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

Abstract

By synchronizing neuronal activity, electrical transmission influences the coordination, pattern, and/or frequency of firing. In the hemaphroditic marine-snail, , 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 Ca 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 Ca current to promote synchronous firing. Electrical coupling is a fundamental form of communication. For the bag cell neurons of , 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 Ca current. Thus, protein kinase C improves the fidelity of electrotonic transmission and promotes synchronous firing by modulating both junctional and membrane conductances.

Organelle calcium-derived voltage oscillations in pacemaker neurons drive food-seeking behavior inAplysia

“…The maximum concentration of DMSO in the final volume reached 0.05%, which in control and previously reported studies did not alter the electrophysiological properties of neurons, the strength of electrical synapses, or intracellular calcium concentrations (Beekharry et al, 2018).…”

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

“…Cyclopianozic acid (CPA, Merck-Sigma-Aldrich), a blocker of the sarco/endoplasmic reticulum Ca 2+ -ATPase pump (SERCA) and Carbonyl cyanide 4- (trifluoromethoxy) phenylhydrazone (FCCP, Merck-Sigma-Aldrich), a protonophoric uncoupler of mitochondrial oxidative phosphorylation that depolarizes the mitochondrial membrane and leads to the organelle’s release of calcium, were diluted to 20 µM in Low Ca+Co saline from stock solutions that were prepared in dimethyl sulfoxide (DMSO) ( Benz and McLaughlin, 1983 ; Geiger and Magoski, 2008 ; Hickey et al, 2010 ). The maximum concentration of DMSO in the final volume reached 0.05%, which in control and previously reported studies did not alter the electrophysiological properties of neurons, the strength of electrical synapses, or intracellular calcium concentrations ( Beekharry et al, 2018 ).…”

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

“…1992; Groten & Magoski, 2015), as well as gap junctions (Beekharry et al . 2018). We now conclude that DAG and IP 3 also influence the H 2 O 2 ‐induced cation current to promote the action potential firing necessary for hormone secretion and reproduction.…”