Mitochondrial Ca2+ Activates a Cation Current 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.

Organelle calcium-derived voltage oscillations in pacemaker neurons drive the motor program for food-seeking behavior in Aplysia

The expression of motivated behaviors depends on both external and internally arising neural stimuli, yet the intrinsic releasing mechanisms for such variably occurring behaviors remain elusive. In isolated nervous system preparations of Aplysia, we have found that irregularly expressed cycles of motor output underlying food-seeking behavior arise from regular membrane potential oscillations of varying magnitude in an identified pair of interneurons (B63) in the bilateral buccal ganglia. This rhythmic signal, which is specific to the B63 cells, is generated by organelle-derived intracellular calcium fluxes that activate voltage-independent plasma membrane channels. The resulting voltage oscillation spreads throughout a subset of gap junction-coupled buccal network neurons and by triggering plateau potential-mediated bursts in B63, can initiate motor output driving food-seeking action. Thus, an atypical neuronal pacemaker mechanism, based on rhythmic intracellular calcium store release and intercellular propagation, can act as an autonomous intrinsic releaser for the occurrence of a motivated behavior.

Mitochondrial F1FO-ATPase and permeability transition pore response to sulfide in the midgut gland of Mytilus galloprovincialis

The molecular mechanisms which rule the formation and opening of the mitochondrial permeability transition pore (mPTP), the lethal mechanism which permeabilizes mitochondria to water and solutes and drives the cell to death, are still unclear and particularly little investigated in invertebrates. Since Ca²⁺ increase in mitochondria is accompanied by mPTP opening and the participation of the mitochondrial F₁F_O-ATPase in the mPTP is increasingly sustained, the substitution of the natural cofactor Mg²⁺ by Ca²⁺ in the F₁F_O-ATPase activation has been involved in the mPTP mechanism. In mussel midgut gland mitochondria the similar kinetic properties of the Mg²⁺- or Ca²⁺-dependent F₁F_O-ATPase activities, namely the same affinity for ATP and bi-site activation kinetics by the ATP substrate, in spite of the higher enzyme activity and coupling efficiency of the Mg²⁺-dependent F₁F_O-ATPase, suggest that both enzyme activities are involved in the bioenergetic machinery. Other than being a mitochondrial poison and environmental contaminant, sulfide at low concentrations acts as gaseous mediator and can induce post-translational modifications of proteins. The sulfide donor NaHS, at micromolar concentrations, does not alter the two F₁F_O-ATPase activities, but desensitizes the mPTP to Ca²⁺ input. Unexpectedly, NaHS, under the conditions tested, points out a chemical refractoriness of both F₁F_O-ATPase activities and a failed relationship between the Ca²⁺-dependent F₁F_O-ATPase and the mPTP in mussels. The findings suggest that mPTP role and regulation may be different in different taxa and that the F₁F_O-ATPase insensitivity to NaHS may allow mussels to cope with environmental sulfide.

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.

Transient heat release during induced mitochondrial proton uncoupling

Non-shivering thermogenesis through mitochondrial proton uncoupling is one of the dominant thermoregulatory mechanisms crucial for normal cellular functions. The metabolic pathway for intracellular temperature rise has widely been considered as steady-state substrate oxidation. Here, we show that a transient proton motive force (pmf) dissipation is more dominant than steady-state substrate oxidation in stimulated thermogenesis. Using transient intracellular thermometry during stimulated proton uncoupling in neurons of Aplysia californica, we observe temperature spikes of ~7.5 K that decay over two time scales: a rapid decay of ~4.8 K over ~1 s followed by a slower decay over ~17 s. The rapid decay correlates well in time with transient electrical heating from proton transport across the mitochondrial inner membrane. Beyond ~33 s, we do not observe any heating from intracellular sources, including substrate oxidation and pmf dissipation. Our measurements demonstrate the utility of transient thermometry in better understanding the thermochemistry of mitochondrial metabolism.

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.

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.

Ca2+ removal by the plasma membrane Ca2+-ATPase influences the contribution of mitochondria to activity-dependent Ca2+ dynamics in Aplysia neuroendocrine cells

