Excitatory neurons in stratum radiatum provide an alternative pathway for excitation flow that escapes perisomatic inhibition

Reviewer #1 (Public Review):

In this manuscript, Lebedeva et al. report the input/output wiring diagram of a population of previously identified giant excitatory neurons (abbreviated as ExNr) in the CA1 region of the rat hippocampus. Overall, Lebedeva et al. report that 1) ExNr are driven by Schaffer collaterals; 2) ExNr do not contact CA1Pyrs; 3) ExNr innervate PV interneurons; 4) ExNr received inhibition from bistratified cells, but not basket cells; and 5) ExNr -> PV synapse is strong enough to massively inhibit CA1Pyrs. Some of the findings reported here appear interesting. However, my appreciation of this manuscript was dampened by the limited scientific novelty, strong statements that are sometimes not supported by data, and vague, imprecise, and oversimplified narratives used throughout.

(1) The identity of ExNr reported here is unclear. It is unclear how ExNr are identified, and how robust the identification criteria are. A single anatomical reconstruction is provided together with depolarization-induced firing. However, whether all cells are consistent with the examples provided is unclear. Giant radiatum cells (previously known as RGCs, here abbreviated as ExNr) were previously identified by Maccaferri (1996) and Gulyas (1998). Based on anatomical criteria alone, it was suggested that these cells could take 4 different forms. The current manuscript mostly ignores this past finding. Given the topic of this paper, a careful anatomical and electrophysiological examination of ExNr is required.

(2) The identity of recorded interneurons is unclear. A major and potentially interesting finding reported here is the differential connectivity of ExNr to basket and bistratified neurons. However, it seems like basket and bistratified cells were mostly identified on the basis of electrophysiological criteria, and that 'only 5 neurons of each group were filed with biocytin, and the identity of interneurons was confirmed by axonal arborization pattern.' First, this significantly departs from the general current practices in the field where interneurons are identified based on combined anatomical and electrophysiological properties. This is because multiple examples in the literature support the extreme heterogeneity of interneurons, and that a combination of criteria is usually required for their appropriate identification. Second, the reconstruction of these neurons should be provided. Since the circuit wiring diagram proposed by the authors is based on these results, proper interneuron classification is necessary.

(3) Multiple conclusions are overstatements. For example, the interpretation that ExNr escapes perisomatic inhibition, as reported in the title, seems to ignore large families of cholecystokinin- or Sncg-expressing basket cells.

(4) Some of the more exciting findings appear preliminary, and the robustness of the findings is hard to evaluate. An example of that is found on Page 8, line 179: 'Thus, ExNR can operate as an amplification relay station for feed-forward inhibition of neurons in the CA area.' This conclusion appears only loosely supported by a few observations, (n = 3), as stated above. Similarly, the next section investigates the downstream effect of ExNr firing on CA1 pyramidal cells. The author reports that 'In 24% of the slices unitary APs in ExNr generated an fIPSP, delayed relative to the peak of the AP by 5.5 ms (n=6; Fig 3D-F).' In my opinion, 24% is a relatively low occurrence, even if we consider potentially cut axons (rightfully acknowledged by the authors) during the slicing procedure. Overall, this clearly doesn't fit the 'massive inhibition of downstream CA1Pyrs' proposed by the authors.

(5) The abstract and introduction are often too vague or oversimplified.

Reviewer #2 (Public Review):

Summary:

This study addresses an intriguing and little-studied population of large excitatory cells that lie in the stratum radiatum, outside the classical cell body layers in the hippocampus. Interestingly, the authors show that these "giant excitatory neurons in stratum radiatum" strongly drive both bistratified and basket interneurons. Activating a single giant cell could induce action potential firing in postsynaptic interneurons, which in turn inhibit their postsynaptic pyramidal cell targets. They appear to receive excitatory input from CA3 but not the entorhinal cortex; at a local level, they are not strongly interconnected with CA1 pyramidal cells, and receive inhibitory input from bistratified but not basket cells.

The lack of perisomatic input from basket cells is unique in comparison with the vast majority of excitatory cells in the hippocampus. It is however not surprising, given the fact that the giant excitatory neurons studied in this paper are defined by their position in a particular hippocampal layer (stratum radiatum), and the axons of inhibitory basket cells are largely restricted to another layer (stratum pyramidale). Nonetheless, the fact that this study draws attention to this unique property, and also provides data to support it, is valuable. As the authors also point out, given the importance of such perisomatic input for rhythmogenesis in the hippocampus, the lack of such input may leave these cells free to operate outside of the dominant rhythm.

In combination with the strong drive onto interneurons, which strongly control the activity of pyramidal cells, the giant excitatory cells in the stratum radiatum appear to be in a unique position to influence the hippocampal circuit. Although clearly such an alternative pathway provides the potential for more diverse functions within the hippocampal circuit, and the connectivity shown in this study will likely be of interest to anyone interested in hippocampal function, the authors do not show a concrete function for this pathway.

Strengths:

Overall, the main value of this study is to demonstrate that this small population of oft-neglected cells could have an unexpectedly large impact on hippocampal function via a uniquely strong excitatory output onto two types of interneurons. Whereas activating a "classical" pyramidal cell produces only subthreshold activity in postsynaptic interneurons, meaning that several pyramidal cells have to be co-active to drive their postsynaptic targets to fire, here the authors show that a single giant excitatory neuron in the stratum radiatum can directly drive at least a subset of its postsynaptic targets to fire.

The authors also show the effect of this output both on the membrane potential of CA1 pyramidal cells and on the extracellular field potential as measured with silicon probes. The fact that the authors identified a relatively large number of these sparse giant excitatory cells in the stratum radiatum and performed paired recordings from them is itself a strength of this study.

