, we investigated the effect of exogenous OAG and IP₃, as well as PLC activation, on cation channel activity and voltage dependence in excised, inside-out patches from cultured bag cell neurons. OAG transiently elevated channel open probability (P_O) when applied to excised patches; however, coapplication of IP₃ prolonged the OAG-induced response. In patches exposed to OAG and IP₃, channel voltage dependence was left-shifted; this was also observed with OAG, but not to the same extent. Introducing the PLC activator, m-3M3FBS, to patches increased channel P_O, suggesting PLC may be physically linked to the channels. Accordingly, blocking PLC with U-73122 ablated the m-3M3FBS-induced elevation in P_O Treatment with m-3M3FBS left-shifted cation channel voltage dependence to a greater extent than exogenous OAG and IP₃ Finally, OAG and IP₃ potentiated the stimulatory effect of PKC, which is also associated with the channel. Thus, the PLC-PKC signaling system is physically localized such that PIP₂ breakdown products liberated during the afterdischarge modulate the cation channel and temporally influence neuronal activity.SIGNIFICANCE STATEMENT Using excised patches from Aplysia bag cell neurons, we present the first evidence of a nonselective cation channel physically associating with phospholipase C (PLC) at the single-channel level. PLC-mediated breakdown of phospholipids generates diacylglycerol and inositol trisphosphate, which activate the cation channel. This is mimicked by exogenous lipids; furthermore, these second messengers left-shift channel voltage dependence and enhance the response of the channel to protein kinase C. PLC-mediated lipid signaling controls single-channel currents to ensure depolarization is maintained for an extended period of firing, termed the afterdischarge, when the bag cell neurons secrete egg-laying hormone to trigger reproduction.

Tyrosine Phosphorylation Determines Afterdischarge Initiation by Regulating an Ionotropic Cholinergic Receptor

Changes to neuronal activity often involve a rapid and precise transition from low to high excitability. In the marine snail, Aplysia, the bag cell neurons control reproduction by undergoing an afterdischarge, which begins with synaptic input releasing acetylcholine to open an ionotropic cholinergic receptor. Gating of this receptor causes depolarization and a shift from silence to continuous action potential firing, leading to the neuroendocrine secretion of egg-laying hormone and ovulation. At the onset of the afterdischarge, there is a rise in intracellular Ca²⁺, followed by both protein kinase C (PKC) activation and tyrosine dephosphorylation. To determine whether these signals influence the acetylcholine ionotropic receptor, we examined the bag cell neuron cholinergic response both in culture and isolated clusters using whole-cell and/or sharp-electrode electrophysiology. The acetylcholine-induced current was not altered by increasing intracellular Ca²⁺ via voltage-gated Ca²⁺ channels, clamping intracellular Ca²⁺ with exogenous Ca²⁺ buffers, or activating PKC with phorbol esters. However, lowering phosphotyrosine levels by inhibiting tyrosine kinases both reduced the cholinergic current and prevented acetylcholine from triggering action potentials or afterdischarge-like bursts. In other systems, acetylcholine receptors are often modulated by multiple signals, but bag cell neurons appear to be more restrictive in this regard. Prior work finds that, as the afterdischarge proceeds, tyrosine dephosphorylation leads to biophysical alterations that promote persistent firing. Because this firing is subsequent to the cholinergic input, inhibiting the acetylcholine receptor may represent a means of properly orchestrating synaptically induced changes in excitability.

Protein Kinase C Enhances Electrical Synaptic Transmission by Acting on Junctional and Postsynaptic Ca2+ Currents

