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Social buffering of fear in zebrafish
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Abstract(s)
A ubiquidade da formação de grupos em animais tem sido explicada, entre outros aspectos, pelos
seus benefícios na proteção contra predadores, nomeadamente devido ao aumento global do
estado de vigilância, diluição do risco e capacidade de confundir o predador. Assim, os grupos
sociais proporcionam um ambiente mais seguro na presença de ameaças, onde a presença de
conspecíficos diminui o medo desencadeado por eventos aversivos que surjam no ambiente, um
fenómeno designado por “social buffering”.
Este fenómeno social tem sido verificado em mamíferos, onde já existe alguma evidência
sobre os mecanismos neurais envolvidos, mas o seu estudo em outros vertebrados ainda é
escasso. Por esta razão, são necessários mais estudos comparativos para melhor compreender a
evolução do “social buffering” entre animais sociais e quão evolutivamente conservados são os
mecanismos subjacentes a este comportamento social.
Diferentes modalidades sensoriais (através de “cues”) podem promover este efeito de
“buffering” e a eficácia de cada canal sensorial neste fenómeno comportamental pode variar entre
espécies. Além disso, do que é sabido, a eficácia ao longo do tempo de diferentes “cues”
sensoriais, bem como a eficiência de grupos sociais que divirjam no seu número de elementos,
nunca foi testada no contexto de “social buffering”. Estas questões adquirem especial
importância se considerarmos que exposições mais prolongadas a ameaças e grupos sociais
maiores, influenciam as probabilidades de sobrevivência do indivíduo.
Dada a sua posição filogenética, os peixes teleósteos permitem explorar o “social
buffering” e os mecanismos envolvidos neste comportamento social, na radiação evolutiva de
maior sucesso entre os vertebrados (paralela à dos tetrápodes). O peixe-zebra é uma espécie
pertencente aos teleósteos que apresenta comportamentos de medo quando submetido a
situações ameaçadoras em contextos de isolamento social, no entanto a ocorrência de “social
buffering” nesta espécie permanece praticamente inexplorada.
Nesta tese, investigámos a ocorrência de “social buffering” no peixe-zebra, submetendo
peixes focais a um estímulo aversivo (substância de alarme) quer na presença ou ausência de
“cues” sensoriais sociais. Numa primeira experiência, mostrámos que quando submetidos à
substância de alarme na presença de “cues” sociais olfactivas (água do cardume) e visuais
(visualização do cardume), os peixe-zebra apresentaram uma resposta de medo inferior do que
quando submetidos à substância de alarme sozinhos (durante 30 minutos de exposição)
revelando a ocorrência de “social buffering” continuado da resposta de medo. No entanto,
análises ao cortisol ou RNA mensageiro da hormona libertadora de corticotrofina (CRF), dos
receptores de glucocorticóides, e receptores de mineralocorticóides, não revelaram evidências de
“buffering” da resposta de stress. Numa segunda experiência, testámos separadamente a eficácia
de “cues” olfactivas e visuais no fenómeno de “social buffering”, e verificámos que a visualização
do cardume de peixes-zebra se revelou a “cue” mais eficaz na diminuição da resposta de medo
provocada pela substância de alarme, quando num cenário de exposição constante à ameaça (30
minutos). Numa terceira experiência, demonstrámos que este “buffering” social não depende do
número de conspecíficos presentes no cardume, uma vez que cardumes mais pequenos se
revelaram igualmente eficazes em diminuir a resposta de medo provocada pela substância de
alarme. Finalmente, usando a expressão génica de um “immediate early gene” (c-fos) como
indicador de atividade neuronal, verificámos que o fenómeno de “social buffering” em peixezebra
desencadeia um padrão específico de co-activação de áreas cerebrais conhecidas pelo seu
envolvimento em respostas de medo e “buffering” em mamíferos.
Concluindo, o conjunto de estudos apresentados nesta tese sugerem uma origem
evolutiva comum nos vertebrados para o comportamento de “social buffering”, providenciando
novas perspectivas sobre os mecanismos comportamentais e neurais envolvidos neste
comportamento social.
