Cerebrospinal fluid (CSF) is increasingly recognized as a dynamic signaling milieu rather than a passive liquid surrounding the brain and spinal cord. It carries ions, peptides, metabolites, and mechanical cues that reflect ongoing physiological states such as posture, breathing, locomotion, and circadian rhythms. Neurons capable of directly sampling this information are ideally positioned to influence motor and autonomic functions. Among them, Cerebrospinal Fluid-contacting Neurons (CSF-cNs) represent one of the most intriguing and evolutionarily conserved sensory populations in the central nervous system.
CSF-cNs line the central canal of the spinal cord, extending a specialized dendritic “bulb” capped with motile cilia into the CSF. This unique morphology allows them to detect chemical signals (pH, osmolarity, neurotransmitters), mechanical forces, and flow dynamics. Their molecular sensory machinery is built upon ion channels such as PKD2L1 and ASIC family members, enabling them to transform environmental changes within the CSF into neuronal activity. Although present across vertebrates, their roles appear adapted to species-specific demands: in zebrafish, they rapidly modulate tail-beat frequency and locomotor reflexes, whereas in mammals they control skilled locomotion and they might integrate autonomic or interoceptive circuits.
Figure 1- The system of spinal CSF-cNs. Top, spinal CSF-cNs in the rabbit spinal cord as depicted by Agduhr in 1922 (left). CSF-cNs in the turtle showing the protrusion (arrows) towards the CC (middle). Mouse CSF-cNs around the CC (dashed line) showing the selective expression of PKD2L1 (green) along with the dendritic marker MAP2 (magenta) in the extension contacting the CSF (arrows). Bottom, the system of mouse CSF-cNs (green) under a light-sheet microscope showing its rostro- caudal distribution from the caudal rhombencephalon to lumbo-sacral spinal cord. Scale bar = 2mm. Top: Adapted from (REFS).
CSF-cNs occupy a strategic position within spinal circuits. They form inhibitory synapses onto premotor interneurons and α-motoneurons, providing a direct route to shaping locomotor output. At the same time, their projections toward sympathetic preganglionic neurons suggest a broader role in adjusting autonomic tone, linking CSF physiology to cardiovascular, respiratory, and metabolic states. Despite their potential importance, their full connectivity, neurotransmitter repertoire, and context-dependent functions remain only partially understood.
To investigates how CSF-cNs sense the internal environment and convert this information into coordinated motor and autonomic responses. Nicolas Wanaverbecq lab integrates electrophysiology, optogenetic/chemogenetic manipulation, single-cell profiling, connectomics, and behavioral assays to reveal important aspects of cSF-cNs physiology:
How chemical and mechanical cues modulate CSF-cN excitability
The precise anatomical organization of CSF-cNs at spinal and suprapinal levels
The synaptic and extrasynaptic mechanisms by which they influence spinal circuits
How their activity contributes to behaviors such as locomotion, postural adjustment, autonomic regulation, and states of vigilance.
- Spinal CSF-cNs and their roles
CSF-contacting neurons (CSF-cNs) are a late-born spinal population with a dual developmental origin, emerging from both dorsal/p2-derived and ventral/p3-derived progenitors. Their immature molecular profile—low NeuN, high input resistance—suggests a neotenic identity that may preserve latent neurogenic potential into adulthood. Positioned along the central canal, they extend a specialized ciliated dendrite into the CSF, placing them at the interface between the spinal parenchyma and the internal milieu. Some studies propose that CSF-cNs could reactivate neurogenic programs after spinal injury, making them an intriguing target for regenerative strategies. Together, these features outline CSF-cNs as a unique developmental and functional module within the spinal cord.
- Neurochemistry of spinal CSF-cNs
Although fundamentally GABAergic, CSF-cNs display remarkable neurochemical diversity, with species- and subtype-specific expression of peptides and neuromodulators. Their rostrocaudal axonal orientation likely contributed to decades of underreporting in transverse spinal studies. Ultrastructural findings hint at a secretory capability from their CSF-exposed dendritic bulb, potentially enabling communication into the CSF itself, though this remains unproven in mammals. Clarifying their transmitter repertoire and secretory capacity is essential to understanding whether CSF-cNs participate in volume transmission and CSF-based signaling loops.
