Pressure-induced depression of synaptic transmission in the cerebellar parallel fibre synapse involves suppression of presynaptic N-type Ca2+ channels.

High pressure induces CNS hyperexcitability while markedly depressing synaptic transmitter release. We studied the effect of pressure (up to 10.1 MPa) on the parallel fibre (PF) synaptic response in biplanar cerebellar slices of adult guinea pigs. Pressure mildly reduced the PF volley amplitude and to a greater extent depressed the excitatory field postsynaptic potential (fPSP). The depression of the PF volley was noted even at supramaximal stimulus intensities, indicating an effect of pressure on the amplitude of the action potential in each axon. Low concentrations of TTX mimicked the effects of pressure on the PF volley without affecting the fPSP. Application omega-conotoxin GVIA (omega-CgTx) reduced the synaptic efficacy by 34.3+/-2.7%. However, in the presence of omega-CgTx the synaptic depression at pressure was significantly reduced. Reduced Ca2+ entry by application of Cd2+ or low [Ca2+]o did not have a similar influence on the effects of pressure. Application of omega-AGA IVA, omega-AGA TK and Funnel-web spider toxin did not affect the synaptic response in concentrations that usually block P-type Ca2+ channels, whilst the N/P/Q-type blocker omega-conotoxin MVIIC reduced the response to 52.7+/-5.0% indicating the involvement of Q-type channels and R-type channels in the non-N-type fraction of Ca2+ entry. The results demonstrate that N-type Ca2+ channels play a crucial role in the induction of PF synaptic depression at pressure. This finding suggests a coherent mechanism for the induction of CNS hyperexcitability at pressure.