Central nervous processing of behaviourally relevant odours in solitary and gregarious fifth instar locusts, Schistocerca gregaria

Physiological and morphological characteristics of antennal lobe neurons of solitary and gregarious fifth-instar nymphs of the desert locust, Schistocerca gregaria, were studied using intracellular recording and staining techniques. Physiological characteristics of antennal lobe neurons of both locust phases responding to stage-dependent aggregation pheromones, egg-laying attractants, a putative sex pheromone and plant-associated volatiles are described. Antennal lobe neurons showed excitatory, inhibitory, combined excitatory and inhibitory and delayed responses. In addition, one neuron␣showing an initial inhibition followed by an excitation and inhibition response was found. Pheromone-specific-, plant-specific- and pheromone-plant-generalist neurons were found in both locust phases. Antennal lobe neurons displayed stage- and phase-dependent differences in the processing of aggregation pheromone component input. Nymphal antennal lobe neurons showed stage-dependent response characteristics highly correlated with the preferential behavioural attraction to the nymphal aggregation pheromone. Phase-dependent differences were found in the response spectra and the sensitivity of the same neuron types. Neurons of solitary locusts responded significantly more frequently to some of the tested components than neurons of gregarious locusts. Furthermore, antennal lobe neurons of solitary locusts showed a higher sensitivity to most of the tested compounds. Accepted: 4 July 1998     

A distinct layer of the medulla integrates sky compass signals in the brain of an insect

Mass migration of desert locusts is a common phenomenon in North Africa and the Middle East but how these insects navigate is still poorly understood. Laboratory studies suggest that locusts are able to exploit the sky polarization pattern as a navigational cue. Like other insects locusts detect polarized light through a specialized dorsal rim area (DRA) of the eye. Polarization signals are transmitted through the optic lobe to the anterior optic tubercle (AOTu) and, finally, to the central complex in the brain. Whereas neurons of the AOTu integrate sky polarization and chromatic cues in a daytime dependent manner, the central complex holds a topographic representation of azimuthal directions suggesting a role as an internal sky compass. To understand further the integration of sky compass cues we studied polarization-sensitive (POL) neurons in the medulla that may be intercalated between DRA photoreceptors and AOTu neurons. Five types of POL-neuron were characterized and four of these in multiple recordings. All neurons had wide arborizations in medulla layer 4 and most, additionally, in the dorsal rim area of the medulla and in the accessory medulla, the presumed circadian clock. The neurons showed type-specific orientational tuning to zenithal polarized light and azimuth tuning to unpolarized green and UV light spots. In contrast to neurons of the AOTu, we found no evidence for color opponency and daytime dependent adjustment of sky compass signals. Therefore, medulla layer 4 is a distinct stage in the integration of sky compass signals that precedes the time-compensated integration of celestial cues in the AOTu.     

Role of Olfactory and Visual Cues in the Attraction/Repulsion Responses to Conspecifics by Gregarious and Solitarious Desert Locusts

Desert locusts [Schistocerca gregaria Forskål (Orthoptera, Acrididae)] change phase in response to population density: solitarious insects avoid one another, but when crowded they change to the gregarious phase and aggregate. The attraction/repulsion responses of gregarious and solitarious locusts maintain phase differences in locust populations. Despite considerable research, the cues for aggregation are poorly understood; moreover, the repulsion response of solitarious locusts has not previously been investigated. This study analyzes the role of visual and olfactory stimuli in triggering these different responses to conspecifics. Isolation-reared insects were repelled by both olfactory and visual stimuli from other locusts. Crowd-reared insects were attracted by the combination of olfactory and visual cues. In addition, olfactory stimuli affected other behaviors in both phases, and behavioral differences between isolation- and crowd-reared locusts were clear even in the absence of conspecifics. The sensory and neurological mechanisms underlying these responses are not well understood and will form the basis for neurobiological investigations of locust phase.     

