Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop

May combine two claims.

CA2 pyramidal neurons have lower input resistance and higher membrane capacitance compared to CA1 or CA3 neurons.

“Thus, the CA2 neuron input resistance was lower and the membrane capacitance was higher compared to CA1 or CA3 neurons (see Table 1).”

May combine two claims.

LIII stimulation with inhibition intact evokes a large net depolarizing response in CA2 neurons, whereas it often evokes a net hyperpolarizing response in CA1 neurons.

“Although LIII stimulation still evoked a large net depolarizing response in CA2 neurons, there was now often a net hyperpolarizing response observed in CA1 neurons (Fig. 2C).”

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CA2 apical dendrite bifurcates close to the soma (31.1 ± 3.6 μm, n = 19) into two or three main branches that extend to stratum lacunosum moleculare (SLM) with few secondary oblique branches in stratum radiatum (SR) but many branches in SLM.

“Thus, the apical dendrite bifurcated close to the soma (31.1 ± 3.6μm, n = 19) into two or three main branches that extended to stratum lacunosum moleculare (SLM) with few secondary, oblique branches in stratum radiatum (SR) but many branches in SLM (Fig. 1C and. Fig. S1).”

May combine two claims.

The large CA2 EPSPs ensure a net depolarizing response even when inhibition is intact, with a 3-4 fold ratio of excitation to inhibition.

“However, the large CA2 EPSPs ensured the presence of a net depolarizing response even when inhibition was intact, with a 3–4 fold ratio of excitation to inhibition (Fig. 2D).”

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The hyperpolarization in CA1 is not due to a lack of excitatory inputs, as all CA1 neurons exhibited a clear EPSP in response to LIII stimulation in the presence of GABA blockers (n = 24 out of 24).

“The hyperpolarization in CA1 was not due to a lack of excitatory inputs as all CA1 neurons exhibited a clear EPSP in response to LIII stimulation in the presence of GABA blockers (n=24 out of 24 neurons).”

May combine two claims.

CA3 neurons fire a burst of action potentials at the beginning of a depolarizing current step, whereas CA1 and CA2 neurons fire during the entire depolarization.

“Moreover, whereas CA3 neurons fired a burst of action potentials at the beginning of a depolarizing current step, CA1 and CA2 neurons fired during the entire depolarization (Fig. 1A).”

May combine two claims.

Stronger stimulation of LII inputs evokes a large inhibitory response, and addition of GABA blockers unmasks polysynaptic EPSPs.

“Stronger stimulation of LII inputs evoked a large inhibitory response, and addition of GABA blockers unmasked polysynaptic EPSPs (Fig. 5A).”

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Activation of LII inputs with a moderate stimulus produces surprisingly large EPSPs in CA2 neurons with little synaptic delay, indicating they are monosynaptic responses.

“Activation of LII inputs with a moderate stimulus produced surprisingly large EPSPs in CA2 neurons with little synaptic delay, indicating they were monosynaptic responses.”

May combine two claims.

SC stimulation never leads to firing in CA2 (n = 0/8) but rather elicits a large net hyperpolarizing response.

“In contrast, SC stimulation never led to firing in CA2 (n = 0/8) but rather elicited a large net hyperpolarizing response (Fig. 4D,E).”

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Blocking inhibition greatly increases the efficiency of spike firing in CA1 neurons (100%, 11/11) and enables SC inputs to trigger spikes in the majority of CA2 neurons (75%, n = 6/8).

“Blocking inhibition greatly increased the efficiency of spike firing in CA1 neurons, with 100% of neurons firing spikes (11/11), and also enabled the SC inputs to trigger spikes in the majority of CA2 neurons (75%, n = 6/8, Fig. 4F).”

May combine two claims.

SC stimulation produces a large net depolarization in CA1 neurons (EPSP/IPSP ratio ~2), whereas inhibition prevails over excitation in CA2 (EPSP/IPSP < 1).

“As a result, SC stimulation produced a large net depolarization in CA1 neurons (EPSP/IPSP ratio ~2), whereas inhibition prevailed over excitation in CA2 (EPSP/IPSP < 1) (Fig. 3D).”

May combine two claims.

With inhibition intact, LIII stimulation produces a net hyperpolarization and fails to elicit a spike in CA1 neurons.

“With inhibition intact, LIII stimulation produced a net hyperpolarization and failed to elicit a spike in CA1 neurons.”

May combine two claims.

