Key points• The intrinsic excitability of a hippocampal CA3 pyramidal cell (CA3-PC), but not CA1-PC, is enhanced by repetitive somatic firing at a physiologically relevant frequency (10 Hz for 2 s).• Such an excitability change is mediated by the Ca 2+ -and Src family kinase-dependent endocytosis of D-type K + channel subunit Kv1.2.• We provide evidence that the surface expression of D-type K + channels is higher in the distal apical dendrites than in the proximal apical dendrites in CA3-PCs.• These results help us understand neuronal computational mechanisms underlying the cognitive functions of the hippocampal CA3 area.Abstract The intrinsic excitability of neurons plays a critical role in the encoding of memory at Hebbian synapses and in the coupling of synaptic inputs to spike generation. It has not been studied whether somatic firing at a physiologically relevant frequency can induce intrinsic plasticity in hippocampal CA3 pyramidal cells (CA3-PCs). Here, we show that a conditioning train of 20 action potentials (APs) at 10 Hz causes a persistent reduction in the input conductance and an acceleration of the AP onset time in CA3-PCs, but not in CA1-PCs. Induction of such long-term potentiation of intrinsic excitability (LTP-IE) was accompanied by a reduction in the D-type K + current, and was abolished by the inhibition of endocytosis or protein tyrosine kinase (PTK). Consistently, the CA3-PCs from Kv1.2 knock-out mice displayed no LTP-IE with the same conditioning. Furthermore, the induction of LTP-IE depended on the back-propagating APs (bAPs) and intact distal apical dendrites. These results indicate that LTP-IE is mediated by the internalization of Kv1.2 channels from the distal regions of apical dendrites, which is triggered by bAP-induced dendritic Ca 2+ signalling and the consequent activation of PTK.

Key pointsr We investigated the cellular mechanisms underlying mossy fibre-induced heterosynaptic long-term potentiation of perforant path (PP) inputs to CA3 pyramidal cells.r Here we show that this heterosynaptic potentiation is mediated by downregulation of Kv1.2 channels.r The downregulation of Kv1.2 preferentially enhanced PP-evoked EPSPs which occur at distal apical dendrites.r Such enhancement of PP-EPSPs required activation of dendritic Na + channels, and its threshold was lowered by downregulation of Kv1.2.r Our results may provide new insights into the long-standing question of how mossy fibre inputs constrain the CA3 network to sparsely represent direct cortical inputs.Abstract A short high frequency stimulation of mossy fibres (MFs) induces long-term potentiation (LTP) of direct cortical or perforant path (PP) synaptic inputs in hippocampal CA3 pyramidal cells (CA3-PCs). However, the cellular mechanism underlying this heterosynaptic modulation remains elusive. Previously, we reported that repetitive somatic firing at 10 Hz downregulates Kv1.2 in the CA3-PCs. Here, we show that MF inputs induce similar somatic firing and downregulation of Kv1.2 in the CA3-PCs. The effect of Kv1.2 downregulation was specific to PP synaptic inputs that arrive at distal apical dendrites. We found that the somatodendritic expression of Kv1.2 is polarized to distal apical dendrites. Compartmental simulations based on this finding suggested that passive normalization of synaptic inputs and polarized distributions of dendritic ionic channels may facilitate the activation of dendritic Na + channels preferentially at distal apical dendrites. Indeed, partial block of dendritic Na + channels using 10 nM tetrodotoxin brought back the enhanced PP-evoked excitatory postsynaptic potentials (PP-EPSPs) to the baseline level. These results indicate that activity-dependent downregulation of Kv1.2 in CA3-PCs mediates MF-induced heterosynaptic LTP of PP-EPSPs by facilitating activation of Na + channels at distal apical dendrites.J. H. Hyun and K. Eom authors contributed equally to this study.

Repetitive action potentials (APs) in hippocampal CA3 pyramidal cells (CA3-PCs) backpropagate to distal apical dendrites, and induce calcium and protein tyrosine kinase (PTK)-dependent downregulation of Kv1.2, resulting in long-term potentiation of direct cortical inputs and intrinsic excitability (LTP-IE). When APs were elicited by direct somatic stimulation of CA3-PCs from rodents of either sex, only a narrow window of distal dendritic [Ca 2ϩ ] allowed LTP-IE because of Ca 2ϩ-dependent coactivation of PTK and protein tyrosine phosphatase (PTP), which renders non-mossy fiber (MF) inputs incompetent in LTP-IE induction. High-frequency MF inputs, however, could induce LTP-IE at high dendritic [Ca 2ϩ ] of the window. We show that MF input-induced Zn 2ϩ signaling inhibits postsynaptic PTP, and thus enables MF inputs to induce LTP-IE at a wide range of [Ca 2ϩ ] i values. Extracellular chelation of Zn 2ϩ or genetic deletion of vesicular zinc transporter abrogated the privilege of MF inputs for LTP-IE induction. Moreover, the incompetence of somatic stimulation was rescued by the inhibition of PTP or a supplement of extracellular zinc, indicating that MF input-induced increase in dendritic [Zn 2ϩ ] facilitates the induction of LTP-IE by inhibiting PTP. Consistently, high-frequency MF stimulation induced immediate and delayed elevations of [Zn 2ϩ ] at proximal and distal dendrites, respectively. These results indicate that MF inputs are uniquely linked to the regulation of direct cortical inputs owing to synaptic Zn 2ϩ signaling.

