Pain is associated with most bony pathologies. Clinical and experimental observations suggest that bone pain can be derived from noxious stimulation of the periosteum or bone marrow. Sensory neurons ...are known to innervate the periosteum and marrow cavity, and most of these have a morphology and molecular phenotype consistent with a role in nociception. However, little is known about the physiology of these neurons, and therefore information about mechanisms that generate and maintain bone pain is lacking. The periosteum has received greater attention relative to the bone marrow, reflecting the easier access of the periosteum for experimental assessment. With the electrophysiological preparations used, investigators have been able to record from single periosteal units in isolation, and there is a lot of information available about how they respond to different stimuli, including those that are noxious. In contrast, preparations used to study sensory neurons that innervate the bone marrow have been limited to recording multi-unit activity in whole nerves, and whilst they clearly report responses to noxious stimulation, it is not possible to define responses for single sensory neurons that innervate the bone marrow. There is only limited evidence that peripheral sensory neurons that innervate bone can be sensitized or that they can be activated by multiple stimulus types, and at present this only exists in part for periosteal units. In the central nervous system, it is clear that spinal dorsal horn neurons can be activated by noxious stimuli applied to bone. Some can be sensitized under pathological conditions and may contribute in part to secondary or referred pain associated with bony pathology. Activity related to stimulation of sensory nerves that innervate bone has also been reported in neurons of the spinoparabrachial pathway and the somatosensory cortices, both known for roles in coding information about pain. Whilst these provide some clues as to the way information about bone pain is centrally coded, they need to be expanded to further our understanding of other central territories involved. There is a lot more to learn about the physiology of peripheral sensory neurons that innervate bone and their central projections.
Pain associated with skeletal pathology or disease is a significant clinical problem, but the mechanisms that generate and/or maintain it remain poorly understood. In this study, we explored roles ...for GDNF, neurturin, and artemin signaling in bone pain using male Sprague Dawley rats. We have shown that inflammatory bone pain involves activation and sensitization of peptidergic, NGF-sensitive neurons via artemin/GDNF family receptor α-3 (GFRα3) signaling pathways, and that sequestering artemin might be useful to prevent inflammatory bone pain derived from activation of NGF-sensitive bone afferent neurons. In addition, we have shown that inflammatory bone pain also involves activation and sensitization of nonpeptidergic neurons via GDNF/GFRα1 and neurturin/GFRα2 signaling pathways, and that sequestration of neurturin, but not GDNF, might be useful to treat inflammatory bone pain derived from activation of nonpeptidergic bone afferent neurons. Our findings suggest that GDNF family ligand signaling pathways are involved in the pathogenesis of bone pain and could be targets for pharmacological manipulations to treat it.
Pain associated with skeletal pathology, including bone cancer, bone marrow edema syndromes, osteomyelitis, osteoarthritis, and fractures causes a major burden (both in terms of quality of life and cost) on individuals and health care systems worldwide. We have shown the first evidence of a role for GDNF, neurturin, and artemin in the activation and sensitization of bone afferent neurons, and that sequestering these ligands reduces pain behavior in a model of inflammatory bone pain. Thus, GDNF family ligand signaling pathways are involved in the pathogenesis of bone pain and could be targets for pharmacological manipulations to treat it.
Deep brain temperature detection by hypothalamic warm-sensitive neurons (WSNs) has been proposed to provide feedback information relevant for thermoregulation. WSNs increase their action potential ...firing rates upon warming, a property that has been presumed to rely on the composition of thermosensitive ion channels within WSNs. Here, we describe a synaptic mechanism that regulates temperature sensitivity of preoptic WSNs and body temperature. Experimentally induced warming of the mouse hypothalamic preoptic area in vivo triggers body cooling. TRPM2 ion channels facilitate this homeostatic response and, at the cellular level, enhance temperature responses of WSNs, thereby linking WSN function with thermoregulation for the first time. Rather than acting within WSNs, we—unexpectedly—find TRPM2 to temperature-dependently increase synaptic drive onto WSNs by disinhibition. Our data emphasize a network-based interoceptive paradigm that likely plays a key role in encoding body temperature and that may facilitate integration of diverse inputs into thermoregulatory pathways.
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•Temperature in the hypothalamic preoptic area (POA) tracks body temperature in mice•TRPM2 participates in warmth detection in the POA in vivo•Presynaptic TRPM2 detects temperature increases to activate warm-sensitive neurons•Warming enhances inhibitory transmitter release via TRPM2 to mediate disinhibition
Kamm, Boffi, et al. found that the ion channel TRPM2 mediates detection of warm temperatures in the mouse hypothalamus. Specifically, TRPM2 enhances the thermosensitivity of hypothalamic warm-sensitive neurons (WSNs) via a presynaptic mechanism. This study highlights the role of WSNs and TRPM2 in body temperature homeostasis under hot environments.