Following block of inhibition, a burst of LIII stimuli triggers spikes in 80% of CA2 neurons (n = 12/15) but remains subthreshold for nearly all CA1 neurons (only 1 in 12 fired a spike).

“Following block of inhibition, the burst of LIII stimuli caused a larger depolarization that triggered spikes in 80% of CA2 neurons (n = 12/15) but remained subthreshold for nearly all CA1 neurons (only 1 in 12 fired a spike; Fig. 4C).”

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Induction of LTP at LIII synapses in both CA1 and CA2 is blocked by the NMDA receptor antagonist APV, indicating these two processes share certain molecular mechanisms.

“The induction of LTP at LIII synapses in both CA1 and CA2 was blocked by the NMDA receptor antagonist APV, indicating these two processes share certain molecular mechanisms (Fig. S3A).”

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LTP at LII and LIII EC inputs to CA2 is pathway specific, as LIII EC inputs display only a small and transient increase in efficacy when HFS is delivered to LII EC inputs.

“Although LII and LIII EC inputs to CA2 terminate in close proximity in SLM, LTP was pathway specific because LIII EC inputs displayed only a small and transient increase in efficacy when the HFS was delivered to LII EC inputs (Fig. S4C).”

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LTP at LII-CA2 synapses does not require activation of CA3 neurons, as it is not altered in slices with mossy fiber inputs cut or with DCG-IV present.

“LTP at the LII –CA2 synapses was not altered in slices in which the mossy fiber inputs were cut, or with DCG-IV present to block glutamate release from the mossy fiber terminals, indicating that the LTP did not require activation of CA3 neurons (Fig. S4B).”

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The large LTP of the fEPSP is due to potentiation of synaptic responses in CA2 pyramidal neurons, confirmed by whole cell recordings (158.7 ± 5.2%, n = 6).

“We confirmed that the large LTP of the fEPSP was actually due to potentiation of synaptic responses in CA2 pyramidal neurons using whole cell recordings (Fig. S3B, 158.7 ± 5.2%, n = 6).”

May combine two claims.

Histograms of the EPSC amplitude distribution showed a prominent peak at 0 pA, representing failures, and a broad distribution of EPSC successes clearly resolvable from the peak of failures.

“Histograms of the EPSC amplitude distribution showed a prominent peak at 0 pA, representing the failures, and a broad distribution of EPSC successes clearly resolvable from the peak of failures.”

• AssumptionRests on an assumption: Synaptic transmission at CA2–CA1 synapses is quantal.

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When activated together, LII and LIII EPSPs show linear summation for small amplitude EPSPs, whereas larger amplitude EPSPs display sublinear summation.

“When activated together, LII and LIII EPSPs showed linear summation for small amplitude EPSPs, whereas larger amplitude EPSPs displayed sublinear summation (Fig. 6A).”

May combine two claims.

LII EC synapses with CA2 neurons generate a correspondingly large LTP (194.3 ± 9.8% increase in fEPSP, n = 6) that is more than two-fold greater than LTP at LII synapses in CA3 (135.1 ± 9.3%, n = 6, p = 0.0008).

“The LII EC synapses with CA2 neurons generated a correspondingly large LTP (194.3 ± 9.8% increase in the fEPSP, n = 6) that was more than two-fold greater than the amount of LTP at LII synapses in CA3 (135.1 ± 9.3%, n = 6, p = 0.0008, Fig. 7C).”

May combine two claims.

When stimulated at different intervals, LII and LIII inputs summated over a symmetrical time window lasting around 20 ms and showed maximum summation when activated simultaneously.

“When stimulated at different intervals, the two inputs summated over a symmetrical time window that lasted around 20 ms and showed a maximum summation when activated simultaneously (Fig. 6B).”

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Induction of LTP at LIII inputs produces a large, ~250% increase in the amplitude of the CA2 population spike elicited by strong stimulation, indicating an enhancement in the efficiency with which cortical inputs drive CA2 firing.

“Finally, induction of LTP at the LIII inputs produced a large, ~250% increase in the amplitude of the CA2 population spike elicited by strong stimulation (Fig. S4A), indicating an enhancement in the efficiency with which cortical inputs drive CA2 firing.”

May combine two claims.

The probability of a CA2-CA1 EPSC success ranged between 0.4 and 0.6, similar to the value at CA3-CA1 synapses.

“The probability of a CA2–CA1 EPSC success ranged between 0.4 and 0.6 (Fig. 8D), similar to the value at CA3–CA1 synapses (Bolshakov and Siegelbaum, 1995).”

