The Establishment of a CSF-Contacting Nucleus "Knockout" Model Animal

Demethylase FTO in the cerebrospinal fluid-contacting nucleus of mice contributes to neuropathic pain via mediating m6A demethylation of P2rx4 mRNA

N6-methyladenosine (m6A) modification plays a crucial role in pain regulation by modulating pain-related gene expression. The cerebrospinal fluid-contacting nucleus (CSF-contacting nucleus) is closely associated with pain, and downregulation of P2X4 receptor (P2X4R) expression in this region alleviates hyperalgesia. However, the relationship between m6A modification and P2X4R in CSF-contacting nucleus remains unclear. This study aims to investigate the role and potential mechanisms of the m6A demethylase fat mass and obesity-associated protein (FTO) and P2X4R in neuropathic pain (NP) induced by spared nerve injury (SNI) in male mice. We observed decreased m6A levels and upregulated FTO expression in the CSF-contacting nucleus of SNI mice. FTO was primarily expressed in neurons of the CSF-contacting nucleus, with symmetrical distribution across its bilateral regions. In CSF-contacting nucleus, FTO overexpression reduced m6A methylation and promoted pain, while FTO inhibition increased m6A levels and alleviated pain hypersensitivity. The administration of the FTO inhibitor meclofenamic acid (MA) into CSF-contacting nucleus alleviated pain. FTO regulated the expression of P2rx4 mRNA and protein in CSF-contacting nucleus. Furthermore, P2rx4 mRNA is a downstream target of FTO-mediated m6A demethylation. In summary, the m6A demethylase FTO contributes to NP by upregulating the expression of P2rx4 mRNA and protein through mediating m6A demethylation of P2rx4 mRNA. Therefore, the m6A demethylase FTO in CSF-contacting nucleus may represent a novel therapeutic target for NP.

Roles of cerebrospinal fluid-contacting neurons as potential neural stem cells in the repair and regeneration of spinal cord injuries

Cerebrospinal fluid-contacting neurons (CSF-cNs) represent a distinct group of interneurons characterized by their prominent apical globular protrusions penetrating the spinal cord's central canal and their basal axons extending towards adjacent cells. Identified nearly a century back, the specific roles and attributes of CSF-cNs have just started to emerge due to the historical lack of definitive markers. Recent findings have confirmed that CSF-cNs expressing PKD2L1 possess attributes of neural stem cells, suggesting a critical function in the regeneration processes following spinal cord injuries. This review aims to elucidate the molecular markers of CSF-cNs as potential neural stem cells during spinal cord development and assess their roles post-spinal cord injury, with an emphasis on their potential therapeutic implications for spinal cord repair.

De novo aging-related NADPH diaphorase positive megaloneurites in the sacral spinal cord of aged dogs

We investigated aging-related changes in nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) in the spinal cord of aged dogs. At all levels of the spinal cord examined, NADPH-d activities were observed in neurons and fibers in the superficial dorsal horn (DH), dorsal gray commissure (DGC) and around the central canal (CC). A significant number of NADPH-d positive macro-diameter fibers, termed megaloneurites, were discovered in the sacral spinal cord (S1-S3) segments of aged dogs. The distribution of megaloneurites was characterized from the dorsal root entry zone (DREZ) into the superficial dorsal horn, along the lateral collateral pathway (LCP) to the region of sacral parasympathetic nucleus (SPN), DGC and around the CC, but not in the cervical, thoracic and lumbar segments. Double staining of NADPH-d histochemistry and immunofluorescence showed that NADPH-d positive megaloneurites co-localized with vasoactive intestinal peptide (VIP) immunoreactivity. We believed that megaloneurites may in part represent visceral afferent projections to the SPN and/or DGC. The NADPH-d megaloneurites in the aged sacral spinal cord indicated some anomalous changes in the neurites, which might account for a disturbance in the aging pathway of the autonomic and sensory nerve in the pelvic visceral organs.

Genetic Architecture and Functional Implications of the CSF-Contacting Nucleus

We previously identified a unique nucleus, the cerebrospinal fluid (CSF)-contacting nucleus. This study aims to understand its gene architecture and preliminarily suggest its functions. The results showed that there were about 19,666 genes in this nucleus, of which 913 were distinct from the dorsal raphe nucleus (non-CSF contacting). The top 40 highly-expressed genes are mainly related to energy metabolism, protein synthesis, transport, secretion, and hydrolysis. The main neurotransmitter is 5-HT. The receptors of 5-HT and GABA are abundant. The channels for Cl-, Na+, K+, and Ca2+ are routinely expressed. The signaling molecules associated with the CaMK, JAK, and MAPK pathways were identified accurately. In particular, the channels of transient receptor potential associated with nociceptors and the solute carrier superfamily members associated with cell membrane transport were significantly expressed. The relationship between the main genes of the nucleus and life activities is preliminarily verified.

