Amir Ayali

Distributed Effects of Dopamine Modulation in the Crustacean Pyloric Networka

Abstract: It is now clear that neuromodulators can reconfigure a single motor network to allow the generation of a family of related movements. Using dopamine modulation of the 14‐neuron pyloric network from the crustacean stomatogastric ganglion as an example, we describe two major mechanisms by which network output is modulated. First, the baseline electrophysiological properties of the network neurons can be altered. Dopamine can affect the activity of each neuron independently. For example, DA modulates IA in nearly every neuron in the pyloric network, but in opposite directions in different cells. Furthermore, DA usually modulates combinations of ionic currents. In some cases, currents with opposing actions on cell excitability are simultaneously affected, and the net response reflects the sum of these opposing effects. Second, neuromodulators can alter the strength of synaptic interactions within the network, quantitatively “rewiring” the network. Every synapse in the network is affected by DA, with some increased and others decreased in strength. DA acts both pre‐ and postsynaptically to affect transmission: these actions are frequently opposing in sign, and the net response arises as the sum of these opposing actions. Finally, spike‐evoked and graded transmission at the same synapse can be oppositely affected by DA. These results emphasize the distributed nature of modulation in motor networks.

Process entanglement as a neuronal anchorage mechanism to rough surfaces

The organization of neurons and glia cells on substrates composed of pristine carbon nanotube islands was investigated using high resolution scanning electron microscopy, immunostaining and confocal microscopy. Neurons were found bound and preferentially anchored to the rough surfaces; moreover, the morphology of the neuronal processes on the small, isolated islands of high density carbon nanotubes was found to be conspicuously curled and entangled. We further demonstrate that the roughness of the surface must match the diameter of the neuronal processes in order to allow them to bind. The results presented here suggest that entanglement, a mechanical effect, may constitute an additional mechanism by which neurons (and possibly other cell types) anchor themselves to rough surfaces. Understanding the nature of the interface between neurons and carbon nanotubes is essential to effectively harness carbon nanotube technology in neurological applications such as neuro-prosthetic and retinal electrodes.

The regulative role of neurite mechanical tension in network development.

A bewildering series of dynamical processes take part in the development of the nervous system. Neuron branching dynamics, the continuous formation and elimination of neural interconnections, are instrumental in constructing distinct neuronal networks, which are the functional building blocks of the nervous system. In this study, we investigate and validate the important regulative role of mechanical tension in determining the final morphology of neuronal networks. To single out the mechanical effect, we cultured relatively large invertebrate neurons on clean quartz surfaces. Applied to these surfaces were isolated anchoring sites consisting of carbon nanotube islands to which the cells and the neurites could mechanically attach. Inspection of branching dynamics and network wiring upon development revealed an innate selection mechanism in which one axon branch wins over another. The apparent mechanism entails the build-up of mechanical tension in developing axons. The tension is maintained by the attachment of the growth cone to the substrate or, alternatively, to the neurites of a target neuron. The induced tension promotes the stabilization of one set of axon branches while causing retraction or elimination of axon collaterals. We suggest that these findings represent a crucial, early step that precedes the formation of synapses and regulates neuronal interconnections. Mechanical tension serves as a signal for survival of the axonal branch and perhaps for the subsequent formation of synapses.

The Insect Frontal Ganglion and Stomatogastric Pattern Generator Networks

Insect neural networks have been widely and successfully employed as model systems in the study of the neural basis of behavior. The insect frontal ganglion is a principal part of the stomatogastric nervous system and is found in most insect orders. The frontal ganglion constitutes a major source of innervation to foregut muscles and plays a key role in the control of foregut movements. Following a brief description of the anatomy and development of the system in different insect groups, this review presents the current knowledge of the way neural networks in the insect frontal ganglion generate and control behavior. The frontal ganglion is instrumental in two distinct and fundamental insect behaviors: feeding and molting. Central pattern-generating circuit(s) within the frontal ganglion generates foregut rhythmic motor patterns. The frontal ganglion networks can be modulated in-vitro by several neuromodulators to generate a variety of motor outputs. Chemical modulation as well as sensory input from the gut and input from other neural centers enable the frontal ganglion to induce foregut rhythmic patterns under different physiological conditions. Frontal ganglion neurons themselves are also an important source of neurosecretion. The neurosecretory material from the frontal ganglion can control and modulate motor patterns of muscles of the alimentary canal. The current and potential future importance of the insect stomatogastric nervous system and frontal ganglion in the study of the neural mechanisms of behavior are discussed.

Intersegmental coordination of cockroach locomotion: adaptive control of centrally coupled pattern generator circuits

Animals’ ability to demonstrate both stereotyped and adaptive locomotor behavior is largely dependent on the interplay between centrally generated motor patterns and the sensory inputs that shape them. We utilized a combined experimental and theoretical approach to investigate the relative importance of CPG interconnections vs. intersegmental afferents in the cockroach: an animal that is renowned for rapid and stable locomotion. We simultaneously recorded coxal levator and depressor motor neurons (MN) in the thoracic ganglia of Periplaneta americana, while sensory feedback was completely blocked or allowed only from one intact stepping leg. In the absence of sensory feedback, we observed a coordination pattern with consistent phase relationship that shares similarities with a double-tripod gait, suggesting central, feedforward control. This intersegmental coordination pattern was then reinforced in the presence of sensory feedback from a single stepping leg. Specifically, we report on transient stabilization of phase differences between activity recorded in the middle and hind thoracic MN following individual front-leg steps, suggesting a role for afferent phasic information in the coordination of motor circuits at the different hemiganglia. Data were further analyzed using stochastic models of coupled oscillators and maximum likelihood techniques to estimate underlying physiological parameters, such as uncoupled endogenous frequencies of hemisegmental oscillators and coupling strengths and directions. We found that descending ipsilateral coupling is stronger than ascending coupling, while left–right coupling in both the meso- and meta-thoracic ganglia appear to be symmetrical. We discuss these results in comparison with recent findings in stick insects that share similar neural and body architectures, and argue that the two species may exemplify opposite extremes of a fast–slow locomotion continuum, mediated through different intersegment coordination strategies.

