Synapse formation among developing sensory neurones from rat nodose ganglia grown in tissue culture

Microfluidic compartmentalization of rat vagal afferent neurons to model gut-brain axis

BACKGROUND

Vagal afferent neurons represent the key neurosensory branch of the gut-brain axis, which describes the bidirectional communication between the gastrointestinal system and the brain. These neurons are important for detecting and relaying sensory information from the periphery to the central nervous system to modulate feeding behavior, metabolism, and inflammation. Confounding variables complicate the process of isolating the role of the vagal afferents in mediating these physiological processes. Therefore, we developed a microfluidic model of the sensory branch of the gut-brain axis. We show that this microfluidic model successfully compartmentalizes the cell body and neurite terminals of the neurons, thereby simulates the anatomical layout of these neurons to more accurately study physiologically-relevant processes.

METHODS

We implemented a primary rat vagal afferent neuron culture into a microfluidic platform consisting of two concentric chambers interconnected with radial microchannels. The microfluidic platform separated cell bodies from neurite terminals of vagal afferent neurons. We then introduced physiologically-relevant gastrointestinal effector molecules at the nerve terminals and assessed their retrograde transport along the neurite or capacity to elicit an electrophysiological response using live cell calcium imaging.

RESULTS

The angle of microchannel outlets dictated the probability of neurites growing into a chamber versus tracking along chamber walls. When the neurite terminals were exposed to fluorescently-labeled cholera toxin subunit B, the proteins were taken up and retrogradely transported along the neurites over the course of 24 h. Additionally, mechanical perturbation (e.g., rinsing) of the neurite terminals significantly increased intracellular calcium concentration in the distal soma. Finally, membrane-displayed receptor for capsaicin was expressed and trafficked along newly projected neurites, as revealed by confocal microscopy.

CONCLUSIONS

In this work, we developed a microfluidic device that can recapitulate the anatomical layout of vagal afferent neurons in vitro. We demonstrated two physiologically-relevant applications of the platforms: retrograde transport and electrophysiological response. We expect this tool to enable controlled studies on the role of vagal afferent neurons in the gut-brain axis.

Cultured Vagal Afferent Neurons as Sensors for Intestinal Effector Molecules

The gut-brain axis embodies the bi-directional communication between the gastrointestinal tract and the central nervous system (CNS), where vagal afferent neurons (VANs) serve as sensors for a variety of gut-derived signals. The gut is colonized by a large and diverse population of microorganisms that communicate via small (effector) molecules, which also act on the VAN terminals situated in the gut viscera and consequently influence many CNS processes. However, the convoluted in vivo environment makes it difficult to study the causative impact of the effector molecules on VAN activation or desensitization. Here, we report on a VAN culture and its proof-of-principle demonstration as a cell-based sensor to monitor the influence of gastrointestinal effector molecules on neuronal behavior. We initially compared the effect of surface coatings (poly-L-lysine vs. Matrigel) and culture media composition (serum vs. growth factor supplement) on neurite growth as a surrogate of VAN regeneration following tissue harvesting, where the Matrigel coating, but not the media composition, played a significant role in the increased neurite growth. We then used both live-cell calcium imaging and extracellular electrophysiological recordings to show that the VANs responded to classical effector molecules of endogenous and exogenous origin (cholecystokinin serotonin and capsaicin) in a complex fashion. We expect this study to enable platforms for screening various effector molecules and their influence on VAN activity, assessed by their information-rich electrophysiological fingerprints.

Activation of Intra-nodose Ganglion P2X7 Receptors Elicit Increases in Neuronal Activity

Vagus nerve innervates several organs including the heart, stomach, and pancreas among others. Somas of sensory neurons that project through the vagal nerve are located in the nodose ganglion. The presence of purinergic receptors has been reported in neurons and satellite glial cells in several sensory ganglia. In the nodose ganglion, calcium depletion-induced increases in neuron activity can be partly reversed by P2X7 blockers applied directly into the ganglion. The later suggest a possible role of P2X7 receptors in the modulation of neuronal activity within this sensory ganglion. We aimed to characterize the response to P2X7 activation in nodose ganglion neurons under physiological conditions. Using an ex vivo preparation for electrophysiological recordings of the neural discharges of nodose ganglion neurons, we found that treatments with ATP induce transient neuronal activity increases. Also, we found a concentration-dependent increase in neural activity in response to Bz-ATP (ED₅₀ = 0.62 mM, a selective P2X7 receptor agonist), with a clear desensitization pattern when applied every ~ 30 s. Electrophysiological recordings from isolated nodose ganglion neurons reveal no differences in the responses to Bz-ATP and ATP. Finally, we showed that the P2X7 receptor was expressed in the rat nodose ganglion, both in neurons and satellite glial cells. Additionally, a P2X7 receptor negative allosteric modulator decreased the duration of Bz-ATP-induced maximal responses without affecting their amplitude. Our results show the presence of functional P2X7 receptors under physiological conditions within the nodose ganglion of the rat, and suggest that ATP modulation of nodose ganglion activity may be in part mediated by the activation of P2X7 receptors.

