Structural and functional studies of the effects of parasympathetic nerve stimulation on rabbit submandibular salivary glands

Muscarinic receptor subtypes in the submandibular gland and the urinary bladder of the rabbit: in vivo and in vitro functional comparisons of receptor antagonists

1. In pentobarbitone-anaesthetized rabbits, the inhibitory effects of muscarinic receptor antagonists with different selectivity profiles were examined on carbachol-evoked submandibular secretion and urinary bladder contractions, and on parasympathetically nerve-evoked secretion. On isolated submandibular gland fragments, the inhibitory effects of the antagonists were studied on carbachol-evoked release of potassium and on the overflow of tritium in response to electrical field stimulation. 2. In vivo, 4-DAMP equipotently inhibited simultaneously carbachol-evoked submandibular secretory and contractile responses of the urinary bladder, while pirenzepine was found to be four times as potent in inhibiting the secretory response compared with the contractile response. 3. The inhibition of carbachol-evoked salivation caused by atropine, 4-DAMP and pirenzepine was as great as their inhibition of parasympathetic nerve-evoked salivation. Methoctramine exerted less inhibitory effect on nerve-evoked salivation than on carbachol-evoked, thus seemingly causing greater presynaptic inhibition. 4. In vitro, pirenzepine was only 30 times less potent in inhibiting carbachol-evoked potassium release than 4-DAMP (pA2, 9.58 vs 8.10). Whereas atropine, 4-DAMP and pirenzepine abolished the overflow of tritium from isolated glands in response to electrical field stimulation, methoctramine increased it. 5. It is concluded that the muscarinic secretory response in the rabbit submandibular gland is exerted via both muscarinic M1 and M3 receptors, while the contractile response of the urinary bladder to muscarinic agonists is exerted via muscarinic M3 receptors. The release of acetylcholine from nerve terminals in the gland can be inhibited via M2 autoreceptors in rabbits.

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.

Morphological changes in the rabbit submandibular gland after parasympathetic or sympathetic denervation

Selective denervation was used to obtain further information about the relative roles of the nerves supplying the gland. Parasympathetic denervation soon caused atrophy in both acinar and granular tubule cells, and a substantial reduction in gland wet weight. The secretory cells had several unusual features, and striated duct cells tended to accumulate glycogen. Three to four weeks after pre-ganglionic parasympathectomy, the parenchymal cells were less abnormal but still smaller than in the control, unoperated gland. Thus, parasympathetic impulses are required to maintain normal parenchymal cells. Chronic sympathectomy caused little or no detectable changes in the secretory cells, and there was no significant change in wet weight. Nevertheless, pre-ganglionic sympathetic axotomy reduced the reflex secretion of acinar mucosubstances in response to feeding. Thus, sympathetic impulses normally contribute to the reflex secretion of acinar mucosubstance from these glands.

Structural and functional studies of the effects of sympathetic nerve stimulation on rabbit submandibular salivary glands

Continuous sympathetic stimulation at 8-10 Hz caused intense vasoconstriction in the gland, so stimulation was generally given in an interrupted pattern to minimize this detrimental effect on secretion. Only a small increase in fluid secretion occurred; it became thick and tended to block the cannula; therefore in later experiments the main duct was not cannulated. After sympathetic stimulation there was substantial degranulation of acinar cells. However, as this was accompanied by little movement of water, the secreted mucosubstance distended the ductal lumina. The granular tubule cells were unchanged by sympathetic stimulation. Use of selective blocking agents revealed that the sympathetically-evoked secretion of acinar mucin was mediated mainly via beta-adrenoreceptor activation. As stimulation of the sympathetic nerves alone caused little additional formation of fluid, the effects of superimposing continuous low frequency sympathetic stimulation onto a background of low parasympathetic secretion were compared with similar parasympathetic stimulation alone of the contralateral gland. These double nerve stimulations did not augment the volume of fluid secreted, or cause morphological changes additional to those from parasympathetic stimulation alone. Nevertheless, it is likely that, under natural reflex conditions, sympathetic impulses can increase the amount of acinar mucosubstance secreted.

The proper role of nerves in salivary secretion: a review

Consideration of the history of this subject has created a perspective which helps one to appreciate how errors in thinking have occurred and why some have persisted, despite subsequent improvements in information. The most enduring misconception has been that the drying of the mouth under stress is due to sympathetic inhibitory fibers, a view that must be eradicated, for such fibers do not exist. The inhibition is due to central influences from higher centers acting on the salivary centers and thereby suppressing reflex activity. Wide variations exist in the neuro-effector arrangements and in the cellular responses in different glands from different species. Myoepithelial cells are usually contracted by both parasympathetic and sympathetic nerves. The blood vessels also receive a dual innervation, but parasympathetic impulses cause vasodilatation as part of secretion, whereas the sympathetic vasoconstrictor fibers are part of a more generalized vascular control system and not a direct part of the reflex secretory sympathetic pathway. Parasympathetic drive usually provides the main stimulus for fluid formation by parenchymal cells, whereas sympathetic nerves tend to increase the output of pre-formed components from certain cells. Absence of nerve impulses causes variable atrophic and other metabolic effects on the parenchymal cells. Evidence is beginning to accrue that certain nerve impulses may influence resynthetic activities. Vacuolation, often found experimentally after strong stimulation, also occurs to a variable extent in certain cells as a normal part of reflex secretion and may therefore have an effect on the components entering the saliva. If rupture of vacuoles occurs, then this may contribute to the salivary amylase that is present in the blood. Recent evidence points to the possibility that, even in a monomorphic gland, not all of the components necessarily enter nerve-induced saliva in "parallel" proportions. Consideration of current information has enabled some provisional generalizations about the roles of the secretory nerves to be suggested in the summary, but they must not be considered immutable.