Mechanisms of the contribution to memory by immature neurons in the adult dentate gyrus

Abstract

Generation of new neurons persists in the dentate gyrus in the mammalian brain in the adult life. New neurons born in the dentate gyrus integrate into the existing network and have been suggested to play a role in memory processing. Experience and learning shape newborn neuron morphology as well as their chance to survive and to functionally integrate in the dentate gyrus network. Maturing neurons have different physiological and morphological properties from mature neurons. It has been suggested that, because of these unique properties, immature neurons might be more easily recruited into networks underlying the memory process. Nevertheless, it is still not clear how maturing neurons contribute to memory. This thesis work aims to reveal mechanisms by which immature neurons contribute to the memory process. In order to manipulate immature neurons after learning we required to develop a technique that would allow us to specifically ablate this neuronal population. Initially we decided to use a prototype lentivirus carrying the diphtheria toxin receptor gene under the control of Ca2+/calmodulin-dependent protein kinase promoter, in order to target the population of excitatory neurons in the dentate gyrus. However, we unexpectedly discovered that this approach kills a subset of maturing neurons without affecting the older population. We assessed which neurons were affected by our ablation and concluded that this approach can be utilized as a method to inducibly ablate immature neurons. We then tested how immature neurons are involved in the memory process after initial acquisition. We improved the aforementioned lentivirus mediated, inducible ablation technique by changing the promoter to the one derived from human doublecortin gene, thus targeting the immature population more specifically. We showed that tour technique allowed us to specifically ablate the population of immature neurons below 4 weeks old. We used this technique to remove immature neurons after training mice in a hippocampus-dependent memory task. We found that the ablation of this population of young neurons impairs the post training memory process of spatial memory evaluated in the water maze task. While control mice changed their platform searching pattern after failing to find the platform in the first probe trial, mice with post training memory ablation were unable to do so. This finding supports the idea that immature neurons in the dentate gyrus play a role in flexible adaption of their behavior in response to changes in the environment, by suppressing previously learned behavior. Lastly to inquire how newborn neurons may participate to spatial memory processing in the dentate gyrus, we performed electrophysiological recordings from the dentate gyrus of rats during spatial exploration. We found place cells more frequently near the hilar border of granule cell layer, and those place cells possess unique firing properties, particularly more frequent burst firing and multiple firing field compared with place cells found in the part of granule cell layer further from the hilar border. We also found that the ablation of immature neurons affects place cells in the dentate gyrus, decreasing their number and biasing the firing properties towards lower prevalence of multiple firing fields. The implications of these findings point to a special role of a subpopulation of maturing neurons in the dentate gyrus, aged one to four weeks. These immature neurons may contribute to flexible behavioral adaptation in response to alterations in the environment which render previously learned behavior obsolete. We propose that immature neurons can exercise this contribution by virtue of their distinct firing properties compared to mature neurons. Indeed the existence of two distinct populations of active neurons in the dentate gyrus (putatively identified as immature neuron population and mature neuron population) is supported by our recordings data and the effects on place cell firing properties that we observe after neurogenesis ablation. Overall this thesis work adds new knowledge on the potential mechanisms through which newly generated neurons are employed for the memory process in the hippocampus.