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

Anatomical differences in nociceptor neurons sensitivity

Background

Dorsal Root Ganglia (DRG) neurons are derived from the neural crest and mainly innervate the skin, while Jugular Nodose Complex (JNC) neurons originate from the placode and innervate internal organs. These ganglia are composed of highly heterogeneous groups of neurons aimed at assessing and preserving homeostasis. Among other subtypes, nociceptor neurons are specialized in sensing and responding to environmental dangers. As form typically follows function, we hypothesized that JNC and DRG neurons would be phenotypically and transcriptomically different.

Methods

Mouse JNC and DRG neurons were cultured ex vivo. Using calcium imaging, qPCR and neurite outgrowth assay, we compared the sensitivity of JNC and DRG neurons. Using in-silico analysis of existing RNA sequencing datasets, we confronted our results to transcriptomic differences found between both ganglia.

Results

We found drastically different expression levels of Transient Receptor Potential (TRP) channels, growth factor receptors and neuropeptides in JNC and DRG neurons. Functionally, naïve JNC neurons’ TRP channels are more sensitive to thermal cues than the ones from DRG neurons. However, DRG neurons showed increased TRP channel responsiveness, neuropeptide release and neurite outgrowth when exposed to Nerve Growth Factor (NGF). In contrast, JNC neurons preferentially responded to Brain-derived neurotrophic factor (BDNF).

Conclusion

Our data show that JNC and DRG neurons are transcriptomically and functionally unique and that pain sensitivity is different across anatomical sites. Drugs targeting NGF signaling may have limited efficacy to treat visceral pain. Bioelectronics nerve stimulation should also be adjusted to the ganglia being targeted and their different expression profile.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42234-022-00088-w.

B-Afferents: A fundamental division of the nervous system mediating homeostasis?

The peripheral nervous system (PNS) has classically been separated into a somatic division composed of both afferent and efferent pathways and an autonomic division containing only efferents. J. N. Langley, who codified this asymmetrical plan at the beginning of the twentieth century, considered different afferents, including visceral ones, as candidates for inclusion in his concept of the “autonomic nervous system” (ANS), but he finally excluded all candidates for lack of any distinguishing histological markers. Langley's classification has been enormously influential in shaping modern ideas about both the structure and the function of the PNS. We survey recent information about the PNS and argue that many of the sensory neurons designated as “visceral” and “somatic” are in fact part of a histologically distinct group of afferents concerned primarily autonomic function. These afferents have traditionally been known as “small dark” neurons or B-neurons. In this target article we outline an association between autonomic and B-neurons based on ontogeny, cell phenotype, and functional relations, grouping them together as part of a common reflex system involved in homeostasis. This more parsimonious classification of the PNS, made possible by the identification of a group of afferents associated primarily with the ANS, avoids a number of confusions produced by the classical orientation. It may also have practical implications for an understanding of nociception, homeostatic reflexes, and the evolution of the nervous system.

Estrogen increases sensory nociceptor neuritogenesis in vitro by a direct, nerve growth factor-independent mechanism

Estrogen affects many aspects of the nervous system, including pain sensitivity and neural regulation of vascular function. We have shown that estrogen elevation increases sensory nociceptor innervation of arterioles in Sprague-Dawley rat mammary gland, external ear and mesentery, suggesting widespread effects on sensory vasodilatory innervation. However, it is unclear whether estrogen elicits nociceptor hyperinnervation by promoting target release of neurotrophic factors, or by direct effects on sensory neurons. To determine if estrogen may promote axon sprouting by increasing release of target-derived diffusible factors, dorsal root ganglia explants were co-cultured with mesenteric arterioles for 36 h in the absence or presence of 17beta-estradiol (E2). Mesenteric arteriolar target substantially increased neurite outgrowth from explanted ganglia, but estrogen had no effect on outgrowth, suggesting that estrogen does not increase the availability of trophic proteins responsible for target-induced neurite outgrowth. To assess the direct effects of estrogen, dissociated neonatal dorsal root ganglion neurons were cultured for 3 days in the absence or presence of E2 and nerve growth factor (NGF; 1-10 ng/mL), and immunostained for the nociceptor markers peripherin or calcitonin gene-related peptide. NGF increased neuron size, survival and numbers of neurons with neurites, but did not affect neurite area per neuron. Estrogen did not affect neuron survival, size or numbers of neurons with neurites, but did increase neurite area per neuron. The effects of these agents were not synergistic. We conclude that estrogen exerts direct effects on nociceptor neurons to promote axon outgrowth, and this occurs through an NGF-independent mechanism.