Piezo2 is a mechanically gated ion-channel that has a well-defined role in innocuous mechanical sensitivity, but recently has also been suggested to play a role in mechanically induced pain. Here we ...have explored a role for Piezo2 in mechanically evoked bone nociception in Sprague Dawley rats. We have used an
in vivo
electrophysiological bone-nerve preparation to record the activity of single Aδ bone afferent neurons in response to noxious mechanical stimulation, after Piezo2 knockdown in the dorsal root ganglia with intrathecal injections of Piezo2 antisense oligodeoxynucleotides, or in control animals that received mismatch oligodeoxynucleotides. There were no differences in the number of Aδ bone afferent neurons responding to the mechanical stimulus, or their threshold for mechanical activation, in Piezo2 knockdown animals compared to mismatch control animals. However, bone afferent neurons in Piezo2 knockdown animals had reduced discharge frequencies and took longer to recover from stimulus-evoked fatigue than those in mismatch control animals. Piezo2 knockdown also prevented nerve growth factor (NGF)-induced sensitization of bone afferent neurons, and retrograde labeled bone afferent neurons that expressed Piezo2 co-expressed TrkA, the high affinity receptor for NGF. Our findings demonstrate that Piezo2 contributes to the response of bone afferent neurons to noxious mechanical stimulation, and plays a role in processes that sensitize them to mechanical stimulation.
Key points
Sensory neurons that innervate the bone marrow provide the CNS with information about pain associated with bone disease and pathology, but little is known of their function.
Here we use a ...novel in vivo bone–nerve electrophysiological preparation to study how they respond to noxious mechanical stimulation delivered by increasing intra‐osseous pressure.
We provide evidence that sensory neurons that innervate the bone marrow respond to high threshold noxious mechanical stimulation, have response properties consistent with a role in nociception, provide information about different features of an intra‐osseous pressure stimulus and express the Piezo2 mechano‐transducer molecule.
Our findings show how some bone marrow nociceptors signal pain in bony diseases and pathologies that involve a mechanical disturbance or increased intra‐osseous pressure, and that the Piezo2 mechano‐transducer may be involved.
Whilst the sensory neurons and nerve terminals that innervate bone marrow have a morphology and molecular phenotype consistent with a role in nociception, little is known about their physiology or the mechanisms that generate and maintain bone pain. In the present study, we provide evidence that Aδ nociceptors that innervate the bone marrow respond to high threshold noxious mechanical stimulation, exhibit fatigue in response to prior stimulation and in some cases can be sensitized by capsaicin. They can be classified on the basis of their response properties as either phasic–tonic units that appear to code for different intensities of intra‐osseous pressure, or phasic units that code for the rate of change in intra‐osseous pressure. Three different subclasses of mechanically sensitive Aδ units were observed: phasic units that were sensitized by capsaicin, phasic units that were not sensitized by capsaicin and phasic–tonic units (that were not sensitized by capsaicin). These could also, in part, be distinguished by differences in their thresholds for activation, mean discharge frequency, latency to peak activation and peak‐to‐peak action potential amplitude. The majority of small‐diameter myelinated sensory neurons projecting to the bone marrow expressed Piezo2. Our findings indicate that Aδ mechano‐nociceptors are likely to play an important role in generating and maintaining pain in response to bony pathologies that involve a mechanical disturbance or increased intra‐osseous pressure, and imply that Piezo2 signalling may be involved in mechano‐transduction in these receptors.
Key points
Sensory neurons that innervate the bone marrow provide the CNS with information about pain associated with bone disease and pathology, but little is known of their function.
Here we use a novel in vivo bone–nerve electrophysiological preparation to study how they respond to noxious mechanical stimulation delivered by increasing intra‐osseous pressure.
We provide evidence that sensory neurons that innervate the bone marrow respond to high threshold noxious mechanical stimulation, have response properties consistent with a role in nociception, provide information about different features of an intra‐osseous pressure stimulus and express the Piezo2 mechano‐transducer molecule.
Our findings show how some bone marrow nociceptors signal pain in bony diseases and pathologies that involve a mechanical disturbance or increased intra‐osseous pressure, and that the Piezo2 mechano‐transducer may be involved.
