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Activity-dependent modulation of electrical transmission typically involves Ca2+ influx acting directly on gap junctions or initiating Ca2+-dependent pathways that in turn modulate coupling. We now describe short-term use-dependent facilitation of electrical transmission between bag cell neurons from the hermaphroditic snail, Aplysia californica, that is instead mediated by changes in postsynaptic responsiveness. Bag cell neurons secrete reproductive hormone during a synchronous afterdischarge of action potentials coordinated by electrical coupling. Here, recordings from pairs of coupled bag cell neurons in culture showed that nonjunctional currents influence electrical transmission in a dynamic manner. Under a dual whole cell voltageclamp, the junctional current was linear and largely voltage-independent, while in current-clamp, the coupling coefficient was similar regardless of the extent of presynaptic hyperpolarization. Moreover, a train stimulus of action potential-like waveforms, in a voltage-clamped presynaptic neuron, elicited electrotonic potentials, in a current-clamped postsynaptic neuron, that facilitated over time when delivered at a frequency approximating the afterdischarge. Junctional current remained constant over the train stimulus, as did postsynaptic voltage-gated Ca2+ current. However, postsynaptic voltage-gated K+ current underwent cumulative inactivation, suggesting that K+ current run-down facilitates the electrotonic potential by boosting the response to successive junctional currents. Accordingly, preventing run-down by blocking postsynaptic K+ channels occluded facilitation. Finally, stimulation of bursts in coupled pairs resulted in synchronous firing, where active neurons could recruit silent partners through short-term use-dependent facilitation. Thus, potentiation of electrical transmission may promote synchrony in bag cell neurons and, by extension, reproductive function. [ABSTRACT FROM AUTHOR]

Many behaviors and types of information storage are mediated by lengthy changes in neuronal activity. In bag cell neurons of the hermaphroditic sea snail Aplysia californica, a transient cholinergic synaptic input triggers an ∼30-min afterdischarge. This causes these neuroendocrine cells to release egg laying hormone and elicit reproductive behavior. When acetylcholine is pressure-ejected onto a current-clamped bag cell neuron, the evoked depolarization is far longer than the current evoked by acetylcholine under voltage clamp, suggesting recruitment of another conductance. Our earlier studies found bag cell neurons to display a voltage-dependent persistent Ca2+ current. Hence, we hypothesized that this current is activated by the acetylcholine-induced depolarization and sought a selective Ca2+ current blocker. Rapid Ca2+ current evoked by 200-ms depolarizing steps in voltage-clamped cultured bag cell neurons demonstrated a concentration-dependent sensitivity to Ni2+, Co2+, Zn2+, and verapamil but not Cd2+ or ω-conotoxin GIVa. Leak subtraction of Ca2+ current evoked by 10-s depolarizing steps using the IC100 (concentration required to eliminate maximal current) of Ni2+, Co2+, Zn2+, or verapamil revealed persistent Ca2+ current, demonstrating persistent current block. Only Co2+ and Zn2+ did not suppress the acetylcholine-induced current, although Zn2+ appeared to impact additional channels. When Co2+ was applied during an acetylcholine-induced depolarization, the amplitude was reduced; furthermore, protein kinase C activation, previously established to enhance the persistent Ca2+ current, extended the depolarization. Therefore, the persistent Ca2+ current sustains the acetylcholine-induced depolarization and may translate brief cholinergic input into afterdischarge initiation. This could be a general mechanism of triggering long-term change in activity with a short-lived input .NEW & NOTEWORTHY Ionotropic acetylcholine receptors mediate brief synaptic communication, including in bag cell neurons of the sea snail Aplysia. However, this study demonstrates that cholinergic depolarization can open a voltage-gated persistent Ca2+ current, which extends the bag cell neuron response to acetylcholine. Bursting in these neuroendocrine cells results in hormone release and egg laying. Thus, this emphasizes the role of ionotropic signaling in reaching a depolarized level to engage Ca2+ influx and perpetuating the activity necessary for behavior. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

After Ca2+ influx, mitochondria can sequester Ca2+ and subsequently release it back into the cytosol. This form of Ca2+-induced Ca2+ release (CICR) prolongs Ca2+ signaling and can potentially mediate activity-dependent plasticity. As Ca2+ is required for its subsequent release, Ca2+ removal systems, like the plasma membrane Ca2+-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 Ca2+ was measured from fura-PE3-loaded cultured bag cell neurons recorded under whole cell voltage clamp. Voltage-gated Ca2+ 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 Ca2+ influx in eliciting mitochondrial CICR. A Ca2+ compartment model recapitulated these findings and indicated that disrupting PMCA-dependent Ca2+ removal increases CICR by enhancing mitochondrial Ca2+ loading. Indeed, carboxyeosin augmented train-evoked mitochondrial Ca2+ uptake. Consistent with their role on Ca2+ dynamics, cell labeling revealed that the PMCA and mitochondria overlap with Ca2+ entry sites. Finally, PMCA-dependent Ca2+ extrusion did not impact endoplasmic reticulum-dependent Ca2+ removal or release, despite the organelle residing near Ca2+ entry sites. Our results demonstrate that Ca2+ removal by the PMCA influences the propensity for stimulus-evoked CICR by adjusting the amount of Ca2+ available for mitochondrial Ca2+ uptake. This study highlights a mechanism by which the PMCA could impact activity-dependent plasticity in the bag cell neurons. [ABSTRACT FROM AUTHOR]

