Diabetic neuropathic pain (DNP) is the most common disabling complication of diabetes. Emerging evidence has linked the pathogenesis of DNP to the aberrant sprouting of sensory axons into the epidermal area; however, the underlying molecular events remain poorly understood. Here we found that an axon guidance molecule, Netrin-3 (Ntn-3), was expressed in the sensory neurons of mouse dorsal root ganglia (DRGs), and downregulation of Ntn-3 expression was highly correlated with the severity of DNP in a diabetic mouse model. Genetic ablation of Ntn-3 increased the intra-epidermal sprouting of sensory axons and worsened the DNP in diabetic mice. In contrast, the elevation of Ntn-3 levels in DRGs significantly inhibited the intra-epidermal axon sprouting and alleviated DNP in diabetic mice. In conclusion, our studies identified Ntn-3 as an important regulator of DNP pathogenesis by gating the aberrant sprouting of sensory axons, indicating that Ntn-3 is a potential druggable target for DNP treatment.
Mice ; Animals ; Diabetes Mellitus, Experimental/metabolism* ; Axons/physiology* ; Diabetic Neuropathies ; Sensory Receptor Cells/metabolism* ; Neuralgia/metabolism*

Chronic pain and itch are a pathological operation of the somatosensory system at the levels of primary sensory neurons, spinal cord and brain. Pain and itch are clearly distinct sensations, and recent studies have revealed the separate neuronal pathways that are involved in each sensation. However, the mechanisms by which these sensations turn into a pathological chronic state are poorly understood. A proposed mechanism underlying chronic pain and itch involves abnormal excitability in dorsal horn neurons in the spinal cord. Furthermore, an increasing body of evidence from models of chronic pain and itch has indicated that synaptic hyperexcitability in the spinal dorsal horn might not be a consequence simply of changes in neurons, but rather of multiple alterations in glial cells. Thus, understanding the key roles of glial cells may provide us with exciting insights into the mechanisms of chronicity of pain and itch, and lead to new targets for treating chronic pain and itch.
Animals ; Chronic Pain ; pathology ; Humans ; Neuralgia ; metabolism ; Pruritus ; pathology ; Sensory Receptor Cells ; physiology ; Spinal Cord ; pathology

The temperature-sensitive transient receptor potential (TRP) channels, is also called thermo TRP, including TRPV1, TRPV2, TRPV3, TRPV4, TRPM8 and TRPA1, which are expressed in sensory neurons and non-neuronal cells (e.g.keratinocyte, mast cell) of the skin. Thermo TRP channels are activated/sensitized by physical and chemical mediators, which participate in thermosensation and thermoregulation, so that they are key players in pruritus or pain pathogenesis. Thermo TRP channels are also involved in cutaneous neurogenic inflammation, thus they are regarded as molecular targets for future therapy in skin inflammation, pruritus and pain. In addition, following a basic syntax and molecular substrate of nociception and pruriception established by TRP channels-centered concept, the sensory categories can be distinguished and re-defined. Thermo TRP channels should be taken into account when analyzing the pathogenesis and management of itch or pruritic dermatosis.
Humans ; Inflammation ; metabolism ; physiopathology ; Inflammation Mediators ; physiology ; Pruritus ; metabolism ; Sensory Receptor Cells ; metabolism ; Skin ; innervation ; metabolism ; Thermoreceptors ; metabolism ; Transient Receptor Potential Channels ; metabolism ; physiology

Arachidonic acid (AA) in the cell membrane produces a variety of metabolites by different enzymatic pathways. These lipid metabolites, along with other mediators, play an important role in the inflammatory processes. Many of them can bind directly to the receptors on the sensory endings and initiate electrical impulses to be transmitted to the central nervous system, causing reflex responses. These bioactive AA metabolites may also alter the lung mechanics (mechanical environment of the sensory ending), and in turn, stimulate sensory afferents. In addition, some metabolites may sensitize the sensory endings and make them more responsive to other mechanical or chemical stimulation. These metabolites may also induce other mediators and modulators to cause physiological effects. Furthermore, some of them may attract inflammatory cells to produce a localized effect. In short, AA metabolites may come from different sources and act through multiple pathways to stimulate airway sensors. This brief review is intended to illustrate the underlying mechanisms and help elucidate the inflammatory process in the airways.
Animals ; Arachidonic Acid ; metabolism ; Humans ; Inflammation ; physiopathology ; Respiratory Physiological Phenomena ; Respiratory System ; metabolism ; Sensory Receptor Cells ; physiology ; Vagus Nerve ; physiology

