An overview of odorant-binding protein functions in insect peripheral olfactory reception

Insect olfactory perception involves many aspects of insect life, and can directly or indirectly evoke either individual or group behaviors. Insect olfactory receptors and odorant-binding proteins (OBPs) are considered to be crucial to insect-specific and -sensitive olfaction. Although the mechanisms of interaction between OBPs or OBP/ligand complex with olfactory receptors are still not well understood, it has been shown that many OBPs contribute to insect olfactory perception at various levels. Some of these are numerous and divergent members in OBP family; expression in the olfactory organ at high concentration; a variety of combinational patterns between different OBPs and ligands, but exclusive affinity for one OBP to specific binding ligands; complicated interactions between OBP/ligand complex and transmembrane proteins (olfactory receptors or sensory neuron membrane proteins). First, we review OBPs' ligand-binding property based on OBP structural research and ligand-binding test; then, we review current progress around the points cited above to show the role of such proteins in insect olfactory signal transmission; finally, we discuss applications based on insect OBP research.     

Unusual microbial xylanases from insect guts

Recombinant DNA technologies enable the direct isolation and expression of novel genes from biotopes containing complex consortia of uncultured microorganisms. In this study, genomic libraries were constructed from microbial DNA isolated from insect intestinal tracts from the orders Isoptera (termites) and Lepidoptera (moths). Using a targeted functional assay, these environmental DNA libraries were screened for genes that encode proteins with xylanase activity. Several novel xylanase enzymes with unusual primary sequences and novel domains of unknown function were discovered. Phylogenetic analysis demonstrated remarkable distance between the sequences of these enzymes and other known xylanases. Biochemical analysis confirmed that these enzymes are true xylanases, which catalyze the hydrolysis of a variety of substituted beta-1,4-linked xylose oligomeric and polymeric substrates and produce unique hydrolysis products. From detailed polyacrylamide carbohydrate electrophoresis analysis of substrate cleavage patterns, the xylan polymer binding sites of these enzymes are proposed.     

Unusual organization of the tropharium in the telotrophic ovarioles of an insect, Saldula saltatoria

The tropharium of the common shorebug Saldula saltatoria consists of 2 zones: the apical mitotic region and the distal one comprising numerous mononucleate nurse cells. Each individual nurse cell is connected to the centrally located trophic core by a thin cytoplasmic projection referred to as a trophic process. The accumulations of a dense material interpreted as the remnants of intercellular bridge rim are observed associated with the trophic process membrane. In the light of these results the establishment of telotrophic ovarioles in hemipterans is discussed.     

Origin and development of unusual insect muscle tension receptors

This work describes the origin and development of about 200 tension receptor cells located around the anterior attachment site of the locust ovipositor muscle and their migration to their final position on the muscle fibres. The locust ovipositor muscle is the only insect system in which more than 100 tension receptor cells are associated with a single muscle. Neuronal precursors of tension receptors are first detectable by horseradish peroxidase immunohistochemistry in fourth instar larvae. Precursors consist of cell clusters (doublets, triplets and quadruplets) located on the anterior attachment site of the muscle. In the early fifth larval stage, cell clusters are absent, although a few sensory neurons that lie embedded between the muscle fibres are apparent. These neurons send their dendrites towards the anterior end of the muscle fibres and their axons posteriorly. By the fourth day of the fifth larval stage, a large number of cell clusters appears on the anterior muscle attachment site. In addition to these assemblies, cells have been identified that extend long processes running exactly along the lateral margin of the attachment site. These cells are thought to provide navigating cues for migrating tension receptors, since they are absent in later stages. By the end of the fifth larval stage, most of the clusters gradually disappear and increasing numbers of differentiated neurons embedded between the muscle fibres become visible. We conclude that the majority of tension receptors develop during the last larval stage from precursors situated on the muscle apodeme. They then migrate from the apodeme to their final place on the muscle fibres where they assume an appropriate orientation.     

Defense posture and leg-position learning in a primitive insect utilize catchlike tension

The capability for conditioning of leg position, using loud sound as an aversive natural reinforcement, was examined in a primitive New Zealand insect, the weta (Orthoptera: Stenopelmatidae). Electromyographic recordings were made during the conditioning. A majority of wetas tested came to occupy stably a metathoracic tibial position window, coupled to turning off the sound, set in either flexion or extension away from the preferred rest position. Steady tensions of up to 7 g in extension and 5 g in flexion were produced. However, no electromyographic activity accompanied the tension. It is concluded that the insects are using a peripheral catchlike mechanism to adjust posture.     

Glial repair in an insect

The repair of cockroach central nervous connectives, following selective glial disruption, involves an initial invasion of the lesion by a novel cell class. The available evidence, including that obtained using monoclonal antibodies, shows that these cells arise from circulating haemocytes. These invasive exogenous cells are restricted to the lesion zone. They are not only involved in initial repair of the peripheral glial elements, but may also be responsible for initiating recruitment and division of endogenous reactive cells. There is a clear anterior polarity in this recruitment, with significantly higher numbers of cells appearing anterior to, and then within, the lesion area. Characteristically, recognizable exogenous cells decline in number after 3 days, although there is no overall reduction in cell numbers within the lesion at this stage, nor has significant cell division begun. This suggests that the haemocyte-derived cells transform into, or are replaced by, functional perineurial glia, between 3 and 5 days, coincident with the restoration of the blood-brain barrier and the onset of endogenous cell division. Glial repair in the insect CNS can thus be divided into three phases which show striking similarities to the repair sequence in vertebrate brain. These include: an initial invasion of the lesion by exogenous cells, subsequent glial proliferation and then longer term fluxes in cell numbers and distribution.     

