Novel Ca2+-Dependent Mechanisms Regulate Spontaneous Release at Excitatory Synapses onto CA1 Pyramidal Cells

Although long thought to simply be a source of synaptic noise, spontaneous, action-potential independent release of neurotransmitter from presynaptic terminals has multiple roles in synaptic function. We explored whether and to what extent the two predominantly proposed mechanisms for explaining spontaneous release, stochastic activation of voltage-gated Ca²⁺ channels (VGCCs) or activation of Ca²⁺-sensing receptors (CaSRs) by extracelluar Ca²⁺, played a role in the sensitivity of spontaneous release to the level of extracellular Ca²⁺ concentration at excitatory synapses at CA1 pyramidal cells of the adult, male mouse hippocampus. Blocking VGCCs with Cd²⁺ had no effect on spontaneous release, ruling out stochastic activation of VGCCs. Although divalent cation agonists of CaSRs, Co²⁺ and Mg²⁺, dramatically enhanced mEPSC frequency, potent positive and negative allosteric modulators of CaSRs had no effect. Moreover, immunoblot analysis of hippocampal lysates failed to detect CaSR expression, ruling out the CaSR. Instead, the increase in mEPSC frequency induced by Co²⁺ and Mg²⁺ was mimicked by lowering postsynaptic Ca²⁺ levels with BAPTA. Together, our results suggest that a reduction in intracellular Ca²⁺ may trigger a homeostatic-like compensatory response that upregulates spontaneous transmission at excitatory synapses onto CA1 pyramidal cells in the adult hippocampus.

from Physiology via xlomafota13 on Inoreader http://ift.tt/2zLNXOc
via IFTTT