Acetylcholinesterase activity in antennal receptor neurons of the sphinx moth Manduca sexta

Genome-wide identification, classification, and expression profiling of serine esterases and other esterase-related proteins in the tobacco hornworm, Manduca sexta

Serine esterases (SEs) are hydrolases that catalyze the conversion of carboxylic esters into acids and alcohols. Lipases and carboxylesterases constitute two major groups of SEs. Although over a hundred of insect genomes are known, systematic identification and classification of SEs are rarely performed, likely due to large size and complex composition of the gene family in each species. Considering their key roles in lipid metabolism and other physiological processes, we have categorized 144 M. sexta SEs and SE homologs (SEHs), 114 of which contain a motif of GXSXG. Multiple sequence alignment and phylogenetic tree analysis have revealed 39 neutral lipases (NLs), 3 neutral lipase homologs (NLHs), 11 acidic lipases (ALs), 3 acidic lipase homologs (ALHs), a lipase-3, a triglyceride lipase, a monoglyceride lipase, a hormone-sensitive lipase, and a GDSL lipase. Eighty-three carboxylesterase genes encode 29 α-esterases (AEs), 12 AEHs (e.g., SEH4-1-3), 20 feruloyl esterases (FEs), 2 FEHs, 2 β-esterases (BEs), 2 integument esterases (IEs), 1 IEH, 4 juvenile hormone esterases, 2 acetylcholinesterases, gliotactin, 6 neuroligins, neurotactin, and an uncharacteristic esterase homolog. In addition to these GXSXG proteins, we have identified 26 phospholipases and 13 thioesterases. Expression profiling of these genes in specific tissues and stages has provided insights into their functions including digestion, detoxification, hormone processing, neurotransmission, reproduction, and developmental regulation. In summary, we have established a framework of information on SEs and related proteins in M. sexta to stimulate their research in the model species and comparative investigations in agricultural pests or disease vectors.

A direct descending pathway informing locomotor networks about tactile sensor movement

Much like visually impaired humans use a white-cane, nocturnal insects and mammals use antennae or whiskers for near-range orientation. Stick insects, for example, rely heavily on antennal tactile cues to find footholds and detect obstacles. Antennal contacts can even induce aimed reaching movements. Because tactile sensors are essentially one-dimensional, they must be moved to probe the surrounding space. Sensor movement is thus an essential cue for tactile sensing, which needs to be integrated by thoracic networks for generating appropriate adaptive leg movements. Based on single and double recordings, we describe a descending neural pathway comprising three identified ON- and OFF-type neurons that convey complementary, unambiguous, and short-latency information about antennal movement to thoracic networks in the stick insect. The neurons are sensitive to the velocity of antennal movements across the entire range covered by natural movements, regardless of movement direction and joint angle. Intriguingly, none of them originates from the brain. Instead, they descend from the gnathal ganglion and receive input from antennal mechanoreceptors in this lower region of the CNS. From there, they convey information about antennal movement to the thorax. One of the descending neurons, which is additionally sensitive to substrate vibration, feeds this information back to the brain via an ascending branch. We conclude that descending interneurons with complementary tuning characteristics, gains, input and output regions convey detailed information about antennal movement to thoracic networks. This pathway bypasses higher processing centers in the brain and thus constitutes a shortcut between tactile sensors on the head and the thorax.

