Chemical deafferentation of the locust flight system by phentolamine

1. Phentolamine was injected into the haemolymph of locusts, Locusta migratoria, and its effects on the flight system were analyzed using electrophysiological techniques. 2.Doses of 150 microliters at 10(-2) M phentolamine inactivated the wing stretch-receptors and tegulae without influencing the central nervous system (CNS). The lack of effect on the CNS was demonstrated by the absence of any effect on the flight motor pattern in animals that had been mechanically deafferented prior to the administration of phentolamine. From these observations we conclude that phentolamine can be used to chemically deafferent the flight system of the locust. Consistent with this conclusion is that the administration of phentolamine in intact animals changed the flight motor pattern so that it resembled the pattern occurring in mechanically deafferented animals. 3. The two main advantages of deafferenting the flight system by injecting phentolamine were a) intracellular recordings from central neurons could be easily maintained during the process of deafferentation, and b) the contribution of different groups of proprioceptors to the generation of the motor pattern could be assessed since not all proprioceptors were inactivated simultaneously. 4. By intracellularly recording from elevator motoneurons and administering phentolamine we confirmed a number of previous results related to the function of the wing stretch-receptors and the tegulae.     

The role of proprioception for motor learning in locust flight

In a muscle-specific flight simulator (simulator driven by muscle action potentials) locusts (Locusta migratoria) show motor learning by which steering performance of the closed-loop muscles is improved. The role of proprioceptive feedback for this motor learning has been studied. Closed-loop muscles were cut in order to disable proprioceptive feedback of their contractions. Since there are no proprioceptors within the muscles, this is a muscle-specific deafferentation. Cut muscles are still activated during flight and their action potentials can be used for controlling the flight simulator. With cut muscles in closed-loop, steering is less reliable as can be seen from the frequent oscillations of the yaw angle. However, periods of stable flight indicate that deafferented muscles are still, in principle, functional for steering. Open-loop yaw stimuli reveal that steering reactions in cut muscles are weaker and have a longer delay than intact muscles. This is responsible for the oscillations observed in closed-loop flight. Intact muscles can take over from cut muscles in order to re-establish stable closed-loop flight. This shows that proprioceptive mechanisms for learning are muscle specific. A hypothetical scheme is presented to explain the role of proprioception for motor learning.     

Flight-correlated activity changes in neurons of the lateral accessory lobes in the brain of the locust Schistocerca gregaria

The study investigates activity changes in neurons of the lateral accessory lobes in the brain of the locust Schistocerca gregaria during wind-elicited tethered flight. Neurons with ascending projections from the ventral nerve cord to the lateral accessory lobes showed flight-associated excitations which were modulated in the flight motor rhythm. Descending neurons with ramifications in the lateral accessory lobes were tonically excited corresponding to flight duration. The onset of wind-elicited responses in the descending neurons preceded the onset of flight motor activity by 22–60 milliseconds. Neurons connecting the lateral accessory lobes with the central body, the anterior optic tubercles, or other brain areas showed a variety of responses including activity changes during flight initiation and flight termination. Activity in many of these neurons was less tightly coupled to the flight situation and often returned to background levels before flight was terminated. Most of the recorded neurons responded, in addition, to stationary visual stimuli. The results suggest that the lateral accessory lobes in the locust brain are integrative links between the central body, visual pathways, and the ventral nerve cord. The possible involvement of these brain areas in flight control is discussed.     

