Extended Data Fig. 4: dCA2 pyramidal neurons project to both dCA1 and LS.

a, Dual retrograde staining of dCA2 by injection of HSV-LSL1–mCherry into dCA1 and CTB-488 into dLS of an Amigo2-Cre mouse. b, Hippocampal coronal section following injections as in a and labelled for RGS14 (blue). Arrowheads denote single-labelled cells (green or red) and arrows dual-labelled ones (white). Scale bar, 50 µm. c, Quantification of the percentage of CA2 pyramidal neurons that project to either dLS or dCA1. Because retrograde labelling efficiency is not complete, the fraction of labelled cells provides a lower limit on the fraction of dCA2 pyramidal neurons that project to these regions (12 sections from 6 mice in each group). Bars show mean ± s.e.m. d, Comparison of the expected percentage of dual-labelled CA2 pyramidal neurons versus the observed percentage. The fraction of dual-labelled cells (21 ± 4%) was almost identical to the fraction predicted under the assumption of random retrograde labelling of a single population of CA2 pyramidal neurons, each of which sends a projection to CA1 and LS ([fraction of labelled CA1 projecting cells] × [fraction of labelled LS projecting cells] = 0.55 × 0.44 = 24%). This is similar to results suggesting that a uniform population of CA3 pyramidal neurons projects to both LS and CA1⁵¹. Thus, it is likely that a single population of CA2 pyramidal cells projects to both LS and CA1. Two-sided Wilcoxon test, P = 0.2. Black dots, individual mice (n = 6). Red dots with error bars, mean ± s.e.m.