The Terminal Nerve : A New Chemosensory System in Vertebrates ?

Ganglion cells of the terminal nerve in goldfish are located in the olfactory nerve and bulb and send peripheral processes into the olfactory epithelium and central processes to the supracommissural nuclei ofthe telencephalon as well as to the retina. Correlations between terminal nerve projections and neurobehavioral studies suggest that the terminal nerve mediates responses to sex pheromones. Pheromones, presumably detected by the olfactory system, influence a number of reproductive responses in fishes (1). The medial olfactory tract (MOT) is specifically implicated in several of these responses; electrical stimulation of the MOT in cod elicits behavioral responses similar to those observed in normal courtship (2); bilateral transection of the MOT in goldfish drastically reduces male responses to pheromones of reproductively active females (3); and stimulation of the MOT in reproductively active male goldfish elicits sperm release (4). However, the axons of another cranial nerve, the terminal nerve (TN), course centrally into the forebrain in association with the MOT in a wide range of vertebrates (5-8). This raises the possibility that pheromone-mediated responses are detected by the TN instead of, or in addition to, the olfactory system. Descriptive studies of the TN in fishes (5-8) have led to the proposal that this cranial nerve consists of ganglion cells located within, or ventral to, the olfactory nerves and bulbs. The peripheral processes of these ganglionic cells are reported to course rostrally and become distributed among the epithelial tissues of the olfactory organs, whereas their central processes course into and terminate among several cell groups closely associated with the anterior commissure of the telencephalon (5-8). Large neuronal cell bodies (Fig. IA) do occur within the medial half of the olfactory nerves and implantation cones of the olfactory bulbs (area of entry of olfactory nerve fibers) in goldfish (Carassius auratus), a pattern that was described from observations of carp (6). In order to determine whether these cells have peripheral processes that ramify in the olfactory epithelium, small pledgets of Gelfoam soaked in horseradish peroxidase (HRP) enzyme (40 percent solution dissolved in 1 percent lysolecithin) were applied unilaterally onto the olfactory epithelium of six goldfish (7 to 10 cm in total length). Two to 4 days later the fish were perfused with 0. 1M phosphate buffer and then with a solution of cold, buffered 4 percent glutaraldehyde in 0.1M phosphate buffer. The brains, including the olfactory bulbs and organs, were removed, embedded in gelatin, and sectioned horizontally at 40 . m. Sections were processed with tetramethylbenzidine in order to visualize the HRP (9). In these cases, HRP-labeled olfactory nerve fibers terminate in the glomerular layer of the ipsilateral olfactory bulb, but no labeling occurred in the mitral cell bodies nor in the majority of the olfacFig. 1. (A) Transverse section of the right olfactory nerve of the goldfish stained by the Bodian method. The cell bodies of the terminal nerve are indicated by the arrows. Dorsal and medial surfaces are to the top and left of the figure, respectively (bar represents 50 p.m). (B) Dark-field illuminated horizontal section through a ventral portion of the telencephalic hemisphere of the goldfish illustrating the central course of the terminal nerve fibers (tn) and their bilateral termination within the supracommissural nucleus of area ventralis (Vs) after HRP application to the left olfactory epithelium. Lateral and rostral planes are to the right and bottom of the figure, respectively (bar represents 100 ,um). 435 o n Ja nu ar y 20 , 2 01 1 w w w . s ci en ce m ag . o rg D ow nl oa de d fr om tory pathways, an indication of the absence of transneuronal transport. However, a bundle of HRP-labeled fibers course more caudally within the ventromedial portion of the ipsilateral olfactory bulb and run in the most medial portion of the medial olfactory tract. On entering the telencephalon, these fibers course lateral to the ventral nucleus of area ventralis and, on reaching the anterior commissure, terminate bilaterally in the supracommissural nucleus of area ventralis (Fig. lB). These results confirm earlier descriptive reports that the peripheral processes of somne ganglion cells of the TN end in the olfactory epithelium, and that the central processes of this nerve terminate in the ventral telencephalon. Similarly located cell bodies in several other piscine species have been labeled retrogradely -after the application of HRP or cobalt unilaterally to the eye or optic nerve (10, 11). These cells and their retinopetal fibers contain luteinizing hormone-releasing hormone (LH-RH) and terminate in the inner nuclear layer of the retina (10, 12, 13). In goldfish, unilateral injections (5 pAl) Qf HRP (40 percent solution dissolved in 1 percent lysolecithin) into the eye or application of pledgets soaked in HRP to the transected optic nerve (five cases) causes bilateral labeling of many cell b,odies in the olfactory nerve; we identify thiese as the ganglion cells of the TN. The tjority of the labeled cells are contralt4eral to the HRP-injected eye or optic nerve. However, a few retrogradely labeled cells also appear in the ipsilateral olfactory nerve, suggesting that each teripinalis ganglion projects bilaterally (Fig. 2). However, double-labeling experiments are needed to ascertain whether any single cell of the terminalis ganglioh projects bilaterally. Whereas HRP injections into the eye retrogradely label many of the ganglionic cells of the terminal nerve, more distal processes, possihly coursing into the olfactory epithelium, were never labeled. Similarly, HRP applied to the olfactory epithelium never labeled fibers caudal to the anterior commissure. Thus, it is possible that our survival times or HRP concentrations were not adequate to label the entire processes of these ganglionic cells or that the TN contains more than one class of neurons, which has been claimed by eaTlier Workers (5, 6). If TN activation is responsible for the mediation of sperm release, then antidromic activation of the terminalis system by optic nerve stimulation could have similar effects. Accordingly, the