Sensory neuron development revealed by taurine immunocytochemistry in the honeybee

Influence of sugar experience during development on gustatory sensitivity of the honey bee

The level of response to sugar plays a role in many aspects of honey bee behavior including age dependent polyethism and division of labor. Bees may tune their sensitivity to sugars so that they maximize collection of high quality nectar, but they must also be able to collect from less profitable sources when high quality food is scarce. However, our understanding of the mechanisms by which bees can change their responsiveness to different sugars remains incomplete. To investigate the plasticity of sensitivity to sugar, bees were raised on different sugars either in vitro or in colonies. Bees raised in the incubator on diets containing mostly either fructose or glucose showed significantly more responsiveness to the majority sugar. In contrast, bees raised in colonies that only foraged on fructose or glucose responded equally well to both sugars. These data suggest that developmental plasticity for responses to sugar is masked by the feeding of worker jelly to larvae and young bees. The production of worker jelly from secretions of the hypopharyngeal and mandibular glands by nurse bees ensures that both glucose and fructose are experienced by young bees so that they respond to both sugars and will be able to exploit all future food sources.

Identification and functional characterization of D-fructose receptor in an egg parasitoid, Trichogramma chilonis

In insects, the gustatory system has a critical function not only in selecting food and feeding behaviours but also in growth and metabolism. Gustatory receptors play an irreplaceable role in insect gustatory signalling. Trichogramma chilonis is an effective biocontrol agent against agricultural insect pests. However, the molecular mechanism of gustation in T. chilonis remains elusive. In this study, we found that T. chilonis adults had a preference for D-fructose and that D-fructose contributed to prolong longevity and improve fecundity. Then, We also isolated the full-length cDNA encoding candidate gustatory receptor (TchiGR43a) based on the transcriptome data of T. chilonis, and observed that the candidate gustatory receptor gene was expressed from the larval to adult stages. The expression levels of TchiGR43a were similar between female and male. A Xenopus oocyte expression system and two-electrode voltage-clamp recording further verified the function analysis of TchiGR43a. Electrophysiological results showed that TchiGR43a was exclusively tuned to D-fructose. By the studies of behaviour, molecular biology and electrophysiology in T. chilonis, our results lay a basic fundation of further study on the molecular mechanisms of gustatory reception and provide theoretical basis for the nutritional requirement of T. chilonis in biocontrol.

Opsin expression patterns coincide with photoreceptor development during pupal development in the honey bee, Apis mellifera

Background

The compound eyes of insects allow them to catch photons and convert the energy into electric signals. All compound eyes consist of numerous ommatidia, each comprising a fixed number of photoreceptors. Different ommatidial types are characterized by a specific set of photoreceptors differing in spectral sensitivity. In honey bees, males and females possess different ommatidial types forming distinct retinal mosaics. However, data are lacking on retinal ontogeny and the mechanisms by which the eyes are patterned. In this study, we investigated the intrinsic temporal and circadian expression patterns of the opsins that give rise to the ultraviolet, blue and green sensitive photoreceptors, as well as the morphological maturation of the retina during pupal development of honey bees.

Results

qPCR and histological labeling revealed that temporal opsin mRNA expression differs between sexes and correlates with rhabdom elongation during photoreceptor development. In the first half of the pupal stage, when the rhabdoms of the photoreceptors are still short, worker and (dorsal) drone retinae exhibit similar expression patterns with relatively high levels of UV (UVop) and only marginal levels of blue (BLop) and green (Lop1) opsin mRNA. In the second half of pupation, when photoreceptors and rhabdoms elongate, opsin expression in workers becomes dominated by Lop1 mRNA. In contrast, the dorsal drone eye shows high expression levels of UVop and BLop mRNA, whereas Lop1 mRNA level decreases. Interestingly, opsin expression levels increase up to 22-fold during early adult life. We also found evidence that opsin expression in adult bees is under the control of the endogenous clock.

