The use of invertebrate peptide toxins to establish Ca super(2+) channel identity of CA3-CA1 neurotransmission in rat hippocampal slices

The relative contribution(s) of different Ca super(2+) channel subtypes to synaptic transmission between Schaffer collaterals of hippocampal CA3 pyramidal cells and CA1 pyramidal cell dendrites has been assessed using the synthetic invertebrate peptide toxins omega -conotoxin GVIA to block N-type Ca super(2+) channels, omega -agatoxin-IVA to block P-type Ca super(2+) channels and omega -conotoxin MVIIC to block N-, P- and Q-type Ca super(2+) channels. omega -Agatoxin-IVA, omega -conotoxin GVIA and omega -conotoxin MVIIC all produced dose-dependent inhibitions of the excitatory post-synaptic field potential (fEPSP) recorded from the CA1 region of transverse hippocampal slices. Application of 300 nM omega -conotoxin GVIA generally produced no further inhibition to that observed with 100 nM, resulting in a maximal 50% inhibition of the fEPSP. By contrast, 30 nM omega -agatoxin-IVA reduced the fEPSP slope by only 4.6 plus or minus 11.1% (mean plus or minus S.D., n = 3), suggesting the lack of involvement of classical P-type Ca super(2+) channels, whereas 300 nM omega -agatoxin-IVA reduced the fEPSP slope by 85.7 plus or minus 15.3% (n = 3) at the end of 44 min application. Similar applications of 100 and 300 nM omega -conotoxin MVIIC reduced the fEPSP slope by 30.9 plus or minus 6.6% and 79.7 plus or minus 5.7% respectively. Application of 30 nM omega -agatoxin-IVA together with omega -conotoxin GVIA (300 nM) produced no greater inhibition of the fEPSP than that observed with omega -conotoxin GVIA alone, suggesting that the omega -agatoxin-IVA-sensitive and omega -conotoxin MVIIC-sensitive component presents a pharmacology similar to the reported Q-type Ca super(2+) channel. The inhibition produced by omega -conotoxin GVIA and omega -conotoxin MVIIC showed no recovery with prolonged washing (1-2 h) whereas that produced by omega -agatoxin-IVA was slowly reversible. The observation that omega -agatoxin-IVA, which does not effect N-type Ca super(2+) channels, is capable of completely suppressing the fEPSP suggests that, whilst N-type Ca super(2+) channels may contribute to normal synaptic transmission at Schaffer collateral-CA1 synapses, they are not capable of supporting transmission when Q-type channels are blocked.