Mesothoracic ventral unpaired median (mesVUM) neurons in the blowfly Calliphora erythrocephala

Single-cell type analysis of wing premotor circuits in the ventral nerve cord of Drosophila melanogaster

To perform most behaviors, animals must send commands from higher-order processing centers in the brain to premotor circuits that reside in ganglia distinct from the brain, such as the mammalian spinal cord or insect ventral nerve cord. How these circuits are functionally organized to generate the great diversity of animal behavior remains unclear. An important first step in unraveling the organization of premotor circuits is to identify their constituent cell types and create tools to monitor and manipulate these with high specificity to assess their function. This is possible in the tractable ventral nerve cord of the fly. To generate such a toolkit, we used a combinatorial genetic technique (split-GAL4) to create 195 sparse driver lines targeting 198 individual cell types in the ventral nerve cord. These included wing and haltere motoneurons, modulatory neurons, and interneurons. Using a combination of behavioral, developmental, and anatomical analyses, we systematically characterized the cell types targeted in our collection. Taken together, the resources and results presented here form a powerful toolkit for future investigations of neural circuits and connectivity of premotor circuits while linking them to behavioral outputs.

Octopamine drives honeybee thermogenesis

In times of environmental change species have two options to survive: they either relocate to a new habitat or they adapt to the altered environment. Adaptation requires physiological plasticity and provides a selection benefit. In this regard, the Western honeybee (Apis mellifera) protrudes with its thermoregulatory capabilities, which enables a nearly worldwide distribution. Especially in the cold, shivering thermogenesis enables foraging as well as proper brood development and thus survival. In this study, we present octopamine signaling as a neurochemical prerequisite for honeybee thermogenesis: we were able to induce hypothermia by depleting octopamine in the flight muscles. Additionally, we could restore the ability to increase body temperature by administering octopamine. Thus, we conclude that octopamine signaling in the flight muscles is necessary for thermogenesis. Moreover, we show that these effects are mediated by β octopamine receptors. The significance of our results is highlighted by the fact the respective receptor genes underlie enormous selective pressure due to adaptation to cold climates. Finally, octopamine signaling in the service of thermogenesis might be a key strategy to survive in a changing environment.

Extensive and diverse patterns of cell death sculpt neural networks in insects

Changes to the structure and function of neural networks are thought to underlie the evolutionary adaptation of animal behaviours. Among the many developmental phenomena that generate change programmed cell death (PCD) appears to play a key role. We show that cell death occurs continuously throughout insect neurogenesis and happens soon after neurons are born. Mimicking an evolutionary role for increasing cell numbers, we artificially block PCD in the medial neuroblast lineage in Drosophila melanogaster, which results in the production of ‘undead’ neurons with complex arborisations and distinct neurotransmitter identities. Activation of these ‘undead’ neurons and recordings of neural activity in behaving animals demonstrate that they are functional. Focusing on two dipterans which have lost flight during evolution we reveal that reductions in populations of flight interneurons are likely caused by increased cell death during development. Our findings suggest that the evolutionary modulation of death-based patterning could generate novel network configurations.

Just like a sculptor chips away at a block of granite to make a statue, the nervous system reaches its mature state by eliminating neurons during development through a process known as programmed cell death.

In vertebrates, this mechanism often involves newly born neurons shrivelling away and dying if they fail to connect with others during development. Most studies in insects have focused on the death of neurons that occurs at metamorphosis, during the transition between larva to adult, when cells which are no longer needed in the new life stage are eliminated.

Pop et al. harnessed a newly designed genetic probe to point out that, in fruit flies, programmed cell death of neurons at metamorphosis is not the main mechanism through which cells die. Rather, the majority of cell death takes place as soon as neurons are born throughout all larval stages, when most of the adult nervous system is built. To gain further insight into the role of this ‘early’ cell death, the neurons were stopped from dying, showing that these cells were able to reach maturity and function. Together, these results suggest that early cell death may be a mechanism fine-tuned by evolution to shape the many and varied nervous systems of insects.

