Cholinergic, somatostatin-immunoreactive interneurons in the guinea pig intestine: morphology, ultrastructure, connections and projections.

The shape, projection, ultrastructure and chemistry of interneurons that were initially identified by their immunoreactivity for somatostatin in the small intestine of the guinea pig were examined. Somatostatin immunoreactive nerve cell bodies and nerve fibres were located in the myenteric plexus. Simultaneous labelling for 2 antigens revealed that the somatostatin immunoreactive interneurons were also immunoreactive for choline acetyltransferase, but not for calbindin or neuropeptide Y. Cell shapes were determined by immunohistochemistry, ultrastructural analysis and intracellular dye filling. The neurons had large cell bodies (38 x 14 microns) which gave rise to long branched filamentous dendrites and a single axon. The projections of the somatostatin immunoreactive interneurons were determined by analysis of patterns of fibre loss and survival following degenerative section of myenteric nerve pathways and by analysis of individual neurons that were injected intracellularly with dye. The axons projected anally, sometimes after a short oral excursion, and were confined to the myenteric plexus. They gave rise to multiple varicose branches within myenteric ganglia: in the majority of cases the first branch was within 100 microns of the cell body. Synaptic inputs to the cells were examined by light and electron microscopy. All somatostatin immunoreactive neurons received numerous somatostatin immunoreactive inputs on the cell body and all filamentous processes. Ultrastructural investigation indicated that these constituted the majority of all inputs. It is concluded that cholinergic, somatostatin immunoreactive, interneurons have a unique soma morphology and form synaptically connected chains that run anally in the myenteric plexus.