Brain and sensory organ morphology in Antarctic eelpouts (Perciformes: Zoarcidae: Lycodinae)

Eelpouts of the family Zoarcidae comprise a monophyletic group of marine fishes with a worldwide distribution. Centers of high zoarcid diversity occur in the North Atlantic and North Pacific, with important radiations into the Arctic, along southern South America, and into the Southern Ocean around Antarctica. Along with snailfishes (Liparidae), zoarcids form an important component of the non-notothenioid fauna in the subzero shelf waters of Antarctica. We document the anatomy and histology of the brains, cranial nerves, olfactory apparatus, cephalic lateral lines, taste buds, and retinas of three Antarctic zoarcid species, living at depths of 310-939 m, representing three of the nine genera from this region. The primary emphasis is on Ophthalmolycus amberensis, and we provide a detailed drawing of the brain and cranial nerves of this species. Although this brain reflects general perciform neural morphology, it exhibits a reduction of the (optic) tecta and the eminentia granulares and crista cerebellares of the lateral line system. Interspecific differences among the three species are slight. The olfactory rosette consists of three to four lamellae and the nasal sac, contrary to the claim of Fanta et al. ([2001] Antarct Rec, Natl Inst Polar Res, Tokyo 45:27-42), is not in communication with the cephalic lateral line system. Primary olfactory neurons are abundant and converge on branches of the olfactory nerve. Numerous taste buds are located in the lips. All three species lack an ocular choroid rete and have relatively thin retinas with a low cell density and a single bank of rods as the only type of photoreceptor. Neural diversification among Antarctic zoarcids has not involved the evolution of sensory specialists; brain and sensory organ morphologies do not approach the condition seen in primary deep-sea fishes, or even that of some sympatric non-perciform secondary deep-sea fishes, including liparids and muraenolepidids (eel cods). There may be phylogenetic constraints on brain morphology in perciforms such that we do not see extreme specialization in sensory and neural systems for deep habitats. We suggest that the brains and sensory organs of Antarctic zoarcids reflect habitation of 500-2,000-m depths and likely reflect morphologies seen in zoarcids living on continental slopes elsewhere in the world. This balance among the sensory modalities makes zoarcids relatively generalized among secondary deep-sea fishes and may be one of the reasons this opportunistic and adaptable group has been successful in colonizing a variety of emergent and ephemeral habitats.

Taste recognition: food for thought

The ability to identify food that is nutrient-rich and avoid toxic substances is essential for an animal's survival. Although olfaction and vision contribute to food detection, the gustatory system acts as a final checkpoint control for food acceptance or rejection behavior. Recent studies with model organisms such as mice and Drosophila have identified candidate taste receptors and examined the logic of taste coding in the periphery. Despite differences in terms of gustatory anatomy and taste-receptor families, these gustatory systems share a basic organization that is different from other sensory systems. This review will summarize our current understanding of taste recognition in mammals and Drosophila, highlighting similarities and raising several as yet unanswered questions.

Temporally staggered glomerulus development in the moth Manduca sexta

Glomeruli, neuropilar structures composed of olfactory receptor neuron (ORN) axon terminals and central neuron dendrites, are a common feature of olfactory systems. Typically, ORN axons segregate into glomeruli based on odor specificity, making glomeruli the basic unit for initial processing of odorant information. Developmentally, glomeruli arise from protoglomeruli, loose clusters of ORN axons that gradually synapse onto dendrites. Previous work in the moth Manduca sexta demonstrated that protoglomeruli develop in a wave across the antennal lobe (AL) during stage 5 of the 18 stages of metamorphic adult development. However, ORN axons from the distal segments of the antenna arrive at the AL for several more days. We report that protoglomeruli present at stage 5 account for only approximately two or three of adult glomeruli with the number of structures increasing over subsequent stages. How do these later arriving axons incorporate into glomeruli? Examining the dendritic projections of a unique serotonin-containing neuron into glomeruli at later stages revealed glomeruli with immature dendritic arbors intermingled among more mature glomeruli. Labeling ORN axons that originate in proximal segments of the antenna suggested that early-arriving axons target a limited number of glomeruli. We conclude that AL glomeruli form over an extended time period, possibly as a result of ORNs expressing new odorant receptors arriving from distal antennal segments.

