Protocols to Study Behavior in Drosophila.

Drosophila melanogaster is an incredibly versatile organism capable of both innate and higher-order behaviors. These behaviors offer not only a way to assay whether or not the animal is physiologically compromised (e.g., feeding, locomotion), but also serve to assess changes in centrally mediated functions. Here we describe several high throughput, reproducible, yet inexpensive and facile behavioral assays for both larval and adult Drosophila. The larval assays all employ an agar substrate in a petri dish; the adult assays are grouped into "vial-based" and "arena-based" paradigms. While these protocols are largely designed to assess individual animals, they are sufficiently rapid that ample numbers can be tested to determine behavioral significance. Importantly, this also allows for one to control for reproductive status, age, and sex, since these factors all have a significant impact on adult behaviors. In general, it is best to designate a dedicated area for any assay, so that lighting conditions are consistent, and all animals should be tested at roughly the same time each day to minimize circadian fluctuations. Temperature and humidity should also be maintained at a constant level to minimize variability in the assays.

Response to stress in Drosophila is mediated by gender, age and stress paradigm.

All living organisms must maintain equilibrium in response to internal and external challenges within their environment. Changes in neural plasticity (alterations in neuronal populations, dendritic remodeling, and synaptic turnover) are critical components of the homeostatic response to stress, which has been strongly implicated in the onset of affective disorders. However, stress is differentially perceived depending on the type of stress and its context, as well as genetic background, age and sex; therefore, an individual's maintenance of neuronal homeostasis must differ depending upon these variables. We established Drosophila as a model to analyze homeostatic responses to stress. Sexually immature and mature females and males from an isogenic wild-type strain raised under controlled environmental conditions were exposed to four reproducible and high-throughput translatable stressors to facilitate the analysis of a large number of animals for direct comparisons. These animals were assessed in an open-field arena, in a light-dark box, and in a forced swim test, as well as for sensitivity to the sedative effects of ethanol. These studies establish that immature and mature females and males represent behaviorally distinct populations under control conditions as well as after exposure to different stressors. Therefore, the neural substrates mediating the stress response must be differentially expressed depending upon the hormonal status of the brain. In addition, an adaptive response to a given stressor in one paradigm was not predictive for outcomes in other paradigms.

Altering the sex determination pathway in Drosophila fat body modifies sex-specific stress responses.

The stress response in Drosophila melanogaster reveals sex differences in behavior, similar to what has been observed in mammals. However, unlike mammals, the sex determination pathway in Drosophila is well established, making this an ideal system to identify factors involved in the modulation of sex-specific responses to stress. In this study, we show that the Drosophila fat body, which has been shown to be important for energy homeostasis and sex determination, is a dynamic tissue that is altered in response to stress in a sex and time-dependent manner. We manipulated the sex determination pathway in the fat body via targeted expression of transformer and transformer-2 and analyzed these animals for changes in their response to stress. In the majority of cases, manipulation of transformer or transformer-2 was able to change the physiological output in response to starvation and oxidative stress to that of the opposite sex. Our data also uncover the possibility of additional downstream targets for transformer and transformer-2 that are separate from the sex determination pathway and can influence behavioral and physiological responses.

Functional analysis of the larval feeding circuit in Drosophila.

The serotonergic feeding circuit in Drosophila melanogaster larvae can be used to investigate neuronal substrates of critical importance during the development of the circuit. Using the functional output of the circuit, feeding, changes in the neuronal architecture of the stomatogastric system can be visualized. Feeding behavior can be recorded by observing the rate of retraction of the mouth hooks, which receive innervation from the brain. Locomotor behavior is used as a physiological control for feeding, since larvae use their mouth hooks to traverse across an agar substrate. Changes in feeding behavior can be correlated with the axonal architecture of the neurites innervating the gut. Using immunohistochemistry it is possible to visualize and quantitate these changes. Improper handling of the larvae during behavior paradigms can alter data as they are very sensitive to manipulations. Proper imaging of the neurite architecture innervating the gut is critical for precise quantitation of number and size of varicosities as well as the extent of branch nodes. Analysis of most circuits allow only for visualization of neurite architecture or behavioral effects; however, this model allows one to correlate the functional output of the circuit with the impairments in neuronal architecture.

Sexually dimorphic recruitment of dopamine neurons into the stress response circuitry.