• cites Bolshakov and Siegelbaum, 1995Not checked yet

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• AssumptionRests on an assumption: Synaptic transmission at CA2–CA1 synapses is quantal.

May combine two claims.

CA2-CA1 EPSCs always showed multiple peaks of successes, which occurred at equally spaced intervals of around −5 pA.

“In addition, whereas a single peak of EPSC successes was reported for CA3–CA1 synapses, the CA2–CA1 EPSCs always showed multiple peaks of successes, which occurred at equally spaced intervals of around −5 pA (Fig. 8B).”

• AssumptionRests on an assumption: Synaptic transmission at CA2–CA1 synapses is quantal.

May combine two claims.

CA1 neurons have a large number of oblique secondary dendrites in SR that shunt synaptic current, whereas CA2 neurons have very few oblique dendrites in SR.

“First, whereas CA1 neurons have a large number of oblique secondary dendrites in SR that shunt the synaptic current as it propagates down the dendrite, CA2 neurons have very few oblique dendrites in SR.”

• AssumptionRests on an assumption: Excitatory synaptic inputs terminating on distal dendrites usually provide weaker somatic drive than proximal inputs due to cable properties.

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Direct LIII EC inputs to CA1 generate weak EPSPs in the CA1 soma, display little LTP, and recruit strong feed-forward inhibition that suppresses CA1 output.

“Although this residual hippocampal function was originally proposed to result from the ability of the direct connection from LIII EC neurons to drive the firing of CA1 neurons, such inputs generate weak EPSPs in the CA1 neuron soma, display little LTP, and recruit strong feed-forward inhibition that suppresses CA1 neuron output.”

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CA2 pyramidal neurons have a reversed synaptic strength rule: they are strongly excited by distal dendritic inputs from entorhinal cortex but only weakly activated by proximal dendritic inputs from hippocampal CA3 neurons.

“Here we report that hippocampal CA2 pyramidal neurons, whose function has remained obscure for 75 years, have a reversed synaptic strength rule. Thus, CA2 neurons are strongly excited by their distal dendritic inputs from entorhinal cortex but only weakly activated by their more proximal dendritic inputs from hippocampal CA3 neurons.”

Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

CA2 (and CA1) neuron dendrites branch along the transverse axis of the hippocampus, whereas CA3 neurons mostly extend their dendrites along the longitudinal axis.

“CA2 neurons could be further distinguished from CA3 neurons in that CA2 (and CA1) neuron dendrites branch along the transverse axis of the hippocampus (parallel to the plane of the slice) whereas CA3 neurons mostly extend their dendrites along the longitudinal axis of the hippocampus (Ishizuka et al., 1995).”

• cites Ishizuka et al., 1995 sourceNot checked yet

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May combine two claims.

CA2 neurons form a potent disynaptic circuit linking cortical input to CA1 neuronal output, independent of the trisynaptic path, that is powerfully regulated by long-term synaptic plasticity.

“In this manner CA2 neurons form the nexus of a highly plastic disynaptic circuit linking the cortical input to the hippocampus to its CA1 neuronal ouput.”

• Also stated in: § INTRODUCTION · plain

  “Finally, CA2 neurons make unusually strong unitary connections with CA1 neurons, forming a potent disynaptic circuit, independent of the trisynaptic path, that is powerfully regulated by long-term synaptic plasticity.”

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LII EC axons form excitatory synapses on granule cells in the molecular layer of dentate gyrus, then project onto the distal dendrites of CA3 neurons, and finally terminate in a U-shaped area near the distal dendrites of CA2.

“LII EC axons form excitatory synapses on granule cells in the molecular layer of dentate gyrus (DG), then project onto the distal dendrites of CA3 neurons, and finally terminate in a U shaped-area near the distal dendrites of CA2 (Niimi et al., 2007; Nishimura-Akiyoshi et al., 2007; Tamamaki and Nojyo, 1995).”

• cites Niimi et al., 2007 sourceNot checked yet

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• cites Nishimura-Akiyoshi et al., 2007 sourceNot checked yet

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• cites Tamamaki and Nojyo, 1995Not checked yet

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May combine two claims.

SC inputs on their own are strong enough to drive firing in CA2 pyramidal neurons, but their impact is tightly controlled by feed-forward inhibition.

“This suggests that SC inputs on their own are strong enough to drive firing in CA2 pyramidal neurons, but their impact is tightly controlled by feed-forward inhibition.”

May combine two claims.

