Satoru Yamashita

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.

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.

Intracellular Na+ and K+ shifts induced by contractile activities of rat skeletal muscles.

The effects of direct and indirect electrical stimulation on intracellular potassium and sodium contents ([K]i and [Na]i, respectively) in rat soleus muscle (SOL) and extensor digitorum longus muscle (EDL) were investigated under in vivo conditions. The changes of [K]i and [Na]i contents in both muscles which were stimulated indirectly reached respective values at 30 min or 1 hr after the beginning of stimulation, whereas those of EDL stimulated with 60 Hz changed gradually through 2 hr stimulation. The shifts of [K]i and [Na]i in EDL occurred during the twitch contraction at considerably lower frequency stimulation (0.5-10 Hz), whereas those in SOL were observed during the tetanus contraction at high frequency stimulation (10-40 Hz). The difference of change in cationic shifts between EDL and SOL under low frequency stimulation was reduced by ouabain treatment, though the difference was still significant. When the muscles were indirectly stimulated 6000 times at 1, 5, 10 and 20 Hz, the cationic shifts in EDL were greater than those in SOL, extending over all frequencies. It was concluded that such a difference in ionic shift between contracting EDL and SOL may be primarily due to the difference in unidirectional ionic fluxes per stimulation and, secondly, to the difference in Na(+)-K+ pump activity.

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.

Gustatory responses in the minnow, Pseudorasbora parva

Abstract Gustatory responsiveness in the Japanese minnow was examined by recording responses to various taste stimuli from the palatine nerve innervating mainly the inner part of the upper lip and the adjacent palate. The receptors responded well not only to salts, acids, quinine and sugars but also to nucleotides and amino acids. The threshold concentrations ranged from 10 −5 to 3 × 10 −5 M for NaCl, from 3 × 10 −5 to 10 −4 M for HCl, from 3 × 10 −5 to 3 × 10 −4 M for quinine HCl, and from 10 −2 to 3 × 10 −2 M for sucrose. Various sugars and other sweet taste substances such as Na saccharin and lead acetate produced good responses. The order of the magnitude of response to 6 sugars at 1 M was mannose⩾fructose⩾maltose>sucrose>galactose>glucose. Differences in the stimulating effectiveness were observed among monovalent sodium salts as well as among monovalent chloride salts. Divalent salts were more effective than monovalent ones. All of the five 5′-ribonucleotides and nine neutral amino acids, at 10 −3 and 10 −2 M respectively produced marked responses. These results suggest that the minnow palatal chemoreceptors possess broad responsiveness to various chemicals, corresponding to the high density of the palatal taste buds.

Chemical sensitivity of lateral line organs in the goby, Gobius giurinus

Abstract 1. 1. Summated neural responses to various chemical stimuli were recorded from the peripheral nerve innervating the lateral line organs which occur on the upper snout of the giurine goby, Gobius giurinus. 2. 2. The threshold for monovalent chloride salts was estimated to be at around 10−3 M, while that for divalent chloride salts, at around 10−4M. The order of the stimulatory effectiveness was RbCl ⩾ NH4Cl ⩾ KCl ⩾ NaCl ⩾ LiCl ⩾ CsCl for monovalent salts and BaCl2 > CaCl2 ⩾ MgCl2 SrCl2 for divalent salts. 3. 3. Of 17 amino acids only two acidic amino acids, l -Glu and l -Asp, were stimulative and the sensitivity of the lateral line organs to them was considered to be due to the hydrogen ion, since response magnitudes to the amino acids depended upon solution pH and were similar to the response magnitude to HCl of the same pH. 4. 4. Many typical free neuromasts were found to form the so called pit-line on the upper snout region, but where no canal organs were observed.

Change of intracellular K+ activity in rat soleus muscle during hypokalemia.

The relationship between intracellular total K+ concentration [( K]i) as determined by a flame spectrophotometer and intracellular K+ activity (aKi) as determined by an ion-selective microelectrode was studied in soleus muscle of rats on a diet deficient in K+ for 40 days. [K]i began to fall immediately from the initial stage of hypokalemia, while aKi was well-maintained for 15 days. Then, aKi decreased gradually. The measured resting potential (Em) hyperpolarized beyond the EK was calculated from aKi in hypokalemic rat muscle from day 20 to 40. A rapid increase in aKi occurred over 3 hours in soleus muscle of hypokalemic rats for 5 to 6 weeks. It was concluded that the bound intracellular K+ acts as a buffer for aKi in hypokalemic rat muscle, that Em exceeds EK because the Na+-K+ pump is stimulated by increased [Na]i and that the increase in aKi after denervation is due to the removal of a Na+-K+ pump inhibitor normally released from nerve ending.