Electrophysiological examination of a non-olfactory, non-gustatory chemosense in the searobin,Prionotus carolinus

Compensatory sensory mechanisms in naïve blind cavefish navigating novel environments after lateral line ablation

Fishes navigating complex aquatic environments rely on various sensory systems, primarily the lateral line system and vision, to guide their movements. One interesting example is the Mexican blind cavefish (Astyanax mexicanus). This fish relies on the lateral line system as it navigates through the environment without the aid of sight. It is unclear, however, how they might navigate through a novel environment when the lateral line is not functional. In this study, we used high-speed videography to quantify whether naïve blind cavefish alter locomotor behavior, navigation patterns, and the use of body and fins to explore a novel environment with obstacles when the lateral line is ablated. Blind cavefish with an intact lateral line demonstrated deliberate slower exploratory movements and navigated around obstacles with fewer touching events. Conversely, fish with ablated lateral line exhibited increased speed to potentially improve flow sensing. Fish with an ablated lateral line also touched obstacles more often, suggesting a reliance on fin and snout mechanoreception for navigation. These results show the blind cavefish have compensatory sensory mechanisms to navigate novel environments when their major sensory system is not functioning.

Evolution of novel sensory organs in fish with legs

How do animals evolve new traits? Sea robins are fish that possess specialized leg-like appendages used to "walk" along the sea floor. Here, we show that legs are bona fide sense organs that localize buried prey. Legs are covered in sensory papillae that receive dense innervation from touch-sensitive neurons, express non-canonical epithelial taste receptors, and mediate chemical sensitivity that drives predatory digging behavior. A combination of developmental analyses, crosses between species with and without papillae, and interspecies comparisons of sea robins from around the world demonstrate that papillae represent a key evolutionary innovation associated with behavioral niche expansion on the sea floor. These discoveries provide unique insight into how molecular-, cellular-, and tissue-scale adaptations integrate to produce novel organismic traits and behavior.

Evolution: Sea robins get a leg up

Sea robins, fish with legs, walk on the ocean bottom. They have evolved taste receptors on their legs that direct digging to access prey. Examining these structures and behaviors advances our understanding of the origin of novel phenotypes.

Sea robins

Allard et al. provide an overview of sea robins, a group of benthic fish that have evolved leg-like appendages that they use to walk on the sea floor and find prey.

The Water to Land Transition Submerged: Multifunctional Design of Pectoral Fins for Use in Swimming and in Association with Underwater Substrate

Fins of fishes provide many examples of structures that are beautifully designed to power and control movement in water; however, some species also use their fins for substrate-associated behaviors where interactions with solid surfaces are key. Here, we examine how the pectoral fins of ray-finned fish with these multifunctional behavioral demands, in water and on solid surfaces, are structured and function. We subdivide fins used in swimming and substrate contact into two general morphological categories, regionalized vs. generalized fins. Regionalized fins have ventral rays that are free from connecting membrane or in which that membrane is reduced. Dorsally they maintain a more typical membranous fin. While all pectoral fins vary somewhat in their morphology from leading to trailing edge, generalized fins do not have the substantial membrane loss between rays that is seen in regionalized fins and the distal edge anatomy changes gradually along its margin. We add a new case study in regionalized fins with the dwarf hawkfish (Cirrhitichthys falco). Hawkfishes are most often found perching and moving on structures in their environments. During perching, the free ventral rays are in contact with the substrate and splayed. We found that unlike other fish with regionalized pectoral fins, hawkfish maintain use of the dorsal membranous region of its pectoral fin for rhythmic swimming. We found that typically hawkfish bend their ventral free rays under, toward the medial hemitrichs or hold them straight during substrate-associated postures. This appears also to be the case for the ventral free rays of other species with regionalized fins. Generalized fin use for substrate contact was reviewed in round gobies (Neogobius melanostomus). In addition, although their lobe fins are not representative of ray-finned fish anatomy, we explored fin contact on submerged substrates in the Senegal bichir (Polypterus senegalus), which has a generalized distal fin (no free fin rays or distinct membrane regions). Both groups use their pectoral fins for swimming. During substrate-based postures, unlike hawkfish, their distal rays generally bend outward toward the lateral hemitrichs and a large swath of the fin membrane can contact the surface. The alternative demands on multifunctional fins suggest specialization of the mechanosensory system. We review mechanosensation related to fin movement and surface contact. These alternative regionalized and generalized strategies for serving aquatic and substrate-based functions underwater provide opportunities to further investigate specializations, including sensory structures and systems, that accompany the evolution of substrate-based behaviors in vertebrates.

