Morphological evidence for a direct projection of trigeminal nerve fibers to the primary gustatory center in the sea catfish Plotosus anguillaris

Primary and secondary sensory trigeminal projections in a cyprinid teleost, carp (Cyprinus carpio)

Primary and secondary sensory trigeminal projections were studied by means of tract-tracing methods in a cyprinid teleost, the carp. Tracer injections into the trigeminal nerve root labeled terminals in the ipsilateral principal sensory trigeminal nucleus, descending trigeminal nucleus, medial funicular nucleus, facial lobe, and medial part of posterior lateral valvular nucleus. The principal sensory trigeminal nucleus is considered a major origin of the secondary sensory trigeminal projections in teleosts. To investigate the secondary sensory trigeminal projections, tracer injections were performed into the principal sensory trigeminal nucleus. The present study suggests that the principal sensory trigeminal nucleus projects to the bilateral ventromedial thalamic nucleus, periventricular pretectal nucleus, stratum album centrale of the optic tectum, caudomedial region of lateral preglomerular nucleus, ventrolateral nucleus of semicircular torus, medial part of rostral and posterior lateral valvular nucleus, oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, superior and inferior reticular formation, descending trigeminal nucleus, medial funicular nucleus, inferior olive, and to the contralateral sensory trigeminal nucleus. These observations indicate that the primary and secondary trigeminal sensory projections of a cyprinid teleost, the carp, are similar to those in percomorph teleosts.

Projections of the sensory trigeminal nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

The sensory trigeminal nucleus of teleosts is the rostralmost nucleus among the trigeminal sensory nuclear group in the rhombencephalon. The sensory trigeminal nucleus is known to receive the somatosensory afferents of the ophthalmic, maxillar, and mandibular nerves. However, the central connections of the sensory trigeminal nucleus remain unclear. Efferents of the sensory trigeminal nucleus were examined by means of tract-tracing methods, in a percomorph teleost, tilapia. After tracer injections to the sensory trigeminal nucleus, labeled terminals were seen bilaterally in the ventromedial thalamic nucleus, periventricular pretectal nucleus, medial part of preglomerular nucleus, stratum album centrale of the optic tectum, ventrolateral nucleus of the semicircular torus, lateral valvular nucleus, prethalamic nucleus, tegmentoterminal nucleus, and superior and inferior reticular formation, with preference for the contralateral side. Labeled terminals were also found bilaterally in the oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, medial funicular nucleus, and contralateral sensory trigeminal nucleus and inferior olive. Labeled terminals in the oculomotor nucleus and trochlear nucleus showed similar densities on both sides of the brain. However, labelings in the trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, and medial funicular nucleus showed a clear ipsilateral dominance. Reciprocal tracer injection experiments to the ventromedial thalamic nucleus, optic tectum, and semicircular torus resulted in labeled cell bodies in the sensory trigeminal nucleus, with a few also in the descending trigeminal nucleus.

Distribution of trigeminal fibers in the primary facial gustatory center of channel catfish, Ictalurus punctatus

Previous studies in several fishes including catfish, have shown that primary trigeminal nerve (NV) axons terminate not only in the principal and spinal trigeminal nuclei, but in the facial (gustatory) lobes. The present study was undertaken to determine the extent and distribution of trigeminal terminations within the facial lobe (FL) and principal trigeminal nucleus (nVpr) in the channel catfish, Ictalurus punctatus. In order to reveal the distribution of trigeminal fibers, the carbocyanine dye, diI, was applied to the central cut stump of the trigeminal root in isolated, paraformaldehyde-fixed brains. After a diffusion period of 10-90 days, the brains were serially sectioned on a vibratome and examined with epifluorescence. The trigeminal motor nucleus (nVm) and principal sensory nucleus lie near the level of entrance of NV. The majority of primary trigeminal fibers, however, sweep caudally after entering into the brain to form the descending root. At the level of the caudal third of the FL, collaterals emitted by the descending root fibers turn medially and dorsally to terminate in the FL. The trigeminal fibers are coarser than the facial nerve (NVII) fibers which terminate within the same structure. The trigeminal fibers terminate throughout the FL except for the lateral-most lobule which contains the representation of taste buds innervated by the recurrent branch of NVII, i.e., those over the trunk and tail of the animal. These results show that in catfish, the trigeminal input to the primary gustatory complex is restricted to those portions of the nucleus receiving chemosensory inputs from the face and barbels, i.e., the trigeminally innervated sensory fields.