To explore this, Pop et al. looked for hints of early cell death in relatives of fruit flies that are unable to fly: the swift lousefly and the bee lousefly. This analysis showed that early cell death is likely to occur in these two insects, but it follows different patterns than in the fruit fly, potentially targeting the neurons that would have controlled flight in these flies’ ancestors.

Brains are the product of evolution: learning how neurons change their connections and adapt could help us understand how the brain works in health and disease. This knowledge may also be relevant to work on artificial intelligence, a discipline that often bases the building blocks and connections in artificial ‘brains’ on how neurons communicate with one another.

Multifunctional Wing Motor Control of Song and Flight

Multifunctional motor systems produce distinct output patterns that are dependent on behavioral context, posing a challenge to underlying neuronal control. Flies use their wings for flight and the production of a patterned acoustic signal, the male courtship song, employing in both cases a small set of wing muscles and corresponding motor neurons. We took first steps toward elucidating the neuronal control mechanisms of this multifunctional motor system by live imaging of muscle ensemble activity patterns during song and flight, and we established the functional role of a comprehensive set of wing muscle motor neurons by silencing experiments. Song and flight rely on distinct configurations of neuromuscular activity, with most, but not all, flight muscles and their corresponding motor neurons contributing to song and shaping its acoustic parameters. The two behaviors are exclusive, and the neuronal command for flight overrides the command for song. The neuromodulator octopamine is a candidate for selectively stabilizing flight, but not song motor patterns.

Dynamic neural control of insect muscle metabolism related to motor behavior

Skeletal muscle innervation differs between vertebrates and insects. Insect muscle fibers exhibit graded electrical potentials and are innervated by excitatory, inhibitory, and also neuromodulatory motoneurons. The latter form a unique class of unpaired neurons with bilaterally symmetrical axons that release octopamine to alter the efficacy of synaptic transmission and regulate muscle energy metabolism by activating glycolysis. Octopaminergic neurons that innervate muscles with a high energy demand, for example, flight muscles that move the wings of a locust up and down, are active during rest but are inhibited during flight and its preparatory phase, a jump. Therefore, it is argued that these neurons are involved in providing locusts with the necessary fuel at takeoff, but then may aid the switch to lipid oxidation during flight. In general, the octopaminergic system may switch the whole organism from a tonic to a dynamic state.

State-dependent performance of optic-flow processing interneurons

Active locomotive states are metabolically expensive and require efficient sensory processing both to avoid wasteful movements and to cope with an extended bandwidth of sensory stimuli. This is particularly true for flying animals because flight, as opposed to walking or resting, imposes a steplike increase in metabolism for the precise execution and control of movements. Sensory processing itself carries a significant metabolic cost, but the principles governing the adjustment of sensory processing to different locomotor states are not well understood. We use the blowfly as a model system to study the impact on visual processing of a neuromodulator, octopamine, which is known to be involved in the regulation of flight physiology. We applied an octopamine agonist and recorded the directional motion responses of identified visual interneurons known to process self-motion-induced optic flow to directional motion stimuli. The neural response range of these neurons is increased and the response latency is reduced. We also found that, due to an elevated spontaneous spike rate, the cells' negative signaling range is increased. Meanwhile, the preferred self-motion parameters the cells encode were state independent. Our results indicate that in the blowfly energetically expensive sensory coding strategies, such as rapid, large responses, and high spontaneous spike activity could be adjusted by the neuromodulator octopamine, likely to save energy during quiet locomotor states.

Motoneurons of the flight power muscles of the blowflyCalliphora erythrocephala: Structures and mutual dye coupling

The morphologies of the motoneurons of the dorsolongitudinal and the three dorsoventral flight power muscles (DLM, DVM 1-3) of Calliphora were investigated by means of cobalt backfills and intracellular biocytin stainings. The DLM is innervated by four prothoracic motoneurons supplying the four ventral muscle fibers and one mesothoracic motoneuron supplying the two dorsal fibers. The three fibers of the DVM 1 and the two fibers of the DVM 2 are innervated by five mesothoracic motoneurons, whereas the two fibers of the DVM 3 are innervated by two prothoracic motoneurons. In general, the motoneurons of each muscle have a common ventral soma cluster located in a characteristic position on the ipsilateral side of the thoracic ganglion, show similar dendritic arborizations in the mesothoracic wing neuropil, and have the same axon pathway. Only the soma of the common motoneuron of two dorsal fibers of the DLM is situated dorsally in the contralateral hemiganglion. The motoneurons of each muscle were found to be strongly dye coupled with each other, indicating that they are connected by gap junctions. In addition, the motoneurons of each muscle establish characteristic coupling patterns with the motoneurons of the other flight power muscles on both sides of the thorax and with two bilateral groups of local mesothoracic interneurons. The revealed coupling patterns are assumed to be of major relevance for the generation the characteristic, rhythmic flight activity of the motoneurons described in previous studies.