Oscillations and slow patterning in the antennal lobe

Odor presentation generates both fast oscillations and slow patterning in the spiking activity of the projection neurons (PNs) in the antennal lobe (AL) of locusts, moths and bees. Experimental results indicate that the oscillations are the result of the interaction between the PNs and the inhibitory local neurons (LNs) in the AL; e.g., blocking inhibition by application of GABA-receptor antagonists abolishes these oscillations. The slow patterning, on the other hand, was shown to be somewhat resistant to such blockage. In a H-H model, we reproduce both the oscillations and the slow patterning. As previously suggested, the oscillations are the result of the interaction between the PNs and LNs. We suggest that calcium and calcium-dependent potassium channels (found in PNs of bees and moths) are sufficient to account for the slow patterning resistant to the application of GABA-receptor antagonists. The intrinsic bursting property of the PNs, resulting from these additional modeled currents, give rise to another network feature that was seen experimentally in locusts: A relatively small increase in the number of additional generated PN action potentials when LN input is blocked. Consequently, the major effect of network inhibition is to redistribute the action potentials of the PNs from bursting to one action potential per cycle of the oscillations. Action Editor: Christiane Linster     

Motor neurone responses during a postural reflex in solitarious and gregarious desert locusts

Desert locusts show extreme phenotypic plasticity and can change reversibly between two phases that differ radically in morphology, physiology and behaviour. Solitarious locusts are cryptic in appearance and behaviour, walking slowly with the body held close to the ground. Gregarious locusts are conspicuous in appearance and much more active, walking rapidly with the body held well above the ground. During walking, the excursion of the femoro-tibial (F-T) joint of the hind leg is smaller in solitarious locusts, and the joint is kept more flexed throughout an entire step. Under open loop conditions, the slow extensor tibiae (SETi) motor neurone of solitarious locusts shows strong tonic activity that increases at more extended F-T angles. SETi of gregarious locusts by contrast showed little tonic activity. Simulated flexion of the F-T joint elicits resistance reflexes in SETi in both phases, but regardless of the initial and final position of the leg, the spiking rate of SETi during these reflexes was twice as great in solitarious compared to gregarious locusts. This increased sensory-motor gain in the neuronal networks controlling postural reflexes in solitarious locusts may be linked to the occurrence of pronounced behavioural catalepsy in this phase similar to other cryptic insects such as stick insects.     

Functional organization of olfactory processing in the accessory lobe of the spiny lobster

An isolated brain preparation was used to characterize neurons innervating the accessory lobe (AL) of the spiny lobster (Panulirus argus). Four distinct classes of neurons responded to electrical stimulation of the olfactory (antennular) nerve. These cells responded to electrical stimulation with a long and variable latency; they also responded to odor stimulation in a nose-brain preparation. Neurons connecting the AL with the olfactory lobe branched in the central AL layer and selectively innervated olfactory lobe glomeruli. These cells had response latencies which were significantly shorter than those of other AL neurons. Intrinsic AL interneurons were heterogeneous as a population, and most arborized in irregular but circumscribed regions of either the lateral or medial layers. The final class of neurons branched ipsilaterally in the deutocerebral neuropil and bilaterally innervated only a few AL glomeruli. The physiology and morphology of these four classes of neurons confirm an olfactory function for the AL and identify the input and output regions of the lobe. Based on these findings, we propose that the AL processes odor information in the context of higher order multimodal input.Abbreviations AL accessory lobe - DCN deutocerebral neuropil - OGT olfactory-globular tract - OGTN olfactory-globular tract neuropil - OL olfactory lobe     

Functional specialization of mouse higher visual cortical areas

The mouse is emerging as an important model for understanding how sensory neocortex extracts cues to guide behavior, yet little is known about how these cues are processed beyond primary cortical areas. Here, we used two-photon calcium imaging in awake mice to compare visual responses in primary visual cortex (V1) and in two downstream target areas, AL and PM. Neighboring V1 neurons had diverse stimulus preferences spanning five octaves in spatial and temporal frequency. By contrast, AL and PM neurons responded best to distinct ranges of stimulus parameters. Most strikingly, AL neurons preferred fast-moving stimuli while PM neurons preferred slow-moving stimuli. By contrast, neurons in V1, AL, and PM demonstrated similar selectivity for stimulus orientation but not for stimulus direction. Based on these findings, we predict that area AL helps guide behaviors involving fast-moving stimuli (e.g., optic flow), while area PM?helps guide behaviors involving slow-moving objects.     

