Biology subjects

Epp, J. R.

Publications and source records attributed to Epp, J. R..

6 recordsLinked to original sources

bioRxiv2021

New neurons in old brains: A cautionary tale for the analysis of neurogenesis in post-mortem tissue.

Adult neurogenesis has primarily been examined in two key regions in the mammalian brain, the subgranular zone of the hippocampus and the subventricular zone. The proliferation and integration of newly generated neurons has been observed widely in adult mammalian species including the human hippocampus. Recent high-profile studies have suggested however, that this process is considerably reduced in humans, occurring in children but declining rapidly and nearly completely in the adult brain. In comparison, rodent studies also show age-related decline but a greater degree of proliferation of new neurons in adult animals. Here, we examine whether differences in tissue fixation, rather than biological difference in human versus rodent studies might account for the diminished levels of neurogenesis sometimes observed in the human brain. To do so we analyzed neurogenesis in the hippocampus of rats that were either perfusion-fixed or the brains extracted and immersion-fixed at various post-mortem intervals. We observed an interaction between animal age and the time delay between death and tissue fixation. While similar levels of neurogenesis were observed in young rats regardless of fixation, older rats had significantly fewer labeled neurons when fixation was not immediate. Furthermore, the morphological detail of the labeled neurons was significantly reduced in the delayed fixation conditions at all ages. This study highlights critical concerns that must be considered when using post-mortem tissue to quantify adult neurogenesis.

neuroscience↗

bioRxiv2021

Toxoplasma infection alters dopamine-sensitive behaviors and host gene expression patterns associated with neuropsychiatric disease

Toxoplasma gondii is a single celled parasite thought to infect 1 in 3 worldwide. During chronic infection, T. gondii can migrate to the brain where it promotes low-grade neuroinflammation with the capacity to induce changes in brain morphology and behavior. Consequently, infection with T. gondii has been linked with a number of neurocognitive disorders including schizophrenia (SZ), dementia, and Parkinsons disease. Beyond neuroinflammation, infection with T. gondii can modulate the production of neurotransmitters, such as dopamine. To further dissect these pathways and examine the impact of altered dopaminergic sensitivity in T. gondii-infected mice on both behavior and gene expression, we developed a novel mouse model, based on stimulant-induced (cocaine) hyperactivity. Employing this model, we found that infection with T. gondii did not alter fear behavior but did impact motor activity and neuropsychiatric-related behaviurs. While both behaviors may help reduce predator avoidance, consistent with previous studies, the latter finding is reminiscent of neurocognitive disorders. Applying RNASeq to two relevant brain regions, striatum and hippocampus, we identified a broad upregulation of immune responses. However, we also noted significant associations with more meaningful neurologically relevant terms were masked due to the sheer number of terms incorporated in multiple testing correction. We therefore performed a more focused analysis using a curated set of neurologically relevant terms revealing significant associations across multiple pathways. We also found that T. gondii and cocaine treatments impacted the expression of similar functional pathways in the hippocampus and striatum although, as indicated by the low overlap among differentially expressed genes, largely via different proteins. Furthermore, while most differentially expressed genes reacted to a single condition and were mostly upregulated, we identified gene expression patterns indicating unexpected interactions between T. gondii infection and cocaine exposure. These include sets of genes which responded to cocaine exposure but not upon cocaine exposure in the context of T. gondii infection, suggestive of a neuroprotective effect advantageous to parasite persistence. Given its ability to uncover such complex relationships, we propose this novel model offers a new perspective to dissect the molecular pathways by which T. gondii infection contributes to neuropsychiatric disorders such as schizophrenia.

microbiology↗

bioRxiv2021

Brain-wide neuronal activation and functional connectivity are modulated by prior exposure to repetitive learning episodes.

Memory storage and retrieval are shaped by past experiences. Prior learning and memory episodes have numerous impacts on brain structure from micro to macroscale. Previous experience with specific forms of learning increases the efficiency of future learning. It is less clear whether such practice effects on one type of memory might also have transferable effects to other forms of memory. Different forms of learning and memory rely on different brain-wide networks but there are many points of overlap in these networks. Enhanced structural or functional connectivity caused by one type of learning may be transferable to another type of learning due to overlap in underlying memory networks. Here, we investigated the impact of prior chronic spatial training on the task-specific functional connectivity related to subsequent contextual fear memory recall in mice. Our results show that mice exposed to prior spatial training exhibited decreased brain-wide activation compared to control mice during the retrieval of a context fear memory. With respect to functional connectivity, we observed changes in several network measures notably an increase in global efficiency. Interestingly, we also observed an increase in network resilience based on simulated targeted node deletion. Overall, this study suggests that chronic learning has transferable effects on the functional connectivity networks of other types of learning and memory. The generalized enhancements in network efficiency and resilience suggest that learning itself may protect brain networks against deterioration.

