The terminal nerve of dolphins: gross structure, histology and luteinizing-hormone-releasing hormone immunocytochemistry.

The terminal nerve (TN) of several dolphins was examined using gross dissection aided by osmium staining, routine light and electron microscopy, and immunocytochemistry with antibodies to mammalian luteinizing-hormone-releasing hormone (LHRH). The TN consists of numerous pial strands which emanate from large paired ganglia located in the dura near the frontal lobe of the hemisphere. The strands are largely composed of myelinated axons which extend to basal forebrain areas including the anterior perforated substance. Peripheral branches of the ganglia run through foramina in the ethmoid bone into the region of the nasal sacs and blowhole. Round to oval ganglion cells are scattered along the nerve and thousands of similar cells are found in the dural ganglia where they are encapsulated by satellite cells. A second, less prevalent cell type is also found in the ganglia. These neurons are fusiform, lack a well-defined capsule and are LHRH-immunoreactive. The results are compared to observations of the anatomy and functions of the TN in other mammals, which unlike toothed whales have retained an olfactory system. Involvement in reproduction and control of secretions and/or circulation of the nasal sac vocalization system are suggested functions of the TN in dolphins.