THE PROJECTIONS OF 5-HYDROXYTRYPTAMINE-ACCUMULATING NEURONS IN THE MYENTERIC PLEXUS OF THE SMALL-INTESTINE OF THE GUINEA-PIG

Retrograde tracing, combined with immunohistochemistry, was used to st udy the projections of 5-hydroxytryptamine (5-HT)-accumulating neurone s within the ileum of the guinea-pig, with confocal microscopy being u sed to characterise further their morphology. Two classes of neurones in the myenteric plexus, capable of taking up 5-HT or analogues, were distinguished. One class had Dogiel type I morphology with lamellar de ndrites, was located on the edge or in the middle of ganglia and lacke d immunoreactivity for somatostatin (SOM). The other class had smooth ovoid cell bodies with multi pie filamentous dendrites and a single ax on and represented a subset of the SOM-immunoreactive interneurones in the myenteric plexus. Varicosities immunoreactive for 5-HT alone, 5-H T/SOM or SOM alone were present in the myenteric ganglia. Both classes of 5-HT-accumulating neurones had long aboral projections within the myenteric plexus (up to 100 mm long) and to the submucous plexus and p robably function as descending interneurones.