Improved specificity of hippocampal memory trace labeling

Abstract

Recent studies have focused on the identification and manipulation of memory traces in rodent models. The two main mouse models utilized are either a CreERT2/loxP tamoxifen (TAM)- or a tetracycline transactivator (tTA)/tetracycline-response element (TRE) doxycycline (DOX)-inducible system. These systems, however, could be improved to label a more specific population of activated neurons corresponding to behavior. Here, we sought to identify an improved selective estrogen receptor (ER) modulator (SERM) in which we could label an individual memory trace in ArcCreERT2 mice. We found that 4-hydroxytamoxifen (4-OHT) is the most selective SERM in the ArcCreERT2 x ROSA26-CAG-stopflox-channelrhodospin (ChR2)-enhanced yellow fluorescent protein (eYFP) mice. The half-life of 4-OHT is also shorter than TAM, allowing for more specificity of memory trace labeling. Furthermore, 4-OHT allowed for context-specific labeling in the dentate gyrus (DG) and CA3. In summary, we believe 4-OHT improves the specificity of memory trace labeling and will allow for refined memory trace studies in the future.

Proximodistal Heterogeneity of Hippocampal CA3 Pyramidal Neuron Intrinsic Properties, Connectivity, and Reactivation during Memory Recall

“…Because the CA3 autoassociative network has been suggested to play a key role in memory storage and recall, especially during pattern completion (Le Duigou et al, 2014; Papp et al, 2007; Rolls, 2013b), we reasoned that CA3b may be most robustly reactivated during recall of a memory trace compared to other CA3 subregions. To test this hypothesis, we used a genetic memory trace strategy to label separately neurons activated during memory encoding and during memory retrieval in the same animals (Cazzulino et al, 2016; Denny et al, 2014). To achieve this, we crossed an ArcCreER T2 mouse line with a Rosa26-CAG-STOP-floxed-ChR2(H134R)-EYFP line.…”

“…To achieve this, we crossed an ArcCreER T2 mouse line with a Rosa26-CAG-STOP-floxed-ChR2(H134R)-EYFP line. We used a contextual fear conditioning (CFC) paradigm to first tag a strongly labeled memory (Cazzulino et al, 2016; Denny et al, 2014). Mice were injected with 4-Hydroxytamoxifen (4-OHT), which is required for CreER activity, 5 hours prior to administering the CFC paradigm.…”

“…Using a genetic memory trace strategy (Cazzulino et al, 2016; Denny et al, 2014; Josselyn et al, 2015), we demonstrated variations in the engagement of CA3 neurons in memory recall along the CA3 transverse axis. Our findings support the results of a recent study in showing that CA3c has weaker reactivation compared to more distal regions of CA3 (Nakazawa et al, 2016).…”

From Engrams to Pathologies of the Brain

“…For instance, there is significantly more background expression with the H2B-mCherry line than with the eYFP line. Moreover, the concentration and timing of a tamoxifen (TAM)/4-hydroxytamoxifen (4-OHT) injection relative to the behavioral experience also produces differences in the labeled populations (Cazzulino et al, 2016). We note these important technical differences because of the following: if an active neuronal population is tagged in a manner that does not recapitulate the endogenous number of active cells in a defined period of time, then the number of tagged cells corresponding to a given experience may produce a diluted percentage.…”

“…In support of this hypothesis, in DG, CA3 and CA1, exposure to the same context twice elicits an above-chance level of overlap in activity whereas exposure to two different contexts elicits either below chance-level overlap, chance-level overlap, or above chance-level overlap but at a magnitude lower than exposing an animal to the same context twice (Guzowski et al, 1999; Guzowski, 2002; Satvat et al, 2011; Liu et al, 2012; Deng et al, 2013; Ramirez et al, 2013; Tayler et al, 2013; Denny et al, 2014; Cazzulino et al, 2016). Testing whether or not newly developed tagging strategies can unveil context-specificity in areas downstream of the hippocampus or valence-specificity within cortico-hippocampus circuits remains an exciting area of inquiry, both within and across brain regions (Adhikari et al, 2010; Fanselow and Dong, 2010; Kheirbek et al, 2013; Wang et al, 2013; Ciocchi et al, 2015).…”