Developmental changes in presynaptic calcium channels coupled to glutamate release in cultured rat hippocampal neurons

Excitatory synaptic transmission in the hippocampus involves the participation of at least two types of presynaptic Ca2+ channels, N-type channels sensitive to omega-conotoxin GVIA (omega-CTx GVIA) and Q-type channels sensitive to omega-agatoxin IVA (omega-Aga IVA). Hippocampal pyramidal neurons in cell culture were used to examine the participation of these two classes of channels at different stages of synapse development. Specific Ca2+ channel toxins were used to block presynaptic Ca2+ channels while whole-cell voltage-clamp recordings were used to record evoked EPSCs in postsynaptic neurons. At immature synapses (cells in culture for 10-15 d), omega-CTx GVIA (1-5 microM) blocked transmission by more than 80% while omega-Aga IVA (1 microM) was less effective. In older cultures, however, omega-Aga IVA (1 microM) was more effective than omega-CTx GVIA (1-5 microM) in blocking synaptic transmission. The pharmacological properties of the omega-Aga IVA sensitive component of synaptic transmission were examined in more detail using omega-Aga IVA and omega-conotoxin MVIIC (omega-CTx MVIIC). The properties of this component of transmitter release indicated that a Q-type Ca2+ channel was involved in presynaptic Ca2+ entry. The results suggest that different classes of presynaptic Ca2+ channels begin to participate in transmitter release at different times during synapse development and maturation.