Pheromone transduction in moths

Calling female moths attract their mates late at night with intermittent release of a species-specific sex-pheromone blend. Mean frequency of pheromone filaments encodes distance to the calling female. In their zig-zagging upwind search male moths encounter turbulent pheromone blend filaments at highly variable concentrations and frequencies. The male moth antennae are delicately designed to detect and distinguish even traces of these sex pheromones amongst the abundance of other odors. Its olfactory receptor neurons sense even single pheromone molecules and track intermittent pheromone filaments of highly variable frequencies up to about 30 Hz over a wide concentration range. In the hawkmoth Manduca sexta brief, weak pheromone stimuli as encountered during flight are detected via a metabotropic PLCβ-dependent signal transduction cascade which leads to transient changes in intracellular Ca²⁺ concentrations. Strong or long pheromone stimuli, which are possibly perceived in direct contact with the female, activate receptor-guanylyl cyclases causing long-term adaptation. In addition, depending on endogenous rhythms of the moth's physiological state, hormones such as the stress hormone octopamine modulate second messenger levels in sensory neurons. High octopamine levels during the activity phase maximize temporal resolution cAMP-dependently as a prerequisite to mate location. Thus, I suggest that sliding adjustment of odor response threshold and kinetics is based upon relative concentration ratios of intracellular Ca²⁺ and cyclic nucleotide levels which gate different ion channels synergistically. In addition, I propose a new hypothesis for the cyclic nucleotide-dependent ion channel formed by insect olfactory receptor/coreceptor complexes. Instead of being employed for an ionotropic mechanism of odor detection it is proposed to control subthreshold membrane potential oscillation of sensory neurons, as a basis for temporal encoding of odors.

Odors Pulsed at Wing Beat Frequencies are Tracked by Primary Olfactory Networks and Enhance Odor Detection

Each down stroke of an insect's wings accelerates axial airflow over the antennae. Modeling studies suggest that this can greatly enhance penetration of air and air-born odorants through the antennal sensilla thereby periodically increasing odorant-receptor interactions. Do these periodic changes result in entrainment of neural responses in the antenna and antennal lobe (AL)? Does this entrainment affect olfactory acuity? To address these questions, we monitored antennal and AL responses in the moth Manduca sexta while odorants were pulsed at frequencies from 10–72 Hz, encompassing the natural wingbeat frequency. Power spectral density (PSD) analysis was used to identify entrainment of neural activity. Statistical analysis of PSDs indicates that the antennal nerve tracked pulsed odor up to 30 Hz. Furthermore, at least 50% of AL local field potentials (LFPs) and between 7–25% of unitary spiking responses also tracked pulsed odor up to 30 Hz in a frequency-locked manner. Application of bicuculline (200 μM) abolished pulse tracking in both LFP and unitary responses suggesting that GABA_A receptor activation is necessary for pulse tracking within the AL. Finally, psychophysical measures of odor detection establish that detection thresholds are lowered when odor is pulsed at 20 Hz. These results suggest that AL networks can respond to the oscillatory dynamics of stimuli such as those imposed by the wing beat in a manner analogous to mammalian sniffing.

Topographic organization of sensory afferents of Johnston's organ in the honeybee brain

Johnston's organ (JO) in insects is a multicellular mechanosensory organ stimulated by movement of the distal part of the antenna. In honeybees JO is thought to be a primary sensor detecting air-particle movements caused by the waggling dance of conspecifics. In this study projection patterns of JO afferents within the brain were investigated. About 720 somata, distributed around the periphery of the second segment of the antenna (pedicel), were divided into three subgroups based on their soma location: an anterior group, a ventral group, and a dorsal group. These groups sent axons to different branches (N2 to N4) diverged from the antennal nerve. Dye injection into individual nerve branches revealed that all three groups of afferents, having fine collaterals in the dorsal lobe, sent axons broadly through tracts T6I, T6II, and T6III to terminate ipsilaterally in the medial posterior protocerebral lobe, the dorsal region of the subesophageal ganglion, and the central posterior protocerebral lobe, respectively. Within these termination fields only axon terminals running in T6I were characterized by thick processes with large varicosities. Differential staining using fluorescent dyes revealed that the axon terminals of the three groups were spatially segregated, especially in T6I, showing some degree of somatotopy. This spatial segregation was not observed in axon terminals running in other tracts. Our results show that projection patterns of JO afferents in the honeybee brain fundamentally resemble those in the dipteran brain. The possible roles of extensive termination fields of JO afferents in parallel processings of mechanosensory signals are discussed.