Evolution of touch and proprioception of the limbs: Insights from fish and humans

The function of the hands is inextricably linked to cutaneous mechanosensation, both in touch and in how hand movement and posture (proprioception) are controlled. The structure and behavior of hands and distal forelimbs of other vertebrates have been evolutionarily shaped by these mechanosensory functions. The distal forelimb of tetrapod vertebrates is homologous to the pectoral fin rays and membrane of fishes. Fish fins demonstrate similar mechanosensory abilities to hands and other distal tetrapod forelimbs in touch and proprioception. These results indicate that vertebrates were using the core mechanosensory inputs, such as fast adapting and slow adapting nerve responses, to inform fin and limb function and behavior before their diversification in fish and tetrapod lineages.

Hindbrain and Spinal Cord Contributions to the Cutaneous Sensory Innervation of the Larval Zebrafish Pectoral Fin

Vertebrate forelimbs contain arrays of sensory neuron fibers that transmit signals from the skin to the nervous system. We used the genetic toolkit and optical clarity of the larval zebrafish to conduct a live imaging study of the sensory neurons innervating the pectoral fin skin. Sensory neurons in both the hindbrain and the spinal cord innervate the fin, with most cells located in the hindbrain. The hindbrain somas are located in rhombomere seven/eight, laterally and dorsally displaced from the pectoral fin motor pool. The spinal cord somas are located in the most anterior part of the cord, aligned with myomere four. Single cell reconstructions were used to map afferent processes and compare the distributions of processes to soma locations. Reconstructions indicate that this sensory system breaks from the canonical somatotopic organization of sensory systems by lacking a clear organization with reference to fin region. Arborizations from a single cell branch widely over the skin, innervating the axial skin, lateral fin surface, and medial fin surface. The extensive branching over the fin and the surrounding axial surface suggests that these fin sensory neurons report on general conditions of the fin area rather than providing fine location specificity, as has been demonstrated in other vertebrate limbs. With neuron reconstructions that span the full primary afferent arborization from the soma to the peripheral cutaneous innervation, this neuroanatomical study describes a system of primary sensory neurons and lays the groundwork for future functional studies.

Sensory cutaneous papillae in the sea lamprey (Petromyzon marinus L.): I. Neuroanatomy and physiology

Molecules present in an animal's environment can indicate the presence of predators, food, or sexual partners and consequently, induce migratory, reproductive, foraging, or escape behaviors. Three sensory systems, the olfactory, gustatory, and solitary chemosensory cell (SCC) systems detect chemical stimuli in vertebrates. While a great deal of research has focused on the olfactory and gustatory system over the years, it is only recently that significant attention has been devoted to the SCC system. The SCCs are microvillous cells that were first discovered on the skin of fish, and later in amphibians, reptiles, and mammals. Lampreys also possess SCCs that are particularly numerous on cutaneous papillae. However, little is known regarding their precise distribution, innervation, and function. Here, we show that sea lampreys (Petromyzon marinus L.) have cutaneous papillae located around the oral disk, nostril, gill pores, and on the dorsal fins and that SCCs are particularly numerous on these papillae. Tract-tracing experiments demonstrated that the oral and nasal papillae are innervated by the trigeminal nerve, the gill pore papillae are innervated by branchial nerves, and the dorsal fin papillae are innervated by spinal nerves. We also characterized the response profile of gill pore papillae to some chemicals and showed that trout-derived chemicals, amino acids, and a bile acid produced potent responses. Together with a companion study (Suntres et al., Journal of Comparative Neurology, this issue), our results provide new insights on the function and evolution of the SCC system in vertebrates.

