Adrenergic and cholinergic nerves of bovine, guinea pig and hamster salivary glands. A light and electron microscopic study

Tyrosine-hydroxylase, dopamine β-hydroxylase and choline acetyltransferase-like immunoreactive fibres in the human major sublingual gland

OBJECTIVE

To study the innervation of the major sublingual gland by means of immunohistochemistry.

DESIGN

Bioptic and autoptic specimens of the major sublingual gland of humans were examined for the presence of immunoreactivity to tyrosine hydroxylase and dopamine-β-hydroxylase, on one hand, and choline acetyltransferase, on the other, to indicate adrenergic and cholinergic nerves, respectively.

RESULTS

Acini and ducts were supplied by both divisions of the autonomic nervous system.

CONCLUSIONS

Mucous and seromucous cells of the human major sublingual glands may respond with secretion not only to parasympathetic activity but also to sympathetic activity. The major sublingual gland is therefore a potential contributor to the mucin secretion recently reported in the literature in response to high sympathetic activity during physical exercise.

Physiology, Pathology and Regeneration of Salivary Glands

Salivary glands are essential structures in the oral cavity. A variety of diseases, such as cancer, autoimmune diseases, infections and physical traumas, can alter the functionality of these glands, greatly impacting the quality of life of patients. To date, no definitive therapeutic approach can compensate the impairment of salivary glands, and treatment are purely symptomatic. Understanding the cellular and molecular control of salivary glands function is, therefore, highly relevant for therapeutic purposes. In this review, we provide a starting platform for future studies in basic biology and clinical research, reporting classical ideas on salivary gland physiology and recently developed technology to guide regeneration, reconstruction and substitution of the functional organs.

Confocal laser scanning microscopic analysis of ectopic sublingual gland-like tissue inside the hamster submandibular gland

Based on its histochemical properties, the secretory portion of the hamster submandibular gland has been classified as seromucous cells. The presence of endogenous peroxidase (PO) reaction was shown in the nuclear envelope, cisternae of endoplasmic reticulum and Golgi apparatus. The 3,3'-diaminobenzidene, tetrahydrochloride (DAB) method revealed bipartite secretory granules containing a PO-positive dense core surrounded by a less dense halo in these cells. In the present investigation, serous and mucous-like cells were found in resin-embedded semi-thin sections of the DAB-reacted hamster submandibular gland. These sections were already on glass slides for routine light microscopic observations, therefore electron microscopic analysis could be unrealizable. We then used reflectance-mode confocal laser scanning microscopy to visualize additional sites of PO activity as detected in these sections. Using this approach, we found mucous cells with PO activity-negative secretory granules and seromucous cells with PO activity-positive spot-like secretory granules of the regular sublingual gland most frequently adjacent to the serous cells with typical electron-dense secretory granules. These cells clearly differ from the seromucous cells with bipartite secretory granules and the granular duct cells with typical electron-dense secretory granules of the hamster submandibular gland. Additionally, secretory endpieces of the ectopic sublingual gland-like tissue empty into the duct of the hamster submandibular gland lobule. Thus, our findings suggest that a mass of sublingual gland tissue extends into the hamster submandibular gland during its development, and PO may be synthesized and secreted into the same duct.

Distribution of sympathetic and afferent neurones innervating the submandibular gland in the sheep

The distribution of sympathetic and sensory neurones innervating the submandibular gland (SMG) in sheep was studied using retrograde tracing technique. The retrograde tracer Fast Blue (FB) was unilaterally injected into the SMG in five juvenile male sheep under general anaesthesia. After a 4-week survival period, all the animals were reanaesthetized, perfused transcardially with 4% buffered paraformaldehyde and ganglia, which could be considered as a potential sources of sympathetic, and afferent innervation of the gland were bilaterally collected. The FB-labelled sympathetic neurones were found in the ipsilateral superior and middle cervical ganglion. Many labelled neurones were distributed in the ipsilateral jugular and nodose ganglia of the vagus nerve and smaller numbers of the nerve cells were also found in ipsilateral C1-C3 dorsal root ganglia (DRG). No labelled neurones were observed in the ipsilateral stellate ganglion, trigeminal ganglion, C4-C8 DRG and in all contralateral ganglia. The present study revealed that the majority of sympathetic neurones projecting to the sheep SMG are found in the superior cervical ganglion but some of them are also distributed in the middle cervical ganglion. Most of the afferent neurones are located in the jugular and nodose ganglia of vagus nerve but C1-C3 DRG also comprise some of these nerve cells.