By synchronizing neuronal activity, electrical transmission influences the coordination, pattern, and/or frequency of firing. In the hemaphroditic marine-snail, Aplysia calfornica, the neuroendocrine bag cell neurons use electrical synapses to synchronize a 30 min afterdischarge of action potentials for the release of reproductive hormone. During the afterdischarge, protein kinase C (PKC) is activated, although its impact on bag cell neuron electrical transmission is unknown. This was investigated here by monitoring electrical synapses between paired cultured bag cell neurons using dual whole-cell recording. Voltage clamp revealed a largely voltage-independent junctional current, which was enhanced by treating with a PKC activator, PMA, before recording. We also examined the transfer of presynaptic action potential-like waveforms (generated in voltage clamp) to the postsynaptic cell (measured in current clamp). For control pairs, the presynaptic spike-like waveforms mainly evoked electrotonic potentials; however, when PKC was triggered, these stimuli consistently produced postsynaptic action potentials. To assess whether this involved changes to postsynaptic responsiveness, single bag cell neurons were injected with junctional-like current mimicking that evoked by a presynaptic action potential. Unlike control neurons, which were less likely to spike, cells in PMA always fired action potentials to the junctional-like current. Furthermore, PKC activation increased a postsynaptic voltage-gated Ca²⁺ current, which was recruited even by modest depolarization associated with an electrotonic potential. Whereas PKC inhibits gap junctions in most systems, bag cell neurons are rather unique, as the kinase potentiates the electrical synapse; in turn, this synergizes with augmented postsynaptic Ca²⁺ current to promote synchronous firing.SIGNIFICANCE STATEMENT Electrical coupling is a fundamental form of communication. For the bag cell neurons of Aplysia, electrical synapses coordinate a prolonged burst of action potentials known as the afterdischarge. We looked at how protein kinase C, which is upregulated with the afterdischarge, influences information transfer across the synapse. The kinase activation increased junctional current, a remarkable finding given that this enzyme is largely considered inhibitory for gap junctions. There was also an augmentation in the ability of a presynaptic neuron to provoke postsynaptic action potentials. This increased excitability was, in part, due to enhanced postsynaptic voltage-dependent Ca²⁺ current. Thus, protein kinase C improves the fidelity of electrotonic transmission and promotes synchronous firing by modulating both junctional and membrane conductances.

Nicotinic acetylcholine receptor inhibitors derived from snake and snail venoms

The nicotinic acetylcholine receptor (nAChR) represents the prototype of ligand-gated ion channels. It is vital for neuromuscular transmission and an important regulator of neurotransmission. A variety of toxic compounds derived from diverse species target this receptor and have been of elemental importance in basic and applied research. They enabled milestone discoveries in pharmacology and biochemistry ranging from the original formulation of the receptor concept, the first isolation and structural analysis of a receptor protein (the nAChR) to the identification, localization, and differentiation of its diverse subtypes and their validation as a target for therapeutic intervention. Among the venom-derived compounds, α-neurotoxins and α-conotoxins provide the largest families and still represent indispensable pharmacological tools. Application of modified α-neurotoxins provided substantial structural and functional details of the nAChR long before high resolution structures were available. α-bungarotoxin represents not only a standard pharmacological tool and label in nAChR research but also for unrelated proteins tagged with a minimal α-bungarotoxin binding motif. A major advantage of α-conotoxins is their smaller size, as well as superior selectivity for diverse nAChR subtypes that allows their development into ligands with optimized pharmacological and chemical properties and potentially novel drugs. In the following, these two groups of nAChR antagonists will be described focusing on their respective roles in the structural and functional characterization of nAChRs and their development into research tools. In addition, we provide a comparative overview of the diverse α-conotoxin selectivities that can serve as a practical guide for both structure activity studies and subtype classification. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Diacylglycerol-mediated regulation of Aplysia bag cell neuron excitability requires protein kinase C

KEY POINTS

In Aplysia, reproduction is initiated by the bag cell neurons and a prolonged period of enhanced excitability known as the afterdischarge. Phosphoinositide turnover is upregulated during the afterdischarge resulting in the hydrolysis of phosphatidylinositol-4,5-bisphosphate by phospholipase C (PLC) and the release of diacylglycerol (DAG) and inositol trisphosphate (IP3 ). In whole-cell voltage-clamped cultured bag cell neurons, 1-oleoyl-2-acetyl-sn-glycerol (OAG), a synthetic DAG analogue, activates a dose-dependent, transient, inward current (IOAG ) that is enhanced by IP3 , mimicked by PLC activation and dependent on basal protein kinase C (PKC) activity. OAG depolarizes bag cell neurons and triggers action potential firing in culture, and prolongs electrically stimulated afterdischarges in intact bag cell neuron clusters ex vivo. Although PKC alone cannot activate the current, it is required for IOAG ; this is the first description of required obligate PKC activity working in concert with PLC, DAG and IP3 to maintain the depolarization required for prolonged excitability in Aplysia reproduction.

