Relative distribution of Ca2+ channels at the crayfish inhibitory neuromuscular junction.
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.
Allana
Tariq N
TN
Department of Biology, Boston University, 5 Cummington St., Boston, MA 02215, USA.
Lin
Jen-Wei
JW
D000818:Animals; D003400:Astacoidea / Q000187:drug effects / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology*; D004533:Egtazic Acid / Q000494:pharmacology; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D009469:Neuromuscular Junction / Q000187:drug effects / Q000502:physiology*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*
D015220:Calcium Channels; D004533:Egtazic Acid