Different physical and chemical stimuli are detected by the peripheral sensory receptors of dorsal root ganglion (DRG) neurons, and the generated inputs are transmitted via afferent fibers into the central nervous system. The gene expression profiles of DRG neurons contribute to the generation, transmission, and regulation of various somatosensory signals. Recently, the single-cell transcriptomes, cell types, and functional annotations of somatosensory neurons have been studied. In this review, we introduce our classification of DRG neurons based on single-cell RNA-sequencing and functional analyses, and discuss the technical approaches. Moreover, studies on the molecular and cellular mechanisms underlying somatic sensations are discussed.
Animals ; Ganglia, Spinal ; cytology ; Gene Regulatory Networks ; Humans ; Pain ; genetics ; metabolism ; pathology ; Sensory Receptor Cells ; metabolism ; Sequence Analysis, RNA ; Transcriptome

BACKGROUNDAlthough flunarizine has been widely used for migraine prophylaxis with clear success, the mechanisms of its actions in migraine prophylaxis are not completely understood. The aim of this study was to investigate the effects of flunarizine on tetrodotoxin-resistant Na(+) channels and high-voltage activated Ca(2+) channels of acutely isolated mouse trigeminal ganglion neurons.

METHODSSodium currents and calcium currents in trigeminal ganglion neurons were monitored using whole-cell patch-clamp recordings. Paired Student's t test was used as appropriate to evaluate the statistical significance of differences between two group means.

RESULTSBoth tetrodotoxin-resistant sodium currents and high-voltage activated calcium currents were blocked by flunarizine in a concentration-dependent manner with the concentration producing half-maximal current block values of 2.89 µmol/L and 2.73 µmol/L, respectively. The steady-state inactivation curves of tetrodotoxin-resistant sodium currents and high-voltage activated calcium currents were shifted towards more hyperpolarizing potentials after exposure to flunarizine. Furthermore, the actions of flunarizine in blocking tetrodotoxin-resistant sodium currents and high-voltage activated calcium currents were use-dependent, with effects enhanced at higher rates of channel activation.

CONCLUSIONBlockades of these currents might help explain the peripheral mechanism underlying the preventive effect of flunarizine on migraine attacks.

Animals ; Calcium ; metabolism ; Cells, Cultured ; Female ; Flunarizine ; pharmacology ; Male ; Mice ; Patch-Clamp Techniques ; Sensory Receptor Cells ; drug effects ; metabolism ; Sodium ; metabolism ; Trigeminal Ganglion ; cytology ; drug effects ; metabolism

The cytoplasmic polyadenylation element (CPE)-binding protein (CPEB) binds to CPE containing mRNAs on their 3' untranslated regions (3'UTRs). This RNA binding protein comes out many important tasks, especially in learning and memory, by modifying the translational efficiency of target mRNAs via poly (A) tailing. Overexpressed CPEB has been reported to induce the formation of stress granules (SGs), a sort of RNA granule in mammalian cell lines. RNA granule is considered to be a potentially important factor in learning and memory. However, there is no study about RNA granule in Aplysia. To examine whether an Aplysia CPEB, ApCPEB1, forms RNA granules, we overexpressed ApCPEB1-EGFP in Aplysia sensory neurons. Consistent with the localization of mammalian CPEB, overexpressed ApCPEB1 formed granular structures, and was colocalized with RNAs and another RNA binding protein, ApCPEB, showing that ApCPEB1 positive granules are RNA-protein complexes. In addition, ApCPEB1 has a high turnover rate in RNA granules which were mobile structures. Thus, our results indicate that overexpressed ApCPEB1 is incorporated into RNA granule which is a dynamic structure in Aplysia sensory neuron. We propose that ApCPEB1 granule might modulate translation, as other RNA granules do, and furthermore, influence memory.
Animals ; Aplysia/genetics/*metabolism ; Fluorescence Recovery After Photobleaching ; RNA/genetics/metabolism ; Sensory Receptor Cells/*metabolism ; mRNA Cleavage and Polyadenylation Factors/genetics/metabolism/*physiology

Peripheral itch stimuli are transmitted by sensory neurons to the spinal cord dorsal horn, which then transmits the information to the brain. The molecular and cellular mechanisms within the dorsal horn for itch transmission have only been investigated and identified during the past ten years. This review covers the progress that has been made in identifying the peptide families in sensory neurons and the receptor families in dorsal horn neurons as putative itch transmitters, with a focus on gastrin-releasing peptide (GRP)-GRP receptor signaling. Also discussed are the signaling mechanisms, including opioids, by which various types of itch are transmitted and modulated, as well as the many conflicting results arising from recent studies.
Action Potentials ; drug effects ; Analgesics, Opioid ; pharmacology ; Animals ; Humans ; Pruritus ; metabolism ; pathology ; Sensory Receptor Cells ; metabolism ; Spinal Cord ; pathology ; Synaptic Transmission ; physiology