ABSTRACT

Following synaptic input, the bag cell neurons of Aplysia undergo a long-term afterdischarge of action potentials to secrete egg-laying hormone and initiate reproduction. Early in the afterdischarge, phospholipase C (PLC) hydrolyses phosphatidylinositol-4,5-bisphosphate into inositol trisphosphate (IP3 ) and diacylglycerol (DAG). In Aplysia, little is known about the action of DAG, or any interaction with IP3 ; thus, we examined the effects of a synthetic DAG analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG), on whole-cell voltage-clamped cultured bag cell neurons. OAG induced a large, prolonged, Ca(2+) -permeable, concentration-dependent inward current (IOAG ) that reversed at ∼-20 mV and was enhanced by intracellular IP3 . A similar current was evoked by either another DAG analogue, 1,2-dioctanoyl-sn-glycerol (DOG), or activating PLC with N-(3-trifluoromethylphenyl)-2,4,6-trimethylbenzenesulfonamide (m-3M3FBS). IOAG was reduced by the general cation channel blockers Gd(3+) or flufenamic acid. Work in other systems indicated that OAG activates channels independently of protein kinase C (PKC); however, we found pretreating bag cell neurons with any of the PKC inhibitors bisindolylmaleimide, sphinganine, or H7, attenuated IOAG . However, stimulating PKC with phorbol 12-myristate 13-acetate (PMA) did not evoke current or enhance IOAG ; moreover, unlike PMA, OAG failed to trigger PKC, as confirmed by an independent bioassay. Finally, OAG or m-3M3FBS depolarized cultured neurons, and while OAG did not provoke afterdischarges from bag cell neurons in the nervous system, it did double the duration of synaptically elicited afterdischarges. To our knowledge, this is the first report of obligate PKC activity for IOAG gating. An interaction between phosphoinositol metabolites and PKC could control the cation channel to influence afterdischarge duration.

Nicotine inhibits potassium currents in Aplysia bag cell neurons

Acetylcholine and the archetypal cholinergic agonist, nicotine, are typically associated with the opening of ionotropic receptors. In the bag cell neurons, which govern the reproductive behavior of the marine snail, Aplysia californica, there are two cholinergic responses: a relatively large acetylcholine-induced current and a relatively small nicotine-induced current. Both currents are readily apparent at resting membrane potential and result from the opening of distinct ionotropic receptors. We now report a separate current response elicited by applying nicotine to cultured bag cell neurons under whole cell voltage-clamp. This current was ostensibly inward, best resolved at depolarized voltages, presented a noncooperative dose-response with a half-maximal concentration near 1.5 mM, and associated with a decrease in membrane conductance. The unique nicotine-evoked response was not altered by intracellular perfusion with the G protein blocker GDPβS or exposure to classical nicotinic antagonists but was occluded by replacing intracellular K(+) with Cs(+) Consistent with an underlying mechanism of direct inhibition of one or more K(+) channels, nicotine was found to rapidly reduce the fast-inactivating A-type K(+) current as well as both components of the delayed-rectifier K(+) current. Finally, nicotine increased bag cell neuron excitability, which manifested as reduction in spike threshold, greater action potential height and width, and markedly more spiking to continuous depolarizing current injection. In contrast to conventional transient activation of nicotinic ionotropic receptors, block of K(+) channels could represent a nonstandard means for nicotine to profoundly alter the electrical properties of neurons over prolonged periods of time.

PKC enhances the capacity for secretion by rapidly recruiting covert voltage-gated Ca2+ channels to the membrane

It is unknown whether neurons can dynamically control the capacity for secretion by promptly changing the number of plasma membrane voltage-gated Ca(2+) channels. To address this, we studied peptide release from the bag cell neurons of Aplysia californica, which initiate reproduction by secreting hormone during an afterdischarge. This burst engages protein kinase C (PKC) to trigger the insertion of a covert Ca(2+) channel, Apl Cav2, alongside a basal channel, Apl Cav1. The significance of Apl Cav2 recruitment to secretion remains undetermined; therefore, we used capacitance tracking to assay secretion, along with Ca(2+) imaging and Ca(2+) current measurements, from cultured bag cell neurons under whole-cell voltage-clamp. Activating PKC with the phorbol ester, PMA, enhanced Ca(2+) entry, and potentiated stimulus-evoked secretion. This relied on channel insertion, as it was occluded by preventing Apl Cav2 engagement with prior whole-cell dialysis or the cytoskeletal toxin, latrunculin B. Channel insertion reduced the stimulus duration and/or frequency required to initiate secretion and strengthened excitation-secretion coupling, indicating that Apl Cav2 accesses peptide release more readily than Apl Cav1. The coupling of Apl Cav2 to secretion also changed with behavioral state, as Apl Cav2 failed to evoke secretion in silent neurons from reproductively inactive animals. Finally, PKC also acted secondarily to enhance prolonged exocytosis triggered by mitochondrial Ca(2+) release. Collectively, our results suggest that bag cell neurons dynamically elevate Ca(2+) channel abundance in the membrane to ensure adequate secretion during the afterdischarge.