Fins as Mechanosensors for Movement and Touch-Related Behaviors

Mechanosensation is a universal feature of animals that is essential for behavior, allowing detection of animals' own body movement and position as well as physical characteristics of the environment. The extraordinary morphological and behavioral diversity that exists across fish species provide rich opportunities for comparative mechanosensory studies in fins. The fins of fishes have been found to function as proprioceptors, by providing feedback on fin ray position and movement, and as tactile sensors, by encoding pressures applied to the fin surface. Across fish species, and among fins, the afferent response is remarkably consistent, suggesting that the ability of fin rays and membrane to sense deformation is a fundamental feature of fish fins. While fin mechanosensation has been known in select, often highly specialized, species for decades, only in the last decade have we explored mechanosensation in typical propulsive fins and considered its role in behavior, particularly locomotion. In this paper, we synthesize the current understanding of the anatomy and physiology of fin mechanosensation, looking toward key directions for research. We argue that a mechanosensory perspective informs studies of fin-based propulsion and other fin-driven behaviors and should be considered in the interpretation of fin morphology and behavior. In addition, we compare the mechanosensory system innervating the fins of fishes to the systems innervating the limbs of mammals and wings of insects in order to identify shared mechanosensory strategies and how different organisms have evolved to meet similar functional challenges. Finally, we discuss how understanding the biological organization and function of fin sensors can inform the design of control systems for engineered fins and fin-driven robotics.

Regional differences in the feeding of the ambush predator Neosebastes pandus and comparisons of diets in the Scorpaenidae, Triglidae and Platycephalidae

This study provides a comprehensive assessment of the dietary composition of the ambush predator Neosebastes pandus and compares the diets of 49 species from 39 studies of three benthic predatory families in the Scorpaeniformes: Scorpaenidae (20 species), Triglidae (19 species) and Platycephalidae (10 species). A total of 275 N. pandus were collected from the west (Rottnest Island) and south (Esperance) coasts of south-western Australia and the percentage frequency and volumetric contribution of the stomach contents identified. Fish from the west coast consumed a greater mean number of broad taxonomic groups and were more diverse in their diet than fish from the south coast. Cephalopods, brachyurans and teleosts were the largest overall contributors to diet, with teleosts being more important to diets of west-coast fish and polychaetes for south-coast fish. This reflects differences in habitat between the two locations. Dietary composition also changed with increasing body size, reflecting morphological changes that allow bigger fish to capture and ingest larger, more mobile prey. Meta-analysis of the diets of 49 species of scorpaenid, triglid and platycephalid revealed that they feed predominantly on teleosts and large crustaceans. Significant differences in diet were detected among families, with platycephalids being the most distinct and feeding more on teleosts than scorpaenids and triglids.

Touch sensation by pectoral fins of the catfish Pimelodus pictus

Mechanosensation is fundamental to many tetrapod limb functions, yet it remains largely uninvestigated in the paired fins of fishes, limb homologues. Here we examine whether membranous fins may function as passive structures for touch sensation. We investigate the pectoral fins of the pictus catfish (Pimelodus pictus), a species that lives in close association with the benthic substrate and whose fins are positioned near its ventral margin. Kinematic analysis shows that the pectoral fins are held partially protracted during routine forward swimming and do not appear to generate propulsive force. Immunohistochemistry reveals that the fins are highly innervated, and we observe putative mechanoreceptors at nerve fibre endings. To test for the ability to sense mechanical perturbations, activity of fin ray nerve fibres was recorded in response to touch and bend stimulation. Both pressure and light surface brushing generated afferent nerve activity. Fin ray nerves also respond to bending of the rays. These data demonstrate for the first time that membranous fins can function as passive mechanosensors. We suggest that touch-sensitive fins may be widespread in fishes that maintain a close association with the bottom substrate.

Molecular phylogenetics of New World searobins (Triglidae; Prionotinae)

Phylogenetic relationships among members of the New World searobin genera Bellator and Prionotus (Family Triglidae, Subfamily Prionotinae) and among other searobins in the families Triglidae and Peristediidae were investigated using both mitochondrial and nuclear DNA sequences. Phylogenetic hypotheses derived from maximum likelihood and Bayesian methodologies supported a monophyletic Prionotinae that included four well resolved clades of uncertain relationship; three contained species in the genus Prionotus and one contained species in the genus Bellator. Bellator was always recovered within the genus Prionotus, a result supported by post hoc model testing. Two nominal species of Prionotus (P. alatus and P. paralatus) were not recovered as exclusive lineages, suggesting the two may comprise a single species. Phylogenetic hypotheses also supported a monophyletic Triglidae but only if armored searobins (Family Peristediidae) were included. A robust morphological assessment is needed to further characterize relationships and suggest classification of clades within Prionotinae; for the time being we recommend that Bellator be considered a synonym of Prionotus. Relationships between armored searobins (Family Peristediidae) and searobins (Family Triglidae) and relationships within Triglidae also warrant further study.