Nerve growth factor and corneal wound healing in dogs

Nerve growth factor in the tear film and corneal epithelium is hypothesized to play an important role in ocular surface maintenance and corneal wound healing. The purpose of this study was to determine the expression of nerve growth factor and its high affinity (trkA) receptor in tears, cornea, and lacrimal glands of normal dogs, the modulation of nerve growth factor and its trkA receptor during corneal wound healing, and the effect of topical nerve growth factor application on canine corneal epithelial wound healing. In the first of three experiments, the nerve growth factor content of tears, corneal epithelium, lacrimal gland, and 3rd eyelid gland was determined in normal dogs by enzyme-linked immunosorbent assay and the expression of nerve growth factor and its trkA receptor were evaluated in the cornea and lacrimal glands by immunohistochemistry. In a second experiment, unilateral corneal epithelial defects were created, and tissues were evaluated for changes in nerve growth factor or trkA expression for 1 week. In a third experiment, bilateral corneal epithelial defects were created and the right eyes in each animal were treated 4 times daily with either recombinant human nerve growth factor, murine nerve growth factor, or nerve growth factor-blocking antibody. The results of this study showed that nerve growth factor levels in normal dog tears, corneal epithelium, third eyelid gland and lacrimal gland were 15.4+/-4.6 ng ml(-1), 33.5+/-12.3, 52.4+/-17.4 and 48.8+/-9.4 ng g(-1), respectively. NGF and trkA receptors were identified by immunohistochemistry in all tissues examined. After unilateral corneal wounding, nerve growth factor concentration increased in the tears bilaterally for 3 days, especially in the wounded eye, and then returned to pre-wounding values. Nerve growth factor content, and immunohistochemical staining for nerve growth factor and trkA, increased significantly in the ipsilateral cornea epithelium following unilateral wounding. Nerve growth factor concentrations in lacrimal and third eyelid glands also increased bilaterally (p<0.01) after unilateral wounding. Time to wound closure and rate of epithelial migration did not differ significantly between nerve growth factor-treated, nerve growth factor antibody-treated, and control eyes. In conclusion, nerve growth factor is present under resting physiologic conditions in normal canine tears, and nerve growth factor and its trkA receptor are present under resting conditions in normal canine corneal epithelium, lacrimal gland and third eyelid gland. Nerve growth factor is elevated in the tears, cornea, and lacrimal glands after corneal epithelial wounding; however, topical application of nerve growth factor, or its blocking antibody does not modulate corneal wound healing in the normal dog eye.

The role of neurotrophins in bronchial asthma: contribution of the pan-neurotrophin receptor p75

Allergic bronchial asthma is characterized by chronic inflammation of the airways, development of airway hyperreactivity and recurrent reversible airway obstruction. Target and effector cells responsible for airway hyperresponsiveness and airway obstruction include sensory and motor neurons as well as epithelial and smooth muscle cells. Although it is well established that the inflammatory process is controlled by T-helper-2 (Th2) cells, the mechanisms by which immune cells interact with neurons, epithelial cells or smooth muscle cells still remain uncertain. Due to growing evidence for extensive communication between neurons and immune cells, the mechanisms of this neuroimmune crosstalk in lung and airways of asthmatic patients are becoming the focus of asthma research. Neurotrophins represent molecules potentially responsible for regulating and controlling the crosstalk between the immune and peripheral nervous system. They are constitutively expressed by resident lung cells and produced in increasing concentrations by immune cells invading the airways under pathological conditions. Neurotrophins modify the functional activity of sensory and motor neurons, leading to enhanced and altered neuropeptide and tachykinin production. These effects are defined as neuronal plasticity. The consequences are the development of neurogenic inflammation.

A study of the cough reflex in idiopathic pulmonary fibrosis

Little is known about the pathogenesis of cough in idiopathic pulmonary fibrosis (IPF). We hypothesized that abnormalities of respiratory tract tachykinin-containing sensory nerves may be implicated. We studied cough response to capsaicin, substance P (SP), and bradykinin in 10 healthy control subjects and 10 patients with IPF. Six patients were tested before and after steroid therapy. Induced sputum cell counts and neurotrophic factor levels were also measured in 13 patients and 13 control subjects. The results show that cough sensitivity to capsaicin was greater in patients (p < 0.01). Neither SP nor bradykinin induced cough in normal subjects. SP and bradykinin induced cough in 7/10 patients (p < 0.002) and 2/10 patients (not significant) with IPF, respectively. Prednisolone caused a reduction in cough sensitivity to capsaicin (p < 0.05) and SP (p < 0.05) in all six patients treated. There were significantly more neutrophils (p = 0.001) and higher levels of nerve growth factor (p < 0.01) and brain-derived neurotrophic factor (p < 0.01) in patient's sputa. These findings suggest functional upregulation of lung sensory neurones in IPF. The cough response to inhaled SP in most patients may reflect disrupted respiratory epithelium. The response to corticosteroids demonstrates that the cough is amenable to therapy.