Local interneuron diversity in the primary olfactory center of the moth <Emphasis Type="Italic">Manduca sexta</Emphasis>

Local interneurons (LNs) play important roles in shaping and modulating the activity of output neurons in primary olfactory centers. Here, we studied the morphological characteristics, odor responses, and neurotransmitter content of LNs in the antennal lobe (AL, the insect primary olfactory center) of the moth Manduca sexta. We found that most LNs are broadly tuned, with all LNs responding to at least one odorant. 70% of the odorants evoked a response, and 22% of the neurons responded to all the odorants tested. Some LNs showed excitatory (35%) or inhibitory (33%) responses only, while 33% of the neurons showed both excitatory and inhibitory responses, depending on the odorant. LNs that only showed inhibitory responses were the most responsive, with 78% of the odorants evoking a response. Neurons were morphologically diverse, with most LNs innervating almost all glomeruli and others innervating restricted portions of the AL. 61 and 39% of LNs were identified as GABA-immunoreactive (GABA-ir) and non-GABA-ir, respectively. We found no correlations between odor responses and GABA-ir, neither between morphology and GABA-ir. These results show that, as observed in other insects, LNs are diverse, which likely determines the complexity of the inhibitory network that regulates AL output.     

Nucleus accumbens neurons are innately tuned for rewarding and aversive taste stimuli, encode their predictors, and are linked to motor output

The nucleus accumbens (NAc) is a key component of the brain's reward pathway, yet little is known of how NAc cells respond to primary rewarding or aversive stimuli. Here, naive rats received brief intraoral infusions of sucrose and quinine paired with cues in a classical conditioning paradigm while the electrophysiological activity of individual NAc neurons was recorded. NAc neurons (102) were typically inhibited by sucrose (39 of 52, 75%) or excited by quinine (30 of 40, 75%) infusions. Changes in firing rate were correlated with the oromotor response to intraoral infusions. Most taste-responsive neurons responded to only one of the stimuli. NAc neurons developed responses to the cues paired with sucrose and quinine. Thus, NAc neurons are innately tuned to rewarding and aversive stimuli and rapidly develop responses to predictive cues. The results indicate that the output of the NAc is very different when rats taste rewarding versus aversive stimuli.     

Stimuli inducing gregarious colouration and behaviour in nymphs of Schistocerca gregaria

Solitarious nymphs of Schistocerca gregaria were reared under various conditions in both Jerusalem and Oxford to tease apart cues involved in behavioural and colour phase change. Treatments included rearing nymphs from the IInd or IIIrd until the final nymphal stadium in physical contact with similarly aged conspecific groups or with another locust species, Locusta migratoria migratorioides, as well as confining single nymphs in mesh cages, which were kept within crowds of S. gregaria or L. migratoria migratorioides, providing visual and olfactory but no physical contact with other locusts. In the Oxford experiments, an extra treatment was included which provided olfactory cues without visual or contact stimulation. Our results confirm that transformation from the solitarious to the gregarious phase of locusts is complex, and that different phase characteristics not only follow different time courses, but are also controlled by different suites of cues. As predicted from earlier studies, behavioural phase change was evoked by non-species-specific cues. Rearing in contact with either species was fully effective in inducing gregarious behaviour, as was the combination of the sight and smell of other locusts, but odour alone was ineffective. Colour phase change was shown to comprise two distinct elements that could be dissociated: black patterning and yellow background. The former of these could be induced as effectively by rearing S. gregaria nymphs in a crowd of L. migratoria migratorioides as by rearing with conspecifics. Sight and smell of other locusts also triggered black patterning and, unlike behavioural change, some black patterning was induced by odour cues alone. Hence, physical contact was not needed to induce gregarious black patterning. Yellow colouration, however, was only fully induced when locusts were reared in contact with conspecifics, implying the presence of a species-specific contact chemical cue.     

Host plant odor extracts with strong effects on oviposition behavior in Papilio polyxenes

Ovipositing females of the black swallowtail butterfly, Papilio polyxenes Fabricius (Lepidoptera: Papilionidae: Papilioninae), have an innate response to contact chemical cues, but it was not known how naïve females would respond to volatile host plant chemicals. Naïve and experienced females were tested in a free-flight experiment using model plants and volatile extracts from two hosts, carrot [ Daucus carota L. (Apiaceae)] and poison hemlock [ Conium maculatum L. (Apiaceae)], and one non-host, fava bean [ Vicia faba L. (Fabaceae)]. Naïve females responded to both host plant extracts with more approaches and landings than to non-host extracts, with poison hemlock receiving more responses than carrot. Females experienced with one of the two host plants also approached and landed frequently on extracts of host plants, but poison hemlock again received more approaches and landings, regardless of a female's host plant experience. Female P. polyxenes thus respond specifically to volatile extracts of their host plants, even in the absence of both relevant contact chemical cues and adult host plant experience.     