Effects of temperature on properties of flight neurons in the locust

High ambient temperatures increase the wing-beat frequency in flying locusts, Locusta migratoria. We investigated parameters of circuit and cellular properties of flight motoneurons at temperatures permissive for flight (20–40 °C). As the thoracic temperature increased motoneuronal conduction velocity increased from an average of 4.40 m/s at 25 °C to 6.73 m/s at 35 °C, and the membrane time constant decreased from 11.45 ms to 7.52 ms. These property changes may increase locust wing-beat frequency by affecting the temporal summation of inputs to flight neurons in the central circuitry. Increases in thoracic temperature from 25–35 °C also resulted in a hyperpolarization of the resting membrane potentials of flight motoneurons from an average of-41.1 mV to -47.5 mV, and a decrease of input resistances from an average of 3.45 M to 2.00 M. Temperature affected the measured input resistance both by affecting membrane properties, and by altering synaptic input. We suggest that the increase in conduction velocity Q₁₀=1.53) and the decrease of membrane time constant (Q₁₀=0.62) would more than account for the wing-beat frequency increase (Q₁₀=1.15). Hyperpolarization of the resting membrane potential (Q₁₀=1.18) and reduction in input resistance (Q₁₀=0.54) may be involved in automatic compensation of temperature effects.Abbreviations ANOVA analysis of variance - CPG central pattern generator - DL dorsal longitudinal muscles - EMG electromyographic - MN motoneuron - PSP post synaptic potential - Q₁₀ temperature coefficient - RMP resting membrane potential - S.D. standard deviation - SR stretch receptor     

Unique,identifiable local nonspiking interneurons in the locust mesothoracic ganglion

The hypothesis that local nonspiking interneurons are unique and identifiable has been tested rigorously for a neuron in the mesothoracic ganglion of the locust. Neurons were physiologically characterized and subsequently stained with cobalt ions. The resulting preparations were examined in whole mounts and serial sections. It is concluded that at least three neurons are unique, based upon a combination of their function, gross morphology, and the location and size of their main processes relative to other neurons. It is strongly suggested that there are other local nonspiking interneurons that are unique and identifiable. A classification system for local nonspiking interneurons is proposed. The implications of this finding for future neuroethological studies are discussed.     

Interneuronal organization in the flight system of the locust

In this paper we describe the characteristics, connections, resetting properties and organization of some identified interneurones in the flight system of the locust. The major conclusions are that: (1) the flight rhythm is generated at the interneuronal level and the flight oscillator is not continuously active (2) the interneurones in the flight pattern generator are distributed within at least 6 segmental ganglia (three thoracic and three fused abdominal ganglia) and are not organized into two homologous groups for the separate control of the forewing and the hindwing (3) this distribution of flight interneurones has no obvious functional significance but could be a consequence of flight having evolved from a segmentally distributed motor behaviour (4) there may be a functional hierarchy among flight interneurones such that premotor interneurones are separate from those generating the rhythm.     

The activity of pleurodorsal muscles during flight and at rest in the moth Manduca sexta (L.)

In the moth Manduca sexta, the paired mesothoracic flight steering muscle II PD2m takes part in the generation of the flight rhythm and is spontaneously active in the non-flying animal. This spontaneous activity is modulated by optomotor stimuli and directionally selective. The directional response characteristics are analyzed. Another spontaneously active steering muscle pair, the III PD2c, is situated in the metathorax. The activities of this pair and of a third muscle pair, the III PD3 are also influenced by visual stimulation.The responses of all 6 muscles to optomotor stimuli which simulate the flight situations yaw, roll, thrust and lift are analyzed. Each situation elicits a unique pattern of activation/deactivation within this set of muscles. The activity pattern in non-flying animals allows the prediction of flight steering mechanisms such as changes of wing area in flight turns and provides a useful basis for the analysis of visuo-motor pathways.     