Conclusions

Our data indicate that the formation of the sex-specific retinal composition of photoreceptors takes place during the second half of the pupal development, and that opsin mRNA expression levels continue to increase in young bees, which stands in contrast to Drosophila, where the highest expression levels are found during the late pupal stage and remain constant in adults. From an evolutionary perspective, we hypothesize that the delayed retinal maturation during the early adult phase is linked to the delayed transition from indoor to outdoor activities in bees, when vision becomes important.

Development of antennal sensilla of Tetragonisca angustula Latreille, 1811 (Hymenoptera: Meliponini) during pupation

The antennal sensilla are sensory organs formed by a group of neurons and accessory cells, which allow perception of environmental cues, which play a role as mechanoreceptors and chemoreceptors. This study describes the post-embryonic development of the antennal sensilla of the stingless Tetragonisca angustula (Hymenoptera: Meliponini) workers. The development of the antennal sensilla begins in the transition stage of the pre-pupae to white-eyed pupae. The sensilla are completely developed at the black-eyed pupae stage, but they are covered by the old cuticle. The sensilla are exposed to the environment only in newly emerged workers of T. angustula, but it is possible that environmental stimuli can be recognized due to the pores in the old cuticle.

Nymphal antennae and antennal sensilla in Aleurodicus dispersus (Hemiptera: Aleyrodidae)

Whiteflies have distinct nymphal stages: their first stage is mobile, whereas the later immature stages are sessile. The developmental and structural changes of antennae and antennal sensilla in whiteflies during these stages have rarely been investigated. This paper describes the morphology of antennae and antennal sensilla in four nymphal stages of Aleurodicus dispersus based on scanning electron microscopy. There were significant differences found in shape and length of the antennae, and differences in type, number, morphological structure and distributional pattern of antennal sensilla in the four nymphal stages of A. dispersus. We found two types of sensilla on the antennae of first-instar nymph, three types on the third-instar nymphal antennae, four types on the second-instar and seven types on the fourth-instar nymphal antennae. Sensilla trichoidea (ST) and elevated sensilla placodea were found on the antennae of each nymphal stage, sensilla chaetica only occurred on the antennae of fourth-instar nymph. Sensilla furcatea occurred on the antennae of second- and third-instar nymphs, and sensilla basiconica were found on the antennae of second- and fourth-instar nymphs. In addition, there were sensilla campaniform and sensilla coeloconica found only on the antennae of fourth-instar nymph, whereas the ST of fourth-instar nymphs included sensilla trichoidea 1 and sensilla trichoidea 2. The possible functions of antennal sensilla are discussed. Our results contribute to a better understanding of the development of the olfactory system of whitefly nymphal stages, and provide a basis for further exploration of chemical communication mechanisms between whiteflies and host plants.

Post-embryonic development of the antennal sensilla in Melipona quadrifasciata anthidioides (Hymenoptera: Meliponini)

The sensilla are sensory organs formed by cuticular and cellular structures specialized in reception of chemical and physical stimuli from the environment and transmission to the insect's central nervous system. In function of the great concentration of sensilla, the antennae are the main organs for interaction between bees and with the environment. This work studied the presence of antennal sensilla in the different phases of pupal development of the stingless bee Melipona quadrifasciata anthidioides by means of scanning electron microscopy and light microscopy. The results showed that antennal sensilla begin their development in the transition of the prepupae to the white-eyed pupae and finish it in the pigmented-body pupae phase. The antennal sensilla were exposed to the environment in the black-eyed pupae when the old cuticle is completely digested, suggesting that only in the final pupal phases can these bees perceive the environmental stimuli.

