BLOCK OF MULTIPLE PRESYNAPTIC CALCIUM-CHANNEL TYPES BY OMEGA-CONOTOXIN-MVIIC AT HIPPOCAMPAL CA3 TO CA1 SYNAPSES

1. The effect of the voltage-dependent Ca channel (VDCC) antagonist omega-conotoxin-MVIIC (omega-CTx-MVIIC) on the presynaptic Ca influx and synaptic transmission was studied in area CA1 of guinea pig hippocampus. The presynaptic Ca transient ([Ca](t)) and the field excitatory postsynaptic potential (fEPSP) evoked by a single electrical stimulus were simultaneously recorded at CA3 to CA1 synapses. 2. omega-CTx-MVIIC dose dependently blocked the fEPSP and the presynaptic [Ca](t) without affecting the presynaptic fiber volley and the presynaptic resting Ca level. During application of omega-CTx-MVIIC, the decrease of both the fEPSP and the presynaptic [Ca](t) had a similar time course, and the initial slope of the fEPSP is proportional to about the fourth power of the amplitude of the presynaptic [Ca](t). These results strongly suggest that omega-CTx-MVIIC inhibits the fEPSP by blocking presynaptic Ca channels at hippocampal CA3 to CAI synapses. 3. Sequential application of high concentrations of omega-CTx-MVIIC (10 mu M)and other VDCC blockers including omega-conotoxin-GVIA (omega-CTx-GVIA, 1 mu M) and omega-agatoxin-IVA (omega-Aga-IVA, 1 mu M) showed that omega-CTx-MVIIC significantly occludes the effects of omega-CTx-GVIA and omega-Aga-IVA. Combined application of omega-CTx-GVIA (1 mu M) and omega-Aga-IVA (1 mu M) largely but not completely occluded the effect of omega-CTx-MVIIC. These results suggest that omega-CTx-MVIIC inhibits multiple types of presynaptic Ca channels, a large fraction of which are the omega-CTx-GVIA- or omega-Aga-IVA-sensitive channels and a small portion of which are channels insensitive to high concentrations of omega-CTx-GVIA and omega-Aga-IVA. 4. Sequential or combined application of the above three toxins at high concentrations showed that similar to 23-29% of the presynaptic [Ca](t) is insensitive to these three toxins. 5. The presynaptic [Ca](t) at CA3 to CA1 synapses was found to have at least four pharmacologically distinct components (expressed as mean): 1) omega-CTx-GVIA sensitive (39%); 2) omega-Aga-OVA sensitive (25%), the majority of which is also sensitive to omega-CTx-MVIIC; 3) omega-CTx-GVIA and omega-Aga-IVA resistant (up to 1 mu M) but omega-CTx-MVIIC sensitive (11%); and 4) omega-CTx-GVIA, omega-Aga-IVA, and omega-CTx-MVIIC resistant (27%). On the basis of this pharmacological evidence, a functional classification of presynaptic Ca channel types including N, Q, and R type is discussed. These Ca channel types trigger transmitter release with a similar efficacy, suggesting that they are randomly distributed around transmitter release sites.