Muscarinic signalling in submandibular salivary acinar cells of ageing rats

Association of a salivary acetylcholinesterase with Alzheimer's disease and response to cholinesterase inhibitors

OBJECTIVES

A decrease in cholinergic activity is a key event in the biochemistry of Alzheimer's disease (AD). The aim of the study was to investigate the expression levels of markers of cholinergic function in saliva, which is a readily accessible body fluid that can be obtained from subjects with minimal distress.

DESIGN AND METHODS

Salivary samples were obtained from people with NINCDS-ARDRA "probable" Alzheimer's disease and age- and sex-matched controls. Salivary acetylcholinesterase enzyme (AChE) activity was determined colorometrically.

RESULTS

Robust AChE catalytic activity was detected in the saliva samples that was stable for up to 6 h at room temperature following the provision of the salivary sample. The activity of the enzyme was significantly lower in people with AD than in age-matched controls. In addition, there were significant differences in activity between those who responded to acetylcholinesterase inhibitor (AChE-I) therapy and those who did not.

CONCLUSIONS

Salivary enzyme activity may therefore prove to be a useful marker of central cholinergic activity.

Muscarinic receptors: What we know

An understanding of muscarinic receptors is tantamount to an understanding of overactive bladder. The M(3) muscarinic receptor subtype is responsible for detrusor smooth muscle contraction and it exerts an exocrine function in the salivary glands. Alterations in the receptor's response to acetylcholine as a result of injury may lead to hypersensitivity and overactivity. The M(2) receptor subtype, which is mainly responsible for cardiac function, is the muscarinic receptor of highest proportion in the detrusor. M(2) also may play a role in detrusor contraction in injury and pathologic states. Muscarinic antagonists are the mainstay of pharmacotherapy for overactive bladder, but those that are available are not tissue specific. Growing knowledge of the nuances of receptor-ligand behavior and interaction between muscarinic receptors subtypes may provide novel targets for future drug development, improve efficacy, and reduce bothersome side effects.

Effects of age on morphology, protein synthesis and secretagogue-evoked secretory responses in the rat lacrimal gland

This study investigated changes in the morphology and protein synthesis and protein and peroxidase secretion due to peptidergic and aminergic stimulation from rat lacrimal gland acinar cells of 3-5, 9, 12, 20 and 24 month old rats. There was a marked reduction in the presence of Golgi apparatus in the acinar cells of glands from the 24 month old rats coupled to dilatation and degeneration of rough endoplasmic reticulum, when compared to that in the acinar cells of glands from 3-5 and 12 month old rats. Following incorporation of tritiated leucine for 360 min (6 h), the amount of newly synthesised protein in acinar cells of the 12 month old rats was significantly (p < 0.01) higher than that in the acinar cells of 3-5 month old animals. However, at 20 months the amount of newly synthesised protein in these acinar cells was significantly (p < 0.01) reduced to less than that in acinar cells of both the 3-5 and 12 month old animals. Immunohistochemical and immunofluorescence studies identified the presence of substance P (SP), vasoactive intestinal peptide (VIP), histamine and 5-hydroxytryptamine (5-HT) in the lacrimal glands of 3-5 month old rats. Stimulation by either SP, VIP, histamine or 5-HT resulted in significant increases in total protein output and peroxidase release from acinar cells of the 3-5 month old rats. However, all responses to the secretagogues were reduced with ageing from 3-5 to 24 months of age. The results indicate that ageing is associated with alteration in the ability of acinar cells to synthesise and secrete proteins.

The effect of ageing on parenchymal cell populations in adult female mouse submandibular gland

The submandibular gland shows an array of responses that accompany ageing, which are usually modest. However, the submandibular acinar-cell mucin shows a substantial decline in total amount per gland. In the submandibular gland, there is also a loss of secretory parenchyma. A number of factors that could influence either parenchymal loss or a change in its cellular composition were examined in three ages of adult female mice. The goal was to see if there are ageing-related cellular changes that might have an effect on mucin production or secretion. The factors examined included DNA, protein, rates of cell division and apoptosis, cell volume and cellular composition of the parenchymal population. The parenchymal cell composition showed significant differences during ageing, with a substantial decrease in the percentage of acinar cells and increases in the percentage of both types of ductal cell components. This decline in the proportion of acinar cells in the parenchyma also reflected an overall reduction in the total number of acinar cells in the gland. Thus, the change in proportions of cells may potentially be a direct cause of the ageing-related decline in the submandibular acinar-cell mucin. The alteration in cellular composition was not attributable to changes in the cell-division indices; however, there was an increased rate of apoptosis for acinar cells that was significantly different between 3 and 28 months. The apoptotic rate doubled for acinar cells but showed no significant change in ductal cells. This selective change in the rate of apoptosis with ageing suggests that it is one of the main reasons for the decline in the proportion of acinar cells in the submandibular gland.