Pan-neurotrophin receptor p75 contributes to neuronal hyperreactivity and airway inflammation in a murine model of experimental asthma

Bronchial asthma represents a severe chronic inflammatory disease with increasing prevalence. The pathogenesis is characterized by complex neuroimmune dysregulation. Although the immunopathogenesis of the disease has been extensively studied, the nature of neuronal dysfunction still remains poorly understood. Recent data indicate that neurotrophins contribute to airway inflammation, broncho-obstruction and airway hyperresponsiveness. Using an established murine model of allergic bronchial asthma, the contribution of the pan-neurotrophin receptor p75(NTR) was defined. This receptor is expressed both in normal and asthmatic lungs and airways. Analysis of p75(NTR-/-) mice, as well as in vivo blocking of p75(NTR), revealed that airway inflammation is to a large extent dependent upon functional receptor expression. Furthermore, neuronal hyperreactivity depends entirely on this receptor. Based on these data, a novel molecular pathway in the neuroimmune pathogenesis of bronchial asthma could be defined.

The role of neurotrophins in bronchial asthma

Allergic bronchial asthma is characterized by chronic inflammation of the airways, development of airway hyperreactivity and recurrent reversible airway obstruction. Target and effector cells responsible for airway hyperresponsiveness and airway obstruction include sensory and motor neurons as well as epithelial and smooth muscle cells. Although it is well established that the inflammatory process is controlled by T-helper (Th) 2 cells and the Th2-derived cytokines interleukin-4, airway hyperresponsiveness-5 and interleukin-13, the mechanisms by which immune cells interact with neurons, epithelial cells or smooth muscle cells still remain uncertain. Since there is growing evidence for extensive communication between neurons and immune cells, the mechanisms of this neuro-immune crosstalk in lung and airways of asthmatic patients are recently becoming the focus of asthma research. Neurotrophins represent candidate molecules regulating and controlling this crosstalk between the immune and peripheral nervous system. They are constitutively expressed by resident lung cells and produced in increasing concentrations by immune cells invading the airways under pathological conditions. They modify the functional activity of sensory and motor neurons, leading to enhanced and altered neuropeptide and tachykinin production. These effects are defined as "neuronal plasticity". The consequences are the development of "neurogenic inflammation" due to neuropeptide and tachykinin activities.

Neurotrophins in bronchial asthma

Allergic bronchial asthma (BA) is characterized by chronic airway inflammation, development of airway hyperreactivity and recurrent reversible airway obstruction. T-helper 2 cells and their products have been shown to play an important role in this process. In contrast, the mechanisms by which immune cells interact with the cells residing in lung and airways, such as neurons, epithelial or smooth muscle cells, still remains uncertain. Sensory and motor neurons innervating the lung exhibit a great degree of functional plasticity in BA defined as "neuronal plasticity". These neurons control development of airway hyperresponsiveness and acute inflammatory responses, resulting in the concept of "neurogenic inflammation". Such quantitative and/or qualitative changes in neuronal functions are mediated to a great extent by a family of cytokines, the neurotrophins, which in turn are produced by activated immune cells, among others in BA. We have therefore developed the concept that neurotrophins such as nerve growth factor and brain-derived neurotrophic factor link pathogenic events in BA to dysfunctions of the immune and nervous system.

In vivo survival requirement of a subset of nodose ganglion neurons for nerve growth factor

The sensory neurons of the nodose ganglion are the classic example of a population of peripheral nervous system neurons that do not require nerve growth factor (NGF) for survival during development but are dependent on other neurotrophins. We have re-examined this assertion by studying the development of the nodose ganglion of mice that have a null mutation in the NGF gene. Compared with wild-type embryos, the number of neurons undergoing apoptosis was elevated in NGF -/- mice, resulting in a significant reduction in the total number of neurons in the ganglion by the end of embryonic development. TrkA, the NGF receptor tyrosine kinase, was expressed in the nodose ganglion throughout development and there was a marked decrease in TrkA mRNA expression in the nodose ganglion of NGF -/- embryos. Although the in vitro survival of the majority of nodose neurons was promoted by brain-derived neurotrophic factor (BDNF), a minor proportion was supported by NGF in cultures established over a range of embryonic stages. These results clearly demonstrate that a subset of nodose ganglion neurons depends on NGF for survival during development. The finding that the expression of tyrosine hydroxylase (TH) mRNA was unaffected in the nodose ganglia of NGF-deficient embryos indicates that this NGF-dependent subset is distinct from the subset of catacholaminergic neurons in the nodose ganglion.