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.

Immunocytochemical localization of choline acetyltransferase and muscarinic ACh receptors in the antenna during development of the sphinx moth Manduca sexta

Immunocytochemistry with monoclonal antibodies was used to investigate the locations of muscarinic acetylcholine receptors (mAChR) and choline acetyltransferase (ChAT) in sections of the developing antennae of the moth Manduca sexta. The results were correlated with a previous morphological investigation in the developing antennae which allowed us to locate different cell types at various stages of development. Our findings indicated that the muscarinic cholinergic system was not restricted to the sensory neurons but was also present in glial and epidermal cells. By day 4-5 of adult development, immunoreactivity against both antibodies was present in the axons of the antennal nerve, and more intense labeling was present in sections from older pupae. At days 4-9, the cell bodies of the sensory neurons in the basal part of the epidermis were also intensely immunolabeled by the anti-mAChR antibody. In mature flagella, large numbers of cells, some with processes into hairs, were strongly labeled by both antibodies. Antennal glial cells were intensely immunolabeled with both antibodies by days 4-5, but in later stages, it was not possible to discriminate between glial and neural staining. At days 4-9, we observed a distinctly labeled layer of epidermal cells close to the developing cuticle. The expression of both ChAT and mAChRs by neurons in moth antennae may allow the regulation of excitability by endogenous ACh. Cholinergic communication between neurons and glia may be part of the system that guides axon elongation during development. The cholinergic system in the apical part of the developing epidermis could be involved in cuticle formation.

A contribution to the functional morphology of the femoral chordotonal organ in the green lacewing Chrysoperla carnea (Neuroptera)

The femoral chordotonal organ (FCO) and the subgenual organ (SGO) of the green lacewing Chrysoperla carnea were examined by conventional light and confocal laser scanning microscopy in order to search for neuroactive substances which are used for neurotransmission in sensory cells of these organs. Antibodies against serotonin, histamine and choline acetyltransferase were tested immunohistochemically. In the FCO, antiserum against serotonin strongly labelled cell bodies and axons of about 16 sensory cells. In the proximal scoloparium all 12 sensory cells showed immunoreaction with antiserotonin. In the distal scoloparium only four of 40 sensory cells were immunoreactive. These results suggest that different neuroactive substances are employed as neurotransmitters in the FCO of the green lacewing and that the proximal scoloparium and the distal scoloparium are functionally differentiated. Contrary to the FCO in the locust, acetylcholine was not found as a neurotransmitter in the FCO of the green lacewing. Additionally, histamine showed a negative result in the sensory cells of the FCO. Other neuroactive substances seem to be used as transmitters in the SGO because none of the tested antibodies showed positive reaction.

Physiological characterisation of antennal mechanosensory descending interneurons in an insect (Gryllus bimaculatus, Gryllus campestris) brain

We investigated five different descending brain interneurons with dendritic arborizations in the deutocerebrum in the crickets Gryllus bimaculatus and G. campestris. These interneurones convey specific antennal mechanosensory information to the ventral nerve cord and all responded to forced antennal movements. These interneurones coded for velocity and showed preferences for distinct sectors of the total range of antennal movements. Their axons descended into the posterior connective either ipsilateral or contalateral to the cell body. Electrical stimulation of sensory nerves indicated that the interneurons received input from different afferents of the two antennal base segments. One interneuron had a particularly large axon with a conduction velocity of 4.4ms−1. This was the only one of the five interneurons that also received visual input. Its activity was reduced during voluntary antennal movements. The reduction in activity occurred even after de-efferentation of the antenna, indicating that it had a central origin. Although we do not have experimental evidence for behavioural roles for the descending antennal mechanosensory interneurons, the properties described here suggest an involvement in the perception of objects in the path of the cricket.

