Sensory Neurons: The Formation of T-Shaped Branches Is Dependent on a cGMP-Dependent Signaling Cascade.

Axon bifurcation - a specific form of branching of somatosensory axons characterized by the splitting of the growth cone - is mediated by a cGMP-dependent signaling cascade composed of the extracellular ligand CNP (C-type natriuretic peptide), the transmembrane receptor guanylyl cyclase Npr2 (natriuretic peptide receptor 2), and the kinase cGKI (cGMP-dependent protein kinase I). In the absence of any one of these components, the formation of T-shaped axonal branches is impaired in neurons from DRGs (dorsal root ganglia), CSGs (cranial sensory ganglia) and MTNs (mesencephalic trigeminal neurons) in the murine spinal cord or hindbrain. Instead, axons from DRGs or from CSGs extend only either in an ascending or descending direction, while axons from MTNs either elongate within the hindbrain or extend via the trigeminal ganglion to the masseter muscles. Collateral formation from non-bifurcating stem axons is not affected by impaired cGMP signaling. Activation of Npr2 requires both binding of the ligand CNP as well as phosphorylation of serine and threonine residues at the juxtamembrane regions of the receptor. The absence of bifurcation results in an altered shape of termination fields of sensory afferents in the spinal cord and resulted in impaired noxious heat sensation and nociception whereas motor coordination appeared normal.

Neurogenesis in the inner ear: the zebrafish statoacoustic ganglion provides new neurons from a Neurod/Nestin-positive progenitor pool well into adulthood.

The vertebrate inner ear employs sensory hair cells and neurons to mediate hearing and balance. In mammals, damaged hair cells and neurons are not regenerated. In contrast, hair cells in the inner ear of zebrafish are produced throughout life and regenerate after trauma. However, it is unknown whether new sensory neurons are also formed in the adult zebrafish statoacoustic ganglion (SAG), the sensory ganglion connecting the inner ear to the brain. Using transgenic lines and marker analysis, we identify distinct cell populations and anatomical landmarks in the juvenile and adult SAG. In particular, we analyze a Neurod/Nestin-positive progenitor pool that produces large amounts of new neurons at juvenile stages, which transitions to a quiescent state in the adult SAG. Moreover, BrdU pulse chase experiments reveal the existence of a proliferative but otherwise marker-negative cell population that replenishes the Neurod/Nestin-positive progenitor pool at adult stages. Taken together, our study represents the first comprehensive characterization of the adult zebrafish SAG showing that zebrafish, in sharp contrast to mammals, display continued neurogenesis in the SAG well beyond embryonic and larval stages.

Renewed proliferation in adult mouse cochlea and regeneration of hair cells.

The adult mammalian inner ear lacks the capacity to divide or regenerate. Damage to inner ear generally leads to permanent hearing loss in humans. Here, we present that reprogramming of the adult inner ear induces renewed proliferation and regeneration of inner ear cell types. Co-activation of cell cycle activator Myc and inner ear progenitor gene Notch1 induces robust proliferation of diverse adult cochlear sensory epithelial cell types. Transient MYC and NOTCH activities enable adult supporting cells to respond to transcription factor Atoh1 and efficiently transdifferentiate into hair cell-like cells. Furthermore, we uncover that mTOR pathway participates in MYC/NOTCH-mediated proliferation and regeneration. These regenerated hair cell-like cells take up the styryl dye FM1-43 and are likely to form connections with adult spiral ganglion neurons, supporting that Myc and Notch1 co-activation is sufficient to reprogram fully mature supporting cells to proliferate and regenerate hair cell-like cells in adult mammalian auditory organs.

Liver X Receptors and Their Implications in the Physiology and Pathology of the Peripheral Nervous System.

Recent research in the last decade has sought to explore the role and therapeutic potential of Liver X Receptors (LXRs) in the physiology and pathologies of the Peripheral Nervous System. LXRs have been shown to be important in maintaining the redox homeostasis in peripheral nerves for proper myelination, and they regulate ER stress in sensory neurons. Furthermore, LXR stimulation has a positive impact on abrogating the effects of diabetic peripheral neuropathy and obesity-induced allodynia in the Peripheral Nervous System (PNS). This review details these findings and addresses certain important questions that are yet to be answered. The potential roles of LXRs in different cells of the PNS are speculated based on existing knowledge. The review also aims to provide important perspectives for further research in elucidating the role of LXRs and assessing the potential of LXR based therapies to combat pathologies of the Peripheral Nervous System.

