Failure of omega-conotoxin to block L-channels associated with [3H]5-HT release in rat brain slices.

The effects of the mixed N- and L-type voltage-sensitive calcium channel (VSCC) antagonist, omega-conotoxin GVIA and the L-type VSCC agonist Bay K-8644 on calcium-dependent, potassium evoked release of [3H]5-hydroxtryptamine ([3H]5-HT) were investigated in slices of rat hippocampus. Bay K-8644 (1 microM) enhanced, whilst omega-conotoxin (10-30 nM) attenuated, but did not abolish, evoked release of [3H]5-HT. The facilitatory actions of Bay K-8644 on evoked release were unaffected by concentrations of omega conotoxin that significantly inhibited [3H]5-HT release. The experiments indicate that concentrations of omega-conotoxin which inhibit neurotransmitter release by blockade of N-type VSCC, may leave L-type calcium channel activity unaffected.