Voltage‐gated ionic currents in an identified modulatory cell type controlling molluscan feeding

Abstract

An important modulatory cell type, found in all molluscan feeding networks, was investigated using two-electrode voltage- and current-clamp methods. In the cerebral giant cells of Lymnaea, a transient inward Na+ current was identified with activation at -58 +/- 2 mV. It was sensitive to tetrodotoxin only in high concentrations (approximately 50% block at 100 microm), a characteristic of Na+ channels in many molluscan neurons. A much smaller low-threshold persistent Na+ current (activation at < -90 mV) was also identified. Two purely voltage-sensitive outward K+ currents were also found: (i) a transient A-current type which was activated at -59 +/- 4 mV and blocked by 4-aminopyridine; (ii) a sustained tetraethylammonium-sensitive delayed rectifier current which was activated at -47 +/- 2 mV. There was also evidence that a third, Ca2+-activated, K+ channel made a contribution to the total outward current. No inwardly rectifying currents were found. Two Ca2+ currents were characterized: (i) a transient low-voltage (-65 +/- 2 mV) activated T-type current, which was blocked in NiCl2 (2 mm) and was completely inactivated at approximately -50 mV; (ii) A sustained high voltage (-40 +/- 1 mV) activated current, which was blocked in CdCl2 (100 microm) but not in omega-conotoxin GVIA (10 microm), omega-agatoxin IVA (500 nm) or nifedipine (10 microm). This current was enhanced in Ba2+ saline. Current-clamp experiments revealed how these different current types could define the membrane potential and firing properties of the cerebral giant cells, which are important in shaping the wide-acting modulatory influence of this neuron on the rest of the feeding network.

Expression and Modulation of an Invertebrate Presynaptic Calcium Channel α1 Subunit Homolog

“…When coexpressed with rat ␤ 1b and ␣ 2 -␦, the functional properties of the modified LCa v 2a-5ЈrbA calcium channel resemble in many ways those of mammalian N-type calcium channels showing similar activation and inactivation behavior, preferential permeation of barium over calcium, a lack of pronounced calcium-dependent inactivation, and complete block by cadmium ions. Consistent with what has been reported for native Lymnaea calcium currents in neurons (24,25), the LCa v 2a-5ЈrbA channel is insensitive to micromolar concentrations of -conotoxin GVIA and does not have characteristics of L-type channels with regard to nifedipine sensitivity. As expected from the lack of an identifiable syntaxin binding site (21), the coexpression of the channel with Lymnaea syntaxin1 did not affect channel function.…”

“…LCa v 2a-5ЈrbA Channels Are -Conotoxin GVIA-resistant-It has been shown previously that native Lymnaea neuronal whole cell calcium currents are insensitive to classical blockers such as -conotoxin GVIA and nifedipine (24,25). Indeed, to date no selective blocker of these native currents has been identified.…”

A Homolog of the Vertebrate Pituitary Adenylate Cyclase-Activating Polypeptide Is Both Necessary and Instructive for the Rapid Formation of Associative Memory in an Invertebrate

“…Previous work in our laboratory and by others has shown that activation of NMDA receptors is necessary for the formation of associative LTM in invertebrates [Aplysia, Murphy and Glanzman (1999); Drosophila, Xia et al (2005); Lymnaea, Jami et al (2007), Wu et al (2007), and Wan et al (2010)], and we now hypothesize that invertebrate PACAP-like peptides play a mechanistic role in the learninginduced activation of NMDA receptors similar to that previously described in the hippocampus. , and K ϩ channels, Staras et al (2002) and Nikitin et al (2008); NOS and NO, Kemenes et al (2002) and Korneev et al (2005); PKA, G. ; RNA and protein synthesis, Fulton et al (2005); sGC, Ribeiro et al (2008) and Michel et al (2008)], are shown boxed. G, G-Protein; DA, dopamine; 5-HT,serotonin;MAPK,mitogen-activatedproteinkinase;CaMKII,calcium/calmodulin-dependentkinaseII;NOS,nitricoxidesynthase;NO, nitric oxide; sGC, soluble guanylyl cyclase; PKG, cGMP-activated protein kinase; PCREB, phosphorylated cAMP response element-binding protein; C/EBP, CCAAT enhancer-binding protein.…”

Retinoic acid affects calcium signaling in adult molluscan neurons

“…The cells were held at Ϫ100 mV, and the voltage increased steadily from Ϫ80 mV to ϩ60 mV over the course of 400 ms. Following this, the cells were treated with 100 M CdCl 2 in an attempt to block high-voltage activated (HVA) channels (Staras et al 2002) and to also observe the effect on total Ba 2ϩ currents elicited by the voltage ramp. The ability of CdCl 2 to block the current was calculated by comparing the peak current density before and at various time points after addition of CdCl 2 , and data were analyzed using a one-way ANOVA.…”