Fin ray sensation participates in the generation of normal fin movement in the hovering behavior of the bluegill sunfish (Lepomis macrochirus)

For many fish species, the pectoral fins serve as important propulsors and stabilizers and are precisely controlled. Although it has been shown that mechanosensory feedback from the fin ray afferent nerves provides information on ray bending and position, the effects of this feedback on fin movement are not known. In other taxa, including insects and mammals, sensory feedback from the limbs has been shown to be important for control of limb-based behaviors and we hypothesized that this is also the case for the fishes. In this study, we examined the impact of the loss of sensory feedback from the pectoral fins on movement kinematics during hover behavior. Research was performed with bluegill sunfish (Lepomis macrochirus), a model for understanding the biomechanics of swimming and for bio-inspired design of engineered fins. The bluegill beats its pectoral fins rhythmically, and in coordination with pelvic and median fin movement, to maintain a stationary position while hovering. Bilateral deafferentation of the fin rays results in a splay-finned posture where fins beat regularly but at a higher frequency and without adducting fully against the side of the body. For unilateral transections, more irregular changes in fin movements were recorded. These data indicate that sensory feedback from the fin rays and membrane is important for generating normal hover movements but is not necessary for generating rhythmic fin movement.

Anatomical and physiological studies of bigheaded carps demonstrate that the epibranchial organ functions as a pharyngeal taste organ

The epibranchial organ (EO) is an enigmatic tubular organ found in the pharyngeal cavity of many filter-feeding fishes. We investigated whether it might function as a taste organ that mediates aggregation and ingestion of planktonic food within the buccal cavity. The EO and associated structures of bighead and silver carps, two successful and invasive planktivorous fishes, were examined using histological and electrophysiological techniques. Both species possess finely structured gill rakers that extend directly via a series of protrusions into each of the four blind canals which are organized as the muscular EO, suggesting that the gill rakers and EO probably function in an integrated manner. Both the interior and exterior surfaces of the EOs of both species are covered with high densities of taste buds and solitary chemosensory cells (SCCs) as well as mucous cells. Conversely, taste buds are scarce in both the buccal cavities and external portions of the head and mouth of both species. Electrophysiological recordings from a caudal branch of the vagus nerve (cranial nerve X) found to innervate the EO showed it to be sensitive to chemicals found in a planktonic diet. l-Amino acids accounted for some, but not all of the neural activity. We conclude that taste buds and SCCs located on the EO and gill rakers probably serve to chemically detect food particles, which the EO then aggregates by mucus secretion before eventually expelling them onto the floor of the pharynx for ingestion. This specialized, pharyngeal chemosensory structure may explain the feeding success of these, and perhaps other planktivorous, filter-feeding fishes.

Evolutionary origins of taste buds: phylogenetic analysis of purinergic neurotransmission in epithelial chemosensors

Taste buds are gustatory endorgans which use an uncommon purinergic signalling system to transmit information to afferent gustatory nerve fibres. In mammals, ATP is a crucial neurotransmitter released by the taste cells to activate the afferent nerve fibres. Taste buds in mammals display a characteristic, highly specific ecto-ATPase (NTPDase2) activity, suggesting a role in inactivation of the neurotransmitter. The purpose of this study was to test whether the presence of markers of purinergic signalling characterize taste buds in anamniote vertebrates and to test whether similar purinergic systems are employed by other exteroceptive chemosensory systems. The species examined include several teleosts, elasmobranchs, lampreys and hagfish, the last of which lacks vertebrate-type taste buds. For comparison, Schreiner organs of hagfish and solitary chemosensory cells (SCCs) of teleosts, both of which are epidermal chemosensory end organs, were also examined because they might be evolutionarily related to taste buds. Ecto-ATPase activity was evident in elongate cells in all fish taste buds, including teleosts, elasmobranchs and lampreys. Neither SCCs nor Schreiner organs show specific ecto-ATPase activity, suggesting that purinergic signalling is not crucial in those systems as it is for taste buds. These findings suggest that the taste system did not originate from SCCs but arose independently in early vertebrates.