Comparison of octopamine-like immunoreactivity in the brains of the fruit fly and blow fly

A serum raised against conjugated octopamine reveals structurally comparable systems of perikarya and arborizations in protocerebral neuropils of two species of Diptera, Drosophila melanogaster and Phaenicia sericata; the latter is used extensively for electrophysiological studies of the optic lobes and their central projections. Clusters of cell bodies in the brain as well as midline perikarya provide octopamine-like immunoreactive processes to the optic lobes, circumscribed regions of the protocerebrum and the central complex, particularly the protocerebral bridge, fan-shaped body, and ellipsoid body. Ventral unpaired median somata provide immunoreactive processes within the subesophageal ganglion and tritocerebrum. Ascending neurites from these cells also supply the antennal lobe glomeruli, regions of the lateral protocerebrum, the mushroom body calyces, and the lobula complex. The mushroom body's gamma lobes contain immunoreactive processes that originate from processes that arborize in the protocerebrum. The present observations are discussed with respect to similarities and differences between two species of Diptera, one of which has neurons large enough for intracellular penetrations. The results are also discussed with respect to recent studies on octopamine-immunoreactive organization in honey bees and cockroaches and the suggested roles of octopamine in sensory processing, learning, and memory.

Distance and force production during jumping in wild-type and mutant Drosophila melanogaster

In many insects renowned for their jumping ability, elastic storage is used so that high forces can be developed prior to jumping. We have combined physiological, behavioural and genetic approaches to test whether elastic energy storage makes a major contribution to jumping in Drosophila. We describe a sensitive strain gauge setup, which measures the forces produced by tethered flies through their mesothoracic legs. The peak force produced by the main jumping muscle of female flies from a wild-type (Canton-S) strain is 101+/-4.4 microN [and this is indistinguishable from a second wild-type (Texas) strain]. The force takes 8.2 ms to reach its peak. The peak force is not affected significantly by altering the leg angle (femur-tibia joint angle) in the range of 75-120 degrees, but the peak force declines as the leg is extended further. Measurements of jumping ability (distance jumped) showed that female Drosophila (with their wings removed) of two wild-type strains, Canton-S and Texas, produced jumps of 28.6+/-0.7 and 30.2+/-1.0 mm (mean +/- s.e.m.). For a female wild-type Drosophila, a jump of 30 mm corresponds to a kinetic energy of 200 nJ on take-off (allowing 20% of the energy to overcome air resistance). We develop equations of motion for a linear force-time model of take-off and calculate that the time to take-off is 5.0 ms and the peak force should be 274 microN (137 microN leg(-1)). We predicted, from the role of octopamine in enhancing muscle tension in several locust muscles, that if stored elastic energy plays no part in force development, then genetic manipulation of the octopaminergic system would directly affect force production and jumping in Drosophila. Using two mutants deficient in the octopaminergic system, TbhnM18 (M18) and TyrRhono (hono), we found significantly reduced jumping distances (20.7+/-0.7 and 20.7+/-0.4 mm, respectively) and force production (52% and 55%, respectively) compared with wild type. From the reduced distance and force production in M18, a mutant deficient in octopamine synthesis, and in hono, a tyramine/octopamine receptor mutant, we conclude that in Drosophila, as in locusts, octopamine modulates escape jumping. We conclude that the fly does not need to store large quantities of elastic energy in order to make its jump because (1) the measured and calculated forces agree to within 40% and (2) the reduction in distances jumped by the mutants correlates well with their reduction in measured peak force.