A multiple Arc (mArc) tagging system to uncover the organizational principles of multiple memories.

Engrams or memory traces are the neuronal ensembles that collectively store individual experiences. Genetic strategies based on immediate early genes (IEGs), such as Arc/Arg3.1 , allow us to tag the ensembles active during memory encoding and compare them to those active during retrieval. However, these strategies only allow for the tagging of one neural ensemble. Here, we developed a multiple Arc (mArc) system that allows for the tagging of two Arc + ensembles. We validated this system by investigating how context, time, and valence influence neuronal ensemble reactivation in the dentate gyrus (DG). We show that similar contextual and valenced experiences are encoded in overlapping DG ensembles. We also find that ensembles are modulated by time, where experiences closer in time are encoded in more similar ensembles. These results highlight the dynamic nature of DG ensembles and show that the mArc system provides a powerful approach for investigating multiple memories in the brain. HIGHLIGHTS: The mArc system allows for the tagging of two Arc + ensembles in the same mouse DG ensembles labeled by the mArc system receive increased excitatory inputContext, valence, and time influence DG ensemble reactivationDG neural ensembles are reactivated less with increasing time.

( R,S )-ketamine's rapid-acting antidepressant effects are modulated by NR2B-containing NMDA receptors on adult-born hippocampal neurons.

Standard antidepressant treatments often take weeks to reach efficacy and are ineffective for many patients. ( R,S )-ketamine, an N -methyl-D-aspartate (NMDA) antagonist, has been shown to be a rapid-acting antidepressant and to decrease depressive symptoms within hours of administration. While previous studies have shown the importance of the NR2B subunit of the NMDA receptor (NMDAR) on interneurons in the medial prefrontal cortex (mPFC), no study has investigated the influence of NR2B-expressing adult-born granule cells (abGCs). In this study, we examined whether ( R,S )-ketamine's efficacy depends upon these adult-born hippocampal neurons using a genetic strategy to selectively ablate the NR2B subunit of the NMDAR from Nestin + cells. To validate our findings, we also used several other transgenic lines including one in which NR2B was deleted from an interneuron (Parvalbumin (PV) + ) population. We report that in male mice, NR2B expression on 6-week-old adult-born neurons is necessary for ( R,S )-ketamine's effects on behavioral despair in the forced swim test (FST) and on hyponeophagia in the novelty suppressed feeding (NSF) paradigm, as well on fear behavior following contextual fear conditioning (CFC). In female mice, NR2B expression is necessary for effects on hyponeophagia in the NSF. We also find that ablating neurogenesis increases fear expression in CFC, which is buffered by ( R,S )-ketamine administration. In line with previous studies, these results suggest that 6-week-old adult-born hippocampal neurons expressing NR2B partially modulate ( R,S )-ketamine's rapid-acting effects. Future work targeting these 6-week-old adult-born neurons may prove beneficial for increasing the efficacy of ( R , S )-ketamine's antidepressant actions.

The Effects of Anesthesia on Adult Hippocampal Neurogenesis.

In animal studies, prolonged sedation with general anesthetics has resulted in cognitive impairments that can last for days to weeks after exposure. One mechanism by which anesthesia may impair cognition is by decreasing adult hippocampal neurogenesis. Several studies have seen a reduction in cell survival after anesthesia in rodents with most studies focusing on two particularly vulnerable age windows: the neonatal period and old age. However, the extent to which sedation affects neurogenesis in young adults remains unclear. Adult neurogenesis in the dentate gyrus (DG) was analyzed in male and female rats 24 h after a 4-h period of sedation with isoflurane, propofol, midazolam, or dexmedetomidine. Three different cell populations were quantified: cells that were 1 week or 1 month old, labeled with the permanent birthdate markers EdU or BrdU, respectively, and precursor cells, identified by their expression of the endogenous dividing cell marker proliferating cell nuclear antigen (PCNA) at the time of sacrifice. Midazolam and dexmedetomidine reduced cell proliferation in the adult DG in both sexes but had no effect on postmitotic cells. Propofol reduced the number of relatively mature, 28-day old, neurons specifically in female rats and had no effects on younger cells. Isoflurane had no detectable effects on any of the cell populations examined. These findings show no general effect of sedation on adult-born neurons but demonstrate that certain sedatives do have drug-specific and sex-specific effects. The impacts observed on different cell populations predict that any cognitive effects of these sedatives would likely occur at different times, with propofol producing a rapid but short-lived impairment and midazolam and dexmedetomidine altering cognition after a several week delay. Taken together, these studies lend support to the hypothesis that decreased neurogenesis in the young adult DG may mediate the effects of sedation on cognitive function.

Neuronal Activity in Non-LNv Clock Cells Is Required to Produce Free-Running Rest:Activity Rhythms in Drosophila.

Circadian rhythms in behavior and physiology are produced by central brain clock neurons that can be divided into subpopulations based on molecular and functional characteristics. It has become clear that coherent behavioral rhythms result from the coordinated action of these clock neuron populations, but many questions remain regarding the organizational logic of the clock network. Here we used targeted genetic tools in Drosophila to eliminate either molecular clock function or neuronal activity in discrete clock neuron subsets. We find that neuronal firing is necessary across multiple clock cell populations to produce free-running rhythms of rest and activity. In contrast, such rhythms are much more subtly affected by molecular clock suppression in the same cells. These findings demonstrate that network connectivity can compensate for a lack of molecular oscillations within subsets of clock cells. We further show that small ventrolateral (sLNv) clock neurons, which have been characterized as master pacemakers under free-running conditions, cannot drive rhythms independent of communication between other cells of the clock network. In particular, we pinpoint an essential contribution of the dorsolateral (LNd) clock neurons, and show that manipulations that affect LNd function reduce circadian rhythm strength without affecting molecular cycling in sLNv cells. These results suggest a hierarchical organization in which circadian information is first consolidated among one or more clock cell populations before accessing output pathways that control locomotor activity.