A Neural Circuit Encoding the Experience of Copulation in Female Drosophila.

Female behavior changes profoundly after mating. In Drosophila, the mechanisms underlying the long-term changes led by seminal products have been extensively studied. However, the effect of the sensory component of copulation on the female's internal state and behavior remains elusive. We pursued this question by dissociating the effect of coital sensory inputs from those of male ejaculate. We found that the sensory inputs of copulation cause a reduction of post-coital receptivity in females, referred to as the "copulation effect." We identified three layers of a neural circuit underlying this phenomenon. Abdominal neurons expressing the mechanosensory channel Piezo convey the signal of copulation to female-specific ascending neurons, LSANs, in the ventral nerve cord. LSANs relay this information to neurons expressing myoinhibitory peptides in the brain. We hereby provide a neural mechanism by which the experience of copulation facilitates females encoding their mating status, thus adjusting behavior to optimize reproduction.

Regulation of the Natriuretic Peptide Receptor 2 (Npr2) by Phosphorylation of Juxtamembrane Serine and Threonine Residues Is Essential for Bifurcation of Sensory Axons.

cGMP signaling elicited by activation of the transmembrane receptor guanylyl cyclase Npr2 (also known as guanylyl cyclase B) by the ligand CNP controls sensory axon bifurcation of DRG and cranial sensory ganglion (CSG) neurons entering the spinal cord or hindbrain, respectively. Previous studies have shown that Npr2 is phosphorylated on serine and threonine residues in its kinase homology domain (KHD). However, it is unknown whether phosphorylation of Npr2 is essential for axon bifurcation. Here, we generated a knock-in mouse line in which the seven regulatory serine and threonine residues in the KHD of Npr2 were substituted by alanine (Npr2-7A), resulting in a nonphosphorylatable enzyme. Real-time imaging of cGMP in DRG neurons with a genetically encoded fluorescent cGMP sensor or biochemical analysis of guanylyl cyclase activity in brain or lung tissue revealed the absence of CNP-induced cGMP generation in the Npr27A/7A mutant. Consequently, bifurcation of axons, but not collateral formation, from DRG or CSG in this mouse mutant was perturbed at embryonic and mature stages. In contrast, axon branching was normal in a mouse mutant in which constitutive phosphorylation of Npr2 is mimicked by a replacement of all of the seven serine and threonine sites by glutamic acid (Npr2-7E). Furthermore, we demonstrate that the Npr27A/7A mutation causes dwarfism as described for global Npr2 mutants. In conclusion, our in vivo studies provide strong evidence that phosphorylation of the seven serine and threonine residues in the KHD of Npr2 is an important regulatory element of Npr2-mediated cGMP signaling which affects physiological processes, such as axon bifurcation and bone growth.SIGNIFICANCE STATEMENT The branching of axons is a morphological hallmark of virtually all neurons. It allows an individual neuron to innervate different targets and to communicate with neurons located in different regions of the nervous system. The natriuretic peptide receptor 2 (Npr2), a transmembrane guanylyl cyclase, is essential for the initiation of bifurcation of sensory axons when entering the spinal cord or the hindbrain. By using two genetically engineered mouse lines, we show that phosphorylation of specific serine and threonine residues in juxtamembrane regions of Npr2 are required for its enzymatic activity and for axon bifurcation. These investigations might help to understand the regulation of Npr2 and its integration in intracellular signaling systems.

Guanylyl cyclase-G is an alarm pheromone receptor in mice.

Many animals respond to threats by releasing alarm pheromones (APs) that warn conspecifics. In mice, detection of the AP 2-sec-butyl-4,5-dihydrothiazole (SBT) is mediated by chemosensory neurons residing in the Grueneberg ganglion (GG) of the anterior nasal region. Although the molecular mechanisms underlying activation of GG neurons by SBT and other substances are still unclear, recent studies have reported an involvement of the transmembrane guanylyl cyclase (GC) subtype GC-G in chemosensory signaling in the GG Here, we show that SBT directly binds with high affinity to the extracellular domain of GC-G and elicits an enhanced enzymatic activity of this protein. In line with this finding, heterologous expression of GC-G renders cells responsive to SBT while activation by SBT was strongly attenuated in GG neurons from GC-G-deficient mice. Consistently, SBT-induced fear-associated behaviors, SBT-evoked elevated blood pressure, and increased serum levels of the stress hormone corticosterone were clearly reduced in GC-G-knockout animals compared to wild-type mice. These observations suggest that GC-G serves as an unusual receptor in GG neurons mediating the detection of the volatile AP substance SBT.