After Ca(2+) influx, mitochondria can sequester Ca(2+) and subsequently release it back into the cytosol. This form of Ca(2+)-induced Ca(2+) release (CICR) prolongs Ca(2+) signaling and can potentially mediate activity-dependent plasticity. As Ca(2+) is required for its subsequent release, Ca(2+) removal systems, like the plasma membrane Ca(2+)-ATPase (PMCA), could impact CICR. Here we examine such a role for the PMCA in the bag cell neurons of Aplysia californica CICR is triggered in these neurons during an afterdischarge and is implicated in sustaining membrane excitability and peptide secretion. Somatic Ca(2+) was measured from fura-PE3-loaded cultured bag cell neurons recorded under whole cell voltage clamp. Voltage-gated Ca(2+) influx was elicited with a 5-Hz, 1-min train, which mimics the fast phase of the afterdischarge. PMCA inhibition with carboxyeosin or extracellular alkalization augmented the effectiveness of Ca(2+) influx in eliciting mitochondrial CICR. A Ca(2+) compartment model recapitulated these findings and indicated that disrupting PMCA-dependent Ca(2+) removal increases CICR by enhancing mitochondrial Ca(2+) loading. Indeed, carboxyeosin augmented train-evoked mitochondrial Ca(2+) uptake. Consistent with their role on Ca(2+) dynamics, cell labeling revealed that the PMCA and mitochondria overlap with Ca(2+) entry sites. Finally, PMCA-dependent Ca(2+) extrusion did not impact endoplasmic reticulum-dependent Ca(2+) removal or release, despite the organelle residing near Ca(2+) entry sites. Our results demonstrate that Ca(2+) removal by the PMCA influences the propensity for stimulus-evoked CICR by adjusting the amount of Ca(2+) available for mitochondrial Ca(2+) uptake. This study highlights a mechanism by which the PMCA could impact activity-dependent plasticity in the bag cell neurons.

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.

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.

Separate Ca2+ sources are buffered by distinct Ca2+ handling systems in aplysia neuroendocrine cells

Although the contribution of Ca(2+) buffering systems can vary between neuronal types and cellular compartments, it is unknown whether distinct Ca(2+) sources within a neuron have different buffers. As individual Ca(2+) sources can have separate functions, we propose that each is handled by unique systems. Using Aplysia californica bag cell neurons, which initiate reproduction through an afterdischarge involving multiple Ca(2+)-dependent processes, we investigated the role of endoplasmic reticulum (ER) and mitochondrial sequestration, as well as extrusion via the plasma membrane Ca(2+)-ATPase (PMCA) and Na(+)/Ca(2+) exchanger, to the clearance of voltage-gated Ca(2+) influx, Ca(2+)-induced Ca(2+)-release (CICR), and store-operated Ca(2+) influx. Cultured bag cell neurons were filled with the Ca(2+) indicator, fura-PE3, to image Ca(2+) under whole-cell voltage clamp. A 5 Hz, 1 min train of depolarizing voltage steps elicited voltage-gated Ca(2+) influx followed by EGTA-sensitive CICR from the mitochondria. A compartment model of Ca(2+) indicated the effect of EGTA on CICR was due to buffering of released mitochondrial Ca(2+) rather than uptake competition. Removal of voltage-gated Ca(2+) influx was dominated by the mitochondria and PMCA, with no contribution from the Na(+)/Ca(2+) exchanger or sarcoplasmic/endoplasmic Ca(2+)-ATPase (SERCA). In contrast, CICR recovery was slowed by eliminating the Na(+)/Ca(2+) exchanger and PMCA. Last, store-operated influx, evoked by ER depletion, was removed by the SERCA and depended on the mitochondrial membrane potential. Our results demonstrate that distinct buffering systems are dedicated to particular Ca(2+) sources. In general, this may represent a means to differentially regulate Ca(2+)-dependent processes, and for Aplysia, influence how reproductive behavior is triggered.

Role for protein kinase C in controlling Aplysia bag cell neuron excitability

Targeting signalling molecules to ion channels can expedite regulation and assure the proper transition of changes to excitability. In the bag cell neurons of Aplysia, single-channel studies of excised patches have revealed that protein kinase C (PKC) gates a non-selective cation channel through a close, physical association. This channel drives a prolonged afterdischarge and concomitant neuropeptide secretion to provoke reproductive behaviour. However, it is not clear if PKC alters cation channel function and/or the membrane potential at the whole-cell level. Afterdischarge-like depolarizations can be evoked in cultured bag cell neurons by bath-application of Conus textile venom (CtVm), which triggers the cation channel through an apparent intracellular pathway. The present study shows that the CtVm-induced depolarization was reduced by nearly 50% compared to control following dialysis with the G-protein blocker, guanosine-5'-O-2-thiodiphosphate (GDP-β-S), or treatment with either the phospholipase C inhibitor, 1-[6-[[(17β)-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122), or the PKC inhibitor, sphinganine. Neurons exposed to the PKC activator, phorbol 12-myristate 13-acetate (PMA), displayed depolarization with accompanying spiking, and were found to be far more responsive to depolarizing current injection versus control. Immunocytochemical staining for the two typical Aplysia PKC isoforms, Apl I and Apl II, revealed that both kinases were present in unstimulated cultured bag cell neurons. However, in CtVm-treated neurons, the staining intensity for PKC Apl I increased, peaking at 10 min post-application. Conversely, the intensity of PKC Apl II staining decreased over the duration of CtVm exposure. Our results suggest that the CtVm-induced depolarization involves PKC activation, and is consistent with prior work showing PKC closely-associating with the cation channel to produce the depolarization necessary for the afterdischarge and species propagation.