Non-neuronal crosstalk promotes an inflammatory response in nodose ganglia cultures after exposure to byproducts from gram positive, high-fat-diet-associated gut bacteria

Vagal afferent neurons (VAN) projecting to the lamina propria of the digestive tract are the primary source of gut-originating signals to the central nervous system (CNS). VAN cell bodies are found in the nodose ganglia (NG). Responsiveness of VAN to gut-originating signals is altered by feeding status with sensitivity to satiety signals such as cholecystokinin (CCK) increasing in the fed state. Chronic high-fat (HF) feeding results in inflammation at the level of the NG associated with a loss of VAN ability to switch phenotype from the fasted to the fed state. HF feeding also leads to compositional changes in the gut microbiota. HF diet consumption notably drives increased Firmicutes to Bacteroidetes phyla ratio and increased members of the Actinobacteria phylum. Firmicutes and Actinobacteria are largely gram positive (GP). In this study, we aimed to determine if byproducts from GP bacteria can induce an inflammatory response in cultured NG and to characterize the mechanism and cell types involved in the response. NG were collected from male Wistar rats and cultured for a total of 72 hours. At 48-68 hours after plating, cultures were treated with neuronal culture media in which Serinicoccus chungangensis had been grown and removed (SUP), lipoteichoic acid (LTA), or meso-diaminopimelic acid (meso-DAP). Some treatments included the glial inhibitors minocycline (MINO) and/or fluorocitrate (FC). The responses were evaluated using immunocytochemistry, qPCR, and electrochemiluminescence. We found that SUP induced an inflammatory response characterized by increased interleukin (IL)-6 staining and increased expression of genes for IL-6, interferon (IFN)γ, and tumor necrosis factor (TNF)α along with genes associated with cell-to-cell communication such as C-C motif chemokine ligand-2 (CCL2). Inclusion of inhibitors attenuated some responses but failed to completely normalize all indications of response, highlighting the role of immunocompetent cellular crosstalk in regulating the inflammatory response. LTA and meso-DAP produced responses that shared characteristics with SUP but were not identical. Our results support a role for HF associated GP bacterial byproducts' ability to contribute to vagal inflammation and to engage signaling from nonneuronal cells.

The metabolic role of vagal afferent innervation

The regulation of energy and glucose balance contributes to whole-body metabolic homeostasis, and such metabolic regulation is disrupted in obesity and diabetes. Metabolic homeostasis is orchestrated partly in response to nutrient and vagal-dependent gut-initiated functions. Specifically, the sensory and motor fibres of the vagus nerve transmit intestinal signals to the central nervous system and exert biological and physiological responses. In the past decade, the understanding of the regulation of vagal afferent signals and of the associated metabolic effect on whole-body energy and glucose balance has progressed. This Review highlights the contributions made to the understanding of the vagal afferent system and examines the integrative role of the vagal afferent in gastrointestinal regulation of appetite and glucose homeostasis. Investigating the integrative and metabolic role of vagal afferent signalling represents a potential strategy to discover novel therapeutic targets to restore energy and glucose balance in diabetes and obesity.

Glutamate-dependent regulation of food intake is altered with age through changes in NMDA receptor phenotypes on vagal afferent neurons

Compared to younger individuals, older human subjects have significantly lower food intakes and an increased satiety response. N-methyl-d-aspartate (NMDA) receptors expressed by vagal afferent neurons originating from nodose ganglia (NG) are involved in modulating the satiety response. The present study investigated how NMDA receptor subunit phenotypes in NG neurons change with age and how these age-related alterations in food intake are modulated by presynaptic NMDA receptors in the NG of male Sprague Dawley rats (six week-old and sixty week-old). Food intake was measured at 30-, 60-, and 120-min following intraperitoneal administration of cholecystokinin (CCK) or the non-competitive NMDA receptor antagonist MK-801. Immunofluorescence was used to determine NMDA receptor subunit expression (NR1, NR2B, NR2C, and NR2D) in the NG. The results showed that, CCK reduced food intake at 30-, 60-, and 120-min post injection in both young and the middle-age animals, with no statistical difference between the groups at 30- and 60-min. In contrast, MK-801 produced an increase in food intake that was significantly higher in middle-age rats compared to young animals at all time points studied. NR1 subunit was expressed by almost all NG neurons in both age groups. In young rats, NR2B, NR2C, and NR2D subunits were expressed in 56.1%, 49.3%, and 13.9% of NG neurons, respectively. In contrast, only 30.3% of the neuronal population in middle-aged rats expressed NR2B subunit immunoreactivity, NR2C was present in 34.1%, and only 10.6% of total neurons expressed the NR2D subunit. In conclusion, glutamate-dependent regulation of food intake is altered with age and one of the potential mechanisms through which this age-related changes in intake occur is changes in NMDA receptor phenotypes on vagal afferent neurons located in NG.

Cytosolic calcium regulation in rat afferent vagal neurons during anoxia

Sensory neurons are able to detect tissue ischaemia and both transmit information to the brainstem as well as release local vasoactive mediators. Their ability to sense tissue ischaemia is assumed to be primarily mediated through proton sensing ion channels, lack of oxygen however may also affect sensory neuron function. In this study we investigated the effects of anoxia on isolated capsaicin sensitive neurons from rat nodose ganglion. Acute anoxia triggered a reversible increase in [Ca2+]i that was mainly due to Ca2+-efflux from FCCP sensitive stores and from caffeine and CPA sensitive ER stores. Prolonged anoxia resulted in complete depletion of ER Ca2+-stores. Mitochondria were partially depolarised by acute anoxia but mitochondrial Ca2+-uptake/buffering during voltage-gated Ca2+-influx was unaffected. The process of Ca2+-release from mitochondria and cytosolic Ca2+-clearance following Ca2+ influx was however significantly slowed. Anoxia was also found to inhibit SERCA activity and, to a lesser extent, PMCA activity. Hence, anoxia has multiple influences on [Ca2+]i homeostasis in vagal afferent neurons, including depression of ATP-driven Ca2+-pumps, modulation of the kinetics of mitochondrial Ca2+ buffering/release and Ca2+-release from, and depletion of, internal Ca2+-stores. These effects are likely to influence sensory neuronal function during ischaemia.

