R-type Ca(2+) currents evoke transmitter release at a rat central synapse

Voltage-dependent Ca(2+ )currents evoke synaptic transmitter release. Of six types of Ca(2+ )channels, L-, N-, P-, Q-, R-, and T-type, only N- and P/Q-type channels have been pharmacologically identified to mediate action-potential-evoked transmitter release in the mammalian central nervous system. We tested whether Ca(2+ )channels other than N- and P/Q-type control transmitter release in a calyx-type synapse of the rat medial nucleus of the trapezoid body. Simultaneous recordings of presynaptic Ca(2+) influx and the excitatory postsynaptic current evoked by a single action potential were made at single synapses. The R-type channel, a high-voltage-activated Ca(2+) channel resistant to L-, N-, and P/Q-type channel blockers, contributed 26% of the total Ca(2+) influx during a presynaptic action potential. This Ca(2+) current evoked transmitter release sufficiently large to initiate an action potential in the postsynaptic neuron. The R-type current controlled release with a lower efficacy than other types of Ca(2+ )currents. Activation of metabotropic glutamate receptors and γ-aminobutyric acid type B receptors inhibited the R-type current. Because a significant fraction of presynaptic Ca(2+ )channels remains unidentified in many other central synapses, the R-type current also could contribute to evoked transmitter release in these synapses.