Ascorbate deficiency impairs the muscarinic-cholinergic and ss-adrenergic receptor signaling systems in the guinea pig submandibular salivary gland

Ascorbic acid is preferentially concentrated in the hypothalamus, pituitary and adrenal glands. Its level in the acini of salivary glands is relatively high. We therefore hypothesized that ascorbate may have a role in salivary gland function. Ascorbate-deficient guinea pigs had lower stimulated whole salivary flow rates than well-fed, age-matched controls (P: < 0.005). Total salivary protein concentration was also markedly (P: < 0.005) reduced in the deficient guinea pigs. SDS-PAGE and densitometric quantification of protein bands confirmed significant reduction in specific salivary proteins (e.g., amylase, proline-rich proteins) in the saliva samples of malnourished guinea pigs. Some protein bands not seen in control saliva were detected in the saliva of malnourished guinea pigs. Ascorbate deficiency also produced a significant (P: < 0.005) reduction in the ss-adrenergic receptor density (subtype 1; 95 +/- 19 fmol/mg protein compared with 179 +/- 27 fmol/mg protein for the controls). No significant difference was observed between the two groups with respect to the ss-adrenergic receptor subtype 2. Additionally, ascorbate-deficient guinea pigs had significantly lower muscarinic-cholinergic receptor densities (50 +/- 5 vs. 74 +/- 8 fmol/mg protein for controls). Our data support the conclusion that diminished membrane receptors might impair the capacity of the transmembrane signaling system, resulting in salivary gland hypofunction in ascorbate-deficient guinea pigs. Without implying extrapolation of our findings in experimental animals to humans, it is perhaps relevant that many conditions often associated with salivary gland hypofunction in humans (e.g., smoking or drug ingestion) deplete cellular ascorbate.

Innervation and myoepithelial arrangements in the submandibular salivary gland of ferret investigated by enzyme, catecholamine and filament histochemistry

Although the submandibular gland of ferret is useful for studying salivary secretory processes which are regulated by nerves and involve myoepithelial activity, little attention has been paid to its parenchymal innervation and myoepithelial arrangements. Therefore, glands obtained postmortem from mature ferrets of both sexes were here examined with the use of light-microscopic histochemical techniques for cholinesterases, phosphatases and phosphorylase, histofluorescence for catecholamines, and milling dyes. Acetylcholinesterase staining was associated with nerve trunks in the interlobular stroma and an extensive intralobular network of nerve fibres, presumably of a cholinergic type, embracing acini and ducts. There were fewer fibres containing fluorescing catecholamines, presumably adrenergic. They were largely associated with acini. Numerous stellate cells with fine branching processes embracing acini, presumably myoepithelial cells, and a few spindle-shaped basal cells, investing striated ducts, were demonstrated on frozen tissue by alkaline phosphatase, but not by adenosine triphosphatase, inosine diphosphatase and phosphorylase. Cells of similar shape and distribution were also demonstrated by staining with milling dyes on fixed tissues, indicating possibly a filamentous constituent conferring mechanical stability and/or contractile ability. Together, these results suggest, firstly, that a cholinergic-type parenchymal innervation is prominent in the submandibular gland of ferret, although many adrenergic nerves are also present, and, secondly that the gland has a very extensive myoepithelial network which is possibly involved in membrane transport, and the support and or contraction of the secretory parenchyma.

Effects of cholinergic and adrenergic agonists on the secretion of fluid and protein by submandibular glands of the guinea-pig and the mouse

Significant differences were observed between the guinea-pig and the mouse in terms of the secretion of fluid, protein and secretory granules from submandibular glands in response to pilocarpine, phenylephrine and isoproterenol. In both the guinea-pig and the mouse, the secretory responses induced by pilocarpine, phenylephrine and isoproterenol were inhibited by pretreatment with 4-DAMP, phentolamine and propranolol, respectively. The results suggest that the submandibular glands of the guinea-pig and the mouse have M3-cholinoreceptors, as well as alpha- and beta-adrenoceptors, and that these receptors play different roles in the secretion of fluid, protein and secretory granules from guinea-pig and mouse submandibular glands.