The associative network of hippocampal CA3 is thought to contribute to rapid formation of contextual memory from one‐trial learning, but the network mechanisms underlying decorrelation of neuronal ensembles in CA3 is largely unknown. Kv1.2 expressions in rodent CA3 pyramidal cells (CA3‐PCs) are polarized to distal apical dendrites, and its downregulation specifically enhances dendritic responses to perforant pathway (PP) synaptic inputs. We found that haploinsufficiency of Kv1.2 (Kcna2+/−) in CA3‐PCs, but not Kv1.1 (Kcna1+/−), lowers the threshold for long‐term potentiation (LTP) at PP‐CA3 synapses, and that the Kcna2+/− mice are normal in discrimination of distinct contexts but impaired in discrimination of similar but slightly distinct contexts. We further examined the neuronal ensembles in CA3 and dentate gyrus (DG), which represent the two similar contexts using in situ hybridization of immediate early genes, Homer1a and Arc. The size and overlap of CA3 ensembles activated by the first visit to the similar contexts were not different between wild type and Kcna2+/− mice, but these ensemble parameters diverged over training days between genotypes, suggesting that abnormal plastic changes at PP‐CA3 synapses of Kcna2+/− mice is responsible for the impaired pattern separation. Unlike CA3, DG ensembles were not different between two genotype mice. The DG ensembles were already separated on the first day, and their overlap did not further evolve. Eventually, the Kcna2+/− mice exhibited larger CA3 ensemble size and overlap upon retrieval of two contexts, compared to wild type or Kcna1+/− mice. These results suggest that sparse LTP at PP‐CA3 synapse probably supervised by mossy fiber inputs is essential for gradual decorrelation of CA3 ensembles.

Distinguishing different contexts is thought to involve a form of pattern separation that minimizes overlap between neural ensembles representing similar experiences. Theoretical models suggest that the dentate gyrus (DG) segregates cortical input patterns before relaying its discriminated output patterns to the CA3 hippocampal field. This suggests that the evaluation of neural ensembles in DG and CA3 could be an important means to investigate the process of pattern separation. In the past, measurement of entorhinal cortex (EC), DG, and CA3 ensembles was largely dependent upon in vivo electrophysiological recording, which is technically difficult. This protocol provides a method to instead measure pattern separation by a molecular method that provides direct spatial resolution at the cellular level.

MLT may prove useful for reducing the genotoxic effects of hypoxia in peripheral lymphocytes and suggest its possible role for ischemic diseases.

Neuronal excitability is a key determinant for recruitment of a neuron to an ensemble. High-frequency mossy fiber (MF) inputs induce prolonged increases in the excitability of a CA3 pyramidal cell (long-term potentiation of intrinsic excitability, LTP-IE), which primes the cell such that weak perforant pathway (PP) inputs can induce long-term potentiation at PP synapses (PP-LTP). However, sustained hyperexcitability can be detrimental, and a mechanism to reverse this state is necessary. Here, we show that burst firings of CA3 pyramidal cells elicited by PP or recurrent inputs reverse the MF-induced LTP-IE. Moreover, the high-frequency PP inputs to CA3-PCs already primed by MF inputs induced not only PP-LTP but also restored the high excitability state. Labeling neuronal ensemble using c-fos promoter in animals exposed to a novel context, we found most CA3 ensemble cells exhibited increased excitability, indicative of LTP-IE. Moreover, when the animals experienced novel contexts twice with a 30 min interval, a substantial subset of putative twice-activated CA3 ensemble cells exhibited reduced excitability, implying depotentiation of LTP-IE. We developed an in silico model based on the experimental results and found that MF-induced LTP-IE and its depotentiation are critical for the association of neuronal ensembles representing temporally spaced events.

Hippocampus is known to be important for episodic memories. Measuring of hippocampal neural ensembles is therefore important for observing hippocampal cognitive processes such as pattern completion. Previous studies on pattern completion had a limitation because the activities of CA3 were not simultaneously observed with the activities of the entorhinal cortex that project to the CA3. In addition, in previous research and modelling, distinct concepts such as pattern completion and pattern convergence have not been considered separately. Here, I used a molecular analysis technique that enables comparison of neural ensembles that evoked two successive events and evaluated neural ensembles in the hippocampal CA3 region and entorhinal cortex. By comparing neural ensembles in hippocampus and entorhinal cortex, I could obtain evidence that suggests pattern completion occurring in the CA3 region was induced by the partial input from EC. Use of the molecular-based ensemble measurement allows measuring two or more brain regions simultaneously, which can lead to insights into the cognitive functions of neural circuits.