The coexistence of TrkA with putative transmitter agents and calcium-binding proteins in the vagal and glossopharyngeal sensory neurons of the adult rat

The presence of the neurotrophin receptor, TrkA, in neurochemically identified vagal and glossopharyngeal sensory neurons of the adult rat was examined. TrkA was colocalized with calcitonin gene-related peptide (CGRP), parvalbumin, or calbindin D-28k in neurons of the nodose, petrosal and/or jugular ganglia. In contrast, no TrkA-immunoreactive (ir) neurons in these ganglia colocalized tyrosine hydroxylase-ir. About one-half of the TrkA-ir neurons in the jugular and petrosal ganglia contained CGRP-ir, whereas only a few of the numerous TrkA-ir neurons in the nodose ganglion contained CGRP-ir. Although 43% of the TrkA-ir neurons in the nodose ganglion contained calbindin D-28k-ir, few or no TrkA-ir neurons in the petrosal or jugular ganglia were also labeled for either calcium-binding protein. These data show distinct colocalizations of TrkA with specific neurochemicals in vagal and glossopharyngeal sensory neurons, and suggest that nerve growth factor (NGF), the neurotrophin ligand for TrkA, plays a role in functions of specific neurochemically defined subpopulations of mature vagal and glossopharyngeal sensory neurons.

Coexistence of s100beta and putative transmitter agents in vagal and glossopharyngeal sensory neurons of the rat

The coexistence of S100beta with calcitonin gene-related peptide (CGRP), substance P (SP), somatostatin (SOM), nicotinamide adenosine dinucleotide phosphate-diaphorase (NADPH-d), and tyrosine hydroxylase (TH) was examined in the glossopharyngeal and vagal sensory ganglia. S100beta immunoreactive (-ir) neurons in the jugular and petrosal ganglia frequently colocalized CGRP- or SP-ir, whereas S100beta-ir neurons in the nodose ganglion infrequently contained CGRP- or SP-ir. No S100beta-ir neurons in the jugular and petrosal ganglia showed SOM-ir while the small number of SOM-ir neurons in the nodose ganglion colocalized S100beta-ir. Many neurons in the nodose ganglion colocalized S100beta-ir and NADPH-d activity, whereas S100beta-ir neurons in the jugular and nodose ganglia infrequently contained NADPH-d activity. S100beta- and TH-ir were frequently colocalized in nodose ganglion but not in petrosal or jugular ganglion neurons. These findings suggest relationships between S100beta and specific putative transmitters in functions of subpopulations of vagal and glossopharyngeal sensory neurons.

Peptidergic innervation of the rat cornea

Corneal nerves regulate corneal epithelial integrity, proliferation, and wound healing. The mechanisms by which the nerves mediate their effects remain poorly understood; however, the release of biologically active neuropeptides has been hypothesized. The purpose of the current investigation was to determine the relative densities, distribution patterns, and origins of rat corneal nerves containing each of eight different neuropeptides, calcitonin gene-related peptide (CGRP), substance P (SP), galanin (GAL), neuropeptide Y (NPY), methionine-enkephalin (M-ENK), vasoactive intestinal polypeptide (VIP), somatostatin (SOM), and cholecystokinin (CCK). In the first set of experiments, immunohistochemical demonstrations of the above neuropeptides were performed on free-floating corneal sections cut tangential to the corneal surface. The results showed that six of the peptides, CGRP, SP, GAL, NPY, M-ENK, and VIP were present in rat corneal nerves. The innervation patterns of corneal nerves containing each of these six peptides were then documented by mapping all fibers in serial sections from select corneal quadrants onto a series of line drawings by using a drawing tube. In the second set of experiments, the origins of the corneal peptidergic nerve fibers were determined by selective ocular denervations. Unilateral combined sensory and sympathetic ocular denervations or unilateral sympathetic ocular denervations were performed in adult rats by transecting the ophthalmomaxillary nerve and/or removing the superior cervical ganglion. After 5-7 days, each of the ipsilateral corneas was sectioned and processed immunohistochemically for the presence of one of the six peptides found in experiment one, and the fibers that survived the ocular denervations were plotted onto line drawings. Ocular denervations revealed that corneal peptidergic nerves have sensory (CGRP, SP, and GAL), sympathetic (NPY), and parasympathetic (GAL, NPY, M-ENK, and VIP) origins. The results of this investigation have shown that the peptidergic innervation of the rat cornea is more extensive and complex than previously reported. This is the first investigation to show the presence of GAL in the rat cornea, and the first to demonstrate the presence of NPY-, VIP-, and M-ENK-IR nerve fibers in the cornea of any species.