The Glutamatergic System in Primary Somatosensory Neurons and Its Involvement in Sensory Input-Dependent Plasticity.

Glutamate is the most common neurotransmitter in both the central and the peripheral nervous system. Glutamate is present in all types of neurons in sensory ganglia, and is released not only from their peripheral and central axon terminals but also from their cell bodies. Consistently, these neurons express ionotropic and metabotropic receptors, as well as other molecules involved in the synthesis, transport and release of the neurotransmitter. Primary sensory neurons are the first neurons in the sensory channels, which receive information from the periphery, and are thus key players in the sensory transduction and in the transmission of this information to higher centers in the pathway. These neurons are tightly enclosed by satellite glial cells, which also express several ionotropic and metabotropic glutamate receptors, and display increases in intracellular calcium accompanying the release of glutamate. One of the main interests in our group has been the study of the implication of the peripheral nervous system in sensory-dependent plasticity. Recently, we have provided novel evidence in favor of morphological changes in first- and second-order neurons of the trigeminal system after sustained alterations of the sensory input. Moreover, these anatomical changes are paralleled by several molecular changes, among which those related to glutamatergic neurotransmission are particularly relevant. In this review, we will describe the state of the art of the glutamatergic system in sensory ganglia and its involvement in input-dependent plasticity, a fundamental ground for advancing our knowledge of the neural mechanisms of learning and adaptation, reaction to injury, and chronic pain.

Rapid neurophysiological screening for sensory ganglionopathy: A novel approach.

Background and Aim: Pure sensory neuropathies involving the dorsal root ganglia are commonly referred to as sensory ganglionopathies (SG). Causes of SG can be inherited (as seen in Friedreich's ataxia) or acquired (e.g. immune-mediated or paraneoplastic). Diagnostic criteria for confirming SG have been published and consist of a combination of clinical and neurophysiological parameters. The aim of our study was to develop a neurophysiological method for rapid screening for diagnosis of SG. Methods: For each subject we obtained the sensory nerve action potentials (SNAPs) of five nerves (median, ulnar, radial, sural and superficial peroneal) bilaterally. In the presence of an entrapment neuropathy we obtained the SNAP of the medial antebrachial cutaneous nerves bilaterally. We estimated the number of pairs of nerves showing a SNAP asymmetry of >50% (difference of SNAPs/ lower SNAP). Results: Sixty-eight subjects, 34 patients with SG and 34 age and sex-matched controls, participated in the study. Among all subjects using a receiver operating characteristic (ROC) curve analysis, the area under the curve was 0.984 (95% CI, 0.960-1.000; SE, 0.012; p < .001). In order to detect SG, presence of SNAP asymmetry of >50% in 2 pairs of nerves, not explained by an entrapment neuropathy, shows a sensitivity of 97.1%, a specificity of 94.1%, a positive predictive value of 94.3% and a negative predictive value of 97.0. Conclusion: The number of pairs of nerves showing a SNAP asymmetry of >50% may be used as a novel rapid screening tool of patients with SG.

Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents.

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.

Identification and modelling of fast and slow Ih current components in vestibular ganglion neurons.

Previous experimental data indicates the hyperpolarization-activated cation (Ih) current, in the inner ear, consists of two components [different hyperpolarization-activated cyclic nucleotide-gated (HCN) subunits] which are impossible to pharmacologically isolate. To confirm the presence of these two components in vestibular ganglion neurons we have applied a parameter identification algorithm which is able to discriminate the parameters of the two components from experimental data. Using simulated data we have shown that this algorithm is able to identify the parameters of two populations of non-inactivated ionic channels more accurately than a classical method. Moreover, the algorithm was demonstrated to be insensitive to the key parameter variations. We then applied this algorithm to Ih current recordings from mouse vestibular ganglion neurons. The algorithm revealed the presence of a high-voltage-activated slow component and a low-voltage-activated fast component. Finally, the electrophysiological significance of these two Ih components was tested individually in computational vestibular ganglion neuron models (sustained and transient), in the control case and in the presence of cAMP, an intracellular cyclic nucleotide that modulates HCN channel activity. The results suggest that, first, the fast and slow components modulate differently the action potential excitability and the excitatory postsynaptic potentials in both sustained and transient vestibular neurons and, second, the fast and slow components, in the control case, provide different information about characteristics of the stimulation and this information is significantly modified after modulation by cAMP.

