Uncoupling of Brain Activity from Movement Defines Arousal States in Drosophila

Prolonged deprivation of sleep-like rest raises metabolic rate in the Pacific beetle cockroach, Diploptera punctata (Eschscholtz)

Rats respond to sustained sleep deprivation with increased mortality preceded by a rise in resting metabolic rate that may or may not be attributed to dysfunction of the thermoregulatory system. The present study was designed to test the hypothesis that deprivation of sleep-like rest will lead to increased metabolic rate in an ectothermic insect, the Pacific beetle cockroach. A mild alerting stimulus consisting of a brief <1% pulse of CO2 and simultaneous 2 s rotation (1 cm motion) of the animal chamber consistently prevented the adoption of a sleep-like resting posture in cockroaches. Two groups of 15 male adult cockroaches were studied; a group targeted for deprivation of sleep-like rest (SD) was presented with one stimulus per minute continuously, and a group of stimulus controls (SC) was given the same number of stimuli per day but scheduled such that the animals received a 3 h interval without stimuli four times per day. This protocol led to significantly increased mortality in the SD group beginning on day 17 (averaging 0.57 deaths per day thereafter), but not in the SC group (averaging 0.17 deaths per day throughout). Oxygen consumption (VO2) increased significantly after 4 weeks in the SD group but not the SC group. VO2 was 82% above pre-deprivation baseline after 35 days in the SD group (P=0.009). Body mass was unchanged throughout. We conclude that sleep-like rest is essential for long-term viability in insects and that prolonged vigilance leads to an increase in whole-animal metabolic rate in this ectothermic species.

Patch-clamp analysis of gene-targeted vomeronasal neurons expressing a defined V1r or V2r receptor: ionic mechanisms underlying persistent firing

Sensory neurons in the mouse vomeronasal organ consist of two major groups, apical and basal, that project to different brain regions, express unique sets of receptors, and serve distinct functions. Electrical properties of these two subpopulations, however, have not been systematically characterized. V1rb2-tau-GFP and V2r1b-tau-GFP tagged vomeronasal sensory neurons (VSNs) were selected as prototypical apical or basal VSNs, respectively, and their biophysical properties were analyzed in acute slices that minimized cell damage. Basal V2r1b-expressing VSNs had voltage-gated conductances, and especially Na(+) (Nav) and Ca(2+) (Cav) currents, that were substantially larger than those observed in apical V1rb2 VSNs, although the resting membrane potential, input resistance, and membrane capacitance were similar in both cell types. Of several types of Cav currents, T-type and L-type Cav currents contributed to action potential firing, and both currents alone were capable of generating oscillatory Ca(2+) spikes. The L-type Cav current was uniquely coupled to a BK large-conductance K(+) current, and interplay between these channels played a critical role in repolarizing spikes and maintaining persistent firing in VSNs. Larger Nav and Cav conductances, along with a more positive inactivation voltage of the Nav current in the V2r1b VSNs, contributed to the larger spike amplitude and higher spike frequency induced by depolarizing current in these cells compared with V1rb2 VSNs. Basal GFP-negative VSNs and V2r1b VSNs responded to prolonged depolarization with persistent, but adapting discharge that could be relevant in sensory adaptation. Collectively, these results suggest a novel mechanism for regulating and encoding neuronal activity in the accessory olfactory system.

A succession of anesthetic endpoints in the Drosophila brain

General anesthetics abolish behavioral responsiveness in all animals, and in humans this is accompanied by loss of consciousness. Whether similar target mechanisms and behavioral endpoints exist across species remains controversial, although model organisms have been successfully used to study mechanisms of anesthesia. In Drosophila, a number of key mutants have been characterized as hypersensitive or resistant to general anesthetics by behavioral assays. In order to investigate general anesthesia in the Drosophila brain, local field potential (LFP) recordings were made during incremental exposures to isoflurane in wild-type and mutant flies. As in higher animals, general anesthesia in flies was found to involve a succession of distinct endpoints. At low doses, isoflurane uncoupled brain activity from ongoing movement, followed by a sudden attenuation in neural correlates of perception. Average LFP activity in the brain was more gradually attenuated with higher doses, followed by loss of movement behavior. Among mutants, a strong correspondence was found between behavioral and LFP sensitivities, thereby suggesting that LFP phenotypes are proximal to the anesthetic's mechanism of action. Finally, genetic and pharmacological analysis revealed that anesthetic sensitivities in the fly brain are, like other arousal states, influenced by dopaminergic activity. These results suggest that volatile anesthetics such as isoflurane may target the same processes that sustain wakefulness and attention in the brain. LFP correlates of general anesthesia in Drosophila provide a powerful new approach to uncovering the nature of these processes.