It is unknown whether neurons can dynamically control the capacity for secretion by promptly changing the number of plasma membrane voltage-gated Ca2+ channels. To address this, we studied peptide release from the bag cell neurons of Aplysia californica, which initiate reproduction by secreting hormone during an after discharge. This burst engages protein kinase C (PKC) to trigger the insertion of a covert Ca2+ channel, Apl Cav2, alongside a basal channel, Apl Cavl. The significance of Apl Cav2 recruitment to secretion remains undetermined; therefore, we used capacitance tracking to assay secretion, along with Ca2+ imaging and Ca2+ current measurements, from cultured bag cell neurons under whole-cell voltage-clamp. Activating PKC with the phorbol ester, PMA, enhanced Ca2+ 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 Cavl. 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 Ca2+ release. Collectively, our results suggest that bag cell neurons dynamically elevate Ca2+ channel abundance in the membrane to ensure adequate secretion during the after discharge. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Although the contribution of Ca2+ buffering systems can vary between neuronal types and cellular compartments, it is unknown whether distinct Ca2+ sources within a neuron have different buffers. As individual Ca2+ 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 Ca2+-dependent processes, we investigated the role of endoplasmic reticulum (ER) and mitochondrial sequestration, as well as extrusion via the plasma membrane Ca2+-ATPase (PMCA) and Na+/Ca2+ exchanger, to the clearance of voltage-gated Ca2+ influx, Ca2+-induced Ca2+-release (CICR), and store-operated Ca2+ influx. Cultured bag cell neurons were filled with the Ca2+ indicator, fura-PE3, to image Ca2+ under whole-cell voltage clamp. A 5 Hz, 1 min train of depolarizing voltage steps elicited voltage-gated Ca2+ influx followed by EGTA-sensitive CICR from the mitochondria. A compartment model of Ca2+ indicated the effect of EGTA on CICR was due to buffering of released mitochondrial Ca2+ rather than uptake competition. Removal of voltage-gated Ca2+ influx was dominated by the mitochondria and PMCA, with no contribution from the Na+/Ca2+ exchanger or sarcoplasmic/endoplasmic Ca2+-ATPase (SERCA). In contrast, CICR recovery was slowed by eliminating the Na+/Ca2+ 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 Ca2+ sources. In general, this may represent a means to differentially regulate Ca2+-dependent processes, and for Aplysia, influence how reproductive behavior is triggered. [ABSTRACT FROM AUTHOR]

Neurons may initiate behavior or store information by translating prior activity into a lengthy change in excitability. For example, brief input to the bag cell neurons of Aplysia results in an approximate 30-min afterdischarge that induces reproduction. Similarly, momentary stimulation of cultured bag cells neurons evokes a prolonged depolarization lasting many minutes. Contributing to this is a voltage-independent cation current activated by Ca(2+) entering during the stimulus. However, the cation current is relatively short-lived, and we hypothesized that a second, voltage-dependent persistent current sustains the prolonged depolarization. In bag cell neurons, the inward voltage-dependent current is carried by Ca(2+); thus we tested for persistent Ca(2+) current in primary culture under voltage clamp. The observed current activated between -40 and -50 mV exhibited a very slow decay, presented a similar magnitude regardless of stimulus duration (10-60 s), and, like the rapid Ca(2+) current, was enhanced when Ba(2+) was the permeant ion. The rapid and persistent Ca(2+) current, but not the cation current, were Ni(2+) sensitive. Consistent with the persistent current contributing to the response, Ni(2+) reduced the amplitude of a prolonged depolarization evoked under current clamp. Finally, protein kinase C activation enhanced the rapid and persistent Ca(2+) current as well as increased the prolonged depolarization when elicited by an action potential-independent stimulus. Thus the prolonged depolarization arises from Ca(2+) influx triggering a cation current, followed by voltage-dependent activation of a persistent Ca(2+) current and is subject to modulation. Such synergy between currents may represent a common means of achieving activity-dependent changes to excitability. [ABSTRACT FROM AUTHOR]