Chemosensory-induced motor behaviors in fish

Chemical sensory signals play a crucial role in eliciting motor behaviors. We now review the different motor behaviors induced by chemosensory stimuli in fish as well as their neural substrate. A great deal of research has focused on migratory, reproductive, foraging, and escape behaviors but it is only recently that the molecules mediating these chemotactic responses have become well-characterized. Chemotactic responses are mediated by three sensory systems: olfactory, gustatory, and diffuse chemosensory. The olfactory sensory neuron responses to chemicals are now better understood. In addition, the olfactory projections to the central nervous system were recently shown to display an odotopic organization in the forebrain. Moreover, a specific downward projection underlying motor responses to olfactory inputs was recently described.

The taste cell-related diffuse chemosensory system

Elements expressing the molecular mechanisms of gustatory transduction have been described in several organs in the digestive and respiratory apparatuses. These taste cell-related elements are isolated cells, which are not grouped in buds, and they have been interpreted as chemoreceptors. Their presence in epithelia of endodermal origin suggests the existence of a diffuse chemosensory system (DCS) sharing common signaling mechanisms with the "classic" taste organs. The elements of this taste cell-related DCS display a site-related morphologic polymorphism, and in the past they have been indicated with various names (e.g., brush, tuft, caveolated, fibrillo-vesicular or solitary chemosensory cells). It may be that the taste cell-related DCS is like an iceberg: the taste buds are probably only the most visible portion, with most of the iceberg more caudally located in the form of solitary chemosensory cells or chemosensory clusters. Comparative anatomical studies in lower vertebrates suggest that this 'submerged' portion may represent the most phylogenetically ancient component of the system, which is probably involved in defensive or digestive mechanisms. In the taste buds, the presence of several cell subtypes and of a wide range of molecular mechanisms permits precise food analysis. The larger, 'submerged' portion of the iceberg is composed of a polymorphic population of isolated elements or cell clusters in which the molecular cascade of cell signaling needs to be explored in detail. The little data we have strongly suggests a close relationship with taste cells. Morphological and biochemical considerations suggest that the DCS is a potential new drug target. Modulation of the respiratory and digestive apparatuses through substances, which act on the molecular receptors of this chemoreceptive system, could be a new frontier in drug discovery.

Medetomidine, ketamine, and sevoflurane for anesthesia of injured loggerhead sea turtles: 13 cases (1996-2000)

OBJECTIVE

To determine safety and efficacy of an anesthetic protocol incorporating medetomidine, ketamine, and sevoflurane for anesthesia of injured loggerhead sea turtles.

DESIGN

Retrospective study.

ANIMALS

13 loggerhead sea turtles.

PROCEDURE

Anesthesia was induced with medetomidine (50 microg/kg [22.7 microg/lb], IV) and ketamine (5 mg/kg (2.3 mg/lb], IV) and maintained with sevoflurane (0.5 to 2.5%) in oxygen. Sevoflurane was delivered with a pressure-limited intermittent-flow ventilator. Heart rate and rhythm, end-tidal partial pressure of CO2, and cloacal temperature were monitored continuously; venous blood gas analyses were performed intermittently. Administration of sevoflurane was discontinued 30 to 60 minutes prior to the end of the surgical procedure. Atipamezole (0.25 mg/kg [0.11 mg/lb], IV) was administered at the end of surgery.

RESULTS

Median induction time was 11 minutes (range, 2 to 40 minutes; n = 11). Median delivered sevoflurane concentrations 15, 30, 60, and 120 minutes after intubation were 2.5 (n = 12), 1.5 (12), 1.25 (12), and 0.5% (8), respectively. Heart rate decreased during surgery to a median value of 15 beats/min (n = 11). End-tidal partial pressure of CO2 ranged from 2 to 16 mm Hg (n = 8); median blood gas values were within reference limits. Median time from atipamezole administration to extubation was 14 minutes (range, 2 to 84 minutes; n = 7).

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that a combination of medetomidine and ketamine for induction and sevoflurane for maintenance provides safe, effective, controllable anesthesia in injured loggerhead sea turtles.