The innervation of salivary glands as revealed by morphological methods

Salivary secretion is nerve mediated. The salivary glands are supplied by parasympathetic and sympathetic efferent nerves which travel to the glands by separate routes. Once in the glands the axons from each type of nerve intermingle and travel together in association with Schwann cells, forming Schwann-axon bundles. Two types of neuro-effector relationships exist with salivary parenchymal and myoepithelial cells: epilemmal (outside the parenchymal basement membrane) and hypolemmal (within the parenchymal basement membrane). Their relative frequencies with either type of nerve differ greatly between glands and species. Salivary blood vessels receive epilemmal innervations by both sympathetic and parasympathetic axons. The classical transmitters--acetylcholine in parasympathetic and noradrenaline in sympathetic axons--are stored in small vesicles. A variety of non-conventional neuropeptide transmitters have also been found in salivary nerves by immunohistochemistry, and they occur in large dense-cored vesicles. Prolonged high frequency stimulation has been found to cause depletion of large dense-cored vesicles from glandular nerves. In recent years afferent nerves have started to be identified and are found in greatest numbers around the main salivary ducts, where they may form a hypolemmal association with the epithelial cells. Functional studies demonstrate complex interactions between parasympathetic and sympathetic nerves. Morphological assessments of changes in the parenchymal cells after nerve stimulations or denervations add greatly to our understanding of the nerve functions. At least four types of influence can be exerted on salivary parenchymal cells by the nerves: hydrokinetic (water mobilizing), proteokinetic (protein secreting), synthetic (inducing synthesis), and trophic (maintaining normal functional size and state). In respect to each role, wide glandular and species differences exist between the relative contributions made by each type of nerve.

Nerve interactions in salivary glands

In the salivary reflex, not only secretory cells are activated, but also myo-epithelial cells are contracted to support these cells and promote the flow of saliva, and blood vessels dilate to meet the increased demands of the tissues. The various effector cells often receive nerves from both parts of the autonomic system, and interactions may occur when the nerves act on the same type of effector, or on different types of effectors. While in an experiment electrical stimulation of the sympathetic trunk may decrease a parasympathetic salivary flow by causing marked vasoconstriction, this does not occur in the salivary reflex, since the vasoconstrictors do not take part. On the contrary, the normal sympathetic vasoconstrictor tone of the resting gland is easily overcome by activity in parasympathetic vasodilator nerves when secretion starts. Pronounced synergism can be demonstrated between sympathetic and parasympathetic secretory nerves. In dogs, for instance, in which sympathetic secretion is beta-adrenoceptor-mediated, this is marked in the case of fluid secretion. In rats and rabbits, in which beta-receptors elicit secretion of amylase, the potentiating interaction among the nerves is striking when amylase secretion is considered. Even the random release of acetylcholine from the post-ganglionic parasympathetic axons, by itself insufficient to evoke secretion, can increase the sympathetic effects. Motor nerves interact with secretory nerves by causing myo-epithelial contraction, mechanically promoting secretion. Interactions between the nerves in their long-term regulatory function on the sensitivity of the acinar secretory and myo-epithelial cells can also be demonstrated.

Adrenergic effects on secretion of epidermal growth factor from Brunner's glands

The influence of the sympathetic nervous system and adrenergic agonists on flow rate and secretion of epidermal growth factor (EGF) from Brunner's glands has been investigated in the rat. Chemical sympathectomy by administration of 6-hydroxydopamine increased volume secretion and output of EGF from Brunner's glands but depleted the glands of EGF. Infusion of noradrenaline, an alpha-adrenergic agonist, inhibited basal and vasoactive intestinal polypeptide (VIP) stimulated flow rate and output of EGF from Brunner's glands and increased the amount of EGF in the tissue. Vasoactive intestinal polypeptide also increased the amount of EGF in Brunner's gland tissue and this was unchanged after simultaneous infusion of VIP and noradrenaline as well as VIP and isoproterenol, a beta-adrenergic agonist. Isoproterenol had no effect on basal and VIP stimulated secretion of EGF from Brunner's glands. The presence of PAS-positive mucus in Brunner's glands was unchanged during infusion of noradrenaline whereas VIP induced a depletion of Brunner's gland mucus which in turn was prevented by simultaneous infusion of noradrenaline. This study indicates that the sympathetic nervous system influence the volume secretion, output of EGF and mucus content in Brunner's glands probably by activation of alpha-adrenergic pathways.

Adrenergic effects on exocrine secretion of rat submandibular epidermal growth factor

The present study was undertaken to investigate the effect of alpha- and beta-adrenergic agonists on secretion of epidermal growth factor (EGF) from the rat submandibular glands and to test the possibility of intestinal absorption of EGF. Alpha-adrenergic agonists increased the concentration of salivary EGF by approximately a hundred times, while the serum concentration of EGF was unchanged. The contents of EGF in the submandibular glands decreased upon administration of the alpha-adrenergic agonist noradrenaline, and this was confirmed on immunohistochemical investigation of the glands. Beta-adrenergic agonists had no effect on secretion of EGF from the submandibular glands. Intestinal absorption of EGF could not be confirmed, as stimulation by noradrenaline with free passage of saliva to the gastrointestinal tract and intrajejunal infusion of EGF had no influence on the concentration of EGF in serum. This study shows that alpha-adrenergic agonists stimulate exocrine secretion of submandibular EGF and that EGF in physiological amounts are not absorbed in the gastrointestinal tract.