Identification and neuropeptide content of trigeminal neurons innervating the rat nasal epithelium

Trigeminal ganglia provides sensory innervation to the rat nasal cavity. The purpose of this study was to identify the location and characterize the neuropeptide content of trigeminal neurons that project specifically to the rat nasal epithelium. The right nasal cavity was instilled with 4 microliters of rhodamine-labelled latex microspheres. Seven, 10 or 14 days after tracer instillation, both trigeminal ganglia were removed and prepared for substance P and calcitonin gene-related peptide immunocyto-chemistry. neurons labelled with microspheres were located in the trigeminal ganglia at the division of the ophthalmic and maxillary nerves and were only found on the side ipsilateral to the instillation. The percentage of labelled cell body profiles in the right trigeminal ganglia averaged 1.61 +/- .04% at seven days. 1.54 +/- .01% at 10 days, and 1.65 +/- .02% at 14 days after instillation. These values were not statistically different, but the fluorescence intensity in labelled neurons was increased after 14 days. The right trigeminal ganglia contained a mean of 166 +/- 13.81 labelled cell body profiles representing 1.60 +/- .09% of the total. Of the labelled profiles, 81.6 +/- 3.27% were immunoreactive for substance P and 35.2 +/- 4.00% for calcitonin gene-related peptide. Sections of nasal mucosa showed that the microspheres were localized only in the epithelial layer and did not enter the lamina propria indicating that the microsphere transport was confined to nerve fibres present in the epithelial layer. These studies have identified specific neurons in the trigeminal ganglia that project to the nasal epithelium. Most of the neurons retrogradely labelled from the nasal cavity contained substance P but some contained calcitonin gene-related peptide, suggesting that the nasal epithelium is predominantly targeted by substance P-containing neurons.

A mechanism for sympathectomy-induced bone resorption in the middle ear

BACKGROUND

Recent investigations have demonstrated a link between sympathectomy and osteoclast-mediated bone resorption. The exact nature of this link, however, is unknown. We hypothesize that substance P, a potent vasoconstrictive neuropeptide found in peripheral sensory fibers, including those innervating bone, is the mediator of this phenomenon. To test this theory, the effects of substance P on in vitro calcium release from cultured neonatal mouse calvaria were assessed. In addition, an in vivo study was conducted whereby gerbils were injected with capsaicin to eliminate substance P-containing fibers before sympathectomy with 6-hydroxydopamine. If the effects of 6-hydroxydopamine were eliminated by prior administration of capsaicin, the role of sensory nerves in sympathectomy-induced resorption would be strongly implicated.

IN VITRO STUDY

Substance P at 10(-8) mol/L was incubated with eight newborn Swiss-Webster mouse hemicalvarial explants and compared with explants incubated in control media alone. The neonatal mice were euthanized at day 3, and their hemicalvaria were preincubated in 2 ml of stock media without treatment for 24 hours at 36.5 degrees C as a stabilization period. After the stabilization period, the stock media were replaced with 2 ml of fresh control media or media containing substance P at 10(-8) mol/L. A similar experiment was performed with the addition of indomethacin at 5 x 10(-7). The explants were then incubated for 72 hours with gassing every 12 hours with a mixture of O2, N2, and CO2. At the end of the 72-hour period, the media were analyzed for calcium content by atomic absorption spectrophotometry and compared by one-way analysis of variance with Bonferroni-corrected post hoc tests.

IN VIVO STUDY

Forty-eight Mongolian gerbils were placed into four groups: group 1 received intraperitoneal injections of 6-hydroxydopamine at 75 micrograms/gm body weight on days 1, 2, 6, 7, and 8; group 2 received identical injections of hydroxydopamine, but 12 hours after receiving subdermal injections of capsaicin at 50 micrograms/gm body weight; group 3 received only subdermal injections of capsaicin; and group 4 received only saline injections to serve as controls. Seven days after treatment, the animals were euthanized, and the ventral wall of each animal's right bulla was resected and quantified for osteoclast number and surface with a computer-based histomorphometry system. Analysis was then made by one-way analysis of variance with Bonferroni-corrected post hoc tests.