Regulation of cyclic GMP elevation in the developing antennal lobe of the Sphinx moth, Manduca sexta

In the moth, Manduca sexta, 3',5'-guanosine monophosphate (cGMP) is transiently elevated during adult development in about 100 neurons of the antennal lobe. We demonstrate that nearly all of these neurons are local interneurons of the lateral cluster I, that their capacity to show a strong cGMP response during development is regulated by the steroid hormone 20-hydroxyecdysone, and that in a subpopulation of these neurons cGMP elevation seems to be controlled directly by the gaseous messenger molecule nitric oxide (NO). Treatment with the acetylcholine esterase inhibitor eserine, antennal nerve transection, and electrical stimulation of the antennae suggest that NO/cGMP signaling during development is an activity-dependent process. Besides input from the antennae, input from the central brain and the ventral ganglia is involved in upregulating cGMP in the antennal-lobe neurons. Possible sources are centrifugal aminergic neurons, since application of serotonin and histamine enhances the GMP signal in local interneurons. Comparing the time course of cGMP elevation with events occurring during development leads us to the hypothesis that the NO/cGMP signaling pathway might be involved in synapse formation of a subset of antennal-lobe neurons.

Localization of choline acetyltransferase-expressing neurons in Drosophila nervous system

A variety of approaches have been developed to localize neurons and neural elements in nervous system tissues that make and use acetylcholine (ACh) as a neurotransmitter. Choline acetyltransferase (ChAT) is the enzyme catalyzing the biosynthesis of ACh and is considered to be an excellent phenotypic marker for cholinergic neurons. We have surveyed the distribution of choline acetyltransferase (ChAT)-expressing neurons in the Drosophila nervous system detected by three different but complementary techniques. Immunocytochemistry, using anti-ChAT monoclonal antibodies results in identification of neuronal processes and a few types of cell somata that contain ChAT protein. In situ hybridization using cRNA probes to ChAT messenger RNA results in identification of cell bodies transcribing the ChAT gene. X-gal staining and/or beta-galactosidase immunocytochemistry of transformed animals carrying a fusion gene composed of the regulatory DNA from the ChAT gene controlling expression of a lacZ reporter has also been useful in identifying cholinergic neurons and neural elements. The combination of these three techniques has revealed that cholinergic neurons are widespread in both the peripheral and central nervous system of this model genetic organism at all but the earliest developmental stages. Expression of ChAT is detected in a variety of peripheral sensory neurons, and in the brain neurons associated with the visual and olfactory system, as well as in neurons with unknown functions in the cortices of brain and ganglia.

Neuropeptides in insect sensory neurones: tachykinin-, FMRFamide- and allatotropin-related peptides in terminals of locust thoracic sensory afferents

Sensory afferents in the thoracic ganglia of the locust Locusta migratoria were labelled with antisera to different neuropeptides: locustatachykinins, FMRFamide and allatotropin. The locustatachykinin-immunoreactive (LTKIR) sensory fibres were derived from the legs and entered the ventral sensory neuropil of each of the thoracic ganglia via nerve 5. In the thoracic neuropil, the LTKIR sensory fibres formed a distinct plexus of terminations ventrally in the ipsilateral hemisphere. The peripheral cell bodies of the sensory neurones could not be revealed, but lesion experiments indicated that origin of the LTKIR fibres was the tarsus of each leg. Possibly the thin fibres are from tarsal chemoreceptors. Double labelling immunocytochemistry revealed that all the LTKIR sensory fibres contained colocalized FMRFamide immunoreactivity. A larger population of sensory fibres reacted with antiserum to moth (Manduca sexta) allatotropin. By means of double labelling immunocytochemistry, we could show that the LTKIR fibres constituted a subpopulation of the larger set of allatotropin-like immunoreactive fibres. Thus some sensory fibres may contain colocalized peptides related to locustatachykinins, FMRFamide-related peptide(s) and allatotropin-like peptide. A separate non-overlapping small set of sensory fibres in nerve 5 reacted with an antiserum to serotonin. Sensory fibres of the other nerves of the ventral nerve cord, including the abdominal ganglia, did not react with the peptide antisera. Since acetylcholine is the likely primary neurotransmitter of insect sensory fibres, it is possible that the peptides and serotonin are colocalized with this transmitter and serve modulatory functions in a subset of the leg afferents.