Study of taurine and tauret content in the compound eye of locust with light and dark adaptation

Taurine as well as tauret (retinyliden taurine) levels were measured in locust Locusta migratoria compound eyes. HPLC measurements revealed relatively low taurine levels (1.9 +/- 0.16 mM) in dark-adapted eyes. Glutamate, aspartate and glycine levels were 2.0 +/- 0.2, 2.7 +/- 0.4 and 3.0 +/- 0.37 mM, respectively, while GABA was present only in trace amounts. After about 4 h of light adaptation at 1500-2000 lx, amino acid levels in the compound eye were as follows: taurine, 1.8 +/- 0.17 mM; glutamate, no change at 2.1 +/- 0.2 mM; aspartate sharply increased to 4.7 +/- 0.7 mM; glycine slightly decreased to 2.8 +/- 0.3 mM; and GABA trace levels. In the compound eye of locust Locusta migratoria, the existence of endogenous tauret in micro-molar range was established. In the dark, levels were several times higher compared with compound eye after light adaptation 1500 lx for 3 h, as estimated by TLC in combination with spectral measurements. Existence of tauret in compound eye is of special interest because in the compound eye, rhodopsin regeneration is based on photoregeneration.

The functions of antennal mechanoreceptors and antennal joints in tactile discrimination of the honeybee (Apis mellifera L.)

Honeybees learn and discriminate excellently between different surface structures and different forms of objects, which they scan with their antennae. The sensory plate on the antennal tip plays a key role in the perception of mechanosensory and gustatory information. It is densely covered with small tactile hairs and carries a few large taste hairs. Both types of sensilla contain a mechanoreceptor, which is involved in the antennal scanning of an object. Our experiments test the roles of the mechanoreceptors on the antennal tip in tactile antennal learning and discrimination. Joints between head capsule and scapus and between scapus and pedicellus enable the bee to perform three-dimensional movements when they scan an object. The role of these joints in tactile antennal learning and discrimination is studied in separate experiments. The mechanoreceptors on the antennal tip were decisive for surface discrimination, but not for tactile acquisition or discrimination of shapes. When the scapus-pedicellus joint or the headcapsule-scapus joint was fixed on both antennae, tactile learning was still apparent but surface discrimination was abolished. Fixing both scapi to the head capsule reduced tactile acquisition.

High Affinity Transport of Taurine by the Drosophila Aspartate Transporter dEAAT2

Excitatory amino acid transporters (EAATs) are structurally related plasma membrane proteins known to mediate the Na(+)/K(+)-dependent uptake of the amino acids l-glutamate and dl-aspartate. In the nervous system, these proteins contribute to the clearance of glutamate from the synaptic cleft and maintain excitatory amino acid concentrations below excitotoxic levels. Two homologues exist in Drosophila melanogaster, dEAAT1 and dEAAT2, which are specifically expressed in the nervous tissue. We previously reported that dEAAT2 shows unique substrate discrimination as it mediates high affinity transport of aspartate but not glutamate. We now show that dEAAT2 can also transport the amino acid taurine with high affinity, a property that is not shared by two other transporters of the same family, Drosophila dEAAT1 and human hEAAT2. Taurine transport by dEAAT2 was efficiently blocked by an EAAT antagonist but not by inhibitors of the structurally unrelated mammalian taurine transporters. Taurine and aspartate are transported with similar K(m) and relative efficacy and behave as mutually competitive inhibitors. dEAAT2 can mediate either net uptake or the heteroexchange of its two substrates, both being dependent on the presence of Na(+) ions in the external medium. Interestingly, heteroexchange only occurs in one preferred substrate orientation, i.e. with taurine transported inwards and aspartate outwards, suggesting a mechanism of transinhibition of aspartate uptake by intracellular taurine. Therefore, dEAAT2 is actually an aspartate/taurine transporter. Further studies of this protein are expected to shed light on the role of taurine as a candidate neuromodulator and cell survival factor in the Drosophila nervous system.