Flow visualization and unsteady aerodynamics in the flight of the hawkmoth,Manduca sexta

The aerodynamic mechanisms employed durng the flight of the hawkmoth, Manduca sexta, have been investigated through smoke visualization studies with tethered moths. Details of the flow around the wings and of the overall wake structure were recorded as stereophotographs and high-speed video sequences. The changes in flow which accompanied increases in flight speed from 0.4 to 5.7 m s^-1 were analysed. The wake consists of an alternating series of horizontal and vertical vortex rings which are generated by successive down- and upstrokes, respectively. The downstroke produces significantly more lift than the upstroke due to a leading-edge vortex which is stabilized by a radia flow moving out towards the wingtip. The leading-edge vortex grew in size with increasing forward flight velocity. Such a phenomenon is proposed as a likely mechanism for lift enhancement in many insect groups. During supination, vorticity is shed from the leading edge as postulated in the ''flex'' mechanism. This vorticity would enhance upstroke lift if it was recaptured diring subsequent translation, but it is not. Instead, the vorticity is left behind and the upstroke circulation builds up slowly. A small jet provides additional thrust as the trailing edges approach at the end of the upstroke. The stereophotographs also suggest that the bound circulation may not be reversed between half strokes at the fastest flight speeds.     

Effects of maturation on synaptic potentials in the locust flight system

We have measured parameters of identified excitatory postsynaptic potentials from flight interneurons in immature and mature adult locusts (Locusta migratoria) to determine whether parameters change during imaginal maturation. The presynaptic cell was the forewing stretch receptor. The postsynaptic cells were flight interneurons that were filled with Lucifer Yellow and identified by their morphology. Excitatory postsynaptic potentials from different postsynaptic cells had characteristic amplitudes. The amplitude, time to peak, duration at half amplitude and the area above the baseline of excitatory postsynaptic potentials did not change with maturation. The latency from action potentials in the forewing stretch receptor to onset of excitatory postsynaptic potentials decreased significantly with maturation. We suggest this was due to an increase in conduction velocity of the forewing stretch receptor. We also measured morphological parameters of the postsynaptic cells and found that they increased in size with maturation. Growth of the postsynaptic cell should cause excitatory postsynaptic potential amplitude to decrease as a result of a decrease in input resistance, however, this was not the case. Excitatory postsynaptic potentials in immature locusts depress more than in mature locusts at high frequencies of presynaptic action potentials. This difference in frequency sensitivity of the immature excitatory postsynaptic potentials may account in part for maturation of the locust flight rhythm generator.Abbreviations EPSP excitatory postsynaptic potential - fSR forewing stretch receptor - IPSP inhibitory postsynaptic potential - SR stretch receptor     

DUM neurons in locust flight: a model system for amine-mediated peripheral adjustments to the requirements of a central motor program

In both vertebrates and invertebrates, multiple effects of biogenic amines on neuromuscular transmission, muscle contraction kinetics and metabolism have been described. Nevertheless, it is not yet known whether and how these different effects work in concert during the performance of a specific behavior. In the locust flight system, the biogenic amine octopamine is released as a neurohormone into the haemolymph, and also delivered directly onto specific target muscles by individually identified dorsal unpaired median neurons. Determining the connectivity of these neurons and their activation during behavior, we show for the first time that different types of dorsal unpaired median neurons are differentially connected to certain components of the flight circuitry. During flight, all types of pterothoracic dorsal unpaired median neurons innervating flight muscles receive inhibitory inputs from tegula proprioceptive afferents and from the central flight circuitry, whereas all other types of dorsal unpaired median neurons are excited by wind-sensitive pathways and by the central pattern generator. Considering the results of other studies which investigated metabolic effects of octopamine, we propose a model in which the differential activation of dorsal unpaired median neurons during flight may lead to an adequately controlled release or removal of octopamine to adjust metabolic processes to the requirements of a specific motor program. Accepted: 24 February 1999     

Output connections of a wind sensitive interneurone with motor neurones innervating flight steering muscles in the locust

