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

Diversity and evolution of serotonergic cells in taste buds of elasmobranchs and ancestral actinopterygian fish

A subset of gustatory cells are serotonin immunoreactive (ir) in the mammalian taste bud. In the taste bud of lamprey, elongated gustatory-like cells are also serotonin-ir. In contrast, flattened serotonin-ir cells are located only in the basal region of the taste buds in the teleosts and amphibians. These serotonin-ir cells are termed as basal cells. To evaluate the evolution and diversity of serotonergic cells in the taste bud of amniote animals, we explored the distribution and morphology of serotonin-ir cells in the taste buds of ancestral actinopterygian fish (spotted gar, sturgeon, Polypterus senegalus) and elasmobranch (stingray). In all examined animals, the taste buds contained serotonin-ir cells in their basal part. The number of serotonin-ir basal cells in each taste bud was different between these fish species. They were highest in the stingray and decreased in the order of the Polypterus, sturgeon, and gar. While serotonin immunoreactivity was observed only in the basal cells in the taste buds of the ancestral actinopterygian fish, some elongated cells were also serotonin-ir in addition to the basal cells in the stingray taste buds. mRNA of tryptophan hydroxylase 1 (tph1), a rate-limiting enzyme of the serotonin synthesis, is expressed in both the elongated and basal cells of stingray taste buds, indicating that these cells synthesize the serotonin by themselves. These results suggest that the serotonin-ir basal cells arose from the ancestor of the cartilaginous fish, and serotonin-ir cells in the elasmobranch taste bud exhibit an intermediate aspect between the lamprey and actinopterygian fish.

Recurrent facial taste neurons of sea catfish Plotosus japonicus: morphology and organization in the ganglion

This study investigated the morphology of the recurrent facial taste neurons and their organization in the recurrent ganglion of the sea catfish Plotosus japonicus. The recurrent ganglion is independent of the anterior ganglion, which consists of trigeminal, facial and anterior lateral line neurons that send peripheral fibres to the head region. The recurrent taste neurons are round or oval and bipolar, with thick peripheral and thin central fibres, and completely wrapped by membranous layers of satellite cells. Two peripheral nerve branches coursing to the trunk or pectoral fin originate from the recurrent ganglion. The results presented here show that the trunk and pectoral-fin neurons are independently distributed to form various sizes of groups, and the groups are intermingled throughout the ganglion. No distinct topographical relationship of the two nerve branches occurs in the ganglion. Centrally, the trunk and pectoral-fin branches project somatotopically in the anterolateral and intermediate medial regions of the trunk tail lobule of the facial lobe, respectively.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

Trigeminal somatosensory innervation of the head of a teleost fish with particular reference to nociception

Trigeminal somatosensory receptors have not been characterised in teleost fish and studies in elasmobranchs have failed to identify nociceptors. The present study examined the trigeminal nerve of a teleost fish, the rainbow trout (Oncorhynchus mykiss) to determine what types of somatosensory receptors were present on the head of the trout specifically searching for nociceptors. Single unit recordings were made from receptive fields on the head of the fish innervated by the trigeminal nerve. Each receptive field was tested for sensitivity to mechanical, thermal and chemical stimulation. Five different receptor types were found: fast adapting receptors responding to mechanical stimulation; slowly adapting receptors responding to mechanical stimuli; polymodal nociceptors responding to mechanical, noxious thermal and chemical stimulation; mechanothermal nociceptors responding to mechanical stimulation and noxious heat; and mechanochemical receptors responsive to mechanical and chemical stimulation. Mechanical thresholds, receptive field diameter, conduction velocities and thermal thresholds of the receptors were determined and there was no significant difference between the receptor types in terms of these properties. Three shapes of action potential (AP) were recorded from these receptors: type 1 with no inflexion; type 2 with an inflexion on depolarisation; and type 3 with an inflexion on repolarisation. Conduction velocity, amplitude and duration of the APs, afterhypolarisation amplitude and duration, as well as the maximum rate of depolarisation were measured for each action potential type. No major differences were found when making comparisons within receptor type and between receptor types. The fish nociceptors had similar physiological properties to nociceptors found in higher vertebrates.

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.

