Survival of the population of NADPH-diaphorase stained myenteric neurons in the small intestine of aged rats.

NADPH-diaphorase staining was performed on wholemount preparations of the muscularis externa (which includes the myenteric plexus) of the rat small intestine taken from animals of 4, 24 and 30 months of age. A population of NADPH-diaphorase-stained neuron cell bodies was observed predominantly in the ganglia of the plexus. There were also many stained axons throughout the plexus and in the muscularis at all ages. The distribution of stained neurons around the circumference of the intestine was the same at all ages, being greater on the mesenteric than on the anti-mesenteric aspect. The numbers of NADPH-diaphorase-positive neurons in the myenteric plexus were counted by a systematic random sampling method. At 4 months there were 3716 +/- 219 stained neurons per cm2. There was no difference in the neuron numbers between the 24- and 30-month animals but they were decreased in number by 15.01% in comparison to the 4-month rats when growth in the length of the intestine was taken into account. This reduction in neuron numbers is markedly less than that previously recorded for the total numbers of myenteric neurons in the aged rat small intestine suggesting that the NADPH-diaphorase neurons are relatively spared in old age. The density of NADPH-diaphorase-positive varicose axons in the myenteric plexus and in the muscularis appeared to be slightly greater in the aged animals. These results provide evidence that the majority of the population of the myenteric NADPH-diaphorase-stained neurons, which may play a part in relaxation of the gut, survive in old age.