CHARACTERIZATION OF ALKALINE PHOSPHATASE-REACTIVE NEURONS IN THE GUINEA-PIG SMALL-INTESTINE

Endogenous alkaline phosphatase activity has been localized histochemi cally on the surface of enteric neurons of the guinea-pig small intest ine by both light and electron microscopy. The enzyme activity was ass ociated with some myenteric neurons that had Dogiel type I morphology, and the histochemical reaction products typically formed a honeycomb- like structure on labelled cell bodies. No Dogiel type II neurons in t he myenteric plexus or submucous neurons showed alkaline phosphatase r eactivity. Nerve fibres reactive for alkaline phosphatase were present in the myenteric plexus and ran in bundles in the circular muscle and deep muscular plexus. In addition, reactive varicose axons supplied t he submucous plexus and non-ganglionated plexus of the mucosa. The res ults of interruption of the enteric neuronal pathways demonstrated tha t alkaline phosphatase-reactive myenteric neurons project anally to ot her myenteric ganglia, to the circular muscle and to the submucous ple xus. Sequential enzyme histochemistry showed that virtually ail alkali ne phosphatase-reactive neurons also contained nitric oxide synthase, revealed by NADPH-diaphorase reactivity. It was estimated that 14-18% of all myenteric neurons showed alkaline phosphatase reactivity. About one-third of nitric oxide synthase-containing myenteric neurons, howe ver, did not contain alkaline phosphatase activity. At the ultrastruct ural level, alkaline phosphatase activity was associated specifically with the plasma membranes of nerve cell bodies, axons and dendrites of some myenteric neurons. Reactive nerve fibres made close appositions with non-reactive submucous neurons and, within myenteric ganglia, pre dominantly with other alkaline phosphatase-reactive neurons. In additi on to its presence in neurons, alkaline phosphatase reactivity was als o present in some endothelial cells in blood vessels in the submucosa and in capillary pericytes. It is concluded, on the basis of the proje ctions and neurochemistry, that in the guinea-pig small intestine alka line phosphatase activity is associated with nitric oxide synthase-con taining neurons which include inhibitory motor neurons to the circular muscle, and anally-directed interneurons to other myenteric and submu cous neurons.