RESULTS

The results of the in vitro study revealed that substance P at 10(-8) mol/L (11.05 +/- 3.37 micrograms/ml) induced significant calcium release from cultured neonatal mouse calvaria when compared with control bone incubated in base media alone (0.92 +/- 2.85 micrograms/ml, p < 0.01). The process was completely inhibited by 5.0 x 10(-7) indomethacin. The results of the in vivo study showed 6-hydroxydopamine treatment significantly increased both the osteoclast number (NOc/TL = 3.14 +/- 1.33/mm) and the osteoclast surface (OcS/BS = 16.04% +/- 6.95%) of bone when compared with bone from saline-treated controls (NOc/TL = 1.77 +/- 0.79/mm, p < 0.01; OcS/BS = 8.88% +/- 4.15%, p < 0.01). These 6-hydroxydopamine-induced increases were eliminated, however, in animals pretreated with capsaicin before sympathectomy (NOc/TL = 1.86 +/- 0.68/mm, p > 0.05; OcS/BS = 9.92 +/- 3.73, p > 0.05), whereas treatment with capsaicin alone had no effect when compared with bone from saline-treated controls (NOc/TL = 2.02 +/- 0.50/mm, p > 0.05; OcS/BS = 10.28% +/- 2.62%, p > 0.05). Substance P has thus been shown to induce calcium release from membranous bone in vitro, whereas capsaicin, a substance P-specific sensory neurolytic chemical, eliminates the in vivo osteoclast-inductive effects of 6-hydroxydopamine when given 12 hours before treatment. The results indicate that substance P is capable of inducing resorption and that substance P-containing sensory ne

Hypertonicity, but not hypothermia, elicits substance P release from rat C-fiber neurons in primary culture

Isocapnic dry gas hyperventilation provokes hyperpnea-induced bronchoconstriction in guinea pigs by releasing tachykinins from airway sensory C-fiber neurons. It is unknown whether dry gas hyperpnea directly stimulates C-fibers to release tachykinins, or whether this physical stimulus initiates a mediator cascade that indirectly stimulates C-fiber tachykinin release. We tested the hypotheses that mucosal hypothermia and/or hyperosmolarity--physical consequences of airway heat and water loss imposed by dry gas hyperpnea--can directly stimulate C-fiber tachykinin release. Neurons isolated from neonatal rat dorsal root ganglia were maintained in primary culture for 1 wk. Cells were then exposed for 30 min at 37 degrees C to graded concentrations of NaCl, mannitol, sucrose, or glycerol (0-600 mOsm) added to isotonic medium, or to isotonic medium at 25 degrees C without or with 462 mOsm mannitol added. Fractional release of substance P (SP) was calculated from supernatant and intracellular SP contents following exposure. Hyperosmolar solutions containing excess NaCl, mannitol, or sucrose all increased fractional SP release equivalently, in an osmolarity-dependent fashion. In marked contrast, hypothermia had no effect on fractional SP release under isotonic or hypertonic conditions. Thus, hyperosmolarity, but not hypothermia, can directly stimulate tachykinin release from cultured rat sensory C-fibers. The lack of effect of glycerol, a solute which quickly crosses cell membranes, suggests that neuronal volume change represents the physical stimulus transduced by C-fibers during hyperosmolar exposure.

Inhibition of axoplasmic transport in the rat vagus nerve alters the numbers of neuropeptide and tyrosine hydroxylase messenger RNA-containing and immunoreactive visceral afferent neurons of the nodose ganglion