Cerebrospinal fluid-contacting neuron tracing reveals structural and functional connectivity for locomotion in the mouse spinal cord

Cerebrospinal fluid-contacting neurons (CSF-cNs) are enigmatic mechano- or chemosensory cells lying along the central canal of the spinal cord. Recent studies in zebrafish larvae and lampreys have shown that CSF-cNs control postures and movements via spinal connections. However, the structures, connectivity, and functions in mammals remain largely unknown. Here we developed a method to genetically target mouse CSF-cNs that highlighted structural connections and functions. We first found that intracerebroventricular injection of adeno-associated virus with a neuron-specific promoter and Pkd2l1-Cre mice specifically labeled CSF-cNs. Single-cell labeling of 71 CSF-cNs revealed rostral axon extensions of over 1800 μm in unmyelinated bundles in the ventral funiculus and terminated on CSF-cNs to form a recurrent circuitry, which was further determined by serial electron microscopy and electrophysiology. CSF-cNs were also found to connect with axial motor neurons and premotor interneurons around the central canal and within the axon bundles. Chemogenetic CSF-cNs inactivation reduced speed and step frequency during treadmill locomotion. Our data revealed the basic structures and connections of mouse CSF-cNs to control spinal motor circuits for proper locomotion. The versatile methods developed in this study will contribute to further understanding of CSF-cN functions in mammals.

WITHDRAWN: The cerebrospinal fluid-contacting nucleus contributes to depression- like behaviors via MKP-1 in rats

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Release of Endocannabinoids into the Cerebrospinal Fluid during the Induction of the Trigemino-Hypoglossal Reflex in Rats

The endocannabinoid system (ECS) plays an important role in pain processing and modulation. Since the specific effects of endocannabinoids within the orofacial area are largely unknown, we aimed to determine whether an increase in the endocannabinoid concentration in the cerebrospinal fluid (CSF) caused by the peripheral administration of the FAAH inhibitor URB597 and tooth pulp stimulation would affect the transmission of impulses between the sensory and motor centers localized in the vicinity of the third and fourth cerebral ventricles. The study objectives were evaluated on rats using a method that allowed the recording of the amplitude of evoked tongue jerks (ETJ) in response to noxious tooth pulp stimulation and URB597 treatment. The amplitude of ETJ was a measure of the effect of endocannabinoids on the neural structures. The concentrations of the endocannabinoids tested (AEA and 2-AG) were determined in the CSF, along with the expression of the cannabinoid receptors (CB1 and CB2) in the tissues of the mesencephalon, thalamus, and hypothalamus. We demonstrated that anandamide (AEA), but not 2-arachidonoylglycerol (2-AG), was significantly increased in the CSF after treatment with a FAAH inhibitor, while tooth pulp stimulation had no effect on the AEA and 2-AG concentrations in the CSF. We also found positive correlations between the CSF AEA concentration and cannabinoid receptor type 1 (CB1R) expression in the brain, and between 2-AG and cannabinoid receptor type 2 (CB2R), and negative correlations between the CSF concentration of AEA and brain CB2R expression, and between 2-AG and CB1R. Our study shows that endogenous AEA, which diffuses through the cerebroventricular ependyma into CSF and exerts a modulatory effect mediated by CB1Rs, alters the properties of neurons in the trigeminal sensory nuclei, interneurons, and motoneurons of the hypoglossal nerve. In addition, our findings may be consistent with the emerging concept that AEA and 2-AG have different regulatory mechanisms because they are involved differently in orofacial pain. We also suggest that FAAH inhibition may offer a therapeutic approach to the treatment of orofacial pain.