Nitric oxide in the amphibian (Ambystoma tigrinum) lateral line

Nicotinamide adenine dinucleotide phosphate reduced-diaphorase (NADPH-d) histochemistry was investigated in the axolotl (Ambystoma tigrinum) lateral line. Hair cells of neuromast organs of the head skin and neurons of the postotic ganglia showed a significant NADPH-d reaction. Multiunit recording of neuromast afferent activity was also performed. Nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) produced an initial slight excitation followed by a significant inhibition of the resting discharge of neuromast afferent neurons. In contrast N(G)-nitro-L-arginine (L-NOARG) produced non-significant actions on the afferent neurons discharge. These findings suggest that afferent neurons and hair cells of the lateral line produce nitric oxide that plays an active role in the mechanisms sustaining basal spike discharge in afferent neurons.

Hydrodynamic detection by cupulae in a lateral line canal: functional relations between physics and physiology

In the present review, signal-processing capabilities of the canal lateral line organ imposed by its peripheral architecture are quantified in terms of a limited set of measurable physical parameters. It is demonstrated that cupulae in the lateral line canal organ can only partly be described as canal fluid velocity detectors. Deviation from velocity detection may result from resonance, and can be characterized by the extent to which a single dimensionless resonance number, N ( r ), exceeds 1. This number depends on four physical parameters: it is proportional to cupular size, cupular sliding stiffness and canal fluid density, and inversely proportional to the square of fluid viscosity. Situated in a canal, a cupula may benefit from its resonance by compensating for the limited frequency range of water motion that is efficiently transferred into the lateral line canal. The peripheral transfer of hydrodynamic signals, via canal and cupula, leads to a nearly constant sensitivity to outside water acceleration in a bandwidth that ranges from d.c. to a cut-off frequency of up to several hundreds of Hertz, significantly exceeding the cut-off frequency of the lateral line canal. Threshold values of hydrodynamic detection by the canal lateral line organ are derived in terms of water displacement, water velocity, water acceleration and water pressure gradients and are shown to be close to the detection limits imposed by hair cell mechano-transduction in combination with the physical constraints of peripheral lateral line signal transfer. The notion that the combination of canal- and cupular hydrodynamics effectively provides the lateral line canal organ with a constant sensitivity to water acceleration at low frequencies so that it consequently functions as a low-pass detector of pressure gradients, supports the appropriateness of describing it as a sense organ that "feels at a distance" (Dijkgraaf in Biol Rev 38:51-105, 1963).

Regulation of tentacle length in snails by odor concentration

The upper tentacle of the snail, bearing the olfactory organ, produces complex movements when the snail explores a new environment. Tentacle trajectories were reconstructed in the presence and absence of odors using two simultaneous video recordings. Reconstructions showed that in the absence of odor, snails constantly scanned the surrounding space with the extended tentacles. Presentation of an odor elicited rapid flexion, independent of the odor concentration, accompanied by concentration-dependent tentacle contractions. Activation of identified motoneuron MtC3 is known to elicit tentacle contraction. Recordings made in semi-intact preparations showed that the dynamics and duration of the spike activity of MtC3 produced in response to odors correlated with the degree of tentacle contraction in response to odors. These data suggest that the central motoneuron MtC3, which triggers tentacle contraction, is involved in controlling the margins of the scanning field. Slow contraction or extension of the tentacle, associated with the level of MtC3 activity, may operate to tune the snail's investigative behavior to the conditions of the sensory environment.

Electrophysiological and behavioural characterization of gustatory responses to antennal ‘bitter’ taste in honeybees

We combined behavioural and electrophysiological experiments to study whether bitter taste is perceived at the antennal level in honeybees, Apis mellifera. Our behavioural studies showed that neither quinine nor salicin delivered at one antenna at different concentrations induced a retraction of the proboscis once it was extended in response to 1 M sucrose solution delivered to the opposite antenna. Bees that extended massively their proboscis to 1 M sucrose responded only partially when stimulated with a mixture of 1 M sucrose and 100 mM quinine. The mixture of 1 m sucrose and 100 mM salicin had no such suppressive effect. No behavioural suppression was found for mixtures of salt solution and either bitter substance. Electrophysiological recordings of taste sensillae at the antennal tip revealed sensillae that responded specifically either to sucrose or salt solutions, but none responded to the bitter substances quinine and salicin at the different concentrations tested. The electrophysiological responses of sensillae to 15 mM sucrose solution were inhibited by a mixture of 15 mM sucrose and 0.1 mM quinine, but not by a mixture of 15 mM sucrose and 0.1 mM salicin. The responses of sensillae to 50 mM NaCl were reduced by a mixture of 50 mm NaCl and 1 mM quinine but not by a mixture of 50 mM NaCl and 1 mM salicin. We concluded that no receptor cells for the bitter substances tested, exist at the level of the antennal tip of the honeybee and that antennal bitter taste is not represented as a separate perceptual quality.

