Absence of tyrosine hydroxylase activity and dopamine β-hydroxylase immunoreactivity in intrinsic nerves of the guinea-pig ileum

The activity of tyrosine hydroxylase and the localization of dopamine β-hydroxylase were determined in the myenteric and submucous plexuses of the normal guinea-pig ileum and in these plexuses after extrinsic denervation. In the normal ileum, the distribution of axons showing immunoreactivity for dopamine β-hydroxylase was not distinguishable from the distribution of noradrenergic axons determined by the fluorescence histochemical localization of catecholamines. The distribution of tyrosine hydroxylase in the different layers of the intestine correlated well with the distribution of dopamine β-hydroxylase and noradrenaline, tyrosine hydroxylase activity being most concentrated in the myenteric and submucous plexuses. Extrinsic denervation resulted in the complete disappearance of both biochemically detectable tyrosine hydroxylase and immunohistochemically demonstrable dopamine β-hydroxylase. It is concluded that if the amine-handling neurons which are known to be intrinsic to the intestine synthesize an aromatic amine, it is almost certainly not a catecholamine, and is probably an indoleamine.