Summary The output connections of a bilaterally symmetrical pair of wind-sensitive interneurones (called A4I1) were determined in a non-flying locust (Schistocerca gregaria). Direct inputs from sensory neurones of specific prosternai and head hairs initiate spikes in these interneurones in the prothoracic ganglion.The interneurone with its axon in the right connective makes direct, excitatory connections with the two mesothoracic motor neurones innervating the pleuroaxillary (pleuroalar, M85) muscle of the right forewing, but not with the comparable motor neurones of the left forewing. The connections can evoke motor spikes.The interneurones also exert a powerful, but indirect effect on the homologous metathoracic pleuroaxillary motor neurones (muscle 114), and a weaker, indirect effect on subalar motor neurones of the hindwings. No connections or effects were found with other flight motor neurones, or motor neurones innervating hindleg muscles, including common inhibitor 1 which also innervates the pleuroaxillary muscle.One thoracic interneurone with its cell body in the right half of the mesothoracic ganglion and with its axon projecting ipsilaterally to the metathoracic ganglion receives a direct input from the right A4I1 interneurone.These restricted output connections suggest a role for the A4I1 interneurones in flight steering.Abbreviations DCMD descending contralateral movement detector - EPSP excitatory postsynaptic potential - TCG tritocerebral commissure giant (interneurone)     

Auditory interneurones in the mesothoracic ganglion of crickets

Auditory interneurone responses in the mesothoracic ganglion of the cricket Gryllus bimaculatus were investigated with special regard to temporal features of the calling song. Units representing five response types were found. One type codes verse syllables and intensity. The second codes syllables of highfrequency verses. The third responds as a pulse marker. The fourth shows adaptation and the response pattern depends on the verse frequency. The fifth fires a burst at verse onset.Responses of mesothoracic units recorded in two other cricket species do not differ markedly from those of Gryllus bimaculatus. Particularly, no tuning is found to species-specific differences in their calling songs.The stimulus direction can affect the threshold in different ways: dependence at all frequencies, dependence only between 3 and 6 kHz, and independence are found. The dependence is mainly expressed by a higher threshold for contralateral sounds.The mesothoracic branching of a few neurones was demonstrated by extracellular CoS-staining. These cells pass through the ganglion as connective fibres giving off small branches into the ventro-medial and dorso-medial neuropiles.     

Activity pattern of suboesophageal ganglion cells innervating the salivary glands of the locust Locusta migratoria

The salivary gland of the locust, Locusta migratoria, is innervated from the suboesophageal ganglion by two neurones, SN1 and SN2 which innervate the gland via the salivary gland nerve (nerve 7B of the suboesophageal ganglion). In addition, like most other peripheral nerves of the head, this nerve carries on its outer surface axons and neurohaemal terminal ramifications of the so called satellite nervous system, established by a group of neurosecretory cells also located in the suboesophageal ganglion. These superficial collaterals ramify over the nerve from its origin in the head to its terminals within the gland in the thoracic segments.Nerve 7B was recorded chronically in freely moving locusts. Both salivary neurones are active during and shortly before feeding, as defined by continuous rhythmic activity of the mandibular closer muscle (M9). The activity of the salivary neurones, particularly that of SN2, thus resembles that of the satellite neurones as described recently. While SN2 ceases firing at the end of a feeding bout, SN1 continues firing for a short period. Also, SN1 fires short bursts of impulses for a few minutes following the end of a feeding bout. Similar bursts also occur at random intervals during the long-lasting phases between feeding events.Abbreviations SN1 salivary neurone 1 - SN2 salivary neurone 2 - M9 mandibular closer muscle - DUM dorsal unpaired median - LMN labral median nerve     

Metabolic integration in locust flight: the effect of octopamine on fructose 2,6-bisphosphate content of flight muscle in vivo