Key points Sensory neurons that innervate the bone marrow provide the CNS with information about pain associated with bone disease and pathology, but little is known of their function. Here we use a ...novel in vivo bone-nerve electrophysiological preparation to study how they respond to noxious mechanical stimulation delivered by increasing intra-osseous pressure. We provide evidence that sensory neurons that innervate the bone marrow respond to high threshold noxious mechanical stimulation, have response properties consistent with a role in nociception, provide information about different features of an intra-osseous pressure stimulus and express the Piezo2 mechano-transducer molecule. Our findings show how some bone marrow nociceptors signal pain in bony diseases and pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and that the Piezo2 mechano-transducer may be involved. Whilst the sensory neurons and nerve terminals that innervate bone marrow have a morphology and molecular phenotype consistent with a role in nociception, little is known about their physiology or the mechanisms that generate and maintain bone pain. In the present study, we provide evidence that Adelta nociceptors that innervate the bone marrow respond to high threshold noxious mechanical stimulation, exhibit fatigue in response to prior stimulation and in some cases can be sensitized by capsaicin. They can be classified on the basis of their response properties as either phasic-tonic units that appear to code for different intensities of intra-osseous pressure, or phasic units that code for the rate of change in intra-osseous pressure. Three different subclasses of mechanically sensitive Adelta units were observed: phasic units that were sensitized by capsaicin, phasic units that were not sensitized by capsaicin and phasic-tonic units (that were not sensitized by capsaicin). These could also, in part, be distinguished by differences in their thresholds for activation, mean discharge frequency, latency to peak activation and peak-to-peak action potential amplitude. The majority of small-diameter myelinated sensory neurons projecting to the bone marrow expressed Piezo2. Our findings indicate that Adelta mechano-nociceptors are likely to play an important role in generating and maintaining pain in response to bony pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and imply that Piezo2 signalling may be involved in mechano-transduction in these receptors. Key points Sensory neurons that innervate the bone marrow provide the CNS with information about pain associated with bone disease and pathology, but little is known of their function. Here we use a novel in vivo bone-nerve electrophysiological preparation to study how they respond to noxious mechanical stimulation delivered by increasing intra-osseous pressure. We provide evidence that sensory neurons that innervate the bone marrow respond to high threshold noxious mechanical stimulation, have response properties consistent with a role in nociception, provide information about different features of an intra-osseous pressure stimulus and express the Piezo2 mechano-transducer molecule. Our findings show how some bone marrow nociceptors signal pain in bony diseases and pathologies that involve a mechanical disturbance or increased intra-osseous pressure, and that the Piezo2 mechano-transducer may be involved.
Small-World Networks (SWNs) represent a fundamental model for the comprehension of many complex man-made and biological networks. In the central nervous system, SWN models have been shown to fit well ...both anatomical and functional maps at the macroscopic level. However, the functional microscopic level, where the nodes of a network are represented by single neurons, is still poorly understood. At this level, although recent evidences suggest that functional connection graphs exhibit small-world organization, it is not known whether and how these maps, potentially distributed in multiple brain regions, change across different conditions, such as spontaneous and stimulus-evoked activities. We addressed these questions by analyzing the data from simultaneous multi-array extracellular recordings in three brain regions of rats, diversely involved in somatosensory information processing: the ventropostero-lateral thalamic nuclei, the primary somatosensory cortex and the centro-median thalamic nuclei. From both spike and Local Field Potential (LFP) recordings, we estimated the functional connection graphs by using the Normalized Compression Similarity for spikes and the Phase Synchrony for LFPs. Then, by using graph-theoretical statistics, we characterized the functional topology both during spontaneous activity and sensory stimulation. Our main results show that: (i) spikes and LFPs show SWN organization during spontaneous activity; (ii) after stimulation onset, while substantial functional graph reconfigurations occur both in spike and LFPs, small-worldness is nonetheless preserved; (iii) the stimulus triggers a significant increase of inter-area LFP connections without modifying the topology of intra-area functional connections. Finally, investigating computationally the functional substrate that supports the observed phenomena, we found that (iv) the fundamental concept of cell assemblies, transient groups of activating neurons, can be described by small-world networks. Our results suggest that activity of neurons from multiple areas of the rat somatosensory system contributes to the integration of local computations arisen in distributed functional cell assemblies according to the principles of SWNs.