Role of GABAergic inhibition in shaping odor-evoked spatiotemporal patterns in the Drosophila antennal lobe

Drosophila olfactory receptor neurons project to the antennal lobe, the insect analog of the mammalian olfactory bulb. GABAergic synaptic inhibition is thought to play a critical role in olfactory processing in the antennal lobe and olfactory bulb. However, the properties of GABAergic neurons and the cellular effects of GABA have not been described in Drosophila, an important model organism for olfaction research. We have used whole-cell patch-clamp recording, pharmacology, immunohistochemistry, and genetic markers to investigate how GABAergic inhibition affects olfactory processing in the Drosophila antennal lobe. We show that many axonless local neurons (LNs) in the adult antennal lobe are GABAergic. GABA hyperpolarizes antennal lobe projection neurons (PNs) via two distinct conductances, blocked by a GABAA- and GABAB-type antagonist, respectively. Whereas GABAA receptors shape PN odor responses during the early phase of odor responses, GABAB receptors mediate odor-evoked inhibition on longer time scales. The patterns of odor-evoked GABAB-mediated inhibition differ across glomeruli and across odors. Finally, we show that LNs display broad but diverse morphologies and odor preferences, suggesting a cellular basis for odor- and glomerulus-dependent patterns of inhibition. Together, these results are consistent with a model in which odors elicit stimulus-specific spatial patterns of GABA release, and as a result, GABAergic inhibition increases the degree of difference between the neural representations of different odors.

Asymmetric Rab 11 endosomes regulate delta recycling and specify cell fate in the Drosophila nervous system

Drosophila sensory organ precursor (SOP) cells are a well-studied model system for asymmetric cell division. During SOP division, the determinants Numb and Neuralized segregate into the pIIb daughter cell and establish a distinct cell fate by regulating Notch/Delta signaling. Here, we describe a Numb- and Neuralized-independent mechanism that acts redundantly in cell-fate specification. We show that trafficking of the Notch ligand Delta is different in the two daughter cells. In pIIb, Delta passes through the recycling endosome which is marked by Rab 11. In pIIa, however, the recycling endosome does not form because the centrosome fails to recruit Nuclear fallout, a Rab 11 binding partner that is essential for recycling endosome formation. Using a mammalian cell culture system, we demonstrate that recycling endosomes are essential for Delta activity. Our results suggest that cells can regulate signaling pathways and influence their developmental fate by inhibiting the formation of individual endocytic compartments.

Inhibitory glutamate receptors in spider peripheral mechanosensory neurons

Most mechanosensory neurons are inhibited by GABAergic efferent neurons. This inhibition is often presynaptic and mediated by ionotropic GABA receptors at the axon terminals. GABA receptor activation opens Cl- channels, leading to membrane depolarization and an increase in membrane conductance. In many invertebrate preparations, efferent neurons that innervate mechanosensory afferents contain glutamate in addition to GABA, suggesting that the sensory neurons are also modulated by glutamate. However, the effects of glutamate on these neurons are not well understood. Peripheral parts of the spider (Cupiennius salei) mechanosensory neurons are surrounded by efferent fibers immunoreactive to antibodies against GABA and glutamate. GABA and its analogue muscimol were shown to effectively inhibit spider mechanosensory neurons innervating lyriform slit sensilla VS-3 that detects cuticular strains in the leg. Here, we show that glutamate also inhibits the VS-3 neurons, but its effects are different from those of GABA or muscimol, suggesting that it acts on a different group of receptors. GABA and muscimol always depolarized these neurons and the inhibitory effect was strongly correlated with the amount of depolarization. In contrast, glutamate inhibited the VS-3 neurons even when it did not depolarize them. In addition, while glutamate inhibited both the axonal action potentials elicited with electrical stimulation and dendritic action potentials produced by mechanical stimulation, muscimol only inhibited the axonal action potentials. Therefore, the inhibitory glutamate receptors in the VS-3 neurons are distinct from and differently distributed than the GABA receptors, providing a subtle control of the neurons' sensitivity in varying behavioural situations.

