TheC. elegansT-type calcium channel CCA-1 boosts neuromuscular transmission

Abstract

SUMMARY Low threshold-activated or T-type calcium channels are postulated to mediate a variety of bursting and rhythmic electrical firing events. However,T-type channels' exact physiological contributions have been difficult to assess because of their incompletely defined pharmacology and the difficulty in isolating T-type currents from more robust high threshold calcium currents. A current in C. elegans pharyngeal muscle displays the kinetic features of a T-type calcium channel and is absent in animals homozygous for mutations at the cca-1 locus (see accompanying paper). cca-1is expressed in pharyngeal muscle and encodes a protein (CCA-1) with strong homology to the α1 subunits of vertebrate T-type channels. We show that CCA-1 plays a critical role at the pharyngeal neuromuscular junction, permitting the efficient initiation of action potentials in response to stimulation by the MC motor neuron. Loss of cca-1 function decreases the chance that excitatory input from MC will successfully trigger an action potential, and reduces the ability of an animal to take in food. Intracellular voltage recordings demonstrate that when wild-type cca-1 is absent, the depolarizing phase of the pharyngeal action potential tends to plateau or stall near -30 mV, the voltage at which the CCA-1 channel is likely to be activated. We conclude that the CCA-1 T-type calcium channel boosts the excitatory effect of synaptic input, allowing for reliable and rapid depolarization and contraction of the pharyngeal muscle. We also show that the pharyngeal muscle employs alternative strategies for initiating action potentials in certain cases of compromised MC motor neuron function.

CCA-1, EGL-19 and EXP-2 currents shape action potentials in theCaenorhabditis eleganspharynx

“…In the pharyngeal muscle, CCA-1 mediates a large inward depolarizing current, which is the first demonstrated role of the T-type calcium channel in C. elegans. This finding, along with the accompanying work (Steger et al, 2005), suggest that CCA-1 activates in response to excitatory inputs from the MC neuron and accelerates the action potential upstroke.…”

“…NGMSR is nematode growth medium (NGM; Sulston and Hodgkin, 1988) containing nystatin and streptomycin to prevent contamination . The wild-type Caenorhabditis elegans (Maupas, 1900) strain was Bristol (N2); mutant strains were: DA1651 cca-1(ad1650) X (Steger et al, 2005 IV (Trent et al, 1983), DA1426 exp-2(sa26 ad1426) V DA1694 egl-19(n582) IV; cca-1(ad1650) X.…”

“…Sharp electrode voltage recordings were performed as described by Steger et al (2005). To determine the slope of the action potential plateau phase, 30 action potentials from five pharynxes (six from each) were analyzed.…”

EGL-19 and EXP-2 Ion Channels Modelling within C. Elegans Pharyngeal Muscle Cell Allows Reproduction of CA2+ Driven Action Potentials, Both Single Events and Trains

“…K.A. Steger and colleagues obtained the value about -73 mV 17 . Other studies 12,18 reported the values from -40 to -50 mV.…”

“…The latter is activated by small depolarizations from resting potential, lifts membrane potential until EGL-19 can be activated and inactivates quickly 17 . The duration of APs in a body wall muscle and in a pharyngeal muscle is about 20 ms and 150 ms correspondingly 7,11 , which is much longer than in vertebrate skeletal muscle (<2 ms).…”