The increase in synaptic terminal [Ca²⁺] that triggers exocytosis is coupled to a decrease in extracellular [Ca²⁺] ([Ca²⁺]_o). While a fall in cleft [Ca²⁺] should reduce subsequent synaptic transmission, sustained fast transmission is a common feature in the cerebral cortex. This led us to hypothesize that at excitatory synapses, a fall in cleft [Ca²⁺] may provide a retrograde compensatory signal to the nerve terminal. Using a modified patch clamp technique we previously demonstrated that falls in [Ca²⁺]_o activate, a novel, non-selective cation channel (NSCC) at neocortical nerve terminals. Using a combination of pharmacological probes and mutant mice we recently identified the receptor in the Ca sensor-NSCC signaling pathway as the calcium-sensing receptor (CaSR), a G-protein coupled receptor that has been localized to nerve terminals. We tested if this receptor could compensate for falls in external [Ca²⁺]. Using paired recordings from neocortical neurons we showed that activation of CaSR inhibited evoked excitatory transmission and that the probability of release was increased in neurons from reduced function CaSR mutants.

Spontaneous release is also dependent on [Ca²⁺]_o but the mechanisms regulating spontaneous vesicle fusion remain unclear. Furthermore, these events and evoked release may arise from distinct vesicle pools. Using patch-clamp methods we have investigated possible mechanisms for the modulation of spontaneous glutamate release by [Ca²⁺]_o from cultured neocortical neurons. We hypothesized that Ca²⁺ entry via voltage-activated Ca²⁺ channels triggered spontaneous release. However blockade of voltage-gated calcium channels had no effect on the enhancement of miniature excitatory postsynaptic current (mEPSC) frequency by elevation of [Ca²⁺]_o. We next tested if Ca entry via the sodium/calcium exchanger or the plasma membrane calcium ATPase mediated the effect of [Ca²⁺]_o. Inhibition of the sodium/calcium exchanger or the plasma membrane calcium ATPase enhanced the facilitation of mEPSC frequency by [Ca²⁺]_o indicating the involvement of an alternative signaling pathway. We tested if CaSR could mediate this response and found that CaSR agonists increased mEPSC frequency, and that neurons from mice with mutant CaSR displayed altered mEPSC frequency dependence on [Ca²⁺]_o. We propose that CaSR links calcium in the synaptic cleft to spontaneous glutamate release. Together these data indicate that CaSR regulates evoked and spontaneous release of glutamate at neocortical nerve terminals.