IMMUNOCYTOCHEMISTRY OF THYROTROPIN-RELEASING-HORMONE IN THE RABBIT MEDULLA-OBLONGATA

The distribution and ultrastructure of thyrotropin-releasing hormone-like immunoreactive (TRH-LI) neurons were examined in rabbit medulla oblongata. TRH-LI cell bodies were located in the ventral region of the medulla oblongata: in the paraolivary and parapyramidal regions, regions in and around the pyramidal tract, the dorsolateral region of the lateral reticular nucleus, and the raphe nuclei. The paraolivary and parapyramidal regions contained most of the TRH-LI cell bodies in the medulla oblongata. TRH-LI neuronal processes were densely distributed in the dorsal vagal complex and the area postrema. Electron-microscopic immunocytochemical studies revealed TRH-LI neurons at the obex level in the paraolivary regions of rabbits. TRH-like immunoreactivity was localized in large granular vesicles. TRH-LI somata and dendrites received synaptic inputs from both TRH-LI and unlabeled axon terminals. More than half of the TRH-Li axon terminals made synapses with somata or processes of TRH-LI neurons. These observations, together with previous reports that TRH causes respiratory facilitation, suggest that TRH-LI neurons in the paraolivary region in rabbits may be involved in respiratory functions.