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Neurobiology

Neurogenesis and Memory

It was less than a decade ago that the belief that the adult brain could not produce new neurons fell apart.  We now know of two cerebral regions where this is not the case, one of which being the hippocampus' dentate gyrus in which stem cells renew the stock constantly. Because of this structure's importance in memory and learning processes, scientists have been investigating the exact role that neurogenesis plays in these faculties.

Until now, most of the research on cellular and molecular mechanisms of memory formation focused on synaptic plasticity (SP)–the rearrangement and reinforcement of connections between existing neurons. But an increasing number of studies suggest that learning processes also stimulate the growth of new neurons in the hippocampus.

“We've shown that the learning processes that depend on the hippocampus are the ones triggering  neurogenesis,” says Serge Laroche, from the Learning, Memory and Communication CNRS laboratory.1 “So we wanted to understand how these new neurons were involved.”

In a 2005 study,2 Laroche's team suggested that these new neurons (which appear, for example, when rats are placed in stimulating “enriched” environments), serve to lay out the system for further learning processes. But in their more recent publication,3 they were able to demonstrate that SP and neurogenesis work as a pair.

For these experiments, the researchers applied electrical stimulation in neuronal circuits afferent to the hippocampus of a rat's brain. This procedure is known to induce synaptic plasticity by emulating the physiological changes that occur in neurons that underlie memory formation, a mechanism known as long-term potentiation (LTP). Using DNA replication markers, the team showed that LTP also boosts the growth of new neurons. Moreover, LTP enhances the survival of pre-existing young neurons.

“Of the 6000 to 9000 neurons that grow every day in the rat's dentate gyrus, only 20 to 30% achieve maturity,” says Laroche. “It seems that during learning processes, neurogenesis is stimulated, the survival of young neurons is enhanced, and the system is laid out to increase synaptic plasticity.”

Laroche believes such research may lead to pharmacological agents capable of improving cognitive functions in patients with neurodegenerative diseases by stimulating neurogenesis.

 

Clémentine Wallace

 

Notes :

1. NAMC: Laboratoire de Neurobiologie de l'Apprentissage, de la Mémoire et de la Communication (CNRS / Université Paris-XI joint lab).
2. E. Bruel-Jungerman et al., “New neurons in the dentate gyrus are involved in the expression of enhanced long-term memory following environmental enrichment.” Eur. J. Neurosci. 21: 513-521. 2005.
3. E. Bruel-Jungerman et al., “Long-term potentiation enhances neurogenesis in the adult dentate gyrus,” J. Neurosci. 26 (22): 5888-93. 2006.

Contacts :

Serge Laroche
Serge.laroche@baic.u-psud.fr


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