Several previous studies in mammalian systems have shown sexually dimorphic behaviors, neuroendocrine changes, and alterations in neurotransmitter release in response to stress. In addition, men and women are differentially vulnerable to stress-related pathologies, which have led to the hypothesis that the stress response circuitry differs depending on sex. The authors used the genetic tractability of Drosophila to manipulate pre- or postsynaptic dopamine signaling in transgenic animals, which were assayed for several parameters of locomotion and heart rate following exposure to 2 environmental stressors: starvation and oxidative stress. Their results show significant differences in the stress response for males and females by analyzing heart rate, centering time, and high mobility in addition to other locomotor parameters with translational relevance. These data demonstrate that both pre- and postsynaptic neurons are differentially recruited into the dopaminergic stress response circuitry for males and females. The results also show that the response circuits differ depending on the stressor and behavioral output. Furthermore, the authors' studies provide a translatable Drosophila model for further elucidation of factors involved in the sexually dimorphic recruitment of neurons into the stress response circuitry.

Early manipulation of juvenile hormone has sexually dimorphic effects on mature adult behavior in Drosophila melanogaster.

Hormones are critical for the development, maturation, and maintenance of physiological systems; therefore, understanding their involvement during maturation of the brain is important for the elucidation of mechanisms by which adults become behaviorally competent. Changes in exogenous and endogenous factors encountered during sexual maturation can have long lasting effects in mature adults. In this study, we investigated the role of the gonadotropic hormone, juvenile hormone (JH), in the modulation of adult behaviors in Drosophila. Here we utilized methoprene (a synthetic JH analog) and precocene (a JH synthesis inhibitor) to manipulate levels of JH in sexually immature male and female Drosophila with or without decreased synthesis of neuronal dopamine (DA). Locomotion and courtship behavior were assayed once the animals had grown to sexual maturity. The results demonstrate a sexually dimorphic role for JH in the modulation of these centrally controlled behaviors in mature animals that is dependent on the age of the animals assayed, and present DA as a candidate neuronal factor that differentially interacts with JH depending on the sex of the animal. The data also suggest that JH modulates these behaviors through an indirect mechanism. Since gonadotropic hormones and DA interact in mammals to affect brain development and later function, our results suggest that this mechanism for the development of adult behavioral competence may be evolutionarily conserved.

Temporally dimorphic recruitment of dopamine neurons into stress response circuitry in Drosophila.

Many studies have pointed to vulnerability to stress and stress-related pathologies at different timepoints during an individual's life span. These sensitive windows are usually during periods of neural development, such as embryogenesis, infancy, and adolescence. It is critical to understand how neural circuitry may change as an individual ages in ways that could affect susceptibility to stress. Here we compare two stages in Drosophila melanogaster: sexual immaturity and sexual maturity. We used the genetic resources available in Drosophila to manipulate pre- and postsynaptic dopamine signaling in sexually immature and mature animals that were then assayed for heart rate and locomotor behavior in response to starvation and oxidative stress. Our results show significant differences in the stress response for sexually immature and mature animals for heart rate, periods of high mobility, mean velocity, and several other parameters of locomotor behavior. Our data show that dopamine neurons are differentially recruited into the stress response circuitry for sexually immature and mature individuals. By observing behaviors that have been previously shown in mammalian models to be affected by stress and altered in models of affective disorders, we provide a genetically tractable model for development and maintenance of the stress response circuitry during sexual maturation.

Comparative approaches to the study of physiology: Drosophila as a physiological tool.

Numerous studies have detailed the extensive conservation of developmental signaling pathways between the model system, Drosophila melanogaster, and mammalian models, but researchers have also profited from the unique and highly tractable genetic tools available in this system to address critical questions in physiology. In this review, we have described contributions that Drosophila researchers have made to mathematical dynamics of pattern formation, cardiac pathologies, the way in which pain circuits are integrated to elicit responses from sensation, as well as the ways in which gene expression can modulate diverse behaviors and shed light on human cognitive disorders. The broad and diverse array of contributions from Drosophila underscore its translational relevance to modeling human disease.

Neurotrophic actions of dopamine on the development of a serotonergic feeding circuit in Drosophila melanogaster.