Early Postnatal Ozone Exposure Alters Rat Nodose and Jugular Sensory Neuron Development

Sensory neurons originating in nodose and jugular ganglia that innervate airway epithelium (airway neurons) play a role in inflammation observed following exposure to inhaled environmental irritants such as ozone (O(3)). Airway neurons can mediate airway inflammation through release of the neuropeptide substance P (SP). While susceptibility to airway irritants is increased in early life, the developmental dynamics of afferent airway neurons are not well characterized. The hypothesis of this study was that airway neuron number might increase with increasing age, and that an acute, early postnatal O(3) exposure might increase both the number of sensory airway neurons as well as the number SP-containing airway neurons. Studies using Fischer 344 rat pups were conducted to determine if age or acute O(3) exposure might alter airway neuron number. Airway neurons in nodose and jugular ganglia were retrogradely labeled, removed, dissociated, and counted by means of a novel technique employing flow cytometry. In Study 1, neuron counts were conducted on postnatal days (PD) 6, 10, 15, 21, and 28. Numbers of total and airway neurons increased significantly between PD6 and PD10, then generally stabilized. In Study 2, animals were exposed to O(3) (2 ppm) or filtered air (FA) on PD5 and neurons were counted on PD10, 15, 21, and 28. O(3) exposed animals displayed significantly less total neurons on PD21 than FA controls. This study shows that age-related changes in neuron number occur, and that an acute, early postnatal O(3) exposure significantly alters sensory neuron development.

N-methyl-D-aspartate receptor subunit phenotypes of vagal afferent neurons in nodose ganglia of the rat

Most vagal afferent neurons in rat nodose ganglia express mRNA coding for the NR1 subunit of the heteromeric N-methyl-D-aspartate (NMDA) receptor ion channel. NMDA receptor subunit immunoreactivity has been detected on axon terminals of vagal afferents in the dorsal hindbrain, suggesting a role for presynaptic NMDA receptors in viscerosensory function. Although NMDA receptor subunits (NR1, NR2B, NR2C, and NR2D) have been linked to distinct neuronal populations in the brain, the NMDA receptor subunit phenotype of vagal afferent neurons has not been determined. Therefore, we examined NMDA receptor subunit (NR1, NR2B, NR2C, and NR2D) immunoreactivity in vagal afferent neurons. We found that, although the left nodose contained significantly more neurons (7,603), than the right (5,978), the proportions of NMDA subunits expressed in the left and right nodose ganglia were not significantly different. Immunoreactivity for NMDA NR1 subunit was present in 92.3% of all nodose neurons. NR2B immunoreactivity was present in 56.7% of neurons; NR2C-expressing nodose neurons made up 49.4% of the total population; NR2D subunit immunoreactivity was observed in just 13.5% of all nodose neurons. Double labeling revealed that 30.2% of nodose neurons expressed immunoreactivity to both NR2B and NR2C, whereas NR2B and NR2D immunoreactivities were colocalized in 11.5% of nodose neurons. NR2C immunoreactivity colocalized with NR2D in 13.1% of nodose neurons. Our results indicate that most vagal afferent neurons express NMDA receptor ion channels composed of NR1, NR2B, and NR2C subunits and that a minority phenotype that expresses NR2D also expresses NR1, NR2B, and NR2C.

Purinergic synapses formed between rat sensory neurons in primary culture

Though there is some evidence to the contrary, dogma claims that primary sensory neurons in the dorsal root ganglion do not interact, that the ganglion serves as a through-station in which no signal processing occurs. Here we use patch clamp and immunocytochemistry to show that sensory neurons in primary culture can form chemical synapses on each other. The resulting neurotransmitter release is calcium dependent and uses synaptotagmin-containing vesicles. On many cells studied, the postsynaptic receptor for the neurotransmitter is a P2X receptor, an ion channel activated by extracellular ATP. This shows that sensory neurons have the machinery to form purinergic synapses on each other and that they do so when placed in short-term tissue culture.

Receptor gene expression of glucagon-like peptide-1, but not glucose-dependent insulinotropic polypeptide, in rat nodose ganglion cells

We previously reported that afferent signals of the rat hepatic vagus increased upon intraportal appearance of insulinotropic hormone glucagon-like peptide-1(7-36) amide (GLP-1), but not glucose-dependent insulinotropic polypeptide (GIP). To obtain molecular evidence for the vagal chemoreception of GLP-1, the concept derived from those electrophysiological observations, receptor gene expressions of GLP-1 and GIP in the rat nodose ganglion were examined by means of reverse transcriptase-mediated polymerase chain reaction (RT-PCR) and Northern blot analysis. Gene expression of the GLP-1 receptor was clearly detected by both RT-PCR and Northern blot analysis. In situ hybridization study confirmed that the expression occurs in neuronal cells of the ganglion. As to the GIP receptor, RT-PCR amplified the gene transcript faintly though Northern blot analysis failed to detect any messages. However, semi-quantitative RT-PCR revealed that the ratio of the gene expression level of the GIP receptor to that of the GLP-1 receptor was less than 1:250, indicating that receptor gene expression of GIP is practically negligible in the ganglion. Additionally, an equal level of GLP-1 receptor gene expressions between left- and right-side ganglia was evidenced by semi-quantitative RT-PCR, implying possible extrahepatic occurrence of vagal GLP-1 reception in addition to the reception through the hepatic vagus (originating from the left-side ganglion). The present results offer, for the first time, the molecular basis for the vagal chemoreception of GLP-1 via its specific receptor.

Expression and distribution of phocein and members of the striatin family in neurones of rat peripheral ganglia

Phocein and members of the striatin family (striatin, SG2NA and zinedin) are intracellular proteins, mainly expressed in neurones of the mammalian central nervous system where they are thought to be involved in vesicular traffic and Ca(2+) signalling. Here, we have investigated whether these proteins are also present in the peripheral nervous system, by analysing their expression and distribution within sensory neurones of the vagal (nodose and jugular) ganglia, the petrosal ganglion, the dorsal root ganglion, and also in the sympathetic neurones of the superior cervical ganglion. RT-PCR experiments showed that mRNAs of phocein, striatin, SG2NA and zinedin are present in all studied peripheral ganglia. Immunocytochemical detections demonstrate that phocein, striatin and SG2NA are expressed in neurones of vagal, petrosal and dorsal root ganglia. Immunoblotting experiments confirm these data and in addition demonstrate that: (1) the proteins phocein, striatin and SG2NA are also present in the superior cervical ganglion and (2) zinedin is detected in all studied ganglia. The distribution appears to differ: immunoreactivity for striatin and SG2NA is found only in soma of sensory neurons, whereas immunoreactivity for phocein is observed in both soma and processes. Our study thus demonstrates that phocein and the members of the striatin family are expressed not only in central nervous system but also in the peripheral nervous system and, in particular, in afferent sensory neurones.

