CA1 pyramidal cell diversity enabling parallel information processing in the hippocampus

Abstract

Hippocampal network operations supporting spatial navigation and declarative memory are traditionally interpreted in a framework where each hippocampal area, such as the dentate gyrus, CA3, and CA1, consists of homogeneous populations of functionally equivalent principal neurons. However, heterogeneity within hippocampal principal cell populations, in particular within pyramidal cells at the main CA1 output node, is increasingly recognized and includes developmental, molecular, anatomical, and functional differences. Here we review recent progress in the delineation of hippocampal principal cell subpopulations by focusing on radially defined subpopulations of CA1 pyramidal cells, and we consider how functional segregation of information streams, in parallel channels with nonuniform properties, could represent a general organizational principle of the hippocampus supporting diverse behaviors.

Fig. 1. Hypothetical nature of parallel channels

a, Left: schematic diagram illustrating hypothetical parallel information-processing channels, where each homogeneous module (f₁) performs essentially similar operation on the inputs. Right: for heterogeneous modules (f₁… f_n), each information-processing channel performs a different operation on the distinct inputs. Each module can be thought of as a microcircuit consisting of particular principal cell and GABAergic interneuron subpopulations, with the principal cells receiving distinct afferents and projecting to different downstream target brain areas. i₁–i_n, inputs; o₁–o_n, outputs. b, Schematic representation of parallel circuits consisting of heterogeneous modules with inhibitory interactions between the modules, with potential for a certain degree of overlap between specific afferent inputs and efferent outputs. The inhibitory interactions can be asymmetric (see Fig. 3).

Fig. 2. Developmental, genetic, morphological, and intrinsic electrophysiological differences between radially defined CA1PC sublayers

a, Top: schematic illustrating coronal section of the hippocampus with major subregions. Middle left: schematic drawing illustrates the deep (blue) and superficial (red) subdivisions of the CA1 somatic layer. Middle right: calbindin expression in the superficial sublayer in the septal CA1. Scale bar, 20 μm. Rad., stratum radiatum; Ori., stratum oriens. Bottom: differences in the cellular compactness and width of the deep and superficial CA1PC sublayers along dorsoventral axis of the hippocampus. Adapted from ref. , Cell Press. b, Superficial and deep CA1PCs are generated in different time-windows during embryonic neurogenesis: in utero intraventricular injections (IUI) of tdTomato-expressing adeno-associated virus at embryonic day 14 (E14, cyan) and E17 (red) label deep and superficial CA1PCs, respectively. Pyramidal cells of the densely packed superficial layer in CA1 arise approximately 2–3 d after those of the deep layer, on average. c, Two example marker genes for superficial and deep sublayers identified with RNA sequencing (top) and cross-validated with in situ hybridization (bottom); gene-expression values (top) are expressed in fragments per kilobase of exon per million fragments mapped (FPKM). Super, superficial. d, Top: the number of enriched genes as a function of the minimum enrichment across all replicates using bulk RNAseq. Note that the dorsoventral axis exhibits the most robust gene-expression differences, but there are many enriched genes across both the superficial–deep and proximodistal axes as well. Bottom: examples of genes enriched in the superficial and deep sublayers. Differences in gene expression may relate to functional differences between layers: calbindin-deficient mice and mice with deletion of the gene encoding the transcription factor Zbtb20, which controls calbindin expression, demonstrate deficits in hippocampal-dependent learning and LTP^–. Zbtb20 expression is maintained throughout adulthood in superficial CA1PCs, and its expression is mutually exclusive with that of Sox5, a marker of deep CA1PCs. Images in b–d adapted from ref. , Cell Press. e, Morphological and intrinsic electrophysiological differences between superficial and deep CA1PCs. Top: representative camera lucida drawings of a superficial CA1PC and a deep CA1PC. L.M., stratum lacunosum-moleculare; Rad., stratum radiatum; Pyr., stratum pyramidale; Ori., stratum oriens. Deep cells have larger soma and more complex basal dendrites (adapted from ref. ). Note that Li et al. reported that calbindin-expressing CA1PCs, primarily located in the superficial sublayer, have more complex apical dendritic arborizations (not shown). Middle: superficial CA1PCs exhibit a more strongly depolarized somatic resting membrane potential (V_rest) and a larger somatic h-current that mediates a depolarizing sag (indicated by arrow) during hyperpolarizing current pulses. Bottom: examples of superficial CA1PC-biased signaling pathway, calcium-binding protein, and a divalent ion. H-current, a major regulator of dendritic excitability and synaptic input integration, is larger in superficial CA1PCs, and recent results indicate that tonic activity of postsynaptic cannabinoid type 1 receptors (CB1R) maintains the amplitude of I_h through a dedicated molecular pathway in these cells. HCN1, hyperpolarization-activated cyclic nucleotide-gated channel.