ABSTRACT : The ubiquity of group formation among animals has been explained among several aspects by its anti-predatory benefits, including an overall increase in vigilance, the dilution of risk and predator confusion. Thus, social groups offer a safer environment in the presence of threats, and the presence of conspecifics is known to decrease the fear response to a detected threatening event, a phenomenon named social buffering. This social phenomenon has been documented in mammals, where there is already some evidence about its neural mechanisms, but its study in other vertebrate taxa is still scarce. Thus, more comparative studies are needed to better understand the evolution of social buffering among social animals and how evolutionary conserved are its underlying mechanisms. For instance, different sensory modalities can convey relevant conspecific social cues used in buffering and the efficiency of each sensory channel may vary across species. Furthermore, to our knowledge, the effectiveness of these sensory cues over time and different conspecific group sizes has never been tested in the context of social buffering, which is of great relevance considering that long exposures to threat and bigger group sizes may influence individual chances of survival. Given their phylogenetic position, teleost fish offer the possibility to investigate the occurrence of social buffering and its underlying mechanisms in the most successful evolutionary radiation among vertebrates, parallel to that of tetrapods. Zebrafish is a teleost species that expresses fear behaviour when individually exposed to threatening situations, however the occurrence of social buffering in this species has been virtually unexplored. In this thesis we investigated social buffering in zebrafish, by exposing focal fish to an aversive stimulus (alarm substance – AS) either in the presence or absence of conspecific cues. In a first experiment, we showed that when exposed to AS in the presence of both olfactory (shoal water) and visual (sight of shoal) conspecific cues, zebrafish exhibited a lower fear response over the 30 min test than when tested alone, indicating sustained social buffering of fear. Nonetheless, analysis of cortisol or mRNA expression of corticotropin-releasing factor, glucocorticoid receptors and mineralocorticoid receptors did not reveal buffering of the stress response. In a second experiment, we separately tested olfactory and visual cues effectiveness on the AS-elicited fear response, and verified that the sight of shoal was more effective in reducing fear responses in a persistent threat scenario (30 min test). In a third experiment, we found that this effect was independent of conspecific number, as smaller shoals were equally efficient as larger shoals at inducing social buffering. Finally, by using the expression of an immediate early gene (c-fos) as a reporter of neuronal activity, we showed that social buffering elicits a distinct pattern of functional connectivity among a set of brain regions known to be involved in fear-like responses and buffering processes in mammals. To conclude, the set of studies on this thesis suggests a shared evolutionary origin for social buffering in vertebrates, bringing new insights into the behavioural and neural mechanisms of this phenomenon.
ABSTRACT : The ubiquity of group formation among animals has been explained among several aspects by its anti-predatory benefits, including an overall increase in vigilance, the dilution of risk and predator confusion. Thus, social groups offer a safer environment in the presence of threats, and the presence of conspecifics is known to decrease the fear response to a detected threatening event, a phenomenon named social buffering. This social phenomenon has been documented in mammals, where there is already some evidence about its neural mechanisms, but its study in other vertebrate taxa is still scarce. Thus, more comparative studies are needed to better understand the evolution of social buffering among social animals and how evolutionary conserved are its underlying mechanisms. For instance, different sensory modalities can convey relevant conspecific social cues used in buffering and the efficiency of each sensory channel may vary across species. Furthermore, to our knowledge, the effectiveness of these sensory cues over time and different conspecific group sizes has never been tested in the context of social buffering, which is of great relevance considering that long exposures to threat and bigger group sizes may influence individual chances of survival. Given their phylogenetic position, teleost fish offer the possibility to investigate the occurrence of social buffering and its underlying mechanisms in the most successful evolutionary radiation among vertebrates, parallel to that of tetrapods. Zebrafish is a teleost species that expresses fear behaviour when individually exposed to threatening situations, however the occurrence of social buffering in this species has been virtually unexplored. In this thesis we investigated social buffering in zebrafish, by exposing focal fish to an aversive stimulus (alarm substance – AS) either in the presence or absence of conspecific cues. In a first experiment, we showed that when exposed to AS in the presence of both olfactory (shoal water) and visual (sight of shoal) conspecific cues, zebrafish exhibited a lower fear response over the 30 min test than when tested alone, indicating sustained social buffering of fear. Nonetheless, analysis of cortisol or mRNA expression of corticotropin-releasing factor, glucocorticoid receptors and mineralocorticoid receptors did not reveal buffering of the stress response. In a second experiment, we separately tested olfactory and visual cues effectiveness on the AS-elicited fear response, and verified that the sight of shoal was more effective in reducing fear responses in a persistent threat scenario (30 min test). In a third experiment, we found that this effect was independent of conspecific number, as smaller shoals were equally efficient as larger shoals at inducing social buffering. Finally, by using the expression of an immediate early gene (c-fos) as a reporter of neuronal activity, we showed that social buffering elicits a distinct pattern of functional connectivity among a set of brain regions known to be involved in fear-like responses and buffering processes in mammals. To conclude, the set of studies on this thesis suggests a shared evolutionary origin for social buffering in vertebrates, bringing new insights into the behavioural and neural mechanisms of this phenomenon.
Description
Tese de Doutoramento em Biologia Comportamental apresentada ao ISPA - Instituto Universitário
Keywords
Social buffering Medo Stress Peixe-zebra Fear Zebrafish