RESUMEN
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 voltage-clamp, 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.NEW & NOTEWORTHY The understanding of how activity can facilitate electrical transmission is incomplete. We found that electrotonic potentials between electrically coupled neuroendocrine bag cell neurons facilitated in a use-dependent fashion. Rather than changes to the junctional current, facilitation was associated with cumulative inactivation of postsynaptic K+ current, presumably augmenting responsiveness. When made to burst, neurons synchronized their spiking, in part by use-dependent facilitation bringing quiescent cells to the threshold. Facilitation may foster en masse firing and neurosecretion.
Asunto(s)
Neuronas , Potenciales Sinápticos , Animales , Neuronas/fisiología , Potenciales de Acción , Aplysia/fisiología , Calcio/metabolismoRESUMEN
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 â¼30-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.
Asunto(s)
Acetilcolina , Aplysia , Animales , Aplysia/fisiología , Acetilcolina/farmacología , Neuronas/fisiología , Colinérgicos , Verapamilo , Hormonas , Calcio/metabolismoRESUMEN
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.