NO in SON contributes to salivary secretion in aged rats

Objectives: The age-related inhibition of salivary secretion has been demonstrated in rats, and the nitric oxide (NO) present in the supraoptic nucleus (SON) has been reported to play an inhibitory role in the regulation of salivary secretion. In the present study, we investigated the age-related changes occurring in the NO synthase (NOS)-expressing neurons in the SON, which is related to the production of NO, and discussed the interrelation between the age-related changes in the NOS-expressing neurons and the age-related inhibition of salivary secretion.

Methods: Nissl staining and reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry were performed for SON in young adult and aged rats. Using a computer-assisted image processing system, the cell number and cell size of the Nissl-stained and the NADPH-d-positive neurons were examined, and the reaction density of the NADPH-d-positive neurons was also measured.

Results: In the young adult group, NADPH-d-positive neurons were found mainly in the dorsal part of the SON, and very few neurons with weak NADPH-d activity were observed in the ventral part of the SON. Further, in the aged group, many neurons with strong NADPH-d activity were observed throughout the entire SON. It was clarified that the number of the Nissl-stained neurons did not change, and that significant age-related increases were detected in the number of cells, cell size and reactive density of the NADPH-d-positive neurons (n=6, p<0.001). Therefore, the production of NO in the SON neurons increased with age.

Conclusion: We concluded that the age-related increase in the NO in the SON might be a factor that contributes to the age-related inhibition of salivary secretion.