Electron microscopic immunocytochemistry has been used to study the distribution and synaptic relationships of substance P (SP)-immunoreactive nerve fibers in ultrathin sections from whole-mount preparations of myenteric ganglia from guinea pig small intestine. At the light microscopic level, myenteric ganglia stained for ultrastructural study contained dense arrays of SP-immunoreactive nerve fibers around and between the neuronal and glial cell bodies. At the electron microscopic level, SP-containing nerve fiber profiles occurred throughout the neuropil of the myenteric ganglia. Both vesicle-containing and nonvesiculated nerve fiber profiles were immunoreactive. The positive vesiculated profiles contained variable proportions of small clear and large granular vesicles. Two-thirds of the vesicle-containing nerve fiber profiles in myenteric ganglia were immunoreactive for SP. Many vesiculated SP-immunoreactive nerve fiber profiles were directly apposed to each myenteric neuron. However, only about 0.6% of these vesiculated profiles formed synapses that showed the ultrastructural specializations of vesicle clustering presynaptically and fuzzy electron-dense material on the postsynaptic membrane. On the other hand, morphological identifiable synapses with SP-immunoreactivity comprised about half the total number of synapses in the ganglia. SP-immunoreactive synapses were observed on nonvesiculated nerve processes in the periphery of the ganglia and on nerve cell bodies. Most of the axosomatic SP synapses occurred on small neurons that lay either on the surfaces of the ganglia or near the fibers of the internodal strands where they traveled through the ganglia. Both SP-positive and SP-negative nerve cell bodies received SP synapses. These results show that synapses in which the presynaptic element is SP-immunoreactive make up a high proportion of the total number of morphologically identifiable synapses in guinea pig myenteric ganglia. They suggest that, although all myenteric neurons are directly contacted by SP-immunoreactive varicosities, a subpopulation of myenteric neurons preferentially receives SP synapses that show clear pre- and postsynaptic specializations.
|Number of pages||8|
|Journal||Journal of Neuroscience|
|Publication status||Published - 1 Jan 1989|