Somatic Acetylcholine Release in Rabbit Nodose Ganglion

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Opening of pannexin- and connexin-based channels increases the excitability of nodose ganglion sensory neurons

Satellite glial cells (SGCs) are the main glia in sensory ganglia. They surround neuronal bodies and form a cap that prevents the formation of chemical or electrical synapses between neighboring neurons. SGCs have been suggested to establish bidirectional paracrine communication with sensory neurons. However, the molecular mechanism involved in this cellular communication is unknown. In the central nervous system (CNS), astrocytes present connexin43 (Cx43) hemichannels and pannexin1 (Panx1) channels, and the opening of these channels allows the release of signal molecules, such as ATP and glutamate. We propose that these channels could play a role in glia-neuron communication in sensory ganglia. Therefore, we studied the expression and function of Cx43 and Panx1 in rat and mouse nodose-petrosal-jugular complexes (NPJcs) using confocal immunofluorescence, molecular and electrophysiological techniques. Cx43 and Panx1 were detected in SGCs and in sensory neurons, respectively. In the rat and mouse, the electrical activity of vagal nerve increased significantly after nodose neurons were exposed to a Ca²⁺/Mg²⁺-free solution, a condition that increases the open probability of Cx hemichannels. This response was partially mimicked by a cell-permeable peptide corresponding to the last 10 amino acids of Cx43 (TAT-Cx43CT). Enhanced neuronal activity was reduced by Cx hemichannel, Panx1 channel and P2X₇ receptor blockers. Moreover, the role of Panx1 was confirmed in NPJc, because in those from Panx1 knockout mice showed a reduced increase of neuronal activity induced by Ca²⁺/Mg²⁺-free extracellular conditions. The data suggest that Cx hemichannels and Panx channels serve as paracrine communication pathways between SGCs and neurons by modulating the excitability of sensory neurons.

The gut-brain axis rewired: adding a functional vagal nicotinic "sensory synapse"

It is generally accepted that intestinal sensory vagal fibers are primary afferent, responding nonsynaptically to luminal stimuli. The gut also contains intrinsic primary afferent neurons (IPANs) that respond to luminal stimuli. A psychoactive Lactobacillus rhamnosus (JB-1) that affects brain function excites both vagal fibers and IPANs. We wondered whether, contrary to its primary afferent designation, the sensory vagus response to JB-1 might depend on IPAN to vagal fiber synaptic transmission. We recorded ex vivo single- and multiunit afferent action potentials from mesenteric nerves supplying mouse jejunal segments. Intramural synaptic blockade with Ca(2+) channel blockers reduced constitutive or JB-1-evoked vagal sensory discharge. Firing of 60% of spontaneously active units was reduced by synaptic blockade. Synaptic or nicotinic receptor blockade reduced firing in 60% of vagal sensory units that were stimulated by luminal JB-1. In control experiments, increasing or decreasing IPAN excitability, respectively increased or decreased nerve firing that was abolished by synaptic blockade or vagotomy. We conclude that >50% of vagal afferents function as interneurons for stimulation by JB-1, receiving input from an intramural functional "sensory synapse." This was supported by myenteric plexus nicotinic receptor immunohistochemistry. These data offer a novel therapeutic target to modify pathological gut-brain axis activity.-Perez-Burgos, A., Mao, Y.-K., Bienenstock, J., Kunze, W. A. The gut-brain axis rewired: adding a functional vagal nicotinic "sensory synapse."

Slow chemical transmission between dorsal root ganglion neuron somata

Somatic sensory neuron somata are located within the dorsal root ganglia (DRG) and are mostly ensheathed by individual satellite glial cell sheets. It has been noted, however, that a subpopulation of these DRG somata are intimately associated, separated only by a single thin satellite glial cell membrane septum. We set out to test whether such neuron-glial cell-neuron trimers (NGlNs) are also linked functionally. The presence of NGlNs in chick DRGs was confirmed by electron microscopy. Selective satellite glial cell immunostains were identified and were used to image the inter-neuron septa in DRG frozen sections. We used a gentle, dispase-based enzymatic method to isolate chick and rat NGlNs in vitro for double patch clamp recordings. In the majority of pairs tested, an action potential-like stimulus train delivered to one soma resulted in a delayed, noisy and long-duration response in its idle partner. The response to a second stimulus train given minutes later was markedly facilitated. Both bidirectional and unidirectional transmission was observed between the paired neurons. Transmission was chemical and block by the general purinergic blocker suramin implicated ATP as a neurotransmitter. We conclude that the two neuronal somata in the NGlN can communicate by chemical transmission, which may involve a transglial, bi-synaptic pathway. This novel soma-to-soma transmission reflects a novel form of processing that may play a role in sensory disorders in the DRG and interneuron communication in the central nervous system.

Catecholamine release from isolated sensory neurons of cat petrosal ganglia in tissue culture

The petrosal ganglion (PG) is entirely constituted by the perikarya of primary sensory neurons, part of which innervates the carotid body via the carotid sinus nerve (CSN). Application of acetylcholine (ACh) or nicotine (Nic) as well as adenosine 5'-triphosphate (ATP) to the PG in vitro increases the frequency of CSN discharges, an effect that is modified by the concomitant application of dopamine (DA). Since a population of PG neurons expresses tyrosine hydroxylase, and DA is released from the cat carotid body in response to electrical stimulation of C-fibers in the CSN, it is possible that DA may be released from the perikarya of PG neurons. Therefore, we studied whether ACh or Nic, ATP and high KCl could induce DA release from PG neurons in culture. Petrosal ganglia were excised from pentobarbitone-anesthetized adult cats, dissociated and their neurons maintained in culture for 7-21 days. Catecholamine release was measured by amperometry via carbon-fiber microelectrodes. In response to KCl, Nic, ACh or ATP application, about 25% of neurons exhibited electrochemical signals compatible with DA release. This percentage increased to 41% after loading the neurons with exogenous DA. The present results suggest that DA release may be induced from the perikarya of a population of PG neurons.

