Philip L. Newland

Parallel somatotopic maps of gustatory and mechanosensory neurons in the central nervous system of an insect

Relatively little is still known about the sense of taste, or contact chemoreception, compared with other sensory modalities, despite its importance to many aspects of animal behaviour. The central projections of the sensory neurons from bimodal contact chemoreceptors (basiconic sensilla) were compared with those from mechanosensory tactile hairs located on similar regions of the middle leg of the locust. Basiconic sensilla are multiply innervated, containing one mechanosensory and several chemosensory neurons, whereas tactile hairs are innervated by a single mechanosensory neuron. We show that the sensory neurons from tactile hairs form a complete 3‐dimensional somatotopic map in the mesothoracic ganglion. Sensory neurons from hairs located on the coxa projected to a region near the midline of the ganglion with neurons from hairs located on progressively more distal parts of the leg arborizing in successively more lateral regions of neuropil. All the neurons from basiconic sensilla, both mechanosensory and chemosensory, also projected in a similar, strictly somatotopic, manner, and the arbors from these neurons overlapped considerably with those from tactile hairs on equivalent parts of the leg to form a continuous region. Thus, the position of a receptor on the leg is preserved in the central nervous system not only for the mechanosensory neurons from both tactile hairs and basiconic sensilla but also for chemosensory neurons. We could observe no anatomical features or small differences in projection region between sensory neurons from individual basiconic sensilla consistent with differences in modality. J. Comp. Neurol. 425:82–96, 2000.

The Neurobiology of Taste in Insects

Abstract Taste or contact-chemoreception is a fundamentally important sense to most insects. Despite its importance there remains a longstanding controversy about how different contact-chemosensory qualities are coded in the central nervous system and several different models have been proposed. Sensory projections from contact-chemosensory sensilla are primarily to the local segmental ganglion or neuromeres. There appears to be a distinction between Orthopteroid insects in which chemosensory and mechanosensory neurones project to the same, somatotopically defined, regions of neuropile, and the Diptera where there is evidence for a modality-specific spatial separation of neurones. Recent advances have been made in understanding central contact-chemosensory processing using the hind leg-metathoracic ganglion of the locust as a model system. This system offers the advantage of defined local behavioural responses controlled by well-understood local neuronal networks. Chemosensory and mechanosensory neurones make monosynaptic connections onto the same spiking local interneurones. All chemicals can elicit withdrawal responses of a leg but the concentration at which different chemicals become effective stimuli varies over several orders of magnitude. The relative size of response of local interneurones to these same chemicals, as well as their outputs onto leg motor neurones, is closely correlated with the probability of eliciting a behavioural withdrawal response. We suggest that contact-chemosensory processing by local circuits in the thoracic ganglia directly assesses a chemosensory quality, that of aversiveness, which is dependent on both chemical identity and concentration and that other chemical qualities such as palatability may be encoded in a similar manner elsewhere in the central nervous system.

Parallel processing of proprioceptive information in the terminal abdominal ganglion of the crayfish

The processing of proprioceptive information from the exopodite-endopodite chordotonal organ in the tailfan of the crayfish Procambarus clarkii (Girard) is described. The chordotonal organ monitors relative movements of the exopodite about the endopodite. Displacement of the chordotonal strand elicits a burst of sensory spikes in root 3 of the terminal ganglion which are followed at a short and constant latency by excitatory postsynaptic potentials in interneurones. The afferents make excitatory monosynaptic connections with spiking and nonspiking local interneurones and intersegmental interneurones. No direct connections with motor neurones were found.Individual afferents make divergent patterns of connection onto different classes of interneurone. In turn, interneurones receive convergent inputs from some, but not all, chordotonal afferents. Ascending and spiking local interneurones receive inputs from afferents with velocity thresholds from 2–400°/s, while nonspiking interneurones receive inputs only from afferents with high velocity thresholds (200–400°/s).The reflex effects of chordotonal organ stimulation upon a number of uropod motor neurones are weak. Repetitive stimulation of the chordotonal organ at 850°/s produces a small reduction in the firing frequency of the reductor motor neurone. Injecting depolarizing current into ascending or non-spiking local interneurones that receive direct chordotonal input produces a similar inhibition.