A dynamic role for the mushroom bodies in promoting sleep in Drosophila

The fruitfly, Drosophila melanogaster, exhibits many of the cardinal features of sleep, yet little is known about the neural circuits governing its sleep. Here we have performed a screen of GAL4 lines expressing a temperature-sensitive synaptic blocker shibire(ts1) (ref. 2) in a range of discrete neural circuits, and assayed the amount of sleep at different temperatures. We identified three short-sleep lines at the restrictive temperature with shared expression in the mushroom bodies, a neural locus central to learning and memory. Chemical ablation of the mushroom bodies also resulted in reduced sleep. These studies highlight a central role for the mushroom bodies in sleep regulation.

Heterogeneity of voltage- and chemosignal-activated response profiles in vomeronasal sensory neurons

Liolaemus lizards were explored to ascertain whether they would make an amenable model to study single-cell electrophysiology of neurons in the vomeronasal organ (VNO). Despite a rich array of chemosensory-related behaviors chronicled for this genus, no anatomical or functional data exist for the VNO, the organ mediating these types of behaviors. Two Liolaemus species (L. bellii and L. nigroviridis) were collected in Central Chile in the Farellones Mountains and transported to the United States. Lizards were subjected to hypothermia and then a lethal injection of sodium pentabarbitol prior to all experiments described in the following text. Retrograde dye perfusion combined with histological techniques demonstrated a compartmentalization of the proportionally large VNO from the main olfactory epithelium (MOE) in cryosections of L. bellii. SDS-PAGE analysis of the VNO of both species demonstrated the expression of three G protein subunits, namely, G(alphao), G(alphai2), and G(beta), and the absence of G(alphaolf), G(alpha11), and G(q), the latter of which are traditionally found in the MOE. Vomeronasal (VN) neurons were enzymatically isolated for whole cell voltage-clamp electrophysiology of single neurons. Both species demonstrated a tetrodotoxin (TTX)-sensitive, rapidly inactivating sodium current and a tetraethylammonium (TEA)-sensitive potassium current that had a transient and sustained component. VN neurons were classified into two types dependent on the ratio of sodium over sustained potassium current. VN neurons exhibited outward and inward chemosignal-evoked currents when stimulated with pheromone-containing secretions taken from the feces, skin, and precloacal pores. Fifty-nine percent of the neurons were responsive to at least one compound when presented with a battery of five different secretions. The breadth of responsiveness (H metric) demonstrated a heterogeneous population of tuning with a mean of 0.29.

Dopaminergic modulation of arousal in Drosophila

BACKGROUND

Arousal levels in the brain set thresholds for behavior, from simple to complex. The mechanistic underpinnings of the various phenomena comprising arousal, however, are still poorly understood. Drosophila behaviors have been studied that span different levels of arousal, from sleep to visual perception to psychostimulant responses.

RESULTS

We have investigated neurobiological mechanisms of arousal in the Drosophila brain by a combined behavioral, genetic, pharmacological, and electrophysiological approach. Administration of methamphetamine (METH) suppresses sleep and promotes active wakefulness, whereas an inhibitor of dopamine synthesis promotes sleep. METH affects courtship behavior by increasing sexual arousal while decreasing successful sexual performance. Electrophysiological recordings from the medial protocerebrum of wild-type flies showed that METH ingestion has rapid and detrimental effects on a brain response associated with perception of visual stimuli. Recordings in genetically manipulated animals show that dopaminergic transmission is required for these responses and that visual-processing deficits caused by attenuated dopaminergic transmission can be rescued by METH.

CONCLUSIONS

We show that changes in dopamine levels differentially affect arousal for behaviors of varying complexity. Complex behaviors, such as visual perception, degenerate when dopamine levels are either too high or too low, in accordance with the inverted-U hypothesis of dopamine action in the mammalian brain. Simpler behaviors, such as sleep and locomotion, show graded responses that follow changes in dopamine level.