The biogenic amine octopamine was injected into the haemolymph of 20-days old male locusts,Locusta migratoria, and the content of fructose 2,6-bisphosphate, a potent activator of glycolysis, was measured in the flight muscle after various time. Octopamine brought about a transient increase in fructose 2,6-bisphosphate. After the injection of 10 l of 10 mmol·l^-1 d, l-octopamine fructose 2,6-bisphosphate was increased by 61% within 2 min. Ten minutes after the injection fructose 2,6-bisphosphate was increased to 6.71±0.89 nmol·g^-1 flight muscle, almost 300% over the control value. Flight caused fructose 2,6-bisphosphate in flight muscle to decrease, but this decrease was counteracted by octopamine injected into the haemolymph of flying locusts. Octopamine and fructose 2,6-bisphosphate may act as signals to stimulate the oxidation of carbohydrate and to integrate muscle performance and metabolism. This mechanism appears particularly significant in the initial stage of flight when carbohydrates are the main fuel.Abbreviations F2,6P₂ fructose 2,6-bisphosphate - F6P fructose 6-phosphate - PFK1 6-phosphofructokinase (EC 2.7.1.11) - P _i inorganic phosphate - PP _i -PFK pyrophosphate dependent fructose 6-phosphate phosphotransferase (EC 2.7.1.90)     

GABA-like immunoreactivity of identified interneurons in the flight system of the locust,Locusta migratoria

Summary The transmitter content of identified inhibitory interneurons in the flight system of the locust, Locusta migratoria, has been characterized using antibodies raised against protein-conjugated gamma aminobutyric acid. Identified flight neurons were filled with the fluorescent dye, Lucifer Yellow. Serial sections of dye-filled neurons were incubated with an antibody to gamma aminobutyric acid which was subsequently tagged with a fluorescent marker. Excitatory motoneurons to wing muscles and 13 flight interneurons (3 excitatory, 7 inhibitory, and 3 with unknown synaptic effect) were examined. Neither the moto-neurons nor any of the 3 excitatory interneurons contained immunoreactive material. Six of the 7 inhibitory interneurons did contain immunoreactive material. All the neurons which contained immunoreactive material and whose synaptic effect is known were inhibitory. We conclude that most of the inhibitory flight interneurons which have been described use gamma aminobutyric acid as their transmitter. Interestingly, at least 1 set of interneurons known to be inhibitory does not use gamma aminobutyric acid. We predict that the 2 interneurons which do contain immunoreactive material and whose synaptic effect is not yet known will be found to have inhibitory roles in the operation of the flight circuitry.     

Visual control of steering in locust flight: the effects of head movement on responses to roll stimuli

The contribution of head movement to the control of roll responses in flying locusts (Locusta migratoria) has been examined (i) on a flight balance, recording the angles through which the locust turns when following an artificial horizon; (ii) by recording activity in a pair of flight muscles in restrained conditions; and (iii) by observations on free flying locusts. Responses were compared when the head was free to turn about the thorax, as normal, and when the head was waxed to the thorax, blocking any relative motion between the two (head-fixed). These experiments suggest that the major signal generating corrective roll manoeuvres is the visual error between the angle of the head and the horizon, rather than a signal that includes a measure of the head-thorax angle.

Differential effects of octopamine and tyramine on the central pattern generator for <Emphasis Type="Italic">Manduca</Emphasis> flight

The biogenic amine, octopamine, modulates a variety of aspects of insect motor behavior, including direct action on the flight central pattern generator. A number of recent studies demonstrate that tyramine, the biological precursor of octopamine, also affects invertebrate locomotor behaviors, including insect flight. However, it is not clear whether the central pattern generating networks are directly affected by both amines, octopamine and tyramine. In this study, we tested whether tyramine affected the central pattern generator for flight in the moth, Manduca sexta. Fictive flight was induced in an isolated ventral nerve cord preparation by bath application of the octopamine agonist, chlordimeform, to test potential effects of tyramine on the flight central pattern generator by pharmacological manipulations. The results demonstrate that octopamine but not tyramine is sufficient to induce fictive flight in the isolated ventral nerve cord. During chlordimeform induced fictive flight, bath application of tyramine selectively increases synaptic drive to depressor motoneurons, increases the number of depressor spikes during each cycle and decreases the depressor phase. Conversely, blocking tyramine receptors selectively reduces depressor motoneuron activity, but does not affect cycle by cycle elevator motoneuron spiking. Therefore, octopamine and tyramine exert distinct effects on the flight central pattern generating network.