Ca2+-induced uncoupling of Aplysia bag cell neurons

Electrical transmission is a dynamically regulated form of communication and key to synchronizing neuronal activity. The bag cell neurons of Aplysia are a group of electrically coupled neuroendocrine cells that initiate ovulation by secreting egg-laying hormone during a prolonged period of synchronous firing called the afterdischarge. Accompanying the afterdischarge is an increase in intracellular Ca2+ and the activation of protein kinase C (PKC). We used whole cell recording from paired cultured bag cell neurons to demonstrate that electrical coupling is regulated by both Ca2+ and PKC. Elevating Ca2+ with a train of voltage steps, mimicking the onset of the afterdischarge, decreased junctional current for up to 30 min. Inhibition was most effective when Ca2+ entry occurred in both neurons. Depletion of Ca2+ from the mitochondria, but not the endoplasmic reticulum, also attenuated the electrical synapse. Buffering Ca2+ with high intracellular EGTA or inhibiting calmodulin kinase prevented uncoupling. Furthermore, activating PKC produced a small but clear decrease in junctional current, while triggering both Ca2+ influx and PKC inhibited the electrical synapse to a greater extent than Ca2+ alone. Finally, the amplitude and time course of the postsynaptic electrotonic response were attenuated after Ca2+ influx. A mathematical model of electrically connected neurons showed that excessive coupling reduced recruitment of the cells to fire, whereas less coupling led to spiking of essentially all neurons. Thus a decrease in electrical synapses could promote the afterdischarge by ensuring prompt recovery of electrotonic potentials or making the neurons more responsive to current spreading through the network.

Electrical coupling between Aplysia bag cell neurons: characterization and role in synchronous firing

In neuroendocrine cells, hormone release often requires a collective burst of action potentials synchronized by gap junctions. This is the case for the electrically coupled bag cell neurons in the reproductive system of the marine snail, Aplysia californica. These neuroendocrine cells are found in two clusters, and fire a synchronous burst, called the afterdischarge, resulting in neuropeptide secretion and the triggering of ovulation. However, the physiology and pharmacology of the bag cell neuron electrical synapse are not completely understood. As such, we made dual whole cell recordings from pairs of electrically coupled cultured bag cell neurons. The junctional current was nonrectifying and not influenced by postsynaptic voltage. Furthermore, junctional conductance was voltage independent and, not surprisingly, strongly correlated with coupling coefficient magnitude. The electrical synapse also acted as a low-pass filter, although under certain conditions, electrotonic potentials evoked by presynaptic action potentials could drive postsynaptic spikes. If coupled neurons were stimulated to spike simultaneously, they presented a high degree of action potential synchrony compared with not-coupled neurons. The electrical synapse failed to pass various intracellular dyes, but was permeable to Cs(+), and could be inhibited by niflumic acid, meclofenamic acid, or 5-nitro-2-(3-phenylpropylamino)benzoic acid. Finally, extracellular and sharp-electrode recording from the intact bag cell neuron cluster showed that these pharmacological uncouplers disrupted both electrical coupling and afterdischarge generation in situ. Thus electrical synapses promote bag cell neuron firing synchrony and may allow for electrotonic spread of the burst through the network, ultimately contributing to propagation of the species.

A potentially novel nicotinic receptor in Aplysia neuroendocrine cells

Nicotinic receptors form a diverse group of ligand-gated ionotropic receptors with roles in both synaptic transmission and the control of excitability. In the bag cell neurons of Aplysia, acetylcholine activates an ionotropic receptor, which passes inward current to produce a long-lasting afterdischarge and hormone release, leading to reproduction. While testing the agonist profile of the cholinergic response, we observed a second current that appeared to be gated only by nicotine and not acetylcholine. The peak nicotine-evoked current was markedly smaller in magnitude than the acetylcholine-induced current, cooperative (Hill value of 2.7), had an EC50 near 500 μM, readily recovered from desensitization, showed Ca2+ permeability, and was blocked by mecamylamine, dihydro-β-erythroidine, or strychnine, but not by α-conotoxin ImI, methyllycaconitine, or hexamethonium. Aplysia transcriptome analysis followed by PCR yielded 20 full-length potential nicotinic receptor subunits. Sixteen of these were predicted to be cation selective, and real-time PCR suggested that 15 of the 16 subunits were expressed to varying degrees in the bag cell neurons. The acetylcholine-induced current, but not the nicotine current, was reduced by double-strand RNA treatment targeted to both subunits ApAChR-C and -E. Conversely, the nicotine-evoked current, but not the acetylcholine current, was lessened by targeting both subunits ApAChR-H and -P. To the best of our knowledge, this is the first report suggesting that a nicotinic receptor is not gated by acetylcholine. Separate receptors may serve as a means to differentially trigger plasticity or safeguard propagation by assuring that only acetylcholine, the endogenous agonist, initiates large enough responses to trigger reproduction.