The role of the gastric afferent vagal nerve in ghrelin-induced feeding and growth hormone secretion in rats

BACKGROUND & AIMS

Visceral sensory information is transmitted to the brain through the afferent vagus nerve. Ghrelin, a peptide primarily produced in the stomach, stimulates both feeding and growth hormone (GH) secretion. How stomach-derived ghrelin exerts these central actions is still unknown. Here we determined the role of the gastric afferent vagal nerve in ghrelin's functions.

METHODS

Food intake and GH secretion were examined after an administration of ghrelin intravenously (IV) to rats with vagotomy or perivagal application of capsaicin, a specific afferent neurotoxin. We investigated Fos expression in neuropeptide Y (NPY)-producing and growth hormone-releasing hormone (GHRH)-producing neurons by immunohistochemistry after administration IV of ghrelin to these rats. The presence of the ghrelin receptor in vagal afferent neurons was assessed by using reverse-transcription polymerase chain reaction and in situ hybridization histochemistry. A binding study on the vagus nerve by (125)I-ghrelin was performed to determine the transport of the ghrelin receptor from vagus afferent neurons to the periphery. We recorded the electric discharge of gastric vagal afferent induced by ghrelin and compared it with that by cholecystokinin (CCK), an anorectic gut peptide.

RESULTS

Blockade of the gastric vagal afferent abolished ghrelin-induced feeding, GH secretion, and activation of NPY-producing and GHRH-producing neurons. Ghrelin receptors were synthesized in vagal afferent neurons and transported to the afferent terminals. Ghrelin suppressed firing of the vagal afferent, whereas CCK stimulated it.

CONCLUSIONS

This study indicated that the gastric vagal afferent is the major pathway conveying ghrelin's signals for starvation and GH secretion to the brain.

Physiological patterns of electrical stimulation can induce neuronal gene expression by activating N-type calcium channels

Activity-dependent neuronal gene expression is thought to require activation of L-type calcium channels, a view based primarily on studies in which chronic potassium (K(+)) depolarization was used to mimic neuronal activity. However, N-type calcium channels are primarily inactivated during chronic depolarization, and their potential contribution to gene expression induced by physiological patterns of stimulation has not been defined. In the present study, electrical stimulation of dissociated primary sensory neurons at 5 Hz, or treatment with elevated K(+), produced a large increase in the percentage of neurons that express tyrosine hydroxylase (TH) mRNA and protein. However, blockade of L-type channels, which completely inhibited K(+)-induced expression, had no effect on TH expression induced by patterned stimulation. Conversely, blockade of N-type channels completely inhibited TH induction by patterned stimulation, whereas K(+)-induced expression was unaffected. Similar results were obtained for depolarization-induced expression of the immediate early genes Nurr1 and Nur77. In addition, TH induction by patterned stimulation was significantly reduced by inhibitors of PKA and PKC but was unaffected by inhibition of the mitogen-activated protein kinase (MAPK) pathway. On the other hand, K(+)-induced TH expression was significantly reduced by inhibition of the MAPK pathway but was unaffected by inhibitors of PKA or PKC. These results demonstrate that N-type calcium channels can directly link phasic membrane depolarization to gene expression, challenging the view that activation of L-type channels is required for nuclear responses to physiological patterns of activity. Moreover, our data show that phasic and chronic depolarizing stimuli act through distinct mechanisms to induce neuronal gene expression.

Nicotinic acetylcholine receptors on vagal afferent neurons

Nicotinic acetylcholine receptors (nAChRs) play an important role in various processes involved in regulating systemic blood pressure. These receptors are expressed at excitatory cholinergic synapses between sympathetic preganglionic neurons and postganglionic sympathetic neurons and link the integrative activities of the CNS with peripheral effector mechanisms of the sympathetic nervous system. Nicotinic AChRs are also expressed on a subset of vagal afferent neurons, including those involved in baroreceptor reflexes. This review discusses the developmental expression of nAChRs on vagal afferent neurons and two factors that influence the differentiation of these neurons: ganglionic satellite cells and neurotrophins. In addition, this review discusses two important properties of neuronal nAChRs: inward rectification and calcium permeability. At the molecular level, intracellular polyamines, acting as gating particles, effectively block the receptor pore in a voltage-dependent manner, producing inward rectification. Moreover, a critical structural determinant underlies both the block by intracellular polyamines and calcium permeability. Finally, this review discusses the modulation and block of neuronal nAChRs by extracellular polyamines and the possible implications for neurodegenerative diseases.

Physiological patterns of electrical stimulation can induce neuronal gene expression by activating N-type calcium channels

Activity-dependent neuronal gene expression is thought to require activation of L-type calcium channels, a view based primarily on studies in which chronic potassium (K(+)) depolarization was used to mimic neuronal activity. However, N-type calcium channels are primarily inactivated during chronic depolarization, and their potential contribution to gene expression induced by physiological patterns of stimulation has not been defined. In the present study, electrical stimulation of dissociated primary sensory neurons at 5 Hz, or treatment with elevated K(+), produced a large increase in the percentage of neurons that express tyrosine hydroxylase (TH) mRNA and protein. However, blockade of L-type channels, which completely inhibited K(+)-induced expression, had no effect on TH expression induced by patterned stimulation. Conversely, blockade of N-type channels completely inhibited TH induction by patterned stimulation, whereas K(+)-induced expression was unaffected. Similar results were obtained for depolarization-induced expression of the immediate early genes Nurr1 and Nur77. In addition, TH induction by patterned stimulation was significantly reduced by inhibitors of PKA and PKC but was unaffected by inhibition of the mitogen-activated protein kinase (MAPK) pathway. On the other hand, K(+)-induced TH expression was significantly reduced by inhibition of the MAPK pathway but was unaffected by inhibitors of PKA or PKC. These results demonstrate that N-type calcium channels can directly link phasic membrane depolarization to gene expression, challenging the view that activation of L-type channels is required for nuclear responses to physiological patterns of activity. Moreover, our data show that phasic and chronic depolarizing stimuli act through distinct mechanisms to induce neuronal gene expression.