Distribution of NADPH-diaphorase-positive ascending interneurones in the crayfish terminal abdominal ganglion

Abstract. Previous neuropharmacological studies have described the presence of a nitric oxide-cGMP signalling pathway in the crayfish abdominal nervous system. In this study we have analysed the distribution of putative nitric oxide synthase (NOS)-containing ascending interneurones in the crayfish terminal abdominal ganglion using NADPH-diaphorase (NADPHd) histochemistry. Ascending intersegmental interneurones were stained intracellularly using the fluorescent dye Lucifer yellow and the ganglia containing the stained interneurones subsequently processed for NADPHd activity. Fluorescence persisted throughout histochemical processing. These double-labelling experiments showed that 12 of 18 identified ascending interneurones were NADPHd positive. Thus many ascending interneurones that process mechanosensory signals in the terminal ganglion may contain NOS, and are themselves likely sources of NO which is known to modulate their synaptic inputs. Three clear relationships emerged from our analysis between the effects of NO on the synaptic inputs of interneurones, their output properties and their staining for NADPH-diaphorase. First were class 1 interneurones with no local outputs in the terminal ganglion, the NE type interneurones, which had sensory inputs that were enhanced by NO and were NADPHd positive. Second were class 1 interneurones with local and intersegmental output effects that had sensory inputs that were also enhanced by NO but were NADPHd negative. Third were class 2 interneurones with local and intersegmental outputs that had synaptic inputs that were depressed by the action of NO but were NADPHd positive. These results suggest that NO could selectively enhance specific synaptic connections and sensory processing pathways in local circuits.

Olfactory selection of Plantago lanceolata by snails declines with seedling age

BACKGROUND AND AIMS Despite recent recognition that (1) plant-herbivore interactions during the establishment phase, (2) ontogenetic shifts in resource allocation and (3) herbivore response to plant volatile release are each pivotal to a comprehensive understanding of plant defence, no study has examined how herbivore olfactory response varies during seedling ontogeny. METHODS Using a Y-tube olfactometer we examined snail (Helix aspersa) olfactory response to pellets derived from macerated Plantago lanceolata plants harvested at 1, 2, 3, 4, 5, 6 and 8 weeks of age to test the hypothesis that olfactory selection of plants by a generalist herbivore varies with plant age. Plant volatiles were collected for 10 min using solid-phase microextraction technique on 1- and 8-week-old P. lanceolata pellets and analysed by gas chromatography coupled with a mass spectrometer. KEY RESULTS Selection of P. lanceolata was strongly negatively correlated with increasing age; pellets derived from 1-week-old seedlings were three times more likely to be selected as those from 8-week-old plants. Comparison of plant selection experiments with plant volatile profiles from GC/MS suggests that patterns of olfactory selection may be linked to ontogenetic shifts in concentrations of green leaf volatiles and ethanol (and its hydrolysis derivatives). CONCLUSIONS Although confirmatory of predictions made by contemporary plant defence theory, this is the first study to elucidate a link between seedling age and olfactory selection by herbivores. As a consequence, this study provides a new perspective on the ontogenetic expression of seedling defence, and the role of seedling herbivores, particularly terrestrial molluscs, as selective agents in temperate plant communities.

l-Arginine enhances immunity to parasitoids in Drosophila melanogaster and increases NO production in lamellocytes

Drosophila melanogaster was used as a model system to explore the link between nutrition and immunity, and to investigate the role of nitric oxide (NO) in enhancing immunity following dietary enhancement with L-arginine. First, we show that adding L-arginine to the food medium increases the ability of D. melanogaster larvae to encapsulate the eggs of the parasitoid Asobara tabida. Secondly, we show that the increase in immunity is specific to L-arginine, and not to an enhanced calorific content, and that immunity decreases when larvae are fed food with added L-NAME, an inhibitor of nitric oxide synthase. Finally, we show that parasitised larvae fed L-arginine have increased haemocyte numbers, and that the lamellocytes (haemocytes which play a key role in encapsulation) show evidence of an increased production of NO. These results suggest that NO plays a key role in immunity and that the effect of NO is mostly targeted via the lamellocytes.