Chemical communication between vagal afferent somata in nodose Ganglia of the rat and the Guinea pig in vitro

The cell bodies of spinal afferents, dorsal root ganglion neurons, are depolarized several millivolts, and their probability of spiking increased when axons of neighboring somata in the same ganglion are electrically stimulated repetitively. This form of neural communication has been designated cross-depolarization (CD) and cross-excitation (CE). The existence of CD and CE between somata of vagal afferents (nodose ganglion neurons, NGNs) of rats and guinea pigs was investigated by electrically stimulating the vagus nerve while recording the electrical activity of NGNs in intact nodose ganglia with sharp intracellular microelectrodes. CD and CE in NGNs were manifested by a membrane depolarization (approximately 4 mV), the presence of spontaneous action potentials, and a decreased spike threshold. CD was dependent on the frequency and intensity of vagal nerve stimulation. Two distinct types of CD were observed: 1) in NGNs with large input resistances (R(in)), CD was dependent on [Ca2+]o, associated with increased membrane conductance, and had an extrapolated reversal potential (E(rev)) value of about -25 mV; and 2) in NGNs with low R(in), CD was independent of [Ca2+]o, not accompanied by a membrane conductance change, or a measurable E(rev) value. These data reveal the existence of a chemical communication pathway between vagal afferent somata and suggest the possibility that communication between different visceral organs may occur at the level of the primary vagal afferent neuron.

Allergic inflammation in isolated vagal sensory ganglia unmasks silent NK-2 tachykinin receptors

Neuroplastic changes in vagal afferents inflicted by allergic inflammation were examined in nodose ganglia (NG) removed from guinea pigs immunized to chick ovalbumin. In control NG neurons, substance P (SP; 0.1-10 microM) produces no discernable changes in membrane electrophysiological properties or [Ca2+]i. After exposing NG from immunized animals to the sensitizing antigen in vitro, 83% of the neurons were depolarized by 100 nM SP. SP also produces an inward current, an increase in membrane conductance, and an elevation of [Ca2+]i. Buffering [Ca2+]i with BAPTA blocked the [Ca2+]i rise and the SP depolarization, indicating that internal stores of Ca2+ are required. When protein synthesis was inhibited >96% (as determined by [3H] leucine incorporation), antigen challenge still unmasked SP responses. The SP response was maximal 30 min after antigen challenge, and it was evident for at least 8 hr in intact ganglia and for 3.5 d in isolated neurons. [beta-Ala8]Neurokinin A ([beta-Ala8]NKA; 10 nM), an NK-2 selective agonist, mimicked SP; selective NK-1 and NK-3 agonists were ineffective. The EC50 values for SP and [beta-Ala8]NKA membrane currents were 78 and 33 nM, respectively. Additionally, SR48968, an NK-2 receptor antagonist, blocked these responses. Thus, antigen challenge appears to unmask an NK-2 tachykinin receptor. These data further support the hypothesis that inflammatory mediators released during immediate hypersensitivity (allergic) reactions can produce profound effects on the excitability of sensory nerves. Unmasked NK-2 receptors may serve an excitatory autoreceptor function, provide a pathway for paracrine signaling between NG neurons, and contribute to ectopic sensory nerve activity.

Synthesis and release of an acetylcholine-like compound by human myoblasts and myotubes

1. Exogenously applied acetylcholine (ACh) is a modulator of human myoblast fusion. Using a chemiluminescent method, we examined whether an endogenous ACh-like compound (ACh-lc) was present in, and released by, pure human myogenic cells. 2. Single, freshly isolated satellite cells and proliferating myoblasts contained 15 and 0.5 fmol ACh-lc, respectively. Cultured myotubes contained ACh-lc as well. Also, ACh-like immunoreactivity was detected in all myogenic cells. 3. Part of the ACh-lc was synthesized by choline acetyltransferase (ChAT), as indicated by the reduction of ACh-lc content when bromoACh was present in the culture medium, and by direct measurements of ChAT activity. Also, ChAT-like immunoreactivity was observed in all myogenic cells. 4. Myoblasts and myotubes released ACh-lc spontaneously by a partially Ca(2+)-dependent mechanism. 5. The application by microperfusion of medium conditioned beforehand by myoblasts (thus presumably containing ACh-lc) onto a voltage-clamped myotube induced inward currents resembling ACh-induced currents in their kinetics, reversal potential, and sensitivity to nicotinic antagonists. 6. In vitro, the spontaneously released ACh-lc promoted myoblast fusion but only in the presence of an anticholinesterase. 7. Our observations indicate that human myogenic cells synthesize and release an ACh-lc and thereby promote the fusion process that occurs in muscle during growth or regeneration.

Enhanced chemiluminescent assays for acetylcholine

Acetylcholine and choline chemiluminescent assays have limitations when these compounds are detected in small areas of mammalian nervous tissue. Use of 7-dimethyl-aminonaphthalene-1,2-dicarbonic acid hydrazide (7-DMAN), instead of luminol, gives a threefold increase in emitted light in the chemiluminescent assay for acetylcholine based on the coupled choline oxidase-peroxidase reaction. Addition of light enhancers, such as para-iodophenol or D-luciferin, to luminol or 7-DMAN further increased the light emission. Under these conditions the detection limit for acetylcholine was 650 femtomoles. This enhanced chemiluminescent assay should be convenient for the detection of in vivo and in vitro acetylcholine release from mammalian neurons.