BACKGROUND: In the fruit fly, Drosophila melanogaster, serotonin functions both as a neurotransmitter to regulate larval feeding, and in the development of the stomatogastric feeding circuit. There is an inverse relationship between neuronal serotonin levels during late embryogenesis and the complexity of the serotonergic fibers projecting from the larval brain to the foregut, which correlate with perturbations in feeding, the functional output of the circuit. Dopamine does not modulate larval feeding, and dopaminergic fibers do not innervate the larval foregut. Since dopamine can function in central nervous system development, separate from its role as a neurotransmitter, the role of neuronal dopamine was assessed on the development, and mature function, of the 5-HT larval feeding circuit. RESULTS: Both decreased and increased neuronal dopamine levels in late embryogenesis during development of this circuit result in depressed levels of larval feeding. Perturbations in neuronal dopamine during this developmental period also result in greater branch complexity of the serotonergic fibers innervating the gut, as well as increased size and number of the serotonin-containing vesicles along the neurite length. This neurotrophic action for dopamine is modulated by the D2 dopamine receptor expressed during late embryogenesis in central 5-HT neurons. Animals carrying transgenic RNAi constructs to knock down both dopamine and serotonin synthesis in the central nervous system display normal feeding and fiber architecture. However, disparate levels of neuronal dopamine and serotonin during development of the circuit result in abnormal gut fiber architecture and feeding behavior. CONCLUSIONS: These results suggest that dopamine can exert a direct trophic influence on the development of a specific neural circuit, and that dopamine and serotonin may interact with each other to generate the neural architecture necessary for normal function of the circuit.

A trophic role for serotonin in the development of a simple feeding circuit.

Correct differentiation and positioning of individual synapses during development is fundamental to the normal function of neuronal circuits. While classical transmitters such as serotonin (5-HT) play a critical trophic role in neurogenesis in addition to their functions as transmitters in the mature nervous system, this process is not well understood. We used a simple model to assess both development and function of a specific behavioral circuit in the larval stage of the fruit fly (Drosophila melanogaster). We show that, as in all other species examined, the neurotransmitter actions of 5-HT depress feeding, and decreased neuronal 5-HT levels increase appetite. However, using transgenic tools, we show that constitutive knockdown of neuronal 5-HT synthesis to reduce 5-HT levels during central nervous system (CNS) development results in increased branching of the serotonergic fibers projecting to the gut, as well as increased size and number of varicosities along the neurite length. As larvae, these animals display decreased feeding rates relative to controls, and, when given exogenous 5-hydroxytryptophan, feeding is significantly enhanced. Late-stage wild-type embryos exposed to 5-HT to augment 5-HT levels during CNS development display, as mature larvae, a significant decrease in gut fiber branching and total varicosity number, as well as increased feeding and a hyposensitivity to the effects of 5-HT. Exposure of embryos unable to synthesize neuronal serotonin to 5-HT during late embryogenesis results in rescue of the feeding behavior and abnormalities in the 5-HT gut fiber architecture. These results demonstrate an inverse relationship between developmental 5-HT levels and complexity of the fiber architecture projecting to gut tissue, which results in a perturbed feeding pattern. We conclude that 5-HT is tightly regulated during CNS development to direct the normal architecture and mature function of this neural circuit.

Feeding Drosophila a biotin-deficient diet for multiple generations increases stress resistance and lifespan and alters gene expression and histone biotinylation patterns.

Energy restriction increases stress resistance and lifespan in Drosophila melanogaster and other species. The roles of individual nutrients in stress resistance and longevity are largely unknown. The vitamin biotin is a potential candidate for mediating these effects, given its known roles in stress signaling and gene regulation by epigenetic mechanisms, i.e. biotinylation of histones. Here, we tested the hypothesis that prolonged culture of Drosophila on biotin-deficient (BD) medium increases stress resistance and lifespan. Flies were fed a BD diet for multiple generations; controls were fed a biotin-normal diet. In some experiments, a third group of flies was fed a BD diet for 12 generations and then switched to control diets for 2 generations to eliminate potential effects of short-term biotin deficiency. Flies fed a BD diet exhibited a 30% increase in lifespan. This increase was associated with enhanced resistance to the DNA-damaging agent hydroxyurea and heat stress. Also, fertility increased significantly compared with biotin-normal controls. Biotinylation of histones was barely detectable in biotin-deprived flies, suggesting that epigenetic events might have contributed to effects of biotin deprivation.

Antioxidants protect PINK1-dependent dopaminergic neurons in Drosophila.

Parkinson's disease (PD) is the most frequent neurodegenerative movement disorder. Mutations in the PINK1 gene are linked to the autosomal recessive early onset familial form of PD. The physiological function of PINK1 and pathological abnormality of PD-associated PINK1 mutants are largely unknown. We here show that inactivation of Drosophila PINK1 (dPINK1) using RNAi results in progressive loss of dopaminergic neurons and in ommatidial degeneration of the compound eye, which is rescued by expression of human PINK1 (hPINK1). Expression of human SOD1 suppresses neurodegeneration induced by dPINK1 inactivation. Moreover, treatment of dPINK1 RNAi flies with the antioxidants SOD and vitamin E significantly inhibits ommatidial degeneration. Thus, dPINK1 plays an essential role in maintaining neuronal survival by preventing neurons from undergoing oxidative stress, thereby suggesting a potential mechanism by which a reduction in PINK1 function leads to PD-associated neurodegeneration.