Ascending spinal systems in the fish, Prionotus carolinus

The fin rays of the pectoral fin of the sea robins (teleostei) are specialized chemosensory organs heavily invested with solitary chemoreceptor cells innervated only by spinal nerves. The rostral spinal cord of these animals is marked by accessory spinal lobes which are unique enlargements of the dorsal horn of the rostral spinal segments receiving input from the fin ray nerves. Horseradish peroxidase (HRP) and 1,1;-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (diI) were used as anterograde and retrograde tracers to examine the connectivity of these accessory lobes and the associated ascending spinal systems in the sea robin, Prionotus carolinus. The majority of dorsal root fibers terminate within the accessory lobes at or nearby their level of entrance into the spinal cord. A few dorsal root axons turn rostrally in the dorsolateral fasciculus to terminate in the lateral funicular complex situated at the spinomedullary junction. The lateral funicular complex also receives a heavy projection from the ipsilateral accessory lobes. In addition, it contains a few large neurons that project back onto the accessory lobes. Injections of either diI or HRP into the lateral funicular complex label fibers of the medial lemniscus which crosses the midline in the caudal medulla to ascend along the ventral margin of the contralateral rhombencephalon. Within the medulla, fibers leave the medial lemniscus to terminate in the inferior olive and in the ventrolateral medullary reticular formation. Upon reaching the midbrain, the medial lemniscus turns dorsally to terminate heavily in a lateral division of the torus semicircularis, in the ventral optic tectum, and in the lateral subnucleus of the nuc. preglomerulosus of the thalamus. Lesser projections also reach the posterior periventricular portion of the posterior tubercle with a few fibers terminating along the ventral, posterior margin of the ventromedial (VM) nucleus of the thalamus. The restricted projection to the ventral tectum is noteworthy in that this part of the tectum maintains the representation of the ventral visual field, that is, the area in which the fin rays lie. A prominent spinocerebellar system is also evident. Both direct and indirect spinocerebellar fibers can be followed through the dorsolateral fasciculus, with or without relay in the lateral funicular nucleus and terminating in a restricted portion of the granule cell layer of the ipsilateral corpus cerebelli. The similarities in connectivity of the spinal cord between the sea robins and other vertebrates are striking. It is especially notable because sea robins utilize the chemosensory input from the fin rays to localize food in the environment. Thus, although these fish use their spinal chemosense as other fishes use their external taste systems, the spinal chemosense apparently relies on the medial lemniscal system to guide this chemically driven feeding behavior.

High-voltage electron microscopy and 3-D reconstruction of solitary chemosensory cells in the anterior dorsal fin of the Gadid fish Ciliata mustela (Teleostei)

Solitary chemosensory cells (SCCs) are secondary sensory cells present in the epidermis of most primary aquatic vertebrates. In rocklings, the epidermis of the anterior dorsal fin (ADF) contains approximately 5 million SCCs. High-voltage electron microscopy and three-dimensional reconstructions from serial sections were used to examine the ultrastructure, arrangement, and synaptic contacts of the SCCs in the rockling ADF. Approximately 15% of all cells in the fin ray epidermis are SCCs, which occupy roughly 30% of the epidermal volume. These spindle-shaped cells are 25-30 microm long and up to 10 microm wide and terminate apically in a microvillus protruding 2-5 microm above the epidermal surface. SCCs contain abundant endoplasmic reticulum and a large Golgi apparatus in their proximal regions. The distal parts of SCCs contain characteristic vesicles, elongate mitochondria, and longitudinal strands of intermediate filaments. Synapses between SCCs and nerves resemble those found in teleost taste buds. One to four synaptic contacts per SCC were found. We hypothesize that the apparent secretory activity of the SCCs serves to replenish the apical membrane and mucus. Furthermore, parallel sampling of several hundred SCCs by single nerve fibers may serve low-threshold detection rather than stimulus localization.

Schreiner organs: a new craniate chemosensory modality in hagfishes

An extensive system of sensory organs resembling taste buds was previously known in the skin of hagfishes. These sensory organs, called here Schreiner organs, are found throughout the epidermis of both Eptatretus stoutii and Myxine glutinosa. They are found also at high densities in the prenasal sinus, nasopharyngeal duct, and pharynx, and at lower densities in the oral and velar chambers. Schreiner organs are multicellular aggregates composed of acetylated tubulin-immunoreactive receptor cells and nonimmunoreactive cells. A considerable range of variation was found in Schreiner organ morphology, but discrete classes of organs could not be recognized. Schreiner organs are innervated by all sensory trigeminal rami, the glossopharyngeal/vagal nerve, and cutaneous rami of spinal nerves, but not by the facial nerve. The central projections of these rami form a continuous tract in the trigeminal sensory zone and the dorsolateral funiculus of the spinal cord. Some Schreiner organs may be represented in the nucleus of the solitary tract, but this structure is certainly not the primary recipient zone of Schreiner organ afferents. In light of these systemic differences between vertebrate taste systems and the Schreiner organ system of hagfishes, it is concluded that Schreiner organs are not homologous to taste buds. This sensory modality of hagfishes has no direct homolog in vertebrates and appears to be a specialization of hagfishes, perhaps derived from the primitive somatosensory system of the earliest craniates.

