Innervation of the rat gastrointestinal sphincters: Changes during development and aging

Abstract

The effect of age on the adrenergic and peptidergic innervation of the lower oesophageal, pyloric and ileocaecal sphincters of the rat was investigated using immunohistochemical techniques. The distribution of nerve fibres containing the neuronal protein, growth associated protein-43, was also studied to determine the integrity of the enteric nervous system during development and aging. The four age groups examined were 2–3 days, 6 weeks, 3 months and 25 months old rats.Using protein gene product 9.5 antibody (a non-specific general neuronal marker), it was revealed that the myenteric ganglia in all sphincter regions were compactly arranged and were smaller in size at neonatal stage getting more spaced out and larger in size with age. There was no obvious change in the structure of the neuronal elements with age. In the lower oesophageal sphincter, calcitonin gene-related peptide- and substance P-like immunoreactive nerve fibres showed notable changes in density and fluorescence intensity with age, decreasing and increasing, respectively, with no obvious change in vasoactive intestinal polypeptide- and growth-associated protein-like immunoreactivity. A slight increase in dopamine-β-hydroxylase-like immunoreactivity was seen in old age. In the pyloric sphincter, there was an increase in calcitonin gene-related peptide- and substance P-like immunoreactivity with a less notable increase in dopamine-β-hydroxylase-like immunoreactivity. A decrease in vasoactive intestinal polypeptide- and growth-associated protein-43-like immunoreactivity in the circular muscle of the sphincter was seen in old age. In the ileocaecal sphincter there was a marked increase in growth associated protein-43-, vasoactive intestinal polypeptide-, dopamine-β-hydroxylase and substance P-like immunoreactivity. There was a decrease in the density of calcitonin gene-related peptide-like immuno-reactive nerve fibres in old age.In summary, two main conclusions can be drawn from the results of the present study. First, there was an age-related differential change in the density of immunoreactive nerve fibres containing various neuroactive substances. This indicates a level of plasticity of the various enteric nerve types and may reflect the degree of importance of the different neurotransmitters in the physiological activities of the specific sphincter. Second, in all three sphincters of aged rats, the density of nerve fibres containing the markers for the excitatory neurotransmitters noradrenaline and substance P (although the precise role of substance P in ileocaecal sphincter is not known) was increased, while the density of nerve fibres containing the inhibitory neurotransmitters calcitonin gene-related peptide in the lower oesophageal and possibly ileocaecal sphincter, and vasoactive intestinal polypeptide in the pylorus were decreased. This is likely to underlie the physiological activity of the sphincters and may be associated with malfunction in old age. Investigations to determine the functional implications of these changes would be of great interest.