ULTRASTRUCTURAL EXAMINATION OF THE TARGETS OF SEROTONIN-IMMUNOREACTIVE DESCENDING INTERNEURONS IN THE GUINEA-PIG SMALL-INTESTINE

Serotonin neurons are descending interneurons in the myenteric plexus of the guinea pig small intestine. Preembedding single- and double-lab el immunocytochemistries at the ultrastructural level were used to ide ntify the targets of these serotonin interneurons. Serial ultrathin se ctions were taken through a myenteric ganglion that had been processed for serotonin immunocytochemistry. The ganglion contained the cell bo dies of 69 neurons, including 2 serotonin neurons and 6 neurons with t he ultrastructural features of Dogiel type II cells. For each cell bod y in the ganglion, the number of serotonin inputs (synapses and close contacts) was determined. About 50% of the cell bodies did not receive any serotonin inputs. The most abundant serotonin terminals were rela ted to two targets: other serotonin descending interneurons and a popu lation of neurons with Dogiel type I morphology, but whose neurochemis try and function is unknown. The serotonin inputs to the serotonin cel l bodies were located predominantly on the lamellar dendrites. Each of the Dogiel type II neurons received 3 or fewer serotonin inputs, and none of the serotonin inputs to Dogiel type II neurons formed a synaps e. Overall, about 40% of the serotonin inputs formed synapses. The ser otonin inputs to neurons that received many serotonin inputs were more likely to show synaptic specializations than serotonin inputs to neur ons that received few serotonin inputs. Inhibitory motor neurons conta in nitric oxide synthase (NOS). At the light microscope level, seroton in nerve fibers do not form dense pericellular baskets around NOS cell bodies. To determine whether there are serotonin inputs to NOS neuron s, serial ultrathin sections were taken through a myenteric ganglion t hat had been processed for preembedding double-label immunocytochemist ry, in which the NOS neurons were labeled with peroxidase-diaminobenzi dine and the serotonin neurons with silver-intensified 1 nm gold. Only 1 out of 9 NOS cells examined in serial section received more than 5 serotonin inputs. The results suggest that, in the guinea pig small in testine, the serotonin descending interneurons are not an essential el ement of the descending inhibitory reflex. (C) 1995 Wiley-Liss, Inc.