Sensory neuron development revealed by taurine immunocytochemistry in the honeybee

The formation of ommatidia in the compound eyes and sensilla on the antennae of the honeybee was followed and the development of their sensory neurons was traced using an antiserum against taurine as a marker. Taurine-like immunoreactivity (Tau-IR) is expressed in sensory neurons of several modalities, namely visual, olfactory, gustatory, and mechanosensory. Staining intensity is very high in the larva and in the first half of the pupal stage and gradually decreases towards the end of metamorphosis. In the photoreceptor cells of the compound eyes, Tau-IR can be detected from the fifth larval instar onwards, prior to differentiation of other components of the ommatidium. Already in the midstage larvae, when the antennal primordia of the adult still lie within the peripodial cavity, a few presumably mechanosensory neurons are labelled in the pedicellus of the developing antenna. The majority of the antennal sensory neurons which are located on the flagellum start to exhibit Tau-IR upon pupation, long before any cuticular specializations such as sensory hairs or plates are detectable. All known types of antennal sensilla were identified and it could be shown that all of them are innervated by Tau-IR sensory neurons. Thus, taurine immunocytochemistry can be applied as a useful label for developing sensory neurons. Functional implications of taurine during development are discussed.

Distribution of acetylcholinesterase activity in the deutocerebrum of the sphinx moth Manduca sexta

We have used a cytochemical technique to investigate the distribution of acetylcholinesterase (AChE) activity in the deutocerebrum of the brain of the sphinx moth Manduca sexta. To distinguish between extra- and intracellular pools of the enzyme, some brains were treated prior to histochemical staining with echothiophate, an irreversible AChE inhibitor which penetrates cell membranes very slowly and, therefore, inhibits only extracellular AChE. In the antennal nerve, fascicles of presumably mechanosensory fibers show echothiophate-insensitive AChE activity. They bypass the antennal lobe and project to the antennal mechanosensory and motor center of the deutocerebrum. In the antennal lobe, fibers in the coarse neuropil, cell bodies in the lateral cell group, and all glomeruli exhibit AChE activity. In most ordinary glomeruli, echothiophate-sensitive AChE activity is concentrated in the outer cap regions, corresponding to the terminal arborizations of olfactory afferents. A previously unrecognized glomerulus in the ventro-median antennal lobe shows uniform and more intense AChE-specific staining that the other glomeruli. No AChE activity appeared to be associated with male-specific pheromone-sensitive afferents in the macroglomerular complex. About 67 interneurons with somata in the lateral cell group of the antennal lobe show echothiophate-insensitive AChE activity. These neurons seem to be members of two types of antennal-lobe projection neurons with fibers passing through the outer-antennocerebral tract to the protocerebrum. AChE-stained arborizations of these neurons appear to invade all glomeruli, including three distinguishable subunits of the male-specific macroglomerular complex. In echothiophate-treated animals, the projections of one of these types of fiber form large terminals in the lateral horn of protocerebrum, which partly protrude into the adjacent glial cell layer. The results suggest that extracellularly accessible AChE is associated with ordinary olfactory receptor terminals but apparently not with pheromone-sensitive afferents. Intracellular AChE appears to be present in antennal mechanosensory fibers and in two types of olfactory projection neurons of the antennal lobe. The study provides further evidence for cholinergic neurotransmission of most antennal afferents. The AChE-containing interneurons might be cholinergic as well or use the enzyme for functions unrelated to hydrolysis of acetylcholine.