Fig. 3. Biased microcircuits and afferent–efferent connectivity of superficial and deep CA1PCs

a, Deep CA1PCs receive stronger feedforward excitation from MEC and from hippocampal area CA2, while superficial CA1PCs receive stronger excitatory drive from LEC. CA3 Schaffer collateral excitation is stronger in calbindin-positive, superficially located CA1PCs. Note that proximodistal differences in cortical innervation between radial sublayers have also been reported^, (not shown). PVBCs (green) provide stronger perisomatic inhibition onto deep CA1PCs and receive stronger excitation from superficial CA1PCs. Despite the stronger innervation of deep CA1PCs by area CA2, which is important for SPWR initiation, deep CA1PCs participate less reliably in SWPRs compared to their superficial counterparts^,. One possible explanation is that CA2PCs may also recruit feedforward inhibition via PVBCs, which can in turn preferentially suppress deep CA1PCs during SPWRs. Biased microcircuit connectivity of other GABAergic interneuron types remains to be determined (open gray symbols). Among these, the second major basket cell class that provides perisomatic innervation to PCs, the regular-spiking cholecystokinin-positive (CCK) basket cells, did not show a preference for either the deep or superficial CA1PCs in mice, although they appeared to provide stronger innervation to superficial PCs in rats. Superficial and deep sublayers provide output both to cortical and subcortical (divided by horizontal dashed line) target areas. mPFC, medial prefrontal cortex; RS, retrosplenial cortex; SC, subicular complex; AMG, amygdala; NAc, nucleus accumbens; LS, lateral septum; LH, lateral hypothalamus. For some of these efferent projections, the soma locations of CA1PCs have been mapped, showing unbiased (for example, MEC) or deep-biased (for example, mPFC) localization of projection neurons. For other cortical and subcortical targets (gray), the precise sublayer distribution remains to be determined. b, Schematic representation of the biased, nonuniform interaction between AMG-projecting and mPFC-projecting CA1PCs via PVBCs. Adapted from ref. . Deep AMG-projecting CA1PCs receive inhibitory inputs three times larger compared to neighboring deep-layer mPFC-projecting CA1PCs. In turn, the differentially projecting CA1PCs from the same (deep) layer provide highly biased innervation to PVBCs, so that the CA1PC population that receives less basket cell inhibition provides stronger excitation of these interneurons. The net result of this microcircuit organization could be that discharges by the mPFC-projecting deep CA1PCs will preferentially enhance the inhibitory drive onto the AMG-projecting cells, a feature that is broadly consistent with the differential activity patterns during fear extinction.

Fig. 4. Differential behavioral functions of radially defined CA1PCs subpopulations

a, Two-photon Ca²⁺ imaging of superficial and deep sublayers during a head-fixed goal-oriented learning task in mice. Top: schematic of the goal-oriented learning task. Head-fixed mice searched for an unmarked reward zone by running on a cue-rich circular treadmill belt, and water rewards were administered only when the mice licked operantly within the goal zone (circled). At the end of Condition I, the reward was moved to a new location of the belt, and the experiment was repeated (Condition II). The same multisensory context (A) was maintained in both conditions. Middle: simultaneous two-photon imaging of Ca²⁺ sensor GCaMP6f in deep and superficial sublayers of dorsal CA1 using a piezoelectric crystal. Time-averaged image sequences from a representative recording session. Planes were separated by 25 μm. Bottom: fraction of licks in reward zone, the behavioral output measure of learning, plotted against the mean distance of deep (left) and superficial (right) place cells relative to the reward. Individual points represent single recording sessions. Dashed line indicates the linear fit, with the 95% confidence interval shaded. A significant relationship was observed between learning performance and mean distance of place cells in the deep sublayer, but not in the superficial sublayer, indicating that goal-directed reorganization (‘reward zone enrichment’) of deep place cell maps is predictive of learning performance. Adapted from ref. , Cell Press b, Top: model of the olfactory cue-based associative Go–No-go task. Middle: schematic of bilateral optogenetic silencing of calbindin-expressing (green) CA1PCs (in Calb2-IRES-Cre::Ai35 mice) in the dorsal hippocampus. Bottom: optogenetically suppressing activities of calbindin-expressing CA1PCs impairs associative learning. Adapted from ref. , Nature Publishing Group c, Comparison of some in vivo physiological properties of superficial and deep CA1PCs.