Somatotopic organization of the facial lobe of the sea catfish Arius felis studied by transganglionic transport of horseradish peroxidase

To reveal the somatotopical organization of the facial lobe (FL), a primary medullary gustatory nucleus in the sea catfish Arius felis, the central projections of the peripheral rami of the facial nerve innervating taste buds located across the entire body surface and rostral oral regions were traced by means of horseradish peroxidase neurohistochemistry. The maxillary barbel, lateral mandibular barbel, medial mandibular barbel, and trunk-tail branches project to four different longitudinal columns (i.e., lobules) extending rostrocaudally in the FL. The trunk-tail lobule, which is located dorsolateral to the barbel lobules, lies in the anterior two-thirds of the FL. The tail is represented in a more rostral portion of the trunk-tail lobule than the trunk, indicating that the rostrocaudal trunk axis is represented in the trunk-tail lobule in a posteroanterior axis. The pectoral fin branch ends in an intermediate region of the FL, whereas the hyomandibular, ophthalmic, lower lip, upper lip, and palatine branches terminate in discrete regions of the caudal one-third of the FL. These results reveal a sharply defined somatotopical organization of the FL of Arius and support the hypothesis that the number and lengths of the barbel lobules within the FL of catfishes are directly related to the number and relative lengths of the barbels. An additional subcolumn, the intermediate nucleus of the FL (NIF), which develops in the medioventral region of the caudal two-thirds of the FL, receives projections in a diffuse somatotopical fashion from the barbels, lower lip, and palatine branches. Trigeminal fibers of the barbel and lower lip branches project in a somatotopic fashion to the FL. The present findings suggest that the FL of Arius is highly organized somatotopically to detect, by tropotaxis, precise spatial information concerning taste and tactile stimuli in the environment.

Convergence of oral and extraoral information in the superior secondary gustatory nucleus of the channel catfish

Neurons within the superior secondary gustatory nucleus (nGS) of the channel catfish were examined electrophysiologically for responses to mechanical and chemical stimulation of neural peripheral receptive fields (RFs). Of the 28 single units sampled, 18 had mechanosensory RFs on the extraoral epithelium, two had RFs within the oropharyngeal cavity, and eight had RFs that included both oral and extraoral surfaces. RF sizes varied from approximately 2 cm2 on the ipsilateral lips and barbels to the whole body surface, bilaterally. No obvious correlation existed between RF pattern and recording location within the nGS. Eight of the mechanosensory nGS units also responded to amino acid taste stimuli with thresholds from micromolar to millimolar concentrations. The convergence of oral and extraoral information within the nGS determined electrophysiologically was corroborated anatomically by HRP labeling experiments. Restricted HRP injections into each of the primary gustatory nuclei of the medulla, the vagal (VL) and facial (FL) lobes, labeled fibers that appeared to terminate diffusely throughout the nGS, and injections into different portions of the nGS retrogradely labeled cells in both the FL and VL. The present electrophysiological and neuroanatomical data distinguish the convergent gustatory representation within the nGS of the catfish from the highly specific somatotopic and viscerotopic sensory maps previously identified in the FL and VL, respectively.

Somatotopical organization of the intermediate nucleus of the facial lobe in the channel catfish, Ictalurus punctatus

Neurons in the intermediate nucleus of the facial lobe (nIF) in the channel catfish that respond to tactile stimulation of oral and/or extra-oral epithelia are somatotopically arranged. Neurons in rostrodorsal portions of the nIF responded to tactile stimulation or deflection of the ipsilateral barbels, whereas neurons arranged in a dorsoventral direction in caudoventral regions of the nIF had receptive fields on the ipsilateral lips and the oral cavity, respectively. Suppression of neuronal activity in response to tactile stimulation of the external skin and/or the oral cavity was indicated for some units. Taste responses were not observed.