CSF-CN contributes to cancer-induced bone pain via the MKP-1-mediated MAPK pathway

Pain is a major complication of cancer and significantly affects the quality of life. Cerebrospinal fluid-contacting nucleus (CSF-CN) has been reported to be involved in the development of neuropathic pain and inflammatory pain. However, whether CSF-CN contributes to cancer-induced bone pain (CIBP) remains unknown. In this study, we aimed to illustrate the role of CSF-CN in the pathogenesis of CIBP and identify its potential mechanism via the MKP-1-mediated MAPK pathway. The Walker 256 cancer cells were injected into the tibia cavity of female Sprague-Dawley rats to induce CIBP models. Intracerebroventricular injection of cholera toxin subunit B- saporin (CB-SAP) was performed to "knockout" the CSF-CN. Morphine and LV-MKP-1 were applied. Mechanical and thermal hyperalgesia behaviors, double immunofluorescence staining and Western blot were conducted after CIBP induction. The results revealed that CIBP significantly reduced the mechanical withdrawal threshold and the thermal threshold. Double immunofluorescence staining revealed that c-Fos-positive neurons in CSF-CN were significantly higher in the CIBP group than that in the sham group. Targeted ablation of CSF-CN dramatically aggravated pain sensitivity. Moreover, MKP-1 was down-regulated in the CSF-CN after CIBP induction. Pharmacological intervention with morphine significantly ameliorated the mechanical and thermal hyperalgesia through reversing the down-expression of MKP-1 in the CSF-CN on day 14 after CIBP induction. Mechanically, overexpression of MKP-1 by LV-MKP-1 injection significantly relieved CIBP via inhibiting the expression of phosphorylated p38, which subsequently decreased the protein levels of Bax, cleaved caspase-3 and Iba-1, and reduced the mRNA levels of IL-1β, TNF-α and IL-6 in CSF-CN. In conclusion, CSF-CN contributed to CIBP via regulating the MKP-1-mediated p38-MAPK pathway. Future therapy targeting the expression of MKP-1 in the CSF-CN may be a promising new choice.

The CSF-Contacting Nucleus Receives Anatomical Inputs From the Cerebral Cortex: A Combination of Retrograde Tracing and 3D Reconstruction Study in Rat

Objective

This study aimed to investigate the direct monosynaptic projections from cortical functional regions to the cerebrospinal fluid (CSF)-contacting nucleus for understanding the functions of the CSF-contacting nucleus.

Methods

The Sprague-Dawley rats received cholera toxin B subunit (CB) injections into the CSF-contacting nucleus. After 7-10 days of survival time, the rats were perfused, and the whole brain and spinal cord were sliced under a freezing microtome at 40 μm. All sections were treated with the CB immunofluorescence reaction. The retrogradely labeled neurons in different cortical areas were revealed under a confocal microscope. The distribution features were further illustrated under 3D reconstruction.

Results

The retrogradely labeled neurons were identified in the olfactory, orbital, cingulate, insula, retrosplenial, somatosensory, motor, visual, auditory, association, rhinal, and parietal cortical areas. A total of 12 functional areas and 34 functional subregions showed projections to the CSF-contacting nucleus in different cell intensities.

Conclusion

According to the connectivity patterns, we conclude that the CSF-contacting nucleus participates in cognition, emotion, pain, visceral activity, etc. The present study firstly reveals the cerebral cortex→CSF-contacting nucleus connections, which implies the multiple functions of this special nucleus in neural and body fluid regulations.

Novel Projections to the Cerebrospinal Fluid-Contacting Nucleus From the Subcortex and Limbic System in Rat

Objective: To identify the novel projections received by the cerebrospinal fluid (CSF)-contacting nucleus from the subcortex and limbic system to understand the biological functions of the nucleus.

Methods: The cholera toxin subunit B (CB), a retrograde tracer, was injected into the CSF-contacting nucleus in Sprague–Dawley rats. After 7–10 days, the surviving rats were perfused, and the whole brain and spinal cord were sliced for CB immunofluorescence detection. The CB-positive neurons in the subcortex and limbic system were observed under a fluorescence microscope, followed by 3D reconstructed with the imaris software.

Results: CB-positive neurons were found in the basal forebrain, septum, periventricular organs, preoptic area, and amygdaloid structures. Five functional areas including 46 sub-regions sent projections to the CSF-contacting nucleus. However, the projections had different densities, ranging from sparse to moderate, to dense.

Conclusions: According to the projections from the subcortex and limbic system, we hypothesize that the CSF-contacting nucleus participates in emotion, cognition, homeostasis regulation, visceral activity, pain, and addiction. In this study, we illustrate the novel projections from the subcortex and limbic system to the CSF-contacting nucleus, which underlies the diverse and complicated circuits of the nucleus in body regulations.