Chemotransduction properties of nodose ganglion cardiac afferent neurons in guinea pigs

To determine the chemotransduction characteristics of ventricular sensory neurites associated with nodose ganglion afferent neurons, various chemicals were applied individually to epicardial sensory neurites associated with individual afferent neurons in anesthetized guinea pigs. The following ion channel-modifying agents were tested: barium chloride, cadmium chloride, calcium chloride, the chelating agent EGTA, nickel chloride, potassium chloride, tetraethylammonium chloride, and veratridine. An acidic solution (pH 6.0) and oxygen-derived free radicals (H(2)O(2)) were tested. The following chemicals were also tested: adenosine, alpha- and beta-adrenergic agonists, angiotensin II, bradykinin, calcitonin gene-related peptide (CGRP), histamine, nicotine, the nitric oxide donor nitroprusside, substance P, and vasoactive intestinal peptide. A total of 102 cardiac afferent neurons was identified, of which approximately 66% were sensitive to mechanical stimuli applied to their epicardial sensory fields. Application of individual ion channel-modifying agents to epicardial sensory fields modified most associated afferent neurons, with barium chloride affecting each neuron studied. Ventricular sensory neurites associated with most identified neurons were also responsive to the other tested chemicals, with hydrogen peroxide, adenosine, angiotensin II, bradykinin, CGRP, clonidine, and nicotine inducing responses from at least 75% of the neurons studied. It is concluded that 1) the ventricular sensory neurites associated with nodose ganglion afferent neurons transduce a much wider variety of chemical stimuli than considered previously, 2) these sensory neurites employ a variety of membrane ion channels in their transduction processes in situ, and 3) adrenergic agents influence on sensory neurites associated with cardiac afferent neurons suggests the presence of a cardiac feedback mechanism involving local catecholamine release by adjacent sympathetic efferent postganglionic nerve terminals.

Spike conduction properties of T-shaped C neurons in the rabbit nodose ganglion

The electrical activity of C-type neurons was recorded intracellularly in the rabbit nodose ganglion maintained in vitro. The initial segment of their axon is spirally wound close to the cell body and a primary branching point divides it into a central process (CP) projecting to the nucleus of solitary tract in the medulla oblongata and a peripheral process (PP) which conveys sensory inputs from the viscera. Stimulation of the CP induced either somatic ("S") spikes or low-amplitude axonal ("A") spikes ("A1" or "A2"). In some cases abrupt changes in the latency of "S" or "A" spikes (jumps) were observed by gradually increasing the stimulus intensity. They are discussed in relation to a secondary branching on the central axon located inside or near the ganglion. Collision experiments showed that antidromic "A" spikes are blocked at the primary bifurcation of the axon (T-shaped neuron). Stimulation of the PP induced either "S" spikes or high amplitude "A" spikes ("A3" or "A4"). Orthodromic spikes could be blocked either before or after the primary bifurcation. When blocking occurs after the bifurcation on the stem axon, the spike can invade the central axon without invading the soma. The study of the refractory periods of the two processes and the application of high frequency stimulation showed that the PP allows higher frequencies than the soma and the CP, and thus that branching and the CP act as low-pass filters. These data support the view that the primary branching point and the CP of these T-shaped cells represent a strategic area to modulate visceral afferent messages.

Somatic Acetylcholine Release in Rabbit Nodose Ganglion

In the rabbit, as in various other species, the presence of a cholinergic vagal afferent contingent has been demonstrated previously using biochemical and immunohistological approaches at the nodose ganglion level, where vagal afferent cell bodies are located. This structure is completely devoid of synaptic contacts. In the present study, somatic acetylcholine release is demonstrated on different types of in vitro rabbit nodose ganglion preparations (fragments of nodose tissue or isolated cell bodies) using chemiluminescent detection. Acetylcholine endogenous content was measured and was shown to be greater in the right nodose ganglion compared to the left. This difference was also observed when spontaneous and potassium chloride-evoked acetylcholine release was measured in extracellular fluid after a 15-min incubation of nodose ganglion fragments. Calcium removal totally blocked this somatic release. A kinetic study of acetylcholine release was also performed by placing the samples (nodose ganglion fragments or isolated cell bodies) directly in front of the photomultiplier, allowing the direct monitoring of (acetylcholine + choline) and choline effluxes. The net acetylcholine release was then deduced by subtraction. Identical kinetics was obtained with the two different nodose ganglion preparations used. This somatic release is calcium-dependent. The occurrence of acetylcholine release at the nodose ganglion level is discussed in comparison with the events occurring in the cholinergic nerve endings. These mechanisms could be implicated in the premodulation of the vagal afferent messages conveyed from the periphery to the central nervous system.

