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Encodoing Social Interaction

Final Report Summary - ENCODING EXPERIENCE (Encodoing Social Interaction)

Every living organism interacts with the physical environment that surrounds it, starting from basic interactions of cells with their extracellular environment and up to complex and diverse interactions that occur between animals in a social group. The ability to adapt to environmental changes is an essential feature of biological systems and is achieved by a coordinated crosstalk between neuronal and hormonal programs, allowing rapid and integrated organismal responses to environmental challenges. One type of adaptation strategies is behavioral adaptation, through which animals integrate their internal physiological state with the changing conditions of the external environment and subsequently choose one action over another to increase their chances of survival and reproduction. Many of the behavioral responses to environmental stimuli are merely innate responses, but when the environmental complexity and uncertainty increases there is a need to incorporate prior experience (learning) and context into the behavioral response. One of the uncertain components of the environment is the social domain, composed of other behaving animals with which the individual needs to interact. The intricate nature of social interaction requires recognizing another member form the same species in the right context, season, sex, age and reproductive state, and to respond appropriately to different social encounters involving different characteristics and outcomes. A fundamental question in neuroscience is what are the mechanisms that allow the brain to execute intricate decisions when it comes to adapting physiology and behavior to the ever-changing environment, in particular to the social environment. Behavior can be considered as an ultimate outcome of orchestrated neuronal activity where single neurons assembled into functional circuits process external and internal information into motor output. We postulate that a particular response to environmental challenge is represented in the neurons participating in the response by molecular mechanisms that sculpt the repertoire of expressed proteins and their function. Several cellular machineries can act to diversify the molecular signature of cells, including transcriptional regulation, post-transcriptional regulation and diverse collection of Previously we have demonstrated that social experience in flies can also modulate their reward systems. Investigating the relationship between social experience and alcohol preference in Drosophila as a proxy for reward-related behavior revealed that chronic copulation vs. chronic courtship rejection, cause opposing changes in ethanol consumption. These changes are mediated through changes in levels of neuropeptide F (NPF, the fly homolog of neuropeptide Y). Our study suggests that NPF signaling represents the internal reward state of flies, and that experiences that alter NPF signaling promote behaviors that restore the reward system to its normal state. The aim of our research was to uncover the molecular and neural mechanisms by which rewarding and non-rewarding experiences are perceived, represented in the brain and converted into modulation of behavior.
We addressed the following specific aims:
1. To Study mechanisms by which social experience modulates the NPF system.
Considering the anatomical location of NPF neurons within the central nervous system, it is not clear how the experience of mating and rejection is relayed to NPF cells, nor how the activity of NPF/R system culminates in altered reward-seeking behavior. To address these questions, we undertook several approaches to identify the neuronal circuits that function upstream and downstream of NPF/R.
To identify the neurons that function upstream of NPF we used a candidate-based approach to identify neurons that control the last steps of copulation and relay the sensory information to NPF neurons. It is not clear which of the multiple sensory and motor responses performed during mating induce the perception of reward. Sexual interactions with female flies that do not reach copulation are not sufficient to reduce ethanol consumption, suggesting that only successful mating encounters are rewarding. Here we uncoupled the initial steps of mating from its final steps, and specifically tested the ability of ejaculation to mimic the rewarding value of full copulation. We induced ejaculation by activating neurons that express the neuropeptide corazonin (CRZ), and subsequently measured different aspects of reward. We show that activating Crz-expressing neurons is rewarding to male flies, as they choose to reside in a zone that triggers optogenetic stimulation of Crz neurons and display conditioned preference for an odor paired with the activation. Reminiscent of successful mating, repeated activation of Crz neurons increases npf levels and reduces the motivation to consume ethanol. These results demonstrate that ejaculation stimulated by Crz/Crz-receptor signaling serves as an essential part of the mating reward mechanism in Drosophila. This study was recently published in Current Biology and was extensively covered by the press.
To identify neuronal circuits downstream of NPF neurons we have preformed molecular profiling of NPF receptor neurons and discovered a subset which co-express a stress related neuropeptide; corticosterone releasing factor homologue DH44. Mimicking rewarding experience via the artificial activation of NPF neurons leads to reduction in DH44 levels and presumably inhibition of stress pathway. The identification of the interplay between NPF/R and DH44 pathway is a wonderful opportunity to look at the interplay between reward and stress pathways.
2. To study olfactory perception of social experience.
We used Drosophila melanogaster as a model organism to explore a basic question in neuroscience: why do different individuals experience the same sensory stimuli, such as smell differently, and moreover, why does one individual experience identical stimuli differently on different occasions? Focusing on sex specific behaviors in fruit flies, we identified odorant binding protein 69a (Obp69a) as a new player in the machinery that promotes behavioral plasticity to the same sensory stimuli in male and female flies.
To identify genes subjected to regulation by rewarding and non-rewarding social experiences, we performed a transcriptome-based analysis for genes that are differentially expressed in rejected and mated males. The screen revealed a strong induction of a subgroup of olfaction-related genes in the heads of rejected male flies. One of these genes is predicted pheromone-binding protein 1 also called Odor Binding Protein 69a (Obp69a). We found that Odor Binding Protein 69a (OPB69a) is dimorphically expressed between male and female flies, it is primarily expressed in the fly’s antennae and that its regulation of expression is affected by different social contexts in males and females. Furthermore, we found that Obp69a expression is affected by olfactory cues and that it is necessary for male-male aggressive behavior. In trying to unravel the mechanism underlying Obp69a’s function, we artificially activated Olfactory Receptor Neurons and found this affected its expression levels differently between male and female flies. These results suggest a novel mechanism for regulation of the pheromone detection system that involves interactions between Sensory Receptor Neurons and Auxiliary cells adjacent to them. The study was recently published in PLOS GENETICS.
3. To explore new behavioral paradigms to study the effect of reward level and olfactory perception on social group dynamics.
Until recently the study of social interaction in animal model systems was simplified to analyzing interaction in pairs. Contemporary technological advances in tracking animals without swapping identities made it possible to analyze spatial and temporal aspects of social interaction in groups. we discovered that sexual challenge such as sexual deprivation is causing significant changes in physiological and social behavioral responses. In the physiological aspect, although rejected males increase their mating duration in comparison to naïve males, there is no increase in the amount of produced sperm cells and the number of progeny as a result. In contrast, they decrease their investment in sperm allocation in response to sexual deprivation, suggesting that under the tested condition, mating with rejected male flies does not contribute to reduced receptivity or increase fertility. Although rejected male flies possess less sperm, they do exhibit a higher level of several accessory gland proteins such as SP and Acp53 which are known to promote post-mating responses in female flies, though the pattern of causality among these factors in not clear yet. In the behavioral aspect, we discover that rejected males increase the distances among other males from the same group and maintain large distances upon touch. They behave more vigorously toward rival males who had have experienced the same sexual challenge, whether they're placed in a group or in pairs, in comparison to the mated counterparts. This reinforces the decrease we received in the transcript levels of Cyp6a20 in the PCR results from the flies' head, since low expression levels of Cyp6a20 are related to the increase in the aggressive behavior. Combining the behavior and physiological responses to the mating challenge, we suggest that sexual deprivation changes the internal state of the fruit fly male, perhaps to provide a sort of advantage while competing with potential rival males and to ensure their paternity.