Sadao Kiyohara

Distribution of taste buds on the lips and inside the mouth in the minnow, Pseudorasbora parva

The distribution and abundance of taste buds were quantitatively examined by observing silver impregnated serial sections. The taste buds were widely dispersed on the skin, the lips, the mucosa in the oro-pharyngeal cavity, the esophagus, and the branchial apparatus. The great majority of them was found on the lips and inside the mouth. The external buds were concentrated especially on the outer lips and the adjacent skin, while their number diminished in a caudal direction. Very few were found on the scaled skin. The total number of external buds in a specimen of 6 cm in length was 1,486. The number of taste buds inside the mouth was 6,600. On the inner lips and the palatal organ densities were found to reach over 140 per mm2. High concentrations of taste buds were also found on the gill arches and rakers. These taste buds varied to some extent in size and shape, depending on the thickness of the epithelial layer. It is suggested that the minnow may use the lips, gills and palatal organ as its main taste organs.

High sensitivity of minnow gustatory receptors to amino acids

Abstract The stimulating effects of amino acids and related compounds on the gustatory receptors were studied in the Japanese minnow, Pseudorasbora parva , by recording electrical responses from the palatine nerve innervating the upper lip and the adjacent palate. All of the 21 amino acids and 6 related compounds elicited responses at a concentration of 10 −3 M. The order of the response magnitude to the 6 most effective of 18 L-amino acids was: proline > lysine-HCl > alanine > arginine-HCl > cysteine > serine. The threshold concentration for proline, the most potent among the amino acids was estimated to range between 10 −11 and 10 −10 M. The relationship between the log response magnitude and the log stimulus concentration for L-proline or L-alanine was linear in a relatively wide concentration range, showing a tendency for the response to be saturated at higher concentrations. The results of this study indicate that the amino acids are the most potent gustatory stimuli in the Japanese minnow among various chemicals so far tested including salts, sugars, quinine-HCl and ribonucleotides.

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.

Mechanical sensitivity of the facial nerve fibers innervating the anterior palate of the puffer,Fugu pardalis, and their central projection to the primary taste center

Summary1.Mechanical and chemical sensitivity of the palatine nerve, ramus palatinus facialis, innervating the anterior palate of the puffer,Fugu pardalis, and their central projection to the primary taste center were investigated.2.Application of horseradish peroxidase (HRP) to the central cut end of the palatine nerve resulted in retrogradely labeled neurons in the geniculate ganglion but no such neurons in the trigeminal ganglion, suggesting that the palatine nerve is represented only by the facial component.3.Tracing of the facial sensory root in serial histological sections of the brain stem suggested that the facial sensory nerve fibers project only to the visceral sensory column of the medulla.4.Peripheral recordings from the palatine nerve bundle showed that both mechanical and chemical stimuli caused marked responses. Mechanosensitive fibers were rather uniformly distributed in the nerve bundle.5.Intra-cranial recordings from the trigeminal and facial nerves at their respective roots revealed that tactile information produced in the anterior palate was carried by the facial nerve fibers.6.Elimination of the sea water current over the receptive field also caused a marked response in the palatine nerve bundle or facial nerve root while this did not cause any detectable responses in the trigeminal nerve root.7.Single fiber analyses of the mechanical responsiveness of the palatine nerve were performed by recording unit responses of 106 single fibers to mechanical stimuli (water flow), HCl (0.005M), uridine-5′-monophosphate (UMP, 0.001M), proline (0.01M), CaCl2 (0.5M), and NaSCN (0.5M). All these fibers responded well to one of the above stimuli; however, most taste fibers did not respond well to the inorganic salts. The palatine fibers (n=36), identified as mechanosensitive, never responded to any of the chemical stimuli, whereas chemosensitive fibers (n=70) did not respond to mechanical stimuli at all. The chemosensitive units showed a high specificity to the above stimuli: they tended to respond selectively to hydrochloric acid, UMP, or proline.8.The responses of the mechanosensitive units consisted of phasic and tonic impulse trains and the sensitivity of the units varied considerably.9.The results reveal that the facial nerve fibers innervating the anterior palate of the puffer contain two kinds of afferent fibers, chemosensory and mechanosensory respectively, and suggest that the convergence of the tactile and gustatory information first occurs in the neurons of the primary gustatory center in the medulla.

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

The central projections of the ramus mandibularis were examined in the Japanese sea catfish, Plotosus anguillaris by using the technique of transganglionic tracing with horseradish peroxidase (HRP). This ramus receives fibers from both the trigeminal and facial nerves and supplies primarily the two mandibular barbels. Two pathways for a direct trigeminal projection to the facial lobe (FL) were found: one from the main descending root of the Vth nerve (MRDV) to the medial portion of the FL, approximately midway between the rostro-caudal axis of the FL and a second, from deep RDV to the intermediate nucleus (NIF), beneath the medial lobule of the FL. The facial fibers project exclusively onto the medial portion of the FL and the NIF. The results show that fibers of these two cranial sensory nerves supplying the mandibular barbels converge centrally on the medial portion of the FL, indicating that the FL of the Japanese sea catfish is a highly differentiated center for both gustation and somatosensation.