The peptide CGRP increases a high-threshold Ca2+ current in rat nodose neurones via a pertussis toxin-sensitive pathway

1. The whole-cell variation of the patch clamp technique was used to study the effect of calcitonin gene-related peptide (CGRP) on voltage-gated calcium currents in acutely dissociated rat nodose ganglion neurones and to determine if its effects were mediated via a guanine nucleotide binding (G) protein. 2. Both low- and high-threshold calcium current components were present in nodose ganglion neurones. CGRP had no effect on the isolated low-threshold current component. However, CGRP (1-1000 nM, ED50 = 50 nM) caused a concentration-dependent increase in high-threshold calcium currents. CGRP (1 microM) increased the peak of these calcium currents 21 +/- 4% over controls. 3. CGRP enhanced a transient high-threshold calcium current evoked from a holding potential of -80 mV but did not affect the slowly inactivating high-threshold current evoked from -40 mV. Multiple high-threshold calcium currents have been reported in sensory neurones. We cannot state unequivocally which high-threshold calcium current component was enhanced by CGRP. However, based on the observation that CGRP increased a transient but not the slowly inactivating high-threshold calcium current, we believe the peptide enhanced primarily the N-type calcium current component. 4. CGRP increased the maximal peak current and caused a modest negative shift of < or = 10 mV in the peak of the current-voltage (I-V) relation in three of six neurones. In the remaining three neurones the peptide increased the maximal peak current without a detectable shift in the peak of the I-V relation. 5. To determine if the CGRP-induced enhancement in calcium current was associated with an increase in calcium conductance, we studied the effect of the peptide on the instantaneous current-voltage (I-V) relation when currents were evoked at a clamp potential (Vc) of +30 mV, positive to the observed maximal current (Vc = 0 to +10 mV). CGRP increased the maximal conductance 23 +/- 4%. 6. The enhancement of calcium current by CGRP was not due to a shift in the voltage dependency of steady-state inactivation of the calcium channels. The stimulatory effect of CGRP on calcium current was evaluated by evoking currents from different holding potentials (Vh) at the same Vc (+10 mV). CGRP-induced increases in calcium currents were similar over the range of (Vh) from -60 to -110 mV, suggesting that the peptide did not alter voltage-dependent steady-state inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

Loss of extrasynaptic channel modulation by protein kinase C underlies the selection of serotonin responses in an identified leech neuron

Pressure-sensitive (P) neurons contacted by serotonergic Retzius (R) neurons of the leech in culture selectively reduce a protein kinase C (PKC)-dependent cation response to serotonin and are innervated by the inhibitory, Cl(-)-dependent synapse seen in vivo. We have examined whether the reduction of extrasynaptic cation channel modulation is due to changes in sensitivity of the channels to second messenger. In inside-out membrane patches from single, uncontacted P cells in culture, cation channel activity was increased by rat brain PKC and cofactors. In contrast, the activity of cation channels in patches isolated from P cells paired with R cells was unaffected by PKC. These results demonstrate the loss of extrasynaptic channel modulation by PKC during synapse formation.

Electrophysiological evidence for the reconstitution of chemosensory units in co-cultures of carotid body and nodose ganglion neurons

The electrophysiological characteristics of nodose ganglion sensory neurons, cultured alone or co-cultured with carotid body tissue, were compared. Some properties of the neurons and their response to acid (a carotid body 'natural' stimulus) changed in the presence of this tissue. (a) The evoked action potential after-hyperpolarization was smaller and longer whereas spike amplitude and duration, and the passive membrane properties remained unaltered. (b) Spontaneously occurring action potentials happened more frequently (16% vs 3%). (c) Acid solutions induced appreciable depolarization, an increased discharge, or both, only in a population of co-cultured neurons. These changes probably arose because of synaptic and/or trophic interactions between neurons and glomus cells.

Basal and stimulated release of substance P from dissociated cultures of vagal sensory neurons

Substance P, the widely distributed 11 amino acid neuropeptide, is present in up to 20% of vagal sensory cell bodies and the fibers emanating from them. To study the factors regulating the release of SP, vagal sensory (nodose or nodose/jugular) ganglia were obtained from neonatal rats and dissociated using neutral protease. Survival of plated neurons on collagen substrate was 10-20% at 2 weeks and 20-30% when neurons were plated over previously dissociated rat atriacytes. Substance P content was low in cultures for the first several days, then rose linearly to 0.1-0.2 pg/surviving neuron. Substance P was released into a 4.5 mM potassium medium at a steady rate of 0.036%/min. In 50 mM K+ supplemented medium, total release during 20 min increased 5-8-fold and steady-state release increased 4-5-fold to 0.15%/min. The sensory neuron specific excitatory neurotoxin, capsaicin, evoked SP release in similar amounts to 50 mM K+. Both net K(+)- and capsaicin-evoked, but not basal release were completely inhibited by 3.5 mM cobalt chloride. Bradykinin, 1-100 nM, stimulated SP release 2-4 times above basal levels. Forskolin and phorbol ester also increased SP release 1.5-3 times basal amounts. In summary, substance P is present in cultured vagal sensory neurons in amounts similar to in vivo and is released in response to sensory specific stimuli. These cultures should allow exploration of some of the tissue specific factors regulating neurotransmitter release in the sensory vagus nerve.

Loss of channel modulation by transmitter and protein kinase C during innervation of an identified leech neuron

When serotonergic Retzius (R) neurons of the leech contact pressure-sensitive (P) neurons in culture, P cells selectively lose a protein kinase C-dependent cationic response to serotonin and the R cell reforms the inhibitory, chloride-dependent synapse seen in vivo. In P cells not contacted by R cells, cell-attached patches contained single cation channels sensitive to serotonin and phorbol ester with characteristic properties and high incidence (present in about one-half of the patches). P cells paired with R cells had a cation channel with similar biophysical properties and incidence, but channel activity was not stimulated by serotonin and phorbol ester. These results suggest that the early clearing of the non-synaptic (excitatory) response to serotonin is due to the loss of activation by protein kinase C (and not the number) of cation channels as a prelude to inhibitory synapse formation.