Biochemical conservation of recombinant Drosophila tyrosine hydroxylase with its mammalian cognates.

Dopamine modulates several behavioral and developmental events; in the fruit fly Drosophila melanogaster, dopamine is a neurotransmitter, a neuromodulator, and a developmental signal. Studies in mammals suggest that these diverse roles for dopamine have been evolutionarily conserved. Fundamental regulation of dopamine occurs via tyrosine hydroxylase (TH), the first and rate-limiting enzyme in the catecholamine biosynthetic pathway. Mammalian TH is acutely regulated via phosphorylation-dephosphorylation mechanisms, which occur as a direct consequence of nerve stimulation. We have shown that the Drosophila homolog of TH, DTH, shares over 50% sequence identity with mammalian TH, and the serine residue corresponding to the major site of phosphorylation is conserved. We demonstrate using recombinant DTH protein generated in E. coli that its regulatory biochemical mechanisms closely parallel those from mammals. Drosophila thus provides a highly conserved and tractable model system in which to test the functional consequences of perturbing TH activity by acute regulatory mechanisms.

Stress affects dopaminergic signaling pathways in Drosophila melanogaster.

Behaviors modulated by dopamine appear to be conserved across species. In the model system Drosophila melanogaster, as in mammals, dopamine modulates female sexual receptivity, a simple form of learning and responses to drugs of abuse. Synthesis, reuptake and binding of dopamine are also evolutionarily conserved. Since stress has been shown to affect dopaminergic signaling pathways in mammals, we investigated the consequences of exposure to diverse stressors on dopaminergic physiology in the fruit fly, D. melanogaster. Animals were exposed to a metabolic stress (starvation), an oxidative stress (via the superoxide anion generator paraquat) or a mechanical stress (gentle vortexing). Sexual maturity, reproductive status, gender and type of stress differentially affected survival. The stress paradigms also resulted in alterations in the activity of tyrosine hydroxylase, the rate-limiting enzyme in dopamine biosynthesis. Exposure to these stressors perturbed female sexual receptivity and ovarian development, which are modulated by dopamine, suggesting that dopaminergic physiology is affected as a consequence of stress. Transgenic Drosophila with reduced levels of neuronal dopamine displayed an altered response to these stressors, suggesting that, as in mammals, dopamine is a key element in the stress response.

Serotonin synthesis by two distinct enzymes in Drosophila melanogaster.

Annotation of the sequenced Drosophila genome suggested the presence of an additional enzyme with extensive homology to mammalian tryptophan hydroxylase, which we have termed DTRH. In this work, we show that enzymatic analyses of the putative DTRH enzyme expressed in Escherichia coli confirm that it acts as a tryptophan hydroxylase but can also hydroxylate phenylalanine, in vitro. Building upon the knowledge gained from the work in mice and zebrafish, it is possible to hypothesize that DTRH may be primarily neuronal in function and expression, and DTPH, which has been previously shown to have phenylalanine hydroxylation as its primary role, may be the peripheral tryptophan hydroxylase in Drosophila. The experiments presented in this report also show that DTRH is similar to DTPH in that it exhibits differential hydroxylase activity based on substrate. When DTRH uses tryptophan as a substrate, substrate inhibition, catecholamine inhibition, and decreased tryptophan hydroxylase activity in the presence of serotonin synthesis inhibitors are observed. When DTRH uses phenylalanine as a substrate, end product inhibition, increased phenylalanine hydroxylase activity after phosphorylation by cAMP-dependent protein kinase, and a decrease in phenylalanine hydroxylase activity in the presence of the serotonin synthesis inhibitor, alpha-methyl-(DL)-tryptophan are observed. These experiments suggest that the presence of distinct tryptophan hydroxylase enzymes may be evolutionarily conserved and serve as an ancient mechanism to appropriately regulate the production of serotonin in its target tissues.

Substrate regulation of serotonin and dopamine synthesis in Drosophila.