Spiking patterns and their functional implications in the antennal lobe of the tobacco hornworm Manduca sexta

Bursting as well as tonic firing patterns have been described in various sensory systems. In the olfactory system, spontaneous bursts have been observed in neurons distributed across several synaptic levels, from the periphery, to the olfactory bulb (OB) and to the olfactory cortex. Several in vitro studies indicate that spontaneous firing patterns may be viewed as "fingerprints" of different types of neurons that exhibit distinct functions in the OB. It is still not known, however, if and how neuronal burstiness is correlated with the coding of natural olfactory stimuli. We thus conducted an in vivo study to probe this question in the OB equivalent structure of insects, the antennal lobe (AL) of the tobacco hornworm Manduca sexta. We found that in the moth's AL, both projection (output) neurons (PNs) and local interneurons (LNs) are spontaneously active, but PNs tend to produce spike bursts while LNs fire more regularly. In addition, we found that the burstiness of PNs is correlated with the strength of their responses to odor stimulation--the more bursting the stronger their responses to odors. Moreover, the burstiness of PNs was also positively correlated with the spontaneous firing rate of these neurons, and pharmacological reduction of bursting resulted in a decrease of the neurons' responsiveness. These results suggest that neuronal burstiness reflects a physiological state of these neurons that is directly linked to their response characteristics.     

Glutamate evokes firing through activation of kainate receptors in chick accessory lobe neurons

Ten pairs of protrusions, called accessory lobes (ALs), exist at the lateral sides of avian lumbosacral spinal cords. Histological and behavioral evidence suggests that neurons are present in ALs and the AL acts as a sensory organ of equilibrium during walking. Neurons in the outer layer of the AL consistently show glutamate-like immunoreactivity and neurons in the central region of the AL show glutamate receptor-like immunoreactivity. However, it is unknown how glutamate acts on the functional activity of AL neurons. In this study, we examined the effects of glutamate on the electrical activities of AL neurons using the patch clamp technique. There are two types of neurons among isolated AL neurons: spontaneously firing and silent neurons. Among silent neurons, 42 % of neurons responded to glutamate and generated repetitive firing. Kainate and glutamate in combination with the NMDA receptor antagonist, MK-801, also induced firing and evoked an inward current. On the other hand, the application of AMPA, NMDA or glutamate in combination with the non-NMDA receptor antagonist, CNQX, did not. These results indicate that chick AL neurons express functional kainate receptors to respond to glutamate and suggest that the glutamatergic transmission plays a role in excitatory regulation of AL neurons of the chick.     

Effect of phase status on responses to AKHI in the desert locust,Schistocerca gregaria gregaria

Hyperlipaemic response to adipokinetic hormone (AKH I) was demonstrated in both solitary and gregarious phases of the desert locust, Schistocerca gregaria gregaria. Time-course studies showed that the gregarious locusts had a faster response to the hormone than their solitary counterparts. At peak response time (90 min), the gregarious locusts were more sensitive to AKH I doses below 2 pmol while the solitary locusts had a higher response above this dose. Upon injection of the hormone, lipoprotein conversion occurred, resulting in the formation of the low density lipoprotein (LDLp). The LDLp formed in the gregarious locusts was much larger than that of the solitary locusts. The fat body lipid reserve (expressed as % fat body dry weight) was significantly (P < 0.01) higher in the gregarious (79.02 ± 2.77%) than in the solitary locusts (65.23 ± 2.55%). Triacylglycerol was the major lipid class representing 83.9 and 73.9% of the total lipids in gregarious and solitary locusts, respectively. The higher fat body lipid reserves and efficient LDLp formation in response to AKH in gregarious locusts compared to solitary locusts suggests a physiological adaptation for prolonged flights. © 1996 Wiley-Liss, Inc.     

