TLX knockdown in the dorsal dentate gyrus of juvenile rats differentially affects adolescent and adult behaviour

Effects of Septin-14 Gene Deletion on Adult Cognitive/Emotional Behavior

While various septin GTPases have been reported for their physiological functions, their roles in orchestrating complex cognitive/emotional functions in adult mammals remained scarcely explored. A comprehensive behavioral test battery was administered to two sexes of 12-week-old Septin-14 (SEPT14) knockout (KO) and wild-type (WT) mice. The sexually dimorphic effects of brain SEPT14 KO on inhibitory avoidance (IA) and hippocampal mGluR5 expression were noticed with greater IA latency and elevated mGluR5 level exclusively in male KO mice. Moreover, SEPT14 KO appeared to be associated with stress-provoked anxiety increase in a stress-related navigation task regardless of animals’ sexes. While male and female WT mice demonstrated comparable cell proliferation in the dorsal and ventral hippocampal dentate gyrus (DG), both sexes of SEPT14 KO mice had increased cell proliferation in the ventral DG. Finally, male and female SEPT14 KO mice displayed dampened observational fear conditioning magnitude and learning-provoked corticosterone secretion as compared to their same-sex WT mice. These results, taken together, prompt us to conclude that male, but not female, mice lacking the Septin-14 gene may exhibit increased aversive emotion-related learning and dorsal/ventral hippocampal mGluR5 expressions. Moreover, deletion of SEPT14 may be associated with elevated ventral hippocampal DG cell proliferation and stress-provoked anxiety-like behavior, while dampening vicarious fear conditioning magnitudes.

The temporal origin of dentate granule neurons dictates their role in spatial memory

The dentate gyrus is one of the only brain regions that continues its development after birth in rodents. Adolescence is a very sensitive period during which cognitive competences are programmed. We investigated the role of dentate granule neurons (DGNs) born during adolescence in spatial memory and compared them with those generated earlier in life (in embryos or neonates) or during adulthood by combining functional imaging, retroviral and optogenetic tools to tag and silence DGNs. By imaging DGNs expressing Zif268, a proxy for neuronal activity, we found that neurons generated in adolescent rats (and not embryos or neonates) are transiently involved in spatial memory processing. In contrast, adult-generated DGNs are recruited at a later time point when animals are older. A causal relationship between the temporal origin of DGNs and spatial memory was confirmed by silencing DGNs in behaving animals. Our results demonstrate that the emergence of spatial memory depends on neurons born during adolescence, a function later assumed by neurons generated during adulthood.

Hippocampal Neural Stem Cell Grafting after Status Epilepticus Alleviates Chronic Epilepsy and Abnormal Plasticity, and Maintains Better Memory and Mood Function

Hippocampal damage after status epilepticus (SE) leads to multiple epileptogenic changes, which lead to chronic temporal lobe epilepsy (TLE). Morbidities such as spontaneous recurrent seizures (SRS) and memory and mood impairments are seen in a significant fraction of SE survivors despite the administration of antiepileptic drugs after SE. We examined the efficacy of bilateral intra-hippocampal grafting of neural stem/progenitor cells (NSCs) derived from the embryonic day 19 rat hippocampi, six days after SE for restraining SE-induced SRS, memory, and mood impairments in the chronic phase. Grafting of NSCs curtailed the progression of SRS at 3-5 months post-SE and reduced the frequency and severity of SRS activity when examined at eight months post-SE. Reduced SRS activity was also associated with improved memory function. Graft-derived cells migrated into different hippocampal cell layers, differentiated into GABA-ergic interneurons, astrocytes, and oligodendrocytes. Significant percentages of graft-derived cells also expressed beneficial neurotrophic factors such as the fibroblast growth factor-2, brain-derived neurotrophic factor, insulin-like growth factor-1 and glial cell line-derived neurotrophic factor. NSC grafting protected neuropeptide Y- and parvalbumin-positive host interneurons, diminished the abnormal migration of newly born neurons, and rescued the reelin+ interneurons in the dentate gyrus. Besides, grafting led to the maintenance of a higher level of normal neurogenesis in the chronic phase after SE and diminished aberrant mossy fiber sprouting in the dentate gyrus. Thus, intrahippocampal grafting of hippocampal NSCs shortly after SE considerably curbed the progression of epileptogenic processes and SRS, which eventually resulted in less severe chronic epilepsy devoid of significant cognitive and mood impairments.

