A DROSOPHILA CIRCUIT FOR HABITUATION OVERRIDE

Habituated animals retain a latent capacity for robust engagement with familiar stimuli. In most instances, the ability to override habituation is best explained by postulating: (a) that habituation arises from the potentiation of inhibitory inputs onto stimulus-encoding assemblies; and (b) fast habituation override occurs through disinhibition. Previous work has shown that inhibitory plasticity contributes to specific forms of olfactory and gustatory habituation in Drosophila. Here we analyze how exposure to a novel stimulus causes override of gustatory (proboscis-extension reflex or ″PER″) habituation. While brief sucrose contact with tarsal hairs causes naīve Drosophila to extend their proboscis, persistent tarsal exposure to sucrose reduces PER to subsequent sucrose stimuli. We show that in so habituated animals, either brief exposure of the proboscis to yeast or direct thermogenetic activation of sensory neurons restores the PER response to tarsal sucrose stimulation. Similar override of PER habituation can also be induced by brief thermogenetic activation of a population of TH (Tyrosine-Hydroxylase) positive neurons, a subset of which send projections to the SEZ. Significantly, sensory-neuron induced habituation override requires transmitter release from these TH-positive cells. Treatments that cause override specifically influence the habituated state, with no effect on the naīve sucrose response across a range of concentrations. Taken together, these and other findings are consistent with a model in which novel taste stimuli trigger activity in dopaminergic neurons which, directly or indirectly, inhibit GABAergic cells that drive PER habituation. The implications of these findings for general mechanisms of attentional and sensory override of habituation are discussed.

Spatial odor discrimination in the hawkmoth, Manduca sexta (L.)

ABSTRACT Flying insects track turbulent odor plumes to find mates, food and egg-laying sites. To maintain contact with the plume, insects are thought to adapt their flight control according to the distribution of odor in the plume using the timing of odor onsets and intervals between odor encounters. Although timing cues are important, few studies have addressed whether insects are capable of deriving spatial information about odor distribution from bilateral comparisons between their antennae in flight. The proboscis extension reflex (PER) associative learning protocol, originally developed to study odor learning in honeybees, was used as a tool to ask if hawkmoths, Manduca sexta, can discriminate between odor stimuli arriving on either antenna. We show moths discriminated the odor arrival side with an accuracy of >70%. Information about spatial distribution of odor stimuli may be available to moths searching for odor sources, opening the possibility that they use both spatial and temporal odor information. This article has an associated First Person interview with the first author of the paper.

Effect of Immune Inducers on Nosema ceranae Multiplication and Their Impact on Honey Bee (Apis mellifera L.) Survivorship and Behaviors

Nosema disease is a major disease of honey bees caused by two species of microsporidia, Nosema apis and N. ceranae. Current control involves using antibiotics, which is undesirable because of possible antibiotic resistance and contamination. In this study, flagellin, zymosan, chitosan, and peptidoglycan were investigated as alternatives for controlling N. ceranae infections and for their effect on bee survivorship and behaviors. Chitosan and peptidoglycan significantly reduced the infection, and significantly increased survivorship of infected bees, with chitosan being more effective. However, neither compound altered the bees’ hygienic behavior, which was also not affected by the infection. Chitosan significantly increased pollen foraging and both compounds significantly increased non-pollen foraging compared to healthy and infected bees. Memory retention, evaluated with the proboscis extension reflex assay, was temporarily impaired by chitosan but was not affected by peptidoglycan, nor was it affected by N. ceranae infection compared to the non-infected bees. This study indicates that chitosan and peptidoglycan provide benefits by partially reducing N. ceranae spore numbers while increasing survivorship compared to N. ceranae infected bees. Also, chitosan and peptidoglycan improved aspects of foraging behavior even more than in healthy bees, showing that they may act as stimulators of important honey bee behaviors.

Spatial odor discrimination in hawkmoth, Manduca sexta (L.)

Flying insects track turbulent odor plumes to find mates, food and egg-laying sites. To maintain contact with the plume, insects are thought to adapt their flight control according to the distribution of odor in the plume using the timing of odor onsets and intervals between odor encounters. Although timing cues are important, few studies have addressed whether insects are capable of deriving spatial information about odor distribution from bilateral comparisons between their antennae in flight. The proboscis extension reflex (PER) associative learning protocol, originally developed to study odor learning in honeybees, was modified to show hawkmoths, Manduca sexta, can discriminate between odor stimuli arriving on either antenna. We show moths discriminated the odor arrival side with an accuracy of >70%. The information about spatial distribution of odor stimuli is thus available to moths searching for odor sources, opening the possibility that they use both spatial and temporal odor information.

