Hippocampal Pallium and Map-Like Memories through Vertebrate Evolution

Abstract

The hippocampus in humans and other mammals is essential for episodic and relational memories. Comparative evidence indicates that a hippocampal pallium homologue is present in birds, reptiles, amphibians, ray-finned fishes, cartilaginous fishes and agnathans. Some of their characteristics, such as the topological position and the pattern of connectivity, appear remarkably well conserved. We review here substantial data showing that in all the vertebrate groups studied up to date, from fish to mammals, the hippocampus plays a fundamental role in spatial memory. In these vertebrates groups, the hippocampal pallium homologue is involved in the use of map-like, relational representations of the objective space that provide stable allocentric frames of reference, thus allowing flexible navigation. These similarities suggest a common evolutionary ancestry and indicate that the functional properties of the hippocampus appear early in the vertebrate phylogenesis and are retained through the independent evolution of the vertebrate lineages.

How do individuals cope with stress? Behavioural, physiological and neuronal differences between proactive and reactive coping styles in fish

“…Interestingly, while bdnf-labelled cells in the Vv of reactive and proactive fish showed similar activation, this appears to be the result of not only the same but also different subpopulations within the neuronal network of the Vv. It has become increasingly clear that telencephalic neuronal populations are highly heterogenic in teleost fishes, where subpopulations within regions, such as the Dl, contain functionally equivalent structures to mammalian nuclei (Broglio et al, 2015). In the present study, the entire Dl, Dm and Vv were sampled; the differential activation within these regions remains to be determined.…”

“…When we extrapolate these functional roles to the fish's proposed telencephalic equivalents, it is tempting to hypothesise that this increase in bdnf may help proactive fish in displaying a greater behavioural reactivity to acute stressors (i.e. active coping), particularly considering that the fish Dl is strongly associated with memory and spatial navigation (Broglio et al, 2015;Vargas et al, 2009). It would therefore be interesting to characterise the learning ability of proactive and reactive individuals in response to different stressful situations to further explore this hypothesis.…”

On the Value of Reptilian Brains to Map the Evolution of the Hippocampal Formation

“…We focus on two classic questions [Smith, 1910]: (i) how much of the reptilian cortex represents the hippocampal formation, and (ii) what is its nature, i.e., what are its subdivisions and functional specializations? For studies on cortical volume changes in relation to spatial navigation [Day et al, 1999;Ladage et al, 2009] we direct the reader to excellent reviews that also incorporate a broader phylogenetic perspective [Roth et al, 2010;Broglio et al, 2015;Murray et al, 2016;Striedter, 2016;Bingman et al, 2017]. Similarly, we omit studies on cortical interneurons in reptiles [reviewed in Reiner, 1991;Guirado and Davila, 1999;Naumann and Laurent, 2017], since, despite extensive knowledge about cell type diversity of mammalian hippocampal interneurons, their function remains poorly understood [Grieves and Jeffery, 2017].…”

Cognitive appraisal in fish: stressor predictability modulates the physiological and neurobehavioural stress response in sea bass

“…In teleost fish, Dl has been established as a homologue of the mammalian hippocampus, with experimental lesions in this area leading deficits in spatial learning, but not emotional or cue learning [62,65]. However, more precise analysis of the available evidence, based on extensive connections with septal nuclei and the preoptic area, the distribution patterns of histochemical and molecular markers and the patterns of neurogenesis and interneuron migration, suggests that this homology should be restricted to its ventral subdivision (Dlv) [66][67][68]. On the other hand, the dorsal subdivision of Dl (Dld) seems to be specialized in the processing of visual information via a tectal loop and in the multimodal integration of visual information with other sensory modalities, given its afferents to other sensory organs [66,68].…”