Cutaneous neurogenic inflammation (CNI) is inflammation that is induced (or enhanced) in the skin by the release of neuropeptides from sensory nerve endings. Clinical manifestations are mainly sensory and vascular disorders such as pruritus and erythema. Transient receptor potential vanilloid 1 and ankyrin 1 (TRPV1 and TRPA1, respectively) are non-selective cation channels known to specifically participate in pain and CNI. Both TRPV1 and TRPA1 are co-expressed in a large subset of sensory nerves, where they integrate numerous noxious stimuli. It is now clear that the expression of both channels also extends far beyond the sensory nerves in the skin, occuring also in keratinocytes, mast cells, dendritic cells, and endothelial cells. In these non-neuronal cells, TRPV1 and TRPA1 also act as nociceptive sensors and potentiate the inflammatory process. This review discusses the role of TRPV1 and TRPA1 in the modulation of inflammatory genes that leads to or maintains CNI in sensory neurons and non-neuronal skin cells. In addition, this review provides a summary of current research on the intracellular sensitization pathways of both TRP channels by other endogenous inflammatory mediators that promote the self-maintenance of CNI.
Animals ; Chronic Disease ; Dendritic Cells ; metabolism ; pathology ; Dermatitis ; metabolism ; pathology ; Gene Expression Regulation ; Humans ; Inflammation ; metabolism ; pathology ; Keratinocytes ; metabolism ; pathology ; Mast Cells ; metabolism ; pathology ; Sensory Receptor Cells ; metabolism ; pathology ; TRPA1 Cation Channel ; biosynthesis ; TRPV Cation Channels ; biosynthesis

Because nerve growth factor (NGF) is elevated during inflammation, plays a causal role in the initiation of hyperalgesia, and is known to activate the sphingomyelin signalling pathway, we examined whether NGF and its putative second messenger, ceramide, could modulate the excitability of capsaicin-sensitive adult sensory neurons. Using the whole-cell patch-clamp recording technique, exposure of isolated sensory neurons to either 100 ng/mL NGF or 1 mmol/L N-acetyl sphingosine (C2-ceramide) produced a 3-4 fold increase in the number of action potentials (APs) evoked by a ramp of depolarizing current in a time-dependent manner. Intracellular perfusion with bacterial sphingomyelinase (SMase) also increased the number of APs suggesting that the release of native ceramide enhanced neuronal excitability. Glutathione, an inhibitor of neutral SMase, completely blocked the NGF-induced augmentation of AP firing, whereas dithiothreitol, an inhibitor of acidic SMase, was without effect. In the presence of glutathione and NGF, exogenous ceramide still enhanced the number of evoked APs, indicating that the sensitizing action of ceramide was downstream of NGF. To investigate the mechanisms of actions for NGF and ceramide, isolated membrane currents were examined. Both NGF and ceramide facilitated the peak amplitude of the TTX-resistant sodium current (TTX-R I(Na)) by approximately 1.5-fold and shifted the activation to more hyperpolarized voltages. In addition, NGF and ceramide suppressed an outward potassium current (I(K)) by ~35%. The inflammatory prostaglandin, PGE2, produced an additional suppression of I(K) after exposure to ceramide (~35%), suggesting that these agents might act on different targets. Based on the existing literature, it is not clear whether this NGF-induced sensitization is mediated by the high-affinity TrkA receptor or the low-affinity p75 neurotrophin receptor. Pretreatment with the p75 blocking antibody completely prevents the NGF-induced increase in the number of APs evoked by the current ramp. Although the sensitization by NGF was blocked, the antibody had no effect on the capacity of ceramide, a putative downstream signalling molecule, to enhance the excitability. Ceramide can be metabolized by ceramidase to sphingosine (Sph) and Sph to sphingosine 1-phosphate (S1P) by sphingosine kinase. It is well established that each of these products of sphingomyelin metabolism can act as intracellular signalling molecules. This raises the question as to whether the enhanced excitability produced by NGF was mediated directly by ceramide or required additional metabolism to Sph and/or S1P. Sph applied externally did not affect the neuronal excitability whereas internally perfused Sph augmented the number of APs evoked by the depolarizing ramp. Furthermore, internally perfused S1P enhanced the number of evoked APs. This sensitizing action of NGF, ceramide, and internally perfused Sph, were abolished by dimethylsphingosine (DMS), an inhibitor of sphingosine kinase. In contrast, internally perfused S1P enhanced the number of evoked APs in the presence of DMS. These observations support the idea that the metabolism of ceramide/Sph to S1P is critical for the sphingolipid-induced modulation of excitability. Thus, our findings indicate that the pro-inflammatory agent, NGF, can rapidly enhance the excitability of sensory neurons. This NGF-induced sensitization is mediated by activation of the sphingomyelin signalling pathway wherein intracellular S1P derived from ceramide, acts as an internal second messenger to regulate membrane excitability, however, the effector system whereby S1P modulates excitability remains undetermined.
Action Potentials ; Animals ; Cells, Cultured ; Ceramides ; pharmacology ; Lysophospholipids ; metabolism ; Nerve Growth Factor ; physiology ; Patch-Clamp Techniques ; Phosphotransferases (Alcohol Group Acceptor) ; metabolism ; Sensory Receptor Cells ; cytology ; Signal Transduction ; Sphingomyelins ; physiology ; Sphingosine ; analogs & derivatives ; metabolism