Fall in density, but not number of myenteric neurons and circular muscle nerve fibres in guinea‐pig colon with ageing

In guinea‐pig ileum, ageing has been associated with a decrease in enteric neurons. This study examined guinea‐pig colon and measured changes in gut dimensions, neuron size, density and ganglionic area. Changes in motor nerve fibres in the circular muscle were also measured. Myenteric neurons in whole‐mount preparations of mid‐colon from 2‐week, 6‐month, and 2‐year‐old guinea‐pigs were labelled immunohistochemically with the neuronal marker human neuronal protein HuC/HuD, and numbers of neurons mm−2, neuronal size, ganglionic area mm−2, gut length, circumference and muscle thickness were measured. Corrected numbers of neurons mm−2 and ganglionic area mm−2 accounting for growth of the colon were calculated. Additionally, nerve fibres in circular muscle cross‐sections were labelled with antibodies against nitric oxide synthase (NOS) and substance P (SP) and the density of nerve fibres in circular muscle was measured. The numbers of neurons mm−2 decreased by 56% (from 2 weeks to 2 years) with no change in neuron size. Total neuron numbers decreased by 19% (P = 0.14) when adjusted for changes in length and circumference with age. The percentage area of NOS‐ and SP‐immunoreactive (IR) nerve fibres in the circular muscle decreased (P