As the gut ages: Timetables for aging of innervation vary by organ in the Fischer 344 rat

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Abstract
To explore the effects of aging on the vagal innervation of the gastrointestinal (GI) tract, male Fischer 344 rats at 3 and 24 months of age were injected in the left nodose ganglion with 3 μl of either 4% wheat germ agglutinin‐horseradish peroxidase (to label sensory endings) or 1% cholera toxin subunit B‐horseradish peroxidase (to label motor endings). The stomach and duodenum were prepared as wholemounts and processed with tetramethyl benzidine. In addition, to study age‐related changes in the myenteric plexus, the stomachs, small intestines, and large intestines from 3‐, 12‐, 21‐, 24‐ and 27‐month‐old rats were prepared as wholemounts and processed with Cuprolinic Blue (to stain the neurons). Vagal afferent endings, motor terminal profiles, and myenteric neurons were counted and mapped with a sampling grid. In the stomach, both the vagal and myenteric innervation were stable between the ages of 3 and 24 months; however, a decrease in the number of myenteric neurons in the forestomach was noted at 27 months. In the small and large intestines, myenteric cell loss occurred by 12 months of age, progressed with age, and appeared to be governed by several general principles: (1) the rate of cell loss was organ‐specific, with a gradient of increasing severity from proximal to distal in the gut; (2) within organs of the GI tract, the rate of cell loss differed between regions; and (3) for given regions, cell losses progressed linearly with increasing age. The findings suggest that a positive relationship exists between the density of vagal extrinsic innervation and myenteric neuron survival; however, whether this results from the vagal innervation and/or other factor(s) protecting or rescuing myenteric neurons from age‐related attrition remains to be determined. J. Comp. Neurol. 434:358–377, 2001.