In Drosophila melanogaster, serotonin (5-hydroxytryptamine, 5-HT) is required for both very early non-neuronal developmental events, and in the CNS as a neurotransmitter to modulate behavior. 5-HT is synthesized, at least in part, by the actions of Drosophila tryptophan-phenylalanine hydroxylase (DTPH), a dual function enzyme that hydroxylates both phenylalanine and tryptophan. DTPH is expressed in numerous tissues as well as dopaminergic and serotonergic neurons, but it does not necessarily function as both enzymes in these tissues. Deficiencies in DTPH could affect the production of dopamine and serotonin, and thus dopaminergic and serotonergic signaling pathways. In this paper, we show that DTPH exhibits differential hydroxylase activity based solely on substrate. When DTPH uses phenylalanine as a substrate, regulatory control (end product inhibition, decreased PAH activity following phosphorylation, catecholamine inhibition) is observed that is not seen when the enzyme uses tryptophan as a substrate. These studies suggest that regulation of DTPH enzymatic activity occurs, at least in part, through the actions of its substrate.

GABAergic modulation of motor-driven behaviors in juvenile Drosophila and evidence for a nonbehavioral role for GABA transport.

We have identified specific GABAergic-modulated behaviors in the juvenile stage of the fruit fly, Drosophila melanogaster via systemic treatment of second instar larvae with the potent GABA transport inhibitor DL-2,4-diaminobutyric acid (DABA). DABA significantly inhibited motor-controlled body wall and mouth hook contractions and impaired rollover activity and contractile responses to touch stimulation. The perturbations in locomotion and rollover activity were reminiscent of corresponding DABA-induced deficits in locomotion and the righting reflex observed in adult flies. The effects were specific to these motor-controlled behaviors, because DABA-treated larvae responded normally in olfaction and phototaxis assays. Recovery of these behaviors was achieved by cotreatment with the vertebrate GABA(A) receptor antagonist picrotoxin. Pharmacological studies performed in vitro with plasma membrane vesicles isolated from second instar larval tissues verified the presence of high-affinity, saturable GABA uptake mechanisms. GABA uptake was also detected in plasma membrane vesicles isolated from behaviorally quiescent stages. Competitive inhibition studies of [3H]-GABA uptake into plasma membrane vesicles from larval and pupal tissues with either unlabeled GABA or the transport inhibitors DABA, nipecotic acid, or valproic acid, revealed differences in affinities. GABAergic-modulation of motor behaviors is thus conserved between the larval and adult stages of Drosophila, as well as in mammals and other vertebrate species. The pharmacological studies reveal shared conservation of GABA transport mechanisms between Drosophila and mammals, and implicate the involvement of GABA and GABA transporters in regulating physiological processes distinct from neurotransmission during behaviorally quiescent stages of development.

Talking the talk: the role of VEGF proteins in cell signaling.

Vascular endothelial growth factors (VEGFs) stimulate endothelial cell growth through interactions with their tyrosine kinase receptors to stimulate intracellular signaling events. This culminates in the expression of specific gene products that induce a cellular response in numerous physiological processes, including hematopoeisis, oncogenesis and embryogenesis. The primordial function of VEGF can be revealed by studying VEGF-mediated signaling pathways in the powerful and tractable model system, Drosophila melanogaster, which has proved invaluable in furthering our understanding of conserved developmental themes.

Pharmacological evidence for GABAergic regulation of specific behaviors in Drosophila melanogaster.

We have identified several GABAergic-modulated behaviors in Drosophila melanogaster by employing a pharmacological approach to disrupt GABA transporter function in vivo. Systemic treatment of adult female flies with the GABA transport inhibitors DL-2,4-diaminobutyric acid (DABA) or R,S-nipecotic acid (NipA), resulted in diminished locomotor activity, deficits in geotaxis, and the induction of convulsive behaviors with a secondary loss of the righting reflex. Pharmacological evidence suggested that the observed behavioral phenotypes were specific to disruption of GABA transporter function and GABAergic activity. The effects of GABA reuptake inhibitors on locomotor activity were dose dependent, pharmacologically distinct, and paralleled their known effects in mammalian systems. Recovery of normal locomotor activity and the righting reflex in DABA- and NipA-treated flies was achieved by coadministration of bicuculline (BIC), a GABA receptor antagonist that supresses GABAergic activity in mammals. Recovery of these behaviors was also achieved by coadministration of gabapentin, an anticonvulsant agent that interacts with mammalian GABAergic systems. Finally, behavioral effects were selective because other specific behaviors such as feeding activity and female sexual receptivity were not affected. Related pharmacological analyses performed in vitro on isolated Drosophila synaptic plasma membrane vesicles demonstrated high affinity, saturable uptake mechanisms for [3H]-GABA; further competitive inhibition studies with DABA and NipA demonstrated their ability to inhibit [3H]-GABA transport. The existence of experimentally accessible GABA transporters in Drosophila that share conserved pharmacological properties with their mammalian counterparts has resulted in the identification of specific behaviors that are modulated by GABA.