Arousal Mechanisms: Speedy Flies Don’t Sleep at Night

Alertness and behavioral performance depend on an animal's level of arousal. In vertebrates, reinforcement and maintenance of arousal in the cortex are ensured by diffuse inputs from neurons releasing biogenic amine neuromodulators. Fruit flies similarly use dopamine for arousal control, indicating an ancient evolutionary origin of this essential feature of the functioning brain.

Cognitive consonance: complex brain functions in the fruit fly and its relatives

The fruit fly, Drosophila melanogaster, has become a model for the study of a growing number of human characteristics because of the power of its genetics. Higher cognitive functions, however, might be assumed to be out of reach for the little fly. But the cumulative history of cognitive studies in insects and some of their arachnid relatives, as well as specific probing of the capabilities of fruit flies, suggests that even in this ethereal realm these creatures have much to contribute. What are the degrees of sophistication in cognitive behavior displayed by these organisms, how have they been demonstrated, and what is their potential for understanding how our own brains work?

The remote roots of consciousness in fruit-fly selective attention?

A mechanistic study of consciousness need not be confined to human complexity. Other animals also display key behaviors and responses that have long been intimately tied to the measure of consciousness in humans. Among them are some very well-defined and measurable endpoints: selective attention, sleep and general anesthesia. That these three variables associated with changes in consciousness might exist even in a fruit-fly does not necessarily imply that a fly is "conscious", but it does suggest that some of the problems central to the field of consciousness studies could be investigated in a model organism such as Drosophila melanogaster. Demonstrating suppression of unattended stimuli, which is central to attention studies in humans, is now possible in Drosophila by measuring neural correlates of visual selection. By combining such studies with an eventual understanding of suppression in other arousal states in the fly, such as sleep and general anesthesia, we might be unraveling mechanisms relevant to consciousness as well.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Systems approaches to biological rhythms in Drosophila

The chronobiological system of Drosophila is considered from the perspective of rhythm-regulated genes. These factors are enumerated and discussed not so much in terms of how the gene products are thought to act on behalf of circadian-clock mechanisms, but with special emphasis on where these molecules are manufactured within the organism. Therefore, with respect to several such cell and tissue types in the fly head, what is the "systems meaning" of a given structure's function insofar as regulation of rest-activity cycles is concerned? (Systematic oscillation of daily behavior is the principal overt phenotype analyzed in studies of Drosophila chronobiology). In turn, how do the several separate sets of clock-gene-expressing cells interact--or in some cases act in parallel--such that intricacies of the fly's sleep-wake cycles are mediated? Studying Drosophila chrono-genetics as a system-based endeavor also encompasses the fact that rhythm-related genes generate their products in many tissues beyond neural ones and during all stages of the life cycle. What, then, is the meaning of these widespread gene-expression patterns? This question is addressed with regard to circadian rhythms outside the behavioral arena, by considering other kinds of temporally based behaviors, and by contemplating how broadly systemic expression of rhythm-related genes connects with even more pleiotropic features of Drosophila biology. Thus, chronobiologically connected factors functioning within this insect comprise an increasingly salient example of gene versatility--multi-faceted usages of, and complex interactions among, entities that set up an organism's overall wherewithal to form and function. A corollary is that studying Drosophila development and adult-fly actions, even when limited to analysis of rhythm-systems phenomena, involves many of the animal's tissues and phenotypic capacities. It follows that such chronobiological experiments are technically demanding, including the necessity for investigators to possess wide-ranging expertise. Therefore, this chapter includes several different kinds of Methods set-asides. These techniques primers necessarily lack comprehensiveness, but they include certain discursive passages about why a given method can or should be applied and concerning real-world applicability of the pertinent rhythm-related technologies.

MHC class I peptides as chemosensory signals in the vomeronasal organ

The mammalian vomeronasal organ detects social information about gender, status, and individuality. The molecular cues carrying this information remain largely unknown. Here, we show that small peptides that serve as ligands for major histocompatibility complex (MHC) class I molecules function also as sensory stimuli for a subset of vomeronasal sensory neurons located in the basal Gao- and V2R receptor-expressing zone of the vomeronasal epithelium. In behaving mice, the same peptides function as individuality signals underlying mate recognition in the context of pregnancy block. MHC peptides constitute a previously unknown family of chemosensory stimuli by which MHC genotypic diversity can influence social behavior.