Spatio-temporal Ca2+ dynamics of moth olfactory projection neurones

We studied the Ca2+ dynamics of odour-evoked glomerular patterns in the antennal lobe of the moth Spodoptera littoralis using optical imaging. Here we selectively stained a large population of antennal lobe output neurones, projection neurones, by retrograde filling with FURA-dextran from the inner antennocerebral tract in the protocerebrum. Different plant-associated odorants evoked distributed patterns of activated glomeruli that were odour dependent and repeatable. These patterns were, however, dynamic during the period of odour exposure. Temporal responses differed across glomeruli and were stimulus dependent. Next we examined how the correlations between patterns evoked by different odorants changed with time. Initially, responses to structurally similar compounds were highly correlated, whereas responses to structurally different compounds differed. Within the period of odour exposure (1 s) we found a significant reduction in similarity of responses evoked by different odours, irrespective of initial similarity, whereas trial-to-trial correlations remained high. Our results suggested an ability for coarse classification at the initial encounter with an odour source. With time, however, the discrimination ability increases and structurally similar odours can be distinguished.

Physiology and glomerular projections of olfactory receptor neurons on the antenna of female Heliothis virescens (Lepidoptera: Noctuidae) responsive to behaviorally relevant odors

The neurophysiology and antennal lobe projections of olfactory receptor neurons housed within short trichoid sensilla of female Heliothis virescens F. (Noctuidae: Lepidoptera) were investigated using a combination of cut-sensillum recording and cobalt-lysine staining techniques. Behaviorally relevant odorants, including intra- and inter-sexual pheromonal compounds, plant and floral volatiles were selected for testing sensillar responses. A total of 184 sensilla were categorized into 25 possible sensillar types based on odor responses and sensitivity. Sensilla exhibited both narrow (responding to few odors) and broad (responding to many odors) response spectra. Sixty-six percent of the sensilla identified were stimulated by conspecific odors; in particular, major components of the male H. virescens hairpencil pheromone (hexadecanyl acetate and octadecanyl acetate) and a minor component of the female sex pheromone, (Z)-9-tetradecenal. Following characterization of the responses, olfactory receptor neurons within individual sensilla were stained with cobalt lysine (N=39) and traced to individual glomeruli in the antennal lobe. Olfactory receptor neurons with specific responses to (Z)-9-tetradecenal, a female H. virescens sex pheromone component, projected to the female-specific central large female glomerulus (cLFG) and other glomeruli. Terminal arborizations from sensillar types containing olfactory receptor neurons sensitive to male hairpencil components and plant volatiles were also localized to distinct glomerular locations. This information provides insight into the representation of behaviorally relevant odorants in the female moth olfactory system.

Drosophila caspase transduces Shaggy/GSK-3beta kinase activity in neural precursor development

Caspases are well known for their role in the execution of apoptotic programs, in which they cleave specific target proteins, leading to the elimination of cells, and for their role in cytokine maturation. In this study, we identified a novel substrate, which, through cleavage by caspases, can regulate Drosophila neural precursor development. Shaggy (Sgg)46 protein, an isoform encoded by the sgg gene and essential for the negative regulation of Wingless signaling, is cleaved by the Dark-dependent caspase. This cleavage converts it to an active kinase, which contributes to the formation of neural precursor (sensory organ precursor (SOP)) cells. Our evidence suggests that caspase regulation of the wingless pathway is not associated with apoptotic cell death. These results imply a novel role for caspases in modulating cell signaling pathways through substrate cleavage in neural precursor development.

Projections of male-specific receptor neurons in the antennal lobe of the Oriental tobacco budworm moth, Helicoverpa assulta: a unique glomerular organization among related species

The macroglomerular complex in the primary olfactory center of male moths receives information from numerous pheromone-detecting receptor neurons housed in specific sensilla located on the antennae. We investigated the functional organization of the three glomeruli constituting this complex in Helicoverpa assulta, a unique species among heliothine moths as concerns the composition of the pheromone blend. By tip recordings from the male-specific receptor neurons combined with cobalt-lysine stainings, the axon terminals in the brain were traced and subsequently reconstructed by camera lucida drawings. Some were also reconstructed in a digital form. The results showed that the sensilla could be classified into two functional types. A major category housed two colocalized receptor neurons, one responding to the primary pheromone component cis-9-hexadecenal and the other to the behavioral antagonists cis-9-tetradecenal and cis-9-hexadecenol. Cobalt-lysine applied to this sensillum type consistently resulted in two stained axons, each terminating in one of the two large subunits of the macroglomerular complex: the cumulus or the dorsomedial glomerulus. The second, less frequently appearing sensillum type contained a receptor neuron responding to the second pheromone component, cis-11-hexadecenal. Dye applied to this type resulted in stained axon projections in the ventral glomerulus. In an evolutionary context it is particularly interesting that differences of related heliothine species are reflected in the functional organization of the MGC compartments.