Whole-cell recording from honeybee olfactory receptor neurons: ionic currents, membrane excitability and odourant response in developing workerbee and drone

Whole-cell recording techniques were used to characterize ionic membrane currents and odourant responses in honeybee olfactory receptor neurons (ORNs) in primary cell culture. ORNs of workerbee (female) and drone (male) were isolated at an early stage of development before sensory axons connect to their target in the antennal lobe. The results collectively indicate that honeybee ORNs have electrical properties similar, but not necessarily identical to, those currently envisaged for ORNs of other species. Under voltage clamp at least four ionic currents could be distinguished. Inward currents were made of a fast transient, tetrodotoxin-sensitive sodium current. In some ORNs a cadmium-sensitive calcium current was detected. ORNs showed heterogeneity in their outward currents: either outward currents were made of a delayed rectifier type potassium current, which was partially blocked by tetraethyl ammonium or quinidine, or were composed of a delayed rectifier type and a transient calcium-dependent potassium current, which was cadmium-sensitive and abolished by removal of external calcium. The proportion of each of the two outward currents, however, was different within the ORNs of the two sexes suggesting a gender-specific functional heterogeneity. ORNs showed heterogeneity in action potential firing properties: depolarizing current steps elicited either one action potential or, as in most of the cells, it led to repetitive spiking. Action potentials were tetrodotoxin-sensitive suggesting they are carried by sodium. Odourant stimulation with different mixtures and pure substances evoked depolarizing receptor potentials with superimposed action potentials when spike threshold was reached. In summary, honeybee ORNs are remarkably mature at early stages in their development.

Taurine: evidence of physiological function in the retina

Taurine is a free amino acid found in high millimolar concentrations in mammalian tissue and is particularly abundant in the retina. Mammals synthesize taurine endogenously with varying abilities, with some species more dependent on dietary sources of taurine than others. Human children appear to be more dependent on dietary taurine than adults. Specifically, it has been established that visual dysfunction in both human and animal subjects results from taurine deficiency. Moreover, the deficiency is reversed with simple nutritional supplementation with taurine. The data suggest that taurine is an important neurochemical factor in the visual system. However, the exact function or functions of taurine in the retina are still unresolved despite continuing scientific study. Nevertheless, the importance of taurine in the retina is implied in the following experimental findings: (1) Taurine exhibits significant effects on biochemical systems in vitro. (2) The distribution of taurine is tightly regulated in the different retinal cell types through the development of the retina. (3) Taurine depletion results in significant retinal lesions. (4) Taurine release and uptake has been found to employ distinct regulatory mechanisms in the retina.

New HPLC evidence on endogenous tauret in retina and pigment epithelium

This investigation was improve the separation for tauret (retinylidene taurine) and to compare its content in the retina under dark and light adaptation. To prevent tauret hydrolysis, retinal samples were quickly frozen and lyophilized. Methanol extracts of dried retina and pigment epithelium from both dark- or light-adapted frogs, Rana ridibunda, were injected onto HPLC. Synthetic standard tauret appeared at 4.7 min after the solvent front. At the same time, an endogenous substance was eluted from the mixed retinal and pigment epithelial samples. The UV spectra of this endogenous compound matched with the spectra of synthetic tauret obtained under identical conditions, with lambda(max) = 446 nm at peak. We conclude that the HPLC system used permitted full separation of tauret from the methanol extracts of the retina and pigment epithelium. TLC and further HPLC analysis have shown that tauret quantities were several times higher in the retina and pigment epithelium of the frogs adapted to dark compared with those light-adapted (about 4 h under 1000 1x illumination). Tauret based vitamin A transport is probably involved in other systems as well, where along with its other known beneficial effects taurine probably is necessary to facilitate vitamin A transport.

