Relative distribution of Ca2+ channels at the crayfish inhibitory neuromuscular junction

J Neurophysiol. 2004 Sep;92(3):1491-500. doi: 10.1152/jn.00287.2004. Epub 2004 May 12.

Abstract

We investigated the Ca(2+) channel-synaptic vesicle topography at the inhibitor of the crayfish (Procambarus Clarkii) neuromuscular junction (NMJ) by analyzing the effect of different modes of Ca(2+) channel block on transmitter release. Initial identification of Ca(2+) channels revealed the presence of two classes, P and non-P-type with P-type channels governing approximately 70% of the total Ca(2+) influx. The remaining Ca(2+) influx was completely blocked by Cd(2+) but not by saturating concentrations of omega-conotoxins MVIIC and GVIA, or nifedipine and SNX-482. To examine the relative spatial distribution of Ca(2+) channels with respect to synaptic vesicles, we compared changes in inhibitory postsynaptic current amplitude and synaptic delay resulting from different spatial profiles of [Ca(2+)](i) around release sites. Specifically, addition of either [Mg(2+)](o), which decreases single-channel current, or omega-Aga IVA, which completely blocks P-type channels, prolonged synaptic delay by a similar amount when Ca(2+) influx block was <40%. Because non-P-type channels are able to compensate for blocked P-type channels, it suggests that these channels overlap considerably in their distribution. However, when Ca(2+) influx was blocked by approximately 50%, omega-Aga IVA increased delay significantly more than Mg(2+), suggesting that P-type channels are located closer than non-P-type channels to synaptic vesicles. This distribution of Ca(2+) channels was further supported by the observations that non-P-type channels are unable to trigger release in physiological saline and EGTA preferentially prolongs synaptic delay dominated by non-P-type channels when transmitter release is evoked with broad action potentials. We therefore conclude that although non-P-type channels do not directly trigger release under physiological conditions, their distribution partially overlaps with P-type channels.

Publication types

  • Comparative Study
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Astacoidea / drug effects
  • Astacoidea / physiology*
  • Calcium Channels / physiology*
  • Egtazic Acid / pharmacology
  • Neural Inhibition / drug effects
  • Neural Inhibition / physiology*
  • Neuromuscular Junction / drug effects
  • Neuromuscular Junction / physiology*
  • Synaptic Transmission / drug effects
  • Synaptic Transmission / physiology*

Substances

  • Calcium Channels
  • Egtazic Acid