Nerve growth factor induces functional nicotinic acetylcholine receptors on rat sensory neurons in culture

Neonatal sensory neurons from rat nodose ganglia express nicotinic acetylcholine receptors when grown in tissue culture without other cell types. The present study investigates the role of nerve growth factor in inducing these receptors. Nerve growth factor has little effect on the growth and survival of nodose neurons in culture, although most neurons were found by quantitative radioautography to have high-affinity nerve growth factor receptors. Nerve growth factor strongly influenced the expression of nicotinic receptors on these neurons: the proportion of acetylcholine-sensitive neurons was approximately 60% in cultures with nerve growth factor compared with 15% in cultures grown without nerve growth factor. The proportion of acetylcholine-sensitive neurons increased over the first week, plateaued by day 12 and remained high for at least three weeks. In contrast, without NGF, the proportion of acetylcholine-sensitive neurons was low throughout the three-week period. The results indicate that nerve growth factor is an important factor in promoting nicotinic receptors on these neurons in culture.

Electrophysiological and morphological properties of type C vagal neurons in the nodose ganglion of the cat

Some passive and active electrical properties of type C neurons were studied intracellularly, in situ, in the nodose ganglia of adult cats. From the neuronal responses to hyperpolarizing and depolarizing rectangular current pulses it was possible to determine the input resistance (34.4 M omega) and specific membrane resistance (2373 omega.cm2). Significant changes in magnitude and duration of the action potential evoked by vagal stimulation result from changes in the resting potential caused by the passage of steady polarizing currents across the cell membrane. The action potentials evoked by infranodose vagal stimulation had a long duration, a long latency and comprised several components. The fast main spike was followed by a long post-hyperpolarization. The double shock technique showed that the fast main potential was composed of an initial segment spike ('A spike') and a somatic spike ('S spike'), and made it possible to determine the somatic refractory periods. After electrical identification of the cells, horseradish peroxidase was injected ionophoretically into the soma, and it was shown that the central processes were about four times smaller in diameter than the peripheral processes.

Parallel processing and selection of the responses to serotonin during reinnervation of an identified leech neuron

In an attempt to define the mechanism of synaptic specificity, we have been studying pairs of identified leech neurons isolated in tissue culture. The cultured neurons reform specific synapses when paired with appropriate partners in the absence of other cell types. In recent studies, we have examined in detail the reformation of a serotoninergic synapse between the Retzius cell and one of its targets, the pressure sensitive (P) cell. The P cell in vivo and its soma in vitro have two types of responses to serotonin (5-HT). From voltage clamp analysis of cultured P cells, we demonstrated the parallel activation of chloride (gCls) and monovalent cation (gCations) channels coupled to distinct receptor subtypes and gated by separate second messengers. Only gCls was activated by 5-HT released from the presynaptic Retzius cell both in vivo and in vitro. This demonstrates the remarkable specificity of the reformation of this synapse in culture since only the correct 5-HT receptor subtype is activated. An 80% reduction of gCations was observed in P cells that had failed to be innervated by Retzius cells in culture, suggesting that gCations may be lost prior to synapse formation. Retzius cells depleted of 5-HT also reduced gCations in the paired P cells and incubating single P cells in 5-HT did not reduce gCations. In addition, aldehyde-fixed Retzius cells were able to selectively reduce gCations when paired with P cells. We conclude that the loss of gCations was due to contact between the neurons. The early clearing of counter-effective receptor subtypes may be a prelude to synapse formation.

Differential regulation of calcitonin gene-related peptide and substance P in cultured neonatal rat vagal sensory neurons

Nodose (inferior vagal sensory) ganglia were removed from neonatal rats, enzymatically dispersed using neutral protease, and maintained on previously dispersed rat atriacytes. After 7-10 days in culture, calcitonin gene-related peptide (CGRP) was present in 1-3 times the molar amount of substance P (SP). The content of SP was doubled by the addition of nerve growth factor (NGF) whereas CGRP was significantly less increased by 50% or less. The addition of forskolin increased SP and CGRP levels in cultures with or without NGF by 60-80 percent. Phorbol ester (PMA) did not alter SP content but significantly raised CGRP content by 40% in NGF supplemented cultures (P less than 0.001). Corticosterone, 0.01-0.1 microM, reduced SP content by 30% independently of NGF but had no effect on CGRP. These studies demonstrate that SP in vagal sensory neurons is more sensitive than CGRP to the effects of NGF or corticosterone. Both peptides are up-regulated by presumed increases in intracellular cyclic AMP, while CGRP (or CGRP neurons) may be independently regulated by protein kinase C.

Substance P content in cultured neonatal rat vagal sensory neurons: the effect of nerve growth factor

To begin to study the factors regulating the synthesis and release of substance P (SP) in the sensory vagus nerve, cultures of neonatal rat nodose ganglia were developed. In microexplant cultures, obtained from small fragments of nodose ganglia, SP was present in low amounts: after 3 weeks, 141 +/- 36 pg per well, 10 ganglia equivalents per well. To enhance neuron survival, nodose ganglia were enzymatically dissociated using neutral protease. Estimated survival at 5 days was 20-30%, with 800-1200 surviving neurons per plated ganglion, and decreased slowly thereafter. Specific SP immunostaining was present in 10-20% of neurons, mostly of small diameter (18-22 micron). SP content was low for 5 days then rose progressively after 14 days to 80-150 pg per plated ganglion. The addition of nerve growth factor (NGF, 100 ng/ml) to the culture medium did not alter neuron survival. However, SP content was doubled in the presence of NGF, or fell rapidly to one-half control levels following its withdrawal: e.g. following 12 days in culture with NGF 1185 +/- 176 pg/well vs NGF withdrawn day 8-12, 592 +/- 118 pg/well, mean +/- S.D., P less than 0.01. Somatostatin, present in one-sixth the amount of SP, was unaltered by NGF. In subsequent studies, plating of neurons onto previously dissociated rat atriacytes increased survival by 50% but did not alter SP content per surviving neurons. These studies demonstrate that SP is present in dissociated cultures of rat vagal sensory neurons; the quantities and estimated net synthesis rate correspond to previous observations in vivo. The studies also demonstrate that SP content but not neuron survival are regulated by NGF in nodose ganglion neurons. This model may prove valuable for the study of SP and other sensory neuropeptides in this important class of visceral afferent neurons.