The Origin, Expression, Function and Future Research Focus of a G Protein-coupled Receptor, Mas-related Gene X2 (MrgX2).

Mas-related genes (Mrgs) belong to a large family of G protein-coupled receptor genes found in rodents. Human MRGX proteins are G protein-coupled 7-transmembrane proteins sharing 41-52% amino acid identity with each other, but have no orthologs in rodents. MrgX2 is a member of the MrgX family. MRGX2 is expressed in the small neurons of sensory ganglia and mast cells. It can interact with a series of factors and genes such as the peptides substance P, vasoactive intestinal peptide, cortistatin (CST), proadrenomedullin N-terminal peptide (PAMP), LL-37, PMX-53 and ß-defensins. MRGX2 is related to nociception, adrenal gland secretion and mast cell degranulation. Recent research on MrgX2 provides insights into its role in nociception and anti-microbial activities. This article reviewed the origin, expression and function of MrgX2, and discussed possible future research focus.

Developmental increase in hyperpolarization-activated current regulates intrinsic firing properties in rat vestibular ganglion cells.

The primary vestibular neurons convey afferent information from hair cells in the inner ear to the vestibular nuclei and the cerebellum. The intrinsic firing properties of vestibular ganglion cells (VGCs) are heterogeneous to sustained membrane depolarization, and undergo marked developmental changes from phasic to tonic types during the early postnatal period. Previous studies have shown that low-voltage-activated potassium channels, Kv1 and Kv7, play a critical role in determining the firing pattern of VGCs. In the present study, we explored the developmental changes in the properties of hyperpolarization-activated current (Ih) in rat VGCs and the role played by Ih in determining the firing properties of VGCs. Tonic firing VGCs showed a larger current density of Ih as compared to phasic firing VGCs, and tonic firing VGCs became phasic firing in the presence of ZD7288, an Ih channel blocker, indicating that Ih contributes to control the firing pattern of VGCs. The amplitude of Ih increased and the activation kinetics of Ih became faster during the developmental period. Analysis of developmental changes in the expression of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels revealed that expression of HCN1 protein and its mRNA increased during the developmental period, whereas expression of HCN2-4 protein and its mRNA did not change. Our results suggest that HCN1 channels as well as Kv1 channels are critical in determining the firing pattern of rat VGCs and that developmental up-regulation of HCN1 transforms VGCs from phasic to tonic firing phenotypes.

Phoenixin: a novel peptide in rodent sensory ganglia.

Phoenixin-14 amide, herein referred to as phoenixin, is a newly identified peptide from the rat brain. Using a previously characterized rabbit polyclonal antiserum against phoenixin, enzyme-immunoassay detected a high level (>4.5 ng/g tissue) of phoenixin-immunoreactivity (irPNX) in the rat spinal cords. Immunohistochemical studies revealed irPNX in networks of cell processes in the superficial dorsal horn, spinal trigeminal tract and nucleus of the solitary tract; and in a population of dorsal root, trigeminal and nodose ganglion cells. The pattern of distribution of irPNX in the superficial layers of the dorsal horn was similar to that of substance P immunoreactivity (irSP). Double-labeling the dorsal root ganglion sections showed that irPNX and irSP express in different populations of ganglion cells. In awake mice, intrathecal injection of phoenixin (1 or 5 µg) did not significantly affect the tail-flick latency as compared to that in animals injected with artificial cerebrospinal fluid (aCSF). Intrathecal administration of phoenixin (0.5, 1.25 or 2.5 µg) significantly reduced the number of writhes elicited by intraperitoneal injection of acetic acid (0.6%, 0.3 ml/30 g) as compared to that in mice injected with aCSF. While not affecting the tail-flick latency, phoenixin antiserum (1:100) injected intrathecally 10 min prior to the intraperitoneal injection of acetic acid significantly increased the number of writhes as compared to mice pre-treated with normal rabbit serum. Intrathecal injection of non-amidated phoenixin (2.5 µg) did not significantly alter the number of writhes evoked by acetic acid. Our result shows that phoenixin is expressed in sensory neurons of the dorsal root, nodose and trigeminal ganglia, the amidated peptide is bioactive, and exogenously administered phoenixin may preferentially suppress visceral as opposed to thermal pain.

Fine structure and primary sensory projections of sensilla located in the labial-palp pit organ of Helicoverpa armigera (Insecta).