Chronic intrahippocampal interleukin-1β overexpression in adolescence impairs hippocampal neurogenesis but not neurogenesis-associated cognition

Both neuroinflammation and adult hippocampal neurogenesis (AHN) are implicated in many neurodegenerative disorders as well as in neuropsychiatric disorders, which often become symptomatic during adolescence. A better knowledge of the impact that chronic neuroinflammation has on the hippocampus during the adolescent period could lead to the discovery of new therapeutics for some of these disorders. The hippocampus is particularly vulnerable to altered concentrations of the pro-inflammatory cytokine interleukin-1β (IL-1β), with elevated levels implicated in the aetiology of neurodegenerative disorders such as Alzheimer's and Parkinson's, and stress-related disorders such as depression. The effect of acutely and chronically elevated concentrations of hippocampal IL-1β have been shown to reduce AHN in adult rodents. However, the effect of exposure to chronic overexpression of hippocampal IL-1β during adolescence, a time of increased vulnerability, hasn't been fully interrogated. Thus, in this study we utilized a lentiviral approach to induce chronic overexpression of IL-1β in the dorsal hippocampus of adolescent male Sprague Dawley rats for 5 weeks, during which time its impact on cognition and hippocampal neurogenesis were examined. A reduction in hippocampal neurogenesis was observed along with a reduced level of neurite branching on hippocampal neurons. However, there was no effect of IL-1β overexpression on performance in pattern separation, novel object recognition or spontaneous alternation in the Y maze. Our study has highlighted that chronic IL-1β overexpression in the hippocampus during the adolescent period exerts a negative impact on neurogenesis independent of cognitive performance, and suggests a degree of resilience of the adolescent hippocampus to inflammatory insult.

Interleukin-1 reduces food intake and body weight in rat by acting in the arcuate hypothalamus

A reduction in food intake is commonly observed after bacterial infection, a phenomenon that can be reproduced by peripheral administration of Gram-negative bacterial lipopolysaccharide (LPS) or interleukin-1beta (IL-1β), a pro-inflammatory cytokine released by LPS-activated macrophages. The arcuate nucleus of the hypothalamus (ARH) plays a major role in food intake regulation and expresses IL-1 type 1 receptor (IL-1R1) mRNA. In the present work, we tested the hypothesis that IL-1R1 expressing cells in the ARH mediate IL-1β and/or LPS-induced hypophagia in the rat. To do so, we developed an IL-1β-saporin conjugate, which eliminated IL-R1-expressing neurons in the hippocampus, and micro-injected it into the ARH prior to systemic IL-1β and LPS administration. ARH IL-1β-saporin injection resulted in loss of neuropeptide Y-containing cells and attenuated hypophagia and weight loss after intraperitoneal IL-1β, but not LPS, administration. In conclusion, the present study shows that ARH NPY-containing neurons express functional IL-1R1s that mediate peripheral IL-1β-, but not LPS-, induced hypophagia. Our present and previous findings indicate that the reduction of food intake after IL-1β and LPS are mediated by different neural pathways.

The orphan nuclear receptor TLX: an emerging master regulator of cross-talk between microglia and neural precursor cells

Neuroinflammation and neurogenesis have both been the subject of intensive investigation over the past 20 years. The sheer complexity of their regulation and their ubiquity in various states of health and disease have sometimes obscured the progress that has been made in unraveling their mechanisms and regulation. A recent study by Kozareva et al. (Neuronal Signaling (2019) 3), provides evidence that the orphan nuclear receptor TLX is central to communication between microglia and neural precursor cells and could help us understand how inflammation, mediated by microglia, influences the development of new neurons in the adult hippocampus. Here, we put recent studies on TLX into the context of what is known about adult neurogenesis and microglial activation in the brain, along with the many hints that these processes must be inter-related.