Autonomic innervation of salivary glands in the armadillo, anteater, and sloth (Edentata)

The intraglandular distribution of adrenergic and cholinergic nerve fibers was studied histochemically in the parotid, mandibular, and sublingual glands of six species of edentates belonging to the three families that comprise the order; namely the Dasypodidae (armadillos), the Myrmecophagidae (anteaters), and the Bradipodidae (sloths). The following histochemical techniques were used: (a) acetylcholinesterase reaction for the demonstration of cholinergic fibers; (b) formaldehyde- and glyoxylic acid-induced fluorescence for the demonstration of adrenergic fibers. In addition, norepinephrine (NE) was assayed fluorimetrically in the mandibular and parotid glands of the armadillo. A network of acetylcholinesterase-positive nerve fibers surrounds the intra- and interlobular ducts and endpieces of all glands; it is of low density in the mandibular and sublingual gland of the sloth, of high density in the sublingual gland of the anteater and of moderate density in the remaining glands. A vascular cholinergic innervation occurs in all salivary glands. Although present around the vessels, adrenergic new fibers were virtually absent from the parenchyma of all glands, even after in vitro incubation of glandular tissue with NE or after administration of NE to armadillos previously treated with a monoamine oxidase (MAO) inhibitor. Consistent with this fact, the amount of NE present in the parotid and mandibular gland of the armadillo was extremely low. These findings may indicate that the salivary secretion in the edentates is regulated by the parasympathetic rather than by the sympathetic nervous system.

Electrophysiology of mouse parotid acini: effects of electrical field stimulation and ionophoresis of neurotransmitters

1. Intracellular micro-electrode recordings of membrane potential and input resistance were made from surface acini of mouse parotid glands placed in a Perspex tissue bath through which oxygenated physiological saline solutions were circulated. The acinar cells were stimulated by microionophoresis of both acetylcholine (ACh) and adrenaline (Ad) from extracellular micropipettes, and by electrical field stimulation via a pair of platinum electrodes. 2. The acinar cells had a mean resting membrane potential of -64.9 mV +/- 0.6 S.E. The input resistance of the unstimulated cell was 4.63 M omega +/- 0.19 S.E. In a number of cells spontaneous miniature depolarizations were observed, associated with synchronous reductions in input resistance. 3. The responses to ionophoresis of both ACh and Ad and the response to supra-maximal field stimulation were identical. Stimulation always evoked a marked decrease in input resistance associated with an initial potential change, generally followed by a delayed hyperpolarization during which the input resistance returned to normal. 4. Field-stimulation responses could be evoked to single shock (1-2 msec) and to low frequency (1-4 Hz) stimulation. The latency for this response was 245 msec +/- 12 S.E. 5. The field-stimulation response was shown to be susceptible to blockade of nerve conduction in sodium-free or tetrodotoxin- (TTX-) containing media; and to blockade of neurotransmitter release in calcium-free media. 6. The field-stimulation and ACh responses were recorded at different levels of membrane potential within the same cells by applying either hyperpolarizing or depolarizing direct current through the recording electrode. The membrane potential at which the initial potential change undergoes reversal, i.e. changes from a depolarization to a hyperpolarization, is known as the equilibrium or reversal potential, EFS and EACh respectively. The field-stimulation (FS) and ACh responses underwent simultaneous reversal at about -60 mV, i.e. EFS = EACh. Equilibrium potentials were also determined indirectly by analysis of the responses evoked by each stimulus in the manner described by Trautwein & Dudel (1958). Using this technique the equilibrium potentials of the responses to all three stimuli, field stimulation, ACh and Ad, were again about -60mV, i.e. EFS = EACh = EAd. 7. Both the field-stimulation and ACh responses were abolished by atrophine (10(-6) M) while the response to Ad persisted. Atropine also abolished all spontaneous activity. The alpha-adrenergic blocker phentolamine (10(-5) M) abolished the response to Ad but left the field-stimulation response unaffected. 8. Electrical field stimulation of isolated segments of salivary gland evoked release of endogenous neurotransmitter as a consequence of neural excitation. The technique of field stimulation thus makes it possible to investigate the functional innervation of a gland using the in vitro preparation. In the mouse parotid gland the field stimulus response was mediated by ACh released from parasympathetic nerve endings.