Spinal and facial innervation of the skin in the gadid fish Ciliata mustela (Teleostei)

The pattern of innervation of the skin of the rockling Ciliata mustela was investigated to sort out spinal from facial nerve innervation of cutaneous chemosensory and mechanosensory systems. This fish has a variety of appendages with different functional sensory specializations, i.e., the chin barbel, pelvic fin, anterior dorsal fin, and dorsal trunk skin. The carbocyanine dye, diI, was applied to nerve stumps in dissected aldehyde-fixed tissue. In the case of the chin barbel, the dye was applied to both the trigeminal and facial nerve components. In the other cases, the dye was applied either selectively to the spinal nerves, to the facial nerves, or to both components. In the chin barbel, diI labeled nerve fibers associated with taste buds (TBs) and solitary chemosensory cells (SCCs) as well as relatively blunt free nerve endings, which closely approach the epidermal surface. In the pelvic fin, anterior dorsal fin, and dorsal trunk skin, taste buds, solitary chemosensory cells, and their innervation were labeled only after diI was applied to the facial nerve stumps. Application of diI to spinal nerves labeled delicate, free nerve endings and nerve fibers associated with small cells deep in the epidermis with features characteristic of Merkel cells. Transmission electron microscopy supports these results; after denervation of the facial component of the anterior dorsal fin, synaptic contacts with Merkel cells remained intact, whereas the synapses with the SCCs vanished.

Receptive fields and gustatory responsiveness of frog glossopharyngeal nerve. A single fiber analysis

Receptive fields and responsiveness of single fibers of the glossopharyngeal (IXth) nerve were investigated using electrical, gustatory (NaCl, quinine HCl, acetic acid, water, sucrose, and CaCl2), thermal, and mechanical stimulation of the single fungiform papillae distributed on the dorsal tongue surface in frogs. 172 single fibers were isolated. 58% of these fibers (99/172) were responsive to at least one of the gustatory stimuli (taste fibers), and the remaining 42% (73/172) were responsive only to touch (touch fibers). The number of papillae innervated by a single fiber (receptive field) was between 1 and 17 for taste fibers and between 1 and 10 for touch fibers. The mean receptive field of taste fibers (X = 6.6, n = 99) was significantly larger than that of touch fibers (X = 3.6, n = 73) (two-tailed t test, P less than 0.001). In experiments with natural stimulation of single fungiform papillae, it was found that every branch of a single fiber has a similar responsiveness. Taste fibers were classified into 14 types (Type N, Q, A, NA, NCa, NCaA, NCaW, NCaAW, NCaWS, NQ, NQA, NQAS, NQWarm, Multiple) on the basis of their responses to gustatory and thermal stimuli. The time course of the response in taste fibers was found to be characteristic of their types. For example, the fibers belonging to Type NQA showed phasic responses, those in Type NCa showed tonic responses, etc. These results indicate that there are several groups of fibers in the frog IXth nerve and that every branch of an individual fiber has a similar responsiveness to the parent fiber.

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.

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.

Multimodal responses of taste neurons in the frog nucleus tractus solitarius

The responses of 216 neurons in the nucleus tractus solitarius (NTS) of the American bullfrog were recorded following taste, temperature, and tactile stimulation. Cells were classified on the basis of their responses to 5 taste stimuli: 0.5 M NaCl, 0.0005 M quinine-HCl (QHCl), 0.01 M acetic acid, 0.5 M sucrose, and deionized water (water). Neurons showing excitatory responses to 1, 2, 3, or 4 of the 5 kinds of taste stimuli were named Type I, II, III, or IV, respectively. Cells whose spontaneous rate was inhibited by taste and/or tactile stimulation of the tongue were termed Type V. Type VI neurons were excited by tactile stimulation alone. Of the 216 cells, 115 were excited or inhibited by taste stimuli (Types I-V), with 35 being Type I, 34 Type II, 40 Type III, 2 Type IV and 4 Type V. The remaining 101 cells were responsive only to tactile stimulation (Type VI). Of those 111 cells excited by taste stimulation (Types I-IV), 106 (95%) responded to NaCl, 66 (59%) to acetic acid, 44 (40%) to QHCl, 10 (9%) to water, and 9 (8%) to warming. No cells responded to sucrose. Of the 111 cells of Types I-IV, 76 (68%) were also sensitive to mechanical stimulation of the tongue. There was some differential distribution of these neuron types within the NTS, with more narrowly tuned cells (Type I) being located more dorsally in the nucleus than the more broadly tuned (Type III) neurons. Cells responding exclusively to touch (Type VI) were also more dorsally situated than those responding to two or more taste stimuli (Types II and III).

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