Ability of the desert locust (Schistocerca gregaria) to compensate for visual deprivation during the final pre‐adult and adult stages

In the desert locust (Schistocerca gregaria Forsskål), vision is a seemingly indispensable prerequisite for many behaviour patterns. The question arises as to whether and to what extent other senses can compensate for the loss of vision, and whether this can take place in the adult stage. To answer this question, both of the compound eyes of nymphs in the final pre‐adult stage are blinded (but not the ocelli), resulting in permanent visual deprivation during adult life. The results are somewhat unexpected: under laboratory conditions, in comparison with sighted controls, the blinded locusts do not exhibit any noticeable change in overall agility relative to daily activity, nor any detriment in terms of the final moult, reproductive rate or longevity. In their search for a specific food source in an experimental arena with a narrow passage between the compartment where the animals are released and the compartment containing food, in the first trial, visually deprived females need significantly more time than the sighted controls. However with an increasing number of trials, each performed after 1 day of food deprivation, the food finding latency of blinded locusts (S. gregaria) approaches or even surpasses that of normally sighted locusts. The blinded locusts use their antennae, mouthparts and tarsi more extensively, suggesting a more frequent use of tactile and chemical cues, which they gradually learn to use more efficiently. The results indicate that pre‐adult and adult S. gregaria can respond to abrupt, permanent changes in their sensory inputs, and have a significant capacity for adaptation.     

Separating natural responses from experimental artefacts: habitat selection by a diadromous fish species using odours from conspecifics and natural stream water

Animals use sensory stimuli to assess and select habitats, mates and food as well as to communicate with other individuals. One way they do this is to use olfaction, whereby they identify and respond to chemical cues. All organisms release odours, which mix with other chemical substances and ambient environmental conditions. The result is that animals are frequently immersed in a complex, highly dynamic sensory environment where they must identify and respond to only some of the potential stimuli they encounter in the face of significant levels of background noise. Understanding how organisms respond to different chemical cues is therefore dependent on knowing how these responses might be influenced by potential interactions with other stimuli. To test this, we examined whether the diadromous fish Galaxias maculatus was attracted to conspecific odours and whether this response differed when cues were offered in an artificial environment lacking other potential chemical stimuli (tap water) or a more natural background environment (stream water). We found that (1) fish responded to both natural stream water odours and those from conspecifics but the response to the latter was stronger; (2) the attraction to conspecific odours was stronger in tap water than in stream water, which indicates the importance of these odours may be overestimated when they are offered in artificial media. We also conducted a brief literature review, which confirmed that artificial media are commonly used in experiments and that the background environment is often not considered. Our results show that future research testing the responses of organisms to auditory, olfactory and visual cues should carefully consider the context in which cues are presented. Without doing so, such studies may inaccurately assess the importance of sensory cues in natural situations in the wild. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Response surfaces for the combined effects of heat shock and smoke on germination of 16 species forming soil seed banks in south‐east Australia

Abstract There is limited understanding of how fire‐related cues such as heat shock and smoke can combine to affect the germination response of seeds from fire‐prone vegetation because combinations of multiple levels of both cues have rarely been investigated. Germination response surfaces were determined for the combination of heat shock and smoke by applying factorial combinations of temperature (up to 100°C) and aerosol smoke (0–20 min) to 16 species that form soil seed banks in the Sydney region of south‐eastern Australia. Duplicate populations of three species were also examined to assess the constancy of a species response surface. Of the 19 populations examined, 16 showed a germination response to both the fire cues, which combined interactively in 14 populations, and independently in two. No population responded only to a single cue; however, seeds of 11 populations responded to heat in the absence of smoke, and nine responded to smoke in the absence of heat. Heat applied in the absence of smoke negatively affected germination in seven populations, either progressively as temperature increased, or above a set temperature. Negative germination responses over part of the temperature range were fully reversed at higher temperatures for unsmoked seeds of four populations (curvilinear heat response). Smoke effects were most frequently positive over all or part of the range of durations used, and when combined with heat frequently fully or partially reversed negative heat effects. Three populations required the obligatory combination of smoke and heat. A novel response to the cues was observed for three species, with smoke reversing negative heat effects at 75°C, being supplanted by a positive heat response of unsmoked seed at 100°C. The response surface for duplicate populations of two of the three species examined was variable. Heat shock and smoke frequently combined to affect germination, in both positive and negative ways. Consequently, to gain an accurate assessment of the response of seeds to fires, an experimental design that samples within the potential response zones of germination cues is essential.     