A histochemical study of catecholamines and cholinesterases in the autonomic nerves of the human minor salivary glands

The cholinergic and adrenergic innervation of human minor sublingual, buccal and labial salivary glands has been studied with histochemical techniques for localizing acetylcholinesterase and catecholamines. A rich cholinergic innervation was observed around the acini, blood vessels and some ducts of the three glands. The adrenergic innervation, however, was virtually absent from the parenchyma although present around the blood vessels, in marked contrast to the dense parenchymal adrenergic innervation observed in the human parotid and submandibular glands. These results suggest that the autonomic nervous mechanism which regulates salivary secretion is more elaborate in the major than in the minor salivary glands.

Innervation of the ventral prostate of the rat

The autonomic innervation of the rat ventral prostate was studied in an attempt to evaluate the role of innervation in the normal function of the gland. Specific histochemical methods for both catecholamines (the formaldehyde-induced fluorescence method, FIF) and acetylcholinesterases (the Gomori-Koelle thiocholine method) were used. The neuro-effector contacts were studied by electron microscopy using both 3% glutaraldehyde and 3% potassium permanganate (KMnO4) as fixatives. It was found that the rat ventral prostate receives dual autonomic innervation. Adrenergic fibers, which formed the majority of the nerves, were often seen in close contact with the smooth muscle cells around both the prostatic alveoli and secretory ducts. The non-adrenergic nerve fibers, which were fewer in number, did not form such intimate contacts with the muscle cells. No direct synapses with epithelial cells were detected.

Light and electron microscopic observations of the autonomic innervation of the mouse gallbladder mucosa. A histochemical, cytochemical, and secretory study

The autonomic innervation of the mouse gallbladder mucosa was studied using histo- and cytochemical methods. In a light microscopic investigation the distribution of acetylcholinesterase (AChE) activity and formaldehyde-induced fluorescence was studied histochemically. Nerve fibres and small varicosities showed concentrations of AChE activity very close to the epithelium in the subepithelial connective tissue. No adrenergic nerves were observed in the mucosa. When using the electron microscope and employing the potassium permanganate fixation/staining technique only one sort of axonal enlargement was encountered, viz. the cholinergic type. These varicosities contained numerous agranular vesicles (500-600 A in diameter). No varicosities of the adrenergic (dense-cored vesicles) type were observed. Signs of increased secretory activity in the epithelium were observed in the first few minutes after cholinergic stimulation. After repeated in vivo stimulation, there was an almost total depletion of glycoprotein granules, best seen when using the cytochemical PA-CrA-silver technique. The findings suggest that the subepithelial connective tissue and the epithelium of the mouse gallbladder mucosa have a cholinergic innervation.

An electron microscopic study on the autonomic innervation of the rabbit parotid gland

As visualized in the electron microscope, the parotid gland of the rabbit has a dual innervation. Both adrenergic and cholinergic nerves are equally distributed in the parenchyma and often run together within the same nerve bundle. Nerve terminals are observed not only subjacent to the basement membrane but interposed between the latter and the acinar cells, where they establish a close membrane to membrane contact with the latter.

Comparison of the secretory processes in the parotid and sublingual glands of the mouse. 1. Regulation of the secretory processes

1. Secretion from the mucous sublingual gland of the mouse has been investigated and compared with the serous parotid gland. The influence of acetylcholine, noradrenalin and adrenalin on the secretion of glycoproteins (e.g. mucins) and proteins (e.g. amylase) from these glands in vitro, and the involvement of cyclic AMP and Ca2+ has been studied. 2. Secretion from the parotid gland could be stimulated by both acetylcholine and the catecholamines. It appears that cyclic AMP plays an important role in the adrenergic secretory process, but not in the cholinergic-induced secretion. In the latter case, exogenous Ca2+ strongly increased the secretion. 3. Mucin secretion from the sublingual gland could be affected by acetylcholine in the presence of exogenous Ca2+. Noradrenalin and adrenalin induced only a slow mucin secretion and, for this secretory process, exogenous Ca2+ is also required. Though cyclic AMP is present in the sublingual gland, no influence on its level could be detected in this gland after stimulation of the adrenergic beta-receptor, whereas, in contrast to the parotid gland, dibutyryl cyclic AMP induced only a slow secretion. Because it was observed that the sublingual gland of the mouse is not innervated sympathetically, it seems reasonable to suppose that the catecholamines stimulate the mucin secretion from this gland via hormonal receptors and not via the adrenergic beta-receptor. 4. The protein secretion from the sublingual gland could be stimulated by both acetylcholine and the catecholamines. An involvement of cyclic AMP in this process was not observed. Addition of exogenous Ca2+ is less important, as was found for the mucin secretion. So it has been concluded that protein and mucin secretion from the sublingual gland are regulated via different pathways.