CCAP regulates feeding behavior via the NPF pathway in Drosophila adults

The intake of macronutrients is crucial for the fitness of any animal and is mainly regulated by peripheral signals to the brain. How the brain receives and translates these peripheral signals or how these interactions lead to changes in feeding behavior is not well-understood. We discovered that 2 crustacean cardioactive peptide (CCAP)-expressing neurons in Drosophila adults regulate feeding behavior and metabolism. Notably, loss of CCAP, or knocking down the CCAP receptor (CCAP-R) in 2 dorsal median neurons, inhibits the release of neuropeptide F (NPF), which regulates feeding behavior. Furthermore, under starvation conditions, flies normally have an increased sensitivity to sugar; however, loss of CCAP, or CCAP-R in 2 dorsal median NPF neurons, inhibited sugar sensitivity in satiated and starved flies. Separate from its regulation of NPF signaling, the CCAP peptide also regulates triglyceride levels. Additionally, genetic and optogenetic studies demonstrate that CCAP signaling is necessary and sufficient to stimulate a reflexive feeding behavior, the proboscis extension reflex (PER), elicited when external food cues are interpreted as palatable. Dopaminergic signaling was also sufficient to induce a PER. On the other hand, although necessary, NPF neurons were not able to induce a PER. These data illustrate that the CCAP peptide is a central regulator of feeding behavior and metabolism in adult flies, and that NPF neurons have an important regulatory role within this system.

Feeding Response to Select Monosaccharides, Sugar Alcohols, and Artificial Sweeteners Relative to Sucrose in Adult House Flies, Musca domestica (Diptera: Muscidae)

Use of insecticidal baits risks the evolution of resistance to the feeding stimulant in the bait, not just to the active ingredient (toxicant). Sucrose-based baits are widely used against house flies, Musca domestica L. The baits are applied as dry granules, but readily liquefy. The proboscis extension reflex (PER) and consumption of alternative sweeteners, dry or in solution, were examined. Fructose, glucose, and xylitol merit further study as alternatives to sucrose. Dry, fructose, glucose, and xylitol elicited PER much more than sucrose, although not when in solution. Furthermore, dry or in solution, females and males ate as much or more fructose as sucrose. In solution, flies ate as much glucose as sucrose; although when dry, consumption was much less for glucose than sucrose. Dry, xylitol elicited as much consumption as sucrose for females, though less for males. In solution, for both sexes, xylitol elicited less consumption than sucrose did. Acesulfame potassium, sodium cyclamate, and sucralose do not look promising as they did not often elicit PER, whether dry or in solution. Erythritol also does not look promising. Erythritol elicited PER no more than sucrose did when dry and elicited PER much less than sucrose when in solution. Flies ate much less erythritol than sucrose whether dry or in solution.

Olfactory Learning in the Stingless Bee Melipona eburnea Friese (Apidae: Meliponini)

Olfactory learning and floral scents are co-adaptive traits in the plant–pollinator relationship. However, how scent relates to cognition and learning in the diverse group of Neotropical stingless bees is largely unknown. Here we evaluated the ability of Melipona eburnea to be conditioned to scent using the proboscis extension reflex (PER) protocol. Stingless bees did not show PER while harnessed but were able to be PER conditioned to scent when free-to-move in a mini-cage (fmPER). We evaluated the effect of: 1) unconditioned stimulus (US) reward, and 2) previous scent–reward associations on olfactory learning performance. When using unscented-US, PER-responses were low on day 1, but using scented-US reward the olfactory PER-response increased on day 1. On day 2 PER performance greatly increased in bees that previously had experienced the same odor and reward combination, while bees that experienced a different odor on day 2 showed poor olfactory learning. Bees showed higher olfactory PER conditioning to guava than to mango odor. The effect of the unconditioned stimulus reward was not a significant factor in the model on day 2. This indicates that olfactory learning performance can increase via either taste receptors or accumulated experience with the same odor. Our results have application in agriculture and pollination ecology.