QUANTITATIVE-ANALYSIS OF INPUTS TO SOMATOSTATIN-IMMUNOREACTIVE DESCENDING INTERNEURONS IN THE MYENTERIC PLEXUS OF THE GUINEA-PIG SMALL-INTESTINE

Somatostatin immunoreactivity occurs in a specific subgroup of choline rgic descending interneurons in the myenteric plexus of the guinea-pig small intestine. In the present work, we made light- and electron-mic roscopic investigations of chemically defined inputs to these neurons, in order that the origins of the connections of other neurons with th em could be deduced. Somatostatin-immunoreactive synapses and close co ntacts were found on the cell bodies and filamentous processes of soma tostatin neurons; these were 84% of all inputs. It is thus confirmed t hat this class of interneuron forms chains that project anally. Descen ding interneurons with immunoreactivity for nitric oxide synthase prov ided 14% of inputs to somatostatin-immunoreactive descending interneur ons. An antiserum against a calcium-binding protein, calbindin, was us ed as marker for the majority of intrinsic primary afferent neurons, A H/Dogiel type II neurons; this class of neurons provided only 2.5% of the inputs to somatostatin-immunoreactive descending interneurons. We conclude that somatostatin-immunoreactive descending interneurons are involved in the conduction of impulses distally along the full length of the small intestine, but receive only a minor input from calbindin- immunoreactive primary afferent neurons.