The Transient receptor potential cation channel subfamily V member 1 (TRPV1) is a non-selective cation channel that is activated by capsaicin, low pH and noxious heat. It has been suggested to have a ...pro-algesic role in a range of conditions that present with bone pain, but the mechanisms by which this occurs are not yet clear. In this study we aimed to determine if TRPV1 is expressed in Aδ and/or C fiber bone afferent neurons, and to explore its role in the activation and/or sensitization of bone afferent neurons to mechanical stimulation. A combination of retrograde tracing and immunohistochemistry was used to determine expression of TRPV1 in the soma of bone afferent neurons that innervate the rat tibial marrow cavity. A novel, in vivo, electrophysiological bone-nerve preparation, recently developed in our laboratory, was used to make recordings of the activity and sensitivity of bone afferent neurons in response to application of the TRPV1 agonist capsaicin to the marrow cavity. We found that a substantial proportion of bone afferent neurons express TRPV1. These include both small-diameter myelinated (neurofilament rich) and unmyelinated (neurofilament poor) neurons that are likely to be Aδ and C fiber neurons, respectively. Electrophysiological recordings revealed that application of capsaicin to the marrow cavity increased ongoing activity of C fiber, and to a lesser extent Aδ fiber, bone afferent neurons. Capsaicin also sensitized both Aδ and C fiber bone afferent neurons to mechanical stimulation. This evidence supports a role for TRPV1 in the pathogenesis of pain associated with bone pathology or disease.
•TRPV1 is expressed in both small-diameter unmyelinated (C fiber) and myelinated (Aδ fiber) bone afferent neurons•Application of capsaicin to bone marrow increases ongoing C, and to a lesser extent Aδ fiber activity•Application of capsaicin to bone marrow sensitizes both C and Aδ fiber bone afferent neurons to mechanical stimulation
Background
Pathologies that affect the bone marrow have a significant inflammatory component; however, it is not clear how inflammatory mediators affect nociceptive nerve terminals within the marrow ...cavity.
Methods
In this study, an in vivo bone‐nerve preparation was used to directly record the physiological response properties of bone marrow nociceptors innervating the tibial marrow cavity of rats, before and after application of the inflammatory agent carrageenan. In addition, endogenous artemin was sequestered by application of an artemin neutralizing antibody to determine if this could prevent the inflammation‐induced physiological changes observed.
Results
A single injection of carrageenan administered into the tibial marrow cavity produced rapid changes in weight bearing (pain‐like behaviour) in conscious animals. Carrageenan, but not saline, activated bone marrow nociceptors in whole‐nerve recordings and sensitized a subtype of Aδ‐bone marrow nociceptors to mechanical stimulation. The activation and sensitization had a rapid time course that matched that of pain‐like behaviours. Sequestration of endogenous artemin significantly reduced carrageenan‐induced increases in ongoing activity and completely abolished sensitization of bone marrow nociceptors to mechanical stimulation.
Conclusions
These observations indicate that inflammation affects the activity and sensitivity of bone marrow nociceptors; that artemin plays a role in these changes; and that artemin might be a promising target for pharmacological manipulations in the treatment of inflammatory bone pain.
Significance
Most pathologies that affect the bone marrow have an inflammatory component. We have used a model of carrageenan‐induced inflammation to show that sequestration of artemin reduces inflammation‐induced activation and sensitization of bone marrow nociceptors. Our findings suggest that artemin signalling is a target for the treatment of inflammatory bone pain.
STOML3 is a membrane bound scaffolding protein that has been shown to facilitate the opening of mechanically sensitive ion channels and contribute to noxious mechanical sensation, allodynia and ...hyperalgesia. In this study, we aimed to determine the role of STOML3 in noxious mechanical sensitivity of bone afferent neurons and carrageenan-induced acute inflammation in the bone. An in vivo, electrophysiological bone-nerve preparation was used to make recordings of the activity and sensitivity of bone afferent neurons that innervate the tibial marrow cavity in anaesthetised rats, in response to noxious mechanical stimuli delivered to the marrow cavity, before and after injection of either the STOML3 oligomerisation inhibitor OB-1 or vehicle, in either naïve animals or animals with carrageenan-induced inflammation of the marrow cavity. A dynamic weight-bearing apparatus was used to measure weight bearing in response to inflammatory pain before and after injection of OB-1 or saline into the tibial marrow cavity in the presence of carrageenan-induced inflammation. Electrophysiological recordings revealed that Aδ, but not C bone afferent neurons have a reduced discharge frequency in response to mechanical stimulation, and that carrageenan-induced sensitisation of Aδ, but not C bone afferent neurons was attenuated by inhibition of STOML3 oligomerisation with OB-1. Animals treated with OB-1 spent a significantly greater amount of time on the limb injected with carrageenan than animals treated with saline. Our findings demonstrate that inhibition of STOML3 oligomerisation reduces inflammatory bone pain by reducing the sensitivity of Aδ bone afferent neurons to mechanical stimulation. Targeting STOML3 may be an effective approach to reduce pain from noxious pressure and/or painful inflammatory pathology in bone.
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