CALCIUM-INDEPENDENT ACTIONS OF ALPHA-LATROTOXIN ON SPONTANEOUS AND EVOKED SYNAPTIC TRANSMISSION IN THE HIPPOCAMPUS

1. The black widow spider venom component, alpha-latrotoxin (alpha-LTx ) (<0.5 nM), increased the frequency of miniature excitatory postsynap tic currents (mEPSCs) in hippocampal CA3 pyramidal cells 14-fold, with out changing their amplitude. 2. This action of alpha-LTx was not affe cted by application of Ca2+-free/ethylene glycol-bis(b-aminoethyl ethe r)-N,N,N',N'-tetraacetic acid-containing saline, 100 mu M Cd2+, or 50 mu M Gd3+ The increase in mEPSC frequency was thus not due to an influ x of Ca2+ into the axon terminal via voltage-dependent Ca2+ channels o r alpha-LTx-induced pores. 3. alpha-LTx did not increase spontaneous r elease when synaptic transmission had been impaired by botulinum toxin /F. 4. alpha-LTx reduced the amplitude of EPSCs, elicited with stimula tion of messy fibers, without affecting paired-pulse facilitation. 5. The Ca2+ ionophore ionomycin (2-2.5 mu M) also enhanced the frequency of mEPSCs, but unlike alpha-LTx, potentiated evoked EPSCs and reduced paired-pulse facilitation. Application of N-methyl-D-aspartate elicite d a high frequency of Ca2+-dependent, tetrodotoxin-sensitive spontaneo us EPSCs, but did not affect evoked EPSC amplitude. Agents that stimul ate vesicular release by increasing presynaptic Ca2+ influx thus do no t mimic the alpha-LTx-induced depression of evoked EPSCs. 6. We conclu de that entry of Ca2+ into presynaptic axon terminals is not responsib le for the effects of low concentrations of alpha-LTx on either sponta neous or evoked transmitter release in the hippocampus. 7. Potential p resynaptic mechanisms that could mediate the opposing actions of alpha -LTx on spontaneous and evoked transmitter release in the hippocampus (i.e., alpha-LTx-induced ionic pores, depletion of synaptic vesicles, actions on exocytotic proteins) are dis cussed.