Previous work showed that axotomy-induced deafferentation of the placode-derived visceral afferent neurons of the nodose ganglion altered their expression of some neuropeptides and tyrosine hydroxylase. The present studies were designed to selectively evaluate the loss of axonal transport on the numbers of vasoactive intestinal polypeptide, tyrosine hydroxylase, and calcitonin gene-related peptide mRNA-containing and immunoreactive neurons in the nodose ganglion of the adult rat. Vinblastine (0.15 mM) application to the cervical vagus nerve was used to block axonal transport between ganglionic perikarya and peripheral targets. In situ hybridization histochemistry with 35S-labeled oligonucleotide probes was used to both quantify the number of mRNA-containing neurons and to assess the density of mRNA expression per neuron, and immunocytochemistry was used to visualize the number of immunoreactive neurons. The efficacy of vinblastine to inhibit axonal transport was verified by evaluating the build-up of calcitonin gene-related peptide immunoreactive in the vagus nerve immediately rostral to the site of drug application. The absence of vinblastine-induced neuronal damage was verified by the relative absence of degenerating nerves in the vagus nerve caudal to the site of drug application. Vinblastine treatment of the vagus nerve increased the numbers of vasoactive intestinal peptide mRNA-containing neurons and vasoactive intestinal peptide-immunoreactive neurons in the nodose ganglion at three, seven and 14 days, and increased the numbers of calcitonin gene-related peptide mRNA-containing and calcitonin gene-related peptide-immunoreactive neurons in the nodose ganglion at one, three and seven days. The average labeling density of vasoactive intestinal peptide mRNA-containing neurons was also increased following vinblastine treatment. Vinblastine treatment of the cervical vagus nerve, however, led to the appearance of low-labeling density calcitonin gene-related peptide mRNA-neurons and resulted in reduction of the average labeling density for calcitonin gene-related peptide mRNA-containing neurons. In contrast, application of vinblastine to the cervical vagus nerve, decreased the number of tyrosine hydroxylase mRNA-containing and tyrosine hydroxylase-immunoreactive neurons in the nodose ganglion. In summary, inhibition of the axoplasmic transport between the periphery and the visceral sensory perikarya appeared to alter vasoactive intestinal peptide, calcitonin gene-related peptide, and tyrosine hydroxylase expression and content in visceral sensory neurons of the nodose ganglion. These data suggest the presence of an axonally transported influence on the regulation of neuropeptide and neurotransmitter enzyme synthesis in mature placode-derived visceral sensory neurons.

Alteration of nerve growth factor-receptor expression in the periodontal ligament of the rat during experimental tooth movement

This pilot study deals with the initial responses of neural elements showing nerve growth factor-receptor (NGFR) immunoreactivity in the periodontal ligament of rats to orthodontic forces. The animals were killed at intervals of 1, 2, 6, 12 h and 1, 3, 5, 7 days after the insertion of elastic bands between the maxillary first and second molars. Serial frozen sections, prepared from each animal, were processed immunohistochemically to demonstrate NGFR, after which the periodontal ligament of the mesial root of the maxillary second molar was examined microscopically. In control sections, NGFR-positive neural elements were predominantly distributed at the apex of the bony socket on the distal side of the periodontal ligament. After 1 h of tooth movement, NGFR positively-stained nerve fibres tended to disappear slightly in both the intermediate and coronal regions of the distal periodontal ligament. By the third day of tooth movement, the periodontal ligament nerve fibres showed more intense NGFR-immunoreactivity; thick, positively stained nerve fibres were recognized on the distal side in which active bone remodelling was occurring, and a few of these fibres were densely distributed around blood vessels or near Howship's lacunae. The distribution of NGFR-positive neural elements on the mesial side increased at 5 days of tooth movement. Finally, at 7 days of tooth movement, staining intensity for NGFR appeared to decrease. These findings demonstrate that an alteration in the distribution and the intensity of immunoreactive staining for NGFR in the periodontal ligament is associated with the bone remodelling induced by orthodontic tooth movement.

Bilateral irradiation of head and neck induces an enhanced expression of substance P in the parasympathetic innervation of the submandibular gland

Substance P and calcitonin gene-related peptide (CGRP) are present in nerve fibers innervating the submandibular gland. Radiotherapy of tumors in the head and neck region usually embraces the salivary glands in the irradiated field and consequently a dramatic decrease in salivary function is seen. In this study, the submandibular glands and ganglia of rats subjected to fractionated irradiation were examined by use of immunohistochemical techniques for demonstration of substance P and CGRP. Irradiation was given on five consecutive days (daily doses of 6-9 Gray) with unilateral or bilateral irradiation techniques. Specimens of control and experimental animals were processed in parallel. A marked increase in the expression of substance P in the ganglionic cells--presumably parasympathetic--and in the number of fibers showing substance P-like immunoreactivity in association with acini and small ducts was seen in response to bilateral irradiation. (Surprisingly, unilateral irradiation of the parotid area had no effect on peptide distribution in the irradiated gland and ganglion). No changes in the pattern of CGRP immunoreactivity occurred. In the trigeminal ganglion, which supplies the submandibular gland with the majority of the sensory substance P- and CGRP-containing nerve fibers, no changes in the expression of substance P or CGRP immunoreactivity were seen. The results suggest that bilateral irradiation leads to an increase in the synthesis of substance P-like substance in the parasympathetic ganglionic cells supplying the submandibular gland with secretory nerves, and can thus be an additional factor in explaining the altered secretory capacity of salivary glands.