Peripheral and central distribution of major branches of the facial taste nerve in the carp

The major pathways of the peripheral facial taste system in the carp, Cyprinus carpio, are the maxillary (Max), mandibular (Mand), palatine (Pal) and recurrent nerve rami. The peripheral distribution of the sensory fibers of these branches (B) was determined by means of electrophysiological techniques. Max.B., Mand.B. and Pal.B., each of which arises from the gasserian-geniculate ganglionic complexes, were found to innervate respectively, the upper lip and the adjacent skin, the internal and external surface of the lower lip region, and the upper lip and the anterior palate, ipsilaterally. The recurrent nerve sends fibers mainly via dorsal and ventral branches of the posterior lateral line nerve (NPLL), and a pectoral branch of the occipito-spinal nerve. The dorsal and ventral branches of NPLL innervate respectively, the dorsal fin and the adjacent body surface, and the remainder of the body surface. The pectoral branch supplies the pectoral fin. The central connections of the above branches were also examined by using the techniques of transganglionic tracing with horseradish peroxidase (HRP). HRP was applied to each of the branches, and its penetration of the brainstem was carefully followed. Labeled fibers were observed only in the ipsilateral region of the brainstem. When Max.B or Mand.B. was treated with HRP, labeled fibers were observed in the facial sensory root and in the descending trigeminal root. When Pal.B. was treated, however, they were traced only to the facial sensory root; thus indicating that the former two branches are trigeminofacial complexes and the latter is a pure facial nerve. Labeled fibers for NPLL were found in the facial sensory root as well as in bundles projecting to the lateral line areas. The facial fibers of Max.B. and Mand.B. innervate respectively in the dorsal-intermediate portion of the rostral half of the facial lobe, and in the ventral portion of the caudal half of the lobe. Those of Pal.B. however, cover a large area of the lobe anteroposteriorly except for the dorsal and ventral portions. The recurrent fibers of NPLL and the pectoral B. end in the dorsal-medial portion of the caudal half of the lobe. Thus the results of this study show that there is a topographical relation between the receptive field of the 6 peripheral nerve branches and their locus of representation in the facial lobe. Similarly, that the gustatory system through Pal.B. is represented on the facial lobe in a disproportionately large area compared to that of the other 5 branches.

Marine teleost locates live prey through pH sensing

Hold your breath or the catfish will find you Finding prey is hard enough in the light of day, but animals that are nocturnal or live in murky conditions face even greater challenges. Caprio et al. describe a sense that allows a marine catfish to detect the mere “breathing” of their prey target. External sensors on the catfishs whiskers detect pH changes generated by hidden, respiring polychaete worms. Science, this issue p. 1154 Japanese eel catfish whiskers help to locate hidden worms by sensing pH increases associated with respiration. We report that the Japanese sea catfish Plotosus japonicus senses local pH-associated increases in H+/CO2 equating to a decrease of ≤0.1 pH unit in ambient seawater. We demonstrated that these sensors, located on the external body of the fish, detect undamaged cryptic respiring prey, such as polychaete worms. Sensitivity is maximal at the natural pH of seawater (pH 8.1 to 8.2) and decreases dramatically in seawater with a pH <8.0.

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.

The ‘goatee’ of goatfish: innervation of taste buds in the barbels and their representation in the brain

Goatfish use a pair of large chin barbels to probe the sea bottom to detect buried prey. The barbels are studded with taste buds but little else is known about the neural organization of this system. We found that the taste buds of the barbel are innervated in a strict orthogonal fashion. The barbel is innervated by a main nerve trunk running in the core of the barbel. A longitudinal nerve bundle originates from the main trunk and, after running a short distance distally, divides into two circumferential nerve bundles (CNB) extending respectively, medially and laterally around the barbel. Approximately 15 CNBs innervate each 1 mm length of barbel. At each transverse level, the CNB innervates two clusters of taste buds, each containing 14 end–organs. The primary taste centre in the brain is similarly extraordinary. The sensory inputs from the barbel terminate in a derived dorsal facial lobe, which has a highly convoluted surface forming a multitude of tubercles. Electrophysiological mapping experiments show that the entire barbel is somatotopically represented in a recurved elongate tubular fashion within the dorsal facial lobe.

Receptor sites for alanine, proline, and betaine in the palatal taste system of the puffer, Fugu pardalis

SummaryTo elucidate the relative independence of the receptor sites for amino acids and betaine in the gustatory system of fish, the neural responses from the ramus palatinus facialis innervating the anterior palate of the puffer, Fugu pardalis, were recorded.There were observed independent amino acidsensitive and betaine-sensitive fibers.Cross-adaptation to pairs of stimulant was studied. The pair stimulants were applied reciprocally, i.e. after adapting with one stimulus the second stimulus was applied and then reversed. There were observed 3 types of cross-effects for the pairs of stimulants tested: (1) a reciprocal profound cross-adaptation; (2) no distinct cross-adaptation; and (3) a reciprocal enhancement of the response between betaine and alanine or glycine. Such an enhancement between betaine and alanine occurred in the amino acid-sensitive fibers, suggesting that betaine has an enhancing effect on the amino acid receptors.The present results suggest at least 3 different groups of receptor sites for the stimulants tested: (1) alanine sites for alanine, glycine and sarcosine; (2) proline sites for proline and dimethylglycine; and (3) betaine sites for betaine and dimethylglycine.