The larger SC synaptic response in CA1 versus CA2 is consistent with the greater number of oblique dendritic branches in the SR of CA1 compared to CA2 neurons and the more extensive branching of CA3 axons in CA1 versus CA2, suggestive of a greater number of synaptic contacts.

“The larger SC synaptic response in CA1 versus CA2 is consistent with the greater number of oblique dendritic branches in the SR of CA1 compared to CA2 neurons and the more extensive branching of CA3 axons in CA1 versus CA2, suggestive of a greater number of synaptic contacts.”

May combine two claims.

The larger SC intracellular EPSP in CA1 versus CA2 neurons is likely to result from a larger excitatory synaptic current, as the local fEPSP in the SR region of CA1 was larger than that in CA2 despite similar fiber volleys.

“The larger SC intracellular EPSP in CA1 versus CA2 neurons is likely to result from a larger excitatory synaptic current as the local fEPSP in the SR region of CA1 was larger than that in CA2 (Fig. 3B), despite similar fiber volleys in both areas (not shown).”

May combine two claims.

The polysynaptic response likely results from recruitment of the DG→CA3→CA2 pathway, as it is blocked by DCG-IV.

“This polysynaptic response likely resulted from the recruitment of the DG→CA3→CA2 pathway either by activation of LII inputs to granule cells or by their direct activation as it was blocked by the mGluR-II agonist DCG-IV, which produces potent presynaptic inhibition at the mossy fiber synapses from DG to CA3 neurons but does not alter SC EPSPs (Kamiya et al., 1996) (Fig. 5A).”

• cites Kamiya et al., 1996 sourceNot checked yet

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May combine two claims.

The stimulating electrodes in DG and CA1 specifically recruited LII and LIII inputs, respectively.

“These data are consistent with the known anatomical projections of LII and LIII EC axons and confirm that our stimulating electrodes in DG and CA1 specifically recruited LII and LIII inputs, respectively.”

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Anatomical reconstructions have shown that CA2 pyramidal neurons send their axons to both SR and SO of CA1, whereas ipsilateral CA3 neurons only send their axons to SR.

“Anatomical reconstructions have shown that CA2 pyramidal neurons send their axons to both SR and stratum oriens (SO) of CA1 (Tamamaki et al., 1988), whereas ipsilateral CA3 neurons only send their axons to SR.”

• cites Tamamaki et al., 1988 sourceNot checked yet

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Both LII and LIII converge onto CA2 neurons where they interact and exert their maximal effect throughout the hippocampus.

“These results show that both LII and LIII converge onto CA2 neurons where they interact and exert their maximal effect throughout the hippocampus.”

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CA2 neurons are a major target of excitatory cortical input to the hippocampus and can potently excite their CA1 neuron targets.

“Altogether, our findings in acute slices reveal that CA2 neurons are a major target of excitatory cortical input to the hippocampus and can potently excite their CA1 neuron targets.”

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CA2 neurons receive uniquely strong, convergent excitatory input from LII and LIII EC neurons, the only site of such convergence in the hippocampus.

“First, CA2 neurons receive uniquely strong, convergent excitatory input from LII and LIII EC neurons, the only site of such convergence in the hippocampus.”

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Both the LII and LIII synapses with CA2 neurons undergo strong LTP, enhancing the efficacy with which these inputs fire CA2 neurons.

“Third, both the LII and LIII synapses with CA2 neurons undergo strong LTP, enhancing the efficacy with which these inputs fire CA2 neurons.”

May combine two claims.

This inhibition is likely important in gating information flow between CA3 and CA2 and may explain why CA2 neurons do not normally mediate a significant quadrisynaptic loop from EC to DG to CA3 to CA2 to CA1.

“This inhibition is likely to be important in gating information flow between CA3 and CA2 and may explain why CA2 neurons do not normally mediate a significant quadrisynaptic loop from EC to DG to CA3 to CA2 to CA1.”

May combine two claims.

Selective decreases in CA2 inhibition are associated with schizophrenia and epilepsy, two pathological conditions in which hippocampal excitability is increased.

“The relevance of this inhibitory drive is suggested by the findings that selective decreases in CA2 inhibition are associated with schizophrenia (Benes et al., 1998) and epilepsy (Williamson and Spencer, 1994), two pathological conditions in which hippocampal excitability is increased.”

• cites Benes et al., 1998 sourceNot checked yet

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• cites Williamson and Spencer, 1994Not checked yet

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The strong inhibition might terminate CA2 neuron firing after its initial activation by the disynaptic loop, and thus prevent excessive activation of CA1 neurons.

