Noradrenergic regulation of plasticity marker expression in the adult rodent piriform cortex

The awakening of dormant neuronal precursors in the adult and aged brain

Beyond the canonical neurogenic niches, there are dormant neuronal precursors in several regions of the adult mammalian brain. Dormant precursors maintain persisting post-mitotic immaturity from birth to adulthood, followed by staggered awakening, in a process that is still largely unresolved. Strikingly, due to the slow rate of awakening, some precursors remain immature until old age, which led us to question whether their awakening and maturation are affected by aging. To this end, we studied the maturation of dormant precursors in transgenic mice (DCX-CreERT2 /flox-EGFP) in which immature precursors were labelled permanently in vivo at different ages. We found that dormant precursors are capable of awakening at young age, becoming adult-matured neurons (AM), as well as of awakening at old age, becoming late AM. Thus, protracted immaturity does not prevent late awakening and maturation. However, late AM diverged morphologically and functionally from AM. Moreover, AM were functionally most similar to neonatal-matured neurons (NM). Conversely, late AM were endowed with high intrinsic excitability and high input resistance, and received a smaller amount of spontaneous synaptic input, implying their relative immaturity. Thus, late AM awakening still occurs at advanced age, but the maturation process is slow.

A complex relation between levels of adult hippocampal neurogenesis and expression of the immature neuron marker doublecortin

We investigated the mechanisms underlying the effects of the antidepressant fluoxetine on behavior and adult hippocampal neurogenesis (AHN). After confirming our earlier report that the signaling molecule β-arrestin-2 (β-Arr2) is required for the antidepressant-like effects of fluoxetine, we found that the effects of fluoxetine on proliferation of neural progenitors and survival of adult-born granule cells are absent in the β-Arr2 knockout (KO) mice. To our surprise, fluoxetine induced a dramatic upregulation of the number of doublecortin (DCX)-expressing cells in the β-Arr2 KO mice, indicating that this marker can be increased even though AHN is not. We discovered two other conditions where a complex relationship occurs between the number of DCX-expressing cells compared to levels of AHN: a chronic antidepressant model where DCX is upregulated and an inflammation model where DCX is downregulated. We concluded that assessing the number of DCX-expressing cells alone to quantify levels of AHN can be complex and that caution should be applied when label retention techniques are unavailable.

Genetic loss of norepinephrine does not alter adult hippocampal neurogenesis in dopamine beta-hydroxylase deficient mice

Norepinephrine (NE), and specific adrenoceptors, have been reported to influence distinct aspects of adult hippocampal neurogenesis, including latent stem cell activation, progenitor proliferation, and differentiation. These findings are predominantly based on the use of pharmacological approaches in both in vitro and in vivo systems. Here, we sought to assess the consequences of genetic ablation of NE on adult hippocampal neurogenesis, by examining dopamine β hydroxylase knockout (Dbh -/-) mice, which lack NE from birth. We find that Dbh -/- mice exhibit no difference in adult hippocampal progenitor proliferation and survival. Further, the number of immature newborn neurons, labeled using stage-specific developmental markers within the hippocampal neurogenic niche, was also unaltered in Dbh -/- mice. In contrast, the noradrenergic neurotoxin DSP-4, which had previously been shown to reduce adult hippocampal neurogenesis in rats, also resulted in a decline in hippocampal progenitor proliferation in C57/Bl6N mice. These findings indicate that pharmacological lesioning of noradrenergic afferents in adulthood, but not the complete genetic loss of NE from birth, impairs adult hippocampal neurogenesis in mice.

LRP4 is required for the olfactory association task in the piriform cortex

Background

Low-density lipoprotein receptor-related protein 4 (LRP4) plays a critical role in the central nervous system (CNS), including hippocampal synaptic plasticity, maintenance of excitatory synaptic transmission, fear regulation, as well as long-term potentiation (LTP).

Results

In this study, we found that Lrp4 was highly expressed in layer II of the piriform cortex. Both body weight and brain weight decreased in Lrp4ECD/ECD mice without TMD (Transmembrane domain) and ICD (intracellular domain) of LRP4. However, in the piriform cortical neurons of Lrp4ECD/ECD mice, the spine density increased, and the frequency of both mEPSC (miniature excitatory postsynaptic current) and sEPSC (spontaneous excitatory postsynaptic current) was enhanced. Intriguingly, finding food in the buried food-seeking test was prolonged in both Lrp4ECD/ECD mice and Lrp4 cKO (conditional knockout of Lrp4 in the piriform cortex) mice.

Conclusions

This study indicated that the full length of LRP4 in the piriform cortex was necessary for maintaining synaptic plasticity and the integrity of olfactory function.