A sensory map based on velocity threshold of sensory neurones from a chordotonal organ in the tailfan of the crayfish

The central projections of sensory neurones innervating a strand chordotonal organ (CO) in the tailfan of the crayfish, Procambarus clarkii (Girard) have been investigated. The CO monitors movement of the exopodite of the tailfan relative to the endopodite. Intracellular recording and staining were used to characterise the response of the sensory neurones to applied stretches of the chordotonal organ and to reveal their morphology. Two gross morphological types of afferents were found: those that terminated in the terminal (6th) abdominal ganglion on the side ipsilateral to the sensory receptor, and those that had branches in the terminal ganglion and an intersegmental axon that ascended rostrally. Afferents responded to position, velocity and direction of imposed CO displacement. Afferents with particular physiological properties had similar morphologies in different crayfish. Irrespective of their directional responses, afferents had central projection areas dependent upon their velocity thresholds. Many afferents responded only during movement of the CO, and those with the lowest velocity thresholds (2°/s) had branches that projected most anteriorly, while those with progressively higher velocity thresholds (up to 200°/s) projected progressively more posteriorly. Afferents that responded to low velocity ramp movements and spiked tonically projected to more posterior areas of the ganglion than those that responded only to movements.

Nitrergic modulation of an oviposition digging rhythm in locusts

SUMMARY In locusts, a central pattern generator underlies the rhythmic movements of the ovipositor valves that serve to drive the abdomen into damp soil in order to lay eggs. We have investigated the role of nitric oxide (NO) in the control of this oviposition digging rhythm. NO increases the frequency of the rhythm by acting via sGC to elevate cGMP, which in turn acts via PKG. Increasing exogenous NO levels using the NO donors SNAP and PAPANONOate increased the cycle frequency of the fictive digging rhythm, as did increasing endogenous NO by bath application of the substrate for NOS, l-arginine. On the other hand, application of the NO scavenger PTIO decreased the cycle frequency, indicating that NO must normally exert a continuous and dynamic role on the central pattern generator underlying the oviposition rhythm. Inhibiting the main molecular target of NO, soluble guanylate cyclase, with ODQ reduced the cycle frequency of the rhythm, suggesting that NO mediated its effects via sGC and cyclic GMP. Further evidence for this was produced by bath application of 8-Br-cGMP, which increased the frequency of the rhythm. Bath application of the generic protein kinase inhibitor and a selective PKG inhibitor, H-7 and KT-5823, respectively, reduced the frequency of the rhythm, suggesting that PKG acted as a target for cGMP. Thus, we conclude that NO plays a key role in regulating the frequency of the central pattern generator controlling rhythmic egg-laying movements in locusts by acting via sGC/cGMP–PKG.

NADPH-diaphorase histochemistry in the terminal abdominal ganglion of the crayfish.

Abstract. Nitric oxide (NO) has an important modulatory role on the processing of sensory signals in vertebrates and invertebrates. In this investigation we studied the potential sources of NO in the terminal abdominal ganglion of the crayfish, Pacifastacus leniusculus, using NADPH-diaphorase (NADPHd) histochemistry, with NADPHd acting as a marker for NO synthase (NOS). In the terminal ganglion a mean of 27 strongly labelled NADPHd-positive cell bodies were found, and of these 8% occurred in three regions located in antero-lateral, central and posterior parts of the ganglion. Ventral and antero-ventral commissures as well as specific dorsal and ventral areas of the dendritic neuropil showed positive staining. Intense labelling was seen in the ventro-medial tract, and in the connective between the terminal ganglion and the 5th abdominal ganglion. In addition, some motor neurones and neurones with branches in the sensory commissures were NADPHd positive. Our finding that NADPHd-positive cells occur in consistent patterns in the terminal abdominal ganglion implies that NO may have a role in mechanosensory processing in the crayfish.

Static electric fields modify the locomotory behaviour of cockroaches

SUMMARY Static electric fields are found throughout the environment and there is growing interest in how electric fields influence insect behaviour. Here we have analysed the locomotory behaviour of cockroaches (Periplaneta americana) in response to static electric fields at levels equal to and above those found in the natural environment. Walking behaviour (including velocity, distance moved, turn angle and time spent walking) were analysed as cockroaches approached an electric field boundary in an open arena, and also when continuously exposed to an electric field. On approaching an electric field boundary, the greater the electric field strength the more likely a cockroach would be to turn away from, or be repulsed by, the electric field. Cockroaches completely exposed to electric fields showed significant changes in locomotion by covering less distance, walking slowly and turning more often. This study highlights the importance of electric fields on the normal locomotory behaviour of insects.