“The strong inhibition might also terminate CA2 neuron firing after its initial activation by the disynaptic loop, and thus prevent excessive activation of CA1 neurons.”

May combine two claims.

The strong excitatory drive of cortical inputs to CA2 neurons, combined with the powerful unitary CA2-CA1 connection, forms a robust disynaptic circuit that may be sufficient to drive CA1 output to mediate key hippocampal functions in spatial learning and memory.

“In contrast, the strong excitatory drive of the cortical inputs to CA2 neurons, combined with the unusually powerful unitary synaptic connection between CA2 and CA1 neurons, forms a robust disynaptic circuit that may be sufficient to drive CA1 neuron output to mediate key hippocampal functions in spatial learning and memory.”

Classified as probably not a substantial claim.

Neurons were identified according to specific electrophysiological properties summarized in table 1.

“Neurons were identified according to specific electrophysiological properties summarized in table 1.”

May combine two claims.

EC inputs to CA2 neurons are strong and undergo robust LTP that enhances the excitatory drive of the disynaptic path.

“In contrast, the EC inputs to CA2 neurons are strong and undergo robust LTP that enhances the excitatory drive of the disynaptic path.”

May combine two claims.

The powerful excitatory disynaptic circuit linking EC to CA2 to CA1, combined with marked feed-forward inhibition from CA3 to CA2, can explain how in vivo electrical stimulation of entorhinal cortex inputs leads to initial firing of CA2 neurons, followed by firing of CA1 and then CA3 neurons.

“Thus the powerful excitatory disynaptic circuit linking EC to CA2 to CA1, combined with the marked feed-forward inhibition from CA3 to CA2, can explain how in vivo electrical stimulation of entorhinal cortex inputs leads to the initial firing of CA2 neurons, followed by firing of CA1 and then CA3 neurons (Bartesaghi and Gessi, 2004; Bartesaghi et al., 2006).”

• cites Bartesaghi and Gessi, 2004Not checked yet

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• cites Bartesaghi et al., 2006 sourceNot checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Series resistance was monitored throughout each experiment; cells with more than 15% change in series resistance were excluded from analysis.

“Series resistance (typically 15–25 MΩ) was monitored throughout each experiment; cells with more than 15% change in series resistance were excluded from analysis.”

Classified as probably not a substantial claim. May combine two claims.

The location of recorded neurons in CA2 was confirmed after fixation and by staining for α-actinin 2.

“The location of the recorded neurons in CA2 was subsequently confirmed after fixation of the slice for 1 hour in 4% paraformaldehyde in phosphate buffer (PBS). We further confirmed the location in CA2 by staining the slice for α-actinin 2, a protein enriched in the CA2 area (Wyszynski et al., 1998)”

• cites Wyszynski et al., 1998 sourceNot checked yet

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May combine two claims.

This excitation may be enhanced upon lesion of CA3 due to loss of feed-forward inhibition, providing a potential explanation for elevated CA1 place cell firing in lesioned animals.

“This excitation may actually be enhanced upon lesion of CA3 due to the loss of feed-forward inhibition, providing a potential explanation for the elevated rate of CA1 neuron place cell firing in the lesioned animals (Nakashiba et al., 2008).”

• cites Nakashiba et al., 2008 sourceNot checked yet

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CA2 neurons are the only hippocampal target of the supramammillary nucleus, a structure involved in controlling theta rhythm and spread of epileptic activity in the hippocampus.

“Finally, CA2 neurons are the only hippocampal target of the supramammillary nucleus (which also targets the dentate gyrus), a structure involved in controlling the frequency of the theta rhythm (Pan and McNaughton, 2002) and the spread of epileptic activity (Saji et al., 2000) in the hippocampus.”

• cites Pan and McNaughton, 2002Not checked yet

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• cites Saji et al., 2000 sourceNot checked yet

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The disynaptic circuit can account for residual hippocampal-dependent memory and high levels of CA1 place cell firing in vivo following surgical or genetic lesions of CA3.

“Moreover, the disynaptic circuit can account for the residual hippocampal-dependent memory and high levels of CA1 place cell firing in vivo following surgical or genetic lesions of CA3 (Brun et al., 2002; Nakashiba et al., 2008).”

• cites Brun et al., 2002 sourceNot checked yet

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• cites Nakashiba et al., 2008 sourceNot checked yet

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