Differential localization of putative amino acid receptors in taste buds of the channel catfish, Ictalurus punctatus

The taste system of catfish, having distinct taste receptor sites for L-alanine and L-arginine, is highly sensitive to amino acids. A previously described monoclonal antibody (G-10), which inhibits L-alanine binding to a partial membrane fraction (P2) derived from catfish (Ictalurus punctatus) taste epithelium, was found in Western blots to recognize a single band, at apparent MW of 113,000 D. This MW differs from the apparent MW for the presumed arginine receptor identified previously by PHA-E lectin affinity. In order to test whether PHA-E lectin actually reacts with the arginine-receptor, reconstituted membrane proteins partially purified by PHA-E affinity were used in artificial lipid bilayers. These reconstituted channels exhibited L-arginine-activated activity similar to that found in taste cell membranes. Accordingly, we utilized the PHA-E lectin and G-10 antibody as probes to differentially localize the L-alanine and L-arginine binding sites on the apical surface of catfish taste buds. Each probe labels numerous, small (0.5-1.0 micron) patches within the taste pore of each taste bud. This observation suggests that each bud is not tuned to a single taste substance, but contains putative receptor sites for both L-arginine and L-alanine. Further, analysis of double-labeled tissue reveals that the PHA-E and G-10 sites tend to be separate within each taste pore. These findings imply that in catfish, individual taste cells preferentially express receptors to either L-arginine or L-alanine. In addition, PHA-E binds to the apices of solitary chemoreceptor cells in the epithelium, indicating that this independent chemoreceptor system may utilize some receptor sites similar to those in taste buds.

The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

Chemoreception of taurocholate in anosmic and sham-operated cod, Gadus morhua

This study investigated the role of the olfactory system in cod, Gadus morhua L., on the general activity level and the responses to the bile salt taurocholate. Ten cod were rendered anosmic by section of the olfactory tracts, while another 10 control fish were sham-operated. The cod were stimulated in a seawater olfactometer which permitted reproducible administration of diluted samples of taurocholate at 5 concentration levels. The activity scores for both groups of cod increased with increasing concentrations of taurocholate. The detection threshold in the sham-operated cod for taurocholate was 7 nM, while the anosmic cod detected the presence of taurocholate at 70 nM. Taurocholate induced orienting reaction and snapping, both in sham-operated and in anosmic cod, indicating convergence of olfactory and other chemosensory pathways to nerve centers mediating these kinds of behavior. The bottom food search was observed only in the control fish. The seawater blanks induced a lower total activity score in the anosmic than in the sham-operated cod, which suggests that the olfactory input augments the general activity level.

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

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. 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. 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. 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. 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. 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. 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 (0.005 M), uridine-5'-monophosphate (UMP, 0.001 M), proline (0.01 M), CaCl2 (0.5 M), and NaSCN (0.5 M). 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. The responses of the mechanosensitive units consisted of phasic and tonic impulse trains and the sensitivity of the units varied considerably. 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.

Organization of motoneuronal pools in the rostral spinal cord of the sea robin, Prionotus carolinus

The functional organization of the motoneurons in the spinal cord of the sea robin, Prionotus carolinus, was studied by means of retrograde transport of horseradish peroxidase (HRP). This species has a complex pectoral apparatus which includes not only a webbed fin, but also three independently mobile fin rays. The motoneurons in the rostral spinal cord fall into two longitudinal columns: dorsal and ventral. The motoneurons of the ventral column innervate the appendicular musculature of the pectoral apparatus. Within the ventral motor column of the rostral spinal cord, four distinct motoneuronal pools were found. The largest pool is situated at the rostral-most end of the spinal cord and contains the motoneurons that innervate the musculature of the webbed pectoral fin. The motoneurons that innervate the fin rays are located in sequentially more posterior pools so that the anteroventral fin ray is controlled by motoneurons situated farthest caudally. The somatotopic arrangement exactly corresponds to the sensory somatotopy determined previously. Furthermore, each fin ray has its sensory representation in a unique accessory spinal lobe which is connected in a reflex fashion to the motoneuronal pool that provides motor output to the same fin ray.