Spontaneous release from mossy fiber terminals inhibits Ni2+‐sensitive T‐type Ca2+channels of CA3 pyramidal neurons in the rat organotypic hippocampal slice

The Low-Threshold Calcium Channel Cav3.2 Mediates Burst Firing of Mature Dentate Granule Cells

Mature granule cells are poorly excitable neurons that were recently shown to fire action potentials, preferentially in bursts. It is believed that the particularly pronounced short-term facilitation of mossy fiber synapses makes granule cell bursting a very effective means of properly transferring information to CA3. However, the mechanism underlying the unique bursting behavior of mature granule cells is currently unknown. Here, we show that Cav3.2 T-type channels at the axon initial segment are responsible for burst firing of mature granule cells in rats and mice. Accordingly, Cav3.2 knockout mice fire tonic spikes and exhibit impaired bursting, synaptic plasticity and dentate-to-CA3 communication. The data show that Cav3.2 channels are strong modulators of bursting and can be considered a critical molecular switch that enables effective information transfer from mature granule cells to the CA3 pyramids.

Diverse synaptic and dendritic mechanisms of complex spike burst generation in hippocampal CA3 pyramidal cells

Complex spike bursts (CSBs) represent a characteristic firing pattern of hippocampal pyramidal cells (PCs). In CA1PCs, CSBs are driven by regenerative dendritic plateau potentials, produced by correlated entorhinal cortical and CA3 inputs that simultaneously depolarize distal and proximal dendritic domains. However, in CA3PCs neither the generation mechanisms nor the computational role of CSBs are well elucidated. We show that CSBs are induced by dendritic Ca²⁺ spikes in CA3PCs. Surprisingly, the ability of CA3PCs to produce CSBs is heterogeneous, with non-uniform synaptic input-output transformation rules triggering CSBs. The heterogeneity is partly related to the topographic position of CA3PCs; we identify two ion channel types, HCN and Kv2 channels, whose proximodistal activity gradients contribute to subregion-specific modulation of CSB propensity. Our results suggest that heterogeneous dendritic integrative properties, along with previously reported synaptic connectivity gradients, define functional subpopulations of CA3PCs that may support CA3 network computations underlying associative memory processes.

Group I metabotropic glutamate receptors are involved in TEA-induced long-term potentiation at mossy fiber-CA3 synapses in the rat hippocampus

Gq-protein-coupled Group I metabotropic glutamate receptors (mGluR) reportedly activate phospholipase C (PLC), leading to Ca(2+) release from intracellular stores and the formation of diacylglycerol (DAG). We electrophysiologically examined the involvement of the Group I mGluR in tetraethylammonium (TEA)-induced long-term potentiation (LTP) at mossy fiber (MF)-CA3 synapses in the rat hippocampus. TEA-induced LTP was almost completely blocked under the selective blockade of either mGluR1 or mGluR5, both of which are Group I mGluR. This result was supported by the blockade of TEA-induced LTP even in the absence of these blockers under low temperature conditions, in which the activation of Group I mGluR is thought not to be fully effective. In addition, the blockade of mGluR1 resulted in lower short-term potentiation (STP) during TEA application compared with the blockade of mGluR5. These results demonstrate the crucial roles of Group I mGluR in the TEA-induced LTP at MF-CA3 synapses and the different contributions of mGluR1 and mGluR5 to the initial component of plasticity.

TEA-induced long-term potentiation at hippocampal mossy fiber-CA3 synapses: characteristics of its induction and expression

Potassium ion channel blockade by tetraethylammonium (TEA) reportedly induces long-term potentiation (LTP) at hippocampal mossy fiber (MF)-CA3 synapses, but the characteristics of induction, expression, and modulation of the LTP remain unclear. In the present study, these features of TEA-induced LTP at MF-CA3 synapses were electrophysiologically examined using rat hippocampal slices. Synaptic responses recorded from MF-CA3 synapses were enhanced long-term by TEA application even under the blockade of NMDA receptors with D-AP5, whereas selective pharmacological blockade of T-type voltage-dependent calcium channels (VDCCs) strongly inhibited TEA-induced LTP. Decrease of the paired-pulse facilitation ratio after LTP induction by TEA suggests the involvement of increased neurotransmitter release probability from MF terminals as LTP expression. The facilitative modulation of MF-CA3 LTP by GABA(A) receptor activation reported previously was reversed when bumetanide, a blocker of Na(+)-K(+)-Cl(-) co-transporters (NKCCs), was applied, suggesting that the region-specific modulation of TEA-induced LTP by GABAergic inputs at MF-CA3 synapses is due to the dominance of NKCC action at MF terminals.