Candidate pheromone receptors of the silkmoth Bombyx mori

Communication via specific chemical signals is vitally important for the reproductive behaviour of many species. The first identified sex-attractant pheromone was bombykol from the silkmoth Bombyx mori. This female-released signalling compound is perceived by the male moth with extreme sensitivity and specificity. Antennal sensory cells supposedly respond to individual bombykol molecules and can efficiently distinguish bombykol from highly related structural analogues like bombykal, a second female-released pheromone component. In the four decades since the discovery of bombykol, the Bombyx mori system has continued to serve as an invaluable model system for unraveling the intricacies of chemical communication. The molecular basis for this extraordinary specific recognition of an extraneous compound is still elusive but probably based on specific receptors of the pheromone-responsive cells. In this study, molecular and bioinformatic approaches were employed to search for candidate pheromone receptors of Bombyx mori. A few receptor types were identified that are related to Heliothis candidate pheromone receptors. They were found to be almost exclusively expressed in male antennae, and double in situ hybridization experiments disclosed a characteristic topographic expression pattern that was reminiscent of pheromone-responsive cells. Furthermore, the receptor-expressing cells were closely associated with cells expressing the pheromone-binding protein. Together, the data support the view that the identified receptor types of Bombyx mori are candidate receptors for pheromone components.

Expression and localization of three G protein alpha subunits, Go, Gq, and Gs, in adult antennae of the silkmoth (Bombyx mori)

In insect olfactory receptor neurons, rapid and transient increases in inositol triphosphate (IP3) and Ca2+ are detected upon stimulation with pheromone or nonpheromonal odorants. This suggests that heterotrimeric guanine nucleotide binding proteins (G proteins) may transduce some odorant responses in insects. We obtained cDNA clones encoding three classes of G protein alpha subunits, Bm Go, Bm Gq, and Bm Gs, from the antennae of the adult male silkmoth (Bombyx mori). RT-PCR experiments showed that the mRNA of these G protein alpha subunits was also present in the various tissues of adult and larval insects. We used immunocytochemistry to localize these G protein alpha subunits in adult male and female antennae. In the adult male antennae, anti-Go antiserum stained the nerve bundles. In contrast, anti-Gq and anti-Gs antisera stained the inner and outer dendritic segments of the putative olfactory receptor neuron. The localizations of Bm Go, Bm Gq, and Bm Gs in the female antennae were the same as in the male antennae. The localizations of Bm Gq and Bm Gs suggest that each subunit mediates a subset of the odorant response.

Expression of muscarinic acetylcholine receptors and choline acetyltransferase enzyme in cultured antennal sensory neurons and non-neural cells of the developing moth Manduca sexta

Antennal sensory neurons of Manduca sexta emerge from epidermal cells that also give rise to sheath cells surrounding the peripheral parts of the neurons and to glial cells that enwrap the sensory axons in the antennal nerve. Reciprocal interactions between sensory neurons and glial cells are believed to aid in axon growth and guidance, but the exact nature of these interactions is not known. We investigated the possibility of cholinergic interactions in this process by locating muscarinic acetylcholine receptors (mAChRs) and choline acetyltransferase (ChAT) enzyme in cultured antennal sensory neurons and non-neural cells. ChAT and mAChRs were present in the sensory neurons from the first day in culture. Therefore, the sensory neurons are probably cholinergic, as previously suggested, but they may also be controlled by ACh. In 7-day-old cultures a subgroup of small non-neural cells with processes expressed ChAT activity, and in 14-day-old cultures non-neural cells that formed lamellipodia and scaffoldlike structures on the culture substrate were labeled with ChAT antibody. mAChR activity was detected in similar non-neural cells but only in areas surrounding the nuclei. In addition, mAChRs were found in flat lamellipodia and filopodia forming cells that were present in 1-day-old cultures and grew in size during the 2 week investigation period. These findings suggest muscarinic cholinergic interactions between the neural and non-neural cells during the development of Manduca antenna.