Density-dependent phase polymorphism affects response to adipokinetic hormone in Locusta

Abstract 1. 1. Haemolymph lipid levels were measured before and after injection of graded doses of corpus cardiacum (CC) extracts, and of synthetic adipokinetic hormone (AKH) I, to solitary and gregarious adult males of Locusta migratoria migratorioides. 2. 2. Gregarious locusts showed higher resting lipid levels and a consistently higher increase of haemolymph lipids, 90–100 min after injection. 3. 3. Haemolymph volume was lower in solitary than in gregarious locusts; therefore, the phase-dependent differences in haemolymph lipid levels and in adipokinetic response were not caused by differences in haemolymph volume. 4. 4. The authors conclude that solitary locusts have lower haemolymph lipid levels and a less intense response to AKH than gregarious locusts.

Biophysical constraints on neuronal branching

We investigate rules that govern neuronal arborization, speci%cally the local geometry of the bifurcation of a neurite into its sub-branches. In the present study we set out to determine the relationship between branch diameter and angle. Existing theories are based on minimizing a neuronal volume cost function, or, alternatively, on the equilibrium of mechanical tension forces, wh ichdepend on branchdiameters. Our experimental results utilizing two-dimensional cultured neural networks partly corroborate both the volume optimization principles and the tension theory. Deviation from pure tension forces equilibrium is explained by an additional force exerted by the anchoring of the junction to the substrate. c

Growth morphology of two-dimensional insect neural networks

How a collection of single neurons self-organize to form a complex functional system, the neural network, is a fundamental question. Two-dimensional in vitro invertebrate preparations o2er an attractive model system to tackle this question due to the large size ofthe neurons, and their ability to grow in relative isolation as well as to develop elaborate networks. We culture locust neurons, monitor and analyze their morphology and growth process under various density conditions. The neurons actively target neighbor cells, and their structure is a2ected by neuronal vicinity. As the network forms there is a tendency for simpli&cation of neuronal morphology. c � 2002 Elsevier Science B.V. All rights reserved.

THE LOCUST FORAGING GENE

Our knowledge of how genes act on the nervous system in response to the environment to generate behavioral plasticity is limited. A number of recent advancements in this area concern food-related behaviors and a specific gene family called foraging (for), which encodes a cGMP-dependent protein kinase (PKG). The desert locust (Schistocerca gregaria) is notorious for its destructive feeding and long-term migratory behavior. Locust phase polyphenism is an extreme example of environmentally induced behavioral plasticity. In response to changes in population density, locusts dramatically alter their behavior, from solitary and relatively sedentary behavior to active aggregation and swarming. Very little is known about the molecular and genetic basis of this striking behavioral phenomenon. Here we initiated studies into the locust for gene by identifying, cloning, and studying expression of the gene in the locust brain. We determined the phylogenetic relationships between the locust PKG and other known PKG proteins in insects. FOR expression was found to be confined to neurons of the anterior midline of the brain, the pars intercerebralis. Our results suggest that differences in PKG enzyme activity are correlated to well-established phase-related behavioral differences. These results lay the groundwork for functional studies of the locust for gene and its possible relations to locust phase polyphenism.

Adipokinetic Hormone and Flight Fuel Related Characteristics of Density-Dependent Locust Phase Polymorphism: A Review

Abstract Recent findings on differences between the gregarious and solitary phases of locusts are reviewed in relation to flight fuel utilization, adipokinetic responses, and adipokinetic hormones. Laboratory results obtained with Locusta migratoria migratorioides show that the amount of lipid reserves, resting levels of haemolymph lipids, and hyperlipaemic responses to flight and to injection of corpus cardiacum extract or of synthetic adipokinetic hormones, are higher in crowded than in isolated locusts. No major phase-dependent differences seem to exist in flight-related carbohydrate metabolism. The adipokinetic hormone content of the corpora cardiaca is higher in younger isolated locusts than in crowded ones. Adipokinetic hormone precursor-related peptide content of the corpora cardiaca is also higher in isolated than in crowded locusts. Crowded locusts have higher lipid reserves and higher hyperlipaemic responses to flight than isolated locusts also in Schistocerca gregaria and, following injection of synthetic adipokinetic hormone, the formation of low density lipophorin is higher in crowded than in isolated locusts of this species. The laboratory results obtained with isolated and crowded locusts are extrapolated to understand the ecophysiology of the migrations of solitary and gregarious field populations of L.m. migratorioides according to available information on the differences in the migration of the two phases. It is inferred that in this species solitary locusts have a rather coarse adipokinetic strategy focused on a single prereproductive long-distance migratory flight, whereas gregarious locusts possess a fine adipokinetic balance for reiterative, sometimes unpredictably long-distance, migrations in the prereproductive, as well as reproductive, periods. The differences between the adipokinetic strategies of solitary and gregarious S. gregaria seem to be less dramatic, nevertheless, they indicate a better adaptation of the gregarious phase to prolonged flights.