Morphological and physiological investigation of the effect of age on the rat lacrimal gland

This study investigates the differences in morphology, innervation pattern and secretory responses in the lacrimal glands of 3-5 months old and aged (20-24 months old) rats. Morphological studies revealed not only large differences in structural damage and inflammatory cell infiltration between young and aged rats but also differences in acinar type. For young rat glands almost all the acini were serous with the presence of a few mucous and seromucous acini. However, in both 20 and 24 month old glands there were marked reductions in the presence of serous acini. Consequently, in 20 month old glands there were large increases in seromucous acini while in the 24 month old glands there were marked increases in mucous acini. Similarly, the glands of young animals have an abundance of VIP-positive nerves compared to aged rats in which the innervation densities were reduced. Aged rats appeared to have more histamine-immunoreactive cells than young rats. Stimulation of lacrimal acinar cells with either acetylcholine (ACh, 10^-13-10^-7 M), γ-amino-butyric acid (GABA, 10^-12-10^-5 M), adrenaline (10^-13-10^-5 M) or 5-hydroxytryptamine (5-HT, 10^-9-10^-4 M) for 30 min resulted in marked increases in total protein output and peroxidase secretion especially in young glands. In contrast, a significant (p < 0.01) reduction in total protein output and peroxidase secretion was observed from cells of 20 month old glands when compared to the output from the young glands. Moreover, total protein output and peroxidase secretion was further reduced from cells from 24 month old glands, with the exception of 5-HT where the total protein output and peroxidase secretion was the same from cells of both groups of aged glands. These results indicate that reduced total protein output and peroxidase secretion from rat lacrimal glands may be associated with changes in morphology including lacrimal acinar type and in innervation densities, and that these changes occur more rapidly as the rats mature between 20 and 24 months.