A subpopulation of nervus terminalis neurons projects to the olfactory mucosa in Xenopus laevis

Cranial nerve 13

Contrary to popular belief, there are 13 cranial nerves. The thirteenth cranial nerve, commonly referred to as the nervus terminalis or terminal nerve, is a highly conserved multifaceted nerve found just above the olfactory bulbs in humans and most vertebrate species. In most forms its fibers course from the rostral portion of the brain to the olfactory and nasal epithelia. Although there are differing perspectives as to what constitutes this nerve, in most species GnRH-immunoreactive neurons appear to be its defining feature. The involvement of this trophic peptide, as well as the nerve's association with the development of the hypothalamic-pituitary-gonadal axis, suggest a primary role in reproductive development and, in humans, disorders such as Kallmann syndrome. In some species, this enigmatic nerve appears to influence sensory processing, sexual behavior, autonomic and vasomotor control, and pathogenic defense (via secretion of nitric oxide). In this review, we provide a general overview of what is known about this neglected cranial nerve, with the goal of informing neurologists and neuroscientists of its presence and the need for its further study.

Tract-tracing study of the extrabulbar olfactory projections in the brain of some teleosts

The extrabulbar olfactory projections (EBOP) is a collection of nerve fibers that originate from primary olfactory receptor neurons. These fibers penetrate into the brain, bypassing the olfactory bulbs (OBs). While the presence of an EBOP has been well established in teleosts, here we morphologically characterize the EBOP structure in four species each with a different morphological relationship of OB with the ventral telencephalic area. Tract-tracing methods (carbocyanine DiI/DIA and biocytin) were used. FMRFamide immunoreactive nervus terminalis (NT) components were also visualized to define any neuroanatomical relationship between the NT and EBOP. Unilateral DiI/DiA application to the olfactory chamber stained the entire olfactory epithelium, olfactory nerve fibers, and ipsilateral olfactory bulb. Labeled primary olfactory fibers running ventromedially as extrabulbar primary olfactory projections reached various regions of the secondary prosencephalon. Only in Moenkhausia sanctaefilomenae (no olfactory peduncle) did lipophilic tracer-labeled fibers reach the ipsilateral mesencephalon. The combination of tracing techniques and FMRFamide immunohistochemistry revealed a substantial overlap of the label along the olfactory pathways as well as in the anterior secondary prosencephalon. However, FMRFamide immunoreactivity was never colocalized in the same cellular or fiber component as visualized using tracer molecules. Our results showed a certain uniformity in the neuroanatomy and extension of EBOP in all four species, independent of the pedunculate feature of the OBs. The present study also provided additional evidence to support the view that EBOP and FMRFamide immunoreactive components of the NT are separate anatomical entities.

Ontogenesis of the extra-bulbar olfactory pathway in Xenopus laevis

Although the development, anatomy, and physiology of the vertebrate olfactory system are fairly well understood, there is still no clear definition of the terminal nerve complex acknowledged by all. Among the most debated matters is whether or not the extrabulbar projections found in anamniotes should or should not be considered part of the terminal nerve complex. In this context, we investigated the early development of the extrabulbar pathway in Xenopus larvae from placodal differentiation to postmetamorphic stages. We showed that the extrabulbar fibers become visible around Stage 42 and are conserved throughout metamorphosis. We confirmed previous reports concerning their central projection patterns. In addition, we showed that these fibers originate from two types of cell bodies located in the olfactory epithelium at premetamorphic stages. Furthermore, in postmetamorphic animals, we showed that the extrabulbar axons originated from both aquatic and aerial cavities. Retrograde tracing experiment also revealed densifications evocating cell bodies along the extrabulbar axons, distributed at different positions along the olfactory nerve depending on the stages of development. These densifications were observed closer to the periphery early in development and always closer to the olfactory bulb up to the metamorphic climax. We discuss these results in light of the latest theories and more recent reports.

Connections of the terminal nerve and the olfactory system in two galeomorph sharks: an experimental study using a carbocyanine dye

In elasmobranchs the terminal nerve courses separately from the olfactory nerve. This characteristic makes elasmobranchs excellent models to study the anatomy and function of these two systems. Here we study the neural connections of the terminal nerve and olfactory system in two sharks by experimental tracing methods using carbocyanine dyes. The main projections from the terminal nerve system (consisting of three ganglia in Scyliorhinus canicula) course ipsilaterally to the medial septal nucleus and bilaterally to the ventromedial telencephalic pallial region. Minor terminal nerve projections were also traced ipsilaterally to diencephalic and mesencephalic levels. With regard to the olfactory connections, our results show that in sharks, unlike ray-finned fishes, the primary olfactory projections are mainly restricted to the olfactory bulb. We also performed tracer application to the olfactory bulb in order to analyze the possible central neuroanatomical relationship between the projections of the terminal nerve and the olfactory bulb. In these experiments labeled neurons and fibers were observed from telencephalic to caudal mesencephalic regions. However, we observe almost no overlap between the two systems at central levels. The afferent and the putatively efferent connections of the dogfish olfactory bulb are compared with those previously reported in other elasmobranchs. The significance of the extratelencephalic secondary olfactory projections is also discussed in a comparative context.