Immature excitatory neurons in the amygdala come of age during puberty

The human amygdala is critical for emotional learning, valence coding, and complex social interactions, all of which mature throughout childhood, puberty, and adolescence. Across these ages, the amygdala paralaminar nucleus (PL) undergoes significant structural changes including increased numbers of mature neurons. The PL contains a large population of immature excitatory neurons at birth, some of which may continue to be born from local progenitors. These progenitors disappear rapidly in infancy, but the immature neurons persist throughout childhood and adolescent ages, indicating that they develop on a protracted timeline. Many of these late-maturing neurons settle locally within the PL, though a small subset appear to migrate into neighboring amygdala subnuclei. Despite its prominent growth during postnatal life and possible contributions to multiple amygdala circuits, the function of the PL remains unknown. PL maturation occurs predominately during late childhood and into puberty when sex hormone levels change. Sex hormones can promote developmental processes such as neuron migration, dendritic outgrowth, and synaptic plasticity, which appear to be ongoing in late-maturing PL neurons. Collectively, we describe how the growth of late-maturing neurons occurs in the right time and place to be relevant for amygdala functions and neuropsychiatric conditions.

Why Would the Brain Need Dormant Neuronal Precursors?

Dormant non-proliferative neuronal precursors (dormant precursors) are a unique type of undifferentiated neuron, found in the adult brain of several mammalian species, including humans. Dormant precursors are fundamentally different from canonical neurogenic-niche progenitors as they are generated exquisitely during the embryonic development and maintain a state of protracted postmitotic immaturity lasting up to several decades after birth. Thus, dormant precursors are not pluripotent progenitors, but to all effects extremely immature neurons. Recently, transgenic models allowed to reveal that with age virtually all dormant precursors progressively awaken, abandon the immature state, and become fully functional neurons. Despite the limited common awareness about these cells, the deep implications of recent discoveries will likely lead to revisit our understanding of the adult brain. Thus, it is timely to revisit and critically assess the essential evidences that help pondering on the possible role(s) of these cells in relation to cognition, aging, and pathology. By highlighting pivoting findings as well as controversies and open questions, we offer an exciting perspective over the field of research that studies these mysterious cells and suggest the next steps toward the answer of a crucial question: why does the brain need dormant neuronal precursors?

Butanol Extract of Tinospora cordifolia Alleviates Acute Sleep Deprivation-Induced Impairments in Cognitive Functions and Neuromuscular Coordination in Middle-Aged Female Rats

Sleep deprivation due to present-day lifestyle and late-hours work commitments are associated with a broad spectrum of neurobehavioral complications. Moreover, women, as they age, become prone to the cumulative effects of menopause such as sleep disturbances, adiposity, and inflammation which are attributed to a compromised immuno-neuro-endocrine axis. So far, no effective therapeutic remedy is available to mitigate the adverse effects of SD. The current study was aimed to elucidate the neuroprotective potential of n-Butanol fraction obtained from hydroalcoholic extract of Tinospora cordifolia stem (B-TCE). Four groups of female rats are (1) Vehicle-undisturbed sleep, (2) Vehicle-sleep deprived (between 6 a.m. and 6 p.m.), (3) B-TCE oral feeding for 2 weeks and sleep deprivation, and (4) B-TCE alone undisturbed sleep group. Novel Object Recognition test was used to study cognitive impairments and Rotarod for motor coordination. Rats were then sacrificed to study the expression of various marker proteins in the hippocampus and piriform cortex regions of the brain by western blotting. SD was observed to impair the exploratory behavior and neuromuscular coordination, whereas, B-TCE pre-treatment was observed to ameliorate these behavioral functions'- impairments and further suppressed the changes in the expression of markers for synaptic plasticity, inflammation, cell survival, and apoptosis pathways. The current data suggest that B-TCE may be effective in the management of acute SD-associated impairments in learning and memory functions and neuromuscular coordination.

Functional Integration of Neuronal Precursors in the Adult Murine Piriform Cortex

The extent of functional maturation and integration of nonproliferative neuronal precursors, becoming neurons in the adult murine piriform cortex, is largely unexplored. We thus questioned whether precursors eventually become equivalent to neighboring principal neurons or whether they represent a novel functional network element. Adult brain neuronal precursors and immature neurons (complex cells) were labeled in transgenic mice (DCX-DsRed and DCX-CreER^T2 /flox-EGFP), and their cell fate was characterized with patch clamp experiments and morphometric analysis of axon initial segments. Young (DCX+) complex cells in the piriform cortex of 2- to 4-month-old mice received sparse synaptic input and fired action potentials at low maximal frequency, resembling neonatal principal neurons. Following maturation, the synaptic input detected on older (DCX−) complex cells was larger, but predominantly GABAergic, despite evidence of glutamatergic synaptic contacts. Furthermore, the rheobase current of old complex cells was larger and the maximal firing frequency was lower than those measured in neighboring age-matched principal neurons. The striking differences between principal neurons and complex cells suggest that the latter are a novel type of neuron and new coding element in the adult brain rather than simple addition or replacement for preexisting network components.

