SYNAPTIC-INTERACTIONS OF RETROGRADELY LABELED HYPOGLOSSAL MOTONEURONSWITH SUBSTANCE P-LIKE IMMUNOREACTIVE NERVE-TERMINALS IN THE CAT - A DUAL-LABELING ELECTRON-MICROSCOPIC STUDY

This study has investigated the synaptic interactions between hypoglos sal motoneurons and substance P (SP)-immunoreactive terminals. Cholera toxin B conjugated to horseradish peroxidase was injected into the ti p of the tongue on the right side of six ketamine-anesthetized cats. T wo to five days later, the animals were killed. Cells containing HRP w ere labeled with a histochemical reaction utilizing tetramethylbenzidi ne (TMB) as the chromogen. TMB forms crystalline reaction products tha t are very distinct at the electron microscopic level. The tissues wer e then processed for immunocytochemisty using an antiserum against SP. The chromogen used in this case, diaminobenzidine, yields amorphous r eaction products. At the light microscopic level, labeled cells were o bserved primarily ipsilaterally in both intermediate and ventrolateral subdivisions of the hypoglossal nucleus. The majority of these labele d cells were seen at the level of obex. At the electron microscopic le vel, both asymmetric and symmetric synapses were observed. SP-immunore active nerve terminals formed asymmetric synapses with labeled dendrit es and symmetric synapses with labeled perikarya. SP-labeled terminals also synapsed on unlabeled dendrites and somata. These are the first ultrastructural studies demonstrating synaptic interactions between hy poglossal motoneurons and SP terminals. These studies demonstrate that hypoglossal motoneurons that innervate intrinsic tongue muscles are m odulated by SP and that SP may play a role in the control of fine move ments of the tongue.