Voltage-gated Ca2+ influx and mitochondrial Ca2+ initiate secretion from Aplysia neuroendocrine cells

Neuroendocrine secretion often requires prolonged voltage-gated Ca(2+) entry; however, the ability of Ca(2+) from intracellular stores, such as endoplasmic reticulum or mitochondria, to elicit secretion is less clear. We examined this using the bag cell neurons, which trigger ovulation in Aplysia by releasing egg-laying hormone (ELH) peptide. Secretion from cultured bag cell neurons was observed as an increase in plasma membrane capacitance following Ca(2+) influx evoked by a 5-Hz, 1-min train of depolarizing steps under voltage-clamp. The response was similar for step durations of ≥ 50 ms, but fell off sharply with shorter stimuli. The capacitance change was attenuated by replacing external Ca(2+) with Ba(2+), blocking Ca(2+) channels, buffering intracellular Ca(2+) with EGTA, disrupting synaptic protein recycling, or genetic knock-down of ELH. Regarding intracellular stores, liberating mitochondrial Ca(2+) with the protonophore, carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazone (FCCP), brought about an EGTA-sensitive elevation of capacitance. Conversely, no change was observed to Ca(2+) released from the endoplasmic reticulum or acidic stores. Prior exposure to FCCP lessened the train-induced capacitance increase, suggesting overlap in the pool of releasable vesicles. Employing GTP-γ-S to interfere with endocytosis delayed recovery (presumed membrane retrieval) of the capacitance change following FCCP, but not the train. Finally, secretion was correlated with reproductive behavior, in that neurons isolated from animals engaged in egg-laying presented a greater train-induced capacitance elevation vs quiescent animals. The bag cell neuron capacitance increase is consistent with peptide secretion requiring high Ca(2+), either from influx or stores, and may reflect the all-or-none nature of reproduction.

Removal of default state-associated inhibition during repetition priming improves response articulation

Behavior is a product of both the stimuli encountered and the current internal state. At the level of the nervous system, the internal state alters the biophysical properties of, and connections between, neurons establishing a "network state." To establish a network state, the nervous system must be altered from an initial default/resting state, but what remains unclear is the extent to which this process represents induction from a passive default state or the removal of suppression by an active default state. We use repetition priming (a history-dependent improvement of behavioral responses to repeatedly encountered stimuli) to determine the cellular mechanisms underlying the transition from the default to the primed network state. We demonstrate that both removal of active suppression and induction of neuron excitability changes each contribute separately to the production of a primed state. The feeding system of Aplysia californica displays repetition priming via an increase in the activity of the radula closure neuron B8, which results in increased bite strength with each motor program. We found that during priming, B8 received progressively less inhibitory input from the multifunctional neurons B4/5. Additionally, priming enhanced the excitability of B8, but the rate at which B8 activity increased as a result of these changes was regulated by the progressive removal of inhibitory input. Thus, the establishment of the network state involves the induction of processes from a rested state, yet the consequences of these processes are conditional upon critical gating mechanisms actively enforced by the default state.

Acetylcholine elongates neuronal growth cone filopodia via activation of nicotinic acetylcholine receptors

In addition to acting as a classical neurotransmitter in synaptic transmission, acetylcholine (ACh) has been shown to play a role in axonal growth and growth cone guidance. What is not well understood is how ACh acts on growth cones to affect growth cone filopodia, structures known to be important for neuronal pathfinding. We addressed this question using an identified neuron (B5) from the buccal ganglion of the pond snail Helisoma trivolvis in cell culture. ACh treatment caused pronounced filopodial elongation within minutes, an effect that required calcium influx and resulted in the elevation of the intracellular calcium concentration ([Ca]i ). Whole-cell patch clamp recordings showed that ACh caused a reduction in input resistance, a depolarization of the membrane potential, and an increase in firing frequency in B5 neurons. These effects were mediated via the activation of nicotinic acetylcholine receptors (nAChRs), as the nAChR agonist dimethylphenylpiperazinium (DMPP) mimicked the effects of ACh on filopodial elongation, [Ca]i elevation, and changes in electrical activity. Moreover, the nAChR antagonist tubucurarine blocked all DMPP-induced effects. Lastly, ACh acted locally at the growth cone, because growth cones that were physically isolated from their parent neuron responded to ACh by filopodial elongation with a similar time course as growth cones that remained connected to their parent neuron. Our data revealed a critical role for ACh as a modulator of growth cone filopodial dynamics. ACh signaling was mediated via nAChRs and resulted in Ca influx, which, in turn, caused filopodial elongation.