The fine structure and primary sensory projections of sensilla located in the labial-palp pit organ of the cotton bollworm Helicoverpa armigera (Insecta, Lepidoptera) are investigated by scanning electron and transmission electron microscopy combined with confocal laser scanning microscopy. The pit organ located on the third segment of the labial palp is about 300 µm deep with a 60-µm-wide opening, each structure containing about 1200 sensilla. Two sensillum types have been found, namely hair-shaped and club-shaped sensilla, located on the upper and lower half of the pit, respectively. Most sensilla possess a single dendrite. The dendrite housed by the club-shaped sensilla is often split into several branches or becomes lamellated in the outer segment. As reported previously, the sensory axons of the sensilla in the labial pit organ form a bundle entering the ipsilateral side of the subesophageal ganglion via the labial palp nerve and project to three distinct areas: the labial pit organ glomerulus in each antennal lobe, the subesophageal ganglion and the ventral nerve cord. In the antennal lobe, the labial pit organ glomerulus is innervated by sensory axons from the labial pit organ only; no antennal afferents target this unit. One neuron has been found extending fine processes into the subesophageal ganglion and innervating the labial palp via one branch passing at the base of the labial palp nerve. The soma of this assumed motor neuron is located in the ipsilateral cell body layer of the subesophageal ganglion. Our results provide valuable knowledge concerning the neural circuit encoding information about carbon dioxide and should stimulate further investigations directed at controlling pest species such as H. armigera.

Odorant-evoked electrical responses in Grueneberg ganglion neurons rely on cGMP-associated signaling proteins.

The Grueneberg ganglion (GG) in the anterior nasal region of mice is considered as an olfactory compartment since its neurons were recently observed to be activated by chemical stimuli, in particular by the odorant 2,3-dimethylpyrazine (2,3-DMP). However, it is unclear whether the GG indeed serves an olfactory function since these findings are solely based on the expression of the activity-dependent gene c-Fos. Consequently, it is yet uncertain whether chemical compounds, such as given odorants, elicit electrical responses in GG neurons which are required to convey the chemosensory information to the brain. Therefore, in the present study, electrical recording experiments on tissue sections through the anterior nasal region of mice were conducted which revealed that 2,3-DMP induces electrical signals in the GG. These responses were restricted to sites harboring GG neurons, indicating that 2,3-DMP elicits an electrical signal only in these but not in other cells of the anterior nasal region. 2,3-DMP-sensitive GG neurons express signaling proteins associated with the second messenger substance cyclic guanosine monophosphate (cGMP); most notably the cyclic nucleotide-gated ion channel CNGA3 and the transmembrane guanylyl cyclase GC-G. Using mice deficient for CNGA3 or GC-G demonstrated that the 2,3-DMP-evoked electrical signals in the GG of these knockout mice were substantially lower than in the GG of wildtype conspecifics, indicating that cGMP signaling plays a crucial role for odorant-induced electrical responses in the GG.

Intercellular communication in sensory ganglia by purinergic receptors and gap junctions: implications for chronic pain.

Peripheral injury can cause abnormal activity in sensory neurons, which is a major factor in chronic pain. Recent work has shown that injury induces major changes not only in sensory neurons but also in the main type of glial cells in sensory ganglia-satellite glial cells (SGCs), and that interactions between sensory neurons and SGCs contribute to neuronal activity in pain models. The main functional changes observed in SGCs after injury are an increased gap junction-mediated coupling among these cells, and augmented sensitivity to ATP. There is evidence that the augmented gap junctions contribute to neuronal hyperexcitability in pain models, but the mechanism underlying this effect is not known. The changes in SGCs described above have been found following a wide range of injuries (both axotomy and inflammation) in somatic, orofacial and visceral regions, and therefore appear to be a general feature in chronic pain. We have found that in cultures of sensory ganglia calcium signals can spread from an SGC to neighboring cells by calcium waves, which are mediated by gap junctions and ATP acting on purinergic P2 receptors. A model is proposed to explain how augmented gap junctions and greater sensitivity to ATP can combine to produce enhanced calcium waves, which can lead to neuronal excitation. Thus this simple scheme can account for several major changes in sensory ganglia that are common to a great variety of pain models.

Bursting activity in myelinated sensory neurons plays a key role in pain behavior induced by localized inflammation of the rat sensory ganglion.

