A comparative analysis of the behavioural and cognitive effects of toxin-induced sickness in the rat (Rattus norvegicus) and the honeybee (Apis mellifera)

Abstract

Tasks designed to measure affective state in animals are valuable in researching the aetiology and treatment of affective disorders in humans. However, many traditional measures are ineffective in assessing affective valence, and judgement bias tasks were designed to overcome this problem. In this thesis I aimed to identify whether rats displayed a judgement bias during sickness, and also whether the task outcomes were translatable to an invertebrate species. A comparative study was performed with a vertebrate (rat) and an invertebrate (honeybee) model exposed to toxin-induced sickness. Behavioural indicators of sickness were assessed following toxin administration, and the animals’ expectations of reward and punishment were measured on a judgement bias paradigm. This thesis includes the first behavioural characterisation of sickness in honeybees. Quinine-induced sickness in the honeybee was accompanied by a biasing of ambiguous information consistent with a negative affective state. A judgement bias was also observed in rats treated with lithium chloride, but this finding was not repeated on replication of the experiment. Methodological problems were identified and the training protocol was revised to accelerate learning of the task and to reduce extinction of responding. In conclusion, evidence of a sickness-induced negative affect in animals was identified in this thesis. This correlates with sickness in humans, thus reinforcing the argument that negative affective states associated with sickness may have an evolutionary basis. In addition, the honeybees’ performance on the task was similar to that seen in vertebrate animals, showing the potential for the honeybee model to be used in investigations of emotion. However, alterations need to be made to the specific protocols to improve the methodology for measuring judgement bias in both honeybees and rats, and recommendations are made for future experimental designs.