Rapid responses of the cupula in the lateral line of ruffe (Gymnocephalus cernuus)

Displacements of cupulae in the supraorbital lateral line canal in ruffe (Gymnocephalus cernuus) have been measured using laser interferometry and by applying transient as well as sinusoidal fluid stimuli in the lateral line canal. The cupular displacement in response to impulses of fluid velocity shows damped oscillations at approximately 120 Hz and a relaxation time-constant of 4.4 ms, commensurate with a quality factor of approximately 1.8. These values are in close agreement with the frequency characteristics determined via sinusoidal fluid stimuli, implying that the nonlinearity of cupular dynamics imposed by the gating apparatus of the sensory hair cells is limited in the range of cupular displacements and velocities measured (100-300 nm; 100-300 microm/s). The measurements also show that cupular displacement instantaneously follows the initial waveform of transient stimuli. The functional significance of the observed cupular dynamics is discussed.

FlyPNS, a database of the Drosophila embryonic and larval peripheral nervous system

Background

The embryonic and larval peripheral nervous system of Drosophila melanogaster is extensively studied as a very powerful model of developmental biology. One main advantage of this system is the ability to study the origin and development of individual sensory cells. However, there remain several discrepancies regarding the organization of sensory organs in each abdominal segment A1-A7.

Description

We have constructed a web site called FlyPNS (for Fly Peripheral Nervous System) that consolidates a wide range of published and unpublished information about the embryonic and larval sensory organs. It communicates (1) a PNS pattern that solves the discrepancies that have been found in the recent literature, (2) the correspondence between the different nomenclatures that have been used so far, (3) a comprehensive description of each sensory organ, and (4) a list of both published and unpublished markers to reliably identify each PNS cell.

Conclusions

The FlyPNS database integrates disparate data and nomenclature and thus helps understanding the conflicting observations that have been published recently. Furthermore, it is designed to provide assistance in the identification and study of individual sensory cells. We think it will be a useful resource for any researcher with interest in Drosophila sensory organs.

Hierarchy of regulatory events in sensory placode development

Cranial placodes are a uniquely vertebrate characteristic; they form the paired sense organs of the eyes, ears and nose, in addition to the distal parts of some of the cranial sensory ganglia. These focal ectodermal thickenings have been studied from an embryological perspective in a diversity of organisms, revealing tissue interactions that are crucial for the morphological formation of the different placodes. In recent times, there has been a renewed interest in understanding the induction and differentiation of these deceptively simple ectodermal regions. This has led to a wealth of information on the molecular cues governing these processes. In particular, the integration of signals at the level of 'placode-specific' enhancers is beginning to provide a glimpse into the complexity of genetic networks that function within this embryonic cell population to generate key components of the peripheral nervous system.

Dual, multilayered somatosensory maps formed by antennal tactile and contact chemosensory afferents in an insect brain

The antennae of most insects move actively and detect the physical and chemical composition of objects encountered by using their associated tactile sensors. Positional information is required for these sensory modalities to interpret the physical environment. Although we have a good understanding of antennal olfactory pathways, little is known about the destinations of antennal mechanosensory and contact chemosensory (gustatory) receptor neurons in the central nervous system. The cockroach Periplaneta is equipped with a pair of long, thin antennae, which are covered in bristles. The distal portions of each antenna possess about 6,500 bimodal bristles that house one tactile sensory and one to four contact chemosensory neurons. In this study, we investigated the morphologies of bimodal bristle receptor afferents by staining individual or populations of bristles. Unlike olfactory afferents, which project exclusively into the glomeruli in the ventral region of the deutocerebrum, both the presumptive mechanosensory and the contact chemosensory afferents projected into the posterior dorsal region of the deutocerebrum and the anterior region of the subesophageal ganglion. Each afferent showed multilayered segmentation and spatial occupation reflecting its three-dimensional position in the periphery. Presumptive contact chemosensory afferents, characterized by their thin axons and unique branching pattern, occupied more medioventral positions compared with the presumptive tactile afferents. Furthermore, projection fields of presumptive contact chemosensory afferents from single sensilla tended to be segregated from each other. These observations suggest that touch and taste positional information from the antenna is precisely represented in primary centers in a modality-specific manner.