Lobule structure and somatotopic organization of the medullary facial lobe in the channel catfish Ictalurus punctatus

Correlation of the somatotopic organization of the facial lobe (FL), a primary medullary gustatory nucleus in the channel catfish Ictalurus punctatus, with its lobular substructure was investigated to examine a possible structural basis for the excellent ability of ictalurid catfishes to localize a food source in the environment. The FL in the channel catfish is composed of six longitudinal columns (i.e., lobules) extending rostrocaudally and differing from each other in their length and location within the lobe. Each lobule receives segregated input from discrete portions of the external body surface. The three more medial lobules in the FL receive input (from medial to lateral) from the medial mandibular barbel, the lateral mandibular barbel, and the maxillary barbel, respectively. The proximal-distal axis of each of the barbels is represented in a posteroanterior lobule axis. The largest lobule, the face-flank lobule, is located dorsolaterally in the FL, whereas the anteroposterior body axis is represented in the posteroanterior lobule axis. This indicates that the neural representation of the external body surface of the channel catfish faces caudally in the FL. The two shortest lobules, positioned ventral to the face-flank lobule, receive input from the nasal barbel and the pectoral fin, respectively. The rostrocaudal dimensions of each of the barbel lobules correlate well with the relative lengths of the barbels. Taste-sensitive portions within the three barbel lobules examined were generally confined to the dorsal region, whereas tactile responses were observed throughout the lobules.2+ primarily tactile, zone.

Topographical organization of taste and tactile neurons in the facial lobe of the sea catfish, Plotosus lineatus

An extraordinary development of the paired medullary facial taste nuclei, the facial lobes, occurs in the sea catfish, Plotosus lineatus. Each of the facial lobes is divided by fiber fascicles into 5 highly distinct lobules or subnuclei, constituting 5 longitudinal columns through the lobe. Extracellular, electrophysiological recordings of neurons within the respective subnuclei of the facial lobe indicate superimposable taste and tactile neural maps organized in a somatotopic manner.

Overlapping taste and tactile maps of the oropharynx in the vagal lobe of the channel catfish, Ictalurus punctatus

Microelectrode mapping experiments indicate an ipsilateral representation of the oropharynx and a well-defined, bilateral input from the proximal portion of the maxillary barbels and snout region within the vagal lobe of channel catfish. The map of the oropharyngeal epithelium is distorted so that the gill arches are rotated through an angle of 90 degrees along the transverse plane, and the dorsally mapped region of the gill rakers is tilted posteriorly in the sagittal plane of the vagal lobe. Multiunit recording studies fail to provide definitive boundaries of adjacently mapped domains of oropharyngeal structures. Gustatory receptive fields of neurons in the vagal lobe correspond to their location on the topological map obtained by tactile stimulation of the oropharyngeal epithelium. A few single unit recordings indicate restricted receptive fields and different response patterns of taste, tactile, and proprioceptive neurons in the vagal lobe of catfish.

Chemosensory anterior dorsal fin in rocklings (Gaidropsarus and Ciliata, Teleostei, Gadidae): somatotopic representation of the ramus recurrens facialis as revealed by transganglionic transport of HRP

The anterior dorsal fin in rocklings consists of a fringe of 50-80 delicate, vibratile rays, which are densely beset with epidermal chemosensory cells. The innervation of these cells is from the dorsal branch of the recurrent facial nerve, which also innervates all other fins and the skin of the trunk. This nerve carries at least three classes of fibres: small (0.5-1.5 micron in diameter), medium (1.5-4 micron), and large (greater than 4 micron). Approximately 12,000 small and weakly myelinated nerve fibres from the recurrent facial nerve innervate the anterior dorsal fin organ. Application of HRP at different locations of the recurrent facial nerve labelled three different sizes of sensory perikarya within the geniculate ganglion--small (6-15 micron in diameter), medium (18-24 micron), and large (greater than 25 micron)--which corresponds to the different size classes of fibres present within the nerve. Retrograde transganglionic transport of HRP revealed somatotopy within the brainstem facial lobe: the delicate nerve fibres innervating the chemosensory anterior dorsal fin terminate exclusively in a distinct, dorsal portion of the facial lobe. Fibres innervating the posterior dorsal fin, the anal and caudal fins, as well as the skin of the trunk terminate within caudal and dorsal areas of the ventral facial lobe; pectoral and pelvic fins are represented in the ventral and caudal portions of the ventral facial lobe. Innervation by a distinct type of fibre and exclusive representation within a distinct, dorsal part of the facial lobe may indicate a peculiar biological role in the anterior dorsal fin chemosensory organ in the rocklings.