Elicitation and abrupt termination of behaviorally significant catchlike tension in a primitive insect

Sustained steady contractural or catchlike tension (CT) occurs in the metathoracic extensor tibiae muscle of the primitive insect the weta (Orthoptera: Stenopelmatidae) during its characteristic leg-extension defense behavior or following leg-position conditioning. Similar action occurs occasionally in semi-intact preparations and is abruptly turned off by a single peripheral inhibitory impulse. These phenomena were reproduced routinely by first infusing saline containing 10^?8M (or stronger) octopamine into the muscle for 12 min, and then stimulating the slow excitatory motor neuron SETi with a brief burst. Direct stimulation of the dorsal unpaired median neuron, innervating the extensor tibiae (DUMETi) prior to SETi stimulation, also led to CT. Both octopamine and DUMETi markedly enhanced the tension developed in response to a burst of impulses in SETi.     

Oxygen reperfusion damage in an insect

The deleterious effects of anoxia followed by reperfusion with oxygen in higher animals including mammals are well known. A convenient and genetically well characterized small-animal model that exhibits reproducible, quantifiable oxygen reperfusion damage is currently lacking. Here we describe the dynamics of whole-organism metabolic recovery from anoxia in an insect, Drosophila melanogaster, and report that damage caused by oxygen reperfusion can be quantified in a novel but straightforward way. We monitored CO(2) emission (an index of mitochondrial activity) and water vapor output (an index of neuromuscular control of the spiracles, which are valves between the outside air and the insect's tracheal system) during entry into, and recovery from, rapid-onset anoxia exposure with durations ranging from 7.5 to 120 minutes. Anoxia caused a brief peak of CO(2) output followed by knock-out. Mitochondrial respiration ceased and the spiracle constrictor muscles relaxed, but then re-contracted, presumably powered by anaerobic processes. Reperfusion to sustained normoxia caused a bimodal re-activation of mitochondrial respiration, and in the case of the spiracle constrictor muscles, slow inactivation followed by re-activation. After long anoxia durations, both the bimodality of mitochondrial reactivation and the recovery of spiracular control were impaired. Repeated reperfusion followed by episodes of anoxia depressed mitochondrial respiratory flux rates and damaged the integrity of the spiracular control system in a dose-dependent fashion. This is the first time that physiological evidence of oxygen reperfusion damage has been described in an insect or any invertebrate. We suggest that some of the traditional approaches of insect respiratory biology, such as quantifying respiratory water loss, may facilitate using D. melanogaster as a convenient, well-characterized experimental model for studying the underlying biology and mechanisms of ischemia and reperfusion damage and its possible mitigation.     

Tracheole migration in an insect wing

Summary Insect tissues are supplied with oxygen by a system of long and highly branched cuticular tubes known as tracheae and tracheoles. During the growth of with imaginal discs in moths and butterflies, tracheole cells migrate distally from the base of the disc. Tracheoles radiate in a distal direction through the extracellular space sandwiched between the upper and lower epithelial surfaces of the wing.Migration of most cells is assumed to be governed by forces intrinsic to the cell. However, the movement of tracheoles is apparently a passive process whose motive force resides in adjacent epithelial cells. After epithelial cells are exposed to ecdysteroid hormones, these cells extend basal processes that are attracted to oxygen-rich tracheoles. By applying traction to the tracheoles with which they establish intimate contact, epithelial cells may control the pattern of their distribution within wing tissue.     

Neuromuscular transmission in a primitive insect: modulation by octopamine, and catch-like tension

The third pair of legs of the primitive New Zealand orthopteran insect, the " weta ", has and innervation and muscle cell distribution exactly similar to that of locusts, but wetas do not jump. Neuromuscular transmission to the slow excitatory axon ( SETi ) is potentiated more than 10-fold by the natural modulator octopamine (OCT). A brief burst of SETi impulses following infusion of as little as 10(-8) M OCT is followed by a very long-lasting plateau of catch-like tension (CT). The plateau is abruptly relaxed by a single inhibitory impulse, or even by a single SETi impulse if this arrives no sooner than about 30 sec following excitation. CT is used by wetas in a defense posture. Locusts and grasshoppers have a different type of modulation by OCT.     

Neuromuscular transmission in an insect visceral muscle

The electrical properties of the muscles of locust oviduct have been examined using intracellular recordings. The muscle cells are both dye and electrically coupled. They possess a wide array of spontaneous electrical activity ranging from slow oscillations of membrane potential to action potentials. In addition to possessing spontaneous electrical activity, certain regions of the oviduct are under motor control. The amplitude of evoked excitatory junction potentials (EJPs) increased step wise revealing innervation from a maximum of three motor units. These EJPs underwent summation and facilitation, and reached a critical threshold at which point the membrane revealed an active response. Bath applied glutamate, aspartate, proctolin, and octopamine were tested for their ability to alter resting potential and EJPs. L-glutamate (1.6 X 10(-5) M and above) produced a dose-dependent depolarization of membrane potential accompanied by a reduction in amplitude of EJPs. Although L-aspartate resulted in similar effects, the concentrations required were higher than those for glutamate. Proctolin (6.3 X 10(-11) M-6.0 X 10(-9) M) resulted in a dose-dependent depolarization but had little or no effect on amplitude of EJPs. Application of D, L-octopamine (3.2 X 10(-5) M-1.7 X 10(-4) M) induced a small hyperpolarization and a reduction in amplitude of EJP. It is suggested that contractions of locust oviduct appear to be regulated by a combination of a classical neurotransmitter such as glutamate, along with the neuromodulators octopamine and proctolin.