Changes in synaptic transmission properties due to the expression of N-type calcium channels at the calyx of Held synapse of mice lacking P/Q-type calcium channels

P/Q-type and N-type calcium channels mediate transmitter release at rapidly transmitting central synapses, but the reasons for the specific expression of one or the other in each particular synapse are not known. Using whole-cell patch clamping from in vitro slices of the auditory brainstem we have examined presynaptic calcium currents (I-pCa) and glutamatergic excitatory postsynaptic currents (EPSCs) at the calyx of Held synapse from transgenic mice in which the alpha(1A) pore-forming subunit of the P/Q-type Ca2+ channels is ablated (KO). The power relationship between Ca2+ influx and quantal output was studied by varying the number of Ca2+ channels engaged in triggering release. Our results have shown that more overlapping Ca2+ channel domains are required to trigger exocytosis when N-type replace P/Q-type calcium channels suggesting that P/Q type Ca2+ channels are more tightly coupled to synaptic vesicles than N-type channels, a hypothesis that is verified by the decrease in EPSC amplitudes in KO synapses when the slow Ca2+ buffer EGTA-AM was introduced into presynaptic calyces. Significant alterations in short-term synaptic plasticity were observed. Repetitive stimulation at high frequency generates short-term depression (STD) of EPSCs, which is not caused by presynaptic Ca2+ current inactivation neither in WT or KO synapses. Recovery after STD is much slower in the KO than in the WT mice. Synapses from KO mice exhibit reduced or no EPSC paired-pulse facilitation and absence of facilitation in their presynaptic N-type Ca2+ currents. Simultaneous pre- and postsynaptic double patch recordings indicate that presynaptic Ca2+ current facilitation is the main determinant of facilitation of transmitter release. Finally, KO synapses reveal a stronger modulation of transmitter release by presynaptic GTP-binding protein-coupled receptors (gamma-aminobutyric acid type B receptors, GABA(B), and adenosine). In contrast, metabotropic glutamate receptors (mGluRs) are not functional at the synapses of these mice. These experiments reinforce the idea that presynaptic Ca2+ channels expression may be tuned for speed and modulatory control through differential subtype expression.