Plasticity and Adult Neurogenesis in Amphibians and Reptiles: More Questions than Answers

Abstract

Studies of the relationship between behavioral plasticity and new cells in the adult brain in amphibians and reptiles are sparse but demonstrate that environmental and hormonal variables do have an effect on the amount of cell proliferation and/or migration. The variables that are reviewed here are: enriched environment, social stimulation, spatial area use, season, photoperiod and temperature, and testosterone. Fewer data are available for amphibians than for reptiles, but for both groups many issues are still to be resolved. It is to be hoped that the questions raised here will generate more answers in future studies.

Sexually Dimorphic Patterns of Cell Proliferation in the Brain Are Linked to Seasonal Life-History Transitions in Red-Sided Garter Snakes

“…The processes that characterize neurogenesis are subject to hormonal regulation, and therefore the influence of sex steroid hormones (or other sexually dimorphic endocrine factors, including glucocorticoid “stress” hormones) on cell proliferation and migration may contribute to the sex differences reported here (see reviews in Balthazart and Ball, 2016 ; Dunlap, 2016 ; Mahmoud et al, 2016 ; Powers, 2016 ). For example, treatment of territorial, but not non-territorial male side-blotched lizards ( Uta stansburiana ) with testosterone decreased the volume of the medial cortex and reduced the number of doublecortin-positive cells, an indicator of immature, migrating neurons (LaDage et al, 2017 ).…”

“…Surprisingly, there were no migration-related changes in cell proliferation within any subregion of the cortex in females, whereas the only migration-related change in cell proliferation observed in males occurred within the medial cortex. The reptilian medial cortex is a structural and functional homolog of the avian and mammalian hippocampus (Butler and Hodos, 2005 ), and variation in cortical neurogenesis and/or volume is positively correlated with territory and home range size in reptiles (Roth et al, 2006 ; LaDage et al, 2009 ; Powers, 2016 ). While it is possible that the increase in cell proliferation within the medial cortex of migrating males is related to changing spatial learning and memory demands, it is unlikely that such a change would be sexually dimorphic, as females also migrate similar distances to summer feeding areas.…”

“…Our results suggest that cell proliferation and/or cell migration are activated earlier or to a greater extent in the hypothalamus compared to other brain regions. Because the processes of neurogenesis are influenced by environmental temperature (Powers, 2016 ), it is possible that the differences reported here are established during low-temperature winter dormancy. For example, in the tropical lizard Tropidurus hispidus , acclimation to low temperatures decreases cell migration but not cell proliferation (Marchioro et al, 2012 ).…”

“…Whether a similar association between neurogenesis and seasonal migration exists in other vertebrate groups has not been directly examined. However, several key studies indicate that variation in hippocampal neurogenesis and/or volume is indeed positively correlated with the spatial memory demands of defending territorial boundaries, establishing social hierarchies, and determining home range size in ectotherms (Roth et al, 2006 ; LaDage et al, 2009 , 2013 ; Holding et al, 2012 , also see review in Powers, 2016 ).…”

“…Thus, we have relatively little information about the potential role of neurogenesis in other neurogenic brain regions, including the hypothalamus and amygdala, which are important in modulating reproductive and social behaviors. An additional area of complexity in understanding the role of neurogenesis is the existence of sex differences in both basal rates of cell proliferation, migration, and differentiation in the adult brain as well as neurogenic responses to environmental and social stimuli (Spritzer et al, 2017 ; see reviews in Duarte-Guterman et al, 2015 ; Frick et al, 2015 ; Holmes, 2016 ; Mahmoud et al, 2016 ; Powers, 2016 ; Heberden, 2017 ). Such results indicate that sex steroid hormones, which also vary seasonally in most vertebrates, play a critical role in modulating the processes that characterize neurogenesis both within the hippocampus and in extra-hippocampal regions.…”