Altered expression of NGF and P75 NGF-receptor by fibroblasts of injured teeth precedes sensory nerve sprouting

Profuse sprouting of sensory nerve fibers occurs in tooth pulp by 1-4 days following dentin injury. A possible role for nerve growth factor (NGF) in that neural response is suggested here by the demonstration that NGF mRNA and protein are increased 6 hr after injury to adult rat molars. The enhanced expression of NGF mRNA was localized to fibroblasts underlying the injury. A concomitant depletion of mRNA encoding the 75 Kd NGF receptor (NGFR) was observed in those fibroblasts. The increase in NGF mRNA was transitory and mRNA levels fell below normal levels by 2 days after injury. Both NGF and NGFR mRNA remained low thereafter in injured pulp. The inverse shifts in fibroblastic mRNA encoding NGF and NGFR were not affected by prior denervation of the tissue, or by pretreatment with dexamethasone. The regulatory mechanisms therefore must involve endogenous, non-neuronal, non-inflammatory factors that are released in response to injury.

Light and electron microscopic distribution of nerve growth factor receptor-like immunoreactivity in the skin of the rat lower lip

Nerve growth factor receptor distribution in the skin of the adult rat was studied by immunocytochemistry with the use of the monoclonal antibody 192-IgG. Immunoreaction occurred in a patchy pattern in the epidermis and outer root sheaths of hair follicles, as well as in nerves and in capsulated and non-capsulated sensory receptors. Electron microscopic analysis revealed that the areas of patchy epithelial immunostaining corresponded to the plasma membrane of groups of keratinocytes, which were always associated with intra-epithelial nerve fibres. Immunostaining was also associated with Merkel cells, hair follicle receptors, and the capsular cells of capsulated receptors. In the nerves, immunostaining was detected in perineurial cells and axons. After sensory denervation of the skin, immunostaining decreased considerably in the epithelia, as well as in the small nerves, although short segments of increased staining were observed in the latter. In larger nerves, immunostaining was markedly increased and mainly associated with newly formed Schwann cell processes. After sympathetic denervation, the pattern of immunostaining was identical to controls, except for augmented immunoreactivity in short segments of degenerating nerves around blood vessels and smooth muscle fibres. From these observations, it is suggested that the epithelial and perineurial cells synthesize and express nerve growth factor receptors, while receptors present in nerve fibres originate from the nerve cell bodies in the gasserian ganglion.

Capsaicin-induced neuronal degeneration in the brain and retina of preweanling rats

Capsaicin is a neurotoxin known for its ability to cause degeneration of small unmyelinated primary sensory neurons in both spinal and cranial nerves. Although lower motor neurons do not degenerate following capsaicin treatment, the extent to which capsaicin may damage neurons in the brain has not been thoroughly evaluated. This study examines the effects of systemic capsaicin (50-150 mg/kg) on the central nervous system of 10-day-old rats. Rat pups were injected with capsaicin or the injection vehicle and sacrificed 6 hours-10 days later. Brains, spinal cords, and retinas were stained with cupric silver to label degenerating neurons. As previously reported for capsaicin-treated rats, degenerating nerve terminals were present in areas receiving primary afferent input: the spinal cord dorsal horn, spinal trigeminal nucleus, nucleus of the solitary tract, and area postrema. However, degenerating terminals were also present in areas not known to receive primary sensory innervation: the inferior olivary nucleus, sphenoid nucleus, medial and olivary pretectal nuclei, interpeduncular nucleus, interfascicular nucleus, caudal linear, dorsal, median, and paramedian raphe nuclei, supramammillary area, lateral habenula, ventrolateral geniculate nucleus, ventral reuniens nucleus, ventromedial hypothalamic nucleus, lateral hypothalamic and preoptic areas, suprachiasmatic nucleus, septohypothalamic nucleus, bed nucleus of the stria terminalis, lateral septal nucleus, accumbens shell, olfactory bulb, and retina. Some areas where capsaicin caused degeneration in rat pups do not appear to be capsaicin-sensitive in adult rats. Results indicate that (1) capsaicin's neurotoxicity is not limited to primary sensory neurons and (2) developmental factors may alter the capsaicin sensitivity of some neuronal projections within the brain.

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.