Analysis of chemical signals by nervous systems

Intraspecific and interspecific communication and recognition depend on olfaction in widely diverse species of animals. Olfaction, an ancient sensory modality, is based on principles of neural organization and function that appear to be remarkably similar throughout the zoosphere. Thus, the "primitives" of olfactory stimuli that determine the input information of olfaction, the kinds of "molecular images" formed at various levels in the olfactory pathway, and the cellular mechanisms that underlie olfactory information processing are comparable in invertebrates and vertebrates alike. A case in point is the male-specific olfactory subsystem in moths, which is specialized to detect and analyze the qualitative, quantitative, and temporal features of the con-specific females' sex-pheromonal chemical signal. This olfactory subsystem can be viewed, and is here presented, as a model in which common principles of organization and function of olfactory systems in general are exaggerated to serve the requirements of a chemical communication system that is crucial for reproductive success.

Acetylcholinesterase activity in neurons of crayfish abdominal ganglia

Acetylcholine is known to be a neurotransmitter in crustacean central nervous systems, but the numbers and distribution of cholinergic neurons in the segmental ganglia have not been described. To begin a census of cholinergic neurons in these ganglia, we used a histochemical assay for acetylcholinesterase to map neurons that contained this enzyme in the six abdominal ganglia of crayfish. In each abdominal ganglion, about 47 cell bodies were stained. The distributions of these stained cells in individual ganglia were similar, and the numbers were not significantly different. None of these stained cell bodies could be identified from their structures or locations as previously identified motor neurons or sensory neurons with central cell bodies. The process of one unpaired midline neuron that occurred only in the first three abdominal ganglia divided to send a pair of axons anteriorly into both halves of the connective. The central projections of afferent axons from many peripheral sensory neurons stained clearly as they entered each ganglion. Terminals of these axons were heavily stained in the horseshoe neuropil and the lateral neuropils. We labeled both gamma-aminobutyric acid (GABA) and acetylcholinesterase in individual ganglia. Only a few neurons in each ganglion were double-labeled. The unpaired midline neurons in the three anterior ganglia that stained for acetylcholinesterase did not show GABA-like immunoreactivity, but cells with similar shapes did label with the GABA antiserum. Acetylcholinesterase is not a definitive marker of cholinergic neurons, but its presence is often associated with the cholinergic phenotype. These stained cells should be considered as putative cholinergic neurons.

FMRFamide-like immunoreactivity in presumptive chemosensory afferents of the spiny lobster, Panulirus argus

Stainings with an antibody against the neuropeptide FMRFamide in the CNS of the spiny lobster revealed strong immunoreactivity in a special class of sensory afferents. These afferents are extremely thin and numerous and innervate all sensory neuropils except the optical and olfactory lobes. In the target neuropils the terminals of the afferents branch in parallel and form very densely labeled net-like structures. Due to their size, number and distribution we conclude that the immunoreactive afferents represent a specialized chemosensory system not related to food detection. We propose that a FMRFamide-related peptide present in the afferent terminals serves as sensory transmitter.

Optical recording of neuronal activity in the insect central nervous system: odorant coding by the antennal lobes of honeybees

Voltage-sensitive dyes and activity-dependent intrinsic optical signals were used to study the spatio-temporal activity in the antennal lobes of honeybees. The intrinsic signals are somewhat slower than the dye signals but show a 10-fold larger intensity change. These intrinsic signals consist of at least two components--one is wavelength-independent and the other strongly wavelength-dependent, with a maximum at approximately 500 nm. Local inhibitory connections within the antennal lobes were examined by recording the activity elicited by an electrical stimulus to the antennal nerve of a slice preparation before and after applying picrotoxin to manipulate GABAergic inhibitory synapses. The inhibition starts with a delay of approximately 10 ms after onset of the response and has at least two components. The spatial distribution of the inhibition is extremely inhomogeneous, with areas of small inhibition adjacent to areas of large inhibition. Thus inhibitory interactions in the antennal lobes are not evenly distributed among the glomerular organization. Stimulation of an in vivo preparation with an odour yields a spatially restricted activity. However, the spatial map appears highly dynamic in time because the size of the activated area is a function of the time during and after the stimulus.