The development and distribution of alpha-bungarotoxin binding sites in rat tectal transplants

The development and distribution of alpha-bungarotoxin (alpha-BTX) binding sites in tectal grafts was examined autoradiographically using the radioligand [125I]-alpha-BTX. High alpha-BTX binding was observed in localized areas within grafts; these areas corresponded to regions which contained high acetylcholinesterase activity and received retinal input. Receptor differentiation also occurred in the absence of specific host afferents. The graft data show that, as in normal superior colliculus, development of high alpha-BTX binding is limited to areas containing presumptive superficial layer cells.

Formation of calyx-like contacts preferentially on appropriate target neurons in culture

The availability of culture systems for both Edinger Westphal and ciliary ganglion neurons has made it possible to examine the interactions in culture between two populations of vertebrate neurons that synapse in vivo. In the chick, Edinger Westphal neurons provide the sole presynaptic input to the ciliary ganglion and, through this projection, are responsible for the control of lens curvature (accommodation), iris constriction, and possibly smooth muscle function in the choroid layer of the eye. When embryonic chick Edinger Westphal and ciliary ganglion neurons were combined in culture and stained for enkephalin-like immunoreactivity to visualize Edinger Westphal terminals, stained calyx-like contacts were observed that resemble the calyciform terminals formed between Edinger Westphal processes and ciliary neurons in the ciliary ganglion in vivo. Although stained calyx-like contacts could also be found in Edinger Westphal-alone and ciliary ganglion-alone cultures, many more were observed when the two cell types were cultured together. The increase depended specifically on the ciliary ganglion neurons since substitution of either dorsal root ganglion or sympathetic ganglion neurons for them in the cocultures did not increase the number of calyx-like contacts staining positive for enkephalin over those present in cultures of Edinger Westphal neurons alone. When Edinger Westphal neurons were grown simultaneously with dorsal root and ciliary ganglion neurons, calyx-like contacts with enkephalin-like immunoreactivity were found to terminate preferentially on the latter. These findings suggest that vertebrate neurons can form morphologically specific contacts preferentially on appropriate target cells in culture in the absence of many of the potential cues present in the intact tissue.

Factors affecting the expression of acetylcholine receptors on rat sensory neurones in culture

Sensory neurones from nodose ganglia of new-born rats were grown in dissociated tissue culture either with or without satellite cells. When cultured without satellite cells, most neurones developed sensitivity to acetylcholine (ACh); time-course experiments indicated that the neurones acquire their sensitivity during the second to third week in culture. Most neurones co-cultured with satellite cells did not develop ACh sensitivity. Delayed removal of satellite cells 8-12 days after plating resulted in few neurones acquiring ACh sensitivity. Delayed addition of satellite cells to neuronal cultures that were initially grown without satellite cells had no effect on the number of ACh-sensitive neurones. The potential to develop ACh sensitivity in culture without satellite cells decreases with the age of the neurones at the time of culturing; few neurones from 2-week-old animals developed sensitivity to ACh when cultured without satellite cells. These results indicate that there is some influence from satellite cells that prevents nodose neurones from developing ACh sensitivity in culture and suggests that this influence may also operate in vivo.

Single-channel analysis of fast transient potassium currents from rat nodose neurones

Single channels that underlie the fast transient potassium current (IA) were recorded, using patch-clamp techniques, from cultured sensory neurones. The open channel conductance was approximately 22 pS, and was constant over most of the physiological voltage range; single-channel conductance decreased at more depolarized levels. Summing single-channel currents resulted in an average current whose kinetics were similar to the macroscopic IA. The inactivation of these currents, at the potentials we studied, was fitted with a single exponential with a time constant of approximately 30 ms. For the currents evoked by large depolarizing steps (to +40 mV), the mean channel open time equals approximately 30 ms. For currents evoked at less depolarized levels (to 0 mV), the mean open time equals approximately 15 ms, half the inactivation time constant.

Placodal sensory neurons in culture: nodose ganglion neurons are unresponsive to NGF, lack NGF receptors but are supported by a liver-derived neurotrophic factor

Explant and dissociated neuron-enriched cultures of nodose ganglia (inferior or distal sensory ganglion of the Xth cranial nerve) were established from chick embryos taken between embryonic Day 4 (E4) and Day 16 (E16). The response of each type of culture to nerve growth factor (NGF) was examined over this developmental range. At the earliest ages taken (E4-E6), NGF elicited modest neurite outgrowth from ganglion explants cultured in collagen gel for 24 hr, although the effect of NGF on ganglia taken from E4 chicks was only marginally greater than spontaneous neurite extension from control ganglia of the same developmental age. The response of nodose explants to NGF was maximal at E6-E7, but declined to a negligible level in ganglia taken from E9-E10 or older chick embryos. In dissociated neuron-enriched cultures, nodose ganglion neurons were unresponsive to NGF throughtout the entire developmental age range between E5 and E12. In contrast to the lack of effect of NGF, up to 50% of nodose ganglion neurons survived and produced extensive neurites in dissociated cultures, on either collagen- or polylysine-coated substrates, in the presence of extracts of late embryonic or early posthatched chick liver (E18-P7). Antiserum to mouse NGF did not block the neurotrophic activity of chick (or rat or bovine) liver extracts. Whether cultured with chick liver extract alone or with chick liver extract plus NGF, nodose ganglion neurons taken from E6-E12 chick embryos and maintained in culture for 2 days were devoid of NGF receptors, as assessed by autoradiography of cultures incubated with 125I-NGF. Under similar conditions 70-95% of spinal sensory neurons (dorsal root ganglion--DRG) were heavily labeled. 2+