A Special Cranial Nucleus (CSF-Contacting Nucleus) in Primates

BACKGROUND

There is a unique nucleus (CSF-contacting nucleus) in the brain of rat. It has been demonstrated in our previous research. The extraordinary feature of this nucleus is that it is not connected to any parenchymal organ but to the CSF. In primates, however, the presence or absence of this nucleus has not been proven. Confirmation of the presence of this nucleus in primates will provide the structural basis for brain-CSF communication and help to understand the neurohumoral regulatory mechanisms in humans.

METHODS

The tracer cholera toxin B subunit conjugated to horseradish peroxidase (CB-HRP) was injected into the CSF in the lateral ventricle (LV) of primate rhesus monkeys. After 48 h, the monkeys were perfused and the brain was dissected out, and sectioned for CB-HRP staining. The CB-HRP positive structures were observed under confocal and electron microscopy. The three-dimensional (3D) structure of the CB-HRP positive neurons cluster was reconstructed by computer software.

RESULTS

(1) CB-HRP labeling is confined within the ventricle, but not leakage into the brain parenchyma. (2) From the midbrain inferior colliculus superior border plane ventral to the aqueduct to the upper part of the fourth ventricle (4V) floor, a large number of CB-HRP positive neurons are consistently located, form a cluster, and are symmetrically located on both sides of the midline. (3) 3D reconstruction shows that the CB-HRP positive neurons cluster in the monkey brain occupies certain space. The rostral part is large and caudal part is thin appearing a "rivet"-like shape. (4) Under electron microscopy, the CB-HRP positive neurons show different types of synaptic connections with the non-CSF-contacting structures in the brain. Some of the processes stretch directly into the ventricle cavity.

CONCLUSION

Same as we did in rats, the CSF-contacting nucleus is also existed in the primate brain parenchyma. We also recommend listing it as the XIII pair of cranial nucleus, which is specialized in the communications between the brain and the CSF. It is significant to the completing of innervation in the organism.

Connection Input Mapping and 3D Reconstruction of the Brainstem and Spinal Cord Projections to the CSF-Contacting Nucleus

Objective

To investigate whether the CSF-contacting nucleus receives brainstem and spinal cord projections and to understand the functional significance of these connections.

Methods

The retrograde tracer cholera toxin B subunit (CB) was injected into the CSF-contacting nucleus in Sprague-Dawley rats according the previously reported stereotaxic coordinates. After 7–10 days, these rats were perfused and their brainstem and spinal cord were sliced (thickness, 40 μm) using a freezing microtome. All the sections were subjected to CB immunofluorescence staining. The distribution of CB-positive neuron in different brainstem and spinal cord areas was observed under fluorescence microscope.

Results

The retrograde labeled CB-positive neurons were found in the midbrain, pons, medulla oblongata, and spinal cord. Four functional areas including one hundred and twelve sub-regions have projections to the CSF-contacting nucleus. However, the density of CB-positive neuron distribution ranged from sparse to dense.

Conclusion

Based on the connectivity patterns of the CSF-contacting nucleus receives anatomical inputs from the brainstem and spinal cord, we preliminarily conclude and summarize that the CSF-contacting nucleus participates in pain, visceral activity, sleep and arousal, emotion, and drug addiction. The present study firstly illustrates the broad projections of the CSF-contacting nucleus from the brainstem and spinal cord, which implies the complicated functions of the nucleus especially for the unique roles of coordination in neural and body fluids regulation.

The CSF-contacting nucleus regulates learning and memory through synaptic connections with the hippocampus

The cerebrospinal fluid (CSF)-contacting nucleus is a special nucleus. To study the mechanism of the CSF-contacting nucleus in learning and memory, we used classic retrograde tracing methods to observe the synaptic connections between the CSF-contacting nucleus and the hippocampus. By injecting cholera toxin B subunit (CB) - saporin (SAP) into the lateral ventricle of animals to exclusively damage this nucleus, a mature CSF-contacting nucleus-deficient model animal was established. Then, the changes in learning and memory behaviors in animals with "damage" or "compensation" after damage to the CSF-contacting nucleus were studied. The results showed that learning and memory abilities in animals decreased significantly after the destruction of the CSF-contacting nucleus, accompanied by a decrease in 5-HT concentrations in hippocampus. However, after compensating for 5-HT in the hippocampus continuously, the learning and memory abilities of the animals were significantly improved. This study suggests that the CSF-contacting nucleus may participate in the regulation of learning and memory through direct synaptic connections with the hippocampus.

Monosynaptic Input Mapping of Diencephalic Projections to the Cerebrospinal Fluid-Contacting Nucleus in the Rat

Objective: To investigate the projections the cerebrospinal fluid-contacting (CSF-contacting) nucleus receives from the diencephalon and to speculate on the functional significance of these connections.

Methods: The retrograde tracer cholera toxin B subunit (CB) was injected into the CSF-contacting nucleus in SD rats according to the experimental formula of the stereotaxic coordinates. Animals were perfused 7–10 days after the injection, and the diencephalon was sliced at 40 μm with a freezing microtome. CB-immunofluorescence was performed on all diencephalic sections. The features of CB-positive neuron distribution in the diencephalon were observed with a fluorescence microscope.

Results: The retrograde labeled CB-positive neurons were found in the epithalamus, subthalamus, and hypothalamus. Three functional diencephalic areas including 43 sub-regions revealed projections to the CSF-contacting nucleus. The CB-positive neurons were distributed in different density ranges: sparse, moderate, and dense.

Conclusion: Based on the connectivity patterns of the CSF-contacting nucleus that receives anatomical inputs from the diencephalon, we preliminarily assume that the CSF-contacting nucleus participates in homeostasis regulation, visceral activity, stress, emotion, pain and addiction, and sleeping and arousal. The present study firstly illustrates the broad projections of the CSF-contacting nucleus from the diencephalon, which implies the complicated functions of the nucleus especially for the unique roles of coordination in neural and body fluids regulations.

Cerebrospinal fluid-contacting neurons in the regenerating spinal cord of lizards and amphibians are likely mechanoreceptors

During spinal cord (SC) regeneration in the tail of amphibians and lizards, small neurons in contact with the central canal and cerebrospinal fluid (CSF) are formed. The present review summarizes previous and recent studies that have characterized most of these neurons as cerebrospinal fluid-contacting neurons (CSFCNs), especially in the regenerating caudal SC of lizards. CSFCNs form tufts of stereocilia immersed in the CSF, secrete exosomes, and are often in contact with a secreted protein-rod indicated as Reissner fiber. Ultrastructural, autoradiographic, immunohistochemical, and behavioral studies strongly indicate that most of these cells are mechanoreceptors that differentiate from ependymal cells within 20-30 days after SC amputation. Numerous CSFCNs are gamma amino-butyric acid (GABA)-ergic, uptake amino acids, receive few synaptic boutons, and contain neurofilaments, fibroblast growth factor (FGFs), and other signaling proteins, the latter likely secreted into the central canal. Similar neurons are formed in the SC of the tuatara (Sphenodon puctatus), anurans, and urodeles during tail regeneration. In lizard, most of their projection remains in the SC close to the regenerated tail, but they form synapses with neurons that receive descending nerves from the brainstem, including vestibular nuclei. CSFCNs, aside a possible neurosecretory activity, might sense liquor movements for maintenance of balance, a role that is supported from recent studies on other caudate vertebrates. The regeneration of these cells also in the nervous system of other vertebrates remains unknown.

Stereotaxic Coordinates and Morphological Characterization of a Unique Nucleus (CSF-Contacting Nucleus) in Rat

A unique nucleus, the cerebrospinal fluid (CSF)-contacting nucleus, has recently been recognized in the brain parenchyma. The outstanding feature of this nucleus is that the neural somas are located in the parenchyma, but their processes stretch into the CSF, implying that it may be a key structure bridging the nervous and body fluids-regulating systems and may play a pivotal role in modulating physiological activities. However, the true biological significance of this nucleus needs to be uncovered. The morphology of a nucleus is one of the most important parameters for neuroscience studies. For this reason, a common experimental animal, Sprague-Dawley (SD) rats, was chosen. The position, adjacent structures, neuronal distribution, size, three-dimensional reconstruction, and core coordinates of the CSF-contacting nucleus in SD rats of different weights (90–400 g) were illustrated for the first time. Furthermore, the formulas for calculating the core coordinates of the CSF-contacting nucleus in rats of different weights were revealed. Finally, the possible biological functions uncovered by past research are reviewed in this paper. This study provides an indispensable methodology and a significant reference for researchers interested in this unique nucleus.