Abnormal spontaneous activity of sensory neurons is observed in many different preclinical pain models, but its basis is not well understood. In this study mechanical and cold hypersensitivity were induced in rats after inflammation of the L5 dorsal root ganglion (DRG), initiated by local application of the immune stimulator zymosan in incomplete Freund's adjuvant. Mechanical hypersensitivity was evident by day 1 and maintained for 2 months. The model also showed reduction of rearing behavior in a novel environment. Microelectrode recordings made in isolated whole DRG on day 3 after inflammation showed a marked increase of spontaneous activity, predominantly with a bursting pattern. The incidence was especially high (44%) in Aαß cells. Spontaneous activity and subthreshold membrane potential oscillations were completely blocked by tetrodotoxin (500 nM) and by riluzole (10 µM), a blocker of persistent sodium currents. In vivo, local perfusion of the inflamed DRG for the first 7 days with riluzole gave long-lasting, dose-dependent reduction in mechanical pain behaviors. Riluzole perfusion did not affect mechanical sensitivity in normal animals. Unmyelinated C cells had a very low incidence of spontaneous activity and were much less affected by riluzole in vitro. Taken together these results suggest that high-frequency and/or bursting spontaneous activity in Aαß sensory neurons may play important roles in initiating pain behaviors resulting from inflammatory irritation of the DRG.

Pathogen-induced heart rate changes associated with cholinergic nervous system activation.

The autonomic nervous system plays a central role in regulation of host defense and in physiological responses to sepsis, including changes in heart rate and heart rate variability. The cholinergic anti-inflammatory response, whereby infection triggers vagal efferent signals that dampen production of proinflammatory cytokines, would be predicted to result in increased vagal signaling to the heart and increased heart rate variability. In fact, decreased heart rate variability is widely described in humans with sepsis. Our studies elucidate this apparent paradox by showing that mice injected with pathogens demonstrate transient bradyarrhythmias of vagal origin in a background of decreased heart rate variability (HRV). Intraperitoneal injection of a large inoculum of Gram-positive or Gram-negative bacteria or Candida albicans rapidly induced bradyarrhythmias of sinus and AV nodal block, characteristic of cardiac vagal firing and dramatically increased short-term HRV. These pathogen-induced bradycardias were immediately terminated by atropine, an antagonist of muscarinic cholinergic receptors, demonstrating the role of vagal efferent signaling in this response. Vagal afferent signaling following pathogen injection was demonstrated by intense nuclear c-Fos activity in neurons of the vagal sensory ganglia and brain stem. Surprisingly, pathogen-induced bradycardia demonstrated rapid and prolonged desensitization and did not recur on repeat injection of the same organism 3 h or 3 days after the initial exposure. After recovery from the initial bradycardia, depressed heart rate variability developed in some mice and was correlated with elevated plasma cytokine levels and mortality. Our findings of decreased HRV and transient heart rate decelerations in infected mice are similar to heart rate changes described by our group in preterm neonates with sepsis. Pathogen sensing and signaling via the vagus nerve, and the desensitization of this response, may account for periods of both increased and decreased heart rate variability in sepsis.

Functional ligand-gated purinergic receptors (P2X) in rat vestibular ganglion neurons.

The expression of purinergic receptors (P2X) on rat vestibular ganglion neurons (VGNs) was examined using whole-cell patch-clamp recordings. An application of adenosine 5'-triphosphate (ATP; 100microM) evoked inward currents in VGNs at a holding potential of -60mV. The decay time constant of the ATP-evoked currents was 2-4s, which is in between the values for rapidly desensitizing subgroups (P2X1 and P2X3) and slowly desensitizing subgroups (P2X2, P2X4, etc.), suggesting the heterogeneous expression of P2X receptors. A dose-response experiment showed an EC(50) of 11.0microM and a Hill's coefficient of 0.82. Suramin (100microM) reversibly inhibited the ATP-evoked inward currents. Alpha, beta-methylene ATP (100microM), a P2X-specific agonist, also evoked inward currents but less extensively than ATP. An application of adenosine 5'-dihosphate (ADP; 100microM) evoked similar, but much smaller, currents. The current-voltage relationship of the ATP-evoked conductance showed pronounced inward rectification with a reversal potential more positive than 0mV, suggesting non-selective cation conductance. However, the channel was not permeable to a large cation (N-methyl-d-glucamine) and acidification (pH 6.3) had little effect on the ATP-evoked conductance. RT-PCR confirmed the expression of five subtypes (P2X2-P2X6) in VGNs. The physiological role of P2X receptors includes the modulation of excitability at the synapses between hair cells and dendrites and/or trophic support (or also neuromodulation) from supporting cells surrounding the VGNs.