Abstract
The role of adult brain neurogenesis (generating new neurons) in learning and memory appears to be quite firmly established in spite of some criticism and lack of understanding of what the new neurons serve the brain for. Also, the few experiments showing that blocking adult neurogenesis causes learning deficits used irradiation and various drugs known for their side effects and the results obtained vary greatly. We used a novel approach, cyclin D2 knockout mice (D2 KO mice), specifically lacking adult brain neurogenesis to verify its importance in learning and memory. D2 KO mice and their wild-type siblings were tested in several behavioral paradigms, including those in which the role of adult neurogenesis has been postulated. D2 KO mice showed no impairment in sensorimotor tests, with only sensory impairment in an olfaction-dependent task. However, D2 KO mice showed proper procedural learning as well as learning in context (including remote memory), cue, and trace fear conditioning, Morris water maze, novel object recognition test, and in a multifunctional behavioral system-IntelliCages. D2 KO mice also demonstrated correct reversal learning. Our results suggest that adult brain neurogenesis is not obligatory in learning, including the kinds of learning where the role of adult neurogenesis has previously been strongly suggested.
Highlights
New neurons are produced in the brains of adult animals, including humans, throughout their lifespan (Altman 1963; Eriksson et al 1998)
Since one of the sites of adult brain neurogenesis is hippocampal formation, a brain structure involved in learning and memory, new neurons were expected to be involved in these phenomena
To determine that our mice had reduced hippocampal neurogenesis, we examined BrdU+ and doublecortin+
Summary
New neurons are produced in the brains of adult animals, including humans, throughout their lifespan (Altman 1963; Eriksson et al 1998). A few experiments demonstrate the effects of new neuron depletion on learning and memory (Shors et al 2001, 2002; Madsen et al 2003; Rola et al 2004; Bruel-Jungerman et al 2005; Snyder et al 2005; Saxe et al 2006; Winocur et al 2006; Zhang et al 2008; Jessberger et al 2009; Table 1). The reason for this is a lack of proper methods to selectively reduce adult brain neurogenesis without affecting other aspects of brain function. We conclude that hippocampal adult neurogenesis is, in general, not crucial in learning
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