Defining hippocampal area CA2 in the fox (Vulpes vulpes) brain

Abstract

Since 1959, the Russian Farm-Fox study has bred foxes to be either tame or, more recently, aggressive, and scientists have used them to gain insight into the brain structures associated with these behavioral features. In mice, hippocampal area CA2 has emerged as one of the essential regulators of social aggression, and so to eventually determine whether we could identify differences in CA2 between tame and aggressive foxes, we first sought to identify CA2 in foxes (Vulpes vulpes). As no clearly defined area of CA2 has been described in species such as cats, dogs, or pigs, it was not at all clear whether CA2 could be identified in foxes. In this study, we cut sections of temporal lobes from male and female red foxes, perpendicular to the long axis of the hippocampus, and stained them with markers of CA2 pyramidal cells commonly used in tissue from rats and mice. We observed that antibodies against Purkinje cell protein 4 best stained the pyramidal cells in the area spanning the end of the mossy fibers and the beginning of the pyramidal cells lacking mossy fibers, resembling the pattern seen in rats and mice. Our findings indicate that foxes do have a "molecularly defined" CA2, and further, they suggest that other carnivores like dogs and cats might as well. With this being the case, these foxes could be useful in future studies looking at CA2 as it relates to aggression.

Identification of hippocampal area CA2 in hamster and vole brain

“…Previously used techniques (e.g., Nissl staining), while allowing researchers to visualize cell sizes and density, are limited in that they cannot distinguish cell types based on molecular characteristics. For example, a recent study in foxes ( Vulpes vulpes ) showed staining for PCP4 in a population of neurons that were very likely in CA2, yet no distinct population of CA2 cells were evident with Nissl staining (Dudek et al., 2023). Similarly, studies using Nissl staining, but not IF, have suggested that domestic cats lack a distinct CA2 cell population based on the anatomical definition, where the large pyramidal cells without input from the mossy fibers were spread out and not easily distinguished from the pyramidal cells of area CA3 (Hirama et al., 1997; Zilli et al., 2022).…”

“…Identification of area CA2 using solely anatomical staining without the additional molecular information that IF provides has faced similar challenges in domestic dogs, rabbits, guinea pigs, and hedgehogs (Hof et al., 1996; Kunzle & Radtke‐Schuller, 2001; Potegal et al., 1993; Rami et al., 1987). However, some studies including single‐nucleus transcriptomic experiments suggest that dogs do have cells with molecular characteristics like those of cells in area CA2 in other animals (Amayasu et al., 1999; Dudek et al., 2023; Hof et al., 1996; Ragbetli et al., 2010; Zhou et al., 2022). More recent work looking at different species of mole rats have demonstrated PCP4 staining that meets the molecular definitions for area CA2 used here (Stöber & Oosthuizen, 2023).…”

“…showed staining for PCP4 in a population of neurons that were very likely in CA2, yet no distinct population of CA2 cells were evident with Nissl staining (Dudek et al, 2023). Similarly, studies using Nissl staining, but not IF, have suggested that domestic cats lack a distinct CA2 cell population based on the anatomical definition, where the large pyramidal cells without input from the mossy fibers were spread out and not easily distinguished from the pyramidal cells of area CA3 (Hirama et al, 1997;Zilli et al, 2022).…”

Identification of hippocampal area CA2 in hamster and vole brain

“…Previously used techniques (e.g., Nissl staining), while allowing researchers to visualize cell sizes and density, are limited in that they cannot distinguish cell types based on molecular characteristics. For example, a recent study in foxes ( Vulpes vulpes ) showed staining for PCP4 in a population of neurons that were very likely in CA2, yet no distinct population of CA2 cells were evident with Nissl staining (Dudek et al, 2023). Similarly, studies using Nissl staining, but not IF, have suggested that domestic cats lack a distinct CA2 cell population based on the anatomical definition, where the large pyramidal cells without input from the mossy fibers were spread out and not easily distinguished from the pyramidal cells of area CA3 (Hirama et al, 1997; Zilli et al, 2022).…”

“…Identification of area CA2 using solely anatomical staining without the additional molecular information that IF provides, has faced similar challenges in domestic dogs, rabbits, guinea pigs and hedgehogs (Hof et al, 1996; Kunzle & Radtke-Schuller, 2001; Potegal et al, 1993; Rami et al, 1987). However, some studies including single-nucleus transcriptomic experiments suggest that dogs do have cells with molecular characteristics like those of cells in area CA2 in other animals (Amayasu et al, 1999; Dudek et al, 2023; Hof et al, 1996; Ragbetli et al, 2010; Zhou et al, 2022). More recent work looking at different species of mole rats have demonstrated PCP4 staining that meets the molecular definitions for area CA2 used here (Stöber & Oosthuizen, 2023).…”