Responsiveness of the cortical taste area neurons to a mixture of the four basic tastants in rats

The taste coding mechanism in the cortical taste area was investigated by analyzing the responses of 59 neurons in the cortical taste area of the anesthetized rat to a mixture of the four basic tastants in both absence and presence of bicuculline methiodide, a specific antagonist to the GABA(A) receptors. The mixture caused response suppression more frequently than response facilitation, both in the control state and during bicuculline application. Cluster analysis revealed that only a group of the neurons with the best response to both NaCl and HCl (group NH) showed the best response to the mixture in the control state, whereas during bicuculline application, in addition to group NH, two other groups of neurons responding to sucrose, or to HCl and quinine responded vigorously to the mixture. Multidimensional scaling located the mixture outside the space of the four basic tastants facing an NaCl-HCl line in both states. GABAergic inhibition caused the group NH to represent the taste of the mixture in the control state. Thus, the mixture probably tastes salty and sour to rats. No cortical neuron was found which specifically responded to the mixture.     

Coordinated Ontogeny of Food Preference and Responses to Chemical Food Stimuli by a Lizard Ctenosaura pectinata (Reptilia: Iguanidae)

When important ecological factors change predictably during the life of an organism, the ontogeny of related behaviors must be timed to maintain appropriate behavioral responsiveness to current ecological conditions. In the brown iguana, Ctenosaura pectinata , hatchlings in natural populations eat primarily insects, consuming little plant matter, whereas adults eat primarily plants, consuming some insects as well. We conducted laboratory experiments on diet preferences and responses to chemical cues that the lizards sampled by tongue-flicking and used to identify food. All hatchlings ate crickets, but only one of six ate romaine lettuce. They responded strongly to chemical cues from prey, as indicated by elevated tongue-flick rates, but not from romaine lettuce. All older individuals ate both crickets and romaine lettuce. They responded much more strongly to chemical cues from both crickets and romaine lettuce than to control chemicals, as indicated by higher proportions of individuals that bit and higher tongue-flick attack scores.
Thus, an ontogenetic change to increased responsiveness to plant chemical stimuli was coordinated with an ontogenetic change to an herbivorous diet. The mechanisms underlying these ontogenetic changes are unknown, but folivory may be unprofitable before juveniles acquire intestinal flora that degrade cellulose by ingestion of feces of adult conspecifics. Possible mechanisms are discussed, including the detection of chemical cues from appropriate food plants during consumption of feces from older individuals. Studies of other squamate reptiles suggest that exposure to these chemicals might affect both future responsiveness to the chemical cues and a tendency to eat the corresponding plants.     

Multi-unit Recording Methods to Characterize Neural Activity in the Locust (Schistocerca Americana) Olfactory Circuits

Detection and interpretation of olfactory cues are critical for the survival of many organisms. Remarkably, species across phyla have strikingly similar olfactory systems suggesting that the biological approach to chemical sensing has been optimized over evolutionary time¹. In the insect olfactory system, odorants are transduced by olfactory receptor neurons (ORN) in the antenna, which convert chemical stimuli into trains of action potentials. Sensory input from the ORNs is then relayed to the antennal lobe (AL; a structure analogous to the vertebrate olfactory bulb). In the AL, neural representations for odors take the form of spatiotemporal firing patterns distributed across ensembles of principal neurons (PNs; also referred to as projection neurons)^2,3. The AL output is subsequently processed by Kenyon cells (KCs) in the downstream mushroom body (MB), a structure associated with olfactory memory and learning^4,5. Here, we present electrophysiological recording techniques to monitor odor-evoked neural responses in these olfactory circuits.First, we present a single sensillum recording method to study odor-evoked responses at the level of populations of ORNs^6,7. We discuss the use of saline filled sharpened glass pipettes as electrodes to extracellularly monitor ORN responses. Next, we present a method to extracellularly monitor PN responses using a commercial 16-channel electrode³. A similar approach using a custom-made 8-channel twisted wire tetrode is demonstrated for Kenyon cell recordings⁸. We provide details of our experimental setup and present representative recording traces for each of these techniques.