neuroscience↗

bioRxiv2021

Neurogenesis-mediated forgetting of complex paired-associates memories

The hippocampus is a critical structure involved in many forms of learning and memory. It is also one of the only regions in the adult mammalian brain that continues to generate new neurons throughout adulthood. This process of adult neurogenesis may increase the plasticity of the hippocampus which could be beneficial for learning but has also been demonstrated to decrease the stability of previously acquired memories. Here we test whether increased production of new neurons following the formation of a gradually acquired paired-associates task will result in forgetting of this type of memory. We trained mice in a touchscreen-based object/location task and then increased neurogenesis using voluntary exercise. Our results indicate that mice with increased neurogenesis show poor recall of the previously established memory. When subsequently exposed to a reversal task we also show that mice with increased neurogenesis require fewer correction trials to acquire the new task contingencies. This suggests that prior forgetting reduces perseveration on the now outdated memory. Together our results add to a growing body of literature which indicates the important role of adult neurogenesis in destabilizing previously acquired memories to allow for flexible encoding of new memories.

animal behavior and cognition↗

bioRxiv2021

Neurogenesis-induced forgetting is associated with reduced neuronal activity in CA1 during context fear memory retrieval

Adult hippocampal neurogenesis has been demonstrated to affect learning and memory in numerous ways. Several studies have now demonstrated that increased neurogenesis can induce forgetting of memories acquired prior to the manipulation of neurogenesis and, as a result of this forgetting can also facilitate new learning. However, the mechanisms mediating neurogenesis-induced forgetting are not well understood. Here, we used a subregion-based analysis of the immediate early gene c-Fos as well as in vivo fiber photometry to determine changes in activity corresponding with neurogenesis induced forgetting. We found that increasing neurogenesis led to reduced CA1 activity during context memory retrieval. We also demonstrate here that perineuronal net expression in areas CA1 is bidirectionally altered by the levels or activity of adult generated neurons in the dentate gyrus. These results suggest that neurogenesis may induce forgetting by disrupting perineuronal nets in CA1 which may otherwise protect memories from degradation.

neuroscience↗

bioRxiv2021

Adult neurogenesis mediates forgetting of multiple types of memory in the rat

The formation and retention of hippocampus-dependent memories is impacted by neurogenesis, a process that involves the production of new neurons in the dentate gyrus of the hippocampus. Recent studies demonstrate that increasing neurogenesis after memory formation induces forgetting of previously acquired memories. Neurogenesis-induced forgetting was originally demonstrated in mice, but a recent report suggests that the same effect may be absent in rats. Although a general species difference is possible, other potential explanations for these incongruent findings are that memories which are more strongly reinforced become resilient to forgetting or that perhaps only certain types of memories are affected. Here, we investigated whether neurogenesis-induced forgetting occurs in rats using several hippocampal dependent tasks including contextual fear conditioning (CFC), the Morris Water Task (MWT), and touchscreen paired associates learning (PAL). Neurogenesis was increased following training using voluntary exercise for 4 weeks before recall of the previous memory was assessed. We show that voluntary running causes forgetting of context fear memories in a neurogenesis-dependent manner, and that neurogenesis-induced forgetting is present in rats across behavioral tasks despite differences in complexity or reliance on spatial, context, or object memories. In addition, we asked whether stronger memories are less susceptible to forgetting by varying the strength of training. Even with a very strong training protocol in the CFC task, we still observed enhanced forgetting related to increased neurogenesis. These results suggest that forgetting due to neurogenesis is a conserved mechanism that aids in the clearance of memories. Significance StatementRecent evidence indicates that hippocampal neurogenesis mediates forgetting of older memories and enhances encoding of new memories free of proactive interference. This evidence comes from multiple rodent species, behavioral tasks, and methods of increasing neurogenesis. However, a recent paper by (Kodali et al. 2016) found that voluntary exercise-induced neurogenesis did not cause forgetting in the Morris Water Task in rats. The results call into question whether the phenomenon is a conserved function of neurogenesis across species. In the present study, we show that voluntary running causes robust forgetting in rats in a neurogenesis-dependent manner and that the effect is present across three different behavioral tasks, confirming the existence of the phenomenon in rats and adding to the growing evidence that forgetting is a conserved function of hippocampal neurogenesis.

neuroscience↗