Differential co-localization with choline acetyltransferase in nervus terminalis suggests functional differences for GnRH isoforms in bonnethead sharks (Sphyrna tiburo)

The nervus terminalis (NT) is a vertebrate cranial nerve whose function in adults is unknown. In bonnethead sharks, the nerve is anatomically independent of the olfactory system, with two major cell populations within one or more ganglia along its exposed length. Most cells are immunoreactive for either gonadotropin-releasing hormone (GnRH) or RF-amide-like peptides. To define further the cell populations and connectivity, we used double-label immunocytochemistry with antisera to different isoforms of GnRH and to choline acetyltransferase (ChAT). The labeling patterns of two GnRH antisera revealed different populations of GnRH-immunoreactive (ir) cell profiles in the NT ganglion. One antiserum labeled a large group of cells and fibers, which likely contain mammalian GnRH (GnRH-I) as described in previous studies and which were ChAT immunoreactive. The other antiserum labeled large club-like structures, which were anuclear, and a sparse number of fibers, but with no clear labeling of cell bodies in the ganglion. These club structures were choline acetyltrasferase (ChAT)-negative, and preabsorption control tests suggest they may contain chicken-GnRH-II (GnRH-II) or dogfish GnRH. The second major NT ganglion cell-type was immunoreactive for RF-amides, which regulate GnRH release in other vertebrates, and may provide an intraganglionic influence on GnRH release. The immunocytochemical and anatomical differences between the two GnRH-immunoreactive profile types indicate possible functional differences for these isoforms in the NT. The club-like structures may be sites of GnRH release into the general circulation since these structures were observed near blood vessels and resembled structures seen in the median eminence of rats.

3D atlas describing the ontogenic evolution of the primary olfactory projections in the olfactory bulb of Xenopus laevis

The adult Xenopus presents the unique capability to smell odors both in water and air thanks to two different olfactory pathways. Nevertheless, the tadpole can initially perceive only water-borne odorants, as the olfactory receptor neurons (ORN) that will detect air-borne odorants develop later. Such a phenomenon requires major reorganization processes. Here we focused on the precise description of the neuroanatomical modifications occurring in the olfactory bulb (OB) of the tadpole throughout metamorphosis. Using both carbocyanine dyes and lectin staining, we investigated the evolution of ORN projection patterns into the OB from Stages 47 to 66, thus covering the period of time when all the modifications take place. Although our results confirm previous works (Reiss and Burd [1997] Semin Cell Dev Biol 8:171-179), we showed for the first time that the main olfactory bulb (MOB) is subdivided into seven zones at Stage 47 plus the accessory olfactory bulb (AOB). These seven zones receive fibers dedicated to aquatic olfaction ("aquatic fibers") and are conserved until Stage 66. At Stage 48 the first fibers dedicated to the aerial olfaction constitute a new dorsomedial zone that grows steadily, pushing the seven original zones ventrolaterally. Only the part of the OB receiving aquatic fibers is fragmented, reminiscent of the organization described in fish. This raises the question of whether such an organization in zones constitutes a plesiomorphy or is linked to aquatic olfaction. We generated a 3D atlas at several stages which are representative of the reorganization process. This will be a useful tool for future studies of development and function.

Extrabulbar olfactory system and nervus terminalis FMRFamide immunoreactive components in Xenopus laevis ontogenesis

The extrabulbar olfactory system (EBOS) is a collection of nerve fibers which originate from primary olfactory receptor-like neurons and penetrate into the brain bypassing the olfactory bulbs. Our description is based upon the application of two neuronal tracers (biocytin, carbocyanine DiI) in the olfactory sac, at the cut end of the olfactory nerve and in the telencephalon of the developing clawed frog. The extrabulbar olfactory system was observed already at stage 45, which is the first developmental stage compatible with our techniques; at this stage, the extrabulbar olfactory system fibers terminated diffusely in the preoptic area. A little later in development, i.e. at stage 50, the extrabulbar olfactory system was maximally developed, extending as far caudally as the rhombencephalon. In the metamorphosing specimens, the extrabulbar olfactory system appeared reduced in extension; caudally, the fiber terminals did not extend beyond the diencephalon. While a substantial overlapping of biocytin/FMRFamide immunoreactivity was observed along the olfactory pathways as well as in the telencephalon, FMRFamide immunoreactivity was never observed to be colocalized in the same cellular or fiber components visualized by tracer molecules. The question whether the extrabulbar olfactory system and the nervus terminalis (NT) are separate anatomical entities or represent an integrated system is discussed.