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Relative Distribution of Ca²⁺ Channels at the Crayfish Inhibitory Neuromuscular Junction

Department of Biology, Boston University, Boston, Massachusetts 02215

Submitted 22 March 2004; accepted in final form 7 May 2004

We investigated the Ca²⁺ channel-synaptic vesicle topography at the inhibitor of the crayfish (Procambarus Clarkii) neuromuscular junction (NMJ) by analyzing the effect of different modes of Ca²⁺ channel block on transmitter release. Initial identification of Ca²⁺ channels revealed the presence of two classes, P and non-P-type with P-type channels governing 70% of the total Ca²⁺ influx. The remaining Ca²⁺ influx was completely blocked by Cd²⁺ but not by saturating concentrations of -conotoxins MVIIC and GVIA, or nifedipine and SNX-482. To examine the relative spatial distribution of Ca²⁺ channels with respect to synaptic vesicles, we compared changes in inhibitory postsynaptic current amplitude and synaptic delay resulting from different spatial profiles of [Ca²⁺]_i around release sites. Specifically, addition of either [Mg²⁺]_o, which decreases single-channel current, or -Aga IVA, which completely blocks P-type channels, prolonged synaptic delay by a similar amount when Ca²⁺ 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 Ca²⁺ influx was blocked by 50%, -Aga IVA increased delay significantly more than Mg²⁺, suggesting that P-type channels are located closer than non-P-type channels to synaptic vesicles. This distribution of Ca²⁺ 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.

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				A. Minami, Y.-f. Xia, and R. S. Zucker Increased Ca2+ influx through Na+/Ca2+ exchanger during long-term facilitation at crayfish neuromuscular junctions J. Physiol., December 1, 2007; 585(2): 413 - 427. [Abstract] [Full Text] [PDF]