Enriched Environment Increases PCNA and PARP1 Levels in Octopus vulgaris Central Nervous System: First Evidence of Adult Neurogenesis in Lophotrochozoa

Organisms showing a complex and centralized nervous system, such as teleosts, amphibians, reptiles, birds and mammals, and among invertebrates, crustaceans and insects, can adjust their behavior according to the environmental challenges. Proliferation, differentiation, migration, and axonal and dendritic development of newborn neurons take place in brain areas where structural plasticity, involved in learning, memory, and sensory stimuli integration, occurs. Octopus vulgaris has a complex and centralized nervous system, located between the eyes, with a hierarchical organization. It is considered the most "intelligent" invertebrate for its advanced cognitive capabilities, as learning and memory, and its sophisticated behaviors. The experimental data obtained by immunohistochemistry and western blot assay using proliferating cell nuclear antigen and poli (ADP-ribose) polymerase 1 as marker of cell proliferation and synaptogenesis, respectively, revealed cell proliferation in areas of brain involved in learning, memory, and sensory stimuli integration. Furthermore, we showed how enriched environmental conditions affect adult neurogenesis

Tool use specific adult neurogenesis and synaptogenesis in rodent (Octodon degus) hippocampus.

We previously demonstrated that degus (Octodon degus), which are a species of small caviomorph rodents, could be trained to use a T-shaped rake as a hand tool to expand accessible spaces. To elucidate the neurobiological underpinnings of this higher brain function, we compared this tool use learning task with a simple spatial (radial maze) memory task and investigated the changes that were induced in the hippocampal neural circuits known to subserve spatial perception and learning. With the exposure to an enriched environment in home cage, adult neurogenesis in the dentate gyrus of the hippocampus was augmented by tool use learning, but not radial maze learning, when compared to control conditions. Furthermore, the proportion of new synapses formed in the CA3 region of the hippocampus, the target area for projections of mossy fiber axons emanating from newborn neurons, was specifically increased by tool use learning. Thus, active tool use behavior by rodents, learned through multiple training sessions, requires the hippocampus to generate more novel neurons and synapses than spatial information processing in radial maze learning

The photograph of tool use and the learning curves.

<p>A) A photograph of a degu using tool during training. The degu has grabbed the tool and is about to pull it to retrieve the food reward. An upward arrow with an asterisk indicates a food reward (a half of sunflower seed). The scale bar indicates 1 cm. B) Left: The learning curve (median ± limits of variation) observed during the tool use training phase. The success rates (ordinate) were plotted for every three sessions (abscissa) of Level 2. Sessions were numbered independently for Level 2 and were aligned at day 0 (abscissa). Each data point represents the average of four animals. Right: The success rate (median ± limits of variation) of the tool use task during the BrdU injection phase C) Left: The learning curve (median ± limits of variation) observed during the radial maze training phase. The average success rates (ordinate) were plotted for every three sessions (abscissa). Each data point represents the average of three animals. Right: The success rate (median ± limits of variation) of the radial maze task during the BrdU injection phase. For both B Left and C left, the criterion for success was an 80% or higher success rate in consecutive sessions. Day 0 is the date the criterion was satisfied. Sessions are numbered based on their alignment at the origin of the abscissa. For both B right and C right, BrdU injections started at Day 0.</p

Tool use learning, but not radial maze learning, promoted maturation of newly generated neurons.

<p>A) Confocal images showing double-labeling for Synapsin-I (green) and PSA-NCAM (red) in the CA3 subfield of the hippocampus. Low magnification images are shown with high magnification insets. Representative PSA-NCAM⁺/Synapsin-I⁺ puncta are indicated by arrowheads in the inset images. B) Effects of tool use or radial maze tasks on formation of clusters of presynaptic vesicles in the CA3 subfield. The numbers of PSA-NCAM⁺/Synapsin-I⁺ puncta in the CA3 subfield were plotted along the y-axis. A schematic of the hippocampus is shown in B (<i>top</i>). Target areas (square) were randomly selected and the images were acquired in the <i>stratum lucidum</i> of the CA3 subfield. Asterisks refer to statistical significance: *p<0.01. Scale bars indicate 50 µm in the low magnification images and 5 µm in the high magnification images.</p

Experimental setups for behavioral analyses.

<p>A) Morphology of Degu hands. Degus have thumb-like structure (top) that allow them to manipulate tools in a dexterous fashion (bottom). B) The homecage of the degus. The degus lived in an “enriched environment” that contained a running wheel, a den, and a tunnel. C) Different levels of tool-use training processes. At Level 1, the degus simply pulled the tools (arrow) toward themselves. At Level 2, the degus had to make lateral movements (arrow*) before pulling the tool toward themselves (arrow**). At Level 2b, the degus had to place the tool beyond the reward before pulling. The degu has grabbed the tool and is about to make a lateral movement and pull the food (down). D) The experimental platform (middle), the TFT monitor for event display (left), and the numeric pad for recording (right). The scale bar is inserted as a rough reference because the photograph is tilted. The degus were placed behind the transparent fence and the experimenter sat facing them from outside of the enclosure. E) A schematic of the radial maze apparatus and the spatial cues. F) A photograph of the radial maze set-up. The experimental platform (middle), the PC for event display (right), and the spatial cues (yellow star, red square and green apple) are visible.</p