Cell Proliferation in the Piriform Cortex of Rats with Motor Cortex Ablation Treated with Growth Hormone and Rehabilitation

Traumatic brain injury represents one of the main health problems in developed countries. Growth hormone (GH) and rehabilitation have been claimed to significantly contribute to the recovery of lost motor function after acquired brain injury, but the mechanisms by which this occurs are not well understood. In this work, we have investigated cell proliferation in the piriform cortex (PC) of adult rats with ablation of the frontal motor cortex treated with GH and rehabilitation, in order to evaluate if this region of the brain, related to the sense of smell, could be involved in benefits of GH treatment. Male rats were either ablated the frontal motor cortex in the dominant hemisphere or sham-operated and treated with GH or vehicle at 35 days post-injury (dpi) for five days. At 36 dpi, all rats received daily injections of bromodeoxyuridine (BrdU) for four days. We assessed motor function through the paw-reaching-for-food task. GH treatment and rehabilitation at 35 dpi significantly improved the motor deficit caused by the injury and promoted an increase of cell proliferation in the PC ipsilateral to the injury, which could be involved in the improvement observed. Cortical ablation promoted a greater number of BrdU+ cells in the piriform cortex that was maintained long-term, which could be involved in the compensatory mechanisms of the brain after injury.

Effects of Dopamine on the Immature Neurons of the Adult Rat Piriform Cortex

The layer II of the adult piriform cortex (PCX) contains a numerous population of immature neurons. Interestingly, in both mice and rats, most, if not all, these cells have an embryonic origin. Moreover, recent studies from our laboratory have shown that they progressively mature into typical excitatory neurons of the PCX layer II. Therefore, the adult PCX is considered a “non-canonical” neurogenic niche. These immature neurons express the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), a molecule critical for different neurodevelopmental processes. Dopamine (DA) is a relevant neurotransmitter in the adult CNS, which also plays important roles in neural development and adult plasticity, including the regulation of PSA-NCAM expression. In order to evaluate the hypothetical effects of pharmacological modulation of dopaminergic neurotransmission on the differentiation of immature neurons of the adult PCX, we studied dopamine D2 receptor (D2r) expression in this region and the relationship between dopaminergic fibers and immature neurons (defined by PSA-NCAM expression). In addition, we analyzed the density of immature neurons after chronic treatments with an antagonist and an agonist of D2r: haloperidol and PPHT, respectively. Many dopaminergic fibers were observed in close apposition to PSA-NCAM-expressing neurons, which also coexpressed D2r. Chronic treatment with haloperidol significantly increased the number of PSA-NCAM immunoreactive cells, while PPHT treatment decreased it. These results indicate a prominent role of dopamine, through D2r and PSA-NCAM, on the regulation of the final steps of development of immature neurons in the adult PCX.

Transgenic models for investigating the nervous system: Currently available neurofluorescent reporters and potential neuronal markers

Recombinant DNA technologies have enabled the development of transgenic animal models for use in studying a myriad of diseases and biological states. By placing fluorescent reporters under the direct regulation of the promoter region of specific marker proteins, these models can localize and characterize very specific cell types. One important application of transgenic species is the study of the cytoarchitecture of the nervous system. Neurofluorescent reporters can be used to study the structural patterns of nerves in the central or peripheral nervous system in vivo, as well as phenomena involving embryologic or adult neurogenesis, injury, degeneration, and recovery. Furthermore, crucial molecular factors can also be screened via the transgenic approach, which may eventually play a major role in the development of therapeutic strategies against diseases like Alzheimer's or Parkinson's. This review describes currently available reporters and their uses in the literature as well as potential neural markers that can be leveraged to create additional, robust transgenic models for future studies.

Structural Changes Observed in the Piriform Cortex in a Rat Model of Pre-motor Parkinson's Disease

Early diagnosis of Parkinson’s disease (PD) offers perhaps, the most promising route to a successful clinical intervention, and the use of an animal model exhibiting symptoms comparable to those observed in PD patients in the early stage of the disease, may facilitate screening of novel therapies for delaying the onset of more debilitating motor and behavioral abnormalities. In this study, a rat model of pre-motor PD was used to study the etiology of hyposmia, a non-motor symptom linked to the early stage of the disease when the motor symptoms have yet to be experienced. The study focussed on determining the effect of a partial reduction of both dopamine and noradrenaline levels on the olfactory cortex. Neuroinflammation and striking structural changes were observed in the model. These changes were prevented by treatment with a neuroprotective drug, a glucagon-like peptide-1 (GLP1) receptor agonist, exendin-4 (EX-4).