Taurine-, aspartate- and glutamate-like immunoreactivity identifies chemically distinct subdivisions of Kenyon cells in the cockroach mushroom body

The lobes of the mushroom bodies of the cockroach Periplaneta americana consist of longitudinal modules called laminae. These comprise repeating arrangements of Kenyon cell axons, which like their dendrites and perikarya have an affinity to one of three antisera: to taurine, aspartate, or glutamate. Taurine-immunopositive laminae alternate with immunonegative ones. Aspartate-immunopositive Kenyon cell axons are distributed across the lobes. However, smaller leaf-like ensembles of axons that reveal particularly high affinities to anti-aspartate are embedded within taurine-positive laminae and occur in the immunonegative laminae between them. Together, these arrangements reveal a complex architecture of repeating subunits whose different levels of immunoreactivity correspond to broader immunoreactive layers identified by sera against the neuromodulator FMRFamide. Throughout development and in the adult, the most posterior lamina is glutamate immunopositive. Its axons arise from the most recently born Kenyon cells that in the adult retain their juvenile character, sending a dense system of collaterals to the front of the lobes. Glutamate-positive processes intersect aspartate- and taurine-immunopositive laminae and are disposed such that they might play important roles in synaptogenesis or synapse modification. Glutamate immunoreactivity is not seen in older, mature axons, indicating that Kenyon cells show plasticity of neurotransmitter phenotype during development. Aspartate may be a universal transmitter substance throughout the lobes. High levels of taurine immunoreactivity occur in broad laminae containing the high concentrations of synaptic vesicles.

A spatiotemporal wave of turnover and functional maturation of olfactory receptor neurons in the spiny lobster Panulirus argus

Olfactory receptor neurons (ORNs) of crustaceans are housed in aesthetasc sensilla that are located on the lateral flagellum of the antennule. We used young adult spiny lobsters to examine turnover of aesthetascs and functional maturation of their ORNs after molting. The proliferation zone for new aesthetascs is located in the proximal part of the aesthetasc-bearing region and progressively moves along a distoproximal axis. Older aesthetascs are lost in the distal part of the aesthetasc-bearing region. As a result, an aesthetasc may be shed three to six molts after it differentiates. Taurine-like immunoreactivity is elevated in ORNs of aesthetascs that have yet to emerge on the cuticular surface and thereafter decreases gradually and asynchronously. ORNs from the distalmost-developing aesthetascs lose taurine-like immunoreactivity immediately before sensillar emergence, whereas ORNs from the most proximal and lateral new aesthetascs retain taurine-like immunoreactivity throughout the intermolt stage after sensillar emergence. Furthermore, taurine-like immunoreactivity is inversely correlated with odor responsiveness. These results suggest that taurine-like immunoreactivity reveals immature ORNs and that their functional maturation is not synchronized with molting and may not be completed until many weeks after sensillar emergence. Our data suggest successive spatiotemporal waves of birth, differentiation and functional maturation, and death of ORNs.

Colocalisation of taurine- with transmitter-immunoreactivities in the nervous system of the migratory locust

The expression of taurine immunoreactivity (TAU-IR) by neurones immunoreactive for octopamine (OA-IR), gamma-aminobutyric acid (GABA-IR), and the C-terminal peptide sequence arginine-phenylalanine (RFamide-IR) was investigated in the migratory locust (Locusta migratoria). TAU-IR is colocalised with OA-IR in the dorsal unpaired median neurones, which are efferent neuroparacrine cells. TAU-IR is not, however, expressed by OA-IR interneurones in the thoracic ganglia and brain. The only other TAU-IR somata found with peripheral axons are the medial neurosecretory cells in abdominal ganglia that project to the neurohaemal organs. These cells exhibit RFamide-IR. The majority of TAU-IR somata in the thoracic abdominal nervous system exhibit GABA-IR. These cells correspond to populations of identified local and intersegmentally projecting inhibitory interneurones. TAU-IR is not, however, exhibited by the well-known GABAergic common inhibitor neurones, which have peripherally projecting axons. This differential distribution of TAU-IR in basically two, functionally different, neuronal subsets (efferent neurosecretory and neuroparacrine cells, inhibitory interneurones) conforms with the concept of taurine acing as a depressive agent to limit excitation during stressful conditions.