Saccharin stimulates the "deterrent" cell in the blowfly: behavioral and electrophysiological evidence

In the attempt to gain more information on the mechanisms underlying bitter and/or sweet taste reception, we have investigated the responses of labellar chemosensilla in the blowfly Protophormia terraenovae to Na-saccharin, as compared to sweet stimuli (sucrose or fructose) and bitter stimuli (denatonium benzoate or amiloride). Electrophysiological and behavioral results indicate that the sweetener Na-saccharin inhibits the "sugar" cell in the labellar taste sensilla of the blowfly P. terraenovae. In multichoice preference tests, flies ingested more of the solutions containing sugar to those with sugar+Na-saccharin. This finding is in good agreement with the spike frequency reduction observed for the "sugar" cell activity. Analysis of the spike discharges also shows a positive dose-response for the "deterrent" cell following stimulation with Na-saccharin and denatonium benzoate. Flies drank any of the Na-saccharin solutions, regardless of their concentration, less than water, thus indicating a weak deterring effect on water drinking. The prevailing activation of the "deterrent" cell by stimulation with Na-saccharin is not directly coupled with a coherent behavioral output. Cross adaptation was found to occur between responses to Na-saccharin and denatonium benzoate or amiloride regardless of the order of adapting stimuli. In the case of sweet stimuli, cross adaptation occurred when the adapting stimulus was Na-saccharin, but it did not when the adapting stimuli were sucrose or fructose. Addition of Na-saccharin to both sugars significantly depressed the spike firing frequency, while an increase was observed with denatonium benzoate or amiloride.

A hidden program in Drosophila peripheral neurogenesis revealed: fundamental principles underlying sensory organ diversity

How is cell fate diversity reliably achieved during development? Insect sensory organs have been a favorable model system for investigating this question for over 100 years. They are constructed using defined cell lineages that generate a maximum of cell diversity with a minimum number of cell divisions, and display tremendous variety in their morphologies, constituent cell types, and functions. An unexpected realization of the past 5 years is that very diverse sensory organs in Drosophila are produced by astonishingly similar cell lineages, and that their diversity can be largely attributed to only a small repertoire of developmental processes. These include changes in terminal cell differentiation, cell death, cell proliferation, cell recruitment, cell-cell interactions, and asymmetric segregation of cell fate determinants during mitosis. We propose that most Drosophila sensory organs are built from an archetypal lineage, and we speculate about how this stereotyped pattern of cell divisions may have been built during evolution.

Amphids: the neuronal ultrastructure of macrocyclic-lactone-resistant Haemonchus contortus

The development of anthelmintic resistance by nematode parasites is a growing problem for veterinarians and producers. The intensive use of the macrocyclic lactones for the treatment of a variety of parasitic diseases has hastened the development of resistance to this family of parasiticides among sheep, goats and cattle. As a result, resistance to ivermectin, moxidectin and doramectin by Haemonchus contortus has been documented throughout the world. While the exact sites of action of the macrocyclic lactones remain incompletely known, a critical point of entry for these drugs may be the terminally exposed sensory major neurons located in the cephalic end of the worms. These neurons, called amphidial neurons, are located in a pair of channels, the amphids, on either side of the pharynx and are exposed to the external environment via pores at the anterior tip of the worm. Through these neurons, important chemical and thermal cues are gathered by the parasite. Examination of serial electron micrographs of ivermectin-susceptible and ivermectin-resistant H. contortus allows for comparison of neuronal structure, arrangement of neurons within the amphidial channel, and distance of the tip of the dendritic processes to the amphidial pore. The latter of these characteristics provides a useful means by which to compare the association between the neurons and the external environment of the worm. Comparison of parental laboratory strains of ivermectin-susceptible H. contortus with related selected, ivermectin-resistant strains and with a wild-type ivermectin-susceptible field strain of H. contortus from Louisiana reveals that the ivermectin-resistant worms examined have markedly shorter sensory cilia than their ivermectin-susceptible parental counterparts. Additionally, the amphidial neurons of ivermectin-resistant worms are characterized by generalized degeneration and loss of detail, whereas other neurons outside of the channels, such as the labial and cephalic neurons, are normal in structure. Similar degeneration was also observed in field strains of doramectin-resistant H. contortus collected from a New Jersey alpaca heard. These findings raise a number of questions regarding the relationship between amphidial structure and macrocyclic lactone resistance as well as the role of amphids as a means of entry for these molecules. While shortened amphidial sensilla are associated with ivermectin resistance, it remains unclear if such a structural modification facilitates survival of nematodes exposed to the macrocyclic lactones.