Another strength is the fact that the authors also investigate the inputs to these giant excitatory cells. The method of paired patch-clamp recordings in rat brain slices enables in principle to record connectivity in both directions, by stimulating one and checking for a response in the other. Recording the interconnectivity of giant excitatory cells with bistratified, basket, and pyramidal cells, as well as the connectivity between pyramidal cells and the two types of interneurons, allows insightful comparisons between "classic" CA1 pyramidal cells and the displaced giant excitatory cells. Although the lack of connectivity between the latter two cell types that the authors report is not so surprising (given the generally very low connectivity between excitatory cells in CA1), it is nonetheless important data. To also check non-local inputs the authors used optogenetics, whereby a Camk2a promoter likely limited cells expressing channelrhodopsin to mostly excitatory cells.

Weaknesses:

The main weakness of this study is perhaps the lack of a clear function for the described circuitry. Although the authors do speculate on this, it remains to be demonstrated what the role of these cells and their connections with the identified interneuron types might be for hippocampal function.

For the first experimental result, it's not fully clear from the evidence the authors present, that indeed the injections were limited to CA3 (for Figure 1c) and to EC (for Figure 1d). This is important since in theory the CA3 injection could also include e.g. CA2 or CA1 itself, which is not that unlikely given the relatively large injected volume of 1ul per side (bilateral). Similarly for the EC injection, it appears the injection may be 2 ul per side (the methods are a bit ambiguous, unfortunately), and this could lead to infection in e.g. Subiculum. Given that these potential mistargeted areas may also project to CA1, this could obviously change the conclusions one can draw from the optogenetic stimulation results the authors present. Furthermore, for the EC result, the authors assume the response they measure is not monosynaptic, which indeed is likely given the long delay, but to interpret this properly a few recordings with pharmacology would be helpful to really show monosynaptic connections (also for the CA3 inputs). One could also cut the inputs to the DG to show that the delayed EC inputs are abolished then (or instead they may be relayed via local CA1 pyramids receiving EC input). Either way, some additional line of evidence beyond simply the delay would be reassuring. A further worry for the EC result relates to the angle of slicing: can the authors give the reader some reassurance that the lack of monosynaptic inputs is not simply a result of cut connections in the slices they used? Especially since only 5 neurons were recorded with stimulation of presumed EC fibers, it is hard to rule out EC input based on the presented evidence. Related to this, one wonders why in Figures 1D and 1E there are no reported connections from EC to CA1 pyramidal cells (while the authors do include CA1 pyramidal cell recordings for the CA3 stimulation experiments); again this might suggest the connections are simply cut in the slice preparation.

For the connectivity results, the data seem to support the claims, but the conclusions would be improved if the terminology of "privileged" and "escaped" could be avoided. More importantly, the exact criteria for distinguishing between bistratified and basket cells are not fully clear; it seems that the amount of current needed to induce AP firing was the main criterion but there is no figure showing this data (only an example in S1A). The input resistance distributions are overlapping, so this was clearly not used as the main criterion. Showing some pictures of the filled cells as supplemental material would also be helpful to give the reader a bit more confidence that the classification is reliable. In the methods, it is mentioned that 10 cells were filled with biocytin, but the authors don't explicitly state (or show) that the identity was confirmed for all 10 filled cells, and what this was based on. Overall, a bit more info on the giant excitatory cells in the stratum radiatum would be helpful (e.g. soma locations, extent of dendrites relative to layers, density/nr of cells); a brief mention of this in the introduction or discussion would help the reader to place the work in context.

The number of tested pairs or cells is also a bit low (or unclear) in some cases. For instance, the relatively low number of recordings (n=30) between CA1 pyramidal cells and giant excitatory cells in the radiatum means a low connectivity rate on the order of a few percent cannot be ruled out; it has been shown that even in CA3, which is classically considered a "reciprocally connected" area, such low connectivity rates can still be functionally important (Guzman et al).

For the feedforward inhibition result, the concept of "amplification relay station" that was introduced is not so clear. It is not unexpected that when you strongly innervate BC cells and bring them to spike, as the giant cells in this study do, this activity will in turn inhibit pyramidal cells (and actually quite a lot of them, so that it is not surprising that you can measure IPSCs). Furthermore, the rationale for doing the silicon probe recordings is not well explained, and it would be helpful if the authors could discuss the significance of performing such LFP recordings in slices.

Conceptually, the presentation of perisomatic inhibition as simply silencing pyramidal and granule cells, forming a "burden" that needs to be "overcome" or "bypassed" via an alternative pathway (as in the example the authors give of having an axon coming from a dendrite instead of the presumably "blocked" soma), is not so convincing. Perisomatic inhibition is much more than that, particularly if one takes timing into account (indeed the authors point to its role in rhythmogenesis). This does not detract from the fact that the lack of perisomatic inhibition (at least from fast-spiking basket cells) is likely to have large functional implications, which the authors rightly emphasize.

Reviewer #3 (Public Review):

Summary:

This paper reexamined large excitatory neurons in the stratum radiatum with optogenetics. The findings are valuable because prior studies of the circuitry were confounded by the use of stimulating electrodes placed in different layers where multiple inputs were stimulated at one time. The strength of the evidence for the conclusions is incomplete because of several concerns with the manuscript.

Strengths:

The strength of the study is the very nice presentation of data. Also, there is a nice combination of patching, LFPs, paired recordings, and microelectrode arrays.

Weaknesses:

The limitations are in the conclusions which don't seem fully justified.