- CSF-cNs as sensory neurons expressing PKD2L1
PKD2L1 defines CSF-cNs as intrinsic CNS sensors tuned to mechanical forces, pH shifts, and osmolarity changes. These neurons can generate action potentials directly from spontaneous channel openings, acting as multimodal spike initiators rather than passive responders. Their sensory properties shape locomotor behavior in zebrafish and may represent an ancient mechanosensory system conserved across bilateria. The presence of PKD2L1-like ciliary neurons in diverse species suggests an atavistic role in fast avoidance behaviors. These attributes challenge the traditional view of the CNS as devoid of interoceptors and position CSF-cNs as core internal sensors.
- CSF-cNs in locomotor regulation
In zebrafish, CSF-cNs dynamically tune locomotor output by inhibiting V0-v interneurons and escape-related motor neurons, adapting speed and posture during both routine swimming and startle responses. Recent evidence in mice indicates that disrupting CSF-cN function impairs skilled locomotion, hinting at a conserved spinal mechanism for fine motor control. Their potential connections to mammalian motor interneurons and motoneurons remain largely unmapped and represent a major gap in spinal circuit biology. Understanding how CSF-cNs integrate mechanical cues from the CSF with descending commands could redefine how internal-state information shapes motor behavior.
- Synaptic inputs onto CSF-cNs
Although best known as sensory detectors, CSF-cNs themselves receive diverse synaptic inputs, including glutamatergic, GABAergic, cholinergic, and monoaminergic signals. Electrophysiology suggests robust spontaneous GABAergic drive, possibly reflecting recurrent local connectivity or slice-dependent biases. Their neuromodulatory inputs imply that CSF-cN activity is not merely reflexive but integrated within broader behavioral states such as arousal, vigilance, or movement initiation. Mapping these inputs—including descending pathways—will clarify how CSF-cNs fit into the larger sensory-motor hierarchy of the spinal cord.
- CSF-cNs and Sympathetic Preganglionic Neurons (SPNs)
The anatomical proximity of CSF-cNs and thoracic SPNs opens the possibility of a direct link between CSF sensing and autonomic output. Defining their connectivity with SPNs could reveal a new spinal hub where internal fluid dynamics shape systemic homeostasis.
- CSF-cNs as interoceptors
CSF-cNs satisfy the key criteria of interoceptors: they detect chemical and mechanical signals and use specialized sensory transduction channels to encode the body’s internal state. As ciliated sensory neurons embedded within the CNS, they challenge the classical boundary between internal and peripheral sensing systems. Their connectivity—with motor, premotor, and autonomic circuits—suggests a role in integrating CSF-borne information into whole-body homeostatic responses. Establishing CSF-cNs as central interoceptors reframes the spinal cord as an active participant in interoception rather than a passive conduit, offering a new conceptual framework for spinal physiology.
- CSF-cNs dialogue with supraspinal centers
Spinal CSF-cNs form an unexpectedly extensive ascending GABAergic system that innervates key autonomic and sensorimotor nuclei in the hindbrain, including the DMV, NTS, Raphe Obscurus, and hypoglossal nucleus. Their projections place them in a position to modulate both parasympathetic output and the processing of visceral inputs—functions traditionally assigned to classical interoceptive pathways. Retrograde tracing from the NTS and lamina X reveals that CSF-cNs interface directly with canonical interoceptive circuits, including neurons involved in nociception. This supraspinal dialogue suggests that CSF-cNs integrate CSF-borne sensory cues with descending and local synaptic inputs to calibrate whole-body responses. By linking spinal mechanics, internal chemistry, and autonomic drive, CSF-cNs emerge as a central node coordinating the interoceptive–autonomic–motor axis.
- Rhombencephalic CSF-cNs: an expanded sensory–autonomic hub
Beyond the spinal cord, a rostral population of CSF-cNs occupies the floor and lateral walls of the fourth ventricle, forming a rhombencephalic extension of the CSF-cN system. These neurons maintain hallmark features—PKD2L1 expression, apical cilia contacting the ventricular CSF, and GABAergic identity—yet reside at the core of interoceptive and autonomic control centers. Their proximity to the NTS, DMV, Raphe nuclei, and parabrachial complex suggests that they could sense CSF dynamics at the brainstem level and gate visceral reflexes, arousal states, or breathing-related rhythms. Because brainstem CSF flow integrates signals from the entire neuraxis, rhombencephalic CSF-cNs may act as high-level “summation sensors” that report global internal-state variables.