Figure 1. The Avp^PVN→CA2 pathway is anatomically distinct.

a, The diagram illustrating ST HSV-LS1L-WGA-CMV-GFP viral injections in the hippocampal CA2 and subsequent expression in the CA2 (fibers) and hypothalamic paraventricular nucleus (PVN, soma and fibers). b–c, Image and diagram of coronal sections show the expression of green fluorescent protein (GFP⁺, green) following a Cre-inducible HSV-GFP injection and vasopressin (AVP⁺, red) immunostaining in the PVN of AVP promoter-driven Cre (Avp-Cre) mice. Cell nuclei are stained with 4′,6-diamidino-2-phenylindole (DAPI, blue). d, Quantification of Avp⁺, GFP⁺, and GFP⁺ neurons that are Avp⁺ in the PVN reveal that a subset of Avp⁺ neurons express the viral GFP⁺ and therefore project to the CA2. Further, all GFP⁺ neurons co-express Avp⁺, indicating that the HSV-GFP selectively labeled Avp⁺ neurons (n = 3 sections from 4 mice). e–f, Image and diagram of coronal sections show that Avp fibers (labeled by expression of GFP⁺) projecting from the PVN innervate the CA2 and immediately adjacent CA3. Arrows are used to identify anatomical boundaries for the CA areas. g, Specifically, GFP⁺ fluorescence intensity, reported in arbitrary units (a.u.), is highest in the CA2, and to a lesser extent immediately adjacent CA3 (aCA3), compared to the surrounding regions, which include the distal CA3 (dCA3), CA1, parietal cortex (PCX), and somatosensory cortex (SCX). h, The schematic illustrates AAV2-DIO-EF1α-hChR2(H134R)-mCherry viral injections and optical stimulation, emitting at a 465-nm wavelength from a LED light, of Avp^PVN→CA2 fibers. i–j, These are images of coronal sections showing expression of ChR2 fused to mCherry (ChR2⁺, red) in the PVN and CA2 of Avp-Cre mice. Arrows are used to identify